Mixing cement with seawater will cause the hydration process of cement to be different from that of ordinary cement, which will significantly affect cement’s mechanical properties and durability. This article investigates the effects of chloride ion concentration, curing temperature, and nano-metakaolin content on the evolution process of Friedel’s salts and ettringite (AFt) crystals in cement pastes. The study was conducted using X-ray diffraction (XRD), thermal analysis (TG), scanning electron microscopy (SEM), and mercury-intrusion porosimetry (MIP). The results show that chlorine salt can increase the production of Friedel’s salt and ettringite, and the delayed AFt production increases by up to 27.95% after the addition of chlorine salt, which has an adverse effect on cement-based materials. Increasing the curing temperature and increasing the nano-metakaolin dosage increased the generation of Friedel’s salt and decreased the delayed AFt generation, which resulted in a decrease in the length and diameter of the AFt crystals. After 28 days of high-temperature curing and the addition of nano-metakaolin, Friedel’s salt production increased by 13.40% and 14.34%, respectively, and ettringite production decreased by 9.68% and 7.93%, respectively. Increasing the curing temperature and adding nano-metakaolin can reduce the adverse effect of delayed ettringite increases due to chloride ion binding.

In order to study the influence of nano-clay on the crack resistance of cement-based materials, two kinds of nano-metakaolin (NMK) and two kinds of nano-attapulgite clay (NMA) were considered. The early cracking process and mechanical properties of nano-clay cement mortar (NCM) was studied by using a plate knife-edge constraint test. Based on fractal theory, the distribution characteristics of NCM surface cracks were revealed, and the calculation method forNCM maximum crack width was given. The results show that the cracking time of the NMK-3 specimen is 2 and 6 h later than that of NMK-1 and NMA-2, respectively; the smaller the particle size of nano-clay, the earlier the cracking time of the specimen. However, nano-clay effectively inhibited the expansion of mortar cracks, and the cracks on the surface of NCM were thin and sparse. At 28 days, the maximum crack width of NMK-3 was 46.7% and 33.3% lower than that of NMK-1 and NMA-2, respectively. NMK hadthe best improvement effect on the mechanical properties cement mortar. The smaller the particle size, the more pronounced the improvement effect.The flexural strength ratio and compressive strength ratio at 7 and 28 days are 76.7%, 67.4%, and 61.2%, respectively.The distribution of surface cracks on NCM has fractal characteristics, and the fractal dimension of surface cracks is smaller than that of ordinary cement mortar. The larger the particle size of nano-clay, the smaller the fractal dimension of cracks. The quantitative relationship between fracture fractal dimension and NCM elastic modulus and shrinkage tensile stress is established.

Freshwater resources are scarce in coastal areas, and using seawater as mixing water can alleviate the scarcity of freshwater resources. However, the presence of chloride ions in seawater affects the generation of hydration products and the durability of concrete structures. In order to investigate the effect of hydrated calcium silicate (C-S-H) gel and calcium hydroxide (CH) generation in seawater-mixed cement pastes under 50 °C curing, their microscopic morphology was investigated using differential scanning calorimetry analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The relationship between the amount of C-S-H gel and CH production and the amount of chloride ion dosing, fly ash dosing, and the age of curing were investigated. The degree of influence between hydration products and influencing factors was analyzed using the grey correlation analysis. It was shown that 50 °C curing promoted the hydration reaction and generated more hydration products compared with ASTM standard. The content of C-S-H gel and CH increased with chloride dosage. The content of C-S-H gel increased by 13.5% under 50 °C curing compared with the control group at a chloride dosage of 1.3%. Fly ash is rich in active SiO₂ and AI₂O₃, and other components, which can react with Ca(OH)₂ generated by cement hydration and then generate C-S-H gel. With the increase of fly ash, the content of C-S-H gel also increases, but the CH content decreases. When 25% of fly ash was doped under 50 °C curing, the C-S-H gel content increased by 5.02% compared to the control group. The CH content decreased by 31.8% compared to the control group. With the growth of the maintenance age, the hydration reaction continues, the generation of C-S-H gel and CH will continue to increase, and their microstructures will become denser. C-S-H gel and CH content increased the most by raising the curing temperature at 7 days of curing, increasing by 10.11% and 22.62%, respectively. C-S-H gel and CH content had the highest gray relation with fly ash dosing. Chloride dosage and age of maintenance had the highest correlation with CH content at room temperature maintenance of 0.788 and 0.753, respectively.

To explore the effect of nano-attapulgite clay (NAC) on the durability of concrete, two kinds of NAC (calcined raw ore and calcined high viscosity ore: calcination at 650 °C for 2 h) were used to study their effects on the durability of concrete, mainly discussing the changes of chloride ion permeability and the resistivity of concrete with NAC. The effect of NAC on the strength of concrete was analyzed by testing the compressive strength of concrete. The two-electrode method, four-electrode method, and concrete resistivity tester were used to analyze the relationship between the testing method and concrete resistivity, and the effect of NAC on concrete resistivity was analyzed. The influence of NAC on the chloride corrosion resistance of concrete was analyzed by measuring the chloride diffusion coefficient, and the relationship between the chloride diffusion coefficient and resistivity was established. The diffusion process of chloride ions in concrete was analyzed by theoretical derivation and numerical simulation. The results show that: calcining raw ore NAC can improve the compressive strength of concrete, while calcining high-viscosity ore reduces the compressive strength of concrete. At the age of 28 days, the strength of concrete mixed with calcined raw ore is about 7.10% higher than that of ordinary concrete, while the compressive strength of concrete mixed with calcined high-viscosity ore is about 4.32% lower than that of common concrete. The resistivity of concrete mixed with calcined raw ore increases the fastest, and the 56 days age is about 15.8% and 29.6% higher than that of ordinary concrete and calcined high-viscosity ore. There is a good negative correlation between concrete resistivity and chloride diffusion coefficient. At 28 days, the incorporation of calcined raw ore concrete decreased by about 19.9% and 49.4% compared with ordinary concrete and calcined high-viscosity ore, respectively. After 10 years of decline, the chloride ion content is 11.1% and 23.2% lower than that of ordinary concrete and concrete mixed with calcined high viscosity ore.

The rapid expansion of fiber-reinforced polymers (FRPs) in wind energy, transportation, and aerospace is generating increasing amounts of accompanied waste, making effective valorization essential to a circular economy. This review compares FRP recovery technologies in terms of recovered-fiber quality, operating conditions, post-treatment, environmental impacts, and industrial applicability. Then, it also examines direct reuse, FRP remanufacturing, and reuse in cementitious composites. Quantitative synthesis indicates that high-quality recycled carbon fibers (rCFs) generally retain more than 90% of their original strength, whereas mechanically recovered glass fibers (rGFs) typically retain approximately 70–90%. The preferred pathway depends on the intrinsic value, damage state, morphology, and residual properties. Components with sufficient residual capacity should be directly reused; high-quality fibers are better suited to polymer remanufacturing; and heterogeneous or lower-grade glass-FRP (GFRP) fractions are more compatible with cementitious applications, where mechanically recycled GFRP can provide interfacial bond strengths comparable to conventional engineering macrofibers. Future research should establish quantitative links among recovered material quality, processing, interfacial behavior, and end-use performance, while adopting consistent environmental and economic assessment boundaries. A graded utilization framework is therefore required to support both large-scale and value-added reuse of FRP waste.