SYNTHESIS OF CALCIUM SILICATE MATERIALS FROM THE RESIDUAL WASTE SLUDGE OF A WATER-PURIFICATION PLANT

Calcium silicate is produced from a mixture of silica sand powder, lime, paper pulp, and Portland cement hydrothermally steamed at 180 °C for about 16 h. This material is considered environmentally friendly and is popular in countries around the world. In this study, quartz sand was replaced with residual waste sludge from water-filtration plants to produce calcium silicate materials. Nowadays, the residual waste sludge from water-filtration plants is an environmental problem that needs to be treated. The results of determining the properties showed that a sample using 10 w/% residual waste sludge gave the best replacement. This sample had a bending strength of 10.95 MPa, a volumetric density of 1.57 g/cm3, and water absorption of 23.67 %. The results of the analysis of the mineral composition (by X-ray diffraction analysis and Fourier-transform infrared spectroscopy) and microstructure (by scanning electron microscopy) showed that all samples formed tobermorite and xonotlite minerals. The tobermorite and the xonotlite are hydro-silicate-calcium minerals characteristic of calcium silicate materials, which are the synthesis products of chemical reactions of SiO2, CaO, and H2O under hydrothermal conditions. Samples using 5–10 w/% of waste residual sludge have even higher mechanical strength than samples without. Therefore, using waste residual sludge from water-filtration plants to replace part of the sand in producing calcium silicate materials can be considered an effective method to treat environmental problems caused by waste residual sludge.