PURPOSE: To provide a lightweight plasterboard retaining its mechanical strength and resistance to water and heat.

CONSTITUTION: The plasterboard is made up two sheets of base paper 1, 2 and a board body 3 sandwiched in between them. The board body is a composite composed of gypsum and a calcium silicate compound [pref. xonotlite and/or tobermorite, being recommended to be agglomerates (e.g. spherical ones ca.40μm in diameter) produced through their association with each other]. Incorporation of 0.5-10wt.% of fine crystalline gypsum dihydrate as seed crystal in the stock slurry in advance makes gypsum dihydrate crystal fine, raising the bond strength of the board body 3 to the sheets of base paper 1, 2.