Influence of iron substitution on the microstructure and properties of ettringite in calcium sulfoaluminate cement: A comprehensive study on the Al/(Fe+Al) ratios

Abstract

Ettringite, as the dominant hydration product of calcium sulfoaluminate cement, significantly contributes to cement strength. Its formation is closely related to the Fe and Al ratio in raw materials. During cement hydration, Fe present in ferrite can replace some of the Al in ettringite, which notably affects the microstructure and properties of the latter. To understand the relationship between the Al/(Fe + Al) ratios and the ettringite phase, we synthesized ettringite with varying Al/(Fe + Al) ratios using a chemical synthesis method. The microstructure was then characterized to determine the formation process of this phase. Various analysis methods, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscope (TEM), and N2 adsorption-Brunauer Emmett Teller (BET), were used to study the solid phases. For aqueous solutions, inductively coupled plasma emission spectroscopy (ICP-OES) was employed. Our results reveal that Fe replacing Al leads to regular changes in the ettringite crystal's microstructure. As the Al/(Fe + Al) ratio decreases, there is a reduction in the ettringite unit cell parameter a, an increase in c, and an enlargement of the unit cell volume. Concurrently, Fe substitution shrinks the crystallinity and grain size of ettringite crystals. SEM-TEM and BET demonstrate that Fe substitution boosts ettringite's particle size and aspect ratio, while the specific surface area contracts with a decreasing Al/(Fe + Al) ratio. Moreover, Fe substitution tends to lower ettringite's thermal stability and OH group binding ability. When the Al/(Fe + Al) ratio reached 80%, the ettringite crystal exhibited optimal thermal stability.