Effect of pH value on the growth and properties of magnesium phosphate cement paste with a large water-to-solid fraction

In this study, the effect of pH value, a neglected but important factor of magnesium phosphate cement (MPC) pastes, was investigated to generate new ideas in the field of large water-to-solid (w/s) fraction MPC preparation. To avoid the introduction of impurities, the pH value was controlled by adjusting the proportions of diammonium hydrogen phosphate (DAP) and ammonium dihydrogen phosphate (ADP). The results showed that the ADP:DAP ratio had strong effects on the setting characteristics, mechanical properties, phase composition and microstructures of the MPC pastes. Regarding the mechanical properties in particular, the variation curve of compressive strength (up to 28 d) exhibited a salient wave peak, which was tremendously different from the variation of the MPC pastes with a low w/s fraction. A speculative theory with a reasonable interpretation is thus proposed.