Properties of fly ash blended magnesium potassium phosphate mortars: Effect of the ratio between fly ash and magnesia

The ratio between fly ash (FA) and magnesia is an important factor for the optimum design of FA blended magnesium potassium phosphate cements (MKPCs). In this study, a high CaO content FA (CaO = 12.5 wt%) was used to partially replace magnesia at 0 wt%, 30 wt%, 50 wt%, 70 wt%, and 90 wt%, respectively. The experimental results showed that a FA replacement of 50 wt% led to the highest compressive strengths. A FA replacement of 70 wt% is considered as upper limit, as the presence of more FA caused significantly lower strength. In the plain and the FA blended MKPCs, K-struvite (MgKPO4⋅6H2O) was the main hydrate. At very high FA contents, additional calcium potassium hydrogen phosphate (CaK3H(PO4)2) was observed as well as the destabilization of K-struvite to cattiite (Mg3(PO4)2⋅22H2O), which could be one of the main factors responsible for the lower strength of high FA blended MKPC mortars stored under water.