## 1 Introduction  
Incineration is one of the most effective technologies used for the treatment of municipal solid waste (MSW) in many industrialized countries [1, 2, 3]. After MSWI, several types of solid residue, such as bottom ash (BA), fly ash (FA), and boiler ash (BLA), are formed [4, 5, 6, 7]. BA constitutes 85-90% of the total weight of all furnace residues [8, 9, 10, 11].  
BA is a highly heterogeneous mixture of slag, nonferrous and ferrous metals, glass and ceramics, other noncombustible matter, and a small amount of unburned organic matter (plastic, fiber, wood, etc.) [5, 12]. The incineration process parameters, such as the furnace configuration, combustion temperature, quenching process, incineration waste composition, etc., affect the properties of BA [13, 14]. Although BA is classified as nonhazardous waste by the European Waste Catalogue [15], most furnace ashes are disposed of in landfills because some elements, such as soluble salts (sulfates and chlorides) or heavy metals (Cu, Zn, Pb, and Ba), are often present in high concentrations in untreated (fresh) bottom ash [8, 16, 17, 18]. BA can be used in civil engineering after pretreatment. The following pretreatment technologies can be applied: Water washing [17, 19], full metal separation [20],vitirification [21; 22], or weathering (natural and accelerated) [1; 16; 23; 24; 25]. Almost all of these technologies incur substantial additional costs. Natural weathering is the most ecologically and economically beneficial method for the treatment of BA.  
During natural or accelerated weathering processes, some characteristics (chemical and mineralogical) of BA particles undergo changes. BA is not an inert material, having a complex set of interactions with the ambient atmosphere and undergoing further chemical and mineralogical transformations [26]. When additional contact with atmospheric air (and thus atmospheric CO\({}_{2}\)) occurs, various chemical reactions begin, resulting in the solidification, volume stability, and reactivity of BA [27]. The main reactions are hydrolysis, oxidation of some metals (e.g., iron, copper, and aluminum), carbonation, dissolution/precipitation of hydroxides and salts, complexation, sorption, neutralization of pH, formation of solid solutions, and mineral neformation [28; 29; 30]. The stabilization processes depend on various conditions, including mineral composition, reaction temperature, the amount of CO\({}_{2}\) in the atmospheric air, BA particle size, etc. Furthermore, the moisture level (amount of water) in BA is an important parameter [11].  
Natural weathering leads to chemical stabilization and reductions in the solubility and mobility of toxic elements through carbonation, oxidation, precipitation, and pH neutralization reactions [31]. However, certain chemical, legal, and technological barriers linked to the potential leaching of toxicity hamper its valorization in many countries because of the heavy metals contained in BA [32]. The environmental characterization we performed demonstrated the effect of increasing the curing time on decreasing the concentrations of leached metals. More detailed research on natural weathering processes should be conducted to ensure the metals remain stabilized over longer periods [33].  
In this study, we investigated the influence of natural weathering on heavy metal (Cu, Zn, and Pb) stabilization for different particle sizes of MSWI bottom ash. We used the following methods in the analysis: Fractionation of BA, XRF, XRD, AAS, and extraction (leaching) tests.