Using Portland Cement to Stabilize/Solidify Sludge Residue from Treatment of Cardboard Plant Wastewater

Abstract This work presents the stabilization/solidification (S/S) process by substituting the portland mortar cement fractions by sludge. This sludge results from the wastewater treatment process of a cardboard plant by infiltration–percolation on different combinations of aggregate supports (matrix: sand/coal bottom ash). The prepared mortars incorporating concentrations 5, 10, 15, 20, and 30 wt% of sludge, have been the subject of mechanical studies (compressive and flexural strength), mineralogical study by XRD, and leaching tests. The mechanical strengths have two regimes: a slow decrease for substitutions up to 20%; and then a significant drop of 36% compared to the normal mortar for a sludge concentration of 30%. This decrease coincides with the inversion of the ratio of the CaCO3/SiO2 intensities deduced from the XRD patterns, and the appearance of a peak identifying a new well-crystallized phase may be hydrogarnet. The leaching of arsenic (As) and the heavy metals (Co, Mo, Ni, Pb, Zn, Cu, and Cr) released after 64 days is at concentrations well below the limit values. The released concentrations of Zn, Cu, and Cr increase significantly up to 16 days and then stabilize at 3.6, 19, and 19.1 mg/cm2, respectively. However, As, Co, Mo, Ni, and Pb, which are initially present in sludge from wastewater filtration, have not been leached.