Thermogravimetrical analysis for monitoring carbonation of cementitious materials Uptake of CO2 and deepening in C-S-H knowledge

Cement paste carbonation, i.e., the reaction between CO2 and the hydrated cement phases, mainly calcium hydroxide or portlandite, can lead to a pH decrease, which in turn can give rise to steel corrosion in reinforced concrete. At the same time, the carbonation reaction contributes to combine CO2 and fix it as calcium carbonate. It is a crucial phenomenon from the point of view of structure durability and also for cement-based materials sustainability. Cement paste specimens with two w/c ratios and eight types of cements were submitted to different environmental conditions for 4 years and the evolution of calcium carbonate formed or carbon dioxide bound was followed by TG performed in inert atmosphere. The amounts of calcium hydroxide, evaporable and C-S-H gel water were also measured. The CO2 bound follows the same trend in all samples and environments: at the beginning there is a sharp increase followed by a very slow stretch and reaching a maximum after less than 2 years in most cases. The calcium hydroxide amounts evolve very differently in each environment. While outside it is almost consumed after 1 year, inside there is a decrease in the first year, but an increase in the next 3 years. The behavior of the C-S-H water in both environments is similar to that of the portlandite inside. The evaporable water diminishes in all cases to 1 %. From the data obtained by TG, the quantification of the C-S-H gel as well as the calculation of the Ca/Si ratio and the hydration of the gel formed by different type of binders has been possible.