Spontaneous Combustion Gangue–Slag Geopolymers: Optimization of Ratio and Performance Characterization Based on Response Surface Methodology

A new binary system affording a geopolymer cementitious material was prepared using spontaneous combustion coal gangue (SCG) and slag as the main raw materials and desulfurization gypsum, sodium silicate, and NaOH as activators. The Box–Behnken design test of the response surface methodology (RSM) and MATLAB modeling were used to visually study the effects of SCG content, desulfurization gypsum content, and sodium silicate modulus on geopolymer fluidity, flexural strength, and compressive strength. The obtained findings shed light on the mechanism of the interaction between factors and its influence on geopolymer properties, additionally revealing the mechanism of polymerization product formation. A quadratic polynomial regression model was established, and the parameters of the influencing factors were optimized. Finally, the composition and microstructure of the polymerized products were characterized by scanning electron microscopy–energy–dispersive X-ray spectroscopy (SEM-EDS), X-ray diffractometry (XRD), and Fourier transform infrared spectroscopy (FTIR). The analysis of variance and model response surface jointly revealed that the interaction between SCG content, desulfurization gypsum content, and sodium silicate modulus were the key factors affecting geopolymer performance. The high correlation coefficient of the quadratic regression model (R2 > 0.95) indicated its high predictive ability. The optimized geopolymer paste (SCG content = 40%, desulfurization gypsum content = 3.5%, and sodium silicate modulus = 1.72 M) featured a fluidity of 154.3 mm and afforded samples with flexural/compressive strengths of 3.21/24.662 MPa (3-day) and 3.57/40.314 MPa (28-day). The relative error of the model during verification tests was less than 5%, indicating high accuracy and strong reliability. The crystallinity of the C-A-S-H gel decreased with the increasing curing time, which improved the stability of C-(A)-S-H and N-A-S-H phases and promoted the formation of a dense 3D network structure support system.