Thermodynamic data for magnesium (potassium) phosphates

Magnesium potassium phosphate (MKP) cements show fast setting and high early strength. MKP cements harden in the presence of water due to the reaction of potassium dihydrogen phosphate (KH2PO4) with magnesia (MgO) resulting in the formation of mainly K-struvite (MgKPO4 center dot 6H(2)O). Other hydrates, newberyite, phosphorrosslerite, cattiite and Mg2KH(PO4)(2)center dot 15H(2)O are formed as intermediate phases during hydration. This paper presents a consistent thermodynamic database for magnesium phosphate hydrates based on solubility measurements for newberyite, phosphorrosslerite, farringtonite, Mg-3(PO4)(2)center dot 4H(2)O, bobierrite, cattiite, K-struvite, KMgPO4 center dot H2O, and Mg2KH(PO4)(2)center dot 15H(2)O carried at different temperature. It was found that the formation of the different hydrates is strongly influenced by pH. Newberyite, phosphorrosslerite and Mg2KH(PO4)(2)center dot 15H(2)O are intermediate phases, which form at low pH as typical at very early hydration times. As pH rises, bobierrite and cattiite may be stabilised, while at high pH K-struvite forms. The phases formed are also strongly influenced by the temperature.