Preparation of Magnesium Oxysulfate Cement with Calcined Phosphate Tailings

Recycling of phosphate tailings (PT) in cement production can reduce the amount of this solid waste and its consequent pollution. This paper used high content of calcined PT as an alternative to light-burned magnesia to prepare magnesium oxysulfate (MOS) cement. Thermal decomposition properties of PT at calcination temperature of 600°C–900°C were studied with X-ray diffraction (XRD) and scanning electron microscopy (SEM). Compressive strength, XRD, Fourier-transform infrared spectroscopy (FT-IR), and SEM were conducted to study the effects of the calcination temperature of PT (600°C–800°C) and the dosage of calcined PT (50%–100%) on the strength development and hydration mechanism of MOS cement. Results showed that the main component of PT, dolomite, partially decomposed into magnesium calcite, periclase, and calcite at 700°C; decomposed further into calcite, periclase, and lime at 800°C; and finally decomposed into lime and periclase at 900°C. The periclase sourced from calcined PT reacted with the MOS system to form brucite and 5Mg(OH)2·MgSO4·7 H2O (517 phase). Gypsum also was formed in the reaction. Brucite, 517 phase, and gypsum worked with the unreacted PT to build the structure and provide the source of strength. MOS paste prepared with 60% PT calcined at 700°C had the highest compressive strength, 63 MPa, at 28 days of all the mixtures. The paste prepared with 50% PT calcined at 700°C had the highest strength retention value, about 68%, after 56 days of water immersion. This study provides a method for the efficient use of PT by incorporating it into MOS cement after calcination.