Tests on magnesium potassium phosphate composite mortars with different water-to-binder ratios and molar ratios of magnesium-to-phosphate

The objective of the present study is to examine the effect of the water-to-binder ratio (W/B) and the molar ratio (Mmp) of magnesium-to-phosphate on the fluid characteristics, compressive strength development, and pH variation of magnesium potassium phosphate composite (MKPC) mortars. A total of 25 mortar mixes were prepared, with the W/B varying from 20% to 40% and Mmp varying from 3.4 to 30.4. Using the present test data, the compressive strength development of MKPC mortars was empirically formulated, which reflects rapid strength gain. Considering practical qualifications, including a relatively good 28-day compressive strength above 30 MPa, delayed setting time, and a near-neutral pH not exceeding 9.4, it can be recommended that the Mmp and W/B be chosen to be below 5.1 and above 25%, respectively, in the MKPC system. With a decrease in the Mmp value, the peak intensities for struvite-k increased slightly, whereas the peak intensities for unreacted MgO were considerably weakened as the Mmp value fell below 5.1. The total porosity of the MKPC pastes was governed by the macro-capillary pores, implying that the 28-day compressive strength is inversely proportional to the macro-capillaries.