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.

Magnesium oxysulfate (MOS) cement is lightweight, offers fire resistance, and has good aesthetics. Although the addition of citric acid improves the cement’s late strength, the concomitant retardation of hydration of MOS by citric acid results in slow setting and low early strength. This may in return decrease the production efficiency of MOS cement products. In this study, 5•1•7 phase (5Mg(OH)2•MgSO4•7H2O) seed crystal (SC) is used as a modifier that works as a seed nucleus and accelerates the hydration of MOS cement in the early stages, in the absence or presence of citric acid. The effect of SC on compressive strength, setting time, and volume change is investigated in detail. X-ray diffraction, scanning electronic microscopy, hydration-heat release rate, and mercury intrusion porosimeter are performed to explain the influence mechanism of SC on the performance of the MOS cement. The results revealed that the in situ formation and growth of the new 5•1•7 phase crystals on the SC surface by the addition of SC can significantly accelerate the setting, improve the 12 h and 28 d compressive strengths, and reduce shrinkage of MOS cement without or with 0.05% citric acid.

Basic magnesium sulfate (BMS) cement was prepared by magnesia with different reactivities derived from calcination of basic magnesium carbonate at 600 to 1000°C. The effects of reactivity of magnesia on the setting time and compressive strength of BMS cement was investigated. Furthermore, XRD, SEM, MIP, hydration heat evolution and the pore solution composition were used to analyze the action mechanism of magnesia reactivity. The results show that the main strength phase of 5Mg(OH)2•MgSO4•7H2O (5•1•7 phase) was formed in BMS cement prepared by magnesia with calcination temperature higher than 600°C. The magnesia with a high BET surface area and low crystallite dimension showed a high adsorption capacity for citrate ions in magnesium sulfate solution, decreasing their concentration in the pore solution. By acting as a soft template for 5•1•7 phase formation, the decrease in citrate ions resulted in a low amount of 5•1•7 phase, forming Mg(OH)2 instead, leading to a low-strength BMS cement.

Magnesium oxysulfate cement (MOS) prepared by light-burned dolomite has excellent performance in terms of light weight, high strength and low carbon dioxide emission. In this research, dolomite ores are used to produce MOS cement at different calcination temperatures and calcination times; the parameters investigated are the rate of loss on ignition, free lime (f-CaO), dihydrate gypsum (CaSO₄×2H₂O) and impact of active magnesium oxide (α-MgO) content exerted on the setting time and compressive strength of the MOS cement. X-ray diffraction analysis, scanning electron microscopy, the time of colour change for citric acid and hydration-heat release rate were adopted to assess the effects of calcination temperature, calcination time and the molar ratio of α-magnesium oxide/magnesium sulfate on the MOS cement. The experimental results showed that the MOS cement prepared by dolomite ores at the calcination temperature of 850°C for 30 min with the α-magnesium oxide/magnesium sulfate molar ratio of 4 demonstrates better mechanical properties than those made in other circumstances. In addition, the main strength phase of MOS cement with tri-sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O) generated by light-burned dolomite ores is the needle-like 5Mg(OH)₂·MgSO₄·7H₂O crystal phase.