The use of water treatment plant sludge ash as a mineral addition

For decades, the sludge produced in water treatment plants (WTP) was dissolved in water and then discharged in watercourses. WTP sludge is rich in pathogens and metals and when discharged in watercourses, it increases the amount of suspended solids, eventually causing the water body to silt up. Existing legislation in Brazil prohibits the discharge of WTP sludge in watercourses, but the practice persists. This study investigated the possibility of using WTP sludge as a mineral addition. First, the pozzolanicity of WTP sludge with Portland cement and the concentration of fixed calcium hydroxide using the Chapelle test were determined after exposing the material to different calcination temperatures and residence times. The samples with the best results were used to investigate the performance of concrete mixes where WTP sludge was substituted for Portland cement in concentrations ranging from 5% to 30% in three different water/binder (w/b) ratios (0.35, 0.50 and 0.65). Results indicate that the use of WTP sludge ash improves the strength of concrete mixes when compared with concrete with rice husk ash or silica fume. By using WTP sludge ash, it is possible to obtain the same strength of a concrete mix with 100% Portland cement and reduce the consumption of cement by 37-200 kg m−3 of concrete, depending on the concentration of substitution and the desired strength level.