Characteristic lengths of the carbonation front in naturally carbonated cement pastes: Implications for reactive transport models

Experimental evidence of a smooth carbonation front is often used to support the inclusion of kinetic reactions in reactive transport models. A new interpretation for a smooth carbonation front is proposed. We first show that the alkalis and sulfur can be used to map a natural carbonation front with energy dispersive spectroscopy. The resulting carbonation front is relatively sharp but tortuous. Most experimental methods takes an average over a volume which smooths the front. To illustrate this new interpretation, we also present a new carbonation model. Instead of using kinetic rates, we obtain a wider apparent carbonation front by considering a distribution of gas diffusion coefficients. The simulated profiles are similar to the experimental ones. This work does not prove that we should get rid of kinetic rates. However it demonstrates that similar macroscopic results can be obtained with very different assumptions. Therefore, these mechanisms must be quantified independently.