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A leaching experiment was designed to evaluate the chemical, mineralogical, and morphological nature of precipitation products formed during the accelerated laboratory weathering of an alkaline fly ash. The ash, classified as alkaline Modic, was leached with 0.005 mol L−1 H2SO4 in a series of five leaching columns for a period of approximately 90 d. The weathered residues were characterized by x‐ray diffraction, differential thermal analysis, infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. Three types of precipitation products were identified in the ash. Calcite formed through the dissolution of Ca from the ash and subsequent reaction with CO2 absorbed by the initial alkaline leaching solutions. Iron, dissolved from the ash under acidic conditions, precipitated as amorphous coatings on fly ash particles. Aluminum and Si dissolved from the glass of the ash was translocated and precipitated within alkaline environments as an amorphous aluminosilicate material best described as proto‐imogolite. Results indicated that the products of weathering of fly ash in terrestrial environments are similar to those documented for alkali soils and some soils of volcanic origin.

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