Messungen der stabilen Isotope des Kohlenstoffs an Kalk-Versinterungen von Beton

The alkaline characteristics of concrete are based essentially on the calcium and alkali hydroxides which are produced during the hydration of the cement and are in equilibrium with the pore solution of the concrete. Calcium hydroxide, dissolved by the action of rain and ground water, reacts with carbon dioxide to form solid calcium carbonate which is deposited on the surface of the concrete as calcite sinter. These calcite sinters are produced either by the absorption of atmospheric CO₂ into the alkaline solutions or by the mixing of such alkaline solutions with ground water rich in dissolved calcium carbonate. The two mechanisms may be differentiated by the stable isotopes of carbon (and of oxygen). CaCO₃ sinters obtained by absorption of atmospheric CO₂ have a δ¹³C value of about -25 ‰ (PDB), whereas sinters deposited from ground water have a δ¹³C value of -13‰ (PDB). The measurements show that the two processes can take place in both above-ground and underground sections of buildings.