*3.1. Chemical Composition*

The average chemical composition of all FA samples (Table 2) is dominated by the oxides CaO (270,000 mg/kg), SO3 (13,000 mg/kg), Na2O (100,000 mg/kg), SiO2 (80,000 mg/kg), K2O (70,000 mg/kg), and Cl (130,000 mg/kg) (full details of chemical data can be found in Tables A1 and A2 and in the Supplementary Materials). Further major constituents are Al2O3 (35,000 mg/kg), Fe2O3 (25,000 mg/kg), P2O5 (10,000 mg/kg), MgO (12,000 mg/kg), and TiO2 (17,000 mg/kg). Of the recoverable elements, Zn (average 36,000 mg/kg) is the most abundant followed by Pb (8000 mg/kg), Cu (2000 mg/kg), and Cd (~200 mg/kg). Precious metals (e.g., Au and Ag) as well as the total content of rare earth elements (REE) show low mg/kg concentration. FA samples have similar constituents, but the content varies heavily due the different waste input (Figure 1). Ca, the dominating contributor to ANC, varies from 150,000 to almost 400,000 mg/kg. S and Cl, which promote the transfer of heavy metals into the flue gas, scatter from 75,000 to 200,000 and 60,000 to 250,000 mg/kg, respectively. Of the total metal content (60,000 to 140,000 mg/kg), the recoverable metals are Zn (15,000–70,000 mg/kg), Pb (2500–16,000 mg/kg), Cu (1000 to 3000 mg/kg), and Cd (100–350 mg/kg). The large concentration range of Zn, Pb, Cu, and Cd indicates again the large differences in the waste input.

### *3.2. Mineralogical Composition*

Phase analyses show that the major solid phases occur in all FA samples. On average, all samples contain an amorphous part of ~41 wt.% including the minor and unidentified phases (Table 3, a complete table can be found in the Supplementary Materials). Crystalline phases are dominated by chlorides, such as halite (NaCl) and sylvite (KCl), which are abundant in all samples (11 wt.% and 4 wt.% on average). K2ZnCl4 (5 wt.% on average) occurs in 23 samples and represents the most important phase, which contains easily recoverable Zn. Gehlenite (Ca2Al2SiO7), belite (Ca2SiO4), and quartz (SiO2) are the dominant silicate minerals (6.6, 4.0, and 2.4 wt.% on average, respectively). The concentrations of the carbonates calcite (CaCO3) and magnesite (MgCO3) are 4.9 and 2.6 wt.% on average. The dominating oxides are mayenite (Ca12Al14O33), perovskite (CaTiO3) (3.3 and 2.8 wt.% on average), and lime (CaO, 1.6 wt.% on average). Rutile (TiO2) and periclase (MgO) are minor constituents (both 0.7 wt.% on average) but occur in almost all FA samples. Among the sulfates, anhydrite (CaSO4) is the only identifiable phase (8.1 wt.% on average). The broad range of concentrations of the major solid

phases in FA is indicated by the large boxes in Figure 2. Beside the main mineralogical constituents, FA samples have many minor phases (<1 wt.%), which cannot be identified or quantified properly.


**Table 2.** Main mineralogical constituents in FA from 29 municipal solid waste incineration plants.

### *3.3. Acid Neutralizing Capacity*

The titration curves of selected FA samples are shown in Figure 3. Seven FA samples had a starting pH below 8 and reached a pH of 2 before 8 moL H<sup>+</sup> was added. Twelve samples started at pH 10–12 but dropped to a pH below 8 after the addition of a 1 moL H+. The remaining 10 samples started at pH 12 and required up to 3 moL H<sup>+</sup> before dropping below pH 8, and an additional 5–10 moL H<sup>+</sup> was needed to reach pH 2. There are three major plateaus apparent. The first one is assigned to the dissolution of portlandite (Ca(OH)2). Lime (CaO) reacts immediately with water, forms portlandite, and elevates the pH above 12 (see Equation (2)). The next major step is assigned to the dissolution of calcite (CaCO3, see Equation (4)). The final major step around pH 4 marks the start of the dissolution of Ca-Si phases like belite (Ca2SiO4) and (partly) gehlenite (Ca2Al2SiO7), and other minor phases start to dissolve. Of all phases, calcite is the dominating phase of the ANC. Ca-Si phases as well as the amorphous part only dissolve partially below pH 4. The required amount of H<sup>+</sup> per kg FA to reach pH 2 with an LS of 10 is shown in Figure 4 as boxplot (a complete table can be found in the Supplementary Materials).


**Table 3.** Approximate annual flow of metals in Swiss FA.

**Figure 2.** Distribution of the main mineral phases in Swiss FA in wt.%. Outliners are marked as circle.

**Figure 3.** Titration curve of selected FA samples (LS 10). The main plateaus are marked with a black line. The black arrow illustrates the big difference of acid that is required to achieve pH 2.

**Figure 4.** Boxplot of the acid neutralizing capacity (ANC) as amount mol H<sup>+</sup> to reach pH 2 at a L/S ratio of 10 and Al0 in Swiss FA (n = 29).
