Potential use of UK sewage sludge ash in cement-based concrete

Sewage sludge ash (SSA) arises as a by-product of the treatment of wastewater by combustion and has been identified for potential use in various construction products, such as brick manufacture, artificial aggregate and cement replacement in concrete. This paper reports on research that looked at replacing the cement or fine aggregate fraction in concretes with SSA from three UK wastewater plants. The first phase of the study established whether the physical and chemical properties of SSA were suitable for use in concrete. The potential reactivity of the material, when used as a cement replacement, was assessed for mortar specimens. In the second phase, SSA was used as a cement replacement in structural grade concrete and as a fine aggregate in foamed concrete. The overall findings of the study were that although the chemical properties of SSA indicated that it had limited reactive abilities, the porous and friable nature of SSA was such that it masked any strength development in conventional concrete and may require further processing to be used effectively. However, this characteristic was a benefit when used as a fine aggregate in foamed concrete, as it enhanced the thermal properties of the concrete by more than a third.