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 m3 of concrete, depending
on the concentration of substitution and the desired strength level.