Understanding hydration properties of magnesium potassium phosphate cement with low magnesium-to-phosphate ratio

Magnesium potassium phosphate cement (MKPC) is used for rapid repair works, but is a candidate for encapsulating intermediate level waste due to its low alkaline or neutral environment. This work aims to at giving insights into the hydration process of MKPC with quite low magnesium-to phosphate ratios. The hydration process of MKPC under low M/P conditions was investigated using XRD identification, pH/conductivity, isothermal calorimetry, FTIR, TG/DSC and SEM methods. The results show that low magnesium-to phosphate ratios (M/P = 0.2–1) of MKPC result in low pH values in the range of 6.3–8.5 which is suitable for the applications in low alkaline or neutral environment. No obvious hydration products were observed at M/P ≤ 0.2, and as M/P increases, MgHPO4•3 H2O primarily formed, followed by Mg2KH(PO4)2•15 H2O. As M/P increase up to 1, Mg3(PO4)2•22 H2O was also found at the initial and later stage of hydration except at the mid-stage of hydration. Mg3(PO4)2•22 H2O, MgHPO4•3 H2O and Mg2KH(PO4)2•15 H2O were the primary hydration products of MKPC at low M/P, where the formation sequence and properties of them may offer the research interests for the application in radioactive waste encapsulation.