Search Results (133)

Fly ash (FA) recycling into cementitious binders offers a promising route to simultaneously reduce the environmental burden of high-impact construction materials, divert industrial waste from disposal pathways, and generate engineering value from an existing aluminosilicate residue. FA has therefore been widely incorporated into ordinary Portland cement (OPC), calcium sulfoaluminate (CSA) cement, magnesium potassium phosphate cement (MKPC), and alkali-activated/geopolymer systems, where it can improve workability, mechanical strength, durability and promote hydration reactions, depending on the binder chemistry. However, these benefits are not always successful: FA addition may also reduce early-age strength, delay setting, limit reactivity, impair fluidity, or produce under-reacted matrices when ash properties are mismatched with the chemistry window of the target binder. Rather than revisiting these systems as isolated application categories, this review develops a property-guided framework for interpreting and selecting FA across major cementitious routes. It first highlights why FA should not be treated as a single material. Key descriptors include glass content, fineness, calcium level, carbon residue, mineralogy, and beneficiation state. The review then compares four representative binder environments to clarify the role of FA shifts from pozzolanic contributor to filler-dominated hydration modifier to functional regulator, and finally to reactive precursor, including the calcium hydroxide (CH)-rich Portland systems, CH-poor CSA systems, phosphate-bonded MKPC systems, and alkali-activated/geopolymer systems. The central conclusion is that the key question is not simply where FA can be used, but which FA is best matched with which binder chemistry and for what performance objective. Full article

This study develops a rapid-hardening and high-fluidity ultra-high-performance cement (UHPC)-based grout by incorporating calcium sulfoaluminate (CSA) cement as an early strength component, along with steel and wollastonite fibers as hybrid reinforcements. The UHPC grout proportions containing different contents of CSA cement and wollastonite fibers were designed to investigate fluidity, mechanical properties, hydration kinetics, microstructure and chloride resistance. The results showed that both CSA cement and wollastonite fibers significantly enhanced the compressive and flexural strength of UHPC grout. The incorporation of CSA cement led to rapid compressive strengths of 23 MPa at 6 h and 75.9 MPa at 1 day, marking a significant enhancement compared to the reference group and indicating excellent early-age performance. CSA cement accelerated the hydration process of the UHPC grout and promoted formation of more ettringite. Wollastonite fibers and U-type expansive agent (UEA) further improved the mechanical performance through bridging and physical filling effects. Moreover, CSA cement and wollastonite fibers effectively optimized expansion behavior and refined pore structure of the UHPC grout, and improved its chloride penetration resistance. Although both components influenced the fluidity of the grout, the UHPC grout still maintained high fluidity, offering a promising outlook for its potential use in demanding engineering applications. Full article

To optimize the shape stability of 3D-printed white calcium sulfoaluminate (WCSA) cement composites, oyster shell powder (OSP) and cuttlebone powder (CBP) were introduced as white admixtures to regulate rheological properties and printability. The setting behavior, rheological properties, and shape stability of the WCSA cement composites were evaluated by Vicat setting-time tests, rotational rheological measurements, three-stage thixotropic recovery tests, and structural deformation measurements, together with mechanical strength tests, XRD, and SEM analyses. The results showed that the incorporation of OSP and CBP shortened the setting time of WCSA cement composites. The initial and final setting times decreased from 41 min and 67 min to 17 min and 30 min in the WCSA cement composites with OSP, and from 42 min and 66 min to 20 min and 33 min in the WCSA cement composites with CBP, which improved printing operability. As the OSP and CBP content increases from 0% to 24%, the dynamic yield stress of WCSA cement composites increased from 48.83 Pa to 530.59 Pa and 60.30 Pa to 1085.80 Pa, respectively. The thixotropic recovery degree of WCSA cement composites increased from 57.89% to 86.46%, and 56.60% to 92.14%, respectively. As the OSP and CBP contents increase from 0% to 24%, the structural deformation decreased from 12.39% to 6.91% and 13.29% to 5.12% respectively, which improved buildability of the printed structures. In addition, although OSP and CBP reduced the mechanical strength of WCSA cement composites compared with the control group, the flexural and compressive strengths gradually recovered as the contents increased from 6% to 24% due to the enhanced filling effect and improved particle packing. This study provides a reference for the application of marine calcareous solid wastes in sustainable 3D-printed cementitious materials. 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

Island reef road construction faces a complex marine service environment characterized by high salinity and high humidity. Meanwhile, rapid construction and prompt subgrade repair are urgently required, creating a strong demand for novel calcareous-sand-based stabilization materials that combine excellent mechanical performance with resistance to seawater erosion. To this end, this study developed an early-strength cemented calcareous-sand reinforcement material for road base construction. Sulfoaluminate cement (SAC) and ferrite-aluminate cement (FAC), both featuring rapid setting/early strength development and superior corrosion resistance, were used to cement calcareous sand (CS) and to investigate its mechanical and microstructural characteristics under different water environments. Unconfined compressive strength tests (UCS) showed that SC-CS and FC-CS could meet subgrade requirements at 1 d and 7 d, with SC-CS and FC-CS reaching 3.12 MPa and 3.44 MPa at 1 d, and 3.26 MPa and 3.67 MPa at 7 d, respectively, under seawater SS conditions. Seawater mixing and immersion were found to promote the early strength and stiffness development of both SC-CS and FC-CS, with a more pronounced effect observed for FC-CS. Based on experimental results, a damage model for the stabilized specimens was established with a fitting accuracy of R² > 0.97. This constitutive model accurately describes the stress–strain relationship of the material and quantitatively characterizes its damage evolution. Microscopic XRD and SEM analyses indicated that the main hydration product in freshwater-cured specimens was ettringite, and the interparticle connection of CS was dominated by bridging through rod-like ettringite. In contrast, under seawater conditions, the ettringite content decreased, while hydrotalcite and calcium aluminate hydrate increased, forming massive and lamellar bridging products. Compared with SC-CS, the bridging structure in FC-CS was denser. Moreover, the compactness of the bridging structure not only affected its mechanical properties but also governed the movement mode of CS particles, thereby influencing the damage evolution and failure mode of the specimens. The findings provide theoretical support for the construction needs of island road. Full article

With the requirement for reducing carbon footprint in engineering construction, porous vegetation concrete is increasingly receiving attention for use in completed slope restoration. Cemented soil is introduced after the completion of porous vegetation concrete stabilization and functions mainly as a revegetation substrate. An important consideration for cemented soil in this application is its ability to maintain strength and water stability and possess moisture retention capacity, without causing much increase in alkali release or diffusion. This present study investigated a newly developed twofold stabilization system involving both cement binders and organic waterborne epoxy resin to meet the requirements of synthetically enhancing slope stabilization and restoration. Changes in the unconfined compressive strength and water stability were analyzed, whilst mineralogical composition and microstructure characteristics were investigated. The results indicated that moderate incorporation of triethylenediamine (TETA)-cured epoxy resin (1–2% by soil mass) moderately reduced strength and increased water stability with controlled alkali release in cemented soil. Mineralogical and microstructural analysis revealed that TETA-cured epoxy resin retarded cement hydration and refined particle bonding, exhibiting less consolidated pore structure characteristics. The twofold stabilization was exceptional in enhancing structural stability exposed to repeated humidity variation, albeit it yielded increased strength reduction rate from <7% to 9–16% under UV irradiation. Potentials of calcium sulfoaluminate cement and Portland slag cement were also investigated. A pilot-scale vegetation trial with representative plant species gave general agreement with effects observed in the laboratory in alkali reduction and moisture retention. The results provided an ecological approach for better restoring completed slopes that were stabilized using porous vegetation concrete. Full article

Calcium–silicate–hydrate (C-S-H), the primary binding phase governing cement paste cohesion, undergoes progressive physicochemical transformation upon carbonation—a process that critically dictates concrete durability in atmospheric environments. When CO₂ penetrates the porous cement matrix, it triggers a cascade of degradation mechanisms: calcium leaching decalcifies the C-S-H structure, inducing polymerization of silicate chains from dimeric to longer-chain configurations, while concurrent precipitation of calcium carbonate and amorphous silica gel fundamentally reconstitutes the nanoscale architecture. These nanoscale alterations propagate to macroscopic property evolution, manifesting as initial strength and stiffness gains due to pore-filling carbonation products followed by eventual deterioration as the cohesive binding network deteriorates. This review synthesizes current understanding of carbonation-induced structural evolution, examining the coupled influences of environmental parameters—CO₂ concentration, relative humidity, and temperature—alongside C-S-H intrinsic chemistry (Ca/Si ratio, aluminum substitution, and alkali content) on reaction kinetics and material performance. However, significant knowledge gaps persist: predictive models for in-service carbonation rates remain elusive due to the disconnect between idealized laboratory conditions and the heterogeneous, cracked reality of field concrete; the causal linkage between nanoscale C-S-H alteration and macroscale cracking patterns along with physical performance is poorly resolved, and most mechanistic studies rely on synthetic C-S-H, neglecting the compositional complexity of real Portland cement systems. We further propose emerging protection strategies, including surface barrier coatings and low-carbon alternative binders (geopolymers, calcium sulfoaluminate cements, carbon-negative materials such as recycled cement), which demonstrate enhanced carbonation resistance. Future research priorities include developing effective coating barriers for carbonation protection, developing operando characterization techniques for real-time reaction monitoring, deploying machine learning algorithms to bridge atomistic simulations with structural-scale predictions, and establishing long-term field performance databases to validate laboratory-derived degradation models. Full article

The development of cementitious materials with multifunctional performance is increasingly important to address environmental demands and durability requirements in modern infrastructure. This study investigates calcium sulfoaluminate (CSA) cement partially substituted with blast furnace slag (BFS), fly ash (FA), and TiO₂ nanoparticles, aiming to combine sustainability with photocatalytic self-cleaning functionality. Phase analysis by X-ray diffraction confirmed the formation of characteristic CSA hydration products, including ettringite, ye’elimite, anhydrite, and calcite, indicating that partial substitution did not disrupt the primary hydration mechanisms. Microstructural observations revealed that the incorporation of BFS, FA, and TiO₂ induced noticeable morphological changes, with increased porosity and microstructural heterogeneity at higher replacement levels. Mechanical testing showed that moderate BFS contents of 5 to 10 wt% enhanced compressive strength in reference mixtures, while systems containing TiO₂ exhibited slightly lower strength values and increased dispersion, particularly at elevated slag contents. The photocatalytic performance, evaluated through Rhodamine B degradation under solar irradiation, demonstrated a marked improvement for TiO₂-containing samples, reaching degradation efficiencies of up to 80%, in contrast to negligible activity in unmodified systems. These results confirm that the combined use of industrial by-products and photocatalytic nanoparticles in CSA-based matrices represents a viable strategy for producing sustainable cementitious materials with added environmental functionality, without compromising fundamental structural performance. Full article

Alkali-activated materials can significantly reduce carbon dioxide emissions compared with cement. However, their durability remains insufficiently understood. This study investigated the effects of calcium hydroxide (Ca(OH)₂, CH), an expansion agent (calcium sulfoaluminate, CSA), and a shrinkage-reducing agent (SRA) on the compressive strength and length change and determined the optimal content levels for each agent. Experiments were conducted to evaluate the compressive strength and length change of 17 mortar mixtures containing CH, CSA, and SRA. The substitution ratios of CH, CSA, and SRA were fixed at three predefined levels for each factor. The microstructural changes induced by the use of each agent were analyzed using pH measurements, porosity analysis, and X-ray diffraction. In addition, the water desorption behaviors associated with CSA and SRA were assessed. Experimental and statistical analyses demonstrated that the optimal contents of CH, CSA, and SRA for simultaneously improving the compressive strength and length change were 8.54, 10.0, and 0.76 wt.%, respectively. The use of CSA significantly enhanced the compressive strength development and dimensional stability of the mortar. This improvement was associated with a reduction in the porosity, which was attributed to ettringite formation. Furthermore, while the SRA slightly reduced the compressive strength, it significantly improved the dimensional stability. Full article

This study investigated the effects of calcium carbide slag (CCS) (0–12 wt%) incorporation on the workability, electrochemical properties, durability, and microstructure evolution of sulfoaluminate cement (SAC) mortar. Results showed that increasing CCS content reduced mortar fluidity and shortened setting time, indicating that CCS accelerates early hydration. A 9% CCS content was determined to be the optimal dosage; at 28 days, compared to the control group, this dosage group exhibited a 6.53% increase in compressive strength, a 22.47% decrease in drying shrinkage, and a 0.279% decrease in mass loss. These performance improvements stemmed from CCS’s ability to inhibit pore connectivity and limit moisture migration. Electrochemical analysis further revealed that the 9% CCS dosage group had the highest charge transfer resistance and resistivity (30.00% higher than the control group), reflecting a denser matrix and greater ion transport resistance. Consequently, chloride ion permeability was significantly reduced, with electrical flux and diffusion coefficient decreasing by 39.98% and 28.89%, respectively. Microstructural observations confirmed that CCS promotes the formation and densification of hydration products, effectively improving the internal pore structure. While 9% CCS can serve as an effective functional supplementary material, its long-term durability and sustainability still face practical application challenges. Future research should focus on establishing predictive models for chloride ion permeation lifetime and conducting quantitative sustainability assessments of CCS-SAC composites, particularly evaluating material cost, energy consumption, and carbon dioxide emissions. Full article

This study addresses the technical challenges of using construction residue sand (CRS) with high mud content in 3D printing by developing a novel sustainable mortar system. The key novelty lies in optimizing the mix design with calcium sulfoaluminate cement (CSA) and defining a quantitative printability window. Results show that at a CSA dosage of 10–15% and a sand–binder ratio of 1.4–1.5, the mortar achieves excellent printability, with a fluidity of 165–195 mm and a slump of 10–40 mm. Mechanistically, CSA promotes AFt formation, refining pore structure, and enhancing mechanical strength. This work provides a high-value pathway for recycling construction waste and advancing green intelligent construction. Full article

Calcium sulfoaluminate–calcium oxide expansive agents (HCSA) are commonly used in mass concrete to compensate for thermal shrinkage. However, the ettringite (AFt) formed by HCSA hydration decomposes when temperatures exceed 70 °C. This study examines the synergistic effects of curing temperature (20 °C to 80 °C), fly ash (FA) content (0%, 40%), and water–binder ratio (w/b, 0.3, 0.4, 0.5) on the expansion behaviour and microstructure of HCSA–cement systems. A critical temperature threshold was identified at 60 °C. Below this limit, elevated temperatures accelerate hydration and enhance expansion, with the restrained expansion ratio peaking at 9.2 × 10⁻⁴ mm/mm under 60 °C curing. Beyond 60 °C, expansion capacity significantly diminishes due to the thermal decomposition of AFt into monosulfoaluminate (AFm), as confirmed by XRD and SEM analysis. Calculations of expansive stress reveal a critical mismatch at temperatures ≥ 40 °C, where the expansive stress exceeds the early-age tensile strength, causing microstructural damage. Furthermore, subsequent cooling to standard curing conditions triggers the reformation of AFt from AFm, leading to Delayed Ettringite Formation (DEF), which poses potential risks for late-stage cracking. AFt morphology shifted from needle-like (2–5 μm) to prismatic (5–8 μm). The incorporation of FA suppressed early-stage expansion but improved expansion stability. above 40 °C, although excessive temperatures (>70 °C) led to pore coarsening and reduced mechanical strength. These findings provide a theoretical basis for optimizing the curing regimes of HCSA-admixed mass concrete to ensure structural integrity. Full article

Glass wool waste constitutes a rapidly increasing fraction of construction and demolition residues, yet it remains one of the most challenging insulation materials to recycle. Its non-combustible nature, extremely low bulk density, and high fibre elasticity preclude energy recovery and severely limit conventional mechanical recycling routes, resulting in long-term landfilling and loss of mineral resources. Converting glass wool waste into a fine mineral powder represents a potentially viable pathway for its integration into low-carbon construction materials, provided that industrial scalability, particle-size control, and chemical compatibility with cementitious binders are ensured. This study investigates the industrial-scale milling of end-of-life glass wool waste in a ventilated horizontal ball mill. It compares two grinding routes: a corundum-free route (BK) and an abrasive-assisted route (ZK) employing α-Al₂O₃ corundum to intensify fibre fragmentation. Particle size distribution was quantified by laser diffraction using cumulative and differential analyses, as well as characteristic diameters. The results confirm that abrasive-assisted milling significantly enhances fragmentation efficiency and reduces the coarse fibre fraction. However, the study demonstrates that this gain in fineness is inherently coupled with the incorporation of α-Al₂O₃ into the milled powder, introducing a chemically foreign crystalline phase that cannot be removed by post-processing. From a cement-oriented perspective, this contamination represents a critical limitation, as α-Al₂O₃ may interfere with hydration reactions, aluminate–sulfate equilibria, and microstructural development in Portland and calcium sulfoaluminate binders. In contrast, the corundum-free milling route yields a slightly coarser, chemically unmodified powder, offering improved process robustness, lower operational complexity, and greater compatibility with circular economy objectives. The study establishes that, for the circular reuse of fibrous insulation waste in cementitious systems, particle fineness alone is insufficient as an optimization criterion. Instead, the combined consideration of fineness, chemical purity, and binder compatibility governs the realistic and sustainable reuse potential of recycled glass wool powders. Full article

Extensive research has addressed concrete deterioration and its countermeasures; however, studies on responsive repair methods and materials remain comparatively limited and less systematic. In this study, six mixtures of repair mortars (RMs) were formulated using aluminate-based binders, specifically calcium sulfoaluminate (CSA) and amorphous calcium aluminate (ACA) cements. The experiment evaluated the mechanical properties and freeze–thaw resistance of these mortars. To accelerate hydration, a controlled amount of anhydrite gypsum was incorporated into each mixture. The fluidity and setting time of fresh RMs were measured, whereas the compressive strength, flexural strength, and ultrasonic pulse velocity (UPV) of hardened RMs were evaluated at 1, 7, and 28 days. In addition, freeze–thaw resistance was assessed as per ASTM C666 by determining the relative dynamic modulus of elasticity. Additionally, the hydration products and microstructural characteristics of paste specimens were qualitatively analyzed. The mechanical performance, including strength and UPV, and freeze–thaw resistance of RMs containing ACA were superior to those of RMs containing CSA. In particular, compared to the CSA-containing specimens exposed to freeze–thaw action were significantly deteriorated, the ACA-containing specimens showed excellent resistance with relatively less cracking and spalling. This may imply that ACA is effective as rapid repair materials for concrete structures in cold regions. Microstructural observations revealed variations in hydration products depending on the aluminate binder employed, which significantly influenced the mechanical and durability properties of the RMs. These results may aid the selection of optimal repair materials for deteriorated concrete structures. Full article

Impressed current cathodic protection (ICCP) systems can experience acidification, which deteriorates the interface between the anode and the anode backfill mortar. This deterioration may necessitate premature intervention to remove and reinstate the backfill and, in some cases, replace the anode. If left unaddressed, acidification ultimately leads to debonding between the anode and the backfill mortar, resulting in the failure of the ICCP system. This paper presents the development of a specialised acid-resistant hybrid mortar designed for ICCP systems used to protect reinforced concrete bridges in marine environments. It also investigates the effects of acidification on the physical and mechanical properties of the proposed anode backfill mortars. Additionally, the study characterises acidification products from both field-extracted ICCP systems and laboratory-based accelerated testing, providing deeper insights into the acidification mechanisms. Novel mortar samples were subjected to varying concentrations of hydrochloric acid (HCl) under accelerated testing conditions. The incorporation of supplementary cementitious materials (SCMs), calcium sulfoaluminate (CSA) cement and zeolite significantly enhanced the strength and durability of the backfill mortars in acidic environments, while maintaining compliance with the electrical resistivity requirements (20–100 kΩ·cm) for ICCP systems. The lowest compressive strength loss observed in the developed hybrid mortar was 54% after 28 days of immersion in 5% HCl and 83% in 15% HCl. Microstructural analyses revealed that gypsum formation and chloride–sulphate competitive binding interactions are key mechanisms contributing to the improved acid resistance, particularly in CSA cement-containing formulations. Full article