Utilization of waste red gypsum in autoclaved aerated concrete preparation

Abstract

Abstract In this study, the utilization feasibility of red gypsum (RG) as a substitute for fly ash to prepare the autoclaved aerated concrete (AAC) was examined. Influences of RG replacement ratio on the gas foaming, mineral components, morphology and mechanical properties were investigated. In addition, analyses of microstructure evolutions, phase compositions and thermal changes were carried out to explore the mechanism of strength improvement for AAC blocks. Results show that the replacement of fly ash by RG reduced the gas-foaming rate, which was resolved by Na2CO3 addition and it’s optimum dosage is 0.75%. The main phases in AAC are C-S-H gel, katoite, tobermorite and anhydrite. A proper amount of RG replacement (5%–20%) promoted the increase of relative strength of AAC samples. The synergistic enhancement of the anhydrite system and tobermorite system ensured an increase in relative strength. The optimum RG replacement ratio in AAC is 20%. With this substitution ratio, density can be obtained around 617\xa0kg/m3 and compressive strength can reach about 4.2\xa0MPa, which satisfies the requirements of B06, A3.5 grade in Chinese national standard GB 11968-2019.