Search Results (675)

Cement production accounts for approximately 8% of global CO₂ emissions, and while calcined clays have attracted growing attention as supplementary cementitious materials, the literature remains fragmented across clay types and performance metrics, with no unified comparative framework examining how mineralogical composition and calcination conditions jointly govern pozzolanic reactivity and downstream performance outcomes. This study addresses that gap through a PRISMA-guided systematic review of 32 peer-reviewed studies, validated by structured expert interviews, and a comparative assessment of five calcined clay categories: metakaolin (MK), limestone-calcined clay blends (LC³), illite-rich clays, montmorillonite (MM)- based clays, and ceramic waste (CW)- derived clays. Findings establish clear performance hierarchies with direct implications for the construction sector. MK at 10–15% cement replacement delivers compressive strength gains of 8–36%, chloride permeability reductions of 61–87%, and sulphate expansion reductions of up to 89%, confirming its suitability for high-performance, chemically aggressive-environment structural concrete. LC³ systems enable 30–50% clinker substitution, yielding an estimated 30–40% embodied CO₂ reduction alongside 6–10% strength gains and 64–90% reductions in chloride migration, representing the most significant decarbonisation opportunity reviewed. Illite-rich clays reduce compressive strength by 6–25%, limiting application to non-structural uses despite moderate durability gains. MM-based clays exhibit highly variable performance, ranging from a 60% strength loss to an 8% gain, with workability penalties of up to a 90% slump reduction, constraining adoption. CW-derived clays achieve 50–69% reductions in chloride diffusion while valorising industrial waste, though strength reductions of 11–20% limit structural applications. Across all clay types, superplasticiser demand increases by 1.5–3.6 times, posing a universal cost and logistics challenge for practitioners in mix design. Full article

This study investigates the development of sustainable dry-mix shotcrete incorporating fly ash and silica fume as partial cement replacements in order to reduce the environmental impact of cement production. A total of 24 mixtures were systematically evaluated, with 10–30% supplementary cementitious material and 0.9–1.8 kg/m³ polypropylene fiber dosages. This research establishes a quantitative framework for optimizing mechanical performance, durability, and Global Warming Potential. Experimental results reveal that silica fume replacement increases 28-day compressive strength by up to 31.13%, while an optimal polypropylene fiber dosage of 0.9 kg/m³ provides a 15.87% strength enhancement through a matrix-bridging effect. Conversely, excessive fiber content (1.8 kg/m³) increases porosity, leading to a 14.94% reduction in strength. Durability analysis demonstrates that silica fume and fly ash significantly refine the microstructure, reducing sorptivity and limiting freeze–thaw strength loss to a range of 18.13% to 41.03%. Crucially, the 30% by volume of the cement replaced with silica fume mixture was identified as the optimum design, achieving the lowest Global Warming Potential per unit strength at 8.82 kg CO₂-eq/m³/MPa, compared to 18.75 for the high-fiber mixture. These findings provide new, specific evidence that these supplementary cementitious material blends can successfully produce dry-mix shotcrete with significantly lower carbon emissions. Full article

The progressive phase-out of coal-fired power plants in Portugal has significantly reduced the availability of fly ash (FA) as a supplementary cementitious material (SCM), reinforcing the need for sustainable alternatives. Waste glass powder (WGP), characterized by its high amorphous silica content, has emerged as a promising candidate; however, most studies focus on ultrafine particles or isolated performance indicators, lacking an integrated technical, environmental, and economic assessment. This study evaluates cement pastes incorporating 25% WGP (by volume) with different particle size distributions, including fineness levels comparable to cement and FA. Mechanical performance, grinding energy demand, carbon footprint, and cost were systematically analyzed. The results indicate that WGP is technically viable as an SCM, with a median particle size (D₅₀) of approximately 48 µm providing the most balanced performance. Although finer particles enhance pozzolanic reactivity, the associated increase in grinding energy and economic cost offsets these gains. The findings demonstrate that optimizing particle size, rather than maximizing fineness, enables a technically robust and industrially realistic use of WGP. This approach supports circular economic strategies and contributes to the decarbonization of the construction sector by identifying an efficient replacement pathway for FA under resource-scarcity conditions. Full article

This study investigates the effect of metakaolin (MK) as a partial replacement of cement (CEM I) in high-workability mortars, with emphasis on fresh-state behaviour, mechanical properties, microstructural development, and carbon footprint implications. Mortars were produced with MK replacement levels ranging from 0 to 50% by mass of binder, under a constant water-to-binder ratio and fixed superplasticiser amount. Fresh-state results showed that increasing MK content reduced flowability due to its high fineness; however, high workability was maintained for replacement levels up to 20%. At 28 days, MK replacement up to 10% retains approximately 90–95% of the control compressive and flexural strength, whereas higher replacement levels lead to gradual strength reductions (to ~55–60% at 50% MK), despite comparable early-age strength gains across all mixes. Durability-related indicators demonstrated reduced water absorption and capillary uptake at moderate MK contents (approximately 20–30%), indicating refined pore structure and reduced pore connectivity. Microstructural analyses using SEM, TGA, and XRD confirmed effective portlandite consumption and the formation of dense C–A–S–H-type hydration products at moderate MK replacement levels, whereas excessive MK contents resulted in unreacted MK. A comparative carbon footprint assessment showed that MK incorporation leads to proportional reductions in embodied CO₂ emissions, with replacement levels of 10–20% providing the most favourable balance between mechanical performance, durability, and environmental benefit. Therefore, the results demonstrate that MK can be used as a supplementary cementitious material for producing low-carbon, high-workability mortars. Full article

The growing demand for sustainable construction has highlighted the need to effectively utilize solid waste materials in concrete production, yet achieving satisfactory workability, strength, and durability simultaneously remains challenging. A multi-parameter mix-design methodology is proposed for solid-waste-based self-compacting concrete (SCC). This method couples minimum water demand, control of paste film thickness, and multi-performance balancing. The ternary solid-waste powder system (silica fume, fly ash, and supersulfated solid-waste-based cement) was first optimized through minimizing water demand to achieve maximum packing density. The resulting composition was then blended with varying dosages of ordinary Portland cement (OPC) to form the final cementitious binder. Aggregate gradation was proportioned to minimize voids, and paste volume was determined using an equivalent-paste-film-thickness model. Under comparable mixture conditions, SCC with OPC contents of 70–40 wt.% and paste film thicknesses of 2.0–2.6 mm was evaluated for fresh performance, compressive strength, freeze–thaw resistance, and material cost. Mixtures with a paste film thickness of 2.4 or 2.6 mm satisfied the self-compactability criterion—the mix with 50 wt.% OPC and a paste film thickness of 2.4 mm showed the best overall performance balance, achieving higher 28 d strength than higher-OPC mixtures while improving freeze–thaw resistance and reducing cost. Results from TGA, XRD, ATR–FTIR, and SEM–EDS analyses indicated enhanced calcium hydroxide (CH) consumption, increased formation of C-(A)-S-H and ettringite, and a denser interfacial transition zone (ITZ), supporting the proposed multi-objective design approach. While the framework was validated for a specific ternary binder system, it provides a reproducible proportioning strategy applicable to a broader range of solid-waste-based concrete systems, with potential for extension to other waste streams and exposure conditions, thus supporting the development of more resource-efficient and environmentally sustainable concrete. Full article

Pozzolanic concrete demonstrates intricate, highly nonlinear material interactions that pose significant challenges for the accurate prediction of compressive strength (CS). This study introduces a novel, interpretable ensemble machine learning (ML) framework for predicting CS based on 759 mixture records encompassing cement, aggregates, supplementary cementitious materials (pozzolans), water/binder (W/B), superplasticizer, water, and curing age. Descriptive analysis and ANOVA were used to identify key predictors, followed by an 80/20 train–test split with 10-fold cross-validation to ensure robust and generalizable modeling. To further enhance model reliability, 5% of outliers were removed using an isolation forest algorithm, after which data were normalized and ensemble hyperparameters optimized. Among the evaluated models, the extra trees algorithm with standard scaling demonstrated the most stable generalization, achieving a coefficient of determination (R²) of 0.978 and a root mean square error (RMSE) of 4.197 MPa on the test set, and R² = 0.966 (RMSE = 5.053 MPa) under 10-fold cross-validation. Feature importance, SHAP, and partial dependence analyses consistently demonstrated that W/B, curing age, and cement are the principal determinants of CS. Finally, multi-objective optimization generated high-strength, low-impact mixtures, confirming the framework’s effectiveness as a transparent decision-support tool for performance- and sustainability-oriented pozzolanic concrete design. This study is novel in combining interpretable ensemble ML with multi-objective optimization to simultaneously achieve precise CS prediction and the formulation of sustainable, performance-optimized pozzolanic concrete mixtures. Full article

This preliminary study investigates the viability of substituting high-performance Aalborg white Portland cement (CEM I 52.5 R) with five diverse industrial byproducts: wood ash, silica waste, clay brick, glass fibre, and calcined sewage sludge ash. Sewage sludge ash was produced in a laboratory from two different sludges from wastewater treatment plants in the Latvian cities of Jelgava and Liepaja. The research evaluates the influence of substitution levels ranging from 5% to 20% on the rheology of fresh material and its early-age mechanical performance (day 7). Results indicate that particle morphology largely dictates workability; porous and angular materials, such as wood ash, clay brick, and sewage sludge ash, reduce flowability, whereas non-absorbent milled glass fibres unexpectedly improve spread diameter. Regarding mechanical performance, glass fibre and clay brick waste demonstrated the highest potential, exceeding the 48–62 MPa reference compressive strengths by achieving up to 69 MPa at a 10% substitution level. Conversely, wood ash and silica waste exhibited significant strength degradation at higher substitution levels, due to agglomeration and high water demand. This approach not only identifies viable waste streams for cement substitution but also diverts significant industrial waste from landfills, thereby reducing CO₂e emissions and advancing more sustainable construction practices. Full article

Fly ash is an effective supplementary cementitious material for reducing clinker consumption and carbon emissions, but its low early reactivity often results in delayed hydration and insufficient early-age strength. This study investigated the age-dependent role of graphene oxide (GO) in fly ash-blended cementitious materials by combining compressive strength testing with X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), ²⁹Si magic-angle spinning nuclear magnetic resonance (²⁹Si MAS NMR), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). Fly ash replacement levels of 10%, 20%, and 30% were considered, and 0.07% GO was introduced to evaluate its effect at 3, 7, and 28 days. The results showed that fly ash reduced the 3-day compressive strength, whereas the strength differences became much smaller at 28 days. GO enhanced the compressive strength of all fly ash-blended mixtures. XRD and TG-DTG results showed that GO refined Ca(OH)₂ crystallization and reduced the retained CH content, indicating more effective CH utilization during hydration and pozzolanic reaction. At 28 days, the incorporation of 0.07% GO increased the compressive strength of the 30% fly ash mixture from 47.38 MPa to 56.58 MPa, while reducing the total CH content from 14.20% to 12.89%, indicating enhanced CH utilization and gel development. ²⁹Si MAS NMR further demonstrated that GO promoted a more mature and polymerized silicate gel structure, as evidenced by lower Q⁰ fractions, higher mean chain length, and higher proportions of more polymerized silicate species. SEM-EDS observations confirmed that GO led to a denser matrix, less dominant coarse CH, and lower Ca/Si and Ca/(Si + Al) ratios. Overall, GO improved the mechanical performance of fly ash-blended cementitious materials through coupled regulation of hydration products, silicate gel polymerization, and matrix densification. Full article

The development of eco-efficient construction materials requires optimisation strategies that reduce cement consumption, valorise industrial by-products, and enhance performance without increasing material demand. Clay–cement sealing suspensions used in geotechnical engineering offer significant sustainability potential due to their high mineral content and compatibility with supplementary cementitious materials such as siliceous fly ash. The early-age rheological properties are essential for the design of geotechnical sealing barriers, yet the influence of chemical additive sequencing on flow behaviour remains poorly understood. This study examines how the priority of sodium silicate addition—introduced either before cement and siliceous fly ash (the “Prior” series) or after them (the “After” series)—affects the flow curves, yield stress, thixotropy, and equilibrium shear stress of clay–cement–fly ash sealing suspensions. Ascending flow curves were fitted to the Casson, Herschel–Bulkley, and Ostwald–de Waele models, and a shear-rate-resolved thixotropic power density analysis was applied to decompose the hysteresis behaviour. The results demonstrate that the Prior series produces deflocculated colloidal clay networks with localised cementitious agglomerates, exhibiting lower shear stresses at low shear rates but markedly higher yield stress amplitudes and larger hysteresis loop areas. The After series yields more uniformly distributed nucleation–coagulation networks with smaller hysteresis loops and pronounced structural rebuilding at low shear rates during the ramp-down phase. These findings provide a physicochemical framework for tailoring the early-age rheology of clay–cement suspensions through controlled additive sequencing, with direct implications for pumpability, injectability, and post-placement structural recovery in geotechnical applications. Full article

The urgent need to decarbonize the global cement industry is compounded by the declining availability of conventional supplementary cementitious materials (SCMs). Limestone-calcined clay cement (LC3) emerges as a highly sustainable alternative, enabling up to 50 percent clinker replacement and an approximate 40 percent reduction in carbon dioxide emissions. Unlike existing reviews that focus on basic material properties, this paper critically bridges the gap between fundamental hydration thermodynamics and next-generation sustainable engineering applications. Through a structured bibliographic analysis of 135 contemporary sources published between 2000 and 2026, it traces the evolution of LC3 from a laboratory innovation to a highly promising solution for large-scale industrial implementation and circular economy integration. The discussion highlights the synergistic alumina carbonate reaction. This reaction forms carboaluminate phases. These phases significantly densify the microstructure and enhance long term durability. Key engineering properties are examined, contrasting rheological challenges from high water demand and carbonation susceptibility against its exceptional chloride resistance in aggressive environments. The transition to field application is thoroughly assessed, emphasizing technological advances in flash calcination, environmental footprint reduction through life cycle assessment (LCA), and production scalability. Finally, rather than restating known challenges, this review exposes the limitations of current empirical mitigation strategies. It proposes a targeted research agenda focused on molecular-level green admixture design and field calibrated durability models to support the integration of LC3 into emerging sustainable technologies such as 3D concrete printing. Full article

Concrete production is one of the main sources of CO₂ emissions. The primary reason for this is the high clinker content of Portland cement. To mitigate this problem, supplementary cementitious materials (SCMs) such as fly ash, silica fume, Ground granulated blast furnace slag (GGBFS), rice husk ash, and natural pozzolans are increasingly being used. These materials are used as partial replacements for cement. SCMs not only reduce the environmental impact of concrete but can also improve its long-term mechanical and durability properties. The aim of this study is to develop a machine learning framework that can accurately predict the compressive strength of concrete containing SCMs. The framework includes the training and evaluation of several machine learning models. Nested cross-validation and Bayesian hyperparameter optimization were used to explore the full capacity of the models and ensure reliable evaluation. Permutation significance testing and learning curve analysis were applied to verify that the models learn meaningful patterns rather than memorize the data. Also, feature importance and SHapley Additive exPlanations analyses were performed and the key variables that influence the prediction of the compressive strength of SCM concrete were identified. The optimized XGBoost model achieved the best generalization performance with a holdout R² of 0.8398. It confirms the effectiveness of the proposed statistically rigorous machine learning framework for reliable compressive strength prediction of SCM-blended concrete. Full article

This study provides new experimental evidence indicating that powdered mineral wool waste traditionally classified as a non-reactive, non-recyclable insulation residue can function as a genuinely pozzolanic supplementary cementitious material when incorporated into Portland cement systems. Unlike previous work that has treated mineral wool exclusively as an inert filler, this research demonstrates that its amorphous silicate–aluminate phase becomes chemically active under high-alkalinity conditions. A combined experimental programme, including mechanical testing, assessment and SEM/EDS microstructural analysis, was used to evaluate replacement levels of 20%, 25%, and 40% in CEM I mortars, with CSA cement employed as a contrasting binder system. The results indicate a potential contribution of powdered mineral wool to strength development; however, this effect cannot be unequivocally attributed to pozzolanic activity alone. It may also be partially related to physical effects such as filler action and particle packing. SEM/EDS observations confirm the formation of secondary C–S–H and C–A–S–H gels, can function as a genuinely pozzolanic supplementary cementitious material. Therefore, the applied assessment approach should be treated as indicative, and further verification using complementary methods is required. This study provides new experimental evidence indicating mineral wool can potentially contribute to cementitious performance as a Supplementary Cementitious Material (SCM). However, these observations should be treated as qualitative and indicative rather than definitive proof of pozzolanic reaction. The study provides an environmentally relevant valorisation pathway for a problematic waste stream, showing that mineral wool residues containing only trace levels of immobilizable formaldehyde can be safely and effectively integrated into low-carbon binder technologies. These findings position powdered mineral wool as a previously overlooked, yet technically viable SCM, offering new opportunities for clinker reduction, waste circularity and sustainable cementitious material design. Full article

The management of hazardous municipal solid waste incineration (MSWI) residues represents a critical challenge for sustainable development due to their increasing generation and environmental risk. At the same time, the cement industry faces urgent pressure to reduce CO₂ emissions associated with clinker production, creating a demand for alternative supplementary cementitious materials. The aim of this study was to evaluate the feasibility of valorising hazardous municipal solid waste incineration (MSWI) air pollution control fly ash (EWC 19 01 07*) as a constituent of Portland composite cement, in line with circular economy principles and the need to reduce CO₂ emissions associated with clinker production. The investigated fly ash, originating from flue gas cleaning processes, is characterised by high alkalinity and elevated concentrations of heavy metals, which currently necessitate controlled landfilling. To enable its safe reuse, the ash was subjected to high-temperature thermal treatment following granulation and subsequently incorporated into cement formulations under semi-industrial conditions. Two Portland composite cements were produced with different ash contents, corresponding to CEM II/A-07 and CEM II/B-07, while a Portland cement manufactured from the same clinker was used as a reference material. The chemical and phase composition of the ash before and after thermal treatment was analysed using XRF and XRD, supported by SEM/EDS observations. The results demonstrate that thermal treatment at 1150 °C induces partial phase stabilisation of APC fly ash without full vitrification, allowing its integration into cement systems under semi-industrial conditions. The incorporation of ash significantly alters hydration behaviour through increased water demand governed by particle porosity, CaO-rich phase composition, and early ionic interactions in the pore solution, leading to reduced workability and mechanical performance. While immobilisation efficiencies exceeding 99.5% were achieved for most heavy metals due to precipitation and incorporation into hydration products, barium exhibited persistent leaching controlled by its solubility under highly alkaline conditions and limited incorporation into C–S–H phases. These findings define both the technological feasibility and the key environmental constraints of APC fly ash utilisation in Portland composite cement. From a sustainability perspective, the proposed approach contributes to the reduction in hazardous waste landfilling and supports clinker substitution in cement production. The results demonstrate the potential of integrating waste management and low-carbon material design within a circular economy framework while highlighting current environmental limitations related to barium leaching. Full article

Construction materials are the primary source of embodied carbon in long-span bridge projects, particularly for steel-intensive structures. This study presents an empirical construction-stage carbon footprint assessment of the Anhsin Bridge, an asymmetric cable-stayed steel truss bridge in Taiwan. Using the emission factor method in accordance with ISO 14067 and Taiwan Environmental Protection Administration guidelines, a cradle-to-gate (A1–A5 equivalent) system boundary was applied, covering material production, transportation, and on-site construction activities. Total construction-stage emissions were estimated at 55,349 tCO₂e, dominated by structural steel (51.8%), followed by reinforcing steel, concrete, and cement. Material-related emissions accounted for over 90% of the total, highlighting the critical role of material selection in embodied carbon reduction. Three practical mitigation strategies were evaluated using verified project data, as follows: 40% cement substitution with supplementary cementitious materials, optimized steel erection methods, and enhanced reuse of formwork and temporary works. The combined scenario achieved a 7.3% reduction in construction-stage emissions without compromising constructability. The findings demonstrate the effectiveness of material-oriented, constructability-aware strategies for reducing embodied carbon in steel-intensive bridge construction. Full article

The escalating production of sewage sludge presents a significant environmental challenge, while the construction industry simultaneously seeks sustainable raw materials to improve its circularity. This review analyses the technical and regulatory landscape for valorizing SS within the Latvian construction sector, set against the divergent strategies of its Baltic neighbours. While global research confirms the technical viability of using SS in fired-clay bricks and as a supplementary cementitious material (SCM), national management approaches differ starkly. Lithuania has adopted widespread incineration, and Estonia has focused on advanced composting. In contrast, Latvia’s national strategy is failing, with 51% of its 2024 sludge production diverted to “temporary storage”. This review identifies this crisis as a unique opportunity, arguing that incorporating dewatered digestate into fired-clay bricks is the most logical and economically viable pathway for Latvia, as it leverages existing industrial infrastructure. The primary obstacle to this circular solution is not technical but legal, specifically the lack of a national “End-of-Waste” (EoW) criterion for sludge-derived construction materials. Therefore, this article proposes a strategic roadmap for Latvia, centred on developing this essential legal framework, creating a national sludge characterization map, and initiating a pilot project to bridge the research-to-industry gap. Although Latvia is the primary focus of this review, the regulatory, infrastructural and material constraints analysed here are common in many small and mid-sized countries, making the insights applicable beyond the Latvian context. Full article