Effect of Magnesium Carbonate on Hydration and Hardened Properties of Portland Cement Paste

The effects of magnesium carbonate on the setting time and compressive strength of Portland cement paste were studied when the content of magnesium carbonate was 0wt%, 0.5wt%, 1wt%, 2wt%, and 3wt% by the weight of cement. The influence mechanism was analyzed through combining with the tests of hydration heat, X-ray diffraction (XRD), differential thermal analysis-thermogravimetric analysis (DTA-TG), and scanning electron microscope (SEM). The results show that magnesium carbonate makes the setting time of cement paste shorten, and makes the compressive strengths of cement paste increase at 1 day and 7 days, but decrease at 28 days. The testing results of the heat of hydration, XRD, SEM, and DTA-TG show that magnesium carbonate promotes the formation of Ettringite (AFt) and the hydration of calcium silicate in the early stage when its content is less than 1wt%, but the addition of magnesium carbonate is unfavorable to the later hydration of Portland cement. Based on the above analysis, it is suggested that the content of magnesium carbonate is not more than 1wt%, which has good reference value for using magnesium carbonate to prepare early strength agent for cement paste.