Different effects for phosphoric acid and calcium citrate on properties of magnesium oxysulfate cement

Abstract

Chemical additive is an indispensable component to improve the properties of magnesium oxysulfate cement (MOSc). In this paper, the differences of the effects of chemical additives phosphoric acid (PA) and calcium citrate (CC) on the performance of MOSc were investigated by analyzing the composition and content of MOSc hydration products and characterizing the microstructure. The results manifest that the hydration rate of MOSc with PA is relatively slow, but the crystalline grain of the hydration product 5 Mg(OH)2·MgSO4·7H2O (5·1·7 phase) is coarse, and the composition of the hydration product is excellent, so it has good mechanical strength and water resistance. The hydration rate of MOSc with CC is relatively fast, and the amount of 5·1·7 phase is relatively high, but its crystalline grain is not as coarse as that of MOSc with PA. Therefore, the MOSc with CC has high early strength, but its later mechanical strength and water resistance are not as excellent as that of MOSc with PA. Meanwhiles, it is found that 5·1·7 phase with low crystallinity and 5·1·n phase with high content of crystal water may exist in MOSc, which is another factor affecting the reduction of its mechanical strength after immersion. The research results are of great significance for promoting the progress of chemical additive modification technology and improving the performance of MOSc.