Biomass ash-based mineral admixture prepared from municipal sewage sludge and its application in cement composites

Abstract

Biomass as the biodegradable fraction of both agricultural products and industrial and municipal waste is currently a versatile energy resource. It can be stored and converted in practically any form of energy carrier and also into biochemicals and biomaterials from which, once they have been used, the energy content can be recovered to generate electricity, heat, or transport fuels. Moreover, the residues of its incineration can often be reused as pozzolanic additions to cement which can be considered as an environmentally friendly way of their disposal. In this paper, municipal sewage sludge with the organic carbon content of 27% was used for the preparation of biomass ash that should potentially find use as environmentally friendly mineral admixture to construction binders. Based on physical and chemical characterization of the raw material that showed a suitable chemical and phase composition, organic matter in a significant amount, and no thermal decomposition processes above 700 °C, biomass ash was produced. The biomass ash was obtained by incineration of sewage sludge at 700 °C, and mechanical activation contained 52% of amorphous phase which correlates with its good pozzolanic activity. The concentration of heavy metals, soluble chlorides, nitrates, and sulfates in biomass ash is found well below the standard permissible values. The composites prepared with a biomass ash dosage of 10, 20, and 30 wt% of Portland cement exhibited good functional properties. The increase in porosity up to 6% and decrease in compressive strength up to 3% were satisfactory. The results of leaching tests showed that composites with biomass ash contain only trace amounts of chlorides, nitrates, and sulfates. Apparently, the chlorides contained in biomass ash (0.7 mg/g) were immobilized in the cement matrix because the amounts of leached chlorides (0.04 wt%) were the same for all composites and well below the permissible limit for concrete. As revealed by the X-ray diffraction and thermogravimetric analyses, a significant decrease in portlandite content with increasing biomass ash content confirmed the pozzolanic reaction in biomass ash containing composite mixes. The environmental assessment showed a significant decrease in both carbon dioxide production and energy consumption with the increasing biomass ash content. For the composite with 30% biomass ash dosage, it was 21% of CO2 and 11% of energy, as compared with the reference mix. The combination of good functional and environmental parameters of the analyzed composites makes good prerequisites for their application in construction industry. Taylor-made mineral admixture on biomass ash basis can find a broad use as eco-efficient admixture to cement- and lime-based binders.