The potential use of drinking water sludge ash as supplementary cementitious material in the manufacture of concrete blocks

The effect of drinking water sludge ash (DWSA) on the properties of DWSA-cement pastes is investigated in this study using x-ray diffraction, scanning electron microscopy coupled with energy-dispersive spectrometry, and thermogravimetry techniques. The pozzolanic reactivity of DWSA is confirmed; the filler effect of DWSA can significantly and rapidly accelerate the hydration reaction at an early curing age. The presence of DWSA in blended pastes promotes the incorporation of aluminum into the C-(A)-S-H gel; the original ‘Al-minor’ C-(A)-S-H gel in pure cement paste is converted to ‘Al-rich’ C-(A)-S-H gel. The added DWSA leads to the formation of aluminum-bearing hydrates, such as ettringite and calcium aluminate hydrates (C-A-H). The compressive strength of sludge-derived concrete blocks is studied; blocks containing 10% DWSA exhibited higher compressive strength at a curing age of 90 d than the reference samples. Such benefits are not observed in block samples with 20 and 30% DWSA, which is attributed mainly to the physical effect of unreacted DWSA particles.