The combination of red mud (RM) and phosphogypsum (PG) can exert the alkalinity of RM and the acidity of PG as a calcium source to promote the gel hardening of composite cementitious material, which effectively improves the reutilization efficiency of RM and. In this study, the effects of the ratio and content of pretreated RM and PG on the non-evaporated water, porosity, hydration products, mechanical properties, pore size distribution, and microstructure of composite cementitious materials were investigated. The results show that, with the incorporation of PG, RM, PG, and RM, the non-evaporable water content, reaction degree, compressive strength, and flexural strength show a downward trend after rising first, and their values reach the maximum with 10% PG and 10% RM, which are higher than a pure cement system, while 70% PG, 70% RM, 70% RM + 10% PG, and 70% PG + 10% RM have the reverse effect. The results of hydration products, pore size distributions, and microstructure indicate that adding an appropriate dosage of RM and PG can efficaciously improve the compactness of cement systems. Nevertheless, the research results can contribute to using the combination of PG and RM to manufacture sustainable cementitious materials with good performance, and achieve the purpose of environmental protection and industrial solid waste resource recycling.

Nano-metallic oxide particles have been found to be potentially effective microstructural reinforcements for cement mortar and have become a research hotspot in recent years for nano-modification technology of building materials. However, different conclusions have been obtained due to various researchers used different research methods, which have resulted in a deficiency for the performance comparison between different nano-metallic oxide particles. In the present study, the effects of five kinds of nano-metallic oxide particles, namely nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO, on the performance of cement mortar at 28 days and 730 days in terms of mechanical, durability, microstructure, and pore size distribution properties by performing different experiments were investigated. Test results show that the dosage of nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO is 2%, 1%, 1%, 1%, and 2%, respectively, where they can significantly prove the compressive and flexural strengths, decrease the porosity, drying shrinkage, and permeability, and refine the pore size distribution of cement mortar. It can be seen through SEM analysis that nano-metallic oxide particles can promote cement hydration, and also refine the size and distribution of Ca(OH)2 crystal, but the specific principles are different. The analysis concluded that the five kinds of nano-metallic oxide particles can play a filling role in cementitious materials to improve the denseness and surface activity role to promote the hydration of cement particles, thus improving the mechanical properties, durability, and pore size distribution of cementitious materials, with the order of their modification effect on cement-based materials being nano-ZrO2 > nano-MgO > nano-Al2O3 > nano-ZnO > nano-CuO.

The main component of phosphogypsum (PG) is CaSO4·2H2O. PG contains a few impurities, heavy metals, and radioisotopes, which limit the use of PG and pose a danger to the environment. In this study, under the excitation of a sodium hydroxide solution, the rheological properties of a paste with granulated blast-furnace slag (GGBS) and PG treated with ultrasonic water washing were investigated. Experimental results showed that the ratio of GGBS to PG and the amount of sodium hydroxide solution significantly affect the density and viscosity of the paste, but the effect patterns of both are different. The maximum viscosity was 498 mPa·s when the ratio of GGBS to PG was 4:1. When the ratio changed from 3:2 to 1:4, the viscosity of the paste gradually decreased by 15.5%, 32.1%, 36.1%, and 46.8%, respectively. In contrast, the ratio of GGBS to PG had a greater effect on the viscosity than the amount of sodium hydroxide solution in terms of the standard consistency water consumption, viscosity, and water release ratio. The larger the PG ratio, the smaller the density, viscosity, and water release ratio of the paste. The variation in the ratio of GGBS to PG had a significant effect on the water film thickness of the paste, demonstrating that the larger the PG mixture, the larger the water film thickness of the paste, which reached 1.122 μm, 2.31 times the minimum water film thickness of the paste. At the same time, the water film thickness of the paste was negatively correlated with the water consumption of the standard consistency, viscosity, and water release ratio, and was positively correlated with the fluidity.

Delayed strength development and long setting times are the main disadvantageous properties of Na2CO3-activated slag cements. In this work, combined auxiliary activators of Ca(OH)2 and Mg(OH)2 were incorporated in one-part Na2CO3-activated slag binders to accelerate the kinetics of alkali activation. The properties and microstructure evolution were investigated to clarify the reaction mechanism. The results showed that the additions of auxiliary activators promoted the hardening of the pastes within 2 h. The 28 days compressive strengths were in the range of 39.5–45.5 MPa, rendering the binders practical cementitious materials in general construction applications. Ca(OH)2 was more effective than Mg(OH)2 in accelerating the kinetics of alkali activation. The dissolution of Ca(OH)2 released more OH− and Ca2+ ions in the aqueous phase to increase alkalinity in the aqueous phase and promote the formation of the main binding gel phase of calcium-aluminosilicate hydrate (C-A-S-H). An increase in the Ca(OH)2/Mg(OH)2 ratios increased autogenous shrinkage and decreased drying shrinkage of the binders. The formation of a compact pore structure restricted the water evaporation from the binders during the drying procedure.

As a by-product of lithium salt mining, the emission of lithium slag increases yearly due to increased demand. Therefore, the utilization of lithium slag faces a huge challenge. In this study, a new approach to using lithium slag as a super-fine aggregate in cement systems was proposed. The use of lithium slag as a super-fine aggregate replacing 0%, 30%, 50%, 70%, and 100% of the standard sand was tested. The main hydration products of cement–lithium slag paste were calcium silicate hydrate gel, calcium hydroxide, unhydrated particles, and a small amount of ettringite. Lithium slag as a super-fine aggregate could significantly reduce the dead load of structures, enhance flexural and compressive strength and the peak stress of mortar, and no more than 50% lithium slag could significantly enhance the permeability of mortar. The study revealed that the replacement rate of lithium slag as a super-fine aggregate could reach 50%, which is five times more than the amount used as supplementary cementitious material. Therefore, the study brings an innovation in the use of lithium slag in cement systems and improves the performance of cement mortar.