Characteristics of Red Mud–Blast Furnace Slag Composite Low-pH Cementitious Material Reinforced Using Urease-Producing Microorganisms

Abstract

Special low-pH cementitious materials are mainly used in the solidification engineering of medium- and low-radioactive waste and heavy metal pollutants, as well as in the engineering of high-level radioactive waste repositories. In this work, red mud (RM) and blast furnace slag (BFS) were used to prepare a red mud–blast furnace slag composite low-pH cementitious material (RM-BFS). The effect of urease-producing microorganisms (UPM) on the pH value and mechanical strength of the RM-BFS was systematically investigated, and the mechanism analyzed by combining X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the UPM enhance the mechanical strength of RM-BFS. As long as a trace amount of UPM is incorporated, the increase of the 120-h compressive strength can reach 141%. Moreover, the increase in strength of RM caused by the combined action of UPM and BFS is significantly greater than the sum of that caused by the action of each of these agents alone: ΔUCS(UPM+BFS)>ΔUCS(UPM)+ΔUCS(BFS). Micromechanism analysis shows that the UPM promotes the hydration reaction of the RM-BFS binary system and generates more C─ A─ S─ H gels; moreover, the CO2 generated by the UPM is distributed in the pores of the RM particles, which contributes to the carbonation reaction in the RM and generates a large number of calcite crystals. The formation of dense C─ A─ S─ H gels and CaCO3 crystals and their tight bonding with RM particles are the main reasons for the strength increase of the composites.