Chemical changes of cement pastes due to the effect of combined carbonation and chloride penetration

Carbonation of concrete is often the cause of corrosion of steel reinforcement, which is actually one of the main causes for the degradation of concrete structures. The degradation might be more significant in the case of chloride penetration. Carbon dioxide (CO2) in the air can penetrate into the concrete; dissolve in the pore solution, and react with calcium hydroxide, leading to water formation and calcium carbonate precipitation. Carbonation is a diffusion based
phenomenon, where the carbonation front moves inwards at a rate proportional to square root of time. The rate of reaction of hardened cement pastes with CO2 is known to depend strongly on the water cement ratio (w/c) of concrete, mixture composition, pore structure, temperature and curing conditions of concrete and its internal humidity.
When concrete is in contact with a chloride solution, chloride ions diffuse in the pore solution. They appear in concrete both as free Cl-ions (meaning water-soluble ions) in the pore interstitial solution and as chemically bound component of hydrate phases (e.g., Friedel’s salt Ca2Al(OH)6Cl•2H2O]), as well as physically adsorbed on CSH. Free chloride ions are the most dangerous due to their capacity to diffuse towards the steel bars
The objective of this paper is to determine the relationship between the chemical changes of hardened cement pastes and accelerated carbonation using different CO2 concentrations with the presence of chloride ions. The chemical changes of the cement pastes were analyzed using XRay diffractometry and thermal analysis.
According to the results of the XRD-analysis all the sample pastes were partly amorphous and, thus, contained only limited amounts of crystalline compounds. The thermal decomposition of the paste samples exposed with sodium chloride NaCl was somewhat different from the behaviour of the other paste samples.