## 3 Results
Such low sulfate concentrations in leaching tests were also found in an Italian study of IBA in which the material from two of the five plants investigated showed low concentrations ranging between 5 and 47 mg/L at an L/S = 10 L/kg [57]. [PERSON] and [PERSON] found that the formation of ettringite in fresh IBA (formed from anhydrite in the unquenched state, i.e., before contact with water in the wet ash discharging system) may be responsible for this behavior [58]. Ettringite is a minimally soluble sulfate species (Ca\({}_{6}\)[Al(OH)\({}_{6}\)]\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)x26 H\({}_{2}\)O (log K\({}_{\rm L}\) = \(-\)44.9 [59; 60]). At lower pH values (i.e., in batch tests with aged IBA), ettringite is no longer stable and other, more soluble Ca-sulfate species are formed [61]. The time-dependent behavior of sulfate can be observed even better in the column tests performed according to DIN 19529. In column tests, the respective concentrations in mg/L are measured at fixed L/S ratios, here at 0.3, 1, 2, 4, 7, and 10 L/kg. The leached amount determined in batch tests can be compared with the cumulative release of substances in a column test. First, the concentrations in the distinct leaching fractions are converted to leached amount (E\({}_{\rm i}\)) and then added together for the cumulative release U\({}_{\rm i}\) (U\({}_{\rm i}\) = \(\Sigma\) E\({}_{\rm i}\)). The maximum cumulative release of sulfate at L/S = 10 L/kg in the column experiment with fresh IBA was around 80 mg/kg (Figure 6b), whereas more than 1250 mg/kg was measured for the aged IBA sample (Figure 6d). The curves in Figure 6c,d can be explained by the presence of gypsum. In practice, the early transformation from ettringite to gypsum is not noticed because leaching tests are rarely performed with fresh bottom ash samples. Figure 4: Leached amount of Sb as a function of Ca ions in batch test with L/S = 10 L/kg. Green: sample A; blue: sample B; black squares and dotted line: modelling results with Visual MINTEQ). Figure 5: Sulfate concentration measured as a function of storage time in leaching tests at L/S = 2 L/kg and 10 L/kg with IBA samples A and B, stored outdoors and indoors. The sulfate concentrations are very low in the beginning (fresh IBA). Figure 6: Measured concentrations (**a**) and calculated cumulative release U (**b**) of sulfate in fresh IBA (sample A) and after 84 days of indoor storage time (**c**,**d**, respectively). The results of the batch tests at L/S = 2 and 10 L/kg are drawn in the graphs (filled and unfilled squares) of cumulative release (**b**,**d**). Note the completely different scale of y-axes of (**a**,**b**) in comparison with (**c**,**d**). The cumulative release for most heavy metals remains almost unchanged over the storage time (i.e., almost no difference between fresh and aged IBA), see the example of Cu in SI (Figure S4). A reasonable agreement between batch and column tests was found for most analyzed components. Discrepancies are in the range of less than one order of magnitude, as shown in Figure 7 (numerical data in SI Table S2). As expected, the batch tests at the higher L/S ratio of 10 L/kg yielded slightly greater results for the leached amount than the batch tests at an L/S = 2 L/kg (see Figure S5 in the SI) simply due to the higher dilution factor.