# Optimal Mixture Designs for Heavy Metal Encapsulation in Municipal Solid Waste Incineration Fly Ash
## 4 Conclusions  
In this paper, compressive strength, X-ray diffraction (XRD), and leaching tests were performed to understand the hydration processes, compressive strength evolution, and heavy metal encapsulation in municipal solid waste incineration fly ash (MSWIFA) pastes with varying amounts of ladle furnace slag (LFS), ground granulated blast-furnace slag (GGBFS), cement, and flue gas desulfurization (FGD) gypsum. The predominant hydration products in these systems are Friedel's salt, ettringite, and C-S-H. Mixtures with high amounts of LFS or MSWIFA have poor early-age strength. This low early-age strength is attributed to several reasons including deleterious agents in LFS and MSWIFA. Mixtures with optimum combinations of cement and GGBFS gain the highest strength. In mixtures with large amount of LFS, the principle hydration products are C-S-H gel and Friedel's salt, while the primary hydration products of mixtures without LFS are C-S-H gel and ettringite. Mixtures with excessive MSWIFA form Friedel's salt because of the large amount of chloride in the system. Cr, Cu, Zn and Cd are safely encapsulated in all mixtures with the concentration of the leachate under the EPA drinking water limit. Zn, Pb and Cd show similarly high encapsulation during the testing period while Cr, Cu and Hg show increasing encapsulation with age. Most heavy metals showed increasing encapsulation with increases in LFS amounts due to calcium aluminate hydrate phase formation with the notable exception of Sb. LFS mixtures show poor Sb encapsulation, and most mixtures show Sb leaching. Among the tested mixtures, mixtures C56B14, C42B28, C28B42 and B70M20 are ideal as they satisfy both leaching and strength requirements. The only zero cement mixture that shows satisfactory behavior is B70M20.  
Conceptualization, Y.W. and W.N.; methodology, Y.W, W.N, S.Z., and J.L.; formal analysis, Y.W, W.N, S.Z., J.L., and P.S.; writing--original draft preparation, Y.W.; writing--review and editing, Y.W, W.N,  
\begin{table}
\begin{tabular}{c c c c} \hline
**Mixtures** & **Major Hydrates** & **Encapsulation Mechanisms** & **Comments** \\ \hline
**C70B0** & C-S-H & Physical encapsulation and adsorption & Pb exceeds EPA limit: Encapsulation high for Zn, Cd. \\ \hline
**C42B28** & C-S-H & Physical encapsulation and adsorption & All ions below EPA limit: Encapsulation high for Zn, Pb. \\ \hline
**B70M20** & C-S-H & Physical encapsulation and adsorption & All ions below EPA limit: Encapsulation high for Zn, Pb. \\ \hline
**B30M60** & Friedel’s salt and C-S-H &
\begin{tabular}{c} Ionic substitution, phase mixing \\ and adsorption \\ \end{tabular} & All ions below EPA limit: Encapsulation high for Zn, Pb. \\ \hline
**L40B20** & Friedel’s salt and Ettringite & Ionic substitution and adsorption & Sb, Hg exceed EPA limit: Encapsulation high for Cr, Zn, Pb and Cd. \\ \hline
**L40B0** & Friedel’s salt & Ionic substitution and adsorption & Sb exceeds EPA limit: Encapsulation high for Cr, Zn, Pb and Cd. \\ \hline
**G2B28** & Friedel’s salt & Ionic substitution and adsorption & All ions below EPA limit: Encapsulation high for Cr, Zn, Pb and Cd. \\ \hline \end{tabular}
\end{table}
Table 5: Major hydrates and encapsulation mechanisms of representative mixtures.  
S.Z., J.L., and P.S.; funding acquisition, W.N. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Technology of the People's Republic of China (NO. 2019 YFC1803500).  
The authors would like to acknowledge [PERSON] at the University of Science and Technology Beijing) for help in performing some experiments.  
The authors declare no conflict of interest.