Technological and environmental behavior of sewage sludge ash (SSA) in cement-based materials

Sewage sludge ash (SSA) is a waste material obtained from the incineration of wastewater sludge. The physical, chemical and mineralogical characteristics of SSA, and the evaluation of its use in cement-based materials, are presented in this paper. Results show that SSA is composed of irregular grains having a high specific surface area and thus leading to a significantly high water demand. A fraction of the ash is rapidly soluble (sulfates, aluminum and silica) and can create new-formed products in presence of lime. SSA induces short delays of cement hydration, probably due to both minor elements of the ash and dilution effect. Compressive strengths of mortars containing 25% and 50% of SSA are always lower than those of reference mortars but it is shown that SSA has a long-term positive effect which might be related to a slight pozzolanic activity. The amount of elements leached from SSA mortars is slightly higher than from the reference mortar without residue but it remains of the same order of magnitude. An extensive literature review was performed in order to compare the residue used in this study with others included in the same category. This analysis highlighted the principal characteristics that must be taken into account to use SSA correctly in cement-based materials.