Optimization and microstructural analysis of carbide slag, sodium silicate, and calcined alum activated type II anhydrite-slag cementitious material

This study employed response surface methodology (RSM) to optimize the mix ratio of Type II anhydrite-based cementitious material (ABCM), utilizing Type II anhydrite as the primary binder activated by carbide slag (CS), sodium silicate (SS), and calcined alum (CA). Results demonstrated significant influences of activator content on ABCM’s water requirement, setting time, and mechanical strength. The initial setting time reached a minimum of 45 minutes when the contents of CS, SS, and CA were 8%, 4%, and 4%, respectively. Considering single-factor effects on mechanical performance, the optimal content for CS, SS, and CA was 4% each, yielding 28-day compressive strengths of 36.0 MPa, 26.8 MPa, and 26.8 MPa for CS, SS, and CA, with corresponding flexural strengths of 7.4 MPa, 5.9 MPa, and 5.9 MPa. RSM determined the overall optimal mix ratio as 3.35% for CS, 3.51% for SS, and 4.22% for CA. Under this ratio, thermogravimetric analysis showed mass losses of 13.13% within 40~120°C, attributed to ettringite (AFt) and calcium aluminosilicate hydrate (C-(A)-S-H) dehydration; and the mass loss was 9.72% within 120~200°C, ascribed to gypsum dihydrate dehydration. Microscopy confirmed dominant hydration products as gypsum dihydrate, C-(A)-S-H gel, sodium aluminosilicate hydrate (N-A-S-H) gel, and AFt. Synergistic coalescence of C-(A)-S-H and N-A-S-H gels formed a densely integrated three-dimensional network, mechanistically contributing to ABCM’s strength development. • Effects of CS, SS, and CA on the properties of ABCM were investigated. • RSM method was adopt to optimize the mix design of ABCM. • Microstructure of ABCM was tested using XRD, SEM, and TG-DTG techniques.