In-Depth Insight into the Effects of Steel Slag and Calcium Hydroxide on the Properties of a Fly Ash-Red Mud Geopolymer

The low mechanical strength of a low-calcium fly ash (FA)-red mud (RM) geopolymer severely limits its application. Steel slag (SS) and Ca(OH)2 can provide calcium and alkali for the hydration process of a low-calcium FA-based geopolymer. In this study, SS was used to replace part of the RM, and Ca(OH)2 was introduced. The effects of SS and Ca(OH)2 on the properties of the FA-RM geopolymer were investigated. The experimental results show that SS promoted the matrix to generate more C(N)-A-S-H and C-S-H gels and optimized the pore structure, thereby improving the mechanical properties of the FA-based geopolymer. The addition of 4 wt.% Ca(OH)2 increased the hydration products of the FA-based geopolymer, the microstructure was denser, and the mechanical properties were significantly improved. The 28 d compressive strength of the FA-based ternary composite geopolymer prepared by replacing part of the RM with SS and adding Ca(OH)2 reached 30.6 MPa, which provided an experimental basis for the resource utilization of various bulk solid wastes.