Effect of Ammonium Citrate Tribasic on the Hydration Reaction and Properties of Magnesium Oxysulfate Cement

In this paper, the hydration reaction and properties of magnesium oxysulfate (MOS) cement modified with ammonium citrate tribasic are discussed and compared with MOS cement without chemical additives. The hydration reaction characteristics, hydration products, mechanical properties, and microstructures of MOS cement prepared with and without chemical additives were characterized by isothermal calorimetry, quantitative X-ray diffraction (QXRD), mercury intrusion porosimetry (MIP), thermogravimetry (TG), and scanning electron microscopy (SEM), and the mechanism of action of the chemical additives–was investigated. The results show that when the molar ratio of α-MgO:MgSO4·7 H2O:H2O is 8∶1∶11, the basic salt hydrate 5 Mg(OH)2·MgSO4·7H2O phase (5·1·7 phase) can be formed in MOS cement with and without chemical additives. However, ammonium citrate tribasic is an effective chemical additive that can (1) adjust the concentration of H+ and OH− ions in the liquid phase through buffering to meet the requirements of the H+ or OH− ion concentration (or pH value) required for the neutralization reaction of MgO particles, (2) strengthen the neutralization reaction of MgO particles, and (3) reduce the hydration reaction of MgO particles to form Mg(OH)2 through the complexation of the complex ions. Therefore, a large amount of the 5·1·7 phase and small amounts of Mg(OH)2 and the amorphous phase are formed in MOS cement with ammonium citrate tribasic, which optimizes the micropore structure and improves its mechanical strength and water resistance.