Comparative studies on the effects of sewage sludge ash and fly ash on cement hydration and properties of cement mortars

Sewage sludge ash (SSA) is the byproduct obtained from incinerating mechanically dewatered sewage sludge. Some published literature mentions that mortars of comparable strength can be produced using a small amount of SSA to replace cement. However, information on how SSA affects the properties of cement mortars is limited given the pozzolanic activities of most SSA being modest. This study identified the mechanisms behind some beneficial effects of the SSA on the strength development of mortars through a comparison study with fine sewage sludge ash (FSSA) and pulverized fly ash (PFA). The findings of this study indicated that the presence of SSA accelerates the rate of heat evolution from cement hydration while PFA does not produce this effect. A higher content of SSA or FSSA produces a greater effect. Replacing cement by SSA or FSSA up to 10% did not induce significant changes to the pore structure of the pastes. The formation of brushite in SSA or FSSA cement mortars contributes to the long-term strength of the mortars. PFA reduces the drying shrinkage of the mortars, but SSA causes greater drying shrinkage due to increasing content of mesopores with sizes less than 0.025 µm. This harmful effect is greater with FSSA.