The chemical and physical origin of incineration ash reactivity in cementitious systems

Municipal solid waste incineration (MSWI) ashes are byproducts of the combustion process at a waste-to-energy facility, often destined for the landfill. These ashes can be compositionally comparable to ordinary Portland cement, blast furnace slag, and coal fly ash. However, these ashes are largely unutilized as a supplementary cementitious material as their effect on cementitious systems is not well understood. This lack of understanding is primarily due to the elemental and mineralogical complexity of these ashes. To solve this problem, here we have analyzed the hydration behavior of ordinary Portland cement blended with 8 distinct and diverse MSWI ashes. Our results indicate that incorporating these MSWI ashes can either accelerate or retard cement hydration depending upon their composition. Specifically, Cu, Fe, Al, Ti, Si, K, Zn, and Sr from the MSWI ash matrix appear to retard cement hydration, while Pb, Br, S, Ca, and Cl appear to accelerate cement hydration. Based on these results, we introduce a new parameter - Incineration Ash Coefficient (I_AC) - which correlates with the 7-day compressive strength of mortars incorporating MSWI ashes reasonably well (R²= 0.79). This new parameter, based on fundamental chemical and physical characteristics of ashes, is the first step towards effective screening and employment of MSWI ashes as cementitious materials.