261 / 2022-08-07 17:13:37

The role of supplementary materials on micro/macro properties and water stability of magnesium phosphate cement

Magnesium phosphate cement,Supplementary materials,Mechanical properties,Water stability,Microstructural progress

Magnesium phosphate cement (MPC) is broadly known as one of ideal repair materials for its high early strength, rapid setting time and good adhesion. However, the existing amorphous phase of MPC could react with water when immersing in water, resulting in large internal stress and cracking in the MPC, thus limiting its application. In this paper, supplementary materials (SMs) including fly ash (FA), silica fume (SF), aluminum silicate (AS) and bauxite (BX) were used to improve both mechanical properties and water stability of MPC. Besides, the hydration mechanism and the microstructural progress were studied. It was found that the addition of SMs to MPC decreased the hydration heat release (Fig. 1) and retarded the final setting time (Fig. 2) compared with pure MPC mixture. Moreover, significant improvement in mechanical strength (including early strength) and sorptivity properties were observed with 5%, 3% and 12% contents of SF, AS and BX mixed in the MPC and FA amalgams.



For water stability, the mass loss behavior, pH characteristics of curing water and mechanical strength of specimens cured in water were studied in this paper. The results showed that with the addition of SF, AS and BX to the MPC-FA composites, the mass loss reduced to 1.56%, 0.93% and 0.92%, respectively as compared to the 6% mass loss of the pure MPC mortar at 28d. Besides, the strength retention coefficients for pure MPC mortar, MPC+FA+5%SF, MPC+FA+3%AS, MPC+FA+12%BX at 28d were 0.87, 0.98, 0.90 and 0.98, respectively for compressive strength and 0.81, 0.94, 0.93 and 0.95, respectively for flexural strength. Furthermore, it was found that the variations of pH behavior of MPC specimens blended with SMs were relatively lower than those of pure MPC mortars, indicating that the penetration of curing water was occurred low amount into the microstructure and dissociated a little amount of materials from insides in curing water. Based on the obtained results, it was revealed that the water stability of MPC was improved by the addition of SMs.



The reason for the improvement of water stability of MPC-SMs was further explained via the investigation of microstructural progress and hydration mechanism. Huge cavities were found in the pure MPC specimen (Fig. 3) due to the high temperature release and water evaporation, leading to high permeability, low mechanical strength and water resistance. However, by investigating the microstructure of MPC-SMs, it was revealed that unreacted FA particles and the formation of intermediate crystals reduced the void spaces and increased the structural compactness, resulting in the enhancement of mechanical strength and water stability of MPC composites. Moreover, it was found that amorphous contents were reduced with the increase in hydration ages. Overall, this research provided reliable theoretical basis and technical support for the subsequent studies and engineering application of MPC blended with SMs.