To effectively utilize a waste formed when producing an artificial light-weight aggregate.

A melted and attached material attached to a wall surface of a rotary kiln without being formed into granules when firing expansive shale is crushed by a drill to provide a crushed lumps in proper shapes. The crushed lump has 0.8-1.7 specific gravity, and can adsorb fine particles because the crushed lump has open pores providing ≥25% porosity and has an water- purifying effect. The crushed lump can be used as a filler 1 for a storing vessel 10 of rain water because the lump can preserve water for a long period. Further, the lump is optimum as a water-retaining material 1 for a plant because the crushed lump has pH 6.5-8.6 which is same as that of a natural lava. The lump which is the waste material at the time of production of the light-weight aggregate can be effectively utilized.