Investigation of magnesium phosphate cement hydration in diluted suspension and its retardation by boric acid

Abstract

Magnesium phosphate cements (MPCs) are used for rapid repair works, but they may also offer prospects for the stabilization/solidification of deleterious waste. MPCs contain calcined magnesium oxide and a water-soluble acid phosphate, such as potassium dihydrogen phosphate (KH2PO4). The main precipitated hydrate is then K-struvite (MgKPO4·6H2O). This work aims at giving new insight into the processes involved in its formation. Since cement hydration is very rapid, the second objective is to understand how boric acid, a common admixture for field application, retards cement hydration. A multi-stage process is evidenced in diluted suspension: MgHPO4·7H2O likely precipitates first. This phase is then destabilized to form Mg2KH(PO4)2·15H2O which is finally converted into K-struvite and cattiite (Mg3(PO4)2·22H2O). Boric acid doesn't slow down the initial dissolution of the reactants, but rather retards the precipitation of the products. Besides, it tends to favor the formation of cattiite against that of K-struvite.