Calcium aluminosulfate (Ca4Al6O16S or C(4)A(3)(S) over bar) was prepared by direct synthesis from calcium and aluminum nitrates, and aluminum sulfate. CaAl4O7 (CA(2)) formed as an intermediate at 900degreesC, and C(4)A(3)(S) over bar was the main phase after calcination at 1100degreesC. The specific surface areas after calcination at 1100degrees and 1300degreesC were similar to2.5 and 1 m(2)/g, respectively. Hydration was investigated using XRD, DSC, SEM, conduction calorimetry, and solid-state Al-27 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 1100degreesC and was 91% complete in the sample calcined at 1300degreesC.