Investigation of water stability and microcharacteristics of magnesium potassium phosphate cement modified by calcium aluminate cement and wollastonite

The early water stability of magnesium potassium phosphate cement (MKPC) blended with calcium aluminate cement (CAC) and wollastonite (WS) was investigated. Macroscopically, the presence of CAC prolongs the setting time of MKPC because of the stimulating effect of the admixture on the composite retarder. The synergistic effect between CAC and WS enhances the early water stability and compressive strength of MKPC subjected to water curing. From the microscopic aspect, the X-ray diffraction and Fourier transform infrared spectroscopy results indicate that the CAC phase participates in the reaction of the MKPC system to produce crystalline (metavariscite) and amorphous (hydroxyapatite and gibbsite) products. In contrast, although WS does not participate in the reaction, it inhibits the development of cracks and strengthens the brittle matrix. Furthermore, the results of the scanning electronic microscopy–energy dispersive spectrometry, mercury intrusion porosimetry, and Brunauer-Emmett-Teller nitrogen adsorption–desorption analysis confirm that the amorphous products coating the surface of struvite-K can change its solubility. Meanwhile, the crystalline product is embedded in the gap of struvite-K crystals, refining the pore structure and preventing water from entering the interior of the hardened paste.