Utilization of acid-washed sewage sludge ash as sand or cement replacement in concrete

Abstract

The transition towards a Circular Economy necessitates the development of methodologies for optimal utilization of secondary resources. One such resource is sewage sludge ash (SSA). SSA can be utilized in construction materials, but as it contains a high concentration of the critical raw material phosphorous (P), extraction of P before the use in construction materials must be considered. EasyMining in Sweden has developed a method for acid extraction of P (Ash2Phos). This work investigates the use of the acid-washed SSA from this process (AW-SSA) in concrete; as partly sand or cement replacement. Castings of mortar prisms showed that a 10% direct replacement of the sand fraction was not possible, as the AW-SSA had a high water demand. Subsequently, the workability was too low for casting. Grinding the AW-SSA before use as cement replacement, on the other hand, gave encouraging results. At 10% replacement, the mortar had a similar 28-day compressive strength as the reference. The Strength Activity Index and Frattini Test did not show conclusive results; however, the AW-SSA is probably not pozzolanic. However, the ground AW-SSA likely has a (latent) hydraulic nature, positively influencing late strength development. The 42-day compressive strength of mortar with 10% replacement exceeded that of the reference with about 5 MPa. This latent hydraulic nature underlines an interesting potential for transforming SSA into two products; phosphorous and material for partly cement replacement in concrete.