The hydration reaction of Ca A-type zeolite and calcium hydroxide was studied. The results revealed that an isostructural material of hydrocalumite (C-A-H) is generated at the early stage of hydration, and following that of stratlingite-vertumnite is generated. Only 0.77 nm of the C-A-H phase was observed at a humidity below 40% while part of the 0.77 nm phase swelled to 0.82 rim. at a humidity of 40% or above.