Effect of early hydration temperature on hydration product and strength development of magnesium phosphate cement (MPC)

Abstract

This paper investigates the effect of early hydration temperatures on hydration products and strength development of magnesium phosphate cement (MPC). MPC paste specimens with borate contents of 3%, 6%, 9% and 12% are prepared and cured in different air temperatures and in the adiabatic condition. The internal hydration temperatures are measured by pre-embedded temperature probes. MPC samples with different hydration temperatures are also obtained by using thin slice samples. The hydration products in MPC samples with different hydration temperatures are analyzed by X-ray diffractometer (XRD) and scanning electron microscope (SEM) and the strength development is also measured. The results show that NH4MgPO4·6H2O is the major hydration product and beneficial to strength development of MPC at hydration temperature below 70°C. NH4MgPO4·H2O is another major product, which significantly decreases the strength, when the temperature is higher than a critical temperature between 70°C and 75°C. NH4MgPO4·H2O can directly form in the MPC paste, and comes from the decomposition of NH4MgPO4·6H2O when the temperature is above 75°C. With temperature elevation and duration extension, NH4MgPO4·6H2O decomposes rapidly, and even entirely when the temperature is above 100°C. The borate content has no effect on the types of major hydration products and the critical temperature.