Characterization of (A/F)H3 Phase Microstructure with Different Al/(Fe+Al) Ratios Based on Calcium Sulfoaluminate Cement

Abstract

The hydration product Fe(OH)3 gel phase or Al-containing Fe(OH)3 [abbreviated as (A/F)H3] gel phase enables calcium sulfoaluminate cement (CSA) to obtain excellent cementitious properties. The formation of these two gels is closely related to the ratio of Al and Fe in the raw materials. The microstructure and properties of hydration products in CSA are seriously affected by the ratio of Al and Fe, and the effect on the gel cannot be ignored. In order to enable the influence relationship between the ratio of Al and Fe and the gel phase was thoroughly investigated. In this study, (A/F)H3 with different Al/(Fe+Al) ratios was synthesized from its chemical composition by sol-gel method, and its microstructure was characterized to determine the formation process of this phase. X-ray diffraction (XRD), thermogravimetric analysis (TGA), inductively coupled plasma emission spectroscopy (ICP), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscope (TEM) and N2 adsorption-Brunauer Emmett Teller (BET) were utilized to investigate the microstructure of (A/F)H3 with different Al/(Fe+Al) ratios. The results show that Fe in Fe(OH)3 was substituted by Al to form the (A/F)H3 phase of goethite crystal structure at pH=12.5. The gradual increase of Al/(Fe+Al) ratios significantly changed the crystal structure parameters of (A/F)H3, and the adsorption capacity and desorption temperature of -OH groups were improved. When the ratio was greater than 40% by mol, the crystal structure of (A/F)H3 was transformed, and the crystallinity was significantly reduced. In addition, the substitution of Al resulted in a significant decrease in the aspect ratio of the (A/F)H3 phase particles, the microscopic morphology changed from needle-rod to granular, and a larger specific surface area was obtained.