Search Results (698)

The manufacturing process of ordinary Portland cement (OPC) is highly resource-intensive and significantly contributes to global CO₂ emissions, thereby exacerbating global warming. In this context, researchers are progressively adopting geopolymer concrete owing to its environmentally friendly production process. However, cracks in OPC and geopolymer concrete structures can substantially reduce their lifespan by exposing reinforcement to the external environment, resulting in concrete deterioration. To mitigate these issues, the self-healing capability of concrete presents an innovative solution to restore structural integrity and minimise maintenance costs. This research delineates various healing techniques and their efficacy for geopolymer concrete, including crystalline admixture, fibres, bacteria, and enzymes. This study primarily examines geopolymer compositions to assess the self-healing efficiency of different healing agents. As many healing agents, including crystalline admixtures and enzyme-based systems, were originally developed for OPC-based concrete and remain underexplored in geopolymers, parallel investigations on OPC systems are also conducted to enable a comparative understanding of the underlying healing mechanisms. The current state of research indicates that crystalline admixture was unable to facilitate crack healing within the geopolymer matrix unless an additional 10% Ca(OH)₂ was incorporated into the binder. The inclusion of fibres embedded with healing agents markedly improved the healing efficiency, achieving a crack width of up to 800 µm when utilised with natural fibres and bacteria. The integration of an optimal quantity of various healing agents enhances the compressive, split tensile, and flexural strength of the concrete. The optimal dosages for the crystalline admixture ranged from 1% to 1.5% by weight of the binder, while the concentration of bacteria ranged from 10⁵ to 10⁷ cells/mL. Furthermore, this review delineates the practical applications and limitations of various healing agents. By integrating appropriate healing agents into geopolymer concrete, this research aims to advance a sustainable approach to durable infrastructure. Full article

In contemporary construction practice, concrete surfaces are commonly coated; however, this factor is often disregarded in durability assessments, particularly with respect to carbonation. Such omission may lead to overly conservative designs and unnecessary material consumption. This study evaluates the actual performance of traditional coatings applied to concrete, considering three types of concrete: ordinary Portland cement (OPC), high-volume fly ash (FA), and high-volume FA with a low water-to-binder ratio. The coatings investigated were mainly based on cement and hydrated lime, with the inclusion of a FA-based alternative. Accelerated carbonation tests were performed on coated and uncoated concretes, as well as on coating mortars, while a sensitivity analysis was undertaken using an empirical and semi-probabilistic model across different exposure classes to simulate real service conditions. The results demonstrate excellent performance, with coated concretes achieving on average more than 52% higher resistance compared with uncoated counterparts. These findings indicate that properly designed coatings can enable reductions in cement content while still satisfying durability requirements, thereby contributing to more sustainable reinforced concrete structures. Full article

Cemented tailings backfill (CTB) is widely used in mining operations due to its operational simplicity, reliable performance, and environmental benefits. However, the poor consolidation of fine tailings with ordinary Portland cement (OPC) remains a critical challenge, leading to excessive backfill costs. This study addresses the utilization of modified manganese slag (MMS) as a supplementary cementitious material (SCM) for fine tailings from an iron mine in Anhui, China. Sodium silicate (Na₂SiO₃) modification coupled with melt-water quenching was implemented to activate the pozzolanic reactivity of manganese slag (MS) through glassy structure alteration. The MMS underwent comprehensive characterization via physicochemical analysis, X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) to elucidate its physicochemical attributes, mineralogical composition, and glassy phase architecture. The unconfined compressive strength (UCS) of the CTB samples prepared with MMS, OPC, tailings, and water (T-MMS) was systematically evaluated at curing ages of 7, 28, and 60 days. The results demonstrate that MMS predominantly consists of SiO₂, Al₂O₃, CaO, and MnO, exhibiting a high specific surface area and extensive vitrification. Na₂SiO₃ modification induced depolymerization of the highly polymerized Q⁴ network into less-polymerized Q² chain structures, thereby enhancing the pozzolanic reactivity of MMS. This structural depolymerization facilitated formation of stable gel products with low calcium–silicon ratios, conferring upon the T-MMS10 sample a 60-day strength of 3.85 MPa, representing a 94.4% enhancement over the T-OPC. Scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS) analysis revealed that Na₂SiO₃ modification precipitated extensive calcium silicate hydrate (C-S-H) gel formation and pore refinement, forming a dense networked framework that superseded the porous microstructure of the control sample. Additionally, the elevated zeta potential for T-MMS10 engendered electrostatic repulsion, while the aluminosilicate gel provided imparted lubrication, collectively improving the flowability of the composite slurry exhibiting a 26.40 cm slump, which satisfies the requirements for pipeline transportation in backfill operations. Full article

The construction industry’s heavy reliance on Ordinary Portland Cement (OPC) significantly contributes to global CO₂ emissions, prompting the search for sustainable alternatives. This study investigates the partial substitution of Portland cement with construction and demolition waste (CDW) powder and concrete waste (CON) powder in mortar mixes. Replacement levels of 5%, 10%, 15%, and 20% by weight were tested following EN 196-1 standards to evaluate the mechanical performance of the resulting materials. X-ray diffraction (XRD), X-ray fluorescence (XRF), and thermo-gravimetric analyses confirmed that CDW and CON powders consist mainly of quartz and calcite, with chemical compositions compatible with cementitious systems. Mechanical testing revealed that compressive strength was maintained or slightly improved at replacement levels up to 10%, while higher substitutions led to moderate reductions due to dilution effects. The use of CDW and CON powders effectively transformed a 52.5 R Type I cement into a 42.5 R Type II equivalent, demonstrating the feasibility of producing sustainable binders with acceptable performance. Full article

This paper presents a microstructural, mineralogical, and mechanical study of low-carbon autoclaved concrete (AC), achieved by partially or fully replacing ordinary Portland cement (OPC) with ground-granulated blast furnace slag (BFS) and substituting lime with calcium carbide slag (CCS). Fourteen mixes were produced and evaluated in the green state and after autoclaving. Quantitative X-ray diffraction (XRD) using the Rietveld method, density, compressive strength, and life cycle assessment (LCA) were conducted. Results show that mixes containing BFS achieve green strengths equal to or higher than the OPC reference, ensuring integrity during autoclaving. Using BFS with an adequate calcium supply promotes the formation of pre-autoclave portlandite, which in turn favors tobermorite development and yields post-autoclave strengths comparable to the OPC reference. Partial lime replacement with CCS (50%) maintains mineralogy and strength, whereas excessive CCS may reduce available portlandite and lower strength. Life-cycle assessment indicates that raw material supply dominates emissions and that removing OPC cuts total CO₂ by 44% without compromising mechanical performance. These findings demonstrate the feasibility of OPC-lean/OPC-free, lime-optimized autoclaved concretes with substantially lower embodied impacts. Full article

Recycling wastewater from washing concrete trucks in concrete production addresses both economic and sustainability needs. In the present article, wastewater from washing concrete trucks was added to cement pastes made with two different types of cement for comparison. OPC type CEM I 42.5 was compared to pozzolanic cement type CEM IV/B (P-W) 32.5 in terms of hydration behavior and compressive strength development. The hydration of ordinary Portland cement (CEM I 42.5) was accelerated, while the hydration of pozzolanic cement (CEM IV 32.5) showed a relatively lower total normalized heat. Cement pastes were produced from both cement types, and compressive strength, thermal analysis, and setting time tests were performed for their characterization. The early-age kinetics and compressive strength development of CEM I 42.5 pastes indicate that hydration with wastewater leads to a slight increase in compressive strength. Test concrete prepared with pozzolanic cement (CEM IV 32.5) exhibited increased capillary voids, which contributed to less favorable mechanical and durability performance. Compared to the reference concrete, compressive strength was reduced by 7% at 28 days. Wastewater utilization increased the initial absorption rate by approximately 20%, but the calculated chloride content at the exposed concrete surface decreased after the addition of wastewater compared to the control mix. The carbonation depth of concrete with wastewater increased by 1–2 mm, with an uneven penetration zone, but the compressive strength after carbonation increased. Overall, the type of cement used appears to significantly influence the performance of concrete prepared with wastewater. For wastewater collected from sedimentation tanks, replacing fresh water at a 100% rate and using it with pozzolanic cement to produce concrete, it seems that the mechanical properties and durability are only slightly affected. Full article

Mine waste remains a persistent challenge for the minerals industry, posing significant environmental concerns if not properly managed. The 1996 Marcopper Mining Disaster in Marinduque, Philippines, left a legacy of mine tailings that continue to threaten local ecosystems and communities. This study investigates the valorization and stabilization of Marcopper river sediments laden with mine tailings using a combined geopolymerization and cement hydration approach. Hybrid mortar samples were prepared with 7.5%, 15%, 22.5%, and 30% mine tailings by weight, utilizing potassium hydroxide (KOH) as an alkaline activator at concentrations of 1 M and 3 M, combined with Ordinary Portland Cement (OPC). The mechanical properties of the hybrid geopolymer cement mortars were assessed via unconfined compression tests, and their crystalline structure, phase composition, surface morphology, and chemical bonding were also analyzed. Static leaching tests were performed to evaluate heavy metal mobility in the geopolymer matrix. The compression tests yielded strength values ranging from 24.22 MPa to 53.99 MPa, meeting ASTM C150 strength requirements. In addition, leaching tests confirmed the effective encapsulation and immobilization of heavy metals, demonstrating the potential of this method for mitigating the environmental risks associated with mine tailings. Full article

Alkali-activated materials (AAMs) or geopolymers have been considered for many years as a sustainable substitution for the traditional ordinary Portland cement (OPC) binder. However, their production needs energy consumption and creates carbon emissions. Since construction and demolition waste (CDW) can become precursors for manufacturing alkali-activated materials, their use as substitutes for traditional AAM (such as metakaolin, blast furnace slag, and fly ash) can solve both the problem of their disposal and the problem of sustainability. Furthermore, CDW can also be used as aggregate replacement, avoiding the exploitation of natural river sand and gravel. A new circular economy could be created based on CDW recycling, creating a new eco-friendly building practice. Unfortunately, this process is quite difficult owing to several variables that should be taken into consideration, such as the possibility of separating and sorting the CDW, the great variability of CDW composition, the cost of the mechanical and thermal treatment, the different parameters that compose an alkali-activated mix-design, and public opinion still being skeptical about the use of recycled materials in the construction sector. This review tries to describe all these aspects, summarizing the results of the most interesting studies performed on this subject. Today, thanks to a comprehensive protocol, the use of building information modeling (BIM) software and machine learning models, a large-scale reuse of CDW in the building industry appears more feasible. Full article

The construction industry faces growing pressure to adopt sustainable materials due to the high CO₂ emissions associated with ordinary Portland cement (OPC) production. Geopolymers synthesized from industrial by-products such as fly ash offer a promising low-carbon alternative. However, the extensive use of commercial sodium silicate (SSC) as an activator remains constrained by its high cost and energy-intensive manufacturing. This study investigates a silica fume-derived sodium silicate alternative (SSA) combined with NaOH as a more sustainable activator for fly ash-based geopolymer mortar. Mortars were prepared with alkali activator-to-precursor (AA/P) ratios of 0.7 and 0.5 and cured at 65 °C and 80 °C. SSA-based mixes exhibited comparable flowability to SSC-based mortars, with slightly longer setting times making them favorable for placement. Mechanical tests showed the superior performance of SSA systems, with AS0.7-65 achieving the highest compressive strength and AS0.7-80 demonstrating greater flexural and tensile strength. Microstructural analyses (SEM, EDX, ATR-FTIR) revealed denser matrices and enhanced sodium aluminosilicate hydrate (N-A-S-H) and calcium-rich N(C)-A-S-H gel formation. Economic assessment indicated approximately 30% cost reduction and a modest (~2%) decrease in CO₂ emissions. These findings highlight SSA as a technically viable and sustainable activator for next-generation geopolymer construction. Full article

The construction sector faces pressure to decarbonise while addressing rising resource demands and agricultural waste. Ordinary Portland cement (OPC) is a major CO₂ emitter, yet biomass residues are often open-burned or landfilled. This study explores corncob ash (CCA) as a sustainable supplementary cementitious material (SCM), examining how calcination conditions influence pozzolanic potential and support circular economy and climate goals, which have not been adequately explored in literature. Ten CCA samples were produced via open-air burning (2–3.5 h) and electric-furnace calcination (400–1000 °C, 2 h), alongside a reference OPC. Mass yield, colour, XRD, XRF, LOI, and LOD were analysed within a process–structure–property–performance–sustainability framework. CCA produced in a 400–700 °C furnace window consistently achieved high amorphous contents (typically ≥80%) and combined pozzolanic oxides (SiO₂ + Al₂O₃ + Fe₂O₃) above the 70% ASTM C618 threshold, with 700 °C for 2 h emerging as an optimal condition. At 1000 °C, extensive crystallisation reduced the expected reactivity despite high total silica. Extended open-air burning (3–3.5 h) yielded chemically acceptable but more variable ashes, with lower amorphous content and higher alkalis than furnace-processed CCA. Simple industrial ecology calculations indicate that valorising a fraction of global CC residues and deploying optimally processed CCA at only 20% OPC replacement could displace 180 million tonnes CC waste and clinker avoidance on the order of 5–6 Mt CO₂ per year, while reducing uncontrolled residue burning and primary raw material extraction. The study provides an experimentally validated calcination window and quality indicators for producing reactive CCA, alongside a clear link from laboratory processing to clinker substitution, circular resource use, and alignment with SDGs 9, 12, and 13. The findings establish a materials science foundation for standardised CCA production protocols and future life cycle and performance evaluations of low-carbon CCA binders. Full article

This study investigates the effect of added sodium sulfate on the performance of high-volume slag-cement mortar (HVSCM). Herein, Na₂SO₄ (0, 1, 2, and 4 wt.% Na₂O) was used to modify HVSCM. The compressive strength, fracture properties, microstructure, and environmental impact of the synthesized samples were analyzed. The results showed that the 1 day compressive strength of HVSCM can be improved by 345.5% with the addition of 4% Na₂O (as Na₂SO₄), compared to samples without Na₂SO₄. However, the 28 day compressive strength of Na₂SO₄-activated HVSCM was 14.3–26.4% lower than that of the non-activated HVSCM, though still comparable to OPC. Regarding fracture properties, the initial fracture toughness of non-activated HVSCM was 45.6% higher than that of Ordinary Portland cement (OPC) mortar. Furthermore, Na₂SO₄ activation further increased initial fracture toughness, with the sample containing 4% Na₂O showing a 101.1% improvement over OPC. In contrast, fracture energy was not significantly influenced by Na₂SO₄ addition. Microstructurally, the enhanced fracture properties of non-activated HVSCM were attributed to a higher degree of C-(A)-S-H polymerization and a denser binder phase. Sodium sulfate introduced sodium ions to strengthen electrostatic attraction and cohesion between C-(A)-S-H globules, offsetting reduced polymerization. Environmental assessment confirms that both activated and non-activated HVSCM substantially reduce embodied energy and CO₂ relative to OPC, while the additional embodied energy associated with Na₂SO₄ activation remains limited (<12%). Overall, this work provides a comprehensive understanding of the fracture behavior of Na₂SO₄-activated HVSCM, elucidating its capacity to enhance early-age strength and fracture toughness while highlighting its limited effect on long-term strength and fracture energy. These findings support the tailored use of Na₂SO₄ activation for sustainable construction applications. Full article

This study presents a comprehensive experimental evaluation of high-performance concretes incorporating calcium sulfoaluminate (CSA) cement as a partial replacement for ordinary Portland cement (OPC). Five CSA replacement levels (0, 15, 30, 45, and 60%) and two water-to-cement ratios (0.40 and 0.45) were examined to assess their effects on mechanical performance and key durability parameters. The experimental program simultaneously investigated compressive strength, tensile splitting strength, water absorption, sorptivity, gas permeability, and freeze–thaw resistance, offering an integrated assessment rarely addressed in previous studies, which typically focus on selected parameters or narrower replacement ranges. The results show that CSA addition enhances microstructural densification, substantially reducing sorptivity and gas permeability and markedly improving freeze–thaw performance even without air entrainment. High CSA contents (45–60%) yielded superior transport-related durability while maintaining competitive 28-day strengths, especially for w/c = 0.40. These findings clarify the interplay between CSA content, transport properties, and frost resistance, highlighting CSA–OPC hybrid binders as a durable and sustainable solution for high-performance concrete applications. Full article

The growing demand for concrete in tropical regions faces two unresolved challenges: the high carbon footprint of ordinary Portland cement (OPC) and limited understanding of how supplementary cementitious materials affect the mechanical performance of laterite rock aggregates concrete. Although metakaolin (MK) is a highly reactive pozzolan, its combined use with laterite rock aggregates concrete and its influence on strength development and microstructure have not been sufficiently clarified. This study investigates the mechanical behavior and sustainability potential of laterite rock aggregate concrete in which OPC is partially replaced by MK at 0%, 5%, 10%, 15%, and 20% by weight. All mixes were prepared at a constant water–binder ratio of 0.50 and tested for workability, compressive strength, split-tensile strength, and flexural strength at 7, 14, and 28 days, with and without a polycarboxylate-based superplasticizer. The results show that MK significantly enhances the mechanical performance of laterite rock concrete, with an optimum at 10% replacement: the 28-day compressive strength increased from 35.6 MPa (control) to 53.9 MPa in the superplasticized mix, accompanied by corresponding gains in tensile and flexural strengths. SEM–EDS analyses revealed microstructural densification, reduced portlandite, and a refined interfacial transition zone, explaining the improved strength and cracking resistance. From an environmental perspective, a 10% MK replacement corresponds to an approximate 10% reduction in clinker-related CO₂ emissions, while the use of locally available laterite rock reduces the dependence on quarried granite and transportation impacts. The findings demonstrate that MK-modified laterite rock concrete is a viable and eco-efficient option for structural applications in tropical regions. The study concludes that MK-enhanced laterite rock aggregate concrete can deliver higher structural performance and improved sustainability without altering conventional mix design and curing practices. Full article

Cemented Paste Backfill (CPB) is a technique that utilizes mine tailings, mining-process water, and a binder, typically Ordinary Portland Cement (OPC), to backfill the opening created in underground mining. However, the use of cement in CPB increases operational costs and has adverse environmental effects. To mitigate these effects, eco-friendly natural pozzolan can be used as a partial replacement for OPC, thereby reducing its consumption and environmental impact. The volcanic region of western Saudi Arabia contains extensive deposits of Saudi natural pozzolan (SNP), which is a promising candidate for this purpose. This study evaluates the mechanical performance of CPB under four scenarios: a control mixture (CTRL), a mixture with untreated SNP (UT), and mixtures with activated SNP, specifically heat-treated (HT) and mechanically treated (MT). Each scenario was tested at replacement levels of 5%, 10%, 15%, and 20% of OPC. The performance was assessed using Uniaxial Compressive Strength (UCS) with Elastic Modulus (E), Ultrasonic Pulse Velocity (UPV), and Indirect Tensile Strength (ITS/Brazilian) tests. The results indicate that the HT scenario at a 5% replacement level delivered the highest performance, slightly outperforming the MT scenario. Both activated scenarios (HT and MT) significantly surpassed the untreated mixture (UT). Overall, the HT scenario proved to be the most effective among all CPB mixtures tested. XRD diffractogram analysis supported HT as the material with the highest strength performance due to the occurrence of more strength phases than other CPB materials, including Alite, Quartz, and Calcite. While UCS and UPV showed a positive correlation across all CPB materials, the relationship between UPV and the modulus of elasticity (E) demonstrated a low correlation. The findings suggest that using activated SNP materials can enhance CPB sustainability by lowering cement demand, stabilizing operating costs, and reducing environmental impacts. Full article

The development of sustainable self-compacting concrete (SCC) requires alternative binders that minimise ordinary Portland cement (OPC) consumption while ensuring long-term performance. This study investigates sulfate-activated SCC (SA SCC) incorporating high volumes of industrial by-products, whereby 72% ground granulated blast furnace slag (GGBS) and 18% fly ash (FA) were activated with varying proportions of OPC and gypsum. Quarry dust was used as a fine aggregate, while granite and electric arc furnace (EAF) slag served as coarse aggregates. Among all formulations, the binder containing 72% GGBS, 18% FA, 4% OPC, and 6% gypsum was identified as the optimum composition, providing superior mechanical performance across all curing durations. This mix achieved slump flow within the EFNARC SF2 class (700–725 mm), compressive strength exceeding 50 MPa at 270 days, and flexural strength up to 20% higher than OPC SCC. Drying shrinkage values remained below Eurocode 2 and ASTM C157 limits, while EAF slag increased density, but slightly worsened shrinkage compared to granite mixes. Microstructural analysis (SEM-EDX) confirmed that strength development was governed by discrete C-S-H and C-A-S-H gels surrounding unreacted binder particles, forming a dense interlocked matrix. The results demonstrate that sulfate activation with a 4% OPC + 6% gypsum blend enables the production of high-performance SCC with 94–98% industrial by-products, reducing OPC dependency and environmental impact. This work offers a practical pathway for low-carbon SCC. Full article