Investigation of the carbonation front shape on cementitious materials: Effects of the chemical kinetics

Abstract

Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO 2. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH) 2 and C–S–H takes place. Carbonation gives rise to almost complete disappearance of C–S–H gel, while Ca(OH) 2 remains in appreciable amount. This may be associated with the CaCO 3 precipitation, forming a dense coating around partially reacted Ca(OH) 2 crystals. The way in which CO 2 is fixed in carbonated samples is studied. The results indicate that CO 2 is chemically bound as CaCO 3, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO 3 is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO 3 are preferentially associated with C–S–H carbonation.