# Optimal Mixture Designs for Heavy Metal Encapsulation in Municipal Solid Waste Incineration Fly Ash
## 3 Results and Discussion
### Hydrate Evolution  
Figure 3 shows the XRD patterns of seven representative pastes hydrated for 3, 7, and 28 days. These mixtures are chosen so they provide an overall picture of the hydration of all pastes in order to keep this work concise. For different mixture designs, the main hydration products include etfumite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\).26H\({}_{2}\)O), Friedel's salt (Ca\({}_{4}\)Al\({}_{2}\)O\({}_{6}\)Cl\({}_{2}\):10H\({}_{2}\)O), sturmanite (Ca\({}_{6}\)Fe\({}_{2}\)(SO\({}_{4}\))\({}_{2}\)[B(OH)\({}_{4}\)](OH)\({}_{12}\).26H\({}_{2}\)O), C-S-H, and calcium magnesium aluminum oxide silicate (C-S-A-M). A small amorphous hump appears in mixtures with higher GGBFS and cement contents. The XRD peak heights are used only as a qualitive indicator as they are affected by numerous factors. The discussion below, based on peak heights, is only intended to provide a relative estimate of the hydrate amounts.  
Figure 3: _Cont._  
Figure 2: Compressive strength evolution of all mixtures during the 90-day period. Error bars are equal to one standard deviation of the mean on each side.  
Figure 3: XRD patterns for pastes hydrated for (**a**) 3 days, (**b**) 7 days, and (**c**) 28 days for chosen groups (PDF#: ettringite—41-1451; Friedel’s salt—31-0245; gypsum—21-0816; portlandite—72-0156; sturmanite—35-0637; C-S-H—14-0035; C-A-S-M—13-0272).  
At 3 days, there is no ettrinigite in C42B28, B30M60 or B70M20, however, ettrinigite is formed in the cement paste without GGBFS (C70B0), suggesting that GGBFS impedes the formation of ettrinigite at early ages [27]. At 3 days, [PERSON]'s salt and gypsum peaks are the most intense in L0B60, while no ettrinigite peaks are found. At later ages, the ettrinigite peaks increase in intensity whereas gypsum and [PERSON]'s salt peaks decrease in intensity. This is probably because of the reaction between sulfates, GGBFS, and [PERSON]'s salt forming ettrinigite [21]. With larger LFS amounts, the aluminum coming from C\({}_{12}\)A\({}_{7}\) has a greater chance to react with Cl\({}^{-}\), forming [PERSON]'s salt before ettrinigite can form [20; 21; 26]. Conversion mechanisms and ionic exchange may also occur between [PERSON]'s salt and ettrinigite, which may explain the inverse relationship between the amounts of these two phases [21]. After 7 days, all pastes show the presence of ettrinigite except B30M60, which has large amounts of MSWIFA. This may be due to the excessive Cl- in the system impeding [PERSON]'s salt transformation to ettrinigite. When there is LFS in the system, [PERSON]'s salt and ettrinigite will form and remain stable. There are no obvious changes in the peak heights of LFS-containing systems during the hydration process, possibly because that the system has abundant amounts of tetrahedrally coordinated Al and SO\({}_{4}\)\({}^{2-}\) which results in relatively stable amounts of [PERSON]'s salt and ettrinigite. Ettrinigite decreases at later ages when there is cement in the system, possibly due to the transformation to monosulfate phases. With excessive MSWIFA, there are large amounts of gypsum in B30M60. Only small amounts of gypsum and ettrinigite are seen in the mixture G2B28, which has a small amount of gypsum to start with. These changes over 3 to 28 days for the different mixtures are summarized in Table 3.