Ceramics-Silikáty 64, (1) 75 - 83 (2020)

In this paper, fly ash (FS), phosphoric acid (H₃PO₄), ferrous sulfate (FeSO₄·7H₂O) and ethyl silicate (TEOS) were incorporated to devote effort for the further improvement of the water resistance of magnesium oxychloride cement (MOC). The strength retention coefficient was tested to evaluate the water resistance of the MOC, in which the addition of FS + H₃PO₄ + FeSO₄·7H₂O + TEOS resulted in a remarkable improvement. The characterisation of the hydration products before and after the water immersion was carried out using X-ray diffraction (XRD), a thermogravimetric (TG) analysis, Fourier-transformed infrared spectroscopy (FTIR) and a scanning electron microscope (SEM). Through the XRD, TG and FTIR analysis, the results showed that the addition of FS + H₃PO₄ + FeSO₄·7H₂O + TEOS resulted in the generation of the amorphous phase during the water immersion. This amorphous gel was identified as a mixture of a magnesium-chloride-silicate-hydrate gel (M-Cl-S-H gel) and an insoluble magnesium-chloride-hydrate gel (M-Cl-H gel) by Energy Dispersive Spectrometer (EDS). The generation of the insoluble M-Cl-S-H gel and M-Cl-H gel and the densification of the microstructure contributed to the valuable and long-term improvement of the water resistance in the MOC. Then the transformation of the morphology might be due to the interaction performance and superposed effect of FS, H₃PO₄, FeSO₄·7H₂O and TEOS.