Phase evolution and mechanical performance of red mud-gypsum waste derived activator composite cementitious materials exposed to various Ca/ Si and Al/S ratios

This study investigates the potential of titanium gypsum (TiG) and red mud (RM) as combined waste-derived activators for blast furnace slag (BFS) in the production of RM-TiG-BFS based cementitious materials (RTB). The mechanical performance, hydration products, microstructure, and environmental properties of RTB are characterized through flexural strength, compressive strength measurements, XRD, FTIR, SEM-EDS, MIP, and ICP. The results demonstrate that a 2:1 ratio of TiG and RM can enhance the 3-d and 28-d strength of RTB when the Ca/Si ratio is no higher than 2.05 and the Al/S ratio is no lower than 0.7. The compressive strength can reach 63.9 MPa. The impact of RTB on strengthening gradually decreases with an increase in the Ca/Si or a decrease in the Al/S ratio, respectively. Changes in the ratios of Ca/Si and Al/S in the cementitious system promote the decomposition of BFS, enhance the formation of C(N)-A-S-H gels and AFt crystals, optimize microstructure AFt crystals, refine pore structure, and improve the mechanical performance of RTB. Furthermore, RTB exhibits a substantially lower level of heavy metal leaching than the Chinese standard for drinking water. This study offers a fresh and innovative perspective on comprehensive utilization of various solid waste and design of wastederived activator composite cementitious materials with excellent mechanical performance and low carbonation emission.