Boosted Performance of Composite Cementitious Materials Derived from Incineration Fly Ash via Thermal Activation

Washing and thermal treatment are among the most promising technologies for managing municipal solid waste incineration fly ash (FA). However, the fate of residual ash after treatment remains uncertain and requires further investigation to achieve full resource utilization. In this study, the feasibility of using raw fly ash (RFA), washed fly ash (WFA), and thermally activated fly ash (TFA) as partial replacements for cement in composite cementitious materials was investigated. The macroscopic properties, strength development, environmental safety, and hydration mechanisms of FA-based composites were systematically examined. After the removal of hazardous substances, the physicochemical properties of WFA and TFA became closer to those of ordinary Portland cement (OPC). The mechanical performance of the composites followed the order TFA > WFA > RFA. At a 30% replacement level, the compressive strength of TFA reached 64.2 MPa, slightly higher than that of pure OPC and 103% greater than that of RFA. The corresponding carbon emission calibrated by strength is 29.5% lower than that of pure OPC, reaching 672.5 kgCO2e/t. In terms of environmental performance, heavy-metal leaching from all FA-based composites met the relevant standards, and the chloride immobilization efficiency exceeded 90%. The crystalline phases of hydration products included ettringite (AFt), calcium silicate hydrate [C-(A)-S-H], and Friedel’s salt. During hydration, the formation of portlandite, precipitation of AFt, and deposition of C-(A)-S-H led to an interlaced microstructure in the TFA-derived composites, effectively refining the microstructure and thereby enhancing the macroscopic performance. These findings provide a reference pathway for the utilization of FA and contribute to the sustainable management of FA.