Low-CO₂ Concrete from Oil Shale Ash and Construction Demolition Waste for 3D Printing

Abstract

To meet 2050 climate targets, the construction sector must reduce CO₂ emissions and transition toward circular material flows. Recycled aggregates (RA) derived from construction and demolition waste (CDW) and industrial byproducts such as oil shale ash (OSA) show potential for use in concrete, although their application remains limited by standardisation and performance limitations, particularly in structural uses. This study aims to develop and evaluate low-strength, resource-efficient concrete mixtures with full replacement of natural aggregates (NA) by CDW-derived aggregates, and partial or full replacement of cement CEM II by OSA–metakaolin (MK) binder, targeting non-structural 3D-printing applications. Mechanical performance, printability, cradle-to-gate life cycle assessment, eco-intensity index, and transport-distance sensitivity for RA were assessed to quantify the trade-offs between structural performance and global warming potential (GWP) reduction. Replacing NA with RA reduced compressive strength by ~11–13% in cement-based mixes, while the aggregate type had a negligible effect in cement-free mixtures. In contrast, full cement replacement by OSA-MK binder nearly halved compressive strength. Despite the strength reductions associated with the use of waste-derived materials, RA-based cement-free 3D-printed specimens achieved ~30 MPa in compression and ~5 MPa in flexure. Replacing CEM II with OSA-MK and NA with RA lowered GWP by up to 48%, with trade-offs in the air-emission, toxicity, water and resource categories driven by the OSA supply chain. The cement-free RA mix achieved the lowest GWP and best eco-intensity, whereas the CEM II mix with RA offered the most balanced multi-impact profile. The results show that regionally available OSA and RA can enable eco-efficient, structurally adequate 3D-printed concrete for construction applications.

1. Introduction

To meet the 2050 climate targets set by international and European policy frameworks, such as the European Green Deal [1], Circular Economy Action Plan [2], and the International Energy Agency [3], the construction sector must reduce both material-related emissions and new CO₂ emissions. Cement-based composites, including concrete, are central to this challenge because they rely heavily on mineral resources and are responsible for a significant share of embodied greenhouse gas emissions. In 2022, global greenhouse gas emissions from human activity were estimated at around 52 Gt of CO₂ equivalents released into the atmosphere [4], of which ~37% are generated by building operations and construction. Manufacturing of construction materials makes up ~11% of that total, while cement and concrete production alone contribute ~6–8% of global CO₂ emissions [5,6]. One effective mechanism for accelerating industrial decarbonisation is to increase the price of CO₂ emission allowances. The Nordic countries are among the most proactive in this regard, maintaining some of the highest carbon allowance prices in Europe and setting strict limits on the carbon footprint of concrete products. Similar regulatory trends are beginning to emerge across the rest of Europe [7,8], where increasingly strict climate requirements in the construction sector are leading to higher demand for low-CO₂ concrete solutions.

At the same time, concrete producers reduce embodied CO₂ by developing low-CO₂ cementitious composites that integrate industrial byproducts and recycled constituents, as a key pathway toward more sustainable construction and toward meeting the 2050 climate goals. Primarily, they have shifted from using CEM I type cement, which consists of 95–100% Portland cement clinker and is very CO₂-intensive, to using CEM II type cement, which is a blended cement with up to 35% supplementary cementitious materials and has a lower carbon footprint. This can be coupled with partial replacement of cement with industrial and agricultural waste products that act as supplementary cementitious materials (SCMs), such as silica fume, fly ash, and ground granulated blast furnace slag, thereby contributing to a circular economy in construction [9,10,11]. The most significant advantage of using such concrete formulations is the drastic reduction in CO₂ emissions, often by 60–80% lower than ordinary Portland cement concrete [12].

One such promising candidate for cement replacement is oil shale ash (OSA), which has emerged as a potential SCM. It is produced as a byproduct of oil shale combustion in power plants—most notably in Estonia, where oil shale is extensively used for energy generation—yielding an estimated 5–7 Mt of OSA annually, which is largely stored in disposal sites near the plants [13]. One of the technologies used for oil shale combustion involves boilers equipped with novel integrated desulphurisation (NID) systems. In these systems, hot flue gases containing oil shale fly ash and sulphur dioxide (SO₂) are directed into a reactor, where a lime-based sorbent is injected. The sorbent reacts with the SO₂ to form anhydrite, resulting in OSA that is enriched in free lime (CaO) and anhydrite [14,15].

Due to its chemical composition, OSA exhibits both pozzolanic and hydraulic properties [16,17]. Its dominant oxides are CaO, SiO₂, and Al₂O₃, which occur as the following mineral phases—calcite, K-feldspar, quartz, and free lime. The ash also contains cementitious phases such as dicalcium silicate (C₂S) and tetracalcium aluminoferrite (C₄AF). The fineness of OSA is comparable to that of cement, with a specific surface area of approximately 0.01–1 m² g⁻¹ and particle sizes ranging from about 5 to 80 µm [13]. When mixed with water, OSA produces a highly alkaline environment, but exhibits a limited binding capacity on its own, as its amorphous aluminosilicate content and reactivity are insufficient to form a continuous gel phase. To address this limitation, metakaolin (MK) was added to increase the reactivity of OSA, as it supplied the highly reactive silica and alumina needed for binder formation [18,19]. Blending metakaolin with OSA can significantly enhance the alkali-activation of the ash, which might otherwise have limited reactivity [18].

Studies indicate that MK can be used as a cement replacement at 5-25% of the cement weight, with significant improvements in various concrete properties [20,21]. Blending OSA with metakaolin has been reported to enhance binder reactivity and strength development by forming aluminosilicate- and calcium–aluminosilicate-type hydrate networks, commonly described as N-A-S-H and C-(A)-S-H/C-S-H gels [18,22,23]. In addition, the production process of MK requires lower temperatures compared to cement, leading to a reduced carbon footprint [24,25]. In this context, it is worth noting that most of the existing research has focused on partial cement replacement, while investigations on completely cementless systems remain limited. Considering the chemical composition of the materials—where metakaolin provides reactive SiO₂ and Al₂O₃ that can react with the CaO present in OSA to form binding phases—it was hypothesised that a self-cementing reaction could occur, even without cement.

Another core element to promote the principles of circular economy in the construction industry is the sustainable management of mineral aggregates, which are extracted in very large quantities. Global demand for construction aggregates increased from about 21 Gt in 2007 to nearly 40 Gt in 2014 [26], and the current annual production is estimated at roughly 46 Gt [4]. This growth is driven by population increase, urbanisation, and infrastructure development, which together increase demand for concrete and, consequently, for aggregates, which account for approximately 60–80% of concrete volume [27,28,29,30]. The extraction and processing of natural aggregates disrupt ecosystems through land use and landscape alteration, habitat loss, and local geological instability [28,31,32]. In addition, quarrying, crushing, and transport of aggregates are energy-intensive processes that contribute to CO₂ emissions [28].

At the same time, construction and demolition waste (CDW) has become one of the largest waste streams, typically accounting for 30–40% of total waste by mass in many countries [33,34,35,36,37]. CDW includes all waste arising from the construction, renovation, and demolition of buildings and infrastructure, and is still predominantly landfilled despite the environmental impacts and the missed opportunities for material recovery [38,39,40,41,42]. A more circular strategy is to produce recycled aggregates (RA) from CDW and use them as a partial or full replacement for natural aggregates (NA) in cement-based composites. However, RA cannot be treated as a direct substitute for NA, because its properties are significantly inferior to NA and may adversely affect composite or concrete performance.

One important limitation arising from RA’s negative impact on concrete performance is a major setback to its widespread adoption in the construction industry. The existing research shows that incorporating RA can reduce the fresh, mechanical, and durability properties of concrete, which are directly influenced by the quality of the aggregate [43]. Compared to NA, RA typically exhibits lower apparent density, higher water absorption, and a higher crushing index. The mortar attached to RA particles has been identified as the key factor for the reduced quality of RA [44,45], mainly due to its porous structure and entrained air [46] and lower mechanical strength [47]. In addition, concrete made with RA may exhibit reduced workability due to the higher angularity and irregular particle shape [48]. Another limitation concerns regulatory restrictions on RA use in structural concrete. European standards restrict the maximum replacement of coarse NA with RA to 0–50%, depending on the RA type, concrete class, and exposure classes [49].

Consequently, RA is generally unsuitable for structural concrete and is instead used in pavements, foundations, and roadbeds [50,51]: a practice that results in downcycling. Such use does not align with circular economy principles, which emphasise recycling materials within the same product category or, at least, keeping them in the same value chain [13]. For example, turning concrete rubble back into new concrete is preferred over downcycling it into lower-value material like road fill.

Although both waste materials discussed, i.e., aggregate from CDW and OSA, are unsuitable for structural concrete, primarily due to standardisation limits, they still hold potential for use in low-strength, non-structural applications. One application that is not yet strictly regulated by standards is 3D-printing of concrete. This is an emerging technology in the construction industry that enables us to create complex geometries, potentially reducing waste and saving on material and labour costs compared to traditional formwork methods [52,53,54,55,56,57].

Unfortunately, such applications typically rely on high-quality cement-based binders. To ensure pumpability and extrudability during 3D-printing, the binder content must be much higher than in conventional cast concrete, which increases the costs and environmental impact [56,57]. This inefficient use of high-quality binders in non-structural components contradicts the purpose of producing these elements, i.e., to save materials and reduce expenses. Therefore, there is a clear need for more resource-efficient materials suitable for 3D-printing.

The number of life cycle assessments (LCAs) focused explicitly on 3D-printed materials has grown, with many reporting significant reductions in environmental impacts, particularly global warming potential (GWP) [58]. These reductions were primarily driven by the utilisation of industrial waste, as well as the adoption of more energy-efficient mixing and printing processes. However, the substitution of binder systems with industrial byproducts other than fly ash and slag, such as OSA, has received little attention. In fact, only two studies were found to discuss this material in the context of 3D-printing, demonstrating that OSA containing mixtures is suitable for 3D-printing and shows adequate cohesion [59] and, in some cases, achieves compressive strength that is comparable to cement-based mixes [13], although neither study included an LCA. Another aspect that is hardly addressed is the use of RA in 3D-printable concrete mixes. In fact, two studies were found that address the effect of RA in printable concrete mixes through LCA, reporting that full replacement of NA with RA can reduce GWP by about 2.5% and that further reductions are possible with lower cement content, although the use of 3D-printing itself can increase GWP by up to 30%, due to higher binder demand [60,61].

Despite this progress, significant research gaps remain. First, few studies fully integrate mechanical performance testing of 3D-printed mixes with LCA, linking hardened strength to the cradle-to-gate impacts of their constituent materials. Second, among the existing LCA studies of 3D-printing, none consider the incorporation of regionally available industrial residues such as OSA nor systematically evaluate the substitution of NA with RA in 3D-printable mixes. Third, the transport and supply-chain effects of RA in 3D-printable mix designs remain largely unexplored in LCA studies.

This study aims to develop concrete with partial or full replacement of cement using waste materials, such as OSA and CDW-derived aggregates. The mechanical performance of said concrete was assessed, along with an LCA and evaluation of its suitability for 3D-printing, with potential applications such as permanent formwork, self-bearing elements, and others. The combination of these objectives aligns with the previously outlined strategies that aim to achieve the 2050 climate targets.

2. Materials

2.1. Admixtures

Tap water, according to EN 1008 [62], was used as mixing water for all concrete mixtures. A superplasticiser (SP) (Floormix, Vincents Polyline Ltd., Kalngale, Latvia) was incorporated to improve the workability of the mortars.

2.2. Aggregates

The fine aggregate used in the mixtures was a 0–2 mm washed, dried, and fractionated natural sand from Sakret Ltd. (Rumbula, Latvia). Coarse RA was obtained from a CDW collection facility in Latvia—Getliņi EKO Ltd. (Rumbula, Latvia). At Getliņi EKO, the CDW is stored outdoors and separated into several size fractions (Figure 1a). The smallest available fraction (0–16 mm) was further manually sieved to obtain the needed 2–8 mm fraction (Figure 1b). The coarse natural aggregate (NA) used as a reference was a commercially available 2–8 mm washed, dried, and fractionated natural gravel from Sakret Ltd. (Rumbula, Latvia).

The visual comparison of NA and RA shows clear differences in particle quality and composition (Figure 2). NA consists of uniform, well-graded, and semi-rounded particles with consistent colour. In contrast, RA is more irregular in shape and texture and contains more angular particles, including pieces of old mortar, brick, and occasionally wood, reflecting its origin from CDW.

2.2.1. Granulometric Analysis

The particle size distribution (PSD) of the used coarse aggregates was performed using a standard sieving method, according to EN 933-1 [63]. A dry sample was placed on a stack of sieves with decreasing opening sizes and mechanically shaken for a specified time duration. After sieving, the material retained on each sieve was weighed, and the percentage passing through each sieve was calculated. The PSD chart of NA and RA (Figure 3) shows that both materials fall within a similar overall size range, with some differences in the finer fractions. RA exhibits a slightly higher proportion of particles passing through the smaller sieves (≤1 mm), indicating a greater fine content compared with NA. Since the RA originates from crushed concrete, the attached mortar and fragmentation during processing most likely contributed to the increased proportion of small particles.

2.2.2. Physical Properties of Aggregates

Water absorption and apparent density were determined according to the EN 1097-6 [64] standard, using a pycnometer method. The results show that RA exhibited a lower apparent density and significantly higher water absorption than NA (

Table 1. Physical properties of used coarse aggregates.

Aggregate	Apparent Density, kg/m3	Water Absorption, %
Natural aggregate	2739	2.74
Recycled aggregate	2625	11.61

).

2.3. Binders

Three types of reactive powders were used in this study: namely, OSA, MK, and cement CEM II (Figure 4). The OSA used in this study originated from a novel integrated desulphurisation (NID) system inside one of the Narva Power Plants (Narva, Estonia): namely, Eesti Power Plant (Eesti Elektrijaam, Narva, Estonia). The NID system is a flue gas treatment unit, where SO₂ that is released during oil shale combustion is captured before it enters the atmosphere. In the NID system, SO₂ is removed from the flue gas by reacting it with the calcium-rich components of the ash, forming anhydrite (CaSO₄), which chemically binds sulphur within the OSA [14,15,65]. For this study, the ash was retrieved from silos at the power plant sites, where it is temporarily stored. Meanwhile, MK powder and cement CEM II/A-LL 42.5 R were supplied by Astra Technologia Betonu, Ltd. (Straszyn, Poland) and Schwenk Ltd. (Brocēni, Latvia), respectively.

2.3.1. Particle Size Distribution (PSD)

The particle size distribution (PSD) for the binding materials was determined by laser diffraction according to ISO 13320 [66], using a Microtrac SYNC 5001 analyser (manufactured by Microtrac MRB, Montgomeryville, PA, United States of America (USA)). Approximately 1 g of each material was placed into the measurement cell and analysed in wet mode, using isopropanol as the dispersion medium.

The differential passing curve (Figure 5a) for OSA showed that the particle size range is 0.5–300 μm with a peak in the particle size range of ~4–5 μm. The MK particle size distribution ranged from 0.25 to 600 µm, with two dominant peaks: a primary peak at ~4–6 µm and a secondary peak at ~60 µm. The cement particle size distribution was estimated to be 0.3–100 µm, with a peak at ~20 µm.

According to the cumulative passing curves (Figure 5b), the median particle size (D₅₀) values are 9.72 µm for OSA, 9.25 µm for PC, and 22.31 µm for MK. This indicates that OSA and PC have comparable fineness. MK contains coarser particles on average, in terms of equivalent particle size.

2.3.2. Morphology

To compare the true geometric size of binders, their particle morphology was examined using scanning electron microscopy (SEM). Samples were prepared by placing a small amount of material onto double-sided conductive carbon tape fixed to a standard SEM stand. Images were taken with a JEOL IT500 LV microscope (manufactured by JEOL Ltd., Akishima, Tokyo, Japan).

The SEM images show that PC particles are predominantly angular, sharp-edged, and irregularly shaped (Figure 6a). In contrast, the OSA powder consists of a mixture of spherical particles with a smooth surface, porous agglomerates, and fine fragmented debris (Figure 6b). Metakaolin shows another distinct morphology (Figure 6c). Particles are mainly irregular, plate-like, and lamellar, and these grains display a wide size distribution.

3. Methods

3.1. Mix Proportions

Four types of mixtures were developed. The first two mixtures contained cement, while the remaining two were cement-free mixtures. The main difference in the dry components between the two cement-containing mixtures was the type of coarse aggregate used: either recycled or natural. The same principle applied for the cement-free mixtures.

The aggregate-to-binder ratio of 1.50 was used for all mixtures. The water-to-binder (W/B) ratio varied between mixtures. Instead of using a fixed ratio, the spread diameter was adjusted to 155–175 mm to achieve comparable workability, based on preliminary testing using the same 3D-printer setup. This approach, instead of consistent W/B ratio, was chosen because all mixtures were supposed to be 3D-printed, and printability was an important consideration in this study. Small variations in spread diameter are normal for 3D-printed concretes and are considered to be acceptable in academic tests. This range ensures that workability is comparable across different mixes while accounting for minor rheological differences [67]. The required water content depended on the particle size distribution, morphology, surface texture, and mineralogical composition of the dry materials [13]. Liquid SP was added at 1% of water.

The mixture proportions of the dry constituents used in the concrete is given in

Table 2. Mass of components per 1000 g of dry materials (g).

Mixture	CEM II	OSA	MK	Sand 0–2 mm	NA 2–8 mm	RA 2–8 mm
CEM + NA	240	100	60	300	300	-
CEM + RA	240	100	60	300	-	300
CEM-free + NA	-	320	80	300	300	-
CEM-free + RA	-	320	80	300	-	300

CEM—cement CEM II; OSA—oil shale ash; MK—metakaolin; NA—natural aggregate; and RA—recycled aggregate.

. These proportions were selected based on preliminary laboratory tests aimed at optimising workability and printability, as well as on the authors’ previous research on OSA and MK binders for 3D-printed concrete, which provided guidance on suitable ratios for achieving adequate strength and consistency [13,68,69].

3.2. Test Methods

3.2.1. Fresh Properties

After adding water to the dry components, a cone flow test was performed according to EN 1015-3 [70]. The test involved filling a cone-shaped mould with the mixture in two layers, compacting each layer, and then lifting the cone vertically to allow the mortar to spread freely on a flat plate. The flow table was then dropped 15 times, and the final spread diameter was measured in two perpendicular directions, recording the average value.

3.2.2. Printability Assessment

Printability of all mixtures was evaluated through a direct buildability test. The direct buildability test was performed ~15 min after adding water to the dry mix by printing a cylinder with a diameter of 250 mm until plastic collapse occurred [71]. The compressive stress at plastic collapse, ${\sigma }_p$ (Pa), and yield stress, ${\tau }_y$ (Pa), are calculated using Equations (1) and (2) [72], respectively:

$${\sigma }_p=\rho n h g$$

(1)

$${\tau }_y={\displaystyle \frac{\rho n h g}{\sqrt{3}}}$$

(2)

where $\rho$ (kg m⁻³) is density, $n$ is the number of layers at the time of collapse, $h$ (m) is the height of the layer, and $g$ (m s⁻²) is the acceleration due to gravity.

This test not only allowed us to compare the buildability between all printed mixtures but also to visually evaluate the surface quality. The thickness of the printed cylinder was also measured to assess the dimensional consistency of the filament width.

Printing was conducted using a custom-made gantry-type laboratory 3D concrete printer at Riga Technical University (RTU). The printer has a working area of 1500 × 1000 mm and a vertical clearance of 1000 mm. The concrete was loaded into a 15 L hopper and extruded through a 25 mm diameter circular nozzle, using a helical screw conveyor. The extrusion rate was adjusted to produce filaments that were approximately 50 mm wide, with a layer height of 10 mm.

3.2.3. Mechanical Properties and Sample Preparation

The preparation of cast samples and the curing of both cast and 3D-printed specimens were carried out in accordance with EN 1015-11 [73]. Fresh concrete was cast into prismatic moulds of 40 × 40 × 160 mm for flexural and compressive strength testing. For the 3D-printed samples, after performing the printability assessment, a rectangular specimen ~200 mm high, ~50 mm wide, and with a layer height of ~10 mm was printed. Twenty-four hours after preparation, the cast specimens were demoulded, and the 3D-printed objects were removed from the printing area and placed in a storage chamber at a relative humidity of 95 ± 5% and a temperature of 20 ± 2 °C until testing. The 3D-printed specimens were removed from the chamber two days before testing to be cut into prisms measuring 40 × 40 × 160 mm for mechanical testing, after which they were returned to the chamber.

Flexural and compressive strength tests were carried out according to standard EN 1015-11 [73]. For the cast specimens, the mechanical tests were performed by applying the load to one of the faces that had been cast against the mould. The 3D-printed specimens were tested parallel to the print direction—applying the load perpendicular to the printed layers (Figure 7). The mechanical tests were performed using a Controls 50-C56Z00 testing machine (manufatured by Controls S.p.A., Milan, Italy). Compressive strength was measured under a force-controlled mode with a stress rate of 0.8 MPa s⁻¹, while flexural strength was determined under a displacement-controlled mode at a rate of 0.5 mm s⁻¹.

3.3. Life Cycle Assessment

3.3.1. Goal and Scope Definition

The primary goal of this attributional LCA is to quantify and compare the environmental impacts of the 3D-printable concrete mixtures containing different proportions of OSA, MK, and RA as partial replacements for cement and NA. The assessment aims to provide insight into how these alternative materials affect the environmental profile of 3D-printed concrete.

This LCA was conducted according to the ISO 14040 [74] and ISO 14044 [75] methodological frameworks. The study’s geographical focus is Latvia, with laboratory-scale data collected at RTU. The temporal scope covers the current production conditions from 2023 to 2025, and the technological representativeness corresponds to the laboratory-scale 3D-printed concrete manufacturing by using research-grade equipment and mix designs that are broadly representative of near-term industrial practice.

The functional unit (FU) was defined as one cubic metre of fresh 3D-printable concrete, representing a generic mix without a specific construction application. The system boundary was set as cradle-to-gate, encompassing all processes from raw material extraction to the production of fresh printable material at the RTU laboratory. This corresponds to modules A1–A3 under the EN 15804 [76] framework. Processes beyond the laboratory gate, including construction, use, and end-of-life, were outside the scope of this study.

3.3.2. System Boundary and Modelling Approach

The modelled life cycle system includes raw material supply (A1), transportation to the production site (A2), and the preparation of the printable mix (A3). The raw material stage covers the extraction and processing of all binder components (cement, OSA, and MK), as well as the processing of NA, RA, and any additional admixtures. The transport stage includes all inbound transport of materials to RTU, based on supplier distance and transport mode. The manufacturing stage (A3) involves material batching, weighing, and mixing in the laboratory environment. Since identical mixing and printing procedures were applied to all six mixes, the associated energy consumption was excluded from the model. Additionally, water used for cleaning the mixer and the pump was omitted. This exclusion does not affect the comparative results between mixes. For transparency,

Table 3. Summary of energy and water required in the 3D printing process at RTU—Latvia.

Input	Purpose	Amount
Electricity	Operating the mixer	4.41 kWh
Electricity	Operating the printer	5 kWh
Water	Cleaning the mixer	15 L
Water	Cleaning the pump	100 L

lists the energy and water requirements for the printing process.

Foreground data, including the mass of all input materials and transport distances, were directly measured or estimated by the research team. Background data, such as upstream production of cement, supplementary cementitious materials, and aggregates, were obtained from different sources, including the Ecoinvent 3.8 database (cut-off system model), the Environmental Product Declaration (EPD), and the published literature. The said data were adapted to Latvian conditions, as applicable. The modelling was performed in SimaPro (version 9.4.0.3) to ensure consistency between foreground inventory and background datasets.

3.3.3. Life Cycle Inventory

The life cycle inventory (LCI) was compiled in

Table 4. Source of life cycle inventory (LCI) data for system inputs.

Raw Material	Source of Inventory Data
CEM II	EPD (CEM II/A-LL 52.5 N)—Schwenk Ltd., Brocēni, Latvia
OSA	Inventory based on the literature data [77]
MK	Inventory based on the literature data [25,78,79,80,81]
Water	Ecoinvent Database—Market for tap water {Europe without Switzerland}
SP	Ecoinvent Database—Market for polycarboxylates, 40% active substance (RoW)
Sand (0–2 mm)	Ecoinvent Database—Market for sand, sand (RoW)
NA (2–8 mm)	Ecoinvent Database—Market for gravel, crushed (RoW)
RA (2–8 mm)	EPD-Mattsson Facility in Upplands Väsby, Sweden

CEM II—cement CEM II; OSA—oil shale ash; MK—metakaolin; NA—natural aggregate; RA—recycled aggregate; and SP—superplasticiser.

. Each of the four mixes was defined by its specific proportions of cement CEM II, OSA, MK, aggregates, admixtures, and water. Material quantities were determined per functional unit of one cubic metre and entered into SimaPro as input flows. The amounts considered herein are listed in Table 2 in Section 3.1. When supplier-specific environmental data were unavailable, appropriate proxy datasets were selected from Ecoinvent based on material type and regional context. For example, data on cement production in Latvia were unavailable; therefore, an EPD produced by Schwenk Ltd. (Brocēni, Latvia) was used, which specified the type of cement used in the mixes under investigation. Transportation was modelled by using standard Ecoinvent processes, corresponding to lorry (EURO 4, 7.5–16 metric ton). All transport distances considered in this study are listed in

Table 5. Transportation mode utilised and travelled distance.

Material	Material Origin *	Distance, km	Mode	Type	Source
CEM II	Schwenk Ltd., Latvia	113.0	Road	Lorry	Transport, freight, lorry 7.5–16 metric ton, Euro 4
OSA	Narva, Estonia	422.0
MK	Astra Technologia Betonu, Ltd., Straszyn, Poland	774.0
SP	Vincents Polyline Ltd., Ādažu Municipality, Latvia	12.7
Sand (0–2 mm)	Sakret, Ltd., Rumbula, Latvia	15.0
NA (2–8 mm)	Rumbula, Latvia	14.0
RA (2–8 mm)	Getlini EKO, Ropaži Municipality, Latvia	16.2

* All materials were delivered to RTU, where mixing and 3D-printing took place.

.

The LCI for OSA was explicitly developed for this study because OSA is not a standard dataset in commercial LCI databases. The approach followed ISO 14044 [75] guidance for multi-output systems, using economic allocation to apportion the impacts of oil shale electricity generation between the main product, which is electricity, and the byproduct, OSA. Primary data for the underlying process of oil shale combustion in Estonia were collected from the literature, particularly the study by Talve and Põld [77], which reports detailed operational and emission characteristics for Estonian thermal power plants operating with fluidised bed combustion [77].

3.3.4. Allocation and Cut-Off Rules

The environmental burdens of the oil shale combustion system were partitioned between the main product, electricity, and the byproduct, oil shale ash, by using economic allocation in accordance with ISO 14044 [75] guidelines for multi-output processes. This method distributes environmental impacts in proportion to each co-product’s economic value, reflecting their relative market importance. Based on the price data for Estonian energy and ash markets, the generation of 2012.5 kWh of electricity (valued at EUR 204.97) and 1000 kg of OSA (valued at EUR 10) yields a total revenue of EUR 214.97, as shown in

Table 6. Economic allocation to partite environmental burdens to the main product, electricity, and the byproduct, OSA.

Products	Quantity	Price	Allocation
Electricity (main product)	2012.5 kWh	EUR 204.97	95.3%
Oil shale ash (byproduct)	1000 kg	EUR 10	4.7%

. The resulting allocation factors are therefore 95.3% and 4.7% for electricity and OSA, respectively.

Accordingly, 4.6% of the total resources used and emissions associated with the oil shale combustion system were attributed to OSA. This fraction was then used as the upstream environmental load in the LCI of OSA as a supplementary cementitious material in the 3D-printed concrete mixes. The use of economics, rather than mass allocation, was justified by the substantial difference in the nature and market value of the co-products. Electricity is the intended output of the process, whereas OSA is an industrial residue of limited commercial worth. This approach ensures that the environmental profile of OSA reflects its real economic significance within the system and prevents an artificial over-allocation of burdens to a low-value byproduct. In this context, the study adopted a consistent cut-off criterion whereby any input or emission contributing less than one percent (1%) of total mass or energy was excluded, provided that the sum of all excluded flows did not exceed five percent in any impact category. The model’s completeness was verified during system checks in SimaPro.

3.3.5. Life Cycle Impact Assessment

The life cycle impact assessment (LCIA) was carried out using the ReCiPe 2016 (H) midpoint method implemented in SimaPro. The selected impact categories included GWP (kg CO₂ eq), ozone depletion (kg CFC11 eq), ionising radiation (kBq Co-60 eq), ozone formation (kg NO_x eq), fine particle formation (kg PM_2.5 eq), terrestrial acidification (kg SO₂ eq), freshwater and marine eutrophication (kg P eq and kg N eq), eco- and human toxicity (kg 1,4-DCB), land use (m²a crop eq), mineral use (kg Cu eq), fossil resource scarcity (kg oil-eq), and water consumption (m³). The results were reported at the midpoint level without normalisation or weighting to maintain transparency and interpretability.

3.3.6. Assumptions and Limitations

Several assumptions underpin the system model. All four concrete mixtures were assumed to have identical mixing and printing procedures, ensuring that energy use and auxiliary materials were comparable across all cases. The printability of the mixes was considered sufficiently similar that no functional differentiation was required within the defined cradle-to-gate boundary. Waste aggregates were assumed to be inert and to require no additional processing beyond crushing and sieving. At the same time, OSA was treated as a co-product of existing industrial operations with burdens assigned through economic allocation. Construction, use, and end-of-life stages were excluded from the analysis, as were potential carbon dioxide (CO₂) uptake effects during curing and the avoided burdens associated with clinker replacement beyond the system boundary. Consequently, the findings are valid for comparative interpretation among the four mixes but should not be extrapolated to represent full life-cycle performance.

3.3.7. Ecological Intensity Index

The ecological intensity index, eco-intensity index, provides a mechanism for standardising environmental and performance data. It measures the amount of carbon dioxide emitted from specific outputs or activities, such as manufacturing goods, transportation, or energy generation [82], which allows for the quantification of the greenhouse gas emissions linked to specific activities, enabling comparisons across industries, processes, or products [82]. For example, CO₂ eq per kilowatt-hour measures emissions from electricity generation.

To carry out a fair comparison between the proposed mixes in this study, this metric was calculated based on each mix’s carbon dioxide emissions and 28-day compressive strength. The ecological intensity index ($C_i$) for concrete, using its compressive strength, is typically expressed as the ratio of its environmental impact (commonly GWP, expressed in kg CO₂ eq emissions per volume of concrete) to its compressive strength ($f_c$, measured in MPa), Equation (3) [83,84,85]. Specifically, one common approach is to divide the emitted CO₂ eq (kg per cubic metre of concrete) by its compressive strength (MPa), yielding units of kg CO₂·m⁻³·MPa⁻¹. This index reflects CO₂ emissions per unit of concrete performance, enabling the assessment of eco-intensity by balancing structural capacity with environmental impact across different mixtures, binders, and aggregates. A lower $C_i$ indicates a better mix proportion for the concrete [58,86,87].

$$C_i={\displaystyle \frac{GWP}{f_c}}$$

(3)

4. Results and Discussion

4.1. Fresh Properties and Printability

Fresh-state properties and direct printability test results are shown in

Table 7. Fresh properties and printability assessment.

Mixture	W/B *	Cone Flow, mm	Direct Buildability Test
			Buildability, Layers	Compressive Stress, Pa	Yield Stress, Pa
CEM + NA	0.35	170	25	5362	3096
CEM + RA	0.39	156	28	5791	3343
CEM-free + NA	0.36	175	18	3744	2162
CEM-free + RA	0.37	162	22	4418	2551

* See Section 3.1, “Mix Proportions,” for an explanation of the varied W/B ratio.

. Despite having a slightly higher water content, the RA-containing mixtures showed lower flowability than the NA-based counterparts. This is reflected in the smaller cone-flow diameters and was also evident during 3D-printing. The 3D-printed RA-containing mixes appeared stiffer and less smooth, which was likely due to the higher angularity of RA particles and the adhered cement paste, which increased water absorption and reduced the amount of effective free water in the mixture [88]. The RA-containing mixtures showed improved buildability by ~10–20% compared to the NA-containing mixtures. This was also likely linked to the more angular shape and rougher surface of the RA, which increased internal friction within the fresh mix [89], reducing flowability and improving layer stability compared with mixtures containing the more rounded NA.

Regarding the binder type, the cement-free mixtures exhibited lower yield stress by ~25–30% and slightly higher cone-flow values, which was consistent with the findings reported in the existing research on fly ash in concrete. Studies show that additions of fly ash reduced the yield stress [90], and that the spherical shape and smooth surface of fly ash particles decreased internal friction and created a lubricating effect, resulting in improved flowability [91,92,93]. A similar mechanism was likely present in the OSA-containing mixtures, as SEM images showed a significant proportion of rounded, smooth-surfaced particles, which reduced resistance to flow. This also reflects in reduced buildability of cement-free mixtures, where they were able to sustain a smaller number of layers before collapse (18–22 layers) compared with the cement-based systems (25–28 layers). The reduced buildability aligns with the lower yield stress and compressive stress obtained in the direct buildability test, indicating that the cement-free binders provide less structural stability.

Despite differences in aggregate type and binder composition, all four mixtures were sufficiently cohesive and demonstrated adequate printability, which was comparable to the performance reported in previous studies that used the same 3D-printer and setup with different mixture formulations [13,94]. The surface texture was comparably smooth across all mixtures, with no visible tearing or segregation, and the printed filaments consistently maintained their shape and thickness during extrusion (Figure 8).

4.2. Density

As shown in Figure 9, NA replacement with RA resulted in a small reduction in density by about 2–4% in both cast and 3D-printed form. This outcome is consistent with the previous findings [44,45], as RA generally exhibited lower density due to their higher porosity, the presence of adhered old mortar, and microcracking introduced during the recycling process, all of which contributed to reducing the overall density of the concrete. Also, it is worth noting that the density of RA used in the mixes was around 4% less than that of NA (see Table 1).

Similarly, cement replacement resulted in a small reduction in density for both NA and RA mixtures, with the average density of cast and 3D-printed samples decreasing by approximately 3–4%. Although only a limited amount of research has investigated the complete substitution of cement with MK and OSA, a slight decrease in density was expected, as both materials have a lower specific gravity than cement and can contribute to a modest increase in matrix porosity [17,95].

In addition, the 3D-printing process itself did not have a substantial effect on the density of the mixes, with reductions generally being limited to about 2% or less. This coincided with the findings of previous research [96], which found that 3D printing did not have a major impact on the average density of concrete samples. However, the observed slight decrease in density was most likely due to interlayer porosity introduced by the printing process, as reported in another study [97].

4.3. Flexural Strength

As illustrated in Figure 10, the mixes containing cement showed a 35% drop in 28-day flexural strength when NA were substituted with RA. This coincided with previous studies [98,99], which consistently show that the use of RA in cement-based concrete leads to lower flexural and tensile performance—the reduction is primarily attributed to the poorer quality of RA compared with natural ones.

On the other hand, in the cement-free mixes, the influence of the aggregate type was considerably smaller and with different trends for cast (3% decrease) and 3D-printed samples (21% decrease). This was explained by the absence of cement hydration products, which normally intensify the contrast between the properties of NA and RA.

Meanwhile, the replacement of cement had a significantly stronger impact on flexural strength than the type of aggregate. Across both cast and printed samples, removing cement resulted in a substantial reduction in 28-day strength, which was more pronounced for mixes containing NA. In the cast samples, the reduction reached about 60% for NA and 41% for RA. In the 3D-printed samples, the decrease was roughly 37% for NA and 22% for RA. Although direct research on concrete with 100% cement replacement is limited, the considerable strength loss was attributed to the absence of the C–S–H-based microstructure that cement normally forms. The reaction products formed by MK and OSA tend to develop more gradually and have a less dense matrix, resulting in lower flexural strength compared with conventional cement binders [18]. In addition, a plausible explanation for the greater loss of flexural strength in the NA mixes was the presence of a stronger cement–aggregate interfacial transition zone (ITZ), which formed during cement hydration. In contrast, RA mixtures already contain a weaker ITZ due to adhered old mortar and microcracking and therefore exhibit a smaller relative reduction when cement is removed.

Furthermore, the flexural strength values of the 3D-printed samples were higher than those of the cast samples for all mixes, likely due to greater compression of the lower layers of the printed samples, resulting in denser material in the areas where tensile forces were applied during testing. This coincided with previous research [100], which found that 3D-printed samples tested parallel to the printing direction showed increased flexural strength compared to their cast counterparts, due to a longer concrete deposition time at the bottom of the tested sample.

As seen in Figure 11, the increase in flexural strength between 28 and 90 days was limited. The cement-based mixtures gained 7–9%, whereas the cementless ones gained just 3–4%. This suggested that most of the flexural strength had already developed within the first month, with limited remaining binder reactivity and moisture loss being likely to prevent further improvement.

4.4. Compressive Strength

As shown in Figure 12, the cement-based mixes showed a noticeable reduction in 28-day compressive strength when NA were replaced with RA. For both cast and 3D-printed samples, the decrease was within the range of 11–13%, a trend commonly observed in RA concrete by previous studies [98,101], and was attributed to the higher porosity and weaker microstructure of RA. Additionally, in the cementless mixtures, the effect of the aggregate type was less distinct, as also observed in the flexural strength results. The cast samples exhibited a reduction of approximately 11%, while the 3D-printed samples showed a small increase of about 3%. However, given that this difference fell within the standard deviation, it was considered negligible. Butler et al. [102] reported that, in higher-strength concretes (C45–C60), failure typically occurs through the aggregates because the ITZ is stronger than the aggregate itself, whereas in lower-strength concretes (around C30), failure shifts to the ITZ. This explains why, in the lower-strength cementless mixes of this study, strength was governed primarily by ITZ quality, rather than the aggregate type.

Next, cement removal had a far more profound impact than aggregate substitution. Replacing cement with MK and OSA led to strength loss of 54% in the cast mixtures and 42–47% in the printed ones, depending on the aggregate type, whereby the mixes containing NA experienced a slightly greater proportional strength decline than those made with RA. Similarly, as with NA replacement, which has a more pronounced effect in cement-containing mixes, this difference was explained by the fact that NA typically establishes a stronger ITZ in the presence of cement. Meanwhile, in the absence of cement, this benefit is voided, making the relative loss more pronounced.

Also, 3D-printed samples of cement-containing mixes showed lower compressive strength than their cast counterparts, with differences of up to 17% at 28 days. This is consistent with the density results and previous studies [97,103], suggesting that the printing process introduced additional porosity and microstructural anisotropy through the layer-by-layer deposition. Conversely, cementless 3D-printed samples showed performance under compressive loading that was very similar to, or even higher than, that of the cast samples. This was attributed to their overall lower compressive strength, which was achieved primarily by the binder and not by a well-developed cementitious ITZ; therefore, the impact of interlayer defects became less significant.

In addition, long-term strength development was modest across all mixes. As illustrated by Figure 13, between 28 and 90 days, the cement-based mixtures showed gains of 6–7%, while cement-free mixtures increased by only 4–11%. Although pozzolanic systems typically show extended strength development, as with the flexural strength results, the limited gains observed here suggested that most reactions occurred at early stages, and that the limited remaining reactivity, moisture loss, and early matrix stiffening prevented further strength increase. Similar observations were made in a study [104] of low-clinker calcined-clay binders, where early-age reactions and densification dominate strength gain, but further development at later ages remained limited..

4.5. Life Cycle Impact Assessment

The LCIA results obtained using the ReCiPe 2016 Midpoint (H) method, considering a cradle-to-gate approach, are presented in

Table 8. Impacts of 1 m³ of 3D-printed concrete mix.

Impact Category	Unit	Mix
		CEM + NA	CEM + RA	CEM-Free + NA	CEM-Free + RA
Global warming	kg CO2 eq	410.1	378.7	237.7	212.9
Stratospheric ozone depletion	kg CFC11 eq	6 × 10−5	6 × 10−5	1 × 10−4	8 × 10−5
Ionising radiation	kBq Co-60 eq	0.9	0.7	1.3	1.1
Ozone formation, human health	kg NOx eq	11.5	10.8	35.4	32.9
Fine particulate matter formation	kg PM2.5 eq	1.4	1.3	4.1	3.8
Ozone formation, terrestrial ecosystems	kg NOx eq	11.6	10.9	35.4	32.9
Terrestrial acidification	kg SO2 eq	4.3	4.1	13.1	12.1
Freshwater eutrophication	kg P eq	0.01	0.01	0.02	0.01
Marine eutrophication	kg N eq	0.02	0.02	0.001	0.003
Terrestrial ecotoxicity	kg 1,4-DCB	1043	900	1580	1388
Freshwater ecotoxicity	kg 1,4-DCB	0.2	0.2	0.3	0.2
Marine ecotoxicity	kg 1,4-DCB	0.8	0.7	1.2	1.1
Human carcinogenic toxicity	kg 1,4-DCB	1.3	1.0	1.8	1.5
Human non-carcinogenic toxicity	kg 1,4-DCB	28.7	24.6	41.1	35.8
Land use	m2a crop eq	6.6	5.7	8.5	7.4
Mineral resource scarcity	kg Cu eq	0.3	0.2	0.4	0.3
Fossil resource scarcity	kg oil eq	39.8	34.7	57.7	50.9
Water consumption	m3	4.2	3.8	9.9	9.0

for the 3D-printed concrete mixtures under investigation. Impacts are expressed per functional unit of one cubic metre of fresh printable concrete under a cradle-to-gate system boundary. Because the mixing and printing stages are identical across all scenarios and excluded on the grounds of equivalence, differences among mixes arise entirely from raw material production and transportation processes. Overall, the results show a consistent pattern of improvement in climate-related indicators with increasing replacement of cement CEM II by OSA and MK, accompanied by rises in categories dominated by the upstream characteristics of the OSA supply chain, particularly those linked to combustion emissions and cooling water demand in oil-shale electricity generation.

4.5.1. Global Warming and Resource-Related Categories

As shown in Table 8, GWP decreases steadily from 410 kg CO₂ eq for Mix CEM + NA to 213 kg CO₂ eq for Mix CEM-Free + RA, corresponding to a 48% reduction. This confirms the central role of clinker substitution in reducing embodied carbon, as cement is the primary contributor to this category in cement-based mixes. The trend reflects the much lower process of greenhouse gas (GHG) emissions associated with OSA and MK compared with clinker manufacture. This trend was also observed in other studies investigating the impact of using innovative cementless concrete or replacing cement with other sustainable SCMs. A study by Jin et al. [105] revealed that the use of limestone calcined clay concrete (LC³) leads to a 36% to 46% reduction in the GWP for concrete 3D-printing. Another study confirmed that the use of LC³ did indeed result in the reduction in GWP by 22% [86]. A study by Ye et al. [106] showed that incorporating incineration bottom ash, limestone powder, and crumb rubber into 3D-printed concrete mixes reduced carbon emissions by 25%, 47%, and 24%, respectively, with lime powder in 3DCP showing the greatest reduction in GWP. This highlights the role of substituting cement with SCMs in reducing the GWP.

While GWP declines, resource-depletion indicators show the opposite trend. Fossil resource scarcity, for example, increases from 39.8 to 57.7 kg oil eq (45%) upon changing the binder from cementitious to cementless in NA-based mixes, and from 39.8 to 50.9 kg oil eq (28%) with synergic cement and RA replacements. A similar trend was observed for mineral resource scarcity, which rose by 38% for binder replacement from cementitious to cementless and by 13% for combined replacement of binder and aggregates. These increases are due to the transportation of OSA to RTU, as shown in Figure 14. Similar trends were reported in the literature [105,106,107]. This indicates that, even as cement substitution substantially lowers the GWP, it ties the system to a resource-intensive process, shifting the impact profile rather than uniformly reducing it.

Aggregate substitution primarily affects mineral resource scarcity, land use, and, to a lesser extent, GWP of the overall impact of the mix. The substitution of NA in Mix CEM + NA with RA, in Mix CEM + RA, led to a 7.7% reduction in GWP. This reduction is slightly higher than the highest reported GWP reduction in the literature when NA was replaced by RA, which is 6.4% [108]. Substituting NA with RA resulted in an almost 89% reduction in the aggregate’s contribution to the overall GWP impact. For example, the GWP of using NA in Mix CEM + NA was 8.9 kg CO₂ eq, corresponding to 2.2% of the GWP of Mix CEM + NA (Figure 14), whereas the GWP of RA in Mix CEM + RA was 1.01 kg CO₂ eq, which accounts for 0.27% of the GWP of Mix CEM + RA. This also applies to the remaining mixes. Although the use of RA reduces GWP, the reduction is minimal, as noted by [109]. The use of RA can sometimes lead to increasing the environmental impact of a mix, simply because more cement, or other additives, are needed to counter the WA’s inferior properties [60]. Despite this shortcoming, using RA instead of NA has the added benefit of avoiding primary extraction and quarrying, thereby reducing the depletion of natural aggregates and associated land occupation.

When RA is locally sourced and lightly processed, these savings are genuine and consistent with circular-economy objectives. However, if RA requires intensive reprocessing, such as multiple crushing, washing, or long-distance transport, the added electricity and fuel use can offset or even reverse the benefits. Under the conditions represented by the Latvian case study, where RA are likely to be available near the point of production and processed at a small scale, substituting NA with RA is expected to yield modest improvements in mineral resource and land-use categories, as well as small additional GWP savings. The effect is secondary to the binder system, but complementary. In fact, when combined, cement replacement and the partial or complete substitution of NA with RA can lead to an over 50% reduction in environmental impacts, as evident by the results presented in this study and other studies [87,110,111]. Additionally, if credits from the avoided disposal of both OSA and RA are included in the assessment, then the impacts are expected to reduce [112].

4.5.2. Air Pollution-Related Impacts

The air-emission categories, including ozone formation (human health and terrestrial ecosystems), fine particulate matter formation, and terrestrial acidification, displayed up to a threefold increase with higher OSA content. Ozone formation for human health grows from roughly 11.5 kg NO_x eq in Mixes 1 to up to 35 kg NO_x eq in Mix CEM-Free + NA, and fine particulate matter formation rises from about 1.3 kg PM_2.5 eq to up to 4.1 kg PM_2.5 eq. Terrestrial acidification shows a threefold increase from low-OSA mixes to OSA-rich ones. These increases are directly attributable to upstream processes in oil shale electricity generation, particularly combustion and flue-gas desulphurisation, which are associated with elevated emissions of sulphur oxides (SO_x) and nitrogen oxides (NO_x) that are partially allocated to OSA as a byproduct [113,114]. In addition, trace metal content that is inherent to oil shale combustion residues contributes to higher human toxicity indicators. As a result, OSA production represents the dominant contributor to terrestrial acidification and toxicity-related impact categories across the mixes, as shown in Figure 14, with the effect becoming more pronounced at higher OSA contents. The effect was more pronounced in mixes utilising more OSA. Consequently, while the climate impact improves markedly, the substitution introduces new airborne burdens that reflect the nature of the energy system from which OSA is derived. A similar pattern is observed with ionising radiation, which increases from 0.9 in Mix CEM + NA to 1.3 kBq Co-60 eq in Mix CEM-Free + NA, again tracing back to power-generation background processes.

4.5.3. Eutrophication and Ecotoxicity

The eutrophication indicators show contrasting responses. Freshwater eutrophication varies modestly, between 0.01 and 0.02 kg P eq, with Mix CEM + RA achieving the lowest value and Mix CEM-Free + NA the highest. In contrast, marine eutrophication decreases sharply in the OSA-rich mixes, from 0.02 kg N eq in Mix CEM + NA to just 0.001 kg N eq in Mix CEM-Free + NA, representing >90% reduction. This divergence indicates that phosphorus-related discharges increase slightly due to processing water and mineral inputs, while nitrogen emissions, which are linked to fuel combustion in clinker manufacture, fall significantly as the clinker is replaced.

The ecotoxicity categories show mixed trends. Terrestrial ecotoxicity peaks at 1580 kg 1,4-DCB eq in Mix CEM-Free + NA, up from 900 kg 1,4-DCB eq in Mix CEM + NA. Freshwater ecotoxicity remains unchanged between mixes 1, 3, and 6, and slightly increased in Mix CEM-Free + NA. Marine ecotoxicity increased when the cement was replaced with OSA and dropped when NA was replaced by WA. The increases likely stem from trace-metal emissions and residual management in the oil-shale combustion process, underscoring that shifting from clinker to OSA reduces direct GHG emissions but not necessarily the broader chemical emissions burden.

4.5.4. Human Health, Land Use, and Water Consumption

Human toxicity indicators also vary with binder composition. Human carcinogenic toxicity is lowest in Mix CEM + RA (1.0 kg 1,4-DCB eq) and highest in Mix CEM-Free + NA (1.8 kg 1,4-DCB eq). Mix CEM-Free + NA has the highest MK and OSA content of all the mixes. In fact, the combined contribution of these two materials and their transportation accounted for almost 74% of the total impact, with OSA transportation dominating (36%). Non-carcinogenic toxicity follows the same pattern, ranging from a low value of 24.6 kg 1,4-DCB eq (Mix CEM + RA) to the highest value of 41.1 kg 1,4-DCB eq (Mix CEM-Free + NA). These changes indicate that moderate OSA/MK substitution alleviates some emissions associated with manufacturing and transportation, while extensive substitution exposes the system to additional combustion-related toxicants from oil shale power generation and the energy-intensive MK manufacturing [112].

Land use and water consumption increase consistently in the OSA-rich mixes. Land use expands from 5.7 to 6.6 m²a crop eq in Mixes 1 and 3 to 7.4–8.5 m²a crop eq in Mixes 4 and 6. Water consumption more than doubles from about 4.2 m³ in Mix CEM + NA to 9.9 m³ in Mix CEM-Free + NA. This is probably due to the water-intensive cooling and flue-gas-cleaning systems inherent in oil-shale power production. These results underscore that industrial byproducts can improve climate performance while simultaneously amplifying regional water and land pressures. This necessitates a comprehensive look at all impact categories, rather than only focusing on one [110].

4.5.5. Synthesis for Decision-Making

Taken together, the LCIA results reveal distinct trade-offs. Substituting cement with OSA and MK yields substantial reductions in climate-related impacts, nearly 48% lower GWP, but introduces higher burdens in categories dominated by the characteristics of oil-shale electricity production. The OSA supply chain transfers emissions of NO_x, SO₂, particulate matter, and water consumption to the byproduct, offsetting part of the environmental benefit. Even though OSA mixes might appear to contribute a lot percentage-wise to these impact categories, the increase is minimal compared to the reductions in GWP that OSA use offers.

Within the studied cradle-to-gate boundary, Mix CEM-Free + RA achieves the lowest GWP and marine eutrophication potential but exhibits the highest or near-highest values in most other categories. Conversely, Mix CEM + RA, which represents a moderate degree of cement replacement, provides the most balanced profile. Its GWP (379 kg CO₂ eq) is 8% lower than Mix CEM + NA, yet it avoids the steep rises in acidification, ozone formation, and water use found in the OSA-rich mixes. This suggests that, from a multi-criteria perspective, an intermediate substitution level may offer the best compromise between carbon mitigation and overall environmental stability.

The verified LCIA results presented in this study demonstrate two main levers for environmental optimisation. These are cement replacements with OSA/MK and NA substitution with WA. Increasing the OSA/MK fraction reduces the GWP sharply but raises several non-climate burdens. Increasing the RA content primarily reduces resource depletion and land use with minimal side effects and contributes to a marginal reduction in the GWP. The data imply that a balanced, mid-range binder substitution, Mix CEM + RA, for example, combined with full aggregate substitution (RA replacing NA) would deliver the most environmentally consistent mix within the studied boundary. Pushing substitution toward full cement replacement would further decarbonise production, but at the expense of higher impacts.

These trade-offs are not purely numerical but structural, reflecting the underlying coupling between industrial byproducts and the systems that generate them. In the Latvian context, where reducing embodied CO₂ is a policy priority [115], the climate gains from OSA/MK substitution justify continued optimisation and scaling. Nevertheless, expanding the assessment beyond cradle-to-gate to incorporate use-phase durability, potential CO₂ uptake, and end-of-life would be essential to confirm whether the overall life-cycle advantage persists under functional equivalence. For a cradle-to-gate analysis, the results clearly demonstrate that incorporating OSA and MK, especially when balanced with waste aggregates, offers a credible pathway toward more sustainable 3D-printed concrete.

It should be emphasised that the environmental results presented in this study are derived from a cradle-to-gate assessment and therefore reflect impacts that are only associated with material production and delivery. As such, the reported reductions in the environmental impacts and improvements in eco-intensity for OSA/MK- and RA-containing mixes should be interpreted within this boundary. Downstream life-cycle stages, including construction, service life, durability-related maintenance, potential carbonation during use, and end-of-life processing, were not included; their consideration could alter the relative performance of the mixes. In particular, differences in mechanical performance and long-term durability may influence the service life and replacement frequency, thereby offsetting or amplifying the upfront environmental benefits identified here. Consequently, the present results demonstrate the environmental potential of OSA- and RA-based 3D-printable concretes at the material production stage; however, a full cradle-to-grave assessment would be required to confirm whether these advantages persist throughout the entire life cycle.

4.5.6. Transport Sensitivity of Recycled Aggregates

To assess the influence of transport distance on WA’s relative environmental performance compared with NA, a transport-sensitivity analysis was conducted, using GWP as the indicator [116]. The study was designed to identify the threshold distance at which the use of RA ceases to be environmentally preferable to NA, due to the increased transport burden. Eight transport scenarios (S1–S8) were modelled, corresponding to distances from 0 to 350 km, in 50 km increments. Road transport assumptions made for the LCA, Section 3.3.3, were maintained for all scenarios.

The study compared four cases:

• Mix CEM-Free + NA (NA-based): composed entirely of OSA and MK as binder and NA transported over distances represented by scenarios S1–S3 (0–100 km) (assuming NA queries are located at a maximum distance of 100 km) (NA content 574.756 kg).
• Mix CEM-Free + RA (RA-based): identical binder composition but produced with RA, modelled under all eight scenarios (0–350 km) (RA content 535.098 kg).
• Mix CEM-Free + RA-NA (NA-based): same mix as Mix CEM-Free + RA but assuming NA instead of RA, with transport distances S1–S8 (NA content 535.098 kg).
• Mix CEM-Free + RA-NA-Fixed (NA-based): same as Mix CEM-Free + RA but with transport distances fixed to S1-S3 to parallel Mix CEM-Free + NA’s baseline (NA content 535.098 kg).

The results presented in

Table 9. Summary of GWP values (kg CO₂ eq/m³) for each scenario.

Scenario	GWP, kg CO2 eq
	S1	S2	S3	S4	S5	S6	S7	S8
Mix CEM-Free + NA	234	240	246	246	246	246	246	246
Mix CEM-Free + RA	211	217	222	228	234	239	245	251
Mix CEM-Free + RA-NA	222	227	233	239	245	251	257	263
Mix CEM-Free + RA-NA-Fixed	222	227	233	233	233	233	233	233

clearly indicate that the transport distance exerts a measurable but nonlinear influence on the GWP of both aggregate types. Comparing Mix CEM-Free + RA and Mix CEM-Free + NA, the mixes considered in this study, across the distance range, reveal a critical threshold.

As shown in Figure 15, the two curves intersect at S7, corresponding to a travel distance of 300 km. At distances below 300 km, RA retains a clear environmental advantage, producing lower GWP values despite its slightly more complex processing chain. Beyond this threshold, the cumulative transport emissions outweigh the avoided impacts from reduced quarrying and primary extraction, making NA the environmentally preferable option. As shown in Figure 15, the impacts of Mix CEM-Free + RA and Mix CEM-Free + RA-NA are parallel to each other, and they never intersect. This is probably due to the fixed difference in the impacts of NA and WA. When Mix CEM-Free + RA is compared to Mix CEM-Free + RA-NA-Fixed, the two intersect around 200 km (S5). Unlike Mix CEM-Free + NA, Mix CEM-Free + RA-NA was made with an NA quantity equal to the RA quantity in the original Mix CEM-Free + RA, which might justify the reason why the distance dropped.

This sensitivity test highlights the importance of regional availability and logistics planning when using secondary materials [110]. In the Latvian context, RA sources are concentrated near Riga, whereas construction sites and concrete production may be geographically dispersed. If RA must be hauled over long distances (>300 km), its embodied carbon surpasses that of locally sourced NA. Conversely, when RA is available within the same region or reused on-site, it yields a meaningful reduction in GWP, up to 10% lower than the equivalent NA mix under short-distance scenarios (S1–S3).

From a methodological perspective, this analysis demonstrates that the environmental merit of circular materials depends not only on their production footprint but also on their spatial system boundary. Incorporating such transport-distance scenarios provides a more realistic assessment of where waste-derived materials can deliver genuine environmental benefits. For future policy or design guidance, this finding suggests that establishing regional reuse hubs for RA within a roughly 200 to 300 km radius of concrete manufacturing sites would maximise both the environmental and logistical efficiency.

4.5.7. Ecological Intensity Index

The eco-intensity analysis reveals clear differences in how binder type and aggregate source affect the balance between environmental impact and mechanical performance in 3D-printed concrete mixes, as shown in

Table 10. GWP, compressive strength ($f_c$), and eco-intensity index ($C_i$) of the 3D-printed concrete mixes.

Mix	GWP,	${\mathit{f}}_{\mathit{c}}$,	${\mathit{C}}_{\mathit{i}}$,
	kg CO2 eq/m3	MPa	kg CO2 eq/m3·MPa
CEM + NA	410.1	55.9	7.3
CEM + RA	378.7	52.9	7.2
CEM-Free + NA	237.7	29.7	8.0
CEM-Free + RA	212.9	30.5	7.0

. The cement-based mix with natural aggregates (CEM + NA) showed the highest GWP (410.1 kg CO₂ eq/m³) and the highest strength (55.9 MPa), resulting in a moderate eco-intensity index ($C_i$ = 7.3). Replacing natural aggregates with recycled aggregates in the cement-based system reduced GWP but led to a moderate decrease in compressive strength, scoring an eco-intensity of 7.2.

In contrast, eliminating cement substantially reduced the GWP of the cement-free mixes (212.9–237.7 kg CO₂ eq/m³), but also significantly lowered the compressive strength (29.7–30.5 MPa). As a result, the cement-free mix with natural aggregates performed worst overall ($C_i$ = 8.0), demonstrating that lower GWP alone does not guarantee good eco-intensity when strength becomes insufficient. Notably, incorporating recycled aggregates into the cement-free system improved the eco-intensity, yielding the lowest $C_i$ value (7.0) among all of the mixes. This suggests a favourable interaction between recycled aggregates and the OSA/MK binder, in contrast to their detrimental effect in cementitious mixes.

Overall, the results show that the cement-free mix with recycled aggregates had the best performance in terms of eco-intensity, while the cement-free mix with natural aggregates performed the worst. These findings highlight that the eco-intensity of 3D-printing concrete requires a careful balance between reducing carbon-intensive materials and maintaining adequate mechanical performance, and that the impact of recycled aggregates is strongly dependent on binder chemistry, rather than universally beneficial or detrimental.

Comparing the eco-intensities of the four mixes developed in this study with values reported in the literature highlights the competitiveness of the proposed mixes and the uniqueness of their performance trends. The $C_i$ values of the cement-based mixes in this study, 7.3 for CEM + NA and 7.2 for CEM + RA, fall within the range that is commonly reported for low-clinker or partially recycled concretes, such as LC³ mixes ($C_i$ = 6.60–8.10) and RA–FA blended systems ($C_i$ = 6.86–6.87), as shown in

Table 11. Summary of GWP and $f_c$ reported in the literature for 3D-printing concrete mixtures (FU = 1 m³), and calculated $C_i$, with notes on binder type and RA content for comparison.

Study	Mix	GWP, kg CO2 eq	${\mathit{f}}_{\mathit{c}}$ (28d), MPa	${\mathit{C}}_{\mathit{i}}$	Comments	RA, %
[87]	P40B10	360.71	50.2	7.19	Recycled sand used	100
[117]	M17	145	21.1	6.87	RA + FA	50
[117]	M18	144	21	6.86	RA + FA	100
[118]	C3G7N1	268	105	2.55	GGBS	0
[86]	LC3	350	53	6.60	LC3	0
[58]	Kaolinite-based calcium sulfoaluminate cement concrete	Not reported	Not reported	6–7.8	Kaolinite-based calcium sulfoaluminate cement concrete	0
[105]	LC3	251	31	8.10	LC3	0
[119]	LC3	283	37	7.65	LC3	0
[110]	Optimised mix	2206.37	42.5	51.91	FA, GGBS, and RA	30

RA = recycled aggregates, FA = fly ash, GGBS = ground granulated blast furnace slag, and LC³ = limestone calcined clay cement.

. These results indicate that the proposed CEM II-based 3D-printing mixes, even when incorporating RA, perform similarly to established low-carbon concretes in terms of eco-intensity. In contrast, the cement-free mixes reflect a more nuanced performance. The CEM-Free + NA mix ($C_i$ = 8.0) has a higher eco-intensity than most of the literature benchmarks listed in Table 11, suggesting that a lower GWP cannot always compensate for a strength reduction associated with removing cement.

However, the cement-free mix incorporating RA (CEM-Free + RA) achieved a $C_i$ of 7.0, placing it among the best-performing mixes in the comparison and closely matching the RA-based mixes reported in the literature (e.g., P40B10 at $C_i$ = 7.19 and M17/M18 at ≈ 6.86 [117]). This demonstrates that the synergy between RA and the OSA/MK binder reduces eco-intensity to levels that are comparable to or superior to those of well-optimised waste-based or LC³ systems. Importantly, none of the published studies, except the very high-performance GGBS-rich mix C3G7N1 ($C_i$ = 2.55) [118], achieved a substantially better combination of strength and environmental performance. At the other extreme, the “optimised” FA—GGBS mix with RA from [110] shows how poor eco-intensity can become when GWP is excessively high ($C_i$ = 51.91). Overall, this comparison shows that the best-performing mix in the present study (CEM-Free + RA) ranks favourably with contemporary low-eco-intensity concretes and highlights RA as a key enabler of sustainability in cement-free 3D-printing materials.

4.5.8. Performance–Environmental Synthesis and Future Research Directions

The work presented in this study integrates material performance and LCA to address the development of eco-efficient 3D-printable concrete. The paper is intentionally structured in two linked stages: first, verifying the printability and mechanical adequacy of the proposed mixes, and second, quantifying their cradle-to-gate environmental impacts. The connection between these stages is explicitly established through performance impact trade-offs, using the eco-intensity index (GWP divided by compressive strength) to evaluate the carbon efficiency, relative to mechanical performance.

Four 3D-printable concrete mixes were developed by combining two binder systems (CEM II and cement-free OSA/MK) with NA or RA. All mixes demonstrated adequate printability and achieved compressive strengths that were suitable for non-structural applications,

Table 12. Environmental impact and fresh and hardened properties of the designed 3D-printed concrete mixes.

Mix	GWP,	Ci,	28-Day fc,	28-Day ff,	Buildability,
	kg CO2 eq/m3	kg CO2 eq/m3·MPa	MPa	MPa	Layers
CEM + NA	410.1	7.3	55.9	9.2	25
CEM + RA	378.7	7.2	52.9	5.9	28
CEM-free + NA	237.7	8.0	29.7	5.8	18
CEM-free + RA	212.9	7.0	30.5	4.6	22

. Cement-free OSA/MK binders achieved substantial reductions in GWP of up to 48% relative to the reference mix, although this was accompanied by a lower compressive strength and increased impacts in several non-climate categories associated with the upstream oil shale ash and metakaolin supply chains. These findings confirm that reductions in embodied CO₂ must be evaluated alongside mechanical performance and broader environmental trade-offs.

The observed differences in fresh and hardened behaviour are explained by the underlying material mechanisms. RAs reduced flowability and, in cement-based mixes, compressive strength, due to higher water absorption and a weaker ITZ, while simultaneously improving buildability through increased internal friction. Cement-free OSA/MK binders formed a less dense binding matrix than clinker-based systems, which explains the reduced strength; however, they still delivered printable mixtures with compressive strengths of approximately 30 MPa, which were sufficient for targeted non-structural applications.

Rather than identifying a single optimal mix, the results support a criteria-based material selection. Where maximum strength is required, cement-based mixes with NA remain preferable. Where minimum GWP and best eco-intensity are prioritised, cement-free OSA/MK mixes with RAs offer the most favourable solution. For applications requiring a balanced compromise between strength, printability, and environmental performance across multiple impact categories, cement-based mixes incorporating RAs provide a strong option.

Building on these findings, several directions for future research are identified. First, the durability performance of cement-free OSA/MK and RA-based 3D-printed concretes should be systematically assessed, including freeze–thaw resistance, carbonation, moisture transport, leaching behaviour, and cyclic environmental exposure. Such studies are essential to confirm functional equivalence and service-life expectations for non-structural applications. Second, microstructural and rheological optimisation of OSA/MK binders represents a key opportunity to improve eco-intensity. Future work should focus on optimising particle packing, binder fineness, OSA/MK ratios, and activation strategies to enhance matrix densification while maintaining printability. Coupling rheological measurements with microstructural characterisation would support a more predictive mix design, tailored for extrusion-based 3D printing. Third, the interaction between RA and binder chemistry warrants deeper investigation. The present results indicate that RAs are less detrimental and, in some cases, beneficial in cement-free systems than in cement-based ones. Research into aggregate pre-treatment, moisture conditioning, and grading optimisation could further exploit this interaction while preserving stable printability and strength development.

From an environmental perspective, extending the assessment from cradle-to-gate to cradle-to-grave is essential. Incorporating factors such as durability-dependent service life, potential CO₂ uptake through carbonation, maintenance requirements, and end-of-life scenarios would allow verification that the identified carbon benefits persist throughout the full life cycle. Finally, element-scale validation, including printing and testing of representative components such as permanent formwork or facade elements, combined with whole-element LCA, is required to bridge the gap between laboratory-scale material development and real-world application.

5. Conclusions

This study developed and evaluated four 3D-printable concrete mixes incorporating oil shale ash (OSA), metakaolin (MK), recycled aggregates (RA), and conventional cement CEM II binders. All mixes satisfied the basic printability requirements, including continuous extrusion, shape retention, and multi-layer buildability. The use of RA reduced flowability but improved buildability, due to higher angularity and water absorption, while cement-free OSA/MK binders increased flowability and reduced yield stress, leading to slightly lower but still acceptable buildability. In the hardened state, replacing natural aggregates with RA in cement-based mixes resulted in an expected 11–13% reduction in compressive strength, whereas the aggregate type had a negligible effect in cement-free mixes. Cement removal had a much stronger impact, approximately halving compressive strength compared with CEM II mixes; however, cement-free 3D-printed samples still achieved ~30 MPa in compression and 5–6 MPa in flexure, indicating suitability for non-structural applications such as permanent formwork and infill elements.

The cradle-to-gate LCA showed that replacing cement with OSA/MK cuts GWP by up to ~48% (≈410 to ≈213 kg CO₂ eq/m³). However, several non-climate midpoint indicators (e.g., air emissions, toxicity, water use, resource scarcity) increased in OSA-rich mixes due to the upstream profiles of oil-shale power generation and metakaolin production. The recycled aggregate provided modest reductions in mineral resource use and land occupation and, for typical Latvian transport distances, a small additional decrease in GWP.

Using eco-intensity (GWP/compressive strength) showed that the lowest GWP is not automatically the best: the cement-free mix with the natural aggregate had higher eco-intensity, whereas the cement-free mix with the recycled aggregate achieved the lowest eco-intensity (≈7.0 kg CO₂ eq·m⁻³·MPa⁻¹) and aligned well with the literature benchmarks. Transport sensitivity indicated that recycled aggregate retains a GWP advantage up to ~300 km, after which transport can negate the benefit.

Overall, the results show that locally available oil shale ash and recycled aggregates can be combined to produce fully cement-free, 3D-printable concretes that achieve adequate mechanical performance for non-structural applications while substantially reducing embodied CO₂. At the same time, the presence of trade-offs in several non-climate impact categories and the sensitivity to transport indicate that mix selection should be application- and supply-chain-specific, rather than based on a single universal “best” option. Future work should therefore prioritise durability validation under relevant exposures (e.g., freeze–thaw, carbonation, moisture cycling and leaching), targeted rheology, and microstructure optimisation of the binder system to improve strength–impact efficiency, and extension of the assessment from material level to printed elements and full life-cycle scenarios to confirm that the observed benefits remain robust in practice.