Preparation and properties of modified magnesium oxysulfate cement derived from waste sulfuric acid

High-strength modified magnesium oxysulfate cement is prepared using light-burned magnesia and industrial waste sulfuric acid resulting from titanium dioxide production. Amino trimethylene phosphonic acid (ATMP) is used as a modifier. Coal fly ash is added as a filler to the magnesium oxysulfate cement in order to further reduce its production cost. The compressive strength, setting time, phase compositions, micro-morphology and volume stability of the modified magnesium oxysulfate cement are comprehensively analysed. The results indicate that the compressive strength of the modified magnesium oxysulfate cement reaches 82·1 MPa when the molar ratio of the active magnesium oxide/sulfuric acid (MgO/H₂SO₄) (M) is 6, the water/cement ratio (R) is 0·6 and the ATMP and coal fly ash dosages are 0·2% and 50%, respectively. Low ATMP doses markedly affect the microstructure, hydration and shrinkage of the magnesium oxysulfate cement. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy analyses prove that the needle-like magnesium hydroxide sulfate hydrate – 5Mg(OH)₂·MgSO₄·7H₂O crystal (5·1·7 phase) – is the predominant strength phase in this cement system.