Abstract
Calcium aluminosulfate (Ca4Al6O16S or C4A3S̄) was prepared by direct synthesis from calcium and aluminum nitrates, and aluminum sulfate. CaAl4O7(CA2) formed as an intermediate at 900°C, and C4A3S̄ was the main phase after calcination at 1100°C. The specific surface areas after calcination at 1100° and 1300°C were ∼2.5 and 1 m2/g, respectively. Hydration was investigated using XRD, DSC, SEM, conduction calorimetry, and solid‐state 27Al MAS‐NMR spectroscopy. Calorimetry showed that the induction period was longer than that of a sample prepared using conventional solid‐state sintering, and this was attributed to the formation of amorphous coatings. Crystalline hydration products, principally calcium monoaluminosulfate hydrate and aluminum hydroxide, appeared subsequently. Although the induction period was very long, complete hydration occurred as early as 3 d in the sample calcined at 1100°C and was 91% complete in the sample calcined at 1300°C.
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