The impact of carbonation on the microstructure and solubility of major constituents in microconcrete materials with varying alkalinities due to fly ash replacement of ordinary Portland cement

The impact of alkalinity on the carbonation reaction in microconcrete mortars was assessed by evaluating the changes in the microstructure, solubility, and migration of major constituents (i.e., calcium, aluminum, and silicon) for cases of partial replacement of the Portland cement with different fly ashes having varying alkalinity. Several experimental techniques (i.e., SEM-EDS, U.S. EPA Method 1313, TIC, and TGA) were used and compared as tools to characterize changes due to the carbonation reaction. The rate and extent of carbonation was inversely related to the alkalinity of the material as evident by the increase in carbonation depth, reduction of the natural pH of the material, extent of the changes in the microstructure, and extent of reaction. Calcium migrated to the carbonated region while conversely silicon migrated from the carbonated region in response to relative solubility and therefore different diffusivity in the carbonated and uncarbonated regions for each constituent.