Search Results (926)

Alkali-activated materials (AAMs) offer a promising low-carbon alternative to Portland cement, but their development has been dominated by fly ash and slag, whose availability is increasingly limited. This research explores waste brick powder (WBP) and metakaolin residue (RN), two abundant yet underutilized by-products, as blended precursors for sustainable binder design. The novelty lies in demonstrating how complementary chemistry between crystalline-rich WBP and amorphous RN can overcome the drawbacks of single-precursor systems while valorizing construction and industrial residues. Pastes were prepared with varying WBP/RN ratios, activated with alkaline solutions, and characterized by Vicat setting tests, isothermal calorimetry, XRD with Rietveld refinement, MIP, SEM, and mechanical testing. Carbon footprint analysis was performed to evaluate environmental performance. Results show that WBP reacts very rapidly, causing flash setting and limited long-term strength, whereas the incorporation of 30–50% RN extends setting times, sustains dissolution, and increases amorphous gel formation. These changes refine the formed reaction products, leading to compressive strengths up to 39 MPa and flexural strengths of 8 MPa at 90 days. The carbon footprint of all blends remained 392–408 kg CO₂e/m³, thus providing about a 60% improvement compared to conventional Portland cement paste. The study establishes clear design rules for waste-derived blended precursors and highlights their potential as circular, low-carbon binders. Full article

Expansive agents (CaO, MgO, C₄A₃Š) were incorporated into coal gangue-slag-fly ash based geopolymer (CSFG). The influence of expansive agents on the properties and microstructure of CSFG was investigated by macroscopic tests including setting time, compressive strength, and shrinkage values, along with microstructural tests including XRD, FTIR, SEM-EDS, and BET. Results showed that CaO and MgO added separately and their combination exhibited similar trends, with CaO added separately yielding the most favorable outcome. In comparison to the control group, the sample with 7% CaO reduced initial and final setting times by 43.6% and 52.8%, increased 28 d compressive strength by 12.6%, and decreased 28 d drying shrinkage and autogenous shrinkage values by 43.5% and 29.9%, respectively. Moderate MgO and CaO enhanced dissolution of precursors (e.g., coal gangue, fly ash), promoting formation of C-A-S-H gel, CaCO₃, and periclase. Incorporating 3% C₄A₃Š shortened initial and final setting times by 41.3% and 17.8%, improved 28 d compressive strength by 32.2%, but increased 28 d drying and autogenous shrinkage values by 58.3% and 12.8%. Exceeding 3% content significantly reduced 3 d strength. Excessive C₄A₃Š promoted rapid ettringite (AFt) formation, leading to microcracking. Correction prediction models for drying shrinkage strain and autogenous shrinkage strain of CSFG were developed, demonstrating good agreement between predictive and actual values. Full article

Three-dimensional concrete printing (3DCP), an innovative fabrication technique, has emerged as an environmentally friendly digital manufacturing process for using recycled waste materials in the construction industry. The aim of this review paper is to critically evaluate the current state of research on the use of recycled materials such as aggregates and powders in 3DCP, correlating the environmental, economic, and performance parameter effects. This review comprehensively evaluates the potential benefits of incorporating recycled waste materials in 3D printing by critically reviewing the existing peer-reviewed articles through a scientometric review. The resulting bibliometric analysis identified 73 relevant papers published between 2018 and 2024. Through the critical review, five main research categories were identified: recycled materials in 3DCP arising mainly from construction demolition in powder and aggregate forms, which investigates the types of recycled materials used, their extraction methods, morphology and physical and chemical properties. The morphology properties of the materials used displayed high irregularities in terms of shape and percentage of adhered mortar. In the second category, printability and performance, the buildability, rheological properties and the mechanical performance of 3DCP with recycled materials were investigated. Category 3 assessed the latest developments in terms of 3D-printed techniques, including Neural Networks, in predicting performance. Category 4 analysed the environmental and economic impact of 3DCP. The results indicated anisotropic behaviour for the printed samples influencing mechanical performance, with the parallel printing direction showing improved performance. The environmental performance findings indicated higher global warming potential when comparing 3DCP to cast-in situ methods. This impact was reduced by 2.47% when recycled aggregates and binder replacements other than cement were used (fly ash, ground slag, etc.). The photochemical pollution impact of 3DPC was found to be less than that of cast-in situ, 0.16 to 0.18 C2H4-eq. This environmental impact category was further reduced up to 0.10 C2H4-eq following 100% replacement. Lastly, category 5 explored some of the challenges and barriers for the implementation of 3DCP with recycled materials. The findings highlighted the main issues, namely inconsistency in material properties, which can lead to a lack of regulation in the industry. Full article

This study examines the influence of steel fiber reinforcement on the mechanical properties of geopolymer concrete incorporating different slag to fly ash binder ratios (75:25, 50:50, and 25:75). Three fiber contents (0%, 1%, and 2%) by volume were used to assess their impact on compressive strength, flexural strength, initial stiffness, and toughness. Compressive tests were conducted at 1, 7, and 28 days, while flexural behavior was evaluated through a four-point bending test at 28 days. The results showed that geopolymer concrete with 75% slag and 25% fly ash experienced the highest compressive strength and modulus of elasticity, regardless of the steel fiber content. The addition of 1% and 2% steel fiber content enhanced the compressive strength by 17.49% and 28.8%, respectively, compared to the control sample. The binder composition of geopolymer concrete plays a crucial role in determining its compressive strength. Reducing the slag content from 75% to 50% and then to 25% resulted in a 15.1% and 33% decrease in compressive strength, respectively. The load–displacement curves of the 2% fiber-reinforced beams display strain-hardening behavior. On the other hand, after the initial crack, a constant increase in load causes the specimen to experience progressive strain until it reaches its maximum load capacity. When the peak load is attained, the curve gradually drops due to a loss in load-carrying capacity known as post-peak softening. This behavior is attributed to steel’s ductility and is evident in specimens 75S25FA2 and 50S50FA2. Concrete with 75% slag and 25% fly ash demonstrated the highest peak load but the lowest ultimate displacement, indicating high strength but brittle behavior. In contrast, concrete with 75% fly ash and 25% slag showed the lowest peak load but the highest displacement. Across all binder ratios, the addition of steel fibers enhanced the flexural strength, initial stiffness, and toughness. This is attributed to the bridging action of steel fibers in concrete. Additionally, steel fiber-reinforced beams exhibited a ductile failure mode, characterized by multiple fine cracks throughout the midspan, whereas the control beams displayed a single vertical crack in the midspan, indicating a brittle failure mode. Full article

Traditional landfill disposal of muck uses a significant amount of land and pollutes the environment, while current solidification methods heavily depend on energy-intensive cement. This study introduces a novel approach for synergistically solidifying muck using cement, fly ash, and steel slag, aiming to utilize waste resources and achieve low-carbon disposal. Experimental optimization identified the optimal ratio (cement:fly ash:steel slag = 2:2:1). The findings indicate that cement is crucial for early strength, while industrial waste materials enhance long-term performance through continued reactions. At a total solidifying agent content of 4–6%, the material exhibits optimal mechanical properties and durability, with only a 4% strength loss after 12 dry–wet cycles. Microscopic analysis indicates that several gels and polymers with cementing properties are produced, collectively enhancing the material’s structure. Additionally, this material effectively immobilizes heavy metals, including chromium, lead, arsenic, and cadmium, with leaching concentrations that are well below safety thresholds. This approach provides a dependable and eco-friendly method for large-scale disposal of construction waste muck and industrial solid waste, offering significant potential for engineering applications. Further studies could investigate additional solid waste types and formulations suitable for high-moisture materials like sludge. Full article

This study demonstrates that synergistic pyrolysis and water-soaking pretreatment transforms municipal solid waste incineration fly ash (MSWI FA) into high-performance alkali-activated materials when combined with ground granulated blast furnace slag (GGBS). Pyrolysis reduced chlorine content by 94.3% and increased reactive components by 44.4%, thereby shifting hydration products from Friedel’s salt to ettringite (AFt). Subsequent water-soaking eliminated expansion-causing elemental aluminum, liberating activators for enhanced reaction completeness (29% higher cumulative heat release) and enabling a denser matrix with 71.5% harmless pores (<20 nm). The dual-treated FA (T-PFA) achieved exceptional mechanical performance—295.6% higher 56-day compressive strength versus untreated FA at a 1:1 ratio—while reducing porosity by 29.1% relative to pyrolyzed-only FA. Despite 22–38% increased total heavy metal content post-pyrolysis, matrix densification and enhanced C-A-S-H/AFt formation reduced Cr/Cd/Cu/Pb leaching by 11.3–66.7% through strengthened physical encapsulation and chemisorption, with all leachates meeting stringent HJ 1134-2020 thresholds. This integrated approach provides an efficient, environmentally compliant pathway for MSWI FA valorization in low-carbon construction materials. Full article

Rapid population growth and accelerating urbanization are intensifying the demand for construction materials, particularly concrete, which is predominantly produced with Portland cement and natural aggregates. This reliance imposes substantial environmental burdens through resource depletion and greenhouse gas emissions. Within the framework of sustainable construction, recycled aggregates and industrial by-products such as fly ash, slags, crushed glass, and other secondary raw materials have emerged as viable substitutes in concrete production. At the same time, three-dimensional concrete printing (3DCP) offers opportunities to optimize material use and minimize waste, yet it requires tailored mix designs with controlled rheological and mechanical performance. This review synthesizes current knowledge on the use of recycled construction and demolition waste, industrial by-products, and geopolymers in concrete mixtures for 3D printing applications. Particular attention is given to pozzolanic activity, particle size effects, mechanical strength, rheology, thermal conductivity, and fire resistance of recycled-based composites. The environmental assessment is considered through life-cycle analysis (LCA), emphasizing carbon footprint reduction strategies enabled by recycled constituents and low-clinker formulations. The analysis demonstrates that recycled-based 3D printable concretes can maintain or enhance structural performance while mix-level (cradle-to-gate, A1–A3) LCAs of printable mixes report CO₂ reductions typically in the range of ~20–50% depending on clinker substitution and recycled constituents—with up to ~48% for fine recycled aggregates when accompanied by cement reduction and up to ~62% for mixes with recycled concrete powder, subject to preserved printability. This work highlights both opportunities and challenges, outlining pathways for advancing durable, energy-efficient, and environmentally responsible 3D-printed construction materials. Full article

The search for sustainable and economical alternative materials has become a top priority in response to the increasing scarcity of natural river sand resources; as a result, a new alkali-activated granulated blast-furnace slag (GGBS)/fly ash (FA) composite cement material innovatively using Tuokexun Desert sand as aggregate has emerged as a good strategy. In this study, GGBS/FA was used in place of cement; the effects of the water glass modulus, alkali equivalent, and FA content on the material’s properties were systematically studied, and the hydration reaction mechanism and durability characteristics were revealed. The material was found to form a stable calcium aluminosilicate hydrate (C-(A)-S-H) gel structure under a specific ratio, which not only displayed excellent mechanical properties (a compressive strength of up to 83.2 MPa), but also showed outstanding resistance to high temperatures (>600 °C) and acid–alkali erosion. Microscopic analysis showed that the phase transition behaviour of C-(A)-S-H was a key factor affecting the material properties under high-temperature and acid–alkali environments. This study provides a new method for the preparation of high-performance building materials using local materials in desert areas, which is of great significance for promoting the construction of sustainable infrastructure in arid areas. Full article

The high density of the internal structure of new-generation cementitious composites, such as high-performance and ultra-high-performance concretes, necessitates the use of advanced methods for evaluating their transport properties, particularly those employing a gaseous medium. The developed gas permeability method for cement pastes, based on a modified RILEM-Cembureau approach, has proven to be highly accurate, reliable, and extremely sensitive to changes in the porosity characteristics of such composites. The article contains the results of a study of the mass transport capabilities of blended cement pastes, characterised by variable water–cement ratios. Two types of cements were used in the study: with the addition of fly ash and blast furnace slag. Ordinary Portland cement was used as the reference binder. The tests were conducted after long-term curing under natural conditions, i.e., after 90 days and 2 years. The assessment of open porosity was carried out through three techniques: helium pycnometry, mercury intrusion porosimetry, and water saturation. Permeability, on the other hand, was measured using a customized approach tailored for uniform paste materials. Microstructural changes were also analysed in the context of natural hydration carbonation progress. The results presented allowed a quantitative description of the effects of the w/c ratio, the presence of additives, and the progress of hydration and carbonation on the porosity of pastes and their permeability to gas flow. The two-year curing period of the pastes exposed to natural CO₂ resulted in a reduction of the permeability coefficient k ranging from 11% to 74%, depending on the type of cement and the water-to-cement (w/c) ratio. This decrease was caused by the continued progress of hydration and simultaneous carbonation. The results of the research presented are of interest from both an engineering and scientific point of view in the context of long-term microstructural changes and the mass transport abilities of cement pastes associated with these processes. The extensive range of materials compositions investigated makes it possible to analyse the durability and tightness of many cementitious composites over long periods of service. Full article

Supplementary cementitious materials (SCMs) provide a practical solution for reducing greenhouse gas emissions associated with Portland cement production while enhancing the economy, performance, and service life of concrete and mortar. Currently, there is a significant disparity in the availability, supply, and utilisation levels of SCMs worldwide, particularly in South Africa. This paper presents an in-depth analysis of the characteristics and performance of various SCMs, including local availability, factors driving demand, production, and utilisation. The findings indicate that fly ash and limestone calcined clay are the most widely available SCM resources in South Africa, with deposits exceeding 1 billion tonnes each. Fly ash stockpiles continuously increase due to the reliance on coal-fired power plants for 85% of generated electricity and a low fly ash utilisation rate of 7%, significantly below international utilisation levels of 10–98%. Conversely, slag resources are depleting due to the steady decline of local steel production caused by energy and input costs, alongside the growing importation of steel products. Combined, the estimated production of slag and silica fume is about 1.4 million tonnes per annum, leading to their limited availability and utilisation in niche applications such as high-performance concrete and marine environments. Furthermore, 216,450 tonnes of SCM could potentially be processed annually from agricultural waste. In addition to quality, logistics, costs, and other challenges, this quantity can only replace 1.5% of clinker in South Africa, raising concerns about the viability of SCMs from agricultural waste. Based on its findings, this study recommends future research areas to enhance the performance, future availability, and sustainability of SCMs. Full article

Three-dimensional concrete printing (3DCP) is advancing rapidly, yet its sustainable adoption requires alignment with circular-economy principles. This study evaluates the substitution of natural aggregates with recycled constituents, 3DCP waste, brick debris, glass cullet, mixed rubble, fly ash, and slag, and the use of lightweight fillers (expanded perlite, lightweight expanded clay aggregate (LECA), and expanded polystyrene (EPS)) to reduce density and improve insulation. Key properties, such as particle-size distribution, printability, mechanical performance, thermal conductivity, and water absorption, were determined. Results indicate that grading strongly affected mixture behavior. Narrow distributions (fly ash, milled 3DCP waste) enhanced extrudability, while broader gradings (glass, rubble, slag) increased water demand and extrusion risks. Despite these differences, all systems remained within the printable window: flow spread decreased with most recycled additions (lowest for brick) and increased with glass. Mechanical responses were composition-dependent. Flexural strength typically decreased. Compressive strength benefited from broader gradings, with replacement levels up to ~6% enhancing strength due to improved packing. Loading anisotropy typical of 3DCP was observed, with perpendicular compressive strength reaching up to 13% higher values than parallel loading. Lightweight fillers significantly reduced thermal conductivity. LECA provided the best compromise between strength and insulation, perlite showed intermediate behavior, and EPS achieved the lowest thermal conductivity but induced significant strength penalties due to weak matrix-EPS interfaces. Water absorption decreased in recycled-aggregate mixes, whereas lightweight systems, particularly with perlite, retained higher uptake. The results demonstrate that non-reactive recycled aggregates and lightweight insulating fillers can be successfully integrated into extrusion-based 3DCP without compromising printability. Full article

3D-printable concretes often require high binder content. This study evaluates the use of industrial gypsum by-products, phosphogypsum (PG) and borogypsum (BG), as partial cement replacements to enhance sustainability without compromising printability. PG and BG were incorporated at 2.5–10 wt% to replace the gypsum fraction in cement-based mortars containing fly ash (FA) or ground granulated blast-furnace slag (GGBS), with and without fibers. The fresh properties (spread flow diameter, open time, air content, density, and pH) and compressive strength were measured. At 28 days, the highest strength was achieved with a 7.5% PG addition to the GGBS system (~51 MPa), which exceeded the strength of the GGBS control C1 (~47.6 MPa). In the FA system, 2.5% PG reached 42.5 MPa, comparable to the FA control C2 (41.2 MPa). BG caused pronounced strength penalties at ≥7.5% across both binder systems, indicating a practical BG ceiling of ≤5%. Open time increased from ~0.75 h in the controls to ~2–2.5 h in BG-FA mixes with fibers, whereas PG mixes generally maintained a stable, printable window close to control levels. Overall, adding 5–7.5% PG, particularly in the presence of GGBS, improved mechanical performance without compromising workability. However, BG should be limited to ≤5% unless extended open time is the primary objective. These findings provide quantitative guidance on selecting PG/BG dosages and FA/GGBS systems to balance strength and printability in cement-based, 3D-printable concretes. Full article

The construction industry is a major contributor to global environmental impacts, particularly through the production and use of cement-based materials. In response to this challenge, this study provides a comprehensive synthesis of recent advances in the integration of Life-Cycle Assessment (LCA) and Artificial Neural Networks (ANNs) for optimizing cementitious composites containing Supplementary Cementitious Materials (SCMs). A total of 14 case studies specifically addressing this topic were identified, reviewed, and analyzed, spanning various binder compositions, ANN architectures, and LCA frameworks. The findings highlight how hybrid ANN–LCA systems can accurately predict mechanical performance while minimizing environmental burdens, supporting the formulation of low-carbon, high-performance cementitious composites. The diverse SCMs explored, including fly ash, slag, silica fume, waste glass powder, and rice husk ash, demonstrate significant potential for reducing CO₂ emissions, energy consumption, and raw material depletion. Furthermore, the systematic comparative matrix developed in this work offers a valuable reference for researchers and practitioners aiming to implement intelligent, eco-efficient mix designs. Overall, this study contributes to advancing digital sustainability tools and reinforces the viability of ANN–LCA integration as a scalable decision-support framework for green construction practices. Full article

Large amounts of spent, radioactive, ion-exchange resins have been generated worldwide, and their production is expected to grow due to a renaissance of nuclear power. Such waste is being stored at individual plant sites around the world, awaiting a reliable disposal route to overcome the downsides of the state-of-the-art management approaches. In this work, a first-of-its-kind pre-disposal strategy is proposed, based on the integration of a heterogeneous Fenton-like treatment with conditioning in an alkali-activated matrix. In particular, the circular economy is pursued by repurposing two industrial by-products, coal fly ash and steel slag, both as catalysts of the Fenton treatment and precursors of the conditioning matrix. The obtained waste forms have been preliminarily tested for leaching and compressive strength according to the Italian waste acceptance criteria for disposal. The proposed technology, tested at laboratory scale up to 100 g of virgin cationic resin, has proven successful in decomposing the waste and synthesizing waste forms with an overall volume increase of only 30%, thereby achieving a remarkable result compared to state-of-the-art technologies. Full article

This study advances alkali-activated cementitious materials (AACMs) by developing a ground-granulated blast furnace slag/high-calcium fly ash (GGBS/HCFA) composite that incorporates Tuokexun desert sand and by establishing a clear linkage between activator chemistry, mix proportions, curing regimen, and microstructural mechanisms. The innovation lies in valorizing industrial by-products and desert sand while systematically optimizing the aqueous glass modulus, alkali equivalent, HCFA dosage, and curing temperature/time, and coupling mechanical testing with XRD/FTIR/SEM to reveal performance–structure relationships under thermal and chemical attacks. The optimized binder (aqueous glass modulus 1.2, alkali equivalent 6%, and HCFA 20%) achieved 28-day compressive and flexural strengths of 52.8 MPa and 9.5 MPa, respectively; increasing HCFA beyond 20% reduced compressive strength, while flexural strength peaked at 20%. The preferred curing condition was 70 °C for 12 h. Characterization showed C-(A)-S-H as the dominant gel; elevated temperature led to its decomposition, acid exposure produced abundant CaSO₄, and NaOH exposure formed N-A-S-H, each correlating with strength loss. Quantitatively, acid resistance was weaker than alkali resistance and both deteriorated with concentration: in H₂SO₄, 28-day mass loss rose from 1.22% to 4.16%, with compressive/flexural strength retention dropping to 75.2%, 71.2%, 63.4%, and 57.4% and 65.3%, 61.6%, 58.9%, and 49.5%, respectively; in NaOH (0.2/0.5/0.8/1.0 mol/L), 28-day mass change was +0.74%, +0.88%, −1.85%, and −2.06%, compressive strength declined in all cases (smallest drop 7.77% at 0.2 mol/L), and flexural strength increased at lower alkalinity, consistent with a pore-filling micro-densification effect before gel dissolution/cracking dominates. Practically, the recommended mix and curing window deliver structural-grade performance while improving high-temperature and acid/alkali resistance relative to non-optimized formulations, offering a scalable, lower-carbon route to utilize regional desert sand and industrial wastes in durable cementitious applications. Full article