Properties and high-temperature resistance of tailing-based magnesium oxysulfate (MOS) cement affected by phosphogypsum and water-binder ratio

Abstract

Magnesium oxysulfate (MOS) cement is considered as a low-carbon cement to produce fireproof building boards. Its large-scale application is limited by the cost and source of the raw material, light-burnt magnesia (LBM). In this paper, 60% of LBM was replaced by calcined phosphate tailings (PT) to prepare MOS cement. Phosphogypsum (PG) and water-binder (W/B) ratio were used to adjust the setting and strength development of the cement. Moreover, strength evolution of MOS cement after high-temperature exposure was investigated. Results showed that PG addition shortened the setting of fresh paste but lowered the early strength. The increase of W/B ratio prolonged the setting time and lowered the strength. Increasing temperature to 400 °C resulted in the dehydration of 5 Mg(OH)2·MgSO4·7H2O phase and gypsum, thereby densifying the microstructure and improving mechanical strength. The highest compressive strength reached 133.8 MPa at 400 °C. Further increasing temperature to 600 °C led to the dehydration of Mg(OH)2, inducing a loose structure as well as a rapid strength reduction. This study aims to recycle bulk industrial solid waste, PT and PG, to produce a high value-added MOS cement with adjustable setting time and high-temperature resistance.