Microstructure and Properties of Ettringite with Different Al/(Fe+Al) Ratios in the C<sub>4</sub>A<sub>3-x</sub>F<sub><i>x</i></sub>$-2C$·H<sub>2</sub> Binary Hydration System
Tian Zeng, Tingting Zhang, Jiangchuan Li, Kai Cui, Jun Chang
Abstract
The solid solution between Al-ettringite and Fe-ettringite (Al/Fe-ettringite), an important hydration product in Ferric-rich calcium sulfoaluminate (FR-CSA) cement, contributed to its desirable properties including excellent compact resistance and seawater erosion resistance. Previous research has given relatively little attention to this phase. Thus, the effect of Al/(Al+Fe) ratio on the crystal structure of ettringite in FR-CSA hydration products is of crucial importance to study. This paper investigates the influence of Fe 3+ doping on the C 4 A 3- x F x $-2C$·H 2 binary hydration system by calcining Fe 2 O 3 -doped ye’elimite, and then explores the microstructure of Al/Fe-ettringite. Various analysis methods, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and microcalorimeter, were used to study the phases. The results indicated that the incorporation of Fe 3+ influenced the hydration process of the C 4 A 3- x F x $-2C$·H 2 samples, with Fe 2 O 3 content exceeding 19.10 wt % retarding hydration kinetics. C 4 A 2.6 F 0.4 $-2C$·H 2 exhibited the highest compressive strength at 28 d, reaching 52.13 MPa. In the C 4 A 3- x F x $-2C$·H 2 samples, the introduction of Fe 3+ decreased the crystallinity of ettringite, reduced thermal stability, and weakened the OH group binding ability; the weight loss peak temperature of ettringite decreased from 123.5 to 117.0 °C. With increasing Fe 3+ content, the mean particle size of ettringite grew steadily from 9786 to 12132 nm. Fe 3+ in C 4 A 3- x F x $ preferred to generate the (A/F)H 3 phase, which included a lamellar stacking morphology with very high Fe 3+ substitution, as well as a fluffy (A/F)H 3 phase with moderate Fe 3+ substitution. Only a small amount of Fe 3+ entered into the ettringite unit cell to form Al/Fe-ettringite, and the actual Al/(Al+Fe) molar ratios of the ettringite were all greater than 0.8.