Isothermal Calorimetry Study of the Effect of Citric Acid on the Hydration Kinetics of Magnesium Oxysulfate Cement

The hydration kinetics of magnesium oxysulfate cement (MOS) with and without citric acid (CA) was evaluated via isothermal calorimetry. A series of tests were conducted with different MgSO4 solution-to-MgO ratios (r) and MgO with different activities at 15 degrees C and 25 degrees C. The test results indicated that the effect of CA on the overall cement hydration rate was determined by the CA dose (w) primarily and can be simulated by a generalized scale factor model. The heat flow and cumulative heat evolution of slurries with different CA dosages could be accurately predicted using the scale factor (f) model. There was a linear relationship between lgf and lg(w/w0), so the individual reaction order in CA could be obtained. The effect of temperature on the hydration kinetics of MOS cement with and without CA was modeled with the original scale factor model developed from the Arrhenius equation, and the apparent activation energy obtained was 66.3 and 74.9 kJ/mol, respectively. This study provided a basis for further in-depth research of the kinetic mechanism of the MOS hydration reaction.