Conversion from metastable phase to stable phase in calcium aluminate cement (CAC) causes volumetric instability, resulting in loss of compressive strength. This study investigated the effect of CaSO4 on the phase conversion of calcium aluminate cement. The specimens with different dosage of CaSO4 were exposed to 60 degrees C to trigger the phase conversion of CAC after curing at 20 degrees C for 7 days. Analysis with X-ray diffraction, thermogravimetry, mercury intrusion porosimetry, and scanning electron microscopy were conducted. The incorporation of CaSO4 inhibited the formation of unstable CAH(10) by direct precipitation of ettringite. As a result, the reduction of strength of CAC specimens due to the increased pore volume by conversion was inhibited by ettringite and monosulfate formation. (C) 2019 Published by Elsevier Ltd.