Use of sewage sludge ash as mineral admixture in mortars

Abstract

This paper presents the physical, chemical and mechanical characteristics of sewage sludge ash (SSA), in order to compare its properties with the standard requirements for coal fly ash (FA). It also provides an evaluation of its environmental behaviour when used as a mineral admixture in cement-based materials. Results show that, compared with FA, SSA has a high amount of calcium and phosphorus but a relatively low fraction of silica. SSA is composed of irregular grains having a high specific surface area and thus leading to a high water demand. The high water absorption of SSA requires an increase of the water–binder ratio of mortars, or the use of a water-reducing admixture. The strength activity index (25% replacement of cement) reaches more than 90% after 28 days, showing a long-term positive effect, which might be related to the pozzolanic activity. The amount of leached elements of SSA mortars is slightly higher than for the reference mortar without residue, but it remains within the same order of magnitude. The comparison of the characteristics of SSA with the FA specifications given by American and European standards shows that SSA does not satisfy the requirements to be directly considered as a mineral admixture. Nevertheless, these preliminary results show that SSA could be classified between filler and pozzolanic admixtures. Thus SSA may not necessarily be excluded from use in cement-based materials.