Sulphoaluminate cement faces several application challenges, such as high heat release during hydration and insufficient strength development at later stages. This paper investigates the effects of SiO2-CaO on the mechanical properties, autogenous shrinkage, and hydration mechanisms of solid waste-based sulphoaluminate cement blended with ordinary Portland cement. The results demonstrated that the addition of 1 wt.% SiO2-CaO increased compressive strength by 25.80% at 28 days and 13.54% at 90 days, while improving carbonation resistance by 71.47% at 60 days. SiO2-CaO mitigated autogenous shrinkage, reducing it by 47.12% at 28 days. In the early hydration stages, the CaO in SiO2-CaO reacted rapidly with water to form calcium hydroxide, which increased the pH of the cement pore solution and promoted the dissolution and hydration of cement clinker. In the later stages, SiO2 reacted with calcium hydroxide to form C-S-H gels, which enhanced gel content, improved pore structure, densified the cement matrix.

Titanium gypsum (TMG) is an industrial by-product generated during the chloride-process production of titanium dioxide (TiO2). Its large output combined with low utilization has led to significant environmental concerns. In this study, high-aspect-ratio calcium sulfate hemihydrate whiskers (CSHW) were synthesized from TMG by a hydrothermal method using lanthanum ions (La3+) and dodecyltrimethylammonium chloride (DTAC) as synergistic modifiers. To the best of our knowledge, this work presents the first systematic comparison of the action mechanisms of La3+ and DTAC as modifiers for TMG-based calcium sulfate hemihydrate whiskers. The mechanism of anisotropic growth regulation was also investigated. X-ray diffraction (XRD) and thermogravimetric (TG)-differential thermogravimetric (DTG) analyses confirmed that all products were alpha-CaSO4.0.5 H2O. The introduction of La3+ and DTAC did not alter the crystal structure or thermal stability, suggesting that modification primarily occurred through surface adsorption. Zeta potential, contact angle, Fourier-transform infrared (FTIR) spectroscopy, energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyses showed that La3 + and the quaternary ammonium group of DTAC preferentially adsorbed on the negatively charged lateral facets. This increased the surface potential and hydrophobicity, enhancing interfacial stability. Scanning electron microscopy (SEM) results revealed that the modifiers were enriched on the (200), (110), and (110) facets, suppressing radial growth while promoting elongation along the c-axis. Under the optimal conditions (2.0 wt% La3+ and 0.25 wt% DTAC), the whiskers achieved a maximum aspect ratio of 98.57, which was approximately 186.62% higher than that of the control sample. Density functional theory (DFT) calculations confirmed stronger co-adsorption on the lateral facets, particularly the (200) facet, where orbital hybridization between La 5d, N 2p, and O 2p stabilized the lateral facets and promoted growth along the c-axis. The present work provides a feasible strategy for preparing high-aspect-ratio CSHW from TMG and further clarifies the crystal growth mechanism governed by the synergistic interaction between inorganic salts and surfactants.

The rapid expansion of renewable energy systems has increased the demand for safe, low-cost, and structurally compatible energy storage technologies. Cement-based supercapacitors are promising candidates; however, their practical deployment is hindered by the intrinsic trade-off between mechanical strength and ionic conductivity. Herein, a tunable porous calcium sulphoaluminate (CSA) foam membrane is engineered via a chemical foaming strategy using H2O2, enabling simultaneous regulation of microstructure and ion transport pathways. The foaming process induces an interconnected pore network that markedly enhances ionic transport while preserving structural integrity. At an optimal H2O2 dosage of 1.0 wt%, the CSA membrane achieves an ionic conductivity of 22.96 mS cm-1, approximately 3.7 times higher than that of the dense membrane, while maintaining adequate mechanical strength. When employed as a separator in a structural supercapacitor, the device delivers a high areal capacitance of 540.44 mF cm-2 at 1 mA cm-2 and retains over 92.43% of its capacitance after 1000 cycles. This work offers a feasible route for developing multifunctional, sustainable, and mechanically robust cement-based structural energy storage devices.

Phosphogypsum is a solid waste generated in the process of manufacturing phosphate fertilizers. A current high-value-added utilization approach involves preparing it into the widely utilized alpha-hemihydrate gypsum (alpha-HH) via the conventional hydrothermal synthesis method. Nevertheless, this method is time-consuming and energy-intensive. Microwave hydrothermal synthesis emerges as a more favorable alternative owing to its strong penetrating power and rapid heating rate. Consequently, in this study, a comparative investigation was conducted on the synthesis of alpha-hemihydrate gypsum using the conventional hydrothermal method and the microwave hydrothermal method. Density-functional theory (DFT) was employed to conduct a systematic analysis of how EDTA affects the synthesis of alpha-HH. In addition, a series of material characterization techniques were employed to clarify the underlying modification mechanism. Experimental results indicate that, under identical parameters, both methods produced alpha-HH gypsum after 4 h of hydrothermal treatment. However, the microwave hydrothermal method yielded products with superior performance. With the addition of 1.2 % EDTA under microwave hydrothermal conditions, the resulting sample exhibited an average L/D ratio of 1.46 and a compressive strength of 39.26 MPa-51.8 % higher than that achieved by the conventional hydrothermal method-while reducing energy consumption by 105.4 % under the same treatment duration. These findings demonstrate that EDTA can ionize carboxyl groups, enabling them to form stable complexes with Ca2 + on the surface of alpha-HH via coordination. This process hinders the growth of the crystal c-axis and thereby regulates the crystal morphology, establishing EDTA as an effective crystal modifier. Consequently, microwave hydrothermal synthesis of alpha-hemihydrate gypsum proves to be more efficient than the conventional hydrothermal method and exhibits favorable environmental and economic advantages.

The substantial accumulation of industrial by-product gypsum, primarily from China's power generation, phosphate fertilizer, and fluorine chemistry sectors, poses a significant environmental challenge. To address this issue and valorize the waste, this study provides, to the best of our knowledge, the first systematic mechanistic comparison of successfully synthesized high-aspect-ratio calcium sulfate hemihydrate whiskers (CSHW) from titanium gypsum (TMG) via a hydrothermal method, using dodecyltrimethylammonium chloride (DTAC) or tetramethylammonium chloride (TMAC) as crystal modifiers. The adsorption behaviors and underlying growth mechanisms were systematically investigated through integrated experimental and theoretical approaches. Optimal aspect ratios were achieved at 0.50 wt% DTAC and 0.75 wt% TMAC, corresponding to significant enhancements of 107.2 % (reaching 71.24) and 48.9 % (reaching 51.19), respectively. Mechanistic analyses revealed that the (CH3)3N+ groups of both surfactants chemically adsorb onto the side facets, inhibiting radial growth and promoting axial elongation. Crucially, DTAC, with its dodecyl chain, forms hemi-micellar or bilayer adsorption structures-a capability lacking in TMAC-which endows the CSHW with markedly enhanced hydrophobicity and a higher surface potential, both of which are crucial for improving interfacial compatibility and reinforcement efficiency in composite materials.

Fluorogypsum (FG, CaSO4-II) originates during hydrofluoric acid production, has low hydration reactivity and contains high levels of free fluoride ions (F-), posing risks to soil and water quality. To enable the secure application of FG, this study evaluates sodium molybdate (Na2MoO4) as a modifier to enhance FG's hydration reactivity and immobilize F-. A novel method was employed, in which the FG paste was centrifuged at different time points prior to initial setting. The supernatants were then analyzed for calcium ions (Ca2+), molybdate ions (MoO42-), sulfate ions (SO42-), and F-, enabling real-time tracking of ion behavior and revealing the mechanism of F- immobilization. Based on these data, we provide a systematic explanation of anhydrite (AH) (CaSO4) hydration and hardening within a precipitation-dissolution equilibrium framework. Results show that MoO42- consume Ca2+ by precipitation transformation, driving the dissolution equilibrium of anhydrite to shift to the right. This induces AH to dissolve until reaching the solubility product constant (Ksp) of dihydrate gypsum (DH) (CaSO4·2H2O) without exceeding that of AH, thereby triggering DH crystallization in a positive feedback loop. Meanwhile, the formation of insoluble calcium molybdate provides heterogeneous nucleation sites that accelerate the growth of DH. With 0.7% Na2MoO4, the 28-day strength reaches 49.1 MPa, approximately 10 times that of untreated FG. Under extreme pH conditions and high temperature, this approach suppresses over 90% of F- leaching by means of chemical fixation and physical encapsulation. This modification offers a solution for stabilizing hazardous FG and converting it into safe outdoor building materials.

The highly polymerized aluminosilicate framework in Class F fly ash limits its reactivity and poses a significant constraint on its use in high-volume fly ash cementitious materials (HVFC). In this study, a novel co-sintering method with Na2SO4 at temperatures of 600, 700, 800 and 900 degrees C was employed to enhance reactivity by depolymerizing the aluminosilicate network within fly ash. The introduction of externally resourced Na2SO4 reduced the content of 4-connected tetrahedra and accelerated the dissolution of fly ash. As a result, the reaction degree of fly ash in 28 days HVFC was promoted from 3.1% of the raw fly ash to 19.3% of that co-sintered with Na2SO4 at 900 degrees C. The initial and final setting time of HVFC reduced from 437 and 580 min to 295 and 418 min, respectively. Additionally, more hydration products were observed in HVFC containing sintered fly ash, leading to an increase in compressive strength by up to 20%. A mechanistic model based on the shielding effect of peripheral tetrahedra linked to 4-connected tetrahedra was developed to account for the enhanced reactivity of depolymerized Class F fly ash.

The development of Low Calcium Carbonatable Binder (LCCB) with high carbonation performance is crucial for mitigating the environmental impact of the cement industry. This study aims to elucidate the mechanism by which tungsten (W) doping enhances the carbonation activity and mechanical properties of LCCB. Employing a combined approach of Density Functional Theory (DFT) computational predictions and experimental validation, we systematically investigated the doping behavior and its multi-scale effects. The results indicate that W6 + preferentially substitute for Si sites in the alpha-CS phase, a process driven by negative formation energy and compatible electronic structures. This substitution induces significant structural distortion in the adjacent Ca-O polyhedra, characterized by elongated bond lengths and a reduced coordination number, which weakens the crystal lattice and facilitates Ca2+ ion migration. At an optimal doping content of 4.6 wt%, this atomic-scale activation translates into accelerated carbonation reaction kinetics, as evidenced by a higher exothermic peak temperature and a shorter time to reach the peak. Consequently, the yield of calcite increases substantially, leading to a remarkable 75.8% improvement in the 8 h compressive strength compared to the undoped sample. This work provides fundamental guidance for the performance-oriented design of advanced carbonatable binders materials through ion doping.

Nano-engineered cementitious materials (NECMs) integrated with superior mechanical performances, enhanced durability and multifunctionality, are represented as the next-generation infrastructure materials. Nevertheless, a considerable gap remains between laboratory-scale investigation and industrial-scale implementation of NECMs. This review aims to provide a comprehensive understanding of NECMs performances with the objective of elucidating their potential for practical application. Firstly, the production and dispersion of nanomaterials are systematically reviewed, which primarily govern the cost and industrial feasibility of NECMs. Afterwards, the progress of the research on the hydration kinetics, microstructure, mechanical performance, durability and functionalities of NECMs is analyzed and summarized. In addition, representative case studies of NECMs in structural, industrial and large-scale applications are presented to illustrate the potential prospects for the development of NECMs. Through the comprehensive evaluation of NECMs, this review aims to offer valuable insights to guide both academic research and practical implementation, promoting the sustainable and large-scale application of NECMs in modern construction.

To address the cross-sensitivity and non-linear coupling issues caused by the coexistence of hydrogen, carbon monoxide, ammonia, and nitrogen dioxide in industrial environments, a flow-through quantitative detection system based on a MEMS gas sensor array was designed and constructed. The steady-state peak sampling method was employed for feature extraction from high-dimensional time-series data, and regression prediction models were developed using a traditional BP neural network and BP neural networks optimized by four swarm intelligence algorithms (ALA, AOO, SFOA, and SDO). The experimental results indicate that the intelligent optimization algorithms excel in decoupling the "cross-response" phenomenon, with all optimized models outperforming the traditional BP network. Among them, the SDOBP (Sledge Dog Optimizer-BP) model demonstrated the best overall performance, achieving the highest accuracy in carbon monoxide and hydrogen detection, with the Root Mean Square Error for hydrogen reduced to 2.17, an 84.2% improvement over the traditional model. The system achieves high-precision quantitative inversion of multi-component gases in complex environments, providing an effective means for industrial environmental safety monitoring.

This study employed a mixed microbial culture (MB) comprising Bacillus subtilis (BS), Bacillus polymyxa (PM), and nitrate-reducing bacteria (NRB) in equal proportions. The mixed microbial culture was used to enhance recycled brick aggregate (RBA) through the microbial-induced carbonate precipitation (MICP) method, thus investigating the effects of this enhancement on both the aggregate and recycled mortar properties. Results indicate that the mineralization activity of the mixed culture significantly exceeded that of individual strains, achieving an 84.64% mineralization rate after 14 days. MICP-enhanced RBA demonstrated markedly improved performance. The compressive strength of the reinforced recycled mortar increased by 32.62% at 3 days and 22.6% at 28 days, with the 28-day compressive strength approaching that of cement mortar using natural aggregates. The interfacial transition zone (ITZ) properties were significantly improved, with their width reduced from 30 to 35 mu m to 20 to 25 mu m. This study provides experimental evidence for RBA reinforcement technology while offering technical supportfor the resource use ofRBAs.

Shrinkage-reducing admixtures (SRAs) can effectively mitigate the drying shrinkage of cement-based materials, but the influence of SRA structures with varying configurations on drying shrinkage remains unclear. This study mainly synthesized two types of SRA by using different shrinkage-reducing monomers and investigated the impact of SRA with distinct branch chain structures on the surface tension, contact angle, and viscosity of solution. Then, the drying shrinkage, mechanical properties, and microstructure of cement-based materials with SRA were further discussed. The findings indicated that Type II SRA with a complex branched-chain structure presents a greater steric hindrance effect, leading to a more significant decrease of surface tension and increase of solution viscosity and contact angle than that of Type I SRA. Meanwhile, samples with Type I SRA presented a smaller drying shrinkage, whereas Type II SRA showed a stronger reduction in surface tension, indicating the mechanism of SRA inhibiting the drying shrinkage is not limited to the reduction of surface tension. Pores below 50 nm are conducive to reducing drying shrinkage, and the pore volume of sample with Type I SRA was significantly reduced by 20.3%, which showed an excellent drying shrinkage inhibition effect.

Sulfoaluminate cement-based grouting materials (SAGMs) are frequently employed in projects concerning reinforcement and repair. Retarders are often added to SAGMs to improve their workability by delaying the hydration process. The current investigation examines the impact of magnesium fluosilicate (MgSiF6), a byproduct of phosphate fertilizer manufacturing, on the hydration and hardening process of SAGMs. The experimental results revealed that a low concentration (0.85-8.5 g/L) of MgSiF6 prolonged the setting time and decreased the compressive strength. In contrast, a high concentration (16.5-25 g/L) of MgSiF6 shortened the setting time and increased the compressive strength, but the cause of this phenomenon above is unclear. Via an examination of the impact of varying MgSiF6 concentrations on the dissolution of the primary mineral (C4A3S and CaSO4) as well as the formation process of their primary hydration products, it was discovered that low MgSiF6 concentrations inhibited the dissolution of C4A3S and CaSO4 while high MgSiF6 concentrations promoted the dissolution of C4A3S and CaSO4. The addition of MgSiF6 inhibited the formation of ettringite, and low and high MgSiF6 concentrations stimulated the formation of calcium fluoride and CaMg2Al2F12, respectively. The hydration product aluminum gel's particle size gradually decreased as the concentration of MgSiF6 increased. A low concentration of MgSiF6 generated CaF2 and hindered the formation of AFt while a high concentration of MgSiF6 promoted the dissolution and hydration of CaSO4 and C4A3S and formed nano Al(OH)3, which altered the pore structure and compressive strength.

This investigation demonstrates a pioneering approach for synthesizing highly carbonation-reactive fibrous alpha-wollastonite (alpha-CS) binder through crystal transformation activation of natural wollastonite, resulting in negative CO2 emission while achieving superior mechanical properties. The rapid crystal transformation activation from natural CS to alpha-CS was accomplished at 1200 degrees C, markedly enhancing carbonation reactivity while maintaining the inherent fibrous morphology. The transformed alpha-CS exhibited remarkable performance improvements compared to natural CS, with CO2 uptake increasing by 1.04 times, while compressive and flexural strengths were enhanced by 13.65 and 5.76 times, respectively. Enhanced carbonation reactivity was attributed to reduced Ca-O bond energy, increased crystal defect concentration, and expanded reactive surface area. The crystal transformation activation methodology presents significant advantages over conventional solid-phase sintering by eliminating carbonate decomposition and the associated sintering process, thereby substantially reducing fossil fuel consumption. Additionally, the carbonation curing process facilitates considerable CO2 sequestration, enabling the development of binders with negative CO2 emissions.

Reducing anthropogenic CO2 emissions remains a global challenge, with silicate minerals in steel slag (SS) significantly influencing its carbonation reactivity. This study synthesized three key mineral phases in SS - larnite ((3-Ca2SiO4), merwinite (Ca3MgSi2O8), and & aring;kermanite (Ca2MgSi2O7) - using solid-phase sintering. Their carbonation reactivity and hardening properties were systematically characterized, with intrinsic differences investigated using density functional theory (DFT). Carbonation reactions for all minerals concentrated in early stages, with 24-hour compressive strength of carbonated (3-Ca2SiO4 reaching 84.70 MPa, significantly higher than Ca3MgSi2O8 (37.89 MPa) and Ca2MgSi2O7 (35.18 MPa). Mg substitution negatively impacted carbonation reactivity and altered products from calcite to Mg-calcite, aragonite, and magnesite. Theoretical simulations showed calcium silicates exhibit higher carbonation reactivity than calcium-magnesium silicates due to Ca/Mg-O bond energy differences. The carbonation reaction involves electrophilic and nucleophilic attacks on ions, with electron cloud localization varying in structures. [Si2O7]6- in Ca2MgSi2O7 may affect ion dissolution during carbonation, potentially explaining its lower reactivity. This study provides valuable guidance for utilizing SS in high-performance CO2 sequestration materials.

This study investigated the carbonation reactivity, carbonation products, and carbonation hardening properties of three low-calcium minerals—larnite (β-C2S), brownmillerite (C4AF), and mayenite (C12A7). The results showed that C12A7 exhibited the highest carbonation reactivity. However, the carbonation hardening of β-C2S (78.2 MPa) was significantly higher than C4AF (25.6 MPa) and C12A7 (21.2 MPa). XRD and SEM analyses revealed that the carbonation of β-C2S produced calcium carbonate and a highly polymerized silica gel that formed a dense microstructure. In contrast, C4AF and C12A7 generated more porous products with aluminum and iron gels having inferior bonding. It is thus evident that the silica gel in carbonated β-C2S induces superior hardening compared to the gels from calcium aluminates. This study demonstrates that silica gel formation imparts exceptional carbonation hardening properties to β-C2S despite its lower carbonation reactivity. The results provide insights into designing the mineral compositions of low-calcium cement for effective CO2 sequestration.

Low Calcium Carbonatable Binder (LCCB) represents an innovative carbon-sequestering cementitious material primarily composed of alpha-CS (pseudo-wollastonite), C3S2 (rankinite), and C2AS (gehlenite). However, LCCB currently faces several challenges, including limited carbonation strength, slow carbonation kinetics, and suboptimal carbon sequestration efficiency. This study investigates a novel approach to enhance the carbonation activity of LCCB through the incorporation of nano-sized calcium carbonate (NC). The carbonation characteristics were systematically examined using advanced analytical techniques, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), pH measurement, conductivity analysis, low-field nuclear magnetic resonance (LF NMR), and scanning electron microscopy (SEM). These methodologies were employed to elucidate the mechanism from multiple perspectives: carbonation product composition, macroscopic properties, microstructural pore characteristics, and carbonation reactivity. The experimental results demonstrate that the optimal NC content of 0.10 % significantly enhances the carbonation performance of LCCB, not only significantly improves its carbonation performance but also promotes calcite growth, fills the pore structure to reduce porosity, and alters the micro-morphology post-carbonation. Furthermore, NC facilitates calcium ion dissolution, accelerates the reaction between CO2 and calcium ions, promotes continuous CO2 reaction within the system, and enhances the early-stage exothermic carbonation reaction. These findings not only advance the fundamental understanding of NC-modified LCCB carbonation mechanisms but also contribute to improving overall CO2 sequestration efficiency in cement-based materials, thereby supporting the sustainable development of the construction industry.

Currently, Low Calcium Carbonatable Binder (LCCB) is a novel building material characterized by its low calcium content, which has the ability to absorb CO2 during the hardening phase. However, its carbonation reaction activity remains relatively weak. This article explores the use of zinc ions doping to enhance its carbonation reaction performance. The preference for Zn ion doping in LCCB and its impact on carbonation properties were systematically investigated through experiments and Density Functional Theory (DFT) calculations. The findings revealed that Zn atoms preferentially occupy Ca sites in C3S2 mineral, with a solid solution limit for Zn2 + approximately at 1.5 mol%. This observation is substantiated by low defect formation energy, small regional structural distortions, comparable electron contributions, and overlapping bond order-bond length distributions between oxygen atoms and guest ions. Additionally, compressive strength, porosity, carbonation products, carbon sequestration rate, and microstructure of LCCB samples with different zinc ions contents were systematically characterized. The results indicated that the incorporation of Zn ions enhances the production of aragonite while concurrently reducing porosity. At a doping level of 1 mol% Zn, the carbonation performance is optimized, resulting in an increase in compressive strength and carbon sequestration rate by 45 % and 15.56 %, respectively. The doping of Zn ions can promote the formation of LCCB carbonation products and enhance carbonation and mechanical properties. Overall, these findings offer significant insights into the impact of ion doping on improving the carbonation characteristics of LCCB.

When gangue is applied to cementitious materials as fine aggregate, the defects of coal gangue fine aggregate (CFA), such as large porosity, high water absorption, and high surface tension, lead to poor mechanical properties of cementitious materials, which limits its large-scale application in the construction field. In this paper, CFA was strengthened using water glass strengthening, microbial-induced mineralization strengthening, and composite strengthening methods, and the effects of the three strengthening methods on the basic properties of CFA, the mechanical properties of mortar, and the interfacial transition zone (ITZ) were explored. The strengthening mechanism was investigated via X-ray diffraction (XRD), scanning electron microscope (SEM), nanoindentation, and other testing methods. The three strengthening methods were evaluated based on the basic principles of life cycle assessment (LCA). Microbial-induced mineralization can utilize the biological activity of bacteria to achieve targeted repair of the tiny pores of aggregates. Water glass strengthening fills CFA pores and coats the CFA surface with a substance such as a gel produced by the water glass. A composite strengthening treatment can utilize the synergistic reinforcement effect of two single treatments to improve the basic properties of the aggregate. The apparent density of the reinforced CFA increased by 7.69

Phosphogypsum (PG) pretreatment with calcium hydroxide (CH) effectively reduces environmental pollution but inversely decreases the mechanical strength of the hardened paste, which leads to its relatively low utilization rate in construction industry. Therefore, in order to enhance the properties of PG pretreated with CH, this study mainly investigate the addition of blast furnace slag (BFS) on the proeperties and microstructure of pretreated hemihydrate phosphogypsum by calcium hydroxide. The results indicated that the incorporation of BFS not only prolonged the setting time of PG, but also significantly enhanced mechanical properties and water resistance. The mechanical strength of samples with 40% BFS was increased to 15.7MPa at 28 days, which was 2.7 times compared with the reference sample. The analysis of phase composition change showed that the incorporation of BFS promoted the formation of new phases, such as ettringite (AFt) and calcium alumino silicate hydrate (C-A-S-H) gel, significantly altering the microstructure by affecting the relative content of hydration products. In addition, the incorporation of BFS could alter the crystallization behavior and morphology of gypsum, mainly caused by the dissolution rate of HPG particles and the nucleation and growth of gypsum due to the hydration environment change. Meanwhile, C-A-S-H gel encapsulated AFt and gypsum, imparting a dense microstructure characteristic. This study provides a theoretical foundation for optimizing PG and advances solid waste utilization on the development of sustainable building materials.