Opportunities for Supplementary Cementitious Materials from Natural Sources and Industrial Byproducts: Literature Insights and Supply Assessment

Abstract

This paper reviews various emerging alternative SCMs derived from minerals and biomass sources, industrial byproducts, and underutilized waste streams. The paper compiles and evaluates physicochemical properties, reaction mechanisms in cementitious systems, resource availability, supply chain dynamics, technology readiness, the impact on concrete performance, and environmental and cost factors for each candidate SCM. Specifically, the review examines wood ash from bioenergy plants, volcanic and sedimentary natural pozzolans, and construction and demolition waste. This includes recycled concrete fines, asphalt plants’ rock dust (baghouse fines), aggregate production fines, and post-consumer waste, particularly municipal solid waste incinerator ash and wastewater sludge ash. Additionally, the paper explores innovative additives such as cellulose and chitin nanomaterials and calcium–silicate–hydrate nanoseeds to address challenges of slower strength development and rheological changes. The key contribution of this review is a multifactor framework for assessing alternative SCMs, emphasizing availability, supply chain, market readiness, and environmental performance, combined with an engineering performance review.

1. Introduction

1.1. Environmental Impacts of Cement Production

Cement is an essential building material for developing and maintaining the built environment to support the growing global population, urbanization trends, infrastructure development, rebuilding, and economic growth. The most common cement type used globally to bind aggregates to form concrete is ordinary portland cement (OPC), which consists of clinker as the primary constituent. The global production of OPC is projected to increase to 5.8 billion metric tons (tonnes) by 2050 from 3.90 billion tonnes in 2020 [1]. However, meeting such immense global demands comes with noteworthy carbon dioxide (CO₂) emissions equal to 7% to 8% of total global anthropogenic CO₂ emissions [2]. The vast majority of CO₂ emissions from cement production come from the decarbonation of limestone to produce clinker, which requires burning fossil fuels to heat the kiln to 1450–1500 °C. The global cement industry must reduce emissions by 24% to avoid increases of 2 °C in global temperature by 2050, a goal proposed in the 2015 United Nations Climate Change Conference [2]. Furthermore, the North American limestone market was valued at 1167 million tonnes in 2024, estimated to reach 1567 million tonnes by 2033 [3]. While limestone itself is abundantly available, local shortages can occur due to transportation costs (high if far from quarries), environmental and permitting regulations (some areas restrict mining), and competing land uses (urbanization limiting quarry expansion).

As part of this direction, the cement and concrete sector faces increasing mandates to reduce emissions. For example, Senate Bill 596, signed by the California governor in September 2021, requires developing a comprehensive strategy for the cement industry to cut emissions by 40% by 2030 and achieve net-zero emissions by 2045 [4]. However, decarbonizing a hard-to-abate heavy industry such as the cement sector requires a multifaceted approach. This approach includes a combination of material optimization, energy efficiency improvements, innovative decarbonization technologies, and other strategies [5].

One material optimization strategy is the more efficient use of OPC in concrete, especially using blended cements. In fact, according to Global Cement Technology, reducing the clinker-to-OPC ratio offers 2.6 billion tonnes CO₂-eq savings globally by 2050 [6], equivalent to a 35% emissions reduction. This solution seems simple: use less cement to make concrete. The use of portland limestone cement (PLC) as a means of reducing the global warming potential (GWP) of concrete is a recent step toward the industry goal of net-zero carbon emissions by 2045 [7].

The ready-mix concrete industry in the U.S. employs clinker reduction strategies by partially substituting OPC or PLC with supplementary cementitious materials (SCMs) in concrete. In other countries, blended cements are produced primarily at the cement plant instead of the concrete plant. In addition to the environmental benefits of this practice, SCMs contribute to improving the hardened concrete properties through hydraulic or pozzolanic activity [8,9,10]. Pozzolans are materials with amorphous silica and alumina (glassy) content. In the presence of water, the amorphous contents of SCMs react with calcium hydroxide (CH), an undesired product of the hydration of OPC, to form additional desired calcium–silicate–hydrate (C-S-H) or calcium–aluminate–silicate–hydrate (C-A-S-H). These products fill the pores and contribute to the strength and durability of concrete. These reaction products enhance concrete’s late strength and durability by pore refinement and reduced permeability [11]. SCM-blended concretes also exhibit lower chloride permeability and improved resistance to alkali–silica reaction (ASR), sulfate attack, freeze–thaw cycling, and salt and deicer exposure durability [12]. Other ways inorganic materials can consume CH and contribute to C-S-H development are through hydraulic reactions similar to OPC. While these materials are not pozzolanic, they consume CH and provide the same benefits as pozzolans to the durability of concrete by producing C-S-H [11]. In addition, inert mineral fillers could serve as clinker substitutions in concrete, enhancing packing density and adding to its strength as long as the durability requirements are maintained. Research shows that a large portion of the clinker can be replaced by ground limestone filler, especially when combined with pozzolanic calcined clay, resulting in blended cement with much lower CO₂ emissions [13]. Limestone filler has much lower GHG emissions than clinker, because it only undergoes milling without the high CO₂ emissions associated with the decarbonation of limestone. Up to 15% limestone filler is allowed in blended cement per ASTM C595 [14] specifications.

1.2. Implications of Traditional SCM Shortages for Concrete Supply Resilience

Coal fly ash is the most widely used SCM in concrete in the U.S., which became widely available in the 1930s, with a documented history of use in concrete in 1937 [15,16]. In 2018, 32.8 million tonnes of coal fly ash was produced, with 18.2 million tonnes beneficially utilized, primarily in concrete and concrete products, according to the American Coal Ash Association (ACAA) [17]. Industry estimates indicate that California used 1.25 million tonnes of SCMs, of which 1 million tonnes was fly ash [18].

However, environmental regulations targeting mercury and CO₂ emissions at coal-fired power plants have led to power plant closures or conversion of plant operations to natural gas [19]. According to the U.S. Energy Information Administration (EIA) projection, the kilowatt-hours of electrical energy generated by coal-fired power plants will drop 23% by 2050 compared to 2020 levels. The EIA predicts that the drop may be even more significant, up to 49%, depending on policy regulations and the cost of other electricity generation alternatives [20]. Additionally, coal production in the U.S. western region is estimated to decline from 322.7 million tonnes in 2021 to 249.5 million tonnes in 2026 [18]. Consequently, a parallel decline in fly ash production would be expected. Despite the decline in production, the ACAA projects that the use of beneficiated coal fly ash, supported by ash harvested from disposal sites, imports, and improvements in technology and logistics, will maintain an average consumption of 26.3 million tonnes per year between 2018 and 2039 [17]. This projection indicates how the concrete industry will tap into various resources to meet the SCM demand despite the supply shortages.

Similarly, a reduced production of slag cement, also known as ground granulated blast furnace slag (GGBFS), results from the decarbonization of the steel sector in the U.S. The U.S. steel industry has transitioned from blast furnace-basic oxygen furnace production from iron ore to scrap steel-based production by electric arc furnaces, which now represent 71% of steel production in the U.S. In 2023, world iron slag production was estimated to be between 330 and 390 million tonnes [19]. Much of the GGBFS used in the U.S. is imported, as there are a few active granulation and pelletizing sites with limited production. These shortages in domestic supplies of GGBFS and coal fly ash require concrete producers to find alternative SCMs to reduce the clinker-to-OPC ratio. Demand for SCMs is expected to grow significantly as various government agencies in the U.S. and worldwide plan to adopt construction materials with lower embodied carbon. Federal Buy Clean initiatives in the U.S. [21], the U.S. Environmental Protection Agency (EPA)’s Label Program for low-embodied carbon construction materials [22], the Green Public Procurement Policy in Canada [23], and similar policies in other countries and state mandates across the U.S. prioritize the use of lower-emission concrete in construction projects. These regulations emphasize reducing the embodied carbon of concrete, making SCMs a critical component in achieving sustainability goals.

Finally, depending on mining and processing, SCMs have different GWP and other environmental impacts compared to fresh coal fly ash, which does not require any processing and is typically considered burden-free to concrete in the life cycle assessment (LCA) of concrete, as they do not have economic value beyond recovering the costs of their collection and transportation [24]. Most SCMs from natural sources, industrial byproducts, or waste streams will require a combination of mechanical, chemical, and thermal treatment to increase fineness and surface area, pozzolanic or hydraulic reactivity, or compatibility with concrete, which are associated with negative environmental impacts and should be accounted for in an LCA.

The motivation for this paper is to address some of the key barriers to the adoption of alternative SCMs and to highlight a selection of potential SCMs that have been overlooked in literature or practice. Additionally, the paper seeks to present a comprehensive assessment framework that considers the multifaceted challenges associated with SCM implementation, with a particular focus on selecting alternatives that offer high source availability.

Moreover, recent advancements in nanomaterials with high surface area to enhance early strength gain, nano reinforcement, and internal curing capabilities provide a promising complementary or alternative approach to traditional SCMs. These innovations promote the hydration of OPC and potentially enable a reduction in cement content in concrete while maintaining or improving performance. This dual focus on alternative SCMs and emerging technologies aims to contribute to addressing practical barriers to the adoption of alternative SCMs.

1.3. Paper Scope and Methodology

This paper aims to identify SCMs, fillers, and admixtures produced from biomass, minerals, construction and demolition waste, and industrial byproducts with currently low recycling rates to identify alternative sources of SCMs other than fly ash and slag cement to address the shortage of these traditional SCMs. The guiding principles were to reduce the GWP of concrete by partially replacing the OPC with the SCMs/fillers, reducing cement content, or achieving the desired properties or performance with the use of customized admixtures. Secondly, to create recycling paths for large waste streams and industrial byproducts, promoting resource efficiency and reducing waste accumulation, while promoting a circular economy. The groups of materials shown in Figure 1 were identified to have viable availability of supplies or potential in terms of engineering, environmental, and cost, and are presented and discussed in this literature review paper.

For each of the reviewed SCMs, the findings for each SCM are summarized in the following organization:

• Description and acting mechanism in concrete.
• Supplies, supply chain, and the National Aeronautics and Space Administration (NASA’s) technology readiness level (TRL).
• The performance of concrete is based on literature with a focus on three aspects: its impact on fresh properties, strength, and durability.
• Cost and environmental impacts considerations.

This framework can serve as a guide for exploring new construction materials, emphasizing the importance of a thorough preliminary investigation before market introduction. The assessment framework application was demonstrated for the Western region of the U.S., particularly California, for example. A similar type of analysis of supplies and supply chains is suggested to be applied to each region specifically.

1.4. Literature Review Methodology

A list of potential SCMs was developed through an extensive initial identification process. The process involved a literature review to assess their engineering performance in concrete and cement-based systems. The comprehensive list of potential SCMs identified is available in detail in another reference [25]. This article focuses on a subset of SCMs identified as viable for further exploration and future implementation.

The description of the SCM, its physicochemical properties, and its potential mechanism of reaction in concrete were gathered from the scientific literature. Information on supply availability and processing methods for each SCM was compiled from diverse sources, including U.S. Geological Survey reports, industry trade associations, communication with industry contacts, publicly available information, and a range of publications (e.g., journal articles, conference papers, books, reports, and specifications). Targeted keyword searches were conducted to assess the impact of SCMs on concrete performance. These included combinations of the SCM name with terms such as “fresh concrete properties”, “fresh cement paste properties”, “fresh mortar properties”, “impact on strength”, and “impact on durability.”

Additional research was performed using scholarly articles, technical reports, and databases to gather cost- and environmental impact-related information for each SCM. This process involved identifying information on production methods, additional processing (e.g., washing, crushing, grinding), and treatment requirements. Keywords used in the searches included “environmental impacts”, “LCCA”, “LCA”, “cost of [SCM name]”, “environmental product declaration”, “global warming potential”, and “greenhouse gas (GHG) emissions.”

2. Potential Supplementary Cementitious Materials

2.1. Wood Ash from Biomass Combustion for Energy Generation

2.1.1. Description and Acting Mechanism in Concrete

Forest and other biomass feedstocks are used as renewable energy sources worldwide to replace fossil fuels with high GHG emissions [26]. Biomass used for heat and electricity production is mainly from forestry and agricultural residues, dedicated energy crops, wood fuel (charcoal, chips, and pellets), municipal solid waste, sewage sludge, and paper sludge. Among these, wood biomass from forestry energy is currently the largest source of biomass [26].

The combustion of these biomass sources produces heat or energy, leaving behind fly ash and bottom ash in various amounts depending on the feedstock and combustion conditions. Ash management presents a challenge for energy plants since large-scale biomass combustion generates significant amounts of ash [27].

The physicochemical properties of the ashes produced from biomass combustion are highly variable and depend heavily on the feedstock type, conditions, and the combustion method [28]. This variability complicates their use in concrete applications, where consistent performance is critical. For example, the effect of wood ash on the long-term durability of concrete is not well understood and remains a general concern [29,30]. Wood ash from combustion energy plants can differ significantly in physiochemical properties from coal ash, requiring thorough characterization covering various geographical areas and tree species, combustion, and treatment methods, so its implications for use as SCM for concrete can be understood [26]. Such comprehensive characterization will help move toward developing a dedicated standard specification, which the lack of it currently poses a challenge for this ash adoption as an SCM for concrete.

The range of oxide contents for woody biomass was collected in reference [26] for a broad range of tropical hardwood, temperate hardwood, softwood, temperate hardwood bark, and softwood bark. The data highlights the significant variability in composition depending on the feedstock and combustion technology (fixed bed, pulverized fuel, or fluidized bed) [31]. The amount of CaO and the three oxides important for pozzolanic reactivity (SiO₂, Fe₂O₃, Al₂O₃), and the sum of the three oxides are shown in Figure 2a. The chemical composition of woody biomass is dominated by CaO, followed by moderate amounts of SiO₂ and lesser amounts of oxides of Mg, Al, K, and P [32]. Figure 2a shows that the sum of the three oxides essential for pozzolanic reactivity is less than 30% for most ashes, which is significantly below the 70% threshold prescribed by ASTM C618 for pozzolanic reactivity of coal ash [33]. However, about 30% of these oxides are present in softwood. A minimum of 18% CaO is required for Class C coal ash, which is met by all wood types. The mean values of some other oxides are also important to the performance of concrete, as shown Figure 2b. Figure 2b shows that softwood, temperate hardwood, and softwood bark have lower amounts of alkalis than the other species.

In contrast, hardwood has a notably high concentration of K₂O, followed by softwood and temperate hardwood and softwood bark. High contents of water-soluble alkalis of potassium (K) and sodium (Na) in a cementitious system could react with the amorphous silica content present in certain aggregates and result in the formation of harmful alkali–silica gel inside the concrete, a reaction known as ASR. Concrete structures such as dams, pavements, and bridges are particularly vulnerable to ASR if they are constructed with reactive siliceous aggregates since they are exposed to water, providing moisture for the reaction. The ASR gel is hydrophilic and expands upon the absorption of water. This expansion could result in physical damage to concrete by micro-hydraulic fracturing of the aggregate and the paste, and lead to the structure’s failure [34]. Mitigating ASR risks includes controlling the level of alkalis introduced into the concrete pore solution and primarily by limiting the amount of alkalis in terms of sodium oxide equivalent [Na₂Oe = Na₂O + 0.658K₂O] to 0.6%, according to ASTM C150 [35]. However, it has been shown that other sources of alkalis, for instance, from SCMs, can also increase the risks of ASR [36]; thus, limiting the maximum alkali content of concrete is the preferred approach. Maximum permissible Na₂Oe contents of between 2.5 and 3 kg/m³ and 3.0 to 3.5 kg/m³ have been specified for high and medium reactive aggregate, respectively [37]. Typically, SCMs that are low in calcium oxide (CaO), such as Class F coal fly ash and natural pozzolans, have been the most successful ASR mitigators [38]. For instance, slag cement is effective in ASR mitigation, but the cement replacement level needs to be more than 50% to reach the mitigation levels of Class F coal fly ash [11].

Excess sulfate ion content can also result in the formation and crystal growth of ettringite in the capillary pores of concrete during external sulfate attacks or in the gel pores during delayed ettringite formation (DEF) [39]. This crystal growth can lead to physical damage to concrete, a phenomenon known as a sulfate attack. While cases of sulfate attack in concrete foundations have been reported in California and other regions, structural failures solely due to sulfate attacks are rare. By comparison, ASR poses a greater risk to concrete than sulfate attack, according to Mehta, who investigated several cases of sulfate attack [40,41]. Nonetheless, to mitigate the risks associated with sulfate attacks, the allowable sulfur trioxide (SO₃) content in cement and SCMs is regulated. For cement, the maximum SO₃ content is capped at 3.0–3.5% based on the C₃A content, as specified in ASTM C150. For SCMs, ASTM C618 limits SO₃ content to 4–5%, depending on the type of SCM, though these prescribed thresholds have been questioned in recent research [42]. Based on the data presented in Figure 2b, the amount of SO₃ is the lowest in temperate hardwood and softwood bark. Temperate hardwood and softwood have a higher SO₃ content than the 4% allowable threshold for SCMs for concrete use, highlighting the necessity for sulfate attack testing for wood ash concrete.

The acting mechanism of wood ash in concrete relies on its oxide composition together with glass content. According to some studies, biomass combustion ashes could react with CH and deliver pozzolanic or hydraulic reactivity in concrete [43]. Both amorphous and crystalline phases were detected in wood fly ash and bottom ash. On average, amorphous content was about 72% (range of 46.6% to 94.2%) in fly ash and 66% (range of 53.8% to 73.3%) in the bottom ash [44]. The hydration of biomass ash resulted in products such as CASH products and calcium carbonate (CaCO₃). The reactive aluminate phase of the ash consumed more CH by pozzolanic reaction and produced strätlingite (C₂ASH₈) [31]. Other studies have shown the potential of biomass ashes as alkali activators due to their high potassium content in alkali-activated concrete with the high content of coal fly ash or GGBFS, and other precursors [45].

2.1.2. Supplies, Supply Chain, and Technology Readiness Level

According to the 2022 Global Bioenergy Association report [46], the supply of biomass globally was estimated to be 3.29 billion tonnes (57.5 exajoules [10¹⁸ joules]) in 2020. California has a biomass resource potential of about 47 million bone-dry tonnes [47]. These estimates are increasing as California and other regions implement strategic forest management plans to remove excess wood and suppress wildfires [48]. California hosts 66 direct combustion biomass facilities, of which 30 were operational at the time of this study [47]. The decline in governmental incentives for bioenergy since 1996, driven by increased focus on other renewable energy sources such as solar, wind, and hydrogen, is the main cause of the reduction in biomass energy production [49]. The trajectory of the biomass-to-electricity industry remains uncertain; thus, the supply of generated ash is correspondingly volatile. Many biomass energy plants in California face financial challenges [50], compounded by the added costs of managing and disposing of their ash. The costs of truck rentals and disposal fees add to their operational burdens, with little to no revenue generated from the ash. Some low-key applications, such as road stabilization and soil amendment, have been tried out by some facilities. As a result, many facilities are actively seeking ways to generate additional income from the benefits of biomass ash. The potential use of this ash in concrete applications presents a promising opportunity. By creating a value-added product, the use of biomass ash in concrete could help offset costs and improve the economic viability of bioenergy plants, potentially tipping the scales toward break-even or maybe profitability.

Currently, however, a supply chain is not in place for using wood ash in concrete. Many wood fly ashes and all bottom ashes require mechanical particle size reduction to enhance their pozzolanic or hydraulic reactivity, and some may require separation of the carbon content by screening. This necessitates the involvement of intermediary handlers equipped with expensive milling and screening facilities. Additionally, the variability in feedstock supply throughout the year or across regions demands frequent testing and certification, further increasing the cost of processing the ash.

Moreover, the logistics of delivering processed ash to end users, such as cement plants or ready-mix concrete suppliers, require the development of appropriate transport networks, including rail or truck systems. Beyond logistical and processing challenges, education and training are essential to ensure wood ash is proportioned correctly in concrete mixtures to achieve target performance specifications for each application.

Based on the authors’ knowledge and experience with wood ash in concrete, its application is at the “proof-of-concept demonstrated, analytically and/or experimentally” stage, which corresponds to Level 1–3 on NASA’s TRL scale. This low TRL underscores the need for further research and development to advance the use of wood ash as SCM in concrete from concept to practical implementation.

2.1.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

A study reported an 18% increase in water demand for wood fly ash-cement paste compared to 100% OPC paste [44]. This increased water demand with the addition of biomass fly ash (BFA) has been consistently observed in several other studies [51,52,53]. Another study showed that the mortar consistency decreased with an increase in biomass percentage in the mixture [53]. Other studies reported a decrease in concrete workability with the addition of waste BFA at a constant water-cement ratio [51,53]. This outcome is attributed to the water absorption of the ashes and the irregular shape and rough texture of the particles. A study showed the unit weight of concrete did not change when 25% of wood fly ash was added to concrete [53]. Furthermore, the setting time of concrete was reported to remain unaffected by BFA at replacement levels lower than 30% [52]. Effects on set time were found to be comparable to coal fly ash [52,54]. Any delay in the set times is attributed to the dilution of the Portland cement as a portion of the cement is replaced by the wood ash [55].

Impact on Strength

One study showed that the initial (3 to 14 days) compressive strength and split tensile strength were much lower in concrete containing 25% wood fly ash compared to the control concrete with no wood fly ash; however, beyond 28 days, those strength properties were comparable to the control [44]. Wood ashes at 30% cement replacement resulted in about 20% lower 56-day compressive and flexural strength than cement alone and cement with coal fly ash at the same replacement level [52]. On the other hand, wood ash at 10% cement replacement resulted in a 12% compressive strength increase versus cement alone after one year [56]. Another study similarly showed that compressive strength decreased with increased ash content [56]. Overall, the literature indicates that wood ash tends to result in lower early strength compared to systems made with 100% cement. However, at moderate replacement levels, wood ash has been shown to provide comparable strength results at later ages [51,52,53,57,58].

Impact on Durability

In general, pozzolans are expected to contribute to ASR mitigation by consuming the CH content, thus reducing the hydroxyl groups in the pore solution that lead to the initial breakdown of reactive silica components in the aggregates, which leads to ASR gel formation or partake in alkali binding [36]. On the other hand, SCMs, including wood ashes, could also bring additional alkalis to the pore solution, thus increasing ASR risks. Whether wood ashes can serve as an ASR mitigator was investigated in one study of co-fired plants that burned 10% to 20% switch grass mixed with 80% to 90% coal [57]. The tested biomass ashes reduced ASR expansion to the acceptable 0.1% limit in 6 months and were more effective in mitigating ASR expansion than Class C fly ash alone. However, more testing of wood ash performance as an ASR mitigator is needed.

Not many studies have investigated the impact of biomass ash on the sulfate attack resistance of concrete. However, one study showed improved sulfate resistance with 10% cement replacement with wood biomass ash in mortars [59]. More research is necessary to determine the effect of wood ash on sulfate attack mitigation.

Other test methods, such as electrical resistivity (ASTM C1876) [60], can provide insights into the pore connectivity of concrete and its ability to resist the permeation of harmful ions, including sulfates, alkalis, and chlorides inside the pores. Pore connectivity and permeability are physical properties that serve as key indicators of concrete durability. The electrical resistivity of the cementitious material is used as a simple, non-destructive method to evaluate its durability. The concept is that higher electrical resistivity indicates a more refined and less interconnected pore network; thus, the pore solution conveys less electrical charge [61]. Overall, cementitious systems with more refined pore systems are more resistant to the penetration of harmful ions (such as sulfates and chlorides) carried by water into the concrete; thus, the concrete with a higher electrical resistivity is considered more durable, but the net charge from the cations and anions in the pore solution of concrete containing various types of SCMs could impact the test results [61,62]. One study investigated the impact of biomass ash on the electrical resistivity of mortar. In that study, using biomass bottom ash in mortar did not have a significant effect on the electrical resistivity, but it increased the chloride diffusion compared to plain mortar [56]. A different study showed wood ash with nearly 53%wt silicon dioxide, 13% alumina, and around 6% iron oxide resulted in better performance in terms of electrical resistivity than the control after 28 days [59]. Conversely, another study showed that wood fly ash concrete had lower chloride permeability than the control OPC concrete [63].

Durability to damage from freeze–thaw cycling in concrete is achieved by entraining a sufficient amount of well-distributed small air voids into concrete [62]. As discussed earlier, a relatively higher loss on ignition (unburnt carbon) in wood biomass ash could interfere with the air-entraining agent functionality and reduce the amount of entrained air [30]. A study showed that BFA at a 15% replacement rate of OPC in concrete increased freeze–thaw durability. However, the source of the biomass ash was not provided in the study. Based on the reported chemical composition in the study, the BFA used had a high (68.5%) silicon dioxide content [64].

Another durability issue in concrete is its potential to crack due to excessive drying shrinkage [65,66]. Many SCMs, especially those with high fineness and unburnt carbon, can lead to a less workable concrete mixture. This issue should be mitigated using water-reducing admixtures and not adding more water. A higher water-to-cement ratio concrete is well known to result in more drying shrinkage and other durability issues. A study that used the same water content for all the mixes showed that adding forest biomass ash proportionally increases the drying shrinkage of mortar [67]. More studies are needed to fully understand how wood biomass ashes affect the drying shrinkage behavior of cementitious systems.

A summary of the effect of biomass ash on the properties of cement-based materials based on the reviewed literature is presented in

Table 1. Authors’ synthesis of the literature on the effect of biomass ash on the properties of cement-based materials.

Property of Concrete	Comparison to 100% OPC Concrete
Water demand	Increase
Early strength	Decrease
Late strength	No change at moderate replacement rates
Pozzolanic reactivity	More research needed
Setting time	No change or delay from the dilution of cement
Drying shrinkage	More research needed
Alkali–silica reaction	More research needed
Sulfate attack resistance	More research needed
Freeze–thaw durability	More research needed

. Based on the reviewed literature, while a comprehensive database of oxide compositions for wood ashes, encompassing a wide range of feedstock types, is available in the literature, the direct testing of wood ashes in concrete remains insufficient. Although these oxide compositions provide a basis for predicting potential effects on concrete performance, the lack of extensive experimental validation has led to speculation, misconceptions, and the absence of standardized guidelines for certification. Standardization is necessary to significantly reduce the risk associated with using wood ash in concrete in its current state of evaluation. Additionally, whether renewable energy from biomass sources will be of more interest in the future in various regions of the world, including the Western U.S., is difficult to predict. This will ultimately affect the supply of ash and research interest in the application of wood ash.

2.1.4. Environmental and Cost Considerations

Biomass combustion ashes can be regarded as a waste product of energy generation because they have no economic value beyond the cost of their processing and transportation. As a waste product, the environmental burdens associated with the ashes’ production can be attributed to energy generation. To assess the environmental impacts of biomass ashes as a concrete mineral admixture, all the required treatments (e.g., grinding) to make the ashes suitable as an SCM must be considered.

One study performed LCA of the reuse of BFA as an SCM in the mortar as an alternative to the scenario of landfilling the ash [68]. The study’s goal was to compare the potential impacts associated with the current waste management of fly ash from biomass combustion with its potential reuse as an SCM in cement formulations as an alternative management strategy. Two scenarios were studied, and a comparative analysis was performed. The first scenario included the use of OPC as the binder material and the transportation of the biomass ash to the landfill, while the second scenario considered the use of BioCement, where the BFA was used as an SCM with a reduced amount of OPC. The declared unit of the study was set to be 1 tonne of binder material. The study concluded that using biomass ash as SCM is preferable over landfilling for all the nontoxic categories. The study also found that where the replacement was 20% and 40% by weight of the concrete, GWP (CO₂-eq) reduction was between 11% and 26% with the use of biomass as an SCM compared to biomass being landfilled. In addition, human toxicity and ecotoxicity midpoint impacts were found to be lower for the biomass ash blended cement scenario.

Another study investigated the suitability of wood BFA to be used as an SCM with OPC [69]. The study evaluated the effects of OPC replacement with high levels of BFA on the environment. The functional unit of the study was 1 tonne of final blended mortar, defined by 0%, 10%, 20%, 30%, 40%, 50%, 60%, and 70% weight of BFA. The study concluded that approximately 25% potential savings could be achieved in primary energy consumption and CO₂ emissions when OPC was partially replaced with 30% BFA. At this replacement level, the mechanical properties of the BFA concrete were maintained at acceptable levels. The overall environmental benefits can reach 50% if higher amounts of BFA are used, but the loss in mechanical properties would limit the application [69].

Biomass ashes have some reuse applications, but these applications are of low or zero value. Based on our conversations with biomass plants, most ashes are disposed of in landfills or transported to farms as soil amendments. Therefore, the current use of biomass combustion ashes provides either minimal economic benefits or imposes expenses on plants as they arrange to transport ashes to farms. However, any treatments required to render the ashes usable as SCM (e.g., grinding of bottom ash, drying) will be added costs.

2.2. Natural Pozzolans with Volcanic Origin

2.2.1. Description and Acting Mechanism in Concrete

Natural pozzolans are a wide range of aluminate-silicate inorganics with sedimentary origins, such as clay, diatomaceous earth (DE), and opaline shales, and volcanic origins [70] with the potential to be used as SCM in their naturally occurring state or following some treatment [71]. Volcanic pozzolans, formed during volcanic eruptions, include pumice, zeolite, perlite, volcanic ash, and scoria. They are predominantly found in regions with a history of volcanic activity, such as the western areas of North and South America, and are widely distributed across other geologically active parts of the world.

Pumice, a highly porous pyroclastic material from the rapid cooling and solidification of volcanic matter, is a colorless or light-gray igneous rock material [70]. Pumice ore comprises SiO₂ (60% to 75%), Al₂O₃ (13% to 17%), Na₂O-K₂O (7% to 8%), Fe₂O₃ (1% to 3%), CaO (1% to 2%), and low amounts of TiO₂ and SO₃ [72]. Because of the high silica and amorphous phase content, pumice powder is considered an SCM [73].

Zeolites are formed from the diagenetic alteration of pyroclastic materials by alkaline waters under high pressure during deposition. However, the alteration induced by less alkaline fluids results in the formation of clay minerals [71]. Zeolites are hydrated alumina-silicates with evenly stacked silica and alumina tetrahedra, resulting in an open, porous, and stable framework of consistent diameter channels [70].

Perlite, unlike zeolite, is an unaltered pyroclastic rock or volcanic glass with high amounts of water [71]. Perlite can expand up to 35 times its original volume when subjected to a temperature of about 1100 °C [74]. Perlites contain approximately 75% SiO₂, 10% to 15% Al₂O₃, and some amounts of alkalis.

Natural volcanic ash forms from the violent separation of molten rock into small pieces during explosive volcanic eruptions [70]. Volcanic ash comprises fragments of glass, minerals, and rock less than 0.08 in. (2 mm) in diameter [75]. Artificially, they are derived from crushing or grinding both loose and consolidated volcanic rocks such as pumice and scoria [75]. The mineralogical and chemical compositions, including the glass content and fineness of the volcanic ash, determine its pozzolanic reactivity [70].

Volcanic scoria is the fragments of vesicular magma with a density lower than 1 g/cm³. The internal structure of scoria contains tight pores and cells. Scoria is characterized by high amounts of vitreous components and high content of SiO₂, which makes it a candidate for a reactive pozzolan. The SiO₂ in scoria can react with the CH present in cement, mortar, or concrete to form C-S-H gel [76]. One study showed that black scoria contains the required amounts of SiO₂ (46.52%), Al₂O₃ (13%), and Fe₂O₃ (11.4%) to be considered a pozzolan. Also, scoria had less than 10% loss on ignition (2.58%) and less than 4% SO₃ content (0.27%) [77].

2.2.2. Supplies, Supply Chain, and Technology Readiness Level

Using natural pozzolans in concrete is mainly influenced by the local availability of suitable deposits and the existing market of traditional industrial byproduct SCMs [71]. Volcanic pozzolans are available in the Western region of North and South America, among other areas of the world with a history of volcanic reactivity [71].

Reports of production amounts of some natural pozzolans are available from the US Geological Survey mineral summary reports [78]. The U.S. produced 580,000 tonnes of pumice in 2021, mined in California, Kansas, Idaho, Oregon, and New Mexico. Most of the mined pumices are concentrated in the western states. Additionally, 87,000 tonnes of natural zeolites was produced in 2021 from nine zeolite mines across Arizona, California, New Mexico, Idaho, Texas, and Oregon. In 2021, 500,000 tonnes of perlite was produced from eight mines in six Western states, indicating an abundance of pyroclastic natural pozzolans in California and neighboring states in the Western U.S., such as Utah and Nevada.

The use of natural pozzolans became popular for the construction of many large infrastructures in the 1940s–1950s, including several dams, according to the United States Bureau of Reclamation [79]. In addition to the dams, pumice powder was used in the Los Angeles aqueduct built in 1912 [70]. Based on these examples of use, volcanic pozzolans are considered to be at a high TRL 8 (“technology has been tested and ready for implementation”) or 9 (“technology fully implemented”).

However, after coal fly ash dominance of the concrete market, the use of natural pozzolans in recent years has been very few. Recently, interest has emerged in natural pozzolans as the production of coal fly ash may decline, as discussed in the Introduction section. Several concrete suppliers incorporate natural pozzolans in their ready-mix concrete and other applications, such as pavers and masonry.

2.2.3. Performance in Concrete Based on the Literature

Impact on Fresh Properties

The effect of some natural pozzolans, such as DE, pumice, zeolite, and volcanic ash, has been investigated in several studies. The findings generally indicate a noticeable increase in water demand for DE and zeolite, while some, such as pumice and volcanic ash, have a moderate impact on water demand and increase water demand only at high replacement rates [73,80,81,82]. Factors that are attributed to higher water demand from natural pozzolans are their morphology, such as their jagged edges and irregular shape, as well as their rough surface texture and high porosity.

In addition, the initial and final setting times of mortar increased when 20% to 30% of cement was replaced with perlite. Another study used volcanic ash as the cement replacement and found that it did not have a noticeable effect on the initial and final setting time [83]. The study also found that a 10% replacement of cement with volcanic ash resulted in similar initial and final setting times to the control.

Impact on Strength

Different studies have evaluated the effect of replacing OPC with natural pozzolans of volcanic origin on the mechanical properties of concrete [84,85,86]. In one study, 25% of cement was replaced with calcined and uncalcined pumices in mortar [87]. The study concluded that using calcined pumice in mortar mixes would cause higher strengths, while using uncalcined pumice would decrease the compressive strength of mortar. Another study investigated the engineering properties of concrete containing zeolites at 10%, 20%, 40%, and 60% of OPC replacement rates [88]. The results showed that sufficient compressive strength and adequate fracture toughness could be achieved with 20% zeolite content.

Another study on the use of Jordanian volcanic tuffs as an SCM found that several of the volcanic tuffs achieved later-age compressive strengths comparable to or exceeding those of mixtures containing 100% OPC [89]. A study on the use of ultra-fine volcanic ash (VAF) as an SCM found that mortars containing VAF20 (VAF without heat treatment at a 20% replacement level) exhibited a slightly higher compressive strength (69.6 MPa) compared to the control mix (68.1 MPa) at 91 days [90]. Another study on the use of volcanic powders originated from natural deposits in the southwest of Algeria (Boukais Massif) as an SCM showed that the strength activity index of the mortar mixes ranged from 82.11% to 91.91% at 28 days of age [91]. Overall, from the reviewed studies, natural pozzolans are expected to provide relatively similar levels of long-term strength to the 100% OPC concrete mix.

Impact on Durability

Volcanic natural pozzolans are expected to consume the CH content in systems that contain hydrated OPC and produce mainly C-S-H. They generally enhance durability by pore filling, pore structure refinement, and CH consumption. One study showed that the addition of pumice powder as an SCM (up to 30%) contributed to pore filling in high-strength concrete, thereby decreasing the concrete’s permeability [92]. Another study showed that the drying shrinkage of the concrete containing blended cement (ASTM Type I Portland cement with 20% finely ground pumice) was slightly lower than the control normal weight concrete [93]. One study showed that the addition of 5–30% zeolite in concrete improved properties such as chloride penetration, shrinkage, sulfate resistance, water permeability, and carbonation resistance [94]. Nevertheless, more experimental studies are required to clarify the impact of volcanic natural pozzolans, particularly on drying shrinkage, and to elucidate the underlying mechanism.

A summary of the effect of natural pozzolans with volcanic origin on the properties of cement-based materials is summarized in

Table 2. Authors’ synthesis of the literature on the impact of volcanic natural pozzolans on concrete properties compiled based on a few studies reviewed above. More test data is needed to further validate and expand these findings.

Property of Concrete	Comparison to 100% OPC Concrete
Water demand	Increase
Early strength	Varied
Late strength	Slight increase or decrease reported. Generally marginal change
Setting time	Delay or no change
Drying shrinkage	More test data needed
Alkali–silica reaction resistance	Increase
Sulfate attack resistance	Increase but more test data needed
Freeze–thaw durability	Increase but more test data needed

Table source: reference [95].

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2.2.4. Environmental and Cost Considerations

Typical steps in the process of producing SCM from a natural pozzolan of volcanic origin are mining, transporting, crushing, screening, and grinding [96]. Pretreatment is generally not a requirement, depending on the level of pozzolanic reactivity.

Using natural pozzolans as SCM in concrete is expected to reduce the environmental impact of concrete made with portland cement, as they do not require the high-temperature pyroprocesses involved in clinker production. If calcination is necessary, it is typically performed at a lower temperature and does not directly emit CO₂. The energy consumption for natural pozzolans is mainly associated with mining, crushing, sorting/grading, and milling. Transportation of volcanic-origin natural pozzolan is anticipated to be a significant source of pollution, particularly due to the use of diesel fuel for long-distance transport. However, the local availability of natural pozzolan in California and the Western United States is expected to reduce the environmental impact of transportation. Additionally, the generation and disposal of reject fines during the mining and milling of pumice and other pozzolans may cause local dust issues at some operations [78].

In a study, it was found that 1 tonne of natural pozzolan powder has a GWP of 89 kg CO₂-eq, significantly lower than the GWP of ordinary portland cement, which is 919 kg CO₂-eq/tonne [97]. Thus, incorporating natural pozzolan as an SCM could be an effective strategy to reduce the GWP of concrete, as it requires only minimal processing, primarily grinding.

2.3. Sedimentary Natural Pozzolans: Clays and Diatomaceous Earth

2.3.1. Description and Acting Mechanism in Concrete

Clay is a widely available natural material with particles smaller than 2 microns. It forms through various geological processes, including weathering. Kaolinitic clay, a subtype with high kaolinite content, has been identified as particularly effective in concrete applications due to its pozzolanic reactivity upon calcination. When pure kaolinitic clay is heat-treated at 700 °C to 850 °C, it transforms into metakaolin (MK), an amorphous pozzolan. MK reacts with calcium hydroxide from cement hydration, producing C-A-S-H and aluminate hydrates and other compounds that enhance concrete durability and strength by filling the pores [98,99,100,101,102]. Lower grades of calcined clays with less kaolin content of 40% and more have been shown to develop sufficient strength as OPC concrete [103]. A blended cement with 50% or less clinker mixed with ground limestone, calcined clay, and gypsum is a recent blended cement developed based on the production of C-A-S-H, carboaluminates, and ettringite hydrate products, together contributing to strength and durability [104].

Another product of clay is Limestone Calcined Clay Cement (LC³), which is an innovative composite cement that integrates calcined clays, particularly kaolinite-rich clays, with limestone to significantly reduce the carbon footprint of cement production. By substituting a large proportion of clinker with these Supplementary Materials, LC³ achieves comparable mechanical properties to OPC while lowering CO₂ emissions by approximately 30%. LC³ usually consists of 50% clinker, 30% calcined clay, 15% limestone, and 5% gypsum. The combination of calcined clay and limestone not only enhances durability through pore refinement but also mitigates issues such as chloride ingress and alkali–silica reactions. Industrial trials in countries like Cuba and India demonstrate LC³’s robustness and scalability, leveraging existing cement production infrastructure to produce sustainable cement alternatives at a lower cost [103,105].

DE, also known as diatomite, is a biogenic material with significant pozzolanic reactivity [106]. It primarily consists of the siliceous frustules or skeletons of diatom microorganisms (a type of hard-shelled algae), along with small amounts of calcareous biogenic material containing calcium carbonate, and detrital sediment such as clay minerals [107]. DE naturally has a high silica content, although the presence of calcareous deposits on its siliceous diatoms can lower the overall silica content [107].

2.3.2. Supplies, Supply Chain, and Technology Readiness Level

Clay reserves are abundant globally, but very few countries produce calcined clay. Many countries, such as India, China, and African nations like Ghana, as well as European countries like Germany, France, and Denmark, and South and Central American countries like Colombia, Cuba, and Argentina, calcinate the clay [108,109,110]. Worldwide reserves exist across South America, Africa, and Asia, although factors like limited capital and skills have restricted their exploitation in some regions [111,112]. In the U.S., deposits of kaolinitic clay are present, particularly in the southeastern regions and parts of California and Washington. The U.S. produced approximately 4.5 million tonnes of kaolin in 2021, making it one of the leading producers globally.

The process of preparing calcined clay for use as SCM involves surface mining, blending, drying, and calcining at around 700–850 °C. Rotary kilns and flash calcination are commonly used to ensure consistent quality. Cooling processes recover heat, while bag filters capture dust generated during production. This process can be scaled to existing cement production infrastructure, making calcined clay a feasible SCM with a TRL of 8 or 9, indicating near-complete industrial integration [113,114,115,116,117,118,119,120].

Around 300 million tonnes of DE is found in western North America [121]. In 2021, U.S. diatomite production was estimated at 830,000 tonnes, representing about 36% of global production [78]. Six companies in the United States extract diatomite from 12 mining sites and nine processing plants located in California, Nevada, Oregon, and Washington. Diatomite is recovered through low-cost, open-pit mining, as it is found near or at the earth’s surface. Companies involved in mining diatomite in California include Imerys Minerals, the Lompoc Plant, and the Celite Corporation. The exact quantity of DE mined at the Lompoc deposits, which is the world’s largest diatomite source, is unknown.

Around 55% of DE is used in filtration products, while the remaining 45% serves in applications such as fillers, absorbents, lightweight aggregates, and others [122]. Less than 1% of diatomite is used for biomedical and pharmaceutical purposes. The unit cost of diatomite varies based on its application. In 2021, the cost ranged from about $10 per tonne for lightweight aggregate in PCC to over $1000 per tonne for niche markets, such as cosmetics, art supplies, and DNA extraction [122].

2.3.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

MK or high kaolin calcined clay has fine particles with a very high surface area of 5500 to 8300 m²/kg compared to 300–400 m²/kg of OPC, leading to reduced workability compared to OPC concrete by increasing the water and superplasticizer demand [123,124]. MK, with its high surface area, also accelerates the cement hydration, leading to a higher heat of hydration and causing potential issues with construction in hot weather at high replacement rates [125]. To avoid the workability and high reaction heat challenges, the replacement rate of MK in concrete is limited to 10–12%, and MK is commonly blended with other SCMs to produce ternary blended cements.

It was shown that replacing cement with diatomite increases the water demand to maintain the same consistency, slump, and flow, due to its high water demand [74]. Milling diatomite powder to a higher fineness can break down the diatom’s skeletal structure, thereby reducing the water demand, particularly when an air-entraining agent is incorporated into the mix [126].

Impact on Strength

Replacing OPC with 10% to 20% MK results in enhanced strength, especially early strength. MK’s fine particles and high reactivity promote rapid strength development. Comparatively, MK mixes typically outperform those with silica fume in terms of early-age strength but show similar long-term strength levels. LC³ applications demonstrate strength improvements due to additional calcium aluminate and carboaluminate phases that form in the presence of limestone [115,127,128,129].

A study [106] investigated the impact of DE on the workability, compressive strength, density, and environmental effects of mortar and concrete mixes. Results showed that increasing DE levels raised both the initial and final setting times and decreased mix density. Compressive strength in mortar increased with DE at 7 and 90 days, with the 30% DE-5% limestone mix showing the highest strength at 28 and 90 days. Concrete mixes containing DE also exhibited higher compressive strength at 28 days, with C-30DE-5LS (a sample containing 30% DE and 5% limestone) marginally outperforming the control mix by 4%.

Impact on Durability

Calcined clay improves the durability of concrete by refining pore structure and reducing permeability. Studies confirm MK’s effectiveness in reducing ASR, sulfate attack, and chloride ion penetration, resulting in enhanced resistance to these durability issues. MK-modified concrete also shows lower drying shrinkage and increased resistance to freeze–thaw cycles, with mixes containing 10% MK displaying improved residual flexural strength and durability factors [130,131,132]. A study assessed chloride-induced reinforcement corrosion in concrete with a 20% diatomite replacement [98]. Steel-reinforced samples were exposed to a NaCl solution for 500 days. The results showed that concrete samples containing 20% diatomite outperformed the control samples in the chloride corrosion test. Generally, pozzolans contribute to a more refined pore structure, reducing the diffusion rate of water and chloride ions into the concrete, making the concrete more resistant to the onset of corrosion and the associated cracking issues.

The effect of calcinated clay and diatomite on the properties of cement-based materials is summarized in

Table 3. Authors’ synthesis of the literature on calcined clay and diatomite performance in cementitious systems.

Property of Concrete	Comparison to 100% OPC Concrete
Workability	Decrease with MK and DE
Early strength	Increase with MK
Late strength	No change
Setting time	Accelerate with MK
Drying shrinkage	Increase but more test data needed especially for DE
Alkali–silica reaction	Decrease
Sulfate attack resistance	Increase
Chloride permeability	Decrease
Freeze–thaw durability	Increase

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2.3.4. Environmental and Cost Considerations

Negative environmental impacts of natural pozzolans are from mining, and any treatment applied to them, which for clays and shale and DE involves calcination (heating to around 700–850 °C), drying, and some milling may also be required to ensure compliance with the applicable standards such as ASTM C618 and C595. The calcination temperature is low compared to clinker production, which occurs around 1450 °C. Furthermore, CO₂ is not released in the clay calcination process as it is produced by clinker from limestone. LCA studies conducted for European kaolinite, following ISO 14025 [133], resulted in a calculated GWP of 350 kg CO₂-eq per tonne [134]. Significant reductions of up to 30% were reported for embodied carbon compared to OPC for ‘LC³-50’, which contains 50% ground clinker, 30% calcined clay, 15% limestone, and 5% gypsum [103].

Clay calcination is less energy-intensive than clinker production, potentially lowering production costs for SCMs compared to clinker. Additionally, since lower temperatures are needed, it is possible to use cheaper fuels such as pet coke and biomass, especially if rotary kilns are used for calcination [103]. The projected growing demand for kaolin in other industries, such as ceramics, plastics, and paints, may limit its availability for SCM production, potentially influencing costs. However, facilities can readily adapt existing cement production equipment, such as ball mills, air cyclones, and kilns for clay calcination, reducing the need for extensive new capital investment.

DE is abundantly available in California and other western states, including Washington, Oregon, and Nevada [106]. Classified as a Class N natural pozzolan according to ASTM C618, DE is suitable for use as a cement replacement. Like other natural pozzolans, DE only requires grinding to exhibit pozzolanic activity [106], although calcination may be necessary to activate any clay or calcite impurities.

2.4. Fine Portion of Recycled Concrete and Crushed Concrete Aggregate from Construction and Demolition Waste

2.4.1. Description and Acting Mechanism in Concrete

Recycled concrete aggregates (RCA) are generated through breaking, removing, crushing, and screening concrete from construction and demolition (C&D) waste [135]. Additionally, excess fresh concrete returned to batching plants is left to harden and then processed into RCA in a similar process. Sometimes, RCA from return concrete is referred to as crushed concrete aggregate.

The coarse fraction of RCA is used in various pavement applications, including base, subbase, fill material, and pavement layers (asphalt and concrete) in at least 43 states [135]. The fine fraction (particle size <150 microns) comprises about 20–30% of RCA production and includes mostly hydrated cement products with very little, if any, anhydrous clinker and also some coarse and fine aggregate fragments [136,137,138]. When used directly as a fine aggregate replacement in concrete, RCA fines can reduce strength and durability and increase water demand due to their attached mortar with high porosity. RCA fines also have variable properties since the concrete source varies, making their use as fine aggregate challenging. However, the fine fraction, if ground, may serve as an SCM, microaggregate, and inert filler to induce some cementitious abilities, improve packing density, enhance the cement hydration and strength development in concrete [139].

2.4.2. Supplies, Supply Chain, and Technology Readiness Level

As of 2012, the annual generation of construction and demolition waste in 40 countries worldwide exceeded 3.0 billion tonnes, with this figure continuing to rise steadily [140]. The U.S. generated over 544 million tonnes of C&D waste in 2018, a large portion of which came from concrete [141]. C&D concrete constituted the largest share at 67.5% (around 367 million tonnes), followed by asphalt concrete at 17.8% (around 96 million tonnes). C&D wood products represented 6.8%, while the remaining materials collectively made up 7.9%. Aggregate was the primary end-of-life application for C&D debris, accounting for 52%, or approximately 284 million tonnes. Of this, concrete alone contributed around 273 million tonnes. Landfills represented the second-largest destination, receiving 24% of the total C&D debris, which amounted to roughly 131 million tonnes [141]. RCA production involves crushing and milling of hardened concrete and generates about 20% to 30% fines, resulting in millions of tonnes being generated each year [136,137,138]. It was estimated that 25–60 million tonnes of RCA powder is produced in China annually, and most of it is landfilled [142]. Additionally, this resource is widely available in most regions. The coarse fraction of generated RCA is commonly used as an aggregate base with demonstrated use in over 100 projects nationwide [135]; the fines are a waste and do not have a value-added use.

TRL for coarse RCA as a base aggregate is fully established at TRL 9 and is in practice in road construction. In contrast, using RCA fines as a ground powder for cement replacement in concrete is still in the proof-of-concept stage, placing it at TRL 1–3. Concrete ready-mix plants and recycling facilities are key suppliers, with operations potentially scaling as demand increases.

2.4.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

RCA fines in cement mortar can reduce air content and flow due to the microporosity of RCA, which raises water demand [143,144]. The initial hydration reaction rate of cement can increase due to the high fineness of RCA fines and high alkali content, acting as an accelerator [145].

Impact on Strength

Replacing cement with RCA fines up to 10% in mortar was shown to yield comparable 28-day compressive strength to OPC mortar [143]. Studies indicate that at replacement rates of less than 30%, the negative impacts of RCA fines are not significant on compressive strength [139,146,147]. Optimal replacement is generally recommended at 20%, though self-compacting concrete with a high-range water reducer demonstrated up to a 30% increase in compressive strength with RCA fine additions [146]. A study on the recycled brick powder [148] showed a slight reduction in early strength (8% at 7 days) at 20% replacement level but an increase in 56-day compressive strength (+5%) and a slight improvement of surface resistivity, potentially reducing chloride ion permeability and corrosion risk.

Impact on Durability

RCA fines can increase the drying shrinkage potential in concrete, as higher water demand and lower hydration product content relative to pure cement contribute to shrinkage cracking. However, RCA powder was shown to have a considerable positive impact on drying shrinkage when used as an inert filler replacing cement. This positive outcome was attributed to the filling ability of the fine RCA powder. Additionally, because RCA powder is inert, it reduces the amount of cement hydration products formed, thereby decreasing the volume of gel pores and contributing to the reduction in drying shrinkage [149]. In self-compacting concrete, RCA fines were found to have a higher drying shrinkage compared to the samples with granulated blast furnace slag and granulated limestone. The increase in drying shrinkage of the sample with recycled fine powder can be attributed to the pore structure in concrete. The addition of ground slag, together with recycled fine powder, reduced the shrinkage of concrete [146]. RCA powder was also shown to have a positive impact on controlling water tightness and chloride ion ingress, reducing permeability to both substances [147]. The effect of RCA on the properties of cement-based materials is summarized in

Table 4. Authors’ synthesis of the literature on RCA powder performance in cementitious systems.

Property of Concrete	Comparison to 100% OPC Concrete
Workability	Decrease
Early strength	No change at optimal replacement levels
Late strength	No change at optimal replacement levels
Setting time	Accelerate
Drying shrinkage	Decrease
Alkali–silica reaction	No information found
Sulfate attack resistance	No information found
Freeze–thaw durability	Decrease but more test data needed

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2.4.4. Environmental and Cost Considerations

RCA fines, though smaller in particle size than the coarse aggregate portion, are still not fine enough to be directly used as a cement replacement and require mechanical size reduction. Achieving a sub-45-micron particle size necessitates significant ball milling, especially if the RCA fines contain a high proportion of aggregate, which increases hardness compared to RCA with a higher hardened cement portion. Additional treatments, such as screening and separation, thermal activation or carbonation, drying, and other processing methods, may also be applied to enhance the performance of RCA fines as a SCM. However, these treatment processes are often associated with negative environmental impacts. Despite these challenges, using RCA fines as an SCM offers notable environmental benefits, including diverting waste from stockpiles, landfills, and ponds, thereby contributing to waste reduction and resource efficiency. Another potential positive environmental impact, which requires quantification, is the carbon uptake by RCA fines, which depends on its surface area, stockpile shapes and depths, age and duration of stockpiling, and environmental conditions. Balancing the environmental burdens of treatment processes with the benefits of waste reduction, resource efficiency, and carbonation is crucial and challenging when quantifying the environmental impacts of RCA fines as an SCM.

LCA studies of RCA fines capturing all these aspects were not found in the literature. The most relevant information found is a published EPD manufacturer in the U.S. for their RCA fines used as SCM in concrete, which is used here as a proxy in the absence of LCA studies. These EPDs, developed following ISO standards, encompass stages from cradle to gate, including raw material extraction, processing, transportation, and manufacturing. According to these EPDs, the use of RCA powder in concrete offers significant environmental advantages over landfilling this C&D waste [150,151]. Other studies took a simpler approach than an LCA and calculated the CO₂ emission reduction by cement avoidance. These studies reported a 30% reduction in CO₂ emissions when partially replacing OPC with RCA fines [152,153].

Regarding cost, RCA fines are a waste product of RCA production with no economic value, and are a resource with availability in most regions. However, utilizing RCA fines as a cement replacement or SCM offers the potential for a value-added byproduct if sold as an SCM. Additional economic gains from using RCA fines as SCM are offsetting some current waste management costs, such as landfill tipping fees, transportation costs to landfill, storage, and stockpile space.

However, as mentioned above, ball milling and any other required treatment will incur additional cost. The supply chain may also include transporting the material to a cement or concrete production plant if RCA is not produced at either of these facilities. Other added costs are associated with any testing, characterization, and certification.

2.5. Rock Dust

2.5.1. Description and Acting Mechanism in Concrete

Two types of rock dust, also known as rock flour, are considered here: one is the rock dust generated as fines during the production of crushed stone aggregates for concrete, asphalt, road base, and subbase, and other aggregate applications. The second source is the rock dust collected as baghouse fines from exhaust gases at HMA plants, with generation occurring in both batch and drum mix plants considered here. The difference between the two rock fines is that baghouse fines are heated during asphalt production, which could potentially alter some of their physicochemical properties and pozzolanic reactivity.

Generally, pozzolanic reactivity is not a desired characteristic in rocks used as aggregate for concrete. Aggregates that contain reactive silica minerals such as opal, chalcedony, volcanic glass, certain types of quartz, and other reactive silicates are not accepted for use in concrete, though this is not a limiting factor for asphalt production. Therefore, the aggregate used for concrete is not expected to have high pozzolanic reactivity. However, rock dust containing some portions of these silicates may exhibit some pozzolanic behavior or may be thermo-mechanically activated to increase reactivity. The physical and chemical properties of rock dust depend on the geological origin of the quarry rock. Granite fines were used in a previous study as quarry or rock dust, along with rice husk ash (RHA), to replace cementitious materials in concrete [154]. The rock dust had a specific gravity of 2.56 and a water absorption of 2.32, and 16.8% of the materials were finer than 75 microns for the as-received rock dust from the granite quarry. No chemical characteristics of granite rock dust were reported in this study. Another study reported the characteristics of rock dust originating from a Dolomite quarry. The as-received rock dust was manually ground using a pestle and mortar to obtain an average particle size of 90 µm. Dolomite rock dust had a higher CaO content (75%) compared to cement (64%). The other dominant oxide ingredient in the dolomite rock dust was MgO (18.3%) [155]. Another study in China applied recycled rock dust as SCM in low-carbon cement [156]. The exact source of this rock dust or the quarry material was not explicitly mentioned in this study. Results indicated that this rock dust had a CaO content of only 5%, whereas a noticeably high amount of SiO₂, 67%. The mean particle size of the rock dust was between 5 and 10 µm. In Nigeria, similar research was conducted by sampling rock dust from a local stone quarry. Characterization yielded a specific gravity of 2.6. [157]. Rock dust from granite quarries usually has low reactivity. As a result, the heat of hydration declines with an increase in rock dust content in the cement mixture [158].

Primarily, rock dust functions as an inert filler in concrete, enhancing packing density and matrix cohesion, potentially increasing concrete’s compressive strength [159]. In some studies, rock dust has replaced sand, improving compressive strength through optimized packing density [160]. Moreover, recycled rock dust has been observed to act as nucleation sites, accelerating the hydration of cement [155].

2.5.2. Supplies, Supply Chain, and Technology Readiness Level

Aggregates constitute approximately 80% of concrete volume, and in the U.S., only 1.6 billion tonnes of crushed stone were produced in 2022. About 70% of this crushed stone was derived from limestone and dolomite sources, with significant amounts allocated to construction, cement, lime manufacturing, and agricultural uses [161].

Additionally, 3600 asphalt plants produce approximately 385 million tonnes of asphalt annually in the U.S., generating around 5% of total aggregate weight as rock dust. This amount translates to an estimated 5000 tonnes of rock dust per asphalt plant per year, amounting to 21 million tonnes nationwide [159,162].

Rock dust generation is well-established as a byproduct of quarrying and asphalt production processes, but its use as SCM or filler in concrete remains at TRL 1–3, representing an early research phase. Aggregate quarries, ready-mix concrete producers, and asphalt plants serve as potential suppliers of this material.

2.5.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

However, research using recycled rock dust with d₅₀ equal to 244.1 microns as a quartz sand (d₅₀ equal to 481.9 microns) replacement at high replacement rates (up to 80%) in ultra-high performance concrete noted no significant alteration in workability or hydration [160]. Other studies suggest that rock dust particles, though non-reactive, can enhance hydration through nucleation owing to their fine particle size and surface area [155]. Dolomite-based rock dust replacements above 20% may reduce the mixture’s flow properties. Additionally, partial cement replacement with rock dust typically delays initial and final setting times, likely due to cement dilution [157].

Impact on Strength

An optimal replacement level of 10% was suggested for maintaining strength at varying water-to-binder ratios in one study [163]. In self-compacting concrete containing granite rock dust and rice husk ash (RHA), early strength gains were hindered, though compressive strength approached that of the control over time [154]. It was also shown that rock dust, when used as SCM by itself, reduces the 28-day compressive strength of concrete [156]. One study successfully produced ultra-high-performance concrete using recycled rock dust as 20%, 40%, 60%, 80%, and 100% replacement of quartz sand [156]. The ultra-high-performance concrete with 80% recycled rock dust achieved 22,349.5 psi (154.1 MPa) compressive strength, even higher than the reference mix (20,493.8 psi [141.3 MPa]) [160]. Another study also showed that replacing 10% of cement and 20% of sand with rock dust resulted in mortar and concrete compressive strength of 44 MPa, nearly equal to the control sample (43.5 MPa) [159].

Impact on Durability

Scanning Electron Microscopy (SEM) analysis revealed that rock dust contributes to matrix compaction by decreasing average pore diameters [156]. Replacing 10% and 20% of cement with granite rock dust and measuring the drying shrinkage over 40 days at 20 °C and 60% relative humidity resulted in a reduction in drying shrinkage by 13.5% and 19.1%, respectively [158]. Additionally, granite rock dust has negligible alkali–silica reactivity. Concrete sulfate resistance slightly improved with 15% cement replacement by rock dust [164], and freeze–thaw durability after 49 repeated freeze-and-thaw cycles improved when sandstone-based rock dust was substituted with cement at ranges up to 15% because of denser structure and durable bonding between cement paste and aggregate [165].

The effect of rock dust on the properties of cement-based materials is summarized in

Table 5. Authors’ synthesis of the literature on the effect of rock dust on cement-based materials properties.

Property of Concrete	Comparison to 100% OPC Concrete
Workability	Decrease
Early strength	Decrease
Late strength	Increases or no change
Setting time	Delay
Drying shrinkage	Decrease, more research needed
Alkali–silica reaction	More research needed
Sulfate attack resistance	More research needed
Freeze–thaw durability	More research needed

.

2.5.4. Environmental and Cost Considerations

Replacing cement with rock dust could offer potential environmental benefits, depending on the treatment processes applied to rock dust. The crushed stone production process includes extraction, crushing, grinding, and screening. Another supply of rocks for construction is sand and gravel sourced from glacial deposits, river channels, and river flood basins. These processes generate fines. A study evaluated the use of rock dust as a sustainable substitute for fine aggregate and cement in concrete for roadway pavements. Using a comprehensive life-cycle cost and environmental impact analysis, it found that replacing 20% of fine aggregate and 10% of cement with rock dust offers significant reductions in costs, energy consumption, GHG emissions, and hazardous waste [159]. The findings highlight that most savings stem from the material production stage, suggesting significant sustainability advantages for pavement construction.

Rock dust’s primary environmental impacts are associated with processing, with a cradle-to-gate LCA conducted by one manufacturer in the U.S. showing GWP and acidification impacts for rock dust as a product. The data in

Table 6. Life cycle inventory and life cycle impact assessment results of the production of 1 Metric Tonne of Rock Dust from a manufacturer in the U.S. [150].

Impact Categories	Unit	2016 Rock Dust	2017 Rock Dust
Global warming	kg CO2 eq	6.06	5.72
Acidification	kg SO2 eq	0.05	0.03
Eutrophication	kg N eq	0.01	0.04
Photochemical ozone creation	kg O3 eq	0.73	0.72
Ozone depletion	kg CFC-11 eq	1.59 × 10−7	4.71 × 10−7
Use of renewable primary energy	MJ	7.54	(1.04 + 13.1) HHV *
Use of nonrenewable primary energy	MJ	90.7	(77.4 + 11.3) HHV *
Use of renewable primary energy resources as raw materials	MJ or kg	0	0.06 (kg)
Use of nonrenewable primary energy resources as raw materials	MJ or kg	0	1000 (kg)

* HHV: high heating value.

summarizes findings from Vulcan’s EPDs for rock dust, indicating that rock dust has a lower environmental impact than traditional aggregates or cement in concrete production [150].

Almost 40% to 50% of HMA plants collect baghouse fines, which may be routed to the asphalt production facility or stored in a silo to be used as a mineral filler additive in asphalt mixes [166]. Rock dust is used as a remineralization material for agricultural soils. Applying rock dust to soils has been reported to stimulate plant growth and increase resistance to pests and diseases, among other benefits [167,168]. However, many plants have to manage this material as waste, which is associated with transportation, landfilling, or ponding fees. However, its potential application as a fine aggregate or SCM in concrete introduces value by repurposing waste material. With demonstrated performance benefits, the use of rock dust in concrete production offers cost advantages through waste reduction and potential material savings, especially in regions where it is readily available.

2.6. Municipal Solid Waste Ash (MSWA) from Post-Consumer Waste

2.6.1. Description and Acting Mechanism in Concrete

MSWA is generated as a byproduct of incinerating municipal solid waste as part of waste management strategies in certain urban areas where waste volumes are high and landfill space is limited. MSWA is a mix of fly ash and bottom ash. Bottom ash is the non-combustible residue that remains after incineration, while fly ash is the particulate matter captured by air pollution control systems [169,170]. Typically, bottom ash accounts for 85% to 95% of the total ash by volume. Its properties and quality rely on the waste, the type of incineration unit, and the nature and type of pollution control system [169]. Though the ash composition is expected to vary from facility to facility, several studies have demonstrated the potential for using MSWA in concrete as a filler material and as an SCM [171,172,173]. Major oxide components in MSWA are CaO, SiO_2, and Al₂O₃ [174]. Due to the high contents of these oxides, MSWA has the potential for use as an SCM or filler in concrete [169,170,171,172,173]. Quartz is the most abundant mineral phase in MSWA, followed by calcite, hematite, magnetite, gehlenite, and heavy metals, including chromium, zinc, and lead [175].

2.6.2. Supplies, Supply Chain, and Technology Readiness Level

Each year, the world produces approximately 2.01 billion tonnes of municipal solid waste, with at least 33% of it not being managed in an environmentally sustainable manner. On average, waste generation per person is 0.74 kg daily, though this varies significantly, ranging from 0.11 to 4.54 kg. High-income nations, despite comprising only 16% of the global population, contribute around 34% of the total waste, equating to about 683 million tonnes. The global waste production is projected to rise to 3.4 billion tonnes by 2050 due to population growth and urbanization [176].

MSW incineration yields approximately 0.275 to 0.325 tonnes of bottom ash and 0.02 to 0.03 tonnes of fly ash per tonne of MSW processed [177]. In California, two facilities actively process MSW and generate MSWA: the Southeast Resource Recovery Facility in Long Beach and the Covanta Stanislaus Incinerator Facility in Stanislaus County. These facilities produce between 250 and 400 tonnes of MSWA daily. Incineration technology has been widely adopted for treating MSW in China. Currently, there are approximately 75 municipal solid waste incineration (MSWI) plants, with a total capacity of 33,000 tonnes per day. At present, the annual output of MSWI fly ash exceeds 300,000 tonnes and continues to increase year by year [172,178,179]. In 2013, the small city-state of Singapore generated more than 7.85 million tonnes of municipal solid waste [180].

The use of MSWA in concrete has been demonstrated in research settings, particularly as an SCM. However, concerns about the leaching of heavy metals from MSWA in concrete have limited its practical applications. The technology readiness level for MSWA use in concrete is TRL 2–3, indicating proof-of-concept (e.g., preliminary research studies) but not yet widespread implementation.

2.6.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

Studies have produced mixed results on the impact of MSWA on fresh concrete properties. For instance, a slump increase was observed with 30% MSWA in dry-ground form, while a slump decrease occurred with wet-ground MSWA [181]. Additionally, ground and washed fly ash from MSW increased water demand in the mix, whereas bottom ash reduced water demand. The authors attributed these variances to fly ash’s porous structure [182]. Moreover, MSWA’s high heavy metal content can retard setting times, especially in bottom ash, which contains higher heavy metal (chromium, zinc, and lead) concentrations that inhibit C₃S hydration [180]. However, the washed fly ash set faster than the OPC control due to anionic ions such as Cl, SO₄, and NO₃ [182]. The same study found that as the replacement percentage increased, the setting time also increased, with an initial setting time of 867 min and a final setting time of 989 min. Similar results were reported in another study [182]. Other studies also reported MSWA results in a different hydration behavior and a significant delay in the onset of the acceleration reaction of C₃S hydration and reduced maximum heat hydration [180,183].

Impact on Strength

One study showed that 30% OPC replacement with MSWA led to good strength development, reaching about 95 MPa after 180 days [181]. Another study focused on the particle size of MSWA showed that generally, strength improved with milled MSWA, especially when ground to finer particle sizes [183]. One study tested stepwise replacement rates of 10%, 20%, 30%, and 40% of cement with pulverized incinerator bottom ash powder, and observed a progressive reduction in compressive strength at all ages at higher replacement rates. Particularly, at 90-day age, the strength decreased from approximately 50 MPa for the control to 48 MPa, 41 MPa, 35 MPa, and 30 MPa at the employed replacement rates. They attributed the strength loss potentially due to poor bonding in the interfacial transition zone between cement and aggregates [184].

Impact on Durability

The durability of ground MSWA concrete was evaluated based on the water absorption rate. Absorption decreased from 0.55 mL/m²s (control) to 0.30 to 0.39 mL/m²s for ground MSWA mixes. It was shown that there is a progressively higher content of MSWA (45% higher shrinkage for mixes with 40% MSW ash after 91 days). resulted in lower initial absorption values than controls, though higher MSWA contents (40%) caused an increase in drying shrinkage [184,185]. Chloride penetration tests indicated that 30% MSWA concrete offers similar resistance to chloride ingress as control concrete [181].

A summary of the effect of MSWA on the properties of cement-based materials is provided in

Table 7. Authors’ synthesis of the literature on the effect of MSWA on various properties of cement-based systems.

Property of Concrete	Comparison to 100% OPC Concrete
Workability	Depends on the ash type and properties
Early strength	Decrease
Late strength	Decrease or no change
Setting time	Delay depending on calcium content and heavy metals
Drying shrinkage	More research needed
Alkali–silica reaction	More research needed
Sulfate attack resistance	More research needed
Freeze–thaw durability	More research needed

.

2.6.4. Environmental and Cost Considerations

MSWA lacks a formal LCA or EPD due to its current waste classification. Due to its coarse nature, MSWA will require milling before use as SCM to enhance its reactivity [172,186]. Milling of bottom ash will add to the environmental impacts and costs. Other environmental concerns primarily stem from the leaching of heavy metals, such as lead, cadmium, and chromium, which could infiltrate groundwater or soil if disposed of improperly. Heavy metal concentrations in MSWA typically meet the inert waste classification standards, but fly ash from MSW poses greater toxicity risks due to high heavy metal contents [187,188,189]. Studies have indicated that MSW bottom ash, when incorporated into concrete, does not exceed regulatory limits for inert waste leachate [190]. The concentration of toxic elements in MSWA and the possibility of their leachate into the environment, if used in the concrete, should be carefully considered, as discussed below.

Leachate of Heavy Metals

Infiltration of water through the cementitious system containing MSW ash could cause the leachate of the concentrations of heavy metals into the environment. The concentration of heavy metals in an MSW bottom ash sample available in a study showed only the cadmium concentration was close to the allowable limits and all other toxic metals were well below the limits [188]. The results of the leachate test from concrete with MSW bottom ash showed all the requirements for inert waste classification were met. However, the study suggests more research on the concentrations of toxic elements from MSWA concrete [188].

Another study similarly reported that MSW bottom ash is non-toxic; however, MSW fly ash has high concentrations of heavy metals from vaporized compounds adsorbed on the large surface area of fine fly ash [189].

As a waste product, MSWA currently holds no intrinsic economic value. Instead, MSWA incurs additional disposal and transportation costs associated with its collection, processing, and landfill deposition. In California, MSWA is often used as a road base material or disposed of in dual-layer landfills. Its potential use as an SCM may present cost-saving opportunities by reducing material waste and landfill needs if safety and regulatory standards are met.

2.7. Municipal Wastewater Sewage Sludge Ash (SSA)

2.7.1. Description and Acting Mechanism in Concrete

Sewage sludge, or biosolids, is the solid residual material produced during municipal wastewater treatment. Typically, biosolids are used as soil amendments or fertilizers. Incineration of sewage sludge, another management option, reduces the sludge by about 70% in mass and 90% in volume, resulting in sewage sludge ash (SSA). With increasing wastewater processing capacity, more SSA is being produced, requiring sustainable and appropriate reuse, particularly as an SCM in the concrete industry, which is expanding to meet decarbonization targets [191,192,193,194]. According to reported chemical compositions for SSA in the literature, SSA is an aluminosilicate or silicate and calcium material with varied contents of other toxic and nontoxic element concentrations [118,195,196]. Crystalline phases of quartz, albite, calcite, magnetite, hematite, and other phases have been detected as the most abundant minerals in SSA, along with iron oxides, iron phosphates, calcium phosphates, and aluminum phosphates. SSA consists primarily of silica, iron, and calcium.

The bulk chemical composition of SSA includes Si, Ca, Fe, Al, and P, along with heavy metal elements like Zn, Cu, Cr, Pb, Ni, and Cd. These elements are of greater importance in terms of the environmental impacts of the material. Studies have reported a large variability in the element concentrations in SSA [197,198,199,200,201]. This variability might be due to differences in the wastewater treatment systems or incineration conditions. The literature also suggests that supplementary processing treatments, like aging and acid washing, can be used to regulate the contents of SSA [193].

SSA comprises irregular particles with rough surface textures and porous microstructures [193,202]. The specific gravity of the SSA is in the range of 1.8 to 2.9, which is somewhat comparable to light sand and less dense than portland cement at 3.15 [199,200,203].

According to a study of the cementing properties of SSA, the amorphous phases are rich in aluminum and iron phosphates that lead to the formation of amorphous hydroxyapatite. The study found that it is likely that these phases produce C-S-H in reaction with portlandite and contribute to strength development [202]. In another study, SSA was found to be a reactive pozzolan along with silica fume, MK, and coal fly ash [204]. Similarly, another study showed that if burned at 800 °C, SSA can possess high pozzolanic reactivity, but reactivity reduces at lower or higher temperatures [205].

2.7.2. Supplies, Supply Chain, and Technology Readiness Level

SSA is generated from municipal wastewater treatment facilities that incinerate biosolids. Global production of municipal wastewater biosolids, also known as sewage sludge, is substantial and varies across regions. Estimates suggest that annual global sewage sludge production is approximately 53 million tonnes of dry solid, with projections indicating an increase to 127.5 million tonnes by 2030 [206]. In the United States, around 12.56 million dry tonnes of municipal wastewater biosolids is produced and managed annually from 15,014 wastewater treatment facilities [207]. Globally, approximately 1.7 million tonnes of SSA is produced annually, primarily in the U.S., the EU, and Japan [196,208]. In California, around 878,510 tonnes of biosolids was produced in 2013, with 3% incinerated to generate SSA [209]. This translates to approximately 19,127 tonnes of SSA produced in the state that year. However, no active sewage sludge incinerators are currently operating in California, and in December 2024, California’s last municipal solid waste incinerator, the Stanislaus County facility operated by Reworld, ceased operations [210].

The production process for SSA involves sludge thickening, dewatering, incineration, air pollution control, and ash handling. Multiple hearths and fluidized bed incinerators are primarily used in the U.S., where the incineration temperature typically ranges from 649 °C to 982 °C. The TRL of SSA as an SCM is at Level 3, with its use demonstrated in research studies but requiring further standardization and durability studies for widespread application.

2.7.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

SSA replacement in concrete tends to reduce workability due to its irregular, porous particle structure, which increases water demand. Studies show a 6% workability reduction per 10% SSA replacement and a 12% slump reduction per 10% replacement [199,211,212,213]. SSA also increased setting times, with an average 35% increase per 10% SSA addition [214,215].

Impact on Strength

SSA in concrete generally results in lower early strength, with an approximate 1 MPa reduction per 1% SSA replacement at 28 days [194,200]. However, SSA concrete showed strength development over time, reaching comparable or even higher strengths than the control after 90 days [216,217]. Similar reductions were observed in flexural strength, with reductions of 5% to 30% depending on the SSA content [196,200,202,218].

Impact on Durability

SSA’s alumina content improved chloride resistance, with SSA mixes (up to 20%) showing increased corrosion resistance [193]. Higher SSA content, however, can decrease corrosion resistance. In terms of drying shrinkage, the mortar with 30 wt% SSA had more than 30% reduced drying shrinkage compared to the control mortar at 90 days [219]. Research also shows improved sulfate resistance and reduced drying shrinkage with SSA. However, findings on porosity and permeability are mixed, with some studies indicating decreased permeability and others suggesting increased porosity [200,202,211,219,220,221,222]. The effect of SSA on the properties of cement-based materials is summarized in

Table 8. Authors’ synthesis of the literature on the effect of SSA on the properties of cement-based materials.

Concrete Property	Compared to 100% OPC Concrete
Workability	Decrease
Early strength	Decrease
Late strength	Increase
Setting time	Delay
Drying shrinkage	Decrease, more research needed
Alkali–silica reaction	More research needed
Sulfate attack resistance	No change, more research needed
Freeze–thaw durability	More research needed

.

2.7.4. Environmental and Cost Considerations

Incineration is used as a method for sewage sludge disposal to recover energy in the form of heat or electricity. When incineration is used primarily for energy recovery, the process is often optimized for combustion efficiency and emissions reduction, ensuring that environmental impacts like GHG emissions and pollutants are minimized [223].

Environmental impacts in both cases can be influenced by the type of incinerator used (e.g., fluidized bed or multiple hearth furnaces), pre-treatment of the sludge (e.g., drying or milling to smaller particles), and post-combustion pollutant controls (e.g., electrostatic precipitators or scrubbers) [223].

Combustion of sewage sludge produces ash, which may vary in particle size depending on the furnace type and combustion conditions. Fine particles are more likely in fluidized bed systems, while coarser ash can result from less optimized systems. If coarse ash requires grinding for subsequent handling or disposal, the associated energy use contributes to the GWP. This is due to the additional fossil fuel consumption and resultant carbon dioxide emissions during the grinding process. Furthermore, the environmental burden of ash management is significant, as heavy metals and other pollutants can leach if not properly treated, making fine particle ash easier to stabilize but more challenging to capture efficiently [223].

The leachate from SSA-containing concrete needs regulation to ensure heavy metal concentrations stay within permissible landfill disposal levels. Studies suggest that heavy metals from SSA are immobilized within the cement matrix, although further confirmation is needed [224,225].

SSA currently has no economic value and is primarily used in low-value applications like soil amendment. Its potential as an SCM, if standardized, could increase its value and make it comparable to other mineral admixtures in the market, depending on demand.

3. Nanomaterial Admixtures

3.1. Cellulose Nanocrystals and Nanofibers

3.1.1. Product Description and Acting Mechanism in Concrete

Cellulose nanomaterials (CNMs), primarily in the forms of cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), can be produced from wood pulp and other sources, each with distinct production methods leading to varied physicochemical properties [226]. CNCs are generally produced through sulfuric acid hydrolysis, which digests the amorphous regions of cellulose, leaving crystalline regions intact, whereas CNFs are obtained through mechanical fibrillation or TEMPO-mediated oxidation [226]. These nanomaterials have been studied for applications in cement-based materials.

In concrete, CNMs provide benefits through potential mechanisms such as (1) bridging and reinforcing due to high-modulus fibers, (2) filling small pores to densify the matrix, (3) providing nucleation sites due to high surface area, and (4) offering internal curing by releasing adsorbed water, which increases the degree of hydration [227,228].

CNCs have a rod-like morphology, typically 6 to 9 nm in width and 127 to 163 nm in length, depending on processing conditions. In contrast, CNFs exhibit an entangled fiber-like structure with an average width of around 30 nm [226]. CNCs have a crystallinity index (CI) of around 88.2%, compared to CNFs with a CI of 81.8%, which reflects more amorphous regions in CNFs. CNCs also exhibit a higher zeta potential of −47.5 ± 2.31 mV (pH 7.4), attributed to the high density of surface hydroxyl groups, while CNFs, produced via nonchemical processes, lack carboxylate groups and have a zeta potential of −50.6 ± 1.48 mV [226].

CNMs are derived from cellulose-rich sources, including wood, cotton, hemp, flax, and even recycled textiles [229,230]. Given these diverse sources, CNMs offer a sustainable and versatile feedstock for various applications.

3.1.2. Supplies, Supply Chain, and Technology Readiness Level

CNMs are produced from cellulosic sources. The source of cellulose can be wood and non-wood plants, such as cotton, hemp, flax, and jute; algae; tunicate; and bacteria [229]. CNMs were also produced from old textiles [230]. Several manufacturers produce CNCs and CNFs at an industrial scale. The list of suppliers is provided in

Table 9. List of Cellulose Nanocrystal and Cellulose Nanofiber Suppliers.

Supplier	Production Capacity (Million Tonnes)	Properties	Source
US Forest Service R&D: Forest Product Laboratory	Pilot	CNC: 5 nm diameter and 150 nm long CNF: 20 nm diameter and 2 μm long	[232]
Kruger	6000	CNF: 80 to 300 nm wide and 100 to 2000 µm long, aspect ratio > 1000	[233]
Sappi North America	N/A *	CNF product with the trademark name of Valida designed as concrete admixture	[234]
CelluForce	300	CNC: Average length of 150 nm and a diameter of 7.5 nm, aspect ratio 20	[235]
American Process	175	CNC: 2–20 nm diameter and 100–600 nm long CNF: 5–30 nm diameter and >1 μm long	[236]
Blue Goose Biorefineries Inc.	N/A	CNC: Length 100–150 nm, width 9–14 nm	[237]
Innotech Materials	N/A	CNC products with various surface functional groups available	[238]

* N/A: Not Available.

. Key CNM suppliers include the U.S. Forest Service’s Forest Product Laboratory, Kruger, Sappi North America, CelluForce, and American Process—some suppliers, like Sappi, market CNMs specifically for concrete applications [231].

CNCs and CNFs are currently used in paints, adhesives, coatings, composites, gels, medical and pharmaceutical applications, and packing [231]. Some are marketed as a concrete admixture, such as the Valida product by Sappi. Research on CNMs in concrete applications is still developing, with the technology at a TRL of 3–4, as studies focus on mix optimization and compatibility with admixtures [226].

3.1.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

CNMs influence the setting and rheology of cement composites. For example, CNFs can act as set accelerators, reducing setting time, while CNCs may have a retarding effect [239]. CNCs at low doses reduce yield stress, functioning as water-reducing agents, but at higher doses, they increase viscosity [240,241]. CNFs, on the other hand, consistently increase yield strength and viscosity due to their fibrous morphology [226,242].

Impact on Strength

CNMs can improve compressive, tensile, and flexural strengths in cement composites. Low doses of CNFs, such as 0.05 wt%, enhance early-age compressive strength, with one study reporting an increase of 13% at 7 days [243]. Another study reported 17% and 18% improvement in compressive strength at 7 and 28 days, respectively, with 0.065 wt% CNF [226]. The study concludes that an optimum dose exists when the strength reaches a maximum value and beyond which compressive strength starts to decrease. Higher doses may lead to fiber agglomeration, reducing strength gains [244]. CNCs also contribute to strength improvements, with one study finding an increase of 17% to 18% in compressive strength for doses around 0.4 wt% [227]. Similarly, another study reported 29% and 17% increase in compressive strength at 7 and 28 days, respectively, with 0.4 wt% CNC [226]. One study found that flexural strength was improved by 21% with the addition of 0.48 wt% CNF [245]. CNC was also found to increase the flexural strength of mortar by 31% with a 0.75 wt% dose [246]. One study also reported 34% and 23% improvement in tensile strength with 0.1% wt% CNF and 0.1 wt% CNC, respectively [247].

Impact on Durability

Studies show that CNFs and CNCs can improve concrete durability. CNFs increase sulfate resistance by reducing ettringite formation and pore refinement, while CNCs enhance freeze–thaw resistance, lowering mass loss under sulfate attack by 35-fold with 1.5 wt% CNC [248,249]. CNMs also reduce pore size and improve chloride penetration resistance in high-performance concrete, with reductions in absorptivity and water vapor permeability observed in some studies [250]. One study reported that 0.096 wt% CNF reduced the pore volume by 29% and water absorption by 6.6% [245]. The study mentioned that the pores in the CSH gels refine into small gel pores with the addition of CNF, reducing the pore volume of the matrix. Similarly, water absorption was reported to be decreased by 51% at 7 days with 0.05 wt% CNC compared to the cement paste without any CNC [251]. The total porosity was reduced, which was attributed to the reduction in the capillary pores in the cement paste. One of the studies demonstrated 47% and 64% reduction in absorptivity with 0.1 wt% CNF and CNC, respectively [247]. The same study reported 45% reduction in open porosity with 0.1 wt% CNF. The same dose of CNC reduced the open porosity by 49%. The sizes were reduced with the incorporation of both CNF and CNC.

The effect of cellulose nanomaterial on the properties of cement-based materials is summarized in

Table 10. Authors’ synthesis of the literature on the effect of cellulose nanomaterials on the properties of cement-based materials.

Property of Concrete	Comparison to 100% OPC Concrete
Water demand	Increase
Early strength	Increase up to a certain dosage
Late strength	Increase up to a certain dosage
Pozzolanic reactivity	None
Setting time	Increase
Drying shrinkage	Decrease needs more research
Alkali–silica reaction	More research needed
Sulfate attack resistance	Increase, needs more research
Freeze–thaw durability	Increase, needs more research

.

3.1.4. Environmental and Cost Considerations

Table 11. Summary of LCA Studies on the Production of Cellulose Nanocrystals and Cellulose Nanofibers.

Ref.	Type	Production Method	GWP (kg CO2 eq)	ME/FE (kg N eq/kg p eq)	TA (kg SO2 eq)	CED Value	Human Toxicity (kg 1,4-DB eq)	Fossil Fuel Depletion (kg Oil eq)	WD (kg or m3 H2O, Other Units Specified)
Hohenthal et al. (2012) [256]	CNF	Enzymatic + HPH	1.2–3.1	0.015–0.016	0.008–0.045	N/A *	N/A	0.3–0.75	50
		TEMPO oxidation + HPH	1.0–1.8	0.018–0.024	0.005–0.0065	N/A	N/A	0.25–0.5	158
		TEMPO oxidation + mechanical refinement	0.75–1.0	0.014–0.015	0.0045–0.005	N/A	N/A	0.20–0.25	120
Li et al. (2013) [252]	CMF	TEMPO oxidation + sonication + centrifuge purifying (TOSO) TEMPO	980 (per kg NC)	N/A	N/A	145.9 MJ	N/A	N/A	N/A
		TEMPO oxidation + homogenization (TOHO)	190 (per kg NC)	N/A	N/A	34.7 MJ	N/A	N/A	N/A
		Chloroacetic acid etherification + sonication + centrifuge purifying (CESO)	1160 (per kg NC)	N/A	N/A	176.1 MJ	N/A	N/A	N/A
		Chloroacetic acid etherification + homogenization (CEHO)	360 (per kg NC)	N/A	N/A	64.9 MJ	N/A	N/A	N/A
Arvidsson et al. (2015) [253]	CNF	Enzymatic pretreatment + microfluidization	0.79	N/A	N/A	87 MJ/kg	N/A	N/A	240
		Carboxymethylation pretreatment + microfluidization	99	N/A	N/A	1800 MJ/kg	N/A	N/A	1000
		Without pretreatment + homogenization treatment	1.2	N/A	N/A	240 MJ/kg	N/A	N/A	130 L/g
De Figueirêdo et al. (2012) [257]	CNC	EUC system	0.122171	0.000320/0.000134	N/A	15.943 MJ for the extraction of raw materials	0.291122	N/A	131 L/g
		EC system		0.000065/0.000024	N/A	1.8 MJ for the extraction of raw materials	0.034797	N/A	138 L/g
Piccinno et al. (2015) [258]	CNF	MFC liberated (Enzymatic + homogenization) + coating MFC with GripX + wet spinning by adding sodium alginate (route 1a)	1.5–1.6 (10 g of MFC)	N/A	N/A	32.2 MJ for production of 10 gr MFC	N/A	N/A	(0.201 for MFC liberation) 0.253 L/g
		MFC liberated (enzymatic + homogenization) + wet spinning by adding sodium alginate (without coating) (route 1b)		N/A	N/A		N/A	N/A	(0.201 for MFC liberation) 0.255 L/g
		MFC liberated (enzymatic + homogenization) + electrospinning by adding PEO as a carrier polymer (route 2)		N/A	N/A		N/A	N/A	(0.201 for MFC liberation) 0.205 L/g
Nascimento et al. (2016) [259]	CNC	Extraction of CNC with high-powered ultrasound (CNU)	0.207	5.68 × 10−5/3.03 × 10−5	0.00045	N/A	0.0477	N/A	0.0023

Notes: GWP: global warming potential; ME: marine eutrophication; FE: water body eutrophication; TA: terrestrial acidification; CED: cumulative energy demand; WD: water depletion; HPH: high-pressure homogenization; TEMPO: 2,2,6,6-tetramethylpiperidine-1-oxyl; EUC: extracted from unripe coconut fiber; EC: extracted from cotton fiber; MFC: micro fibrillated cellulose; PEO: poly(ethylene) oxide. * N/A: Not Available. The economics of CNMs depend on the production method. Enzymatic pretreatment for CNFs costs about $0.42/kg, while sulfuric acid hydrolyzed CNCs are priced around $15 to $30/kg [231]. The University of Maine Process Development Center offers CNFs for $34/kg, with CNCs priced at $136/kg. At the commercial scale, prices are higher; for instance, CelluForce offers CNCs at $350/kg for 40 L of suspension [231,260].

summarizes the LCA studies of the production of cellulose nanocrystals and cellulose nanofibers. LCA of CNM production shows variable impacts, largely dependent on the production method. For instance, TEMPO oxidation incurs high cumulative energy demand (CED) and GWP, while enzymatic and mechanical fibrillation are less energy-intensive [252,253]. Although production impacts differ, cellulose nanomaterials are generally biocompatible, biodegradable, and nontoxic [254]. Another study [255] analyzed GHG emissions from 23 nanocellulose production processes. Mechanical treatment and sulfuric acid hydrolysis were identified as the lowest-emission methods for CNFs and CNCs, respectively. While nanocellulose increases emissions, its mechanical benefits can offset this in composite applications [255].

3.2. Chitin Nanomaterials

3.2.1. Product Description and Acting Mechanism in Concrete

Chitin, the second most abundant amino polysaccharide polymer in nature, provides structural integrity to crustacean exoskeletons and insect shells and is also found in fungal cell walls [261,262,263]. Chitin nanomaterials (ChNMs) can be derived from crustacean shells and used as reinforcement components in composites, including concrete. These nanomaterials exist in two forms: rod-like chitin nanocrystals (ChNC) and fibrous chitin nanofibers (ChNF), produced through chemical or chemo-mechanical extraction methods. ChNMs may serve as concrete admixtures, offering control over setting time and rheology, as well as potential cement reduction [264]. Chitin nanomaterials can enhance cement hydration by providing high-surface-area nucleation sites, which accelerates the formation of calcium–silicate–hydrate (C-S-H) structures. Additionally, ChNMs can modulate the setting time in concrete by inducing electrostatic repulsion, which refines the microstructure, reduces porosity, and increases strength. Their reinforcing potential, based on fiber length and the increased crystallinity of the composite polymer matrix, contributes to improved stiffness and strength [265,266,267].

The morphology and size of chitin nanomaterials are summarized in

Table 12. Physical and Chemical Properties of Chitin Nanocrystal and Chitin Nanofiber.

Chitin Nanocrystal (ChNC)	Chitin Nanofiber (ChNF)
Rod/whisker type	Fibrillous
Length: 211 ± 80 nm Width: 8.7 ± 4 nm Aspect ratio: 24 ± 20 nm	Length: 1063 ± 765 nm Width: 16 ± 10 nm Aspect ratio: 67 ± 90 nm

[268]. Chitin nanocrystals typically exhibit a rod-like morphology, with an average length of 211 ± 80 nm, a width of 8.7 ± 4 nm, and an aspect ratio of 24 ± 20. Chitin nanofibers are fibrillous, with an average length of 1063 ± 765 nm, a width of 16 ± 10 nm, and an aspect ratio of 67 ± 90. Both ChNC and ChNF share a high crystallinity index (92%), with notable differences in zeta potential and surface charge density due to their extraction processes.

Chitin extraction typically involves deproteination with an alkaline solution, demineralization with an acidic solution, and organic solvent treatment for decolorization, resulting in pure white chitin fibers [269,270]. TEMPO-mediated oxidation can be applied to produce chitin nanocrystals by selectively oxidizing hydroxyl groups on the chitin surface into carboxylate groups [271]. This process allows for controlled oxidation levels to produce chitin nanocrystals with varying degrees of functionality. ChNF production often involves mechanical fibrillation, where grinding disks generate nanofibers without chemical treatment, preserving the native amino and hydroxyl groups and providing a stable structure in aqueous environments with pH-dependent positive charges [272].

3.2.2. Supplies, Supply Chain, and Technology Readiness Level

Chitin is abundantly produced in nature, estimated at a global scale of 100 billion tonnes annually. The fishing and seafood industry generates approximately 6.6 to 8.8 million tonnes of chitin-rich waste each year [273], most of which is discarded or used in low-value products like animal feed and fertilizers [269]. Chitin content in crustacean shells varies from 20% to 30%, depending on species and season [274]. Without treatment, this waste can harm aquatic ecosystems due to its nitrogen and phosphorus content [275]. Shrimp, crab, and lobster shells are primary sources, with the U.S. processing roughly 200,000 tonnes of shrimp, 35,000 tonnes of lobster, and 90,000 tonnes of crab annually [276]. Chitin nanomaterials offer valuable properties such as high specific surface area, stiffness, reactive surface chemistry, and biodegradability, making them suitable for sustainable reinforcement applications [270,275,277].

Some global chitin and chitosan suppliers, with manufacturers including Vanson, Inc., DCV, Inc., Biopolymer Engineering, Inc., and Marine Polymer Technologies in the U.S., and companies in Japan, China, and Norway, where chitin production is more developed. Chitosan, a derivative of chitin, is widely available and soluble in dilute acids, enhancing its applicability in various industries [278,279,280].

Some uses for chitosan and chitin are in medical applications: tissue engineering, wound healing, surgical sutures, bandages, food additives, supplements, vaccine adjuvants, and drug delivery [262,263]. Chitosan is used in stormwater treatment [267]. Chitin nanomaterial applications in concrete remain at the laboratory research stage (TRL 2–3), as further studies are required to assess compatibility and optimize concrete formulations.

3.2.3. Performance in Concrete Based on Literature

Impact on Fresh Properties

Studies indicate that ChNCs increase setting time, with the highest delay of 56 min for the initial and 106 min for the final setting at a 0.055 wt% concentration. ChNF, with a more entangled morphology, showed a smaller delay of 35 min for the initial and 78 min for the final setting at 0.035 wt% [268]. Additionally, ChNF demonstrated higher plastic viscosity than ChNC due to its fiber structure, which increases entanglement and enhances viscosity.

Impact on Strength and Durability

Both ChNC and ChNF improved the 7-day compressive strength of concrete, with ChNC achieving a 31% increase at 0.15 wt% and ChNF a 21% increase at 0.05 wt%. The 28-day strength gain was less prominent due to fiber entanglement at higher doses. ChNMs significantly enhanced flexural strength and fracture energy, with improvements ranging from 16% to 40% for ChNC and up to 41% for ChNF at 28 days, attributed to the reactive groups on chitin enhancing bonding with cement hydrates [267,268,281].

The effect of Chitin on the properties of cement-based materials is described in this Section and summarized in

Table 13. Authors’ synthesis of the literature on the effect of chitin nanomaterials on the properties of cement-based materials.

Property of Concrete	Comparison to 100% OPC Concrete
Water demand	Increase
Early strength	Increase up to a certain dosage
Late strength	No change
Pozzolanic reactivity	Increase or decrease
Setting time	Increase
Drying shrinkage	More research needed
Alkali silica reaction	More research needed
Sulfate attack resistance	More research needed
Freeze–thaw durability	More research needed

.

3.2.4. Environmental Considerations

Chitin is a biodegradable, nontoxic, and renewable material with environmental impacts similar to cellulose nanomaterials. The production of chitin nanomaterials is expected to be environmentally favorable due to minimal chemical processing requirements for ChNFs and sustainable feedstock from seafood waste [270,275]. The production methods of chitin nanomaterials from chitin are similar to those described for cellulose nanomaterials from a cellulosic source. The differences lie in the processing of the source material, i.e., extraction of chitin and isolation from protein and calcium carbonates, versus the pulping of cellulose.

3.3. Calcium–Silicate–Hydrate (C-S-H) Seeding: Description and Acting Mechanism in Concrete

3.3.1. Description and Acting Mechanism

More SCMs are used to replace the clinker content in concrete to lower the GWP of concrete and enhance its durability. However, some aluminosilicate SCMs typically exhibit lower reactivity, leading to slower early strength development in concrete [282,283]. This behavior can cause delays in fast construction and mold removal, and in paving applications, it can impact traffic opening times. Therefore, accelerating admixtures are desired for high-SCM concrete to address the low early strength.

C-S-H is the main hydration product responsible for concrete’s strength and durability [284,285]. To enhance concrete’s early strength, synthetic C-S-H nanomaterials, or C-S-H seeds, can be added to stimulate hydration and for C-S-H to grow more of its own. These synthesized C-S-H seeds can be added, similar to other chemical admixtures, to concrete during production. C-H-S can be synthesized in the laboratory by various methods, such as the sol–gel method, and through pozzolanic and precipitation reactions. The raw materials needed are generally sources of lime, silica, and water. Experimental studies have shown that these seeds accelerate the hydration of silicate phases in clinker, especially C₃S, and increase the degree of hydration within the first 24 h. This acceleration is due to C-S-H seeds providing additional nucleation sites, allowing C-S-H products to grow both on clinker particles and within the pore solution, reducing or eliminating the dormant period of cement hydration [286,287,288,289,290].

3.3.2. Performance in Concrete Based on Literature

Impact on Hydration and Strength Development

C-S-H seeding has shown strong potential to enhance the hydration of cement, especially at early ages, by promoting the formation of calcium–silicate–hydrate through accelerated hydration of clinker’s silicate phases, notably C₃S [286]. By providing additional nucleation sites, C-S-H seeds facilitate the growth of C-S-H in the pore solution rather than directly on clinker surfaces, which stimulates early strength gain and can shorten the induction or dormant phase of hydration [287,288,289,290]. Studies have documented that this mechanism allows seeded mixtures to reach compressive strength benchmarks within the first 24 h, which are typically achieved much later in unseeded mixes. This accelerated hydration is particularly useful in construction scenarios where early strength is critical, such as precast operations, pavement construction, and emergency repairs in highways and airfields [291,292].

Long-Term Strength and Microstructural Development

C-S-H seeding has mixed results for long-term strength. While certain types of seeds can contribute to late-age strength through continued hydration, others may only improve early strength without further benefit at 28 days and beyond [287,293,294,295]). One challenge has been the reduction in porosity and formation of a dense microstructure without compromising strength development at later stages. Experimental work with various types of C-S-H seeds—such as afwillite, gyrolite, hillebrandite, and xonotlite—has shown that seed characteristics, including crystal structure, Ca/Si ratio, and morphology, greatly influence their effectiveness. For example, the fibrous morphology of some seeds appears to support better bonding and microstructural reinforcement, which can positively impact flexural strength in applications where flexural performance is essential [296,297,298].

Suitability in Blended and Alternative Cement Systems

C-S-H seeding is particularly advantageous when combined with SCMs, which are often slower to develop early strength due to lower reactivity. By enhancing the hydration of SCM-blended systems, C-S-H seeding could make it viable to use higher levels of SCMs, such as fly ash or slag, without compromising the strength requirements. This allows for a greater reduction in OPC content, thereby reducing the overall environmental impact [282,283].

Comparison to Traditional Accelerators

Compared to traditional accelerators like calcium salts, C-S-H seeding offers significant benefits. Calcium-based accelerators can improve early strength but may lead to issues with corrosion in reinforced concrete and sometimes reduce strength at later ages due to the formation of less stable hydration phases [299,300]. C-S-H seeding, in contrast, facilitates a balanced strength gain across early and late stages without risking durability or corrosion in reinforced elements. Additionally, it offers an alternative to proprietary rapid-setting cement products for accelerated construction projects, making it a versatile option for a variety of concrete applications, including cold-weather concreting, shotcrete jobs, and masonry fabrication [291,292].

This body of research indicates that C-S-H seeding presents a promising approach for both enhancing performance and potentially lowering the environmental footprint of cementitious systems. Further studies on optimizing seed characteristics and exploring their effects in blended cement systems could broaden the applications of C-S-H seeding in sustainable construction practices.

3.3.3. Environmental and Cost Considerations

While the introduction of nanomaterials like C-S-H seeds in cementitious systems typically increases energy demand, studies suggest that the improvement in material performance can offset these impacts. For instance, LCAs of seeded cement pastes indicate a 7–12% increase in GHG emissions, but when normalized for strength, they demonstrate up to 30% lower CO₂ intensity compared to unseeded pastes [290]. Opportunities for sustainable production, such as co-locating C-S-H production with industries that generate steam as a byproduct (e.g., paper mills), could further reduce emissions associated with C-S-H seeding. In a study, the environmental impacts in terms of GWP of producing 1 kg of two types of C-S-H: foshagite (FOS) and tobermorite (TOB) from new steam vs. recycled steam were shown to decrease from 10.8 down to 3.35 kg of CO₂ eq for FOS and from 9.07 down to 2.5 kg of CO₂ eq for TOB [301]

Though C-S-H seeds add to production costs due to additional material and synthesis requirements, they could reduce costs associated with energy-intensive curing methods. Additionally, producing seeds via hydrothermal methods using abundant materials like lime and diatomaceous earth could control costs [302].

4. Discussion and Future Directions

The findings from this study have been summarized in

Table 14. Summary of Gathered Information About the Studied SCMs.

Studied Material	Estimated Supply	Select Suppliers in the U.S.	Required Treatment	TRL	Function (SCM, Filler, Admixture)
Biomass energy plant ash	Potential 3.29 billion tonnes of biomass feedstock but not all combusted to produce ash (global)	25 biomass energy plants	Grinding/milling for bottom ash and some fly ash	4	SCM
Pumice, perlite, zeolite, volcanic ash	500,000 tonnes of perlite/year (U.S.) 87,000 tonnes of zeolites/year (U.S.) 580,000 tonnes of pumice/year (U.S.)	Mines across Arizona, California, New Mexico, Idaho, Texas, and Oregon	Grinding	8	SCM
Diatomaceous earth	830,000 tonnes/year (U.S.)	Imerys Minerals California’s Lompoc Plant and Celite Corporation	Mining, drying, screening	3	SCM
Metakaolin	4.5 million tonnes of kaolin/year (U.S.)	Mines across Arizona, California, Idaho, and Oregon	Calcinating, grinding	8	SCM
Recycled concrete fines	347 million tonnes/year (U.S.)	Concrete Ready-Mix Producers and Concrete Recycling Plants	Grinding/milling	4	SCM and Filler
Rock dust	21 million tonnes of asphalt rock dust/year (U.S.)	Aggregate quarries, ready-mix concrete producers	None	3	SCM and Filler
Municipal solid waste ash	300,000 tonnes/year (U.S.)	MSW incineration plants like the Southeast Resource Recovery Facility in Long Beach and the Covanta Stanislaus Incinerator Facility in Stanislaus County	Grinding/milling may be needed	3	SCM
Sewage sludge ash	12.56 million tonnes/year (U.S.)	No active plant in California A few are still active in other states, but the number is declining	None	3	SCM
Cellulose nanomaterials	Unlimited source of cellulose, but few CNM producers (global)	Kruger, Sappi North America, CelluForce, American Process Inc.	Chemical or mechanical treatments of cellulose into nanomaterials	4	Admixture
Chitin nanomaterials	100 billion tonnes of chitin/year (global)	Tidal Vision, Scandinavian Formulas, Creative Enzymes	Chemical or mechanical treatments of chitin into nanomaterials	3	Admixture
C-S-H seed	Unlimited supplies of lime and silica from various sources, but a limited admixture producer (global)	Master X-seed 55 (Master Builders)	Thermochemical process of lime and silica source into C-S-H seeds	6–8	Admixture

, covering information regarding the annual SCM supplies, treatment type, TRL, and their likely function in concrete as SCM, filler, or admixture. As seen in the table, biomass energy plant wood ash has an annual production of 380,151 tonnes from 25 biomass energy plants across California. This material requires grinding to process some fly ashes and all bottom ashes into usable particle sizes for concrete applications. Bottom ash has a use as a road base but fly ash does not have a value-added use. The chemical and physical properties vary significantly based on species and combustion method, and the pozzolanic or hydraulic reactivity could vary accordingly. With a TRL of 4, wood ash is in the developmental stage, demonstrating potential for use in specific applications. While promising, further research and process optimization are necessary to establish its viability as an SCM fully.

Natural pozzolans have substantial annual production, with 526,000 tonnes of pumice and 453,000 tonnes of perlite mined across Arizona, California, New Mexico, Idaho, Texas, and Oregon. Treatments such as drying, thermal or mechanical activation, and screening are typically required to prepare these materials for SCM applications. With a TRL of 8, these pozzolans are well-established and proven for use in concrete, offering significant pozzolanic reactivity that enhances the strength and durability of concrete. Their availability and high performance make them highly viable as SCMs, particularly in regions like California, where they are readily accessible.

Metakaolin is produced in large quantities, with an annual supply of 4.1 million tonnes. Its preparation involves calcination followed by grinding and screening, which contributes to its cost and environmental impacts. However, with a TRL of 8, metakaolin is a mature material widely recognized for its high pozzolanic reactivity and its ability to significantly enhance concrete performance. Calcined clay with less kaolin (up to 40%) has been shown to give par strength to the control. Other sedimentary natural pozzolans include diatomaceous earth, which has an annual production of 752,000 tonnes in the U.S. However, with a TRL of 3, it remains in the early stages of exploration for cementitious applications. As a pozzolan, diatomaceous earth shows potential for enhancing concrete properties, but additional research is needed to validate its performance and establish its suitability as a viable SCM, especially with considerations for higher water and admixture demand.

RCA fines are abundantly available, with an annual supply of 367 million tonnes. However, producing SCM powder from the fines requires a significant amount of grinding to achieve the appropriate particle size for incorporation into cementitious systems, driving up the cost and complexity of production. With a TRL of 4, recycled concrete fines are still in the exploration phase for use as SCMs, though they primarily act as fillers rather than reactive materials. While their significant availability makes them appealing, their role is largely limited to non-chemical functions, reducing their suitability as traditional SCMs.

Rock dust has an annual production of 19.05 million tonnes and is readily available from aggregate quarries and ready-mix concrete producers. Unlike many other SCMs, rock dust does not require additional treatment, which reduces its processing costs. However, with a low TRL of 3, it is still in the early stages of exploration for use in cementitious systems. Functioning primarily as a filler, rock dust lacks significant chemical reactivity, which limits its potential as a traditional SCM. Despite its abundance and ease of use, further research is required to expand its applications.

MSWA is produced at an annual rate of 215,229 tonnes from MSW incineration plants. The material does not require additional treatment, making it a cost-effective option for initial evaluation. With a TRL of 3, MSW ash is still in the experimental stage, requiring further investigation to confirm its performance and reliability. It can function as a pozzolan depending on its composition and processing.

The production of sewage sludge ash in the U.S. is not well-documented; however, it is known to be generated by multiple facilities. This material does not require additional processing for initial use, offering a straightforward entry point for exploration. With a TRL of 3, it is in the early stages of research. Wastewater treatment sludge ash has potential as a pozzolan (SCM). However, its mainstream use as an SCM depends on further studies to address challenges related to logistics, performance variability, and consistency.

Cellulose nanomaterials are derived from wood and agricultural sources and have virtually unlimited production potential. Major suppliers include Kruger, Sappi North America, CelluForce, and American Process Inc. These materials require chemical or mechanical treatment of cellulose fibers for use in concrete systems. With a TRL of 4, cellulose nanomaterials are emerging as a promising admixture, capable of modifying concrete properties at the nanoscale.

Chitin nanomaterials have a small annual market in the U.S., with only 68 tonnes produced by companies like Tidal Vision, Scandinavian Formulas, and Creative Enzymes. They require chemical or mechanical treatment of chitin to prepare them for use. With a TRL of 3, these nanomaterials are still in the exploration phase. Functioning as an admixture, chitin nanomaterials have the potential for specialized applications, but their limited supply and early-stage development restrict their broader viability as SCMs.

C-S-H seed refers to particles used as nucleation agents to accelerate the hydration process in cementitious materials. These particles act as substrates that promote the formation of additional C-S-H, the primary hydration product responsible for concrete’s strength and durability [300]. By enhancing hydration, C-S-H seeding can offset delays caused by other additives without compromising long-term strength. This makes C-S-H seeds a highly effective additive for improving the performance of cement-based systems [300].

Introducing an alternative SCM to coal fly ash or slag cement (especially those from new source materials) and advancing it to full implementation in construction remains a slow and complicated process, hindered by diverse challenges, as identified in previous review papers [303]. Extensive laboratory evaluations across multiple scales are necessary to determine whether a potential material is useful as an SCM or filler for concrete from an engineering performance standpoint. Such an assessment should cover all engineering performance aspects, including durability in various adverse climatic conditions, given that much of the built environment is designed to last at least 40 to 60 years [304].

Standardizing new SCMs, particularly those derived from waste streams and industrial byproducts, presents a significant challenge due to the variability in their composition and properties, which directly affect concrete performance. Achieving standardization requires extensive collaboration and the voluntary contribution of expertise from industry and academia. Experts must analyze scientific and rigorous experimental data to reach a consensus on performance criteria, ensuring that these highly variable SCMs meet the necessary standards for consistent and reliable concrete performance. Standards play a critical role in building market confidence, accelerating market acceptance, enabling global trade, and reducing the risks of adopting new materials.

Additionally, scalability to support regional or national demand is a critical factor in the successful commercialization of SCMs. Large-scale production of new SCMs to meet concrete demand necessitates long-term contracts to secure raw materials, operational plants with appropriate permits in place, and coordinated logistics for delivery to end users. Transport logistics and the lack of an established supply chain present additional obstacles, particularly for SCMs sourced far from cement or concrete plants. Developing a robust and integrated supply chain, including feedstock suppliers, intermediary processors for thermochemical or mechanical treatment, certification laboratories, storage, handling, and distribution centers, is essential to provide the concrete industry with a consistent and reliable supply of high-quality SCMs.

Education and training are equally important in addressing deviations in behavior that alternative SCMs may exhibit compared to traditional materials like coal fly ash or slag cement. For instance, some SCMs may have higher water demand, cause increased bleeding, or exhibit slower reaction rates, resulting in delayed strength gain. Concrete producers must be equipped with the knowledge and tools to adapt to these differences. Additionally, agency specifications may require updates to accommodate the use of alternative SCMs. Regional or state-specific supply limitations may further necessitate localized education campaigns to facilitate adoption.

Innovations in admixtures and nanotechnology could play a crucial role in overcoming these challenges, enabling alternative SCMs to perform more effectively and accelerating their acceptance in the market. These advancements could mitigate issues such as slow development of strength or rheological inconsistencies, broadening the implementation of alternative materials. However, the tradeoff with the added costs and GWP from these admixtures should be considered.

5. Conclusions

This paper presents a literature review of 10 groups of materials with the potential to replace OPC in concrete. These materials included SCMs, as SCMs with low pozzolanic or hydraulic reactivity, and more functioning mineral fillers, as well as some admixtures used to enhance rheology and early strength development. The data was collected from diverse sources, including published literature, reports, fact sheets, and online resources. Additionally, input was gathered through interviews with professional organizations, associations, and suppliers to assess the scale of potential feedstock supply, material availability in California, and the estimated NASA-developed TRL scale. The review also explored information from the literature and suppliers regarding the type and extent of treatments required to produce SCMs for concrete, also obtaining insights into the economic and environmental impacts of these treatments.

• Most studied materials have physicochemical properties (high in silica, alumina, or calcium) and are suitable for acting as pozzolanic or hydraulic SCMs and replacing cement in concrete. Some of the studied materials, such as RCA fines or rock dust, may have low pozzolanic reactivity but are still viable as mineral fillers, which can reduce resource use in concrete while offering packing and densifying properties that enhance concrete performance.
• Most SCMs identified here, apart from natural pozzolans like pumice, perlite, zeolite, volcanic ash, and calcined clay, are primarily waste products. For example, biomass energy plant ash (380,151 tonnes annually) and municipal solid waste ash (215,229 tonnes annually) are landfilled or used in limited, low-value applications. Developing supply chains to reuse these waste products as value-added SCMs to replace OPC in concrete mixes can offer significant environmental and societal benefits. The reuse of waste materials has the potential to reduce the consumption of natural resources required to produce OPC, help preserve landfill space, and reduce methane and other emissions (to air, water, and land) in landfills. Local availability may help reduce transportation distances, local landfill use, and pollution from unregulated processing and may also help create local employment opportunities. These waste products usually have zero environmental allocation of impacts in LCA if used without additional processing and minimal transportation of construction materials. However, some may require further processing, such as washing, drying, crushing, grinding, long transportation distances, and other steps that significantly diminish their environmental benefits or even make them worse than current practice.
• Most of the materials studied were at TRL 3 or 4, meaning initial research was conducted to prove feasibility. Examples include diatomaceous earth (TRL 3), recycled concrete fines (TRL 4), and cellulose nanomaterials (TRL 4). Further laboratory testing at the concrete scale is necessary to advance these materials toward industrial-scale implementation. While materials like metakaolin (TRL 8) and natural pozzolans (TRL 8) are closer to market readiness, more demonstration and pilot projects, training, and education campaigns are needed to make their use mainstream.
• California has an abundant supply of feedstock for many of the studied materials, such as recycled concrete fines (367 million tonnes annually) and rock dust (19 million tonnes annually), which exceed the current annual demand for coal fly ash as an SCM in concrete. However, most of these materials need centralized producers for processing, treatment, testing, and certification and require distributors to make their implementation possible.
• Though admixture science and technology have shown feasibility in the research phase from nanomaterials, their mainstream production and application are behind and need more effort.