Search Results (481)

The technological performance of bentonite in foundry applications is strongly influenced by the nature of its exchangeable interlayer cations, with sodium bentonites generally exhibiting superior swelling, plasticity, and bonding properties compared with calcium bentonites. Given the limited availability of natural sodium bentonite, upgrading abundant calcium-rich bentonite resources holds significant industrial interest. In this study, a natural Ca-rich bentonite from the Bijelo Polje deposit (Bar, Montenegro) was upgraded by alkali activation using Na₂CO₃ and evaluated as a binder for green sand foundry molds. The raw bentonite was characterized by physicochemical, mineralogical, and structural analyses, confirming its Ca-type character and suitability for sodium activation. Activation was performed using 2–6 wt.% Na₂CO₃, with the optimum treatment achieved at 5 wt.% Na₂CO₃. The activated bentonite was subsequently characterized using structural, textural, thermal, and physicochemical methods. Alkali activation significantly improved the key technological properties of the material, increasing the free swelling capacity from 7 to 20 cm³, the specific surface area from 27.4 to 45.8 m² g⁻¹, the cation exchange capacity from 74.6 to 89.5 meq/100 g, and the plasticity index from 79.6% to 193.4%. XRD, ATR–FTIR, and thermal analyses confirmed successful sodium activation while preserving the fundamental montmorillonite structure. Evaluation of foundry-relevant properties, including refractoriness, methylene blue adsorption, gas permeability, thermal stability, and bonding strength, demonstrated that the activated bentonite satisfies the technological requirements for green sand molding of both ferrous and non-ferrous alloys. These findings demonstrate that Na₂CO₃ activation is an effective and resource-efficient approach for converting natural Ca-rich bentonite into a high-performance foundry binder. Full article

Zinc processing generates large volumes of tailings enriched with potentially toxic elements such as zinc, lead, arsenic, and antimony, creating environmental challenges. Conventional disposal in tailings dams is associated with land occupation, contamination risks, and geotechnical concerns, reinforcing the need for more sustainable management strategies. This study presents a bibliometric and semi-systematic review of alkali-activated binders for the stabilization and solidification of zinc mine tailings, based on nine studies published between 2019 and 2026. The results indicate that this is a recent and expanding research field, with a marked concentration of studies in China. Current research mainly focuses on the links between microstructure, heavy metal immobilization, and mechanical performance. Alkali-activated systems, commonly based on blast furnace slag, fly ash, and coal gangue, can produce dense matrices with compressive strengths of up to 100.77 MPa and high immobilization efficiency. Their performance is largely governed by the type of reaction products formed, particularly calcium silicate hydrate, calcium aluminosilicate hydrate, and sodium aluminosilicate hydrate gels, which control microstructural development and stabilization mechanisms such as encapsulation, structural incorporation, and secondary phase formation. Overall, the reviewed studies suggest that alkali-activated binders have potential as alternative binders to Portland cement for the management and valorization of zinc mine tailings. Full article

This study investigates the use of mussel shells (MSs), a biogenic by-product of the food industry, as a partial replacement for ground granulated blast furnace slag (GBFS) in alkali-activated mortars. Given their high CaCO₃ content, MSs represent a sustainable secondary raw material that reduces both waste disposal burden and reliance on natural resources, while offering a low-carbon alternative to conventional cement-based binders. Alkali-activated mussel shell/slag mortars (AAMSs) were produced with MS replacement ratios of 0%, 5%, 10%, 15%, and 20% by mass of GBFS. Sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) were used as alkaline activators. Fresh specimens were cured at 60 °C for 48 h. The experimental program included workability, compressive and flexural strength, water absorption, porosity, density, capillarity, ultrasonic pulse velocity (UPV), and freeze–thaw (F-T) resistance tests. Increasing MS content slightly reduced flowability and mechanical strength, while increasing water absorption, porosity, and capillarity. The M0 series achieved the highest 28-day compressive strength (54.06 MPa), while M15 exhibited the highest flexural strength (5.23 MPa). Following F-T cycling, the 5% and 10% MS series demonstrated the best compressive strength (30 MPa). The 10% MS exhibits a relatively balanced overall performance, providing the best balance between mechanical performance, F-T resistance, and microstructural stability, as confirmed by scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS) analyses showing elevated Ca/Si ratios and the formation of Ca-rich crystalline phases. Full article

Three-dimensional concrete printing (3DCP) has emerged as a promising digital construction technology that reduces material waste, eliminates formwork, and enables complex geometries. However, its sustainability remains constrained by the extensive use of ordinary Portland cement (OPC) and natural aggregates. This review comprehensively evaluates waste utilization in extrusion-based 3D printed concrete, classifying applications into three pathways: cement replacement in OPC-based systems, waste-derived precursors in alkali-activated/geopolymer binders, and fine aggregate replacement. Industrial, agricultural, and marine wastes are assessed regarding their effects on rheology, printability, mechanical performance, interlayer bonding, and durability. The reviewed literature investigated waste incorporation levels reaching up to 50% for cement replacement, up to 70% for alkali-activated/geopolymer systems, and up to 100% for aggregate replacement, depending on the material type and application pathway. Industrial wastes, particularly fly ash, slag, silica fume, and metakaolin, represent the most mature materials and generally improve printability and long-term performance. Agricultural and marine wastes show promising sustainability potential but remain insufficiently investigated. Despite encouraging laboratory-scale results, challenges related to material variability, early-age performance, standardization, and scalability continue to limit practical implementation. The review identifies critical research gaps and outlines future directions for developing sustainable and field-ready 3DCP technologies. Full article

The successful realization of a circular economy in the cement industry, coupled with a substantial reduction in carbon emissions, relies on the development of sustainable alternative binder systems. This study investigated the physicomechanical performance and sulfate resistance of composites produced by alkali activation of natural perlite and blast furnace slag. The aim of the research was to improve mechanical properties under low- and medium-alkalinity conditions (5–10 M NaOH). The samples were cured at an ambient temperature of 20 °C and then treated with heat at 60 °C. These samples were then mechanically processed and subjected to five soak–dry cycles in 5% and 10% Na₂SO₄ solutions. The results showed that heat treatment resulted in the formation of a dense C-A-S-H gel, increasing compressive strength approximately eightfold, from 11.64 MPa to 92 MPa. However, perlite’s porous and brittle structure limits its flexural strength to 0.27 MPa; this value is insufficient for structural applications. Under severe sulfate attack (10% Na₂SO₄), samples cured at ambient temperature showed a 12% mass increase in the first cycle due to solution infiltration into capillary voids. As a consequence of extensive ettringite and gypsum formation, the specimens experienced severe deterioration, resulting in a complete loss of mechanical integrity and a residual compressive strength of 0 MPa. In contrast, heat-treated samples showed limited ion diffusion due to a denser matrix and an improved aggregate interface transition zone, resulting in a 2.6% mass increase and a residual compressive strength of 5.17 MPa. Consequently, the obtained findings indicate that thermally treated alkali-activated slag–perlite composites exhibit high resistance against sodium sulfate attack and may have potential for use in specific industrial environments with high sulfate concentrations. However, the performance of these materials under more complex aggressive conditions, such as mining environments involving magnesium sulfate exposure and acidic drainage waters, should be further validated through future studies. Full article

This research investigates the relationship between mineralogical composition and compressive strength in alkali-activated cement–sand mortars incorporating palm oil fuel ash (POFA) as a partial replacement of Portland cement. POFA was introduced at 5 wt.% and 10 wt.% of the binder, and activation was achieved using a NaOH–Na₂SiO₃ solution (3:1 mass ratio). Compressive strength and bulk density were evaluated at 7 and 28 days, while phase evolution was analyzed by X-ray diffraction (XRD) coupled with Rietveld refinement. The results demonstrate that POFA incorporation significantly modified the CaO–SiO₂–Al₂O₃ balance of the system, promoting the consumption of portlandite and the formation of Na- and K-rich aluminosilicate phases such as albite and muscovite. The control mixture exhibited the highest compressive strength values, whereas increasing POFA content reduced both strength and density due to calcium dilution, lower gel compactness, and increased porosity. Nevertheless, all mixtures exhibited progressive strength development over time, indicating continued hydration and geopolymerization reactions associated with the formation of hybrid C–(N,K)–A–S–H gels. These findings demonstrate that POFA can effectively participate in alkali-activated hybrid binders when applied at controlled replacement levels, highlighting its potential as a sustainable supplementary material for lower-carbon cementitious systems. Full article

The copper industry generates nearly 25 million tons of slag annually, which is stockpiled or landfilled, leading to land occupation and the potential for soil and water contamination alongside the environmental burden of the construction sector, which accounts for up to 9% of global CO₂ emissions and massive raw material consumption. The need for low-carbon, resource-efficient binders has spurred interest in geopolymerization, or the alkali activation of aluminosilicate residues, as a pathway to valorize industrial by-products. The objective of this review is to analyze, synthesize, and critically evaluate the scientific evidence on alkali-activated materials derived from Cu slag, emphasizing the synthesis parameters, mechanical and durability behavior, and environmental performance. The review applies the PRISMA 2020 methodology. The analysis of the 57 reports shows that copper slag—used alone or with metakaolin or blast furnace slag—can produce alkali-activated materials with high compressive strength, refined pore structures, and cradle-to-gate CO₂ reductions of up to 80%. Cu slag is not a chemically homogeneous precursor, and its influence on performance depends on the activation strategy and dosage rather than the slag content alone. Overall, this review consolidates dispersed findings, identifies research gaps, and proposes a framework for sustainable valorization in the form of low-carbon construction materials. Full article

Although geopolymer and alkali-activated binders are promoted as low-carbon OPC alternatives, their resource-centric performance remains complex and geographically dependent. This review examines these systems from a resource-efficiency perspective and evaluates alkaline activator demand; precursor availability, including fly ash, slag, calcined clays, and mining residues; and embodied energy across mix designs and curing regimes. Recent mechanical and durability analyses, together with life cycle assessments, reveal important trade-offs in alkali-activated geopolymer systems. Customized precursors may unintentionally compromise their inherent resource efficiency, while the declining availability of industrial waste increasingly competes with alternative waste valorization processes. Developing one-part activator systems and implementing data- or machine-optimized mix designs capable of handling extremely highly variable waste streams will be necessary to achieve meaningful reductions in mineral consumption, energy demand, and emissions. The study reframes these binders as enablers of urban mining and industrial symbiosis. Policy changes toward resource-oriented governance, including performance-based standards, carbon-responsive procurement, and more transparent end-of-waste legislation, are also needed to promote a circular material economy. Strategic, large-scale deployment requires the integration of regional resource mapping with predictive performance modeling to navigate resource constraints in the construction sector. Full article

Clinker-free binders derived from industrial solid wastes are promising for low-carbon construction, but many binder designs still rely on reagent-grade activators. This study investigates carbide slag (CS) as a substitute for a conventional alkali activator route in a waste-derived clinker-free binder composed of fly ash, coal gasification slag, and blast furnace slag. The CS-based binder is benchmarked against unactivated, mechanically processed, and Ca(OH)₂-activated reference binders. The CS-based route shows sustained strength development from 3 to 28 d and achieves 20.04 MPa compressive strength at 28 d, slightly higher than the Ca(OH)₂-activated reference (18.78 MPa). Mercury intrusion porosimetry reveals clear pore refinement: the fraction of pore throats ≤ 50 nm increased to 40.96% in the CS-based binder, compared with 1.50% in the unactivated milled-CGS reference, and the median pore throat decreased to 70.01 nm. Calorimetric kinetic fitting showed that the CS-based binder had a higher fitted cumulative heat release, 58.75 J·g⁻¹, than the Ca(OH)₂-activated reference, 23.36 J·g⁻¹, indicating a more sustained reaction process. FTIR, TG-DTG, XRD, and SEM-EDS further supported differences in gel development and Ca-bearing phase evolution. In particular, the CS-based binder showed a high-temperature mass loss above 600 °C of 14.11%, compared with 5.83% for the Ca(OH)₂-activated reference, and a stronger relative calcite signal. These results show that CS substitution is not equivalent to simple Ca(OH)₂ addition and provides binder-scale evidence for designing waste-derived clinker-free binders with reduced reliance on reagent-grade activation. Full article

Copper slag (CS) was considered a major by-product produced from the copper refining industry, which estimates about 2.2 to 3 tons generated during the production of every one ton of copper. At the same time, continuous dumping and improper disposal of this byproduct have led to serious environmental problems, especially due to the leaching of heavy metals into soil and water. This review carefully studies the potential of CS as a sustainable construction material through a clear distinction of its performance, especially when used as a fine aggregate and as a supplementary cementitious material (SCM). Due to the presence of higher content of iron and silica, higher hardness, and very low water absorption, it was found that CS helps in improving the density and durability of concrete. When used as a fine aggregate, CS enhances workability, strength, and durability at an optimum level of about 40%, mainly due to better particle packing and reduced pore connectivity. On the other hand, when used as an SCM, CS contributes to long-term strength through pozzolanic reactions and the formation of C–S–H gel, but its replacement level should be limited to about 20% to avoid loss of early-age strength caused by reduced alkalinity. In terms of durability, the use of CS can reduce water absorption by up to 60%, lower chloride penetration, and improve resistance to sulfate attack. Environmental Life Cycle Assessment studies show that CS can reduce global warming potential by about 12–19% and also decrease overall energy consumption. Statistical validation using multi-criteria decision analysis (MCDA) and separate regression modeling with an R² value of about 0.965, which supports these optimum replacement levels up to 40% for fine aggregate and 20% for cement, providing a good balance between strength, durability, environmental benefits, and cost. Overall, this review shows that CS is a valuable and multi-functional material that supports circular economy practices when used with a proper mix design based on specific applications. Full article

Silicomanganese slag (SiMnS), a Mn-bearing by-product from silicomanganese alloy production, is often stockpiled in large quantities and may pose environmental concerns due to potential metal leaching. This study develops an OPC-rich hybrid SiMnS–steel slag–fly ash–OPC-activated composite binder, referred to as SMSAB, in which OPC accounts for 55% of the solid precursor mass. Different alkali contents and sodium silicate moduli were investigated, and the optimised paste was characterised in terms of mechanical strength, reaction products, pore structure, carbon-footprint and heavy-metal leaching. The best performance was obtained at an alkali content of 4% and a sodium silicate modulus of 1.0, giving 28-day compressive and flexural strengths of 65.13 MPa and 3.37 MPa, respectively. XRD, SEM-EDS, FTIR and MIP results showed that the main reaction products were C-(A)-S-H, N-A-S-H and C-N-A-S-H gels, which refined the pore structure and produced a dense matrix. The reduction in Mn leaching may be associated with physical encapsulation, possible charge-balancing interactions within gel structures, changes in Mn-related bonding environments and the presence of Mn-bearing phases. Leaching concentrations of Zn, Mn, Cr, Cu and Ni satisfied the Grade III groundwater limits used in China. The calculated carbon intensity of SMSAB was 3.97 kg·(m³·MPa)⁻¹, indicating a favourable strength-to-emission balance compared with the reference systems considered. It should be noted that the present work examines paste specimens only; aggregate skeleton, traffic loading, freeze–thaw cycling and wet–dry/moisture cycling were not included. Therefore, the results demonstrate binder-level potential rather than direct qualification of SMSAB as a pavement base or subbase material. Full article

The present study reports the variation of the mechanical properties of engineered metakaolin-based geopolymers synthetized using NaOH-KOH alkali activators and sodium disilicate, investigated after 7 and 28 days of aging by means of unconfined compression tests for mechanical strength analysis. The geopolymers were synthetized by mixing KOH and NaOH in different proportions in the alkaline activating solution, from 0% to 100% of KOH addition, fixing the Si/Al ratio and water content. The binders were synthetized with different curing temperatures. A novel composition using quarry-derived materials (silt from sedimentation lakes) was developed to realize an innovative composite. The materials were characterized by XRD, ESEM-EDS and unconfined compression tests. The mechanical results underlined that the addition of the filler tends to preserve the mechanical properties of the composite. Generally, curing at 40 °C followed by a 28-day aging period for the mixed Na-K geopolymers demonstrated the highest mechanical strength of all the synthesized products, with a maximum strength of 21 MPa. Mixed NaOH-KOH composites generally exhibited lower performances compared to sample consisting solely of 100% NaOH when cured at a temperature of 85 °C. Nonetheless, the synthetized composites reported in this study can have diverse applications across various technological fields requiring low-strength materials. Full article

The construction sector consumes significant amounts of natural resources and contributes substantially to global CO₂ emissions, making it necessary to develop materials with a reduced environmental impact. In this context, the valorization of reusable industrial waste as secondary raw materials represents a strategic direction for applying circular economy principles and for decarbonizing the construction materials industry. The scientific problem addressed in this review is the urgent need to develop construction materials with a reduced environmental footprint, given that the construction sector is a major consumer of natural resources and a significant contributor to global CO₂ emissions. This challenge requires the identification and critical evaluation of sustainable solutions that support decarbonization and the transition toward a circular economy. The main findings indicate that the valorization of industrial waste offers high decarbonization potential: supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag and fly ash, can reduce CO₂ emissions by approximately 20–50%, while alkali-activated binders and geopolymers achieve reductions of 40–80% compared to Portland cement. These materials also enhance durability, extending service life by 10–20% in aggressive environments, although early-age strength may decrease by 10–30%; recycled aggregates derived from construction and demolition waste (CDW) can substitute up to 100% of natural aggregates, while rubber fibers can increase impact resistance by 30–50% and reduce density by 10–20%. However, key limitations relate to waste variability, heavy metal leaching risks (requiring immobilization efficiencies > 90%), and the relatively low technological maturity of many solutions (TRL < 7), leading to the TRL–CO₂ paradox and highlighting the need for standardization and performance-based regulatory frameworks. The synthesized results indicate that the appropriate integration of industrial waste enables a significant reduction in clinker content, lowers associated CO₂ emissions, and decreases primary energy consumption while maintaining physical–mechanical properties and durability characteristics comparable to or in some cases superior to those of traditional materials, if mix design is based on clear performance criteria, stratified according to the type of waste, dosage used, curing regime, binder chemistry, and the target application. Full article

Utilizing lunar regolith as a raw material for structural components offers significant potential for future lunar exploration. Direct manufacturing from unprocessed regolith reduces the need for specialized refining equipment compared to element extraction methods. At present, the mechanical properties of long-term alkali-activated CQU-1 lunar regolith simulant geopolymer (LRSG) columns have not been studied. To address this, forty-eight CQU-1 LRSG cylindrical specimens were prepared and tested under axial compression in this study. The effects of the curing temperature (60 °C and 80 °C), curing time (3 d, 7 d, 14 d and 28 d), and water–binder ratio (0.325 and 0.455) on the failure modes and stress–strain behavior were investigated. The alkali-activated CQU-1 LRSG achieved a maximum compressive strength of 33.89 MPa under optimal conditions. Elevated curing temperatures and extended curing times enhanced peak stress and elastic modulus while reducing peak and ultimate strains, indicating greater stiffness and brittleness. Conversely, increased water–binder ratios flattened stress–strain curves, diminishing slope and peak stress while elevating peak and ultimate strains. Based on these test results, the stress–strain model, elastic modulus model and peak strain model of alkali-activated CQU-1 LRSG were proposed. The proposed models can accurately predict the stress–strain relationship, compressive strength and ultimate strain of alkali-activated CQU-1 LRSG. The influence of curing temperature, curing time, and water–binder ratio on the performance of alkali-activated CQU-1 LRSG is also discussed in detail. This work confirms the viability of the alkali-activated CQU-1 LRSG and lunar regolith-based geopolymers for future extraterrestrial construction. Full article

In 2026, sustainable construction materials research is focused on optimization of the resources’ circularity, carbon reduction, and performance improvements through advanced materials. Biochar, nanomaterials, and recycled aggregates (RA) are enhancing concrete by improving strength, durability, and carbon capture, while supporting low-carbon, circular practices. When used in low-carbon alkali-activated materials (AAMs), these materials reduce greenhouse gas emissions by approximately 30–60% compared to Portland cement (PC). Despite challenges in cost, standardization, and large-scale production, these innovations are advancing the construction industry towards sustainable, carbon-neutral solutions. RA helps reduce landfill waste and converse resources, though issues like quality variability and potential contaminants must be addressed. Biochar’s (0.5–2 wt.% of binder) adoption is limited by inconsistent properties, while nanomaterials (0.01 to 3 wt.% of binder) offer improved mechanical properties (5–20%) but face high production costs and limited long-term data. In the coming years, efforts will focus on standardizing production, improving nanoparticle dispersion, and refining RA processing. The integration of AI and machine learning may further optimize material design, leading to greener, low-carbon materials for large-scale, sustainable infrastructure by 2036. Full article