* 40% ash;
* 40% melted products (slag).  
The APC residues contain considerably more heavy metals and organic pollutants, such as dioxins and furans, which hampers use of the material [13]. Using IBA, usually after a treatment in which at least the elemental metals are separated, is widely accepted [14]. The treatment processes are described in detail elsewhere [8; 15; 16; 17; 18; 19]. An important use pathway is as a sub-base material for road construction [20; 21].  
A three-month aging period has been established as the common practice for initial treatment of bottom ash before reuse applications. In the course of this aging, contaminants are immobilized by processes like carbonation, hydration, and oxidation. In particular, the leaching of heavy metals is reduced to environmentally acceptable levels as required for disposal of IBA on landfills [22]. The leaching behavior of ashes is influenced by parameters such as the pH of the leaching solution, the redox potential (pE), the liquid-to-solid ratio (L/S in L/kg), and the contact time or flow velocity (in column percolation tests) [23; 24; 25]. For most heavy metals, the pH value during the leaching test is the dominant factor. In contact with water, IBA reacts alkaline due to the presence of Ca(OH)\({}_{2}\), a reaction product from CaO formed during the incineration process. From the solubility product of Ca(OH)\({}_{2}\) (K\({}_{L}\) = 5.5 \(\times\) 10\({}^{-6}\) mol\({}^{3}\) L\({}^{-3}\)[26]), a maximum pH of 12.35 can be calculated. Observed pH values in standard leaching tests are even higher. During aging, calcium oxide and hydroxide in IBA react with CO\({}_{2}\) from the atmosphere (or rain in the case of outdoor storage) and forms CaCO\({}_{3}\), which is almost insoluble, and therefore does not react alkaline. The observed pH values in the leaching test decrease, and so does the concentration of most heavy metals. The leaching of chloride and sulfate is less influenced by the pH [27], but is also relevant for use. The legal requirements for using IBA in Europe have recently been reviewed by an expert group [14]. The set limit values address the release of contaminants during use (mainly heavy metals and salts), emphasizing the importance of the leaching behavior. It has been stated that the overall utilization rate of IBA is approximately 54%. An upcoming issue is the classification of IBA according to the European List of Waste (LoW). The assessment of the ecotoxicity (hazardous property HP 14) of IBA is still under discussion [28; 29]. Requirements for the total content of constituents are rarely defined for landfilling or using IBA; instead, leaching limit values are set. Therefore, the leaching behavior of IBA has been the topic of various research and review articles [30; 31; 32; 33; 34; 35; 36].  
The present study was performed with three main objectives: (1) to investigate the leaching behavior of IBA in the course of the aging process for up to 281 days through batch tests with constant L/S ratios and column leaching tests, allowing a comparison of the different leaching approaches; (2) to compare the leaching data obtained with modelling results using the geochemical software Visual MINTEQ 3.1 [37], based on MinteqA2 [38], to identify relevant species in the leaching processes and to enable the prediction of heavy metal concentrations in the leachates; and (3) to perform nearly complete chemical analyses of leachates and solid contents, making it possible to elucidate the reaction mechanisms in the aging process. Finally, conclusions are drawn on the use of IBA as secondary building material.