Factory trial with SSA as partial substitute for clay in fired bricks

Sewage sludge ash (SSA) is the byproduct of wastewater treatment processes, it is an underutilized material and mainly landfilled. A potential reuse of SSA is as a partial supplement in clay brick manufacturing. Previous studies of reusing SSA in brick manufacturing have been carried out on a laboratory scale [1]. Laboratory scale studies provide bases for utilization potential, where it is possible to test multiple variables with high accuracy and limited time, material and equipment. However, many of the manufacturing processes are often simulated with significant changes compared to the factory scale production. Laboratory scale incineration is typically carried out in an electrical laboratory furnace, with a set heating rate and last 12-24 h. The factory scale incineration is done in large tunnel kilns, over 3-5 days. The aim of this trial study was to investigate the potential and challenges of utilizing sewage sludge ash (SSA) as a partial substitute for clay on a factory scale. The study was conducted in collaboration with a brick factory in the southern part of Denmark. The factory produces handmade bricks, and bricks on machines that resemble the way in which bricks were originally made by hand. Bricks with yellow or red brick clay were produced, with 24% (dry weight) SSA of the clay mixture. The clay and SSA were mixed with water and rested for 24 hours. After 24 hours more water was added, because the SSA had absorbed a lot, and dried the mixture overnight. The bricks were hand-molded using wooden molds, and dried at room temperature for 2 days, then subsequently placed in a drying chamber at 75°C for 5 days. Firing was carried out in a tunnel gas oven over 4.5 days under low-oxygen conditions at 1035°C and 1060°C. Porosity, initial rate of absorption (IRA), capillary suction, water absorption and compressive strength of the fired bricks were investigated. SSA, characterized by larger and more porous grains compared to typical clay, altered the clay mixture’s properties. SSA has a very porous structure and can absorb high quantities of water, therefore excess water had to be added before the mixture could be molded. The clay/SSA mixture had a more sand-like feeling than the reference mixture. After firing, the bricks with SSA had a rough surface compared to reference bricks, however the reference bricks had large cracks that did not appear on the bricks with SSA. Bricks with SSA had higher porosity and IRA compared to reference bricks. Despite increased porosity, the compressive strength of SSA bricks were higher than the declared values by the manufacture and the reference bricks Using SSA in fired clay bricks has potential, although mixing SSA and clay at factory scale can pose a challenge with increased water requirements during production. Future research should focus on improving SSA and clay mixing techniques, as the workability of the clay and SSA gave challenges in the forming procedure.