Alkali and sulfate effects on mechanical properties and microscopic mechanisms of slag and fly ash geopolymers

Aiming at the problem that it is difficult to realize low-cost, high-performance and large-scale utilization of cementitious materials prepared from bulk solid wastes, this paper constructs a set of composite cementitious system based on alkaline activation of slag and fly ash (FA) by calcium carbide slag (CCS) and synergistic activation of sodium sulfate (Na₂SO₄) as a chemical dopant. The influence of factors such as solid waste type, mixing ratio, and Na₂SO₄ content on the mechanical properties of composite cementitious systems was investigated by assessing compressive strength and analyzing microstructure using XRD, SEM-EDS, and FTIR. The test results indicate that CCS and Na₂SO₄ exert significant influences on the strength of the composite cementitious system. CCS acts as an alkali activator, enhancing the system's hydration with an optimal dosage of 25%. Low Na₂SO₄ content also promotes hydration, but higher concentrations disrupt the internal structure of the cementitious system post-coagulation, with an optimal content of 6%. The Projection Pursuit Regression (PPR) strength prediction model can fit the actual experimental data very well, which provides a feasible method for the proportion design and mechanical strength prediction of all-solid-waste cementitious systems.