Reuse of by-product gypsum with solid wastes-derived sulfoaluminate cement modification for the preparation of self-leveling mortar and influence mechanism of H3PO4

Abstract

Preparing beta-hemihydrate gypsum and then using it for gypsum-based self-leveling mortar (GSLM) in building engineering is one of the promising approaches to consume by-product gypsum such as flue gas desulfurization gypsum and phosphogypsum. In order to overcome the deficiencies of low mechanical strengths and large fluidity loss of beta-hemihydrate gypsum, solid wastes-derived sulfoaluminate cement (WSAC) was used to modify beta-hemihydrate gypsum and prepare GSLM in this work. The effects of various additives on the fluidity and setting characteristics of GSLM prepared using WSAC-modified beta-hemihydrate flue gas desulfurization gypsum was firstly studied, and an optimal formula was obtained. The WSAC-enhanced GSLM had good mechanical performance due to the well-connected structure of the hydration products gypsum and ettringite. However, the hydration speed and degree of GSLM prepared using WSAC-modified beta-hemihydrate phosphogypsum were seriously weakened by phosphorus impurities, which resulted in poor mechanical performance. PO43- would react with Ca2+ and Al3+ dissolved from ye'elimite in WSAC to form CaHPO4·2H2O and AlPO4 precipitates, which seriously inhibited ettringite formation. This negative effect was found mainly acting on the hydration of WSAC and could last until later hydration.