Hydraulic activity of municipal solid waste incinerator fly-ash-slag-blended eco-cement

The melting of municipal solid waste incinerator (MSWI) fly ash can produce an extremely stable glassy slag that exhibits pozzolanicity if activated. Such slag, therefore, could be recovered in more valuable form as a cement replacement than as a roadbed aggregate. This study investigates the hydraulic activity, compressive strength development, composition variation, and heavy metal leachability of such fly-ash-slag-blended cement (FASBC). The results indicate that FASBC pastes, with cement replacement ratios of less than 20%, show a smaller early strength but a larger later strength at the age of 28 days. It was found that early cement hydration, followed by the breakdown and dissolving of the slag, enhanced the formation of calcium silicate hydrates (C-S-H), which contributes to the later strength. The XRD and 29Si MAS/NMR results also indicate that, after 28 days, both the hydration degree and the average length of the linear polysilicate anions would increase with an increasing cement replacement ratio, implying an increased strength development. On the other hand, the targeted heavy metal leaching concentrations in the synthetic slag and the FASBC paste were significantly low, thus clearing the EPA's regulatory thresholds. These factors suggest the feasibility and safety of using MSWI fly ash slag in blended cement.