High temperature calcined red mud-cement mortar: Workability, mechanical properties, hydration mechanism, and microstructure

Abstract

To efficiently utilize red mud (RM) in cement-based materials, this study employed high-temperature calcination to enhance RM's pozzolanic activity. Calcined RM replaced cement in RM-cement mortar, examining the effects of different calcination temperatures and RM addition amounts on the workability and mechanical properties of the mortar. Additionally, XRD, FTIR, SEM-EDS, TAM-Air, and TG were used to explore the hydration mechanism and microstructure of RM-cement mortar. Research showed that as the calcination temperature increased, minerals like Calcite, Hematite, and Cancrinite in RM began to decompose, and the glassy structure increased. At 800 °C, the serrated and amorphous diffraction peaks significantly increased. Calcined RM positively influenced the setting and hardening of cement mortar but negatively affected its fluidity. The optimal calcination temperature for RM was 800 °C, with an optimal addition amount of 10%. The compressive strengths of RM-cement mortar at 3, 7, and 28d, with optimal conditions, are 9.62, 29.94, and 40.12 MPa, respectively, showing increases of 4.25, 1.72, and 1.39 times over untreated RM-cement mortar. When RM was calcined to 800 °C, the diffraction peak intensities of Portlandite, C–S–H, and Ettringite in the RM-cement paste were the highest, and the hydration products interwove to form a dense microstructure. Adding calcined RM extended the induction period of cement, increased the rate of heat release, shortened the acceleration period, and further accelerated the rate of heat release. Nevertheless, the alkaline components in calcined RM pose a certain hindrance to the later stages of cement hydration. In summary, calcining RM to 800 °C enhanced its pozzolanic activity, improved the workability, early hydration, and early strength of cement, and reduced cement consumption and CO2 emissions.