Preliminary investigation of the hydration mechanism of MgO-SiO_2-K_2HPO_4 cement

摘要

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 center dot 6H(2)O (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. (C) 2019 Elsevier Ltd. All rights reserved.