Microstructure Control of AH3 Gel Formed in Various Calcium Sulfoaluminate Cements as a Function of pH

Calcium sulfoaluminate (CSA) cements, as one of the most promising low-CO2 alternatives to Portland cements, have been receiving increasing attention worldwide. Aluminum hydroxide (AH3) gel is a vital hydration product and contributes significantly to the performances of CSA cements. This work investigated the behavior of AH3 gel in different CSA cements as a function of pH, used for regulating different CSA cement performances applied in various civil engineering areas. Different CSA cement clinkers (ss-ye'elimite, Sr-ye'elimite, and Ba-ye'elimite) were sintered and hydrated in various alkaline environments, and the evolution process of the formed AH3 gel was studied by laboratory techniques, including X-ray diffraction, Rietveld method, thermogravimetry, field-emission scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction image. Results show that Ba-ye'elimite always had the rapidest hydration rate regardless of the change of pH, compared with ss-ye'elimite and Sr-ye'elimite. The final hydration products formed by the three ye'elimite changed largely as a function of pH. The AH3 phase of the three ye'elimite pastes exhibited a low crystallinity degree and had a similar content at pH ≤ 13; however, its crystallinity degree was improved largely (e.g., the well-crystalline AH3 was formed with a polycrystalline structure), but its content was decreased at pH > 14. In addition, the AH3 phase in the Ba-ye'elimite sample exhibited the rapidest growth rate as a function of pH, but was formed with the least content among the three ye'elimite samples at each pH. The compositions of AH3 were different in the three ye'elimite samples, and the contents of Ca/Sr/Fe within AH3 changed as a function of alkalinity. Thus, the AH3 phase can be controlled by the crystal structure of ye'elimite and pH, which would be beneficial to design different performances of CSA cement-based construction materials.