ABSTRACT
Significant interest in waste-cement recycling has been stimulated because of the high contents of calcium and silicon in waste cement. The reactivity of calcium and silicon in the raw material is one of the important factors for the hydrothermal synthesis of xonotlite. Therefore, the effect of the reactivity of calcium and silicon in the waste cement on the hydrothermal synthesis of xonotlite was studied in this paper. Portland cement that was hydrated for 6 months, with the aim of simulating the waste cement, was used for the first time as the calcium and silicon source in the hydrothermal synthesis of xonotlite. As calcination would raise the reactivity of the hydrated cement, the effect of calcination of the Portland hydrated cement on the hydrothermal synthesis of xonotlite was investigated. The hydrated cement was calcined at 900 °C, and the hydrothermal synthesis was carried out at 220 °C for different times. The phases of the hydrothermal products were analyzed by XRD and TG-DSC, and it was noted that the calcination of hydrated cement affected the formation rate of xonotlite. The content of xonotlite increased from 18% (synthesized with hydrated cement without calcination) to 74% (synthesized from hydrated cement with calcination at 900 °C) during a reaction time of 24 h. Furthermore, the micromorphologies of xonotlite using calcined and hydrated cement were compared and discussed from the perspective of the reactivity of the starting materials.
ABSTRACT
Additions of andalusite aggregates (19 wt%) were shown in previous literature to enhance the antioxidation of Al2O3-SiC-C (ASC) castables. This work aims to investigate whether micronized andalusite has a greater influence on antioxidation improvement than andalusite aggregates. Various low contents (5 wt% and below) of micronized andalusite (≤5 µm) were introduced as a substitute for brown fused alumina in the matrix of ASC castables. The antioxidation of castable specimens was estimated by the oxidized area ratio on the fracture surface after a thermal shock test. The microstructure and phases of micronized andalusite and the castable specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results suggest that the antioxidation effects of ASC castables with a low addition of micronized andalusite are effectively enhanced. The heat-induced transformation of andalusite produces SiO2-rich glass, favoring the sintering of the castable matrix and impeding oxygen diffusion into the castable's interior. Therefore, the castable antioxidation is enhanced without deteriorating the hot modulus of rupture.
