Potable water filtration sludge: Use as set retarder in one-coat plastering mortars

This work describes the study of the potential use of the sludge generated in potable water filtration/cleaning operations (waste material) as set retarder and workability-aid in rendering/plastering mortars. A commercial mortar was used as reference and several formulations were investigated, containing the waste material and sodium phosphate instead of sodium or calcium phosphate enriched bentonites, traditionally used as set retarder and workability-aid in cement based compositions. Sodium tri-polyphosphate was also tested alone for the same purpose. Fresh mortars were prepared with a water-to-cement weight ratio of 1.1 and a sand-to-cement weight ratio of 4:1.The results obtained in set time tests suggest that the retarding effect is associated with the presence of sodium ions. The same set delay can be obtained even with lower contents of the new waste-based additive (0.25–0.50 wt%), but the workability window tends to be narrower than that observed with the use of sodium enriched bentonite. The potable water filtration sludge can be a strong candidate in this application, with economical and environmental benefits.