The use of fly ashes from waste-to-energy processes as mineral CO2 sequesters and supplementary cementitious materials.

This study explores the simultaneous application of fly ash (FA) generated from the thermal treatment of municipal solid waste as a CO2 sequester through aqueous mineral carbonation and as a supplementary cementitious material (SCM) for the development of green construction materials. Two types of FAs are tested, namely an incineration fly ash (IFA) collected from electrostatic precipitator of an incineration plant and a gasification fly ash (GFA) collected from air pollution control unit of a high temperature slagging gasification waste-to-energy (WTE) plant. Ground waste glass (GWG) is used as a tertiary SCM. GFA demonstrates favorable sequestration capacity (87.5 mg/g) and high carbonation degree (74.1 %) while the IFA is found to be inactive during carbonation (3.1 mg/g, 4.6 %). Mortars blended with the wastes have shown delay in the cement hydration but eventually achieve compressive strength comparable to the control specimen. The mixing of GWG and GFA synergistically improves the performance of mortars which highlights the importance of strategic coupling of different waste streams. Most of the hazardous heavy metals, chloride and sulfate in FAs were stabilized in the mortars suggesting the potential for safe re-utilization of carbonated FAs as sustainable SCMs to concurrently close the waste loop and combat climate change.

Characteristics of incineration ash for sustainable treatment and reutilization.

Municipal solid waste incineration (MSWI) generates bottom ash, fly ash (FA), and air pollution control (APC) residues as by-products. FA and APC residues are considered hazardous due to the presence of soluble salts and a high concentration of heavy metals, and they should be appropriately treated before disposal. Physicochemical characterization using inductively coupled plasma mass spectroscopy (ICP-MS), X-ray diffraction (XRD), and X-ray fluorescence (XRF) have shown that FA and APC have potential for reuse after treatment as these contain CaO, SiO2, and Al2O3. Studies conducted on treatment of FA and APC are categorized into three groups: (i) separation processes, (ii) solidification/stabilization (S/S) processes, and (iii) thermal processes. Separation processes such as washing, leaching, and electrochemical treatment improve the quality and homogeneity of the ash. S/S processes such as chemical stabilization, accelerate carbonation, and cement solidification modify hazardous species into less toxic constituents. Thermal processes such as sintering, vitrification, and melting are effective at reducing volume and producing a more stable product. In this review paper, the treatment processes are analyzed in relation to ash characteristics. Issues concerning mixing FA and APC residues before treatment, true treatment costs, and challenges are also discussed to provide further insights on the implications and possibilities of utilizing FA and APC as secondary materials.