Carbonation rates of alkali-activated and cement based concretes

The reduction of pH from ~12.5 to ~9 by carbonation of the pore solution of reinforced cementbased concrete structures results in the reinforcement corrosion. The rate of carbonation is an important input for design of the concrete cover depth and the service life prediction of reinforced concrete structures because the initiation of reinforcement corrosion is usually considered as the end of service life of concrete infrastructure. The information from the field carbonation of alkali activated concrete is in most cases limited and related to exposure shorter than 40 years. In this paper, a comparative study regarding accelerated and natural carbonation of alkali-activated concretes and cement-based concretes has been carried out. The pH and carbonation depths are periodically measured. The results show that, despite the low porosity of alkali-activated concrete with 50 wt. % slag, these concretes must have an appropriate curing in order to be used in exposure classes where carbonation is an issue, due to their lower carbonation resistance compared to cement-based concrete. Regardless the exposure conditions, the pH of carbonated alkali-activated concrete was maintained above 9. Finally, recommendations for alkali activated concrete applications and their improved carbonation resistance are given.