CMA

48 Leaching mechanisms for major, minor and trace elements can be assessed by applying thermodynamic modelling to address the long-term behaviour (Engelsen et al., 2009). Furthermore, the sensitivity of the soil, groundwater, and surface water together with the transport pathways (in the vadose and saturated zone) determine the acceptable concentration level in the cement (Engelsen et al., 2012). 4.5 Volatilization Quantitative data regarding air emissions from cement manufacturing using MSWI ash as a raw ingredient are seldom reported and represent a large gap in the literature. The research indicates that the presence of alkalis and chloride in MSWI ashes may act to volatilize many trace metals that tend to form volatile salts with these compounds. In a study where MSWI fly ash samples were melted at temperatures of 1300–1500 0 C, it was found that volatile metals such as Cd and Pb were evaporated and less volatile metals such as Cu and Cr were retained in the end product. Wu et al. (2011) noted in lab-synthesized clinkers containing 30% fly ash and heated at temperatures up to 1300 0 C that K, Na, Cd, Zn, Pb, As, and Se all volatilized more than 80% of the original mass present in the fly ash while elements such as Cr, Cu, Ni, and Mn were considerably less volatile (less than 25%). In a study where MSWI fly ash replaced 5–35% of raw materials and was heated up to 1450 0 C, Al, Si, Ca, Mg, Fe, Ti, and Cr were all fully incorporated into the clinker, while volatilization of S, Cl, and Pb increased with increased addition of MSWI fly ash; washing the ash decreased the amount of volatilized S and Cl (Bogush et al., 2020). Chloride availability strongly influences volatility, especially for trace metals that form chloride salts with low boiling points, such as Pb, Cd, and Zn. Water washing of MSWI fly ash, for instance, has been reported to decrease Pb volatility in the manufacture of ash-amended cement by up to 50% and eliminate Zn volatility, attributed to the influence of salts on melting behaviour (Bogush et al., 2020). Arsenic volatilization was strongly influenced by chloride content in raw meal, and As and Sb incorporation was enhanced by recirculation of cement kiln dust during the manufacturing process (Wang et al., 2018). Dioxin and furan compounds are formed during the combustion process of MSWI and may be present in MSWI ash (Karstensen, 2008). These harmful organic compounds have been re-ported to volatilize at clinkerization temperatures; one study reported that 94% of poly-chlorinated dibenzo-p-dioxins and dibenzofurans present in a feedstock were eliminated at cement kiln temperatures (Liu et al., 2015). Mercury cycling in cement kilns happens because of Hg occurring largely in the coal fly ash used as a raw material for cement production. This process, similar to chloride and alkali cycling in a cement kiln, can cause mercury enrichment and excessively high Hg emissions in cement kiln flue gas. Some cement operations mitigate this issue using “dust shuttling”, whereby mercury enriched dust is removed from the operation and blended in as an inorganic process addition to the cement (Wang et al., 2016). Though no studies to our knowledge have reported Hg related issues with MSWI ash-amended clinker production, authors have reported total Hg in MSWI fly ashes are higher than coal fly ash, and that stack emissions of Hg associated with MSWI fly ash as a raw material are elevated for as little as 2% replacement of the raw mix (Yan et al., 2018). Furthermore, MSWI fly ash addition to cement production in Austria has been projected to increase Hg content by up to 90% (Lederer et al., 2017). Therefore, to avoid air emissions and enrichment issues associated with Hg when utilizing MSWI fly ash as a raw material, careful consideration should thus be given to the selection of supplementary raw materials and fuels that are low in Hg content. 4.6 Life cycle impacts The benefits of waste usage in cement manufacture are generally well understood and research points to waste utilization as an effective means of increasing resource productivity and decreasing environmental impact associated with cement production (Viczek et al., 2020), but the use of MSWI ash as a cement kiln feed is an area for which there is comparatively few LCA studies. The use of alternative raw materials such as MSWI ash may reduce the production of CO2. The calcium composition in MSWI ashes is in the form of CaO, rather than CaCO3, and thus the contribution of Ca to the clinker formation process does not require the calcination process responsible for CO2 emissions. Krammart and Tangtermsirikul (2004) found that the use of 5–10% MSWI bottom ash as a raw material for cement production would save 25–49 kg of CO2 per ton of clinker produced. Saikia et al. (2007) report that all phases present in ordinary cement can be synthesized at lower temps