Study on the effects and mechanism of gypsum on the setting, hardening, and mechanical properties of fly ash-cement composites

The activation of fly ash (FA) by gypsum provides a low-cost and compatible route for developing low-carbon cementitious materials, yet the dependence of gypsum efficiency on FA replacement level and gypsum type remains unclear. This study systematically investigated the effects of gypsum dosage and type on fly ash-cement composites (FAC) with different FA replacement levels through macroscopic tests and microstructural analyses. The results show that the role of gypsum varies significantly with FA dosage. In low-FA systems (≤30%), gypsum mainly regulates cement hydration, and the optimal dosage is about 6%. In high-FA systems (≥60%), gypsum acts more prominently as a chemical activator for FA, and the optimal dosage shifts to 10%. Among the tested materials, anhydrite gypsum (AG) exhibited a superior strengthening effect to dihydrate gypsum (DG), with 10% AG increasing the 28 d compressive strength of FAC containing 80% FA by 151.83% relative to the control group. Microstructural analyses indicate that AG, owing to its slower and more sustained dissolution behavior, is better matched with the reaction kinetics of FA, thereby promoting hydrate formation and matrix densification more effectively than DG. This work provides a technically viable and economically valuable pathway for the large-scale application of high-volume FA in concrete.