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Aluminum sulfate shows a positive effect on preventing efflorescence of magnesium potassium phosphate (MKP) cements. Its influence on the MKP cement properties and hydration is explored in this study. Compared to a reference cement (Mg/PO4 molar ratio = 4, w/c ratio = 0.25), aluminum sulfate retards setting, increases compressive strength, and has little influence on the short-term volume stability. Aluminum sulfate slows down the hydration kinetics, decreases the pH values, accelerates the depletion of KH2PO4, and thus mitigates the formation of efflorescence. It also modifies the hydrate assemblage. In addition to K-struvite, aluminum sulfate leads to the formations of arcanite and aluminum-containing hydrates such as amorphous aluminum phosphate, possibly phosphate containing LDH (layered double hydroxide) and minyulite-like phases. The formation of amorphous MgHPO4 and minor amounts of bobierrite and brucite as observed in the reference paste, are suppressed in the presence of aluminum.
Xu et al. (Thu,) studied this question.