Sludge-derived biochar as an additive in cement mortar: Mechanical strength and life cycle assessment (LCA)

Abstract

Given the growing concern about environmental contamination risks, the management of sewage sludge (SS) from wastewater treatment plants is becoming more challenging. Thus, alternative pathways that offer value-added opportunities with minimal environmental impacts are needed. This study investigates the use of 0.52 wt% biochar (BC) powders generated from the pyrolysis of the SS in an N2 or CO2 environment, as an additive in cement mortar mixtures. The effect of dry and pre-wetted BC on the fresh properties, phase formation, compressive and tensile strength properties of cement mortar mixtures were assessed. Thereafter, life cycle assessment (LCA) of Plain and BC-modified mortar mixtures were evaluated, and the environmental impacts arising thereof were quantified. Results showed that whereas the BC had an infinitesimal effect on mix fresh properties, it caused approximately 1734% and 10–18% increases in compressive and tensile strength, respectively. Regardless of the pyrolysis condition, pre-wetted BC had a superior impact on mechanical strength evolution. LCA results showed that except for minor, net increases in particulate matter/water consumption potential, the rest of the impact categories associated with the production of 1 m3 of cement mortar decreased with the addition of the BC. In terms of Global Warming Potential (GWP) per unit 28d compressive strength, the BC-modified mixtures yielded values that were lower than that of the Plain mortar by about 1629%. Therefore, these research findings indicate that the use of SS-derived BC as a low-dose additive in cement composites could help the construction industry mitigate its negative environmental footprints.