Carbonation and Leaching Behaviors of Cement-Free Monoliths Based on High-Sulfur Fly Ashes with the Incorporation of Amorphous Calcium Aluminate

[Image: see text] The high sulfate content in various alkaline wastes, including those from fossil fuel and biomass combustion, and other industrial processes, necessitates careful management when used in cementitious systems to prevent potential deterioration of construction materials and environmental safety concerns. This study explores the under-researched area of high-sulfur fly ash (HSFA) utilization in the production of cement-free monoliths through accelerated carbonation and further examines the effect of niobium slag (NS)—a calcium aluminate-containing slag—as an additive on the strength development and the mobility of SO(4)(2–). The methodology involves mineralogical and microstructural analyses of monoliths before and after carbonation, accounting for the effects of accelerated carbonation treatment and NS addition. The findings suggest that accelerated carbonation significantly improves the initial compressive strength of the HSFA monoliths and generally immobilizes heavy metals, while the effect on sulfate immobilization can vary depending on the ash composition. Moreover, the addition of NS further enhances strength without substantially hindering CO(2) uptake, while reducing the leaching values, particularly of sulfates and heavy metals. These findings suggest that it is feasible to use calcium aluminate-containing NS in HSFA-based carbonated monoliths to immobilize sulfates without compromising the strength development derived from carbonation. This research contributes to the understanding of how accelerated carbonation and NS addition can enhance the performance of HSFA-based materials, providing valuable insights for the development of sustainable construction materials.