Chemical degradation of magnesium potassium phosphate cement pastes during leaching by demineralized water: Experimental investigation and modeling

The long-term durability of magnesium potassium phosphate cement (MKPC) pastes was investigated by examining their leaching behavior. MKPC comprised magnesium oxide (MgO) and potassium dihydrogen phosphate (KH2PO4) in equimolar amounts and yielded K-struvite (MgKPO4·6H2O) and a nearly neutral pore solution pH upon hydration. Semi-dynamic leaching tests were performed on MKPC paste samples using demineralized water with a pH set at 7, and the leached solids were analyzed using XRD, SEM/EDS, 11B and 31P MAS-NMR spectroscopy. Leaching was mainly governed by diffusion of dissolved species through the pore network of the paste. Three main zones were observed in the leached solids: (i) a poorly cohesive residual layer where K-struvite was fully depleted, (ii) an intermediate zone where K-struvite coexisted with cattiite (Mg3(PO4)2·22H2O), and (iii) a third zone without any cattiite. Reactive transport modeling made it possible to predict the extent of degradation and the phase evolution in the MKPC paste samples. •Magnesium phosphate cement paste was investigated under leaching conditions.•A zonation process occurred in the leached solid.•K-struvite dissolved and cattiite precipitated transiently.•A reactive transport model helped describing the paste evolution under leaching.