Search Results (545)

To investigate the impact of underground multiple stress disturbances on the long-term stability of cemented tailings backfill (CTB), this study conducted experiments under different disturbance levels (20–80% of static strength) and frequencies (1–4 times). By comprehensively utilizing mechanical testing, wave velocity monitoring, digital image correlation (DIC), and scanning electron microscopy (SEM), the “heterogeneous” evolution mechanism of macro-micro damage was revealed. The results indicate that disturbance level and frequency exert distinctly different driving effects on the deterioration of CTB, rather than a simple linear superposition. Specifically, low-frequency disturbance produces a compaction strengthening effect, microscopically promoting the generation of Ca(OH)₂ and ettringite (increased Ca/Si ratio). In contrast, the combination of high disturbance and high frequency induces free water extrusion and inhibits hydration, leading to an advanced damage threshold based on energy evolution and the accelerated coalescence of microcracks, which favors the formation of C-S-H gel (decreased Ca/Si ratio). Within this heterogeneous mechanism, the disturbance level acts as the dominant controlling factor. This study clarifies the nonlinear mechanical and chemical evolution paths under composite disturbances, providing theoretical support for the dynamic stability control of backfill in deep multi-step mining. Full article

To investigate the corrosion resistance and deterioration mechanism of cast-in situ concrete incorporating iron ore tailings aggregate (IOT), specimens with IOT replacement ratios of 0%, 30%, and 50% were exposed to distilled water, endogenous Cl⁻-SO₄²⁻ corrosion, exogenous Mg²⁺-SO₄²⁻ corrosion, and endogenous-exogenous coupled corrosion. The evolution of mass, size, compressive strength, and flexural strength was evaluated, while Nuclear Magnetic Resonance (NMR), Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS), X-ray Diffraction (XRD), and Thermogravimetric Analysis/Derivative Thermogravimetry (TG/DTG) were used to characterize pore structure and phase transformation. Results show that distilled water causes limited variation, whereas exogenous and coupled corrosion accelerate product accumulation, size expansion, pore coarsening, and strength degradation. Under exogenous Mg²⁺-SO₄²⁻ corrosion, the peak compressive strengths of specimens with 0%, 30%, and 50% IOT reach 43.30 MPa, 45.60 MPa, and 46.93 MPa, respectively, with the 50% IOT specimen showing an 8.38% increase compared with the specimen without IOT. TG/DTG results show that the Ca(OH)₂ related mass loss decreases from 5.42% under distilled water immersion to 4.37% under exogenous Mg²⁺-SO₄²⁻ corrosion, confirming calcium consumption during sulfate–magnesium attack. Microstructural characterization reveals that sulfate reaction, chloride binding, and Mg²⁺-induced decalcification jointly promote the formation of gypsum, ettringite, Friedel’s salt, magnesium silicate hydrate (M-S-H), and magnesium-associated corrosion products. Overall, 30% IOT provides better pore refinement and mechanical stability under endogenous and exogenous corrosion, whereas 50% IOT improves residual skeleton support under coupled corrosion. These findings provide guidance for durability design and sustainable utilization of IOT aggregate in cast-in situ concrete. Full article

In this study, the rapidly setting and hardening high-belite sulfoaluminate cement (HBSAC) is used as the cementitious material, with natural river sand as the fine aggregate, and a high-performance repair mortar is prepared through the synergistic use of different polymers and admixtures. The influences of two polymers (VAE and HPMC) on the working performance, mechanical properties, and hydration characteristics of HBSAC mortars are systematically studied. The results showed that the two polymers had a significant improvement effect on the setting time, mortar flowability, and water retention rate of HBSAC mortar. Among them, VAE had a significant effect on the mortar flowability, and a 5% content could increase the flowability of HBSAC mortar by 29.8%. HPMC has a significant improvement effect on setting time and water retention rate; at 0.1% content, it can delay the initial setting time by 6.5 min and achieve a water retention rate of over 90%. As the polymer to binder ratio increases, both polymers, except for 2.5% VAE, which can slightly improve the flexural strength of mortar, will reduce the flexural and compressive strength of mortar, with VAE causing greater damage to strength. On the contrary, the polymer significantly enhanced the bond strength of the mortar. Compared with the cement control group, the 28 d bond strength of 5% VAE and 0.1% HPMC groups increased by 56.7% and 15.1%, respectively. Moreover, the addition of polymers delayed the occurrence of the exothermic peaks of HBSAC dissolution and ettringite formation, but the total amount of hydration heat released within 48 h was higher than that of pure cement. The diffraction peaks of AFt in the hydration products of VAE-HBSAC paste at 3d and 28d showed significant enhancement, and the peak intensity increased with higher doping levels, while the diffraction peak intensity of C₂S showed a certain decrease. The polymer significantly increased the weight loss peak intensity and mass loss after heating of AFt, AH₃, AFm, and C-S-H gel. The SEM images indicate that VAE can form a mesh on the surface of hydration products and refine the crystal size of AFt; HPMC wraps more flocculent substances around the hydration products, thereby improving the compactness of paste. This study can provide scientific reference for improving the performance and promoting the practical application of high-performance rapid repair mortar for concrete structure damage. 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 study developed a composite solidification system for silt from the Yellow River Basin, utilizing calcium carbide slag, ground granulated blast-furnace slag, and desulfurized gypsum, in conjunction with the xanthan gum biopolymer. A Box–Behnken Design and Response Surface Methodology were employed, and the 14-day unconfined compressive strength was set as the focus of this investigation, with four variables examined: the content of the soil stabilizer, the xanthan gum-to-soil stabilizer ratio, the calcium carbide slag-to-soil stabilizer ratio, and the ratio of desulfurized gypsum to the soil stabilizer. The regression model demonstrated high significance (R² = 0.9798), with the xanthan gum ratio exerting the most substantial influence on the soil strength. The optimal proportions were determined to be 4.01% soil stabilizer content, 0.080 xanthan gum ratio, 0.143 calcium carbide slag ratio, and 0.110 desulfurized gypsum ratio. Microstructural analysis revealed that xanthan gum maintained hydration humidity through hydrogen bonding, facilitating the formation of C-(A)-S-H gels and ettringite crystals. This organic–inorganic structure effectively reduces porosity, although excess xanthan gum can impede hydration. This approach advances the sustainable utilization of industrial waste and environmentally friendly stabilization of the Yellow River silt. Full article

Bacteria in concrete has been studied as an additive to repair microcracks and reduce permeability, as well as increase compressive strength. Within the broad spectrum of bacteria, two types promise to be effective agents against external sulfate attack: (i) Bacillus subtilis, which could indirectly prevent the entry of sulfates through the mechanism of sealing by calcium precipitation, and (ii) Bacillus licheniformis, which could encapsulate the sulfates that enter by diffusion and prevent the consequences of the pathology, such as expansion and loss of strength. This research evaluates the impact of B. subtilis and B. licheniformis on the performance of cementitious mixes against external sulfate attack, measuring compressive strength, expansion, permeability, and effects on the microstructure. Results show that both bacteria can produce compressive strength improvements of up to 20% at 28 days and 50% at 180 days. Moreover, in the presence of sulfates, improvements of up to 90% can be observed over control mixes. However, this result should be carefully evaluated because although B. licheniformis produces better results in the long term, it results in lower strength in the presence of sulfates in the short term. At the same time, B. licheniformis significantly reduces expansion against external sulfate attack, decreasing it by up to 80%, because it generates less ettringite and gypsum. Thus, B. licheniformis is an effective agent against external sulfate attack. Based on the results, it is estimated that both bacteria can be used to improve performance; however, care must be taken with concentration, which affects homogeneity or generates negative effects. In particular, it is noteworthy that calcium carbonate loss was observed from the mixes due to continuous curing and that calcium precipitation can generate negative effects against sulfates in the long term. Full article

A quaternary solid-waste-based binder was prepared from low-titanium slag, municipal solid waste incineration (MSWI) fly ash, steel slag, and flue-gas desulfurization gypsum (FGDG) to clarify the activating effect of MSWI fly ash on low-titanium slag and its influence on hydrate evolution. Unlike conventional solid-waste-based binders in which MSWI fly ash is mainly regarded as a hazardous residue requiring stabilization, this study demonstrates its specific role as a Ca-rich alkaline activator for promoting low-titanium slag depolymerization and coordinated hydrate formation. The results showed that the compressive strength first increased and then decreased with increasing MSWI fly ash content. Considering both strength development and MSWI fly ash utilization, the optimum mixture was identified as low-titanium slag:MSWI fly ash:steel slag:FGDG = 43.0:17.2:25.8:14.0, with compressive strengths of 9.51 and 46.32 MPa at 3 and 90 d, respectively. These values corresponded to 5.66 and 1.04 times those of the reference mixture without MSWI fly ash, respectively. Ettringite and C-(A)-S-H gel were the main strength-contributing hydration products, while Friedel’s salt was identified as a chloride-bearing AFm phase. Moderate MSWI fly ash addition promoted alkaline activation and low-titanium slag depolymerization, leading to increased formation of ettringite, C-(A)-S-H gel, and Friedel’s salt, which contributed to improved compressive strength. In contrast, excessive MSWI fly ash disturbed the Ca-Si-Al balance and inhibited effective hydrate formation. These results demonstrate that MSWI fly ash can serve as an effective Ca-rich activator for low-titanium-slag-based low-carbon cementitious materials and provide a feasible route for the synergistic utilization of multiple solid wastes. Full article

To balance ultra-early strength development and workable time in cement-based grouting materials for rapid repair applications, an ultra-early high-strength grout system was developed by regulating the dosage of an accelerating agent (CF), retarder content, and water-to-binder ratio (w/b). The effects of these parameters on setting behavior, workability, mechanical properties, volumetric stability, and durability were systematically investigated. X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM/EDS) were further conducted to qualitatively evaluate the hydration characteristics and microstructural evolution of the optimized system. The results showed that CF accelerated early hydration and promoted the rapid formation of ettringite (AFt), which contributed to the development of ultra-early strength. The incorporation of a retarder effectively prolonged the workable time and improved slurry workability. Increasing the w/b ratio enhanced flowability and toughness, although excessive w/b reduced compressive strength. The optimal mixture contained 30% CF, 0.02% retarder, and a w/b ratio of 0.19. Under this condition, the grout exhibited a flowability of 312 mm and compressive strengths of 81.4 MPa at 1 h and 121.3 MPa at 28 d. In addition, low air shrinkage (0.027% at 28 d) and excellent chloride penetration resistance (12 C at 28 d) were achieved. Microstructural observations suggested that the dense structure formed by AFt and C–S–H gel contributed to the improved macroscopic performance. This study provides an engineering-oriented reference for the mix design and performance optimization of ultra-early high-strength cement-based grouting materials for rapid repair applications. Full article

To promote the high-value utilization of industrial solid wastes and address the disposal of excavated soils, a novel low-carbon composite cementitious material, solid waste-based geopolymer cement (SGPC), was developed, consisting of soda residue (SR), granulated blast furnace slag (GGBS), phosphogypsum (PG), and ordinary Portland cement (PC) in a mass ratio of 10:81:9:25, with industrial solid wastes accounting for 80% of the binder. The effects of water-to-solid ratio (W/S = 0.41–0.49) on the workability, mechanical performance, and microstructural evolution of SGPC-stabilized soil were systematically investigated to provide a sustainable alternative to conventional cement-based stabilizers. The results indicate that the optimum water-to-solid ratio is 0.43 (SGPC43), with a 28-day unconfined compressive strength of 1450 kPa, exceeding the engineering requirement of 0.8 MPa and reaching over 85% of that of a pure cement system (C43). The flowability remained 163 mm after 60 min, with initial and final setting times of 43 h and 58 h, respectively. Microstructural analysis revealed that the alkalinity provided by soda residue promotes the hydration of slag and phosphogypsum, forming interwoven calcium (alumino) silicate hydrate (C–(A)–S–H) and ettringite (AFt), which fill pores and form a dense structure, thereby enhancing mechanical performance. Environmental and economic assessments show that the CO₂ emission of SGPC43 per ton of binder decreases from 930 kg CO₂-e/t to 235 kg CO₂-e/t (approximately 74.7% reduction), while the material cost decreases from 110 USD/t to 53 USD/t (approximately 51.8% reduction). A simplified uncertainty analysis indicates that the carbon reduction remains at 70% ± 5% and the cost reduction at 50% ± 5%, confirming the robustness of the results. Overall, SGPC43 demonstrates excellent engineering performance, environmental benefits, and economic feasibility, highlighting its potential as a low-carbon and sustainable stabilizing material. Full article

Seawater and sea sand as constituents in concrete are valuable alternatives to freshwater and river sand. Further, the use of seawater and sea sand in projects located in the proximity of a sea/ocean can reduce the overall project cost and lower the carbon footprint. Nevertheless, seawater contains high concentrations of chloride (Cl⁻), sulphate (SO₄²⁻) and magnesium (Mg²⁺), which can react with tricalcium aluminate (C₃A) in cement and the byproduct calcium hydroxide (Ca(OH)₂), and form Friedel’s salt, delayed ettringite and brucite, respectively. These chemical compounds are aggressive and can degrade the strength and durability of the concrete. Differences in the physical properties of sea sand compared to river sand can also lead to weak and porous concrete. In reinforced concrete, steel bars are susceptible to corrosion due to the formation of corrosion products as a result of high concentrations of Cl⁻. Whilst mitigation strategies such as the use of supplementary cementitious materials (SCMs) and fibre-reinforced polymer (FRP) reinforcements have been investigated in the literature, no validated method that enables the use of concrete with seawater and sea sand has been established. Based on research reported in the literature, the present study investigates the chemistry, strength and microstructure of concrete mixed with seawater and sea sand as a means of establishing their use in concrete without compromising the properties of the concrete. The study shows that the compressive strength of seawater–sea sand mixed concrete (SWSSC) is increased in the short term (up to 28 days) due to the formation of additional chemical compounds in the former. However, the long-term (i.e., beyond 28 days) compressive strength of concrete reduces by up to 20% after one year due to the weakening of the microstructure (more flaws/expansions), which further reduces the durability of the reinforced concrete. Although the long-term degradation of SWSSC has been noticed, the underlying causes are not fully understood. The present critical review study provides chemical and microstructural insight into the degradation of concrete with seawater and sea sand, and the current developing understanding is used to develop a mitigation strategy towards the use of seawater and sea sand in real-world concrete applications. Full article

Cemented backfill (CTB) in mining operations is frequently exposed to blasting-induced dynamic disturbances. These disturbances compromise CTB stability. This study employed split Hopkinson pressure bar (SHPB) tests, scanning electron microscopy (SEM), and high-speed imaging to characterize dynamic mechanical response and failure modes of CTB. The results showed that dynamic compressive strength (DCS) increased with impact load. However, damage severity also increased with load. High-speed imaging of crack evolution yielded fractal dimensions between 0.5 and 1.3. Higher fractal dimensions correlated with more extensive macroscopic cracking and greater CTB damage. They also indicated an increased probability of CTB failure and loss of load-bearing capacity. Microstructural observations identified ettringite (AFt) and C-S-H gel as the principal hydration products that provided cohesion and strength. However, pervasive microcracks and micropores accounted for the CTB’s low inherent strength. The findings suggest that reducing dynamic loads during mining operations can mitigate damage to CTB. Full article

Granulated blast furnace slag is a commonly used supplementary cementitious material in cement-based materials. The raw materials for ironmaking and the cooling process affect its composition, thereby influencing its reactivity. Three types of slag were selected and incorporated at replacement ratios of 15%, 30%, and 50% to investigate the influence of chemical composition on the activity index of slag at different ages and the mechanisms. The results indicate that in the early hydration stage, slag primarily plays a mechanical filling and dilution role (inert volumetric occupation without significant heterogeneous nucleation), while the pozzolanic effect dominates at later stages. Al₂O₃ in the slag is activated at early ages to form ettringite; at replacement ratios of 30%, C-A-S-H gel is also formed at later ages; when the replacement ratio reaches 50%, the significant reduction in cement clinker content leads to dropping in system alkalinity—corresponding to a 50% reduction in cement-derived Ca(OH)₂, the activation of Al₂O₃ in the slag is not significant at early ages. The effects of glass content, alkali content, specific surface area, CaO + MgO content, quality coefficient, and basicity coefficient on the reactivity become prominent at longer ages. No additional crystalline phases beyond those present in pure cement paste were detected in the cement paste after slag incorporation. This study provides a theoretical basis and data support for the high-value utilization of industrial solid waste in green building materials. Full article

Sodium sulfate-activated slag cement is considered a highly promising low-carbon cementitious material; however, its application is limited by low early-age activation efficiency and slow strength development. This study aims to systematically elucidate the coupled regulatory mechanism of alkalinity (2% and 4% Na₂O equivalent) and sodium sulfate dosage on the performance of alkali-activated slag (AAS). Under standard curing conditions (20 ± 2 °C, relative humidity ≥ 95%), the macroscopic properties of the samples (workability, setting time, and compressive strength) and the evolution of their microstructure (analyzed by XRD, FTIR, and SEM-EDS) were evaluated. The results indicate that the effect of sodium sulfate on alkali-activated slag (AAS) strongly depends on the alkalinity. Under low-alkalinity conditions (2% Na₂O), sodium sulfate exhibits a synergistic activation effect by increasing the ionic concentration, promoting slag depolymerization and the nucleation of ettringite (AFt). Specifically, compared with the control, incorporating 6 wt% sodium sulfate (N2S6 mix) increased compressive strength by approximately 82% at 3 days and 21% at 28 days. In contrast, under high-alkalinity conditions (4% Na₂O), excessive sodium sulfate (≥2 wt%) shows an inhibitory effect. This is likely because an excess of sodium sulfate interferes with the normal polymerization pathways of the aluminosilicate network, suppressing the formation of the primary C-(A)-S-H gel and thus significantly reducing later-age strength. Microstructural analysis revealed that the hydration products in the composite-activated system mainly consist of C-(A)-S-H gel, ettringite (AFt), monosulfate (AFm), and hydrotalcite. This study investigates the observed kinetic trends of anion-competitive hydration under different alkalinity conditions, providing a theoretical basis for the mix design of low-carbon alkali-activated materials and the valorization of coal chemical industrial salts. Full article

In recent years, there has been an urgent need for integrated solutions to synergistically manage industrial solid waste stockpiling and CO₂ emissions. Single-component solid waste mineralization, such as those using only fly ash (FA) or carbide slag (CS), often encounters performance bottlenecks, typically characterized by a compressive strength of less than 2 MPa and a carbonation efficiency of under 10%. Furthermore, a systematic quantitative understanding of the synergistic interactions within multi-component systems remains absent. This study employs Response Surface Methodology to investigate the interactive effects of solid waste ratios, the water-to-solid ratio, and alkali content, aiming to elucidate the synergistic mineralization mechanism and overcome the bottlenecks of single solid waste mineralization. Under optimized conditions—specifically, 34% CS, 30% phosphogypsum (PG), a water-to-solid ratio of 0.48, and an alkali content of 27%—the system achieved a 7-day compressive strength of 3.5 MPa and a CO₂ mineralization efficiency of approximately 16%, representing a significant improvement over typical single solid waste mineralization materials. Microstructural and spectroscopic analyses indicate that CS serves a dual function as both a calcium source for CaCO₃ precipitation and an alkaline activator for FA. FA constructs a dense aluminosilicate network via pozzolanic reactions, while SO₄²⁻ released from PG promotes the formation of ettringite, facilitating efficient pore filling and early strength development. Additionally, it was observed that surface pores were filled with more products compared to the interior, forming a gradient pore structure that is dense on the outside and sparse on the inside. The AFt and silicate gel were identified as the key microstructural driver for the performance enhancement. This study not only explores the ternary synergistic mechanism of FA, CS, and PG but also provides a viable pathway for developing high-performance solid waste-based mineralization materials that combine mechanical properties with efficient CO₂ sequestration. Full article

This study evaluates the technical feasibility, environmental sustainability, and economic viability of producing sulfoaluminate cement (SW-SAC) by co-utilizing bauxite and industrial solid wastes—phosphogypsum, calcium carbide residue (CCR), and red mud—with the solid wastes accounting for approximately 75% of the raw meal. CCR replaces limestone as the primary CaO source, releasing H₂O instead of CO₂, while phosphogypsum supplies SO₃; the raw meal is directly calcined in a single step at 1300–1350 °C, 100–150 °C below that of ordinary Portland cement (OPC). Calcination temperature and holding time were optimized through phase analysis, microstructural observation, free lime (f-CaO) determination, and strength testing. SW-SAC meeting the 42.5 strength class was then prepared using phosphogypsum as a setting regulator and phosphorus slag or limestone powder as Supplementary materials. X-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscopy (SEM) were used to examine hydration products and microstructural evolution. The optimized clinker was dominated by ye’elimite (${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$) and belite (C₂S). Phosphorus slag favored the formation of gel-like products at later ages, whereas limestone powder promoted ettringite (AFt) stabilization and monocarboaluminate (Mc) formation. SW-SAC exhibited a lower carbon footprint than both Type P·I Portland cement and conventional SAC, and a lower production cost than conventional SAC. These results demonstrate a promising low-carbon route for high-value utilization of industrial solid wastes. Full article