Fluxing properties of sewage sludge ash in brick manufacturing: Effect of phosphorus extraction and brick clay composition

Sewage sludge ash (SSA) from wastewater treatment plants has previously shown potential as a secondary resource in fired clay brick production. Previous work has hypothesised but not investigated that P, Fe, and Ca in SSA can act as fluxing agents, thereby improving brick properties. However, because P is a critical element and abundant in SSA, it should be extracted for fertiliser use before using the SSA in brick production. This study examines SSA’s fluxing properties, with a focus on SSA before and after P extraction, as a partial substitute (30%) for brick clay, to determine the influence of P in SSA on fluxing properties. SSA from two wastewater treatment plants, where P is precipitated by iron salts, was tested with two brick clays, with high (yellow) or low (red) carbonate content. Specimens were produced and fired (1020°C, 1030°C, 1050°C), and morphology and fired properties were analysed. Results showed that SSA increased sintering and melt phase in the fired bricks, which improved fired properties. For yellow brick clay (high Ca content), adding SSA with a lower Ca content would result in the highest level of sintering, especially between 1030°C and 1050°C. For red bricks (low Ca content), vitrification increased at all firing temperatures with SSA addition. The SSA's fluxing effect was mainly due to Fe and Na, while P did not contribute to fluxing. Therefore, it can be concluded that the extraction of P from SSA before replacing clay with SSA does not compromise brick properties. Brick properties for samples with SSA exhibit similar or superior properties to reference samples. There is potential for the production of bricks incorporating SSA, both for resource preservation and waste utilisation, and to utilise the intrinsic fluxing properties of SSA for the benefit of brick properties. • Sewage sludge ash (SSA) addition increases vitrification in fired bricks. • P in SSA does not show fluxing properties. • Fe and Na in SSA act as fluxing agents. • Low Ca content resulted in higher shrinkage