A clean approach through sustainable utilization of cement kiln dust, hazardous lead-bearing, and sewage sludges in the production of lightweight bricks

Hazardous wastes produced from different industrial activities mainly contribute to serious environmental problems, notably their negative impact on the human health. Therefore, there is a great need to implement an innovative protocol for the safe disposal of these wastes in the production of widely-used materials. This study focused on the sustainable consumption of hazardous lead-bearing and dry-bed sewage sludges (LBS and DBS, respectively) in the production of lightweight bricks using alkali-activation process in which cement kiln dust (CKD) used as alkali-rich-waste. Very simple procedures were carried out to prepare the hardened bricks comprising wet mixing sludge-CKD blend and humidity-curing at 23±2 °C for 24 h. The performance of the prepared bricks mainly depends on CKD content, chemical composition and physical characteristics of sludges, and thermal treatment. The increase of CKD content (up to 50 wt %) within LBS-CKD mixtures leads to a significant improvement in the mechanical properties of the produced bricks; whereas, it demonstrated a marginal effect on DBS-CKD set. The main reason behind too low compressive strength (≤1.5 MPa) of DBS-CKD is the low pozzolanic activity caused by the high organic matter content (∼42 wt %) within DBS composition. Nevertheless, all DBS-CKD bricks recorded the lowest bulk density values ranging from 0.92 to 0.99 g/cm³ at 28-days of humidity curing. The full replacement of untreated sludges with thermally-treated one, yields hardened bricks with higher bulk density and compressive strength. Achieving eco-sustainability, low energy demand, and cost minimization, we recommend to use the LBS-CKD (50-50 wt %) mixture in the fabrication of lightweight bricks as it showed desirable mechanical properties, relatively low bulk density, low drying shrinkage, and low Pb-concentration in leachate.