PURPOSE: To effectively utilize industrial waste, to eliminate the defficiency of the treatment plant thereof and to facilitate the refilling of an underground cavity.

CONSTITUTION: Since the sludge discharged from an intermediate treatment plant of waste acid and waste alkali has clayey consistency and flowability, it has almost no mechanical strength. When cement is added to this sludge and the resulting mixture is cured, mechanical strength as aggregate is imparted to the sludge and the chemical stability of the sludge is also increased. Since the mixture of the sludge and cement or a filler prepared by adding water to said mixture has flowability, the filling work of an underground cavity is easy and the underground cavity can be filled without generating gaps.