Calcium monosulfate hydrates, Ca-4[Al-2(OH)(12)][1-x)SO4 . x(OH)(2)]. 6H(2)O were synthesized at 20 degrees C from aluminate hauyne, Ca8Al12O24(SO4)(2) by hydration under the presence of gypsum and portlandite. The experimental water/solid ratio was 0.80. Quantitative analysis by EPMA revealed that the x reaches up to 0.16 under the present synthetic conditions where ettringites are considered to be stable at lower water/solid ratios. Stabilities of accompanied phases were discussed in terms of phase assemblage.