Effect of mixture proportioning on the strength and mineralogy of magnesium phosphate cements

Abstract

Magnesium potassium phosphate cement (MKPC) has properties advantageous over ordinary portland cement such as quick setting and rapid strength gain. Although the effect of mixture proportioning on MKPC pastes has been studied, there are conflicting reports on how calcination of magnesia, parameters like magnesium-to-phosphate ratio (M/P) and water-to-binder ratio (W/B), added materials like borax and fly ash, or the addition of sand influence mineralogy and properties like setting and strength. These factors are evaluated over a practical range of values for each using X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy on paste samples. Strength development is evaluated using both pastes and mortars. Calcination of magnesia forms some forsterite with impurity SiO2 which may be linked to the reduction of reactivity. Increasing W/B increases setting time more significantly at lower M/P. Increasing M/P increases early strength but not so much ultimate strength. The main final phases in the pastes are K-struvite and unreacted MgO. Addition of borax or low-lime fly ash does not alter the mineralogy of hydrated pastes. Varying M/P simply changes the relative amounts of these solid phases. Unlike in Portland systems, adding up to ~60 vol% sand to pastes does not decrease strength considerably.