A method is provided for advantageously altering the rate of hardening of a magnesium silico-phosphate cement (MSPC). Addition of on the order of 1% of an [MF6]n− salt or acid to an MSPC, obtained by adding the salt or acid either directly to the dry mix or to the water used to effect hydraulic hardening of the cement, significantly alters the hardening rate without adversely affecting the physical properties of the final set cement. In preferred embodiments, Na2TiF6 and/or K2TiF6 are used as retardants, while K3AlF6 is used as an accelerant. Other embodiments use M′n-MF6 compounds wherein M′ is an alkali metal, an alkaline earth metal, or H, and M is chosen from inter alia Ti (n=2), Zr (n=2), Si (n=2), P (n=1), Al (n=3), and Sb (n=1).