Low-Carbon Concrete Development Through Incorporation of Carbonated Recycled Aggregate and Carbon Dioxide During Concrete Batching and Curing

Abstract

The accelerated carbonation of fresh concrete and recycled aggregates is one of the safest methods of CO₂ sequestration as it mineralizes CO₂, preventing its escape into the atmosphere. CO₂ injection during batching of concrete improves its strength and may partially replace Portland cement, as with supplementary cementitious materials (SCMs). The curing of concrete by incorporation of CO₂ also accelerates early strength development, which may enable early stripping of formwork/moulds for precast and in situ construction. The carbonation process may also be used for the beneficiation of recycled aggregates sourced from demolition waste. The CO₂ mineralization technique may also be used for producing low-carbon, carbon-neutral, or carbon-negative concrete constituents via the carbonation of mineral feedstock, including industrial wastes like steel slag, mine tailings, or raw quarried minerals. This research paper analyses various available technologies for CO₂ storage in concrete, CO₂ curing and mixing of concrete, and CO₂ injection for improving the properties of recycled aggregates. Carbon dioxide can be incorporated into concrete both through reaction with hydrating cement and through incorporation in recycled aggregates, giving a product of similar properties to concrete made from virgin materials. In this contribution we explore the various methodologies available to incorporate CO₂ in both hydrating cement and recycled aggregates and develop a protocol for best practice. We find that the loss of concrete strength due to the incorporation of recycled aggregates can be mitigated by CO₂ curing of the aggregates and the hydrating concrete, giving no negative strength consequences and sequestering around 30 kg of CO₂ per cubic metre of concrete.

1. Introduction

Global warming poses a serious threat to the existence of humankind. Global warming is directly linked to climate change caused by greenhouse gas (GHG) emission. GHGs include carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), with CO₂ being the dominant greenhouse gas. The Intergovernmental Panel on Climate Change (IPCC) cumulative CO₂ emission data shows a near linear relationship with global warming [1]. The Paris agreement commits signatories to limit global warming to well below 2 °C, preferably 1.5 °C, compared to pre-industrial levels [2]. In 2024, the global CO₂ emission was 38.60 billion tonnes (bt), and 386.73 million tonnes (Mt) for Australia. Australia’s per capita CO₂ emission is 14.48 tonnes (t), which is approximately three times the world’s per capita CO₂ emission of 4.72 t [3]. Australian construction industry is responsible for approximately 18% of total greenhouse gas emissions [4]. Therefore, decarbonization of this sector needs to be prioritised to achieve Australia’s Net Zero goal by 2050. Approximately 29 million m³ of concrete is used per year in Australia, with a total embodied carbon (EC) of approximately 12 Mt CO_2e. A concrete mix typically consists of 12% to 15% of Portland cement (PC), which is responsible for approximately 90% of concrete’s embodied carbon (EC). The decarbonization of cement and concrete, therefore, needs to be prioritised for the decarbonization of construction industry.

Figure 1 presents the decarbonization pathways for the Australian cement and concrete sector recommended by Verein Deutscher Zementwerke (VDZ) [5]. This study shows that decarbonization of cement and concrete can be achieved by decarbonizing electricity and transport (14% reduction in CO₂ footprint), use of alternative fuels (6% reduction in CO₂ footprint), innovation in concrete and cement technology (20% reduction in CO₂ footprint), innovation in design and construction (21% reduction in CO₂ footprint), carbon capture (33% reduction in CO₂ footprint), and accounting for concrete re-carbonation (6% reduction in CO₂ footprint). This study envisages that carbon capture will be essential for decarbonization of the cement and concrete industry.

CO₂ may be captured and stored permanently in concrete and recycled aggregates by chemically reacting with hydrating oxides and producing stable carbonates. This research paper analyses the following technologies currently under development and trialling and proposes a methodology for integrating them with low-carbon concrete (LCC) production technologies to further lower the EC for improving sustainability.

• CO₂ storage in concrete for sustainability;
• CO₂ used for concrete curing and mixing;
• CO₂ injection for improving properties of recycled aggregates.

2. Carbon Dioxide (CO₂) Recycling in Concrete

2.1. CO₂ Storage in Concrete for Sustainability

2.1.1. CO₂ Mineralization

Mineral carbonation is one of the safest methods for the sequestration of CO₂ as it prevents the escape of CO₂ to the atmosphere for long periods of time. Ex situ carbonation involves the carbonation of pretreated feedstock above ground (e.g., in a plant) to produce a value-added material for specific use. The carbonation process involves initial dissolution of both the carbon dioxide and the metal ions, which subsequently react to form carbonates (e.g., MgCO₃, CaCO₃). Owing to their high alkalinity, cementitious material waste stream materials commonly used as supplementary cementitious materials (SCMs), including steel slags and fly ashes, are valuable carbonation feedstocks. The concrete carbonation process can be carried out on both cement slurry (i.e., the solution with water and cementitious binder) or fresh concrete, which includes additives and aggregates. The carbonation processes involve hydrated gel (portlandite and C-S-H), ettringite, and un-hydrated phases, and affect the final composition of the hardened material and the strength development due to the hydration process [6].

Accelerated carbonation curing is the most widely used method to store CO₂ in cement-based materials. In this method, the fresh cement or concrete mix (within a few hours after mixing) is subjected to a carbonation reaction. CO₂ storage in concrete due to accelerated carbonation curing depends on the free lime content of the cementitious binder system including Portland cement and supplementary cementitious materials (SCMs) such as GGBFS, fly ash, and glass powder. Other factors, including particle size distributions and moisture contents, also affect the CO₂ storage capacity. The maximum amount of CO₂ that can be stored in cement-based materials depends on the chemical composition of the cement. The Steinour formula, as given in Equation (1), may be used for estimating the maximum amount of CO₂ that can be stored in cement-based materials [7].

$${CO}_2 (\%,\mathrm{m}\mathrm{a}\mathrm{x})=0.785(CaO-0.7{SO}_3)+1.091MgO+1.420{Na}_{2}O+0.935K_{2}O$$

(1)

where CaO, SO₃, MgO, Na₂O, and K₂O are the mass percentages of relevant constituent oxides.

The carbonation reaction products are a hybrid of calcium silicate hydrate (C-S-H) and calcium carbonate (CaCO₃). High early strength can be obtained from a few minutes to a few hours. The carbonation reaction occurring in hardened concrete is usually considered detrimental because calcium silicate hydrate (C-S-H) is decomposed, reducing the concrete strength. However, if carbonation takes place at an early stage, it could be beneficial for CO₂ utilisation and performance (mechanical strength and durability properties) improvement [8].

Approximately 100 kg slag/ton of steel is produced during the steel-making process. Steel slag mainly consists of CaO-SiO, CaO-MgO-SiO₂, and Fe₂O₃ phases. Steel slag is not commonly used as an SCM because it does not show hydraulic behaviour and has high free lime content. It is reported that the carbonation activation of steel slag is required before using it as an SCM. When steel slag is carbonated by exposing slag panels to 99.5% pure CO₂ gas at the ambient temperature and 0.15 MPa pressure for a duration of 2 to 24 h, the average CO₂ uptake has been reported to be 3.4% in 2 h carbonation and 4.6% in 24 h carbonation. One kilo of basic oxygen furnace slag can capture up to 215 g of carbon dioxide [9].

This implies that by adding CO₂ sourced from the flue gases, the industry wastes or raw materials may be upcycled to carbonate and silicate materials. These materials may be used to produce low-carbon concrete and manufacturing of low-carbon industrial products. In addition, by adding CO₂ at an early stage, the performance of concrete may be improved.

2.1.2. MCi Mineral Carbonation Process

In Australia, MCi Carbon has developed a mineral carbonation process which combines CO₂ captured from flue gases with a mineral feedstock, including industrial wastes like steel slag, mine tailings, or raw quarried minerals, to produce magnesium carbonate, calcium carbonate, and amorphous silica. MCi Carbon has established a pilot plant in Newcastle for the upcycling of industrial waste. A conceptual process of MCi materials beneficiation, as described on the MCi Carbon website [10], is shown in Figure 2.

The low-carbon carbonate materials may be used for the manufacturing of products including paper, glass, plastics, and paint. However, the silicates (amorphous silica) may be used as a supplementary cementitious material (SCM) which can deliver comparable mechanical performance at lower replacement levels, such as 10% replacement. It will have the potential to achieve the same embodied carbon reduction as a FA substitution greater than 10%, enabling concrete producers to minimise clinker use without compromising target strengths [11].

2.2. Use of CO₂ for Concrete Curing and Mixing

2.2.1. CO₂ Curing Process

Curing freshly cast concrete allows for rapid cement hydration, which improves early-age performance and durability properties. CO₂ curing of fresh concrete may be used on a standalone basis or in conjunction with water curing. This not only enables the storage of CO₂ as a solid calcium carbonate phase in concrete but also accelerates the curing process and improves the mechanical properties. The conceptual CO₂ curing process flow is presented in Figure 3.

During CO₂ curing, calcium silicate hydration generates a mixture of calcium silicate hydrate (C-S-H) and calcium carbonate (CaCO₃), instead of only C-S-H, which is generated during the normal hydration process. The rapid generation of C-S-H is responsible for early strength generation. CO₂ curing of concrete is an exothermic process and it results in a compressive strength growth rate similar to that of elevated temperature curing, while being more sustainable. CO₂ curing reduces permeability and improves the freeze–thaw, chloride, and sulphate resistance of concrete. CO₂ curing reduces the alkalinity of concrete, increasing the vulnerability of embedded steel reinforcement to corrosion [12]. CO₂ curing is generally undertaken in a pressurised curing chamber. CO₂ is injected into the chamber, and the gas pressure is maintained at 0.2 MPa, for example, for a duration of 3 h to 28 days. CO₂ curing may be followed by water curing. It is reported that the compressive strength of carbonated concrete increases significantly with dry curing. However, if CO₂ curing is followed by water curing, the 1-day compressive strength of concrete increases further, by 30% [13]. The use of CO₂ during batching and CO₂ curing of concrete are already being trialled. An overview of the technology developed by Solidia Technologies and CarbonCure is included here.

2.2.2. Solidia Low-Carbon Cement and CO₂ Curing Technology

Solidia Technologies [14] has developed Solidia Cement™, which is a non-hydraulic cement composed of low-lime-containing calcium silicate phases, such as wollastonite/pseudowollastonite (CaO.SiO₂) and rankinite (3CaO.2SiO₂). Solidia cement clinker is produced at 1200 °C (PC clinker is produced at 1450 °C) and has a carbon footprint 30% lower than that of PC [15]. Solidia cement uses approximately 50% limestone, compared to the 70% required by PC. It is reported that Solidia cement consists of 46.6% of CaO, 47.9% of SiO₂, 2.6% of Al₂O₃, 0.8% of Fe₂O₃, 0.8% of MgO, 0.4 of Na₂O, 0.7 of K₂O, and 0.2 of SO₃.

This cement sets and hardens by a reaction between CO₂ and the calcium silicates (in the presence of water), locking CO₂ permanently. During the carbonation process, calcite (CaCO₃) and silica (SiO₂) are formed as per the chemical reactions given below, which are responsible for the strength development in concrete [15,16].

CaO.SiO₂ + CO₂ + H₂O → CaCO₃ + SiO₂ + H₂O

3CaO.2SiO₂ + 3CO₂ + H₂O → 3CaCO₃ + 2SiO₂ + H₂O

CO₂ used during the curing process needs to be pressurised, which makes it more relevant for precast concrete applications. The mechanical properties of Solidia cement concrete comprising 350 Kg/m³ of Solidia cement, 821 Kg/m³ of sand, 9.5 mm aggregate 414 Kg/m³, 19 mm aggregate 737 Kg/m³, water 136 Kg/m³, and admixtures (high-range water reducer @ 4 mL/Kg of cement, air-entraining admixture @ 3 mL/kg of cement, and set retarding admixture @ 5 mL/kg of cement), as reported by Sahu et al. [16], are presented in Figure 4. This figure shows that the performance of Solidia cement concrete is similar to that of PC concrete. Sahu et al. [16] also reported that Solidia cement concrete demonstrated equivalent or better freezing-and-thawing, alkali–silica reaction (ASR), and sulphate resistance compared to PC concrete.

Solidia cement concrete can develop full strength in 1 day, and therefore can also increase precast production output significantly. Solidia Concrete™ products are more durable and have a wider colour palette and no efflorescence. Unreinforced precast concrete products such as pavers or blocks produced with Solidia Cement™ and cured with CO₂ reduce its carbon footprint by 60% (30% due to the use of Solidia cement and 30% due to CO₂ curing). These unreinforced precast products can be produced in traditional precast factories with a 24 h production cycle time. Solidia cement can be produced in existing rotary kilns with minor adjustments, removing barriers to its adoption. This technology is being trialled via pilot projects in collaboration with cement producers including LafargeHolcim, and may potentially be trialled in other places around the world [17,18].

2.2.3. CarbonCure Technology

CarbonCure [19] has developed CO₂ sequestration technology that involves the addition of an optimum dose of liquid CO₂ to ready-mixed concrete during the batching process. This results in the formation of mineralized calcium carbonate (CaCO₃) due to the reaction of CO₂ with calcium carbonate and calcium silicate hydrate gel (formed during the hydration of Portland cement), storing the CO₂ permanently in the concrete. The process flow is shown in Figure 5.

Monkman et al. [20] reported that the addition of CO₂ @ 0.06% by weight of cement to a ternary concrete mix (design 28 days compressive strength of 48.2 MPa) comprising cement (50%), slag (37%), and fly ash (13%) resulted in a cement reduction of 55 kg/m³ (20%) for similar mechanical properties of concrete. CO₂ addition resulted in 1% and 3% greater compressive strength than that of the control mix at 7 days and 28 days, respectively. This example mix resulted in a reduction of 142 g/m³ of atmospheric CO₂ and 49.8 Kg/m³ of embodied carbon reduction. The carbon footprint of enabling works, capture and liquefaction of CO₂, pipeline transportation, and tanker transport was estimated to be 31.7 Kg/t.km and 0.38 Kg/t.km. This technology provides dual benefit, namely, it enables the recycling of CO₂ sourced from flue gases and the partial replacement of PC in concrete mixes, functioning as an SCM. The technology was used in the production of concrete used in a 34,000 m² (36,000 m³ concrete) mid-rise office tower at 725 Ponce de Leon Avenue NE in Atlanta, Georgia, in the United States.

2.3. CO₂ Injection for Improving Properties of Recycled Aggregates

In 2022–2023, 29.2 Mt from the construction and demolition (C & D) stream (39% of the total) was generated in Australia, which included 27 Mt of building and demolition materials including asphalt, bricks, concrete, and pavers; ceramics, tiles, and pottery; plasterboard and cement sheeting; uncontaminated soil, sand and rock; rubble, etc. The estimated resource recovery for building and demolition materials was 84%. In total, 4.6 Mt of building and demolition materials was disposed in landfill [21,22].

The properties of recycled aggregate (RA) are typically inferior to those of natural aggregates due to the existence of adhered old mortar and multiple interfacial transition zones (ITZs) of RA. It is reported that the fresh, mechanical, and durability properties of recycled aggregate concrete (RAC) are inferior to those of concrete with virgin aggregates with the same water binder ratio. The adhered old mortar on RA is generally removed by mechanical treatment, including mechanical grinding, heating, and pre-soaking in acid solution. This mechanical treatment may induce micro-cracks in the RA, which adversely affect the properties of RAC. To mitigate this issue, the accelerated carbonation technique is used for improving the properties of RA. CO₂ gas curing of RA enhances the properties of the adhered old mortar and the ITZ. In this process, CO₂ gas penetrates adhered mortar via pores and cracks, and reacts with the primary hydration products, producing CaCO₃ and silica gel, enhancing the strength of the adhered old mortar of RA. CO₂ curing of RA is generally undertaken in a carbonation chamber at a relative humidity (RH) of 50%, a temperature of 25 °C, and a pressure of 0.5 to 0.6 bar [23]. The compressive strength of RCA depends on the CO₂ exposure time of the RA and the proportion of RA in the concrete mix. Pu et al. [24] reported that the carbonation occurs rapidly to 17.65% (62% of total carbonation) in first 0.5 h; then, carbonation occurs at a slower rate of 2.9% per hour up to 3 h from the commencement of the process. After 3 h, carbonation stabilises and continues for a long period of time. This finding may be used for designing an efficient RA carbonation process, allowing for a minimum 0.5 h exposure time. A conceptual RA carbonation setup is presented in Figure 6. For efficient performance, the volume of RA should be approximately 20% of the carbonation chamber volume to maximise the high contact between RA and CO₂. The concentration of CO₂ in the gas mix should be minimum 20% for an optimum carbonation. This threshold allows for selecting industrial flue gas that can be used with minimum processing. The carbonation of RA requires a continuous supply of CO₂, so the RA carbonation plant should be located close to an industrial or power plant emiting CO₂ to minimise transportation costs and CO₂ emission.

The effect of carbonation treatment parameters, % RA in aggregate, CO₂ gas pressure, and exposure time on compressive strength is presented in Figure 7. It may be seen from this that the carbonation treatment of recycled aggregates provides RAC mixes incorporating up to 70% RA with a compressive strength of ±10% of that of concrete with 100% virgin aggregate.

Pan et al. [26] reported that for optimal CO₂ curing results, the CaOH (CH) pretreatment should be carried out with a 0.01–0.05 mol/kg CH solution, a CO₂ concentration of 70%, and a moisture content of 5%. After the CO₂ curing process, the powder content of the recycled fine aggregates (RFAs) decreased from 14.2% to 9.1%, the water absorption decreased from 4.35% to 1.65%, and the crush value decreased from 18% to 13%. In addition, the water-demand ratio of the cured RFAs decreased from 1.17 to 1.10, and the compressive-strength ratio increased from 0.95 to 1.04.

Shi et al. [27] reported that CO₂ curing increases the apparent density of RA by 5.6–7.8%. The post-CO₂ curing apparent density was only 1.5–2.6% lower than that of the natural aggregate. Generally, the drying shrinkage of RAC is higher than that of natural aggregate concrete (NAC), and it increases with an increase in RA content. However, carbonation treatment reduces the dry shrinkage of RAC by 8–13% [23].

It is expected that carbonation would lower the alkalinity of concrete, which may reduce the pH of concrete, increasing the susceptibility of embedded steel to corrosion. However, in the carbonation process, CO₂ penetrates the loosely attached mortar of RAs through the pores or cracks, and dissolves in the pore water, producing carbonic acid (H₂CO₃). The CO₃ ions released from H₂CO₃ react with calcium ions from the hydration of PC, producing carbonate (CaCO₃) and silica gel, which fill the cracks, improving the microstructure and reducing the porosity, increasing the resistance to the penetration of detrimental ions into the concrete. This mitigates the corrosion risk of embedded reinforcement to some extent. Liang et al. [23] reported that the chloride diffusion coefficient of concrete with carbonated RA decreased by 41–46% compared with that with RA. The carbonation depth of concrete with carbonated RA decreased by approximately 20% to 29% compared with that with RA.

During the natural carbonation process, one ton of RA can absorb 11–20 kg of CO₂ [23]. This implies that the embodied carbon of naturally carbonated RA is approximately −15 KgCO_2e/t if no preprocessing is undertaken. Assuming accelerated carbonation will be able to achieve CO₂ absorption equivalent to that by natural carbonation over a long period of time, the estimated embodied carbon of carbonated RA (excluding carbon footprint associated with carbonation system building) for 0.5 h carbonation (17.65% of total carbonation) is presented in Figure 8. This figure shows that the carbonation of RA makes the aggregates more or less carbon-neutral when carbon emissions relating to CO₂ processing and transport are added. The embodied carbon of carbonated RA concrete may be carbon-negative when its cured with CO₂ (due to reduction in binder quantity and CO₂ absorption).

3. Limitations and Challenges

Despite the significant potential of CO₂ mineralisation, carbonation curing, and recycled aggregate (RA) beneficiation for reducing the embodied carbon of concrete, there exist several limitations and implementation challenges.

3.1. Availability, Purity, and Logistics of CO₂ Supply

The feasibility of CO₂ utilisation in concrete depends on access to a reliable CO₂ stream and the infrastructure required for capture, conditioning, compression, storage, and distribution. While sectoral roadmaps indicate that carbon capture is expected to play a substantial role in decarbonising the cement and concrete industry, deployment at scale remains contingent on broader industrial CCS buildout and associated logistics [5]. In addition, the performance of carbonation-based processes can be sensitive to CO₂ concentration, moisture, and processing conditions, so variability in CO₂ supply and handling can translate into variability in uptake and performance [6,8,12,13].

3.2. Variability in Carbonation Capacity and Kinetics

The maximum CO₂ uptake in cementitious systems is constrained by reactive oxide availability (primarily CaO and MgO), but realised uptake is governed by kinetics and transport (e.g., moisture content, particle size distribution, and diffusion) [6,7,8]. Consequently, theoretical capacity estimates (e.g., using oxide-based formulae) may overpredict achievable uptake under practical curing or treatment timeframes [7,8]. Similar constraints apply to RA carbonation, where CO₂ ingress and reaction depend on pore structure and adhered mortar characteristics, which vary widely across sources [24,25,26,27].

3.3. Durability and Reinforcement Corrosion Risks

While early-age carbonation curing can improve early strength and reduce permeability, carbonation reduces alkalinity, which may increase the vulnerability of embedded reinforcement to corrosion if carbonation depth approaches the steel cover zone [12]. This consideration tends to make CO₂ curing most straightforward for non-reinforced or lightly reinforced precast elements or concrete reinforced with non-ferrous reinforcement, unless combined curing regimes or additional durability design measures are applied [12,13]. Long-term durability under representative exposure conditions (chlorides, cyclic wetting/drying, carbonation progression) requires further field validation to support broader structural uptake [12,13].

3.4. Recycled Aggregate Quality and Consistency

RA properties are inherently variable due to differences in parent concrete strength, demolition/processing methods, adhered mortar content, and contamination. Although accelerated carbonation can improve RA density and reduce water absorption, the magnitude of improvement and its translation into concrete performance depends strongly on the specific RA stream and treatment protocol [24,26,27]. Studies reporting strength recovery and shrinkage reductions following carbonation treatment are promising, but achieving consistent performance at scale will require robust quality control and specification frameworks [24,27].

3.5. Energy Demand, Throughput, and Economic Viability

Accelerated carbonation processes commonly require controlled environments (CO₂ concentration, humidity, and sometimes pressure) and sufficient residence time to achieve meaningful uptake and performance gains [12,13,24]. These requirements imply added infrastructure and operational complexity that can affect throughput and cost—especially in ready-mix contexts where curing chambers are less readily integrated than in precast production [12,13]. The overall cost–benefit also depends on how much clinker can be displaced (e.g., through strength gains enabling cement reduction) and the achievable uptake per m³ under real production constraints [20].

3.6. Standards, Codes, and Industry Adoption

Even where laboratory and pilot results are positive, widespread adoption depends on acceptance pathways within standards, performance verification, and risk management. Roadmaps highlight that carbon capture and technology innovation are essential components of sector decarbonisation, but translating emerging CO₂ utilisation approaches into routine practice will require consistent test protocols and evidence bases that satisfy regulators and industry stakeholders [5]. This is particularly important for structural applications where durability and long-term performance must be demonstrated [12,13].

3.7. System Boundaries and LCA Uncertainty

Claims of “carbon-neutral” or “carbon-negative” concrete depend on system boundaries and assumptions regarding CO₂ sourcing, capture energy, transport, and avoided burdens (e.g., cement reduction, landfill diversion). While mineral carbonation is generally considered a safe long-term storage pathway, the net benefit must account for the emissions associated with implementing the carbonation process and any additional processing steps [6,7,8,12,13,24]. Therefore, cradle-to-grave assessments are needed to quantify overall climate impact consistently across alternative decarbonisation levers [5].

4. Mitigation Strategy and Integrated Approach

A potential mitigation strategy could address the identified limitations by integrating multiple CO₂ utilisation pathways across the concrete production lifecycle rather than relying on a single carbonation mechanism. Low-dosage CO₂ injection during batching and short-duration curing reduces dependence on continuous high-purity CO₂ supply while still delivering strength gains and enabling clinker reduction [12,13,20], consistent with sectoral decarbonisation roadmaps [5]. Carbonation treatment of recycled aggregates targets highly reactive adhered mortar, improving aggregate quality and interfacial transition zones and mitigating performance variability associated with high recycled aggregate contents [24,25,26,27]. Durability risks linked to early-age carbonation are reduced by limiting carbonation to controlled early stages and, where required, combining CO₂ curing with subsequent water curing to preserve later-age hydration [12,13]. By combining cement reduction, recycled aggregate beneficiation, and permanent mineralisation of CO₂ within existing precast and ready-mix infrastructure, the integrated approach enhances robustness, scalability, and the likelihood of achieving meaningful embodied carbon reductions at the system level [5,12,13,20].

Embedded steel corrosion is considered a major risk to the service life of reinforced concrete structures. To eliminate the corrosion risks for steel reinforcement due to carbonated RA and CO₂ curing, carbonated concrete may be adopted for the construction of unreinforced concrete elements such as pavers and concrete elements reinforced with non-ferrous steel reinforcements such as glass fibre-reinforced polymer (GFRP) reinforcement. Carbonated RA and CO₂ curing may also be used for low-risk concrete structures with as design life of 25 to 50 years designed according to AS 3600, including footpaths, cycleways, medians, safety barriers, drainage structures, etc. These infrastructures require 20 to 32 MPa concrete, which represent a significant portion (>50%) of the concrete supply. In Australia, transport authorities allow the controlled use of RA and carbonated RA. In October, 2023 a 1 m by 25 m section of the pathway at Transport for NSW’s Edmondson Park North Commuter Carpark in New South Wales (NSW) was installed by AW Edwards in collaboration with Holcim and Western Sydney University. In this installation, 50% carbonated RA was used. The carbonated RA was also trialled at Maidstone Tram Maintenance Facility, Victoria, Blacktown Animal Rehoming Centre, New South Wales, and Western Sydney University Hawkesbury Farm’s concrete slabs in similar low-risk concrete infrastucture in at-grade structures [29].

It is noted that further research is required to demonstrate the compliance of the performance of carbonated aggregates with Australian Standards, including AS 2758 (Aggregates and rock for engineering purposes), AS 3600 (Concrete structures), etc., for stimulating the adoption of carbonated RA. A long-term data set for carbonated RA is required to support lab-based measured and modelled data to demonstrate compliance of its properties with relevant standards, including design life (>60 years), degradation factor (>60), attrition resistance (Los Angeles value < 30–40%), sodium sulphate soundness (average material loss 6–12%), etc. The strength improvement effect of CO₂ when added during mixing and curing stages needs to be included in the concrete standard, AS 3600.

5. Conclusions

We have examined the potential for reducing the embodied carbon of concrete through the integrated recycling of atmospheric and industrial carbon dioxide using carbonation-based technologies applied to cementitious systems and recycled aggregates. Mineral carbonation was shown to provide a safe and durable pathway for permanent CO₂ storage through the formation of stable carbonate phases, while simultaneously enabling the valorisation of industrial by-products such as steel slag, mine tailings, and demolition waste. When used as supplementary cementitious materials or aggregate feedstocks, these carbonated materials can contribute to clinker reduction and landfill diversion.

We find that the use of CO₂ during concrete batching and curing can deliver dual benefits, permanent mineralisation of CO₂ and measurable improvements in early-age strength, enabling reductions in Portland cement content without compromising mechanical performance. Industrial examples such as CO₂ injection during batching and carbonation curing of precast elements demonstrate that these approaches are already technically feasible within existing production infrastructure. However, durability considerations, particularly related to alkalinity reduction and reinforcement corrosion, necessitate controlled application and, in some cases, combined curing regimes.

Carbonation treatment of recycled aggregates was shown to significantly improve aggregate quality by densifying adhered mortar and enhancing interfacial transition zones. Evidence from the literature indicates that recycled aggregate concrete incorporating up to approximately 70% carbonated recycled aggregates can achieve compressive strengths within ±10% of those of equivalent natural aggregate concrete, while also reducing shrinkage and water demand. This provides a viable pathway for increasing recycled aggregate utilisation in structural and non-structural applications.

Importantly, we find that the greatest benefit arises from integrating these approaches rather than applying them singularly. Strength gains from CO₂ injection during batching can partially offset performance losses associated with high recycled aggregate contents, while mineralisation of CO₂ across multiple process stages reduces reliance on any single carbonation mechanism. This integrated strategy improves robustness against variability in CO₂ supply, material composition, and processing conditions, and aligns with recommended decarbonisation pathways for the cement and concrete sector.

While the potential for embodied carbon reduction is substantial, realising these benefits at scale will require further work to address CO₂ supply logistics, energy demand, standardisation, and lifecycle assessment uncertainties. Future research should focus on validated mix-design case studies, cradle-to-grave carbon accounting, and performance-based standards development to support wider industry adoption. Overall, the integrated CO₂ recycling framework presented in this paper provides a pragmatic and scalable pathway toward low-carbon concrete consistent with net-zero objectives.