CMA

47 34% MSWI fly ash addition created cement pastes with comparable strength to a control at 28 days, and higher strength than a control at 6 months of curing; 35% replacement of unwashed fly ash created cement paste with poor strength development. There is some research to suggest that creating eco- cement from 100% waste incineration residues is plausible from a physical performance perspective, if appropriate chemical supplements (Ca(OH)2 and SiO2) and CO2 activation are provided to promote reactivity; authors report that these cements had comparable strength to ordinary portland cement (approximately 55 MPa compared to 58 MPa compressive strength at 28 days for ash-amended and control, respectively) (Ashraf et al., 2019). Researchers have posited that compressive strength issues are a consequence of insufficient alite and calcium oxide formation in MSWI-ash amended clinkers, but higher alite concentrations in MSWI ash- amended cement have also been connected to faster setting times and lower late age compressive strength (Clavier et al., 2019). Higher late-age strength in ash-amended specimens compared to control has been attributed to increased proportions of belite (Bogush et al., 2020). Most studies have noted that performance related issues with MSWI ash-amended products are magnified with increasing addition of MSWI ash. 4.4 Leaching Generally, available literature indicates that trace elements in MSWI ash are stabilized during cement manufacturing; stabilized refers to the “binding” of concerning elements in the structure of the clinker nodules during the clinkerization or cement product manufacturing process. Stabilization mechanisms may be physical or mineralogical and lead to retention in the clinker and minimal leaching of trace elements (Mao et al., 2020); cement kilns have utilized this phenomenon to act as treatment methods for hazardous MSWI ash and cement-based solidification is a common method for immobilizing heavy metals in waste products. Wu et al., 2011 manufactured sulfoaluminate cement containing 30.89% MSWI fly ash as a raw ingredient and found that Toxicity Characteristic Leaching Procedure (TCLP) and deionized water extractions showed small amounts of Cu and Zn (deionized water) and Ni and Pb (TCLP) leaching; no heavy metals were detected in monolithic extractions (trace element mass release from a hardened concrete or mortar specimen suspended in deionized water) and Ni, Cu, and Cr were reported as stabilized in the clinkers. Lam et al. (2011) noted similar TCLP extractions for clinkers containing 8% of MSWI fly ash, washed MSWI fly ash, or MSWI bottom ash; ash-amended clinkers had slightly higher Ba (1.86–3.84 ppm), Cr (2.40 ppm in 8% bottom ash mixes), Pb (3.412 ppm in 8% bottom ash mixes), and Tl (0.872–1.17 ppm) than the control specimen containing no ash (1.327 ppm Ba, 0.263 Cr, non-detect for Pb, 0.056 Tl), but reported values were below regulatory thresholds. Washed fly ash mixes had lower Ba concentrations and Tl concentrations than unwashed fly ash mixes. Several other studies have noted minimal leaching for TCLP and TCLP-like extractions on MSWI ash- amended specimens (Mao et al., 2020), including in eco-cements made entirely from mixed MSWI ashes (Ashraf et al., 2019). Clinker retention may reduce leachability, but total trace metal content, especially for elements such as Cd and Pb found in MSWI ash, may jeopardize end of life recycling of MSWI-ash amended concrete products (Lederer et al., 2017; Joseph et al., 2018). Special attention should thus be given to total trace element content in cements manufactured using MSWI ash. Austrian regulations, for instance, limit heavy metal content of raw materials but allow up to 10% secondary raw material addition if the cement produced has total trace metal contents below regulatory limits. For instance, a MSWI fly ash is allowed if the cement produced from that material has less than 15 mg/kg of As and 200 mg/kg of Pb, with similarly set limits for other elements. Cd, Hg, Pb, and Zn concentrations have been found considerably higher due to MSWI ash in- corporation into cement (Lederer et al., 2017). Cd and Pb contents in Austrian cements are expected to increase by 310% and 170% respectively due to widespread MSWI fly ash incorporation. Jones et al. (2012) reported that As concentrations in coal fly ash, a common raw ingredient for cement, were as high as 58.2 mg/kg, while MSWI bottom ash As concentrations are reported as lower (27.5 mg/kg) (Clavier et al., 2019). This means that even though an intuitive thought may be that MSWI ash will cause elevated As concentrations in clinker, industry standard mix designs may already manufacture clinker with even higher.