# Optimal Mixture Designs for Heavy Metal Encapsulation in Municipal Solid Waste Incineration Fly Ash
## 2 Materials and Methods
### Experimental Methods  
The materials were mixed mechanically in a mixer according to ASTM C305-14 [15; 21]. The fresh paste was then cast into 5-cm cubic molds after tamping and vibration [15; 21]. Pastes were demolded at one day, and then cured at temperature of \(30\pm 2\)\({}^{\circ}\)C and a relative humidity of 95%. The temperature of 30 \({}^{\circ}\)C was used to accelerate hydration and strength processes, which is an important consideration for non-cement mixtures. Three specimens were cast for each mixture, and the curing times were 3, 7, 28, and 90 days for group LB and GB, with an extended curing time for groups CB and MB until 180 days. The compressive strength test was performed according to ASTM C109-20a on all pastes at 3, 28, and 90 days in order to determine the strength evolution. Broken pieces of paste were stored in absolute ethyl alcohol for 2 days to stop hydration. After 2 days, filtration was performed, followed by drying under vacuum at 40 \({}^{\circ}\)C for 1 day [24]. The pieces were then ground and sieved through a 45-\(\upmu\)m mesh and the resulting powder was used for XRD and leaching tests. XRD was performed at 3, 7, and 28 days to determine the nature of the hydrates formed in the different mixtures in a qualitative manner. The scanning was performed from 3\({}^{\circ}\) to 90\({}^{\circ}\) 20 and the device was operated at 40 kV and 40 mA and had a Cu K\(\alpha\) anode. Leaching tests were performed based on ASTM D8155-17 at 3, 7, 28, 90, and 180 days to evaluate the extent of encapsulation of the heavy metals in the MSWIFA. Around 10 g of powder samples was mixed with 100 mL of deionized water in a closed conical flask. Then, the flask was placed on a shaking table at 23 \({}^{\circ}\)C for 8 h and it was shaken at 110 rpm and then allowed to rest for 16 h. The leachate was filtered with a 0.45-\(\upmu\)m membrane filter at the end of the extraction. The concentration of heavy metals in the leachate was detected by inductively coupled plasma mass spectrometry (ICP-MS). While the leached concentrations of heavy metals, sulfate, and chloride were all measured, only data for heavy metals are reported here in detail. The chloride encapsulation is discussed in detail elsewhere [21]. ICP-MS was also run on an MSWIFA-water mixture under the same conditions. Under these conditions, the MSWIFA had very high leached amounts (\(C_{0}\)), and the concentrations of many of the heavy metals were higher than the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mixture** & **MSWIFA** & **GGBFS** & **LFS** & **Gypsum** & **Cement** & **Water/Binder Ratio** \\ \hline
**C70B0** & 20 & 0 & 0 & 10 & 70 & 0.3 \\ \hline
**C56B14** & 20 & 14 & 0 & 10 & 56 & 0.3 \\ \hline
**C42B28** & 20 & 28 & 0 & 10 & 42 & 0.3 \\ \hline
**C28B42** & 20 & 42 & 0 & 10 & 28 & 0.3 \\ \hline
**B70M20** & 20 & 70 & 0 & 10 & 0 & 0.3 \\ \hline
**B50M40** & 40 & 50 & 0 & 10 & 0 & 0.3 \\ \hline
**B30M60** & 60 & 30 & 0 & 10 & 0 & 0.3 \\ \hline
**L0B60** & 30 & 60 & 0 & 10 & 0 & 0.4 \\ \hline
**L20B40** & 30 & 40 & 20 & 10 & 0 & 0.4 \\ \hline
**L40B20** & 30 & 20 & 40 & 10 & 0 & 0.4 \\ \hline
**L60B0** & 30 & 0 & 60 & 10 & 0 & 0.4 \\ \hline
**G14B16** & 30 & 16 & 40 & 14 & 0 & 0.4 \\ \hline
**G6B24** & 30 & 24 & 40 & 6 & 0 & 0.4 \\ \hline
**G2B28** & 30 & 28 & 40 & 2 & 0 & 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mixture designs used in the study (in mass %); water/binder is unitless [21]. EPA drinking water limitation. The concentration of heavy metals was determined for given mixtures and heavy metals (\(C_{x}\)), and the encapsulation percentage (E) for each heavy metal was obtained using Equation (1) [21]:  
\[\mathrm{E}\ =\ \left(1-\frac{C_{x}}{P\times C_{0}}\right)\times 100 \tag{1}\]  
where \(C_{x}\) is the mixture leachate concentration, \(P\) is the mass proportion of MSWIFA in the paste, and \(C_{0}\) is the leachate concentration in the MSWIFA and water mixture.