This work describes the study of the potential use of the sludge generated in potable water filtration/cleaning operations (waste material) as workability-aid in rendering/plastering mortars. A commercial mortar was used as reference and several formulations were investigated, containing sodium phosphate enriched waste material instead of the enriched bentonite traditionally used as workability-aid and setting retarder in cement-based compositions.Fresh mortars were characterised in terms of plasticity as a function of elapsed time. Rheological behaviour was studied with a specific rheometer. The same setting delay can be obtained with the use of the enriched sludge even by using lower contents of the additive (0.25%), but the workability window tends to be narrower. Fluidity assured by the tested admixtures is mostly controlled by changes in yield stress values of the paste.The replacement of bentonite by an equivalent amount of sodium phosphate-added waste material (ca. 0.25wt.%) is technically feasible.