This study aimed to improve the mechanical properties and water resistance of calcined gypsum from phosphogypsum (CGP) by incorporating organic additives and inorganic admixtures. The effects of the dosage of these additives—including kaolin, nano-SiO₂, polycarboxylic acid superplasticizer, and sodium methyl silicate—on the properties (flexural strength, compressive strength, water absorption, and softening coefficient) of CGP composites (CGPCs) were investigated. A high water resistance of the CGPCs was achieved using nano-SiO₂ and sodium methyl silicate modification, superplasticizer addition, and the partial replacement of gypsum with mineral admixtures. The results showed that the flexural and compressive strength of the composites hit 4.61 MPa and 19.54 MPa, respectively, while the softening coefficient was 0.70 and the water absorption rate was 19.85%. Microstructural investigation confirmed that the combination of nano-SiO₂ and kaolin led to the formation of calcium silicate hydrate. Additionally, the superplasticizer played a crucial role in reducing the water-to-cement ratio, while unhydrated mineral particles had a filling effect, thereby enhancing the density of the hardened paste. The sodium methyl silicate formed a hydrophobic film on the surface of the hardened paste, increasing the contact angle to 109.01° and improving the water resistance of the CGPCs.

This study investigates the preparation and performance of ultra-high-performance concrete (UHPC) incorporating manufactured sand as a full replacement for quartz sand. The mix design was optimized by integrating the compressible packing model (CPM) with an orthogonal experimental design. The influence of stone powder content in manufactured sand—0, 5, 10, and 15% by mass of fine aggregate—on fresh-state fluidity and 7d-mechanical properties was systematically evaluated. Hydration products and microstructural features were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP). Results show that the manufactured sand-based UHPC achieved a fresh-state fluidity of 185 mm and a 7-day compressive strength of 152.4 MPa. Both fluidity and compressive strength exhibited a unimodal trend with increasing stone powder content, reaching maxima at 10%. Microstructural analysis revealed intimate interfacial bonding between unhydrated particles and calcium silicate hydrate (C–S–H) gel; notably, the UHPC matrix with 10% stone powder displayed the densest microstructure. MIP results further demonstrated that an optimal stone powder content effectively reduced total porosity, with the lowest overall porosity and the highest volume fractions of harmless (≤20 nm) and less harmful (20–100 nm) pores observed at 10%. These microstructural refinements collectively underpin the superior mechanical performance of manufactured sand-based UHPC.

Lightweight aggregates have a porous structure and high water absorption, which may lead to underestimation of the powder content in conventional mix design methods for lightweight ultra-high-performance concrete (LUHPC). To address this issue, this study used ceramsite sand as the lightweight aggregate and combined the excess paste theory with the particle packing method to design and evaluate ceramsite-sand-based LUHPC mixtures based on the modified Andreasen packing model (APM) and the compressible packing model (CPM). By optimizing the particle size distribution of ceramsite sand and the binder composition, a mix design method suitable for ceramsite-sand-based LUHPC was developed. The workability, apparent density, mechanical properties, elastic modulus, and shrinkage behavior of the material with different steel fiber contents were systematically investigated. The results showed that the total binder content, water-to-binder ratio, and paste volume of the mixtures designed using the two models differed only slightly. However, the aggregate skeleton formed by CPM was denser, and its skeleton packing volume was approximately 3.5% lower than that obtained using APM. At the same steel fiber content, the main mechanical properties of the CPM-designed LUHPC were generally superior to those of the APM-designed mixtures. Specifically, the 28-day cube compressive strength increased by 5.0–7.6%, the axial compressive strength by 8.8–12.2%, the axial tensile strength by 6.4–25.8%, the flexural strength by 14.1–17.2%, and the shear strength by 3.1–6.5%. The elastic modulus was also slightly higher, while the shrinkage remained consistently lower. The CPM-2.0 LUHPC mixture achieved a 28-day cube compressive strength of 124.6 MPa and an apparent density of approximately 1982 kg/m³, realizing a compressive strength above 120 MPa at a density below 2000 kg/m³. The 28-day cube compressive strength of the CPM-3.0 mixture further increased to 131.7 MPa. As the steel fiber content increased from 1.5% to 3.0%, the workability of LUHPC decreased, whereas its compressive, tensile, flexural, and shear properties generally improved, and the elastic modulus increased slightly. Steel fibers effectively restrained shrinkage deformation, but the improvement showed diminishing marginal benefits with increasing fiber content. Considering the mechanical performance, shrinkage control, and material economy, a steel fiber content of approximately 2.0% is recommended as a reference range for ceramsite-sand-based LUHPC. Overall, CPM is more suitable than APM for the mix design of ceramsite-sand-based LUHPC and can provide guidance for mix optimization and performance regulation of lightweight ultra-high-performance concrete.

The presence of impurities directly affects the properties of α-hemihydrate gypsum (α-HH) prepared from phosphogypsum (PG) as a raw material. However, the effect of soluble fluorine impurities on the properties of α-HH by autoclaving remains insufficiently understood. This study investigated the influence of sodium fluoride on the morphology, hydration, and hardening properties of α-HH, using XRD, XPS, SEM, MIP, and tests of setting time, evolution of hydration temperature increase, and strength. The results showed that during the preparation of α-HH, some F⁻ reacted with Ca²⁺ to form CaF₂, which adhered to the surface of the α-HH crystal, hindering the growth and development of the crystal and resulting in small crystals with rough surfaces. When α-HH hydrated, sodium fluoride caused the early, rapid nucleation of dihydrate gypsum (DH) crystals, accelerating the crystallization process of DH. The introduction of sodium fluoride inhibited the early hydration of α-HH and promoted its later hydration. The increase in sodium fluoride content caused the initial setting time of α-HH hydration to first increase and then decrease, while the final setting time continued to decrease. In the absence of sodium fluoride, the average pore diameter of the hardened paste was approximately 617.99 nm. When the NaF content was 0.2%, the DH crystals were prismatic and densely packed, which resulted in a decrease in the average pore diameter to 449.35 nm. When the NaF content was 0.6%, the DH crystals exhibited a plate-like morphology and were loosely interlocked, leading to an increase in the average pore diameter to 1169.58 nm. Based on these results, the sodium fluoride content in PG should be controlled below 0.2%.

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.