PURPOSE: To improve bending strength, impact strength, frost damage resistance, etc. by using a cementite mixture composed of specific weight ratios of porous lightweight aggregate, hydraulic cement, water, bending strength promoter and a water-holding capacity promoter for the lightweight aggregate.

CONSTITUTION: The porous lightweight aggregate (e.g. expanded clay, hearth ash), (B) the hydraulic cement (e.g. ordinary Portland cement), (C) water, (D) the bending strength promoter (e.g. styrene acrylic resin) and (D) the water- holding capacity promoter for the component A (e.g. cellulose ether) are mixed at such ratios at which the product having good impact strength and bending strength are obtd. The more specific examples of the mixing ratios include 44 to 74 wt.% component A, 10 to 40 wt.% component B, 4 to 14 wt.% component C, 0.5 to 15 wt.% component D and 0.01 to 1 wt.% component E. The resulted cementite mixture is extrusion molded, by which the lightweight concrete products for roof tiles, etc. are produced.