Effect of limestone powder on the water stability of magnesium phosphate cement-based materials

Addition of limestone to Porland cement can significantly increase the hydration and high early strength of Porland cement. However its application in the magnesium phosphate cement is less known. In this paper, magnesium potassium phosphate cement (MKPC) pastes in which magnesium oxide was replaced by up to 30% limestone were studied. The effect of limestone content on the hydration temperature, setting time, compressive strength, and early water stability were investigated. The deterioration mechanism in water was explored by analyzing the pH value of the immersed solution, leached substances, total phosphorus content and microstructural analyses including X-ray diffraction (XRD), scanning electron microscope (SEM), thermogravimetric analysis (TG/DTG) and mercury intrusion porosimetry (MIP). The results showed that addition of limestone into the magnesium phosphate cement matrix accelerated the setting time, decreased the total heat evolution of MKPC, and clearly degraded the strengths. The poor water stability of MKPC with limestone powder might be caused by the poor crystallinity and crystal morphologies of hydration products, worsened pore structure and increased dissolution of hydration products. Considering the materials costs and the strength development, the optimum addition of limestone powder should be no more than 10%.