Microstructure of Fe(OH)3 phase in hydration products of calcium sulfoaluminate cement

The Fe(OH)3 phase is one of the important hydration products in Ferric-rich calcium sulfoaluminate (FR-CSA) cement and is an essential contributor to the desirable properties of FR-CSA. However, Fe(OH)3 phase has always been investigated as an auxiliary component of the cement hydration system, and the understanding of the microstructure of Fe(OH)3 phase seems to have not been clarified. Therefore, in order to thoroughly investigate the microstructure of the Fe(OH)3 phase to determine the formation process of this phase. In this study, the Fe(OH)3 phase was synthesized from chemical composition using the sol–gel method, and compared with the hydration product Fe(OH)3 of the ferric phase prepared by solid-phase sintering. The microstructure of the Fe(OH)3 phase was investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-Brunauer Emmett Teller (BET) and Mössbauer spectroscopy. The results indicate that Fe(OH)3 phase exists in the form of crystals at different pH, and the crystallinity of Fe(OH)3 phase gradually increases with the increase of pH. At low pH (<10), the Fe(OH)3 phase is microcrystalline (ferrihydrite), while high pH (>10) favours stable Fe(OH)3 (goethite) crystal structures to be obtained. In addition, with the increase of pH, the micro-morphology of Fe(OH)3 phase changes from granular to needle-like, and the particle size (length) gradually increases and the specific surface area decreases. No evidence of amorphous Fe(OH)3 was found during hydration, and XRD, TEM and Mössbauer spectroscopy indicated that Fe(OH)3 has a distinct crystal structure. Compared with the chemically synthesized Fe(OH)3 phase, the hydrated Fe(OH)3 phase exists in a microcrystalline state with a flaky microscopic morphology and smaller particle size.