To properly adjust the flowability of backfill soil with a smaller blending amount of material by producing the material through a process that comprises recovering sludge which is in a slurry state and has a high water content and particles which are mostly fine clay/silt particles, subjecting a recovered sludge to filtration treatment, grinding dewatered solid matter captured by a filter medium into a powder, and mixing the powder with a prescribed amount of a rock powder.

The production of this material comprise storing sludge in a slurry state, which has a high water content and the particles of which are mostly fine clay/silt particles, in a thickener 1 to separate solid matter by sedimentation; then, discharging the solid matter in the form of a concentrated slurry from the thickener 1 and transferring the concentrated slurry to a sand screen 2 and separating and removing impurities such as grit from the concentrated slurry in the sand screen 2, charging the resulting concentrated slurry into a slurry collection vessel 3, transferring this concentrated slurry to a mixing vessel 4 to mix the concentrated slurry with an adequate amount of cement, thereafter, transferring the concentrated slurry contg. cement to a filter press 6 to separate the concentrated slurry into a filtrate and dewatered solid matter A captured by a filter medium in the filter press 6; storing the dewatered solid matter A in a yard 7 and thereafter, finely grinding the solid matter A by subjecting it to primary and secondary grinding treatment into a powder A'; and charging the powder A' into a mixer 8 and mixing the powder A' with a rock powder B in a prescribed mixing ratio.