Raman identification of CaCO3 polymorphs in concrete prepared with carbonated recycled concrete aggregates

The urge to preserve natural resources, to reduce cement production CO2 emissions and to recycle concrete waste conducted to the French national program FastCarb. It is aimed at using recycled concrete aggregates (RCAs), once carbonated with CO2 coming from cement production sites, as a replacement for natural aggregates. The carbonation step serves to reduce the porosity of the old cement paste and to improve future concrete properties. Two different carbonation processes (rolling drum (P1), fluidized bed (P2)) were tested and the resulting RCAs were mixed in different weight proportions with natural aggregates to elaborate new concretes. Raman investigations were then conducted on some sections to analyze the carbonated phases and their spatial distribution. Results indicated a difference in polymorphs distributions. Process P1 seems to generate more vaterite than process P2, which mainly generates calcite and aragonite. They also allowed to appreciate the thickness of the interface between the old and the new cement pastes.