Phase development of different magnesium phosphate cements at room temperature and elevated temperatures

This work focuses on the study of the development of the solid phase assemblage in four different magnesium phosphate cements (MPC) at room temperature and elevated temperatures. Rapid setting pastes were prepared by mixing magnesium oxide with four different phosphate powders, namely KH2PO4, Mg(H2PO4)2, Al(H2PO4)3, and Ca(H2PO4)2. The phase formation at room temperature was investigated qualitatively and quantitatively by X-ray diffraction, whereas samples subjected to temperatures of 85, 120, 250 and 1000 °C were analyzed only qualitatively by the former method. TGA and FTIR were employed complimentary to X-ray for the investigation of the samples treated at room temperature. At room temperature, XRD and TGA revealed the presence of struvite as the main hydration product in the MgO - KH2PO4 mixture. From the ATR-FTIR spectra of the same system, vibration bands related to an X-ray amorphous precursor of K-struvite were identified. Upon heating, in the temperature range from 85 to 700 °C, struvite was found to collapse to a non-crystalline phase. After exposure at 1000 °C, the long–range crystallographic order was restored and the presence of KMgPO4 was identified. For the Mg, Al and Ca based MPCs, newberyite was identified as main hydration product at room temperature. Additionally, the precipitation of other phases such as cattite or brushite took place depending on the type of phosphate used. In all three cases, changes in the phase assemblage upon exposure at different temperatures, could be linked with the decomposition of newberyite and formation of Mg3(PO4)2 (farringtonite) through the reaction of pyrophosphate and excess MgO. Finally, SEM/EDX analysis revealed that sample treated at 20 °C exhibit irregular shaped crystals, whereas the presence of a rounded morphology like structure related to the Mg3(PO4)2 phase was identified in Mg, Al and Ca based samples subjected at 1000 °C.