Effect of additives on properties of magnesium phosphosilicate cement

Magnesium phosphate-based cements are rapid setting materials. Retarders are often used to prolong the setting time of magnesium phosphate-based cements. Magnesium phosphosilicate cement (MPSC) is a new type of magnesium phosphate-based cement, which is composed of magnesia, phosphate and silicate materials, such as fly ash. MPSC has the characteristics of rapid setting and high early strength. In the present study, the effect of different additives, sodium tripolyphosphate (ST), borax (NB) and boric acid (BA) on the properties of MPSC was investigated, including the setting time, temperature rise of cement during initial hydration and strength development. In comparison with the MPSC sample without additive, ST had an acceleration effect on the setting process, whereas NB and BA had a retarding effect on the hydration. BA had a longer retardation time than NB. The higher the rate of temperature rise, the higher was the reaction rate, the shorter was the setting time, and the higher was the early strength. Hydration products of MPSC were analysed by X-ray diffraction. The results showed that a cementitious hydrate, MgKPO₄·6H₂O (MKP), formed in the hardened cement paste. There was no new product formed after the additives were incorporated in the hydration system, they might influence the formation rate of MKP during reaction.