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[Display omitted] • Chlorine in MSWI fly ash is the main barrier to its recycling in cement kiln. • CO 2 injection during washing promoted decomposition of hardly soluble salts. • CO 2 -aided washing increased Cl removal rates by 6–9% than water washing. • CO 2 -aided washing could promote recycling of fly ash in cement kiln. Municipal solid waste incineration (MSWI) fly ash contains many harmful components that may limit its potential for recycling. An effective pretreatment is therefore required before any recycling can be implemented. In this study, the effects of four pretreatment methods (water washing, CO 2 -aided washing, CO 3 2–-aided washing, and CO 2 and CO 3 2–-aided washing) on the extraction behavior of chloride, sulfate, and heavy metals were evaluated. Water washing was found to be effective for the extraction of all easily and moderately soluble Cl-bearing salts, achieving Cl extraction ratios of 88%, 90%, and 96% for ash from Chongqing (CQ), Qingdao (QD), and Tianjin (TJ), respectively. Injection of CO 2 during washing facilitated decomposition of the hardly soluble Cl-bearing salts, increasing the Cl extraction efficiency by 6% for CQ ash and 9% for QD ash. However, for the TJ ash that contained few insoluble Cl-bearing minerals, CO 2 injection decreased the Cl extraction rate. The addition of CO 3 2– had a negative influence on Cl extraction for all ashes, but it slightly promoted sulfate extraction. Despite the high Cl removal rate, only 23–37% of the sulfate and 0.1–12% of heavy metals were removed. Overall, water-based pretreatment, especially CO 2 -aided washing, significantly altered the physical, chemical, and mineralogical characteristics of the fly ash, making it more suitable for recycling. Consequently, the blending ratio of the fly ash for cement clinker manufacture increased from 0.2 to 0.3% in the raw ash to 3.5–5.5% in the treated ash, enabling the extensive use of ash materials. [ABSTRACT FROM AUTHOR]