Optimization design and characterization of slag cementitious composites containing carbide slag and desulfurized gypsum based on response surface methodology

Since many independent factors can affect the property of geopolymer, it is of great significance to seek optimization technologies to maximize the recovery and utilization of solid waste resources. In this work, a cementitious material was prepared using ground granulated blast furnace slag (GGBS), carbide slag (CS) and desulfurized gypsum (DG). Utilizing the Box-Behnken design under the response surface method, the content of DG and CS, along with the water-binder ratio, were selected as impact factors. Regression models were established to investigate their impact on fluidity, setting time, and compressive strength. In addition, the changes in hydration products, microstructure and pore characteristics of hardened paste were explored through microscopic tests. The results show that the multivariate correlation coefficients of each model are greater than 0.99, the coefficient of variation is less than 10%, and the signal-to-noise ratio is greater than 4, which indicates that the fitting accuracy of established models is extremely high. Microstructural testing shows that the main hydration products of cementitious material are ettringite (AFt), calcium hydroxide and calcium-aluminosilicate (C-A-S-H) gel. CS provided an alkaline environment for GGBS dissolution to promote C-A-S-H gel formation, while DG provide SO42− for AFt formation. Under optimal state, the C-A-S-H gel and AFt crystals in matrix are gradually formed and promoted by each other, making paste structure more compact, which improves mechanical property. This work serves as a valuable reference for the optimal design of cementitious composites under multiple factors and expands the scope of solid waste utilization.