Abstract: A process for preparing a solid material containing coal ash includes a first step of mixing coal ash with a calcium compound to obtain a mixture, a second step of molding the mixture to obtain a molded article, and a third step of subjecting the molded article obtained in the second step to a hydrothermal treatment at a temperature of at least 120.degree. C. under high pressure. In the first step, 40 to 95 parts by weight of the coal ash is mixed with 60 to 5 parts by weight of the calcium compound, and the coal ash to be used has a bulk density of at least 0.8 g/cm.sup.3, an average particle diameter of 5 to 40 .mu.m and an aluminum content of 35% by weight or less in terms of Al.sub.2 O.sub.3. By the use of a blowing agent or the like, it is also possible to obtain a porous lightweight solid.