Abstract: In natural environment, where a long-term interaction of potentially toxic elements with surface water or groundwater systems may be of concern, ash can also be affected by weathering and certain secondary minerals can be formed. In weathering processes, the structure of fly-ash changes considerably which determines their physical and chemical properties. Simulating the weathering by the freeze-thaw test allows the changes occurring in ash particles structure to be produced. The experiment presented shows that ash solubility during a long period of time provides evidence that in the ash-water system, a variety of precipitation processes dominate the solubility of solid mass. The simulation of field conditions in laboratory research explains the behaviour of fly ash on deposits. It has been confirmed that weathering is the reason for a specific grain destruction. As a consequence, the grains are more reactive and more reaction products encircle the individual/single grain. The freeze-thaw test can be used as a helpful tool for predicting the behaviour of combustion by-product deposited on landfill, or temporal deposits and those subjected to weathering under natural conditions. The changes in ash particles observed on SEM images after 5 to 10 freeze-thaw cycles were comparable to those typical of ashes exposed to weathering during field test for a period not shorter than 12 months. The practical conclusion is that disposed fly ashes, taken from landfill site or store place for further application, are characterized by another peculiarity and therefore cannot uncritically be used for a partial replacement of cement in concrete or for other applications.
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