PURPOSE: To obtain a hydraulic object having a high strength using diatomaceous earth by adding lime and water to a diatomaceous earth component, converting aluminum oxide therein into calcium aluminate and adding cement thereto.

CONSTITUTION: Lime and water are added to diatomaceous earth powder to convert aluminum oxide therein into calcium aluminate and cement is then added thereto. Portland cement-based, alumina cement, jet cement, mixed cement-based or high-sulfate slag cement-based cement, a mixture thereof, etc., are cited as the cement used. Alternatively, the lime and water may be added to the diatomaceous earth powder to convert the aluminum oxide therein into the calcium aluminate while stirring the resultant mixture at ordinary temperature or by heating. An acid may be added to neutralize the resultant substance and alumina cement may be added thereto. The above-mentioned hydraulic object can be utilized as building materials, such as ceiling board, interior or exterior finish, of buildings, roofing tiles and all the cement and concrete products and uncalcined ceramic products. The specific gravity thereof can be suppressed to a low value, such as about 1.5.