Preliminary investigation of the hydration mechanism of MgO-SiO2-K2HPO4 cement

Abstract

Different from making ordinary magnesium phosphate cements (MPCs) with potassium dihydrogen phosphate (KH2PO4, PDP), magnesium oxide (MgO, M) and borate as retarder, a novel high-strength Magnesium Silicon Potassium Phosphate Cement (MSPPC) created by mixing dipotassium hydrogen phosphate (K2HPO4, DHP (P)), silica fume (SF), MgO and Water (W), without the use of a retarder, is proposed for the first time in this study. The mechanical properties of several MSPPC mixtures prepared with different proportions of P/M (weight ratio of K2HPO4 to MgO), W/C (weight ratio of water to cement paste (C = P + M + SF)) and mass fraction of silica fume were investigated. The hydration reaction mechanism of MgO-SiO2-K2HPO4 system was analyzed with a variety of techniques including pH, X-ray diffraction (XRD), environmental scanning electron microscope-energy dispersive spectrometer (ESEM-EDS) and thermo-gravimetric and differential scanning calorimeter (TG-DSC). The results show that the cement paste with P/M of 1/3, 15 wt% SF and W/C ratio of 0.16, the compressive strength was measured to be 113.16 MPa. The maximum flexural and compressive strength of MgO-SiO2-K2HPO4 system is 16.82 MPa and 101.26 MPa respectively, which is higher than those of MgO-K2HPO4 system. In MgO-K2HPO4 system, the reaction generates a large amount of Mg(OH)2 with poor gelation and expansion, and a small amount of hydration product MgKPO4·6H2O (MKP) with low internal crystallinity, so the structural strength is very low. Inversely, in MgO-SiO2-K2HPO4 system, silica fume plays an important role, and the active silicon components in silica fume are fully involved in the acid-base reaction, improving the mechanical properties and compactness of microstructure of MSPPC. Main hydration product struvite-K has good crystallization, compact accumulation, high density of cross-section structure and good structural integrity of MSPPC. Expected secondary hydration products are magnesium silicate gel (e.g., MgSiO3) and amorphous silicon phosphate phases, contributing to the strength in MSPPC pastes.