Performance and microstructure of sustainable cementitious materials mixed by municipal sewage sludge ash, slag, and fly ash

Abstract

Substituting cement with sewage sludge ash greatly benefits the environment and the economy, but the improved methods of the performance of sludge ash cementitious materials should be further studied. Systematic experiments were designed to investigate the effects of typical municipal sewage sludge ash (SSA) in combination with slag and fly ash on cementitious material properties, including workability, mechanical strength, water absorption, chloride penetration, and drying shrinkage. Furthermore, the hydration and microstructure of the cement pastes were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption and desorption (NAD) tests to better understand the properties of sludge ash cementitious materials. The results demonstrate that, 30 wt% SSA resulted in 18.6 % fluidity loss, 16.01 % lower compressive strength, and higher water absorption coefficient and chloride migration coefficient (DRCM) at 28 days compared with the reference specimen. However, the performance of the sludge ash mortar was improved with the combination of SSA, slag, and fly ash. The optimum combination proportion was found to be 10 wt% SSA, 10 wt% fly ash, and 10 wt% slag, and the corresponding mortar demonstrated ideal strength and resistance to water absorption and chloride migration. Additionally, the mortar with 30 wt% SSA had more than 30 % reduced drying shrinkage compared to the reference cement mortar at 90 days. The slag converted monosulfate into hemi/monocarboaluminate, thus increasing the abundance of hydrates, and the incorporation of slag and fly ash promoted pozzolanic reactions and refined the pore structure of the blended pastes. This study proposes a reliable and practical method to promote the utilization of sludge ash.