The mechanism of carbonate growth on concrete structures, as elucidated by carbon and oxygen isotope analyses

Carbonate mineral growths on concrete structures are endemic throughout the world. The growths take the form of calcitic crusts and stalactites, the formation mechanism for which has not been well defined. We have applied carbon and oxygen isotope analyses to calcites collected from structures in the Midland Valley of Scotland. Carbonate δ¹³C_PDB-values are in the range of −28.9 to −18.8‰ with δ¹⁸O_SMOW in the range of +8.5 to + 16.5‰. These results indicate that the calcites are produced by the interaction of atmospheric carbon dioxide (δ¹³C_PDB = −7‰ δ¹⁸O_SMOW = + 41‰ with hyper-alkaline water, in an isotopically open system. The water attains a high pH as it percolates through and interacts with the concrete, dissolving portlandite. Atmospheric carbon dioxide is fixed almost instantaneously as calcite in the water. A large kinetic carbon isotope fractionation of −10‰ occurs as carbon dioxide molecules cross the gas-liquid interface. The final δ¹³C-value of the calcites produced in the concrete geochemical environment is also shifted by another kinetic fractionation mechanism that may be pH controlled.