Enhancing early strength of cement mortar using MSWI fly ash under accelerated carbonation curing

Metallic Al in municipal solid waste incineration fly ash (MSWIFA) generates hydrogen gas in cement mortars, creating internal pores that typically reduce mechanical strength and necessitate pretreatment before use. This study aims to utilize these gas-induced pores as diffusion pathways for CO₂ during carbonation curing to maximize its effectiveness in cement mortars. Cement Paste and mortar specimens were prepared by replacing cement with 2.5, 5, and 10 wt% of untreated MSWIFA. Their physical and chemical properties were evaluated through flowability, calorimetry, compressive strength, porosity, water absorption, Fourier-transform infrared spectroscopy, thermogravimetric-differential thermal analysis, and inductively coupled plasma tests. The chlorides in MSWIFA accelerated cement hydration but reduced flowability owing to increased viscosity. Although gas-induced pores reduced strength, they facilitated CO 2 penetration, which contributed to strength improvement through densification. Notably, the sample with 5% MSWIFA (AFA5) showed the highest compressive strength after carbonation. In all specimens, Ca(OH) 2 and C-S-H transformed into CaCO 3 , with AFA5 forming more metastable CaCO 3 . Heavy metals such as Cu, Zn, and Pb were effectively immobilized by hydration products, while Cr leaching slightly increased owing to electrostatic repulsion. Carbonation resulted in a slight increase in heavy metal leaching; however, all values remained below the regulatory limits established by the United States Environmental Protection Agency, confirming the environmental safety.