Valorization of MSWI Fly Ash for Use in Cement-Based Materials

The production of cement emits large amounts of CO₂ to the atmosphere. It is estimated that cement production accounts for 8% of the global yearly anthropogenic CO₂ emissions. One way to reduce these emissions is to replace part of the cement used for concrete manufacturing with secondary cementitious materials  (SCMs). However, limited availability and supplies call for the identification of new cement replacements. Fly ash residues originating from municipal solid waste incineration (MSWI) could potentially be used as a new secondary cementitious material. However, treating the residues may be required before they can be used as a cement replacement, as MSWI fly ash residues are considered hazardous and pose an environmental problem. If the treatment also extracts the valuable metals from the MSWI fly ash residues, it could be beneficial. Therefore, this PhD project aimed to develop a treatment process that could extract valuable metals from the MSWI residues and enable their use as a partial cement replacement.
Five MSWI fly ash residues (four fly ashes and one air pollution control (APC) residue) were acquired from three waste incinerators for this PhD project. The five residues were characterised based on their chemical composition and particle size distribution and subsequently screened for use in cement-based materials. The residues were very different from the commonly used fly ash from coal incineration, and that they were not suitable for use as a cement replacement. Multivariate data analysis showed that one of the residues, the MSWI APC residue, had an average chemical composition for MSWI fly ash residues. Three residues had less common compositions. The fifth, a sulphur-rich MSWI fly ash residue, was an outlier in the analysis compared to a range of samples from the literature. The MSWI APC residue, one of the three MSWI fly ashes and the sulphur-rich MSWI fly ash were chosen for further study, as this would provide a broad interpretation of how MSWI fly ash residues perform.
Electrodialytic remediation was chosen to treat the three residues, as this method can potentially extract the valuable metals from the residues and improve the residues performance in cement-based materials. Although the treatment improved the characteristics of the residues, the treated residues were still very different from the traditionally used SCMs, and that they may be characterised as high Ca mineral additions. Furthermore, the electrodialytic remediation treatment did not recover a large amount of the valuable metals from the residues compared to acid leaching. Acid leaching had a combined average removal of Cd, Cr, Cu, Pb and Zn of 50-65% at lab-scale, while remediation removed between 32-37% at lab-scale and 18% at bench-scale. Therefore, electrodialytic remediation may be an inefficient treatment for recovering the valuable metals from MSWI fly ash residues.
Remediation did improve the residues performance in blended binders with 10% cement replaced with one of the residues. It reduced the setting time delay and expansion caused by the residues. For the MSWI fly ash and sulphur-rich fly ash, it improved the compressive strength and resulted in a higher strength than 10% cement replacement with inert quartz. The treated residues were not pozzolanic. Therefore, the contribution to the compressive strength may be hydraulic and due to the formation of additional ettringite and monocarbonate, binding more water. The untreated APC residue resulted in as high compressive strength as a control without any cement replacement.
Heavy metals leaching from the blended binders with MSWI fly ash was a concern for their potential use since external environmental factors such as chlorides and sulphates may increase leaching. It was determined that the monolithic heavy metal leaching was below regulatory limits, even with external environmental factors. Cr was the only metal leaching from crushed binder material in higher concentrations than the regulatory limits permit. However, the control specimen with only cement leached as high a Cr concentration as the specimens with MSWI residues, indicating that the residues were not an issue but that the cement itself may be.
During the experiments, it was observed that the untreated APC residue could function as an accelerator and that the treated sulphur-rich residue may replace the gypsum in cement. Additional experiments then showed that the APC residue could indeed function as an accelerator, although not as efficiently as a commonly used accelerator, and that the treated sulphur-rich residue can replace gypsum.
In summary, it was concluded that the EDR process shows potential as a treatment for using MSWI fly ash residues in cement-based material but may require additional research to optimise the process and results, especially to increase metal extraction.