Long-Term Natural Carbonation in Concretes with Fly Ash and Limestone Calcined Clay Systems

The use of supplementary cementitious materials (SCMs) in concrete is a sustainable solution. However, many practitioners and researchers perceive the lower resistance of SCM-based concretes against carbonation as a concern for large-scale implementation. It must be noted that most literature discusses the carbonation resistance of SCM-based concretes performed in accelerated carbonation. These accelerated carbonation results should be compared with natural carbonation results to predict the reliable performance of SCM-based concretes against carbonation in natural exposure conditions. Therefore, studies on the long-term natural carbonation of concrete with various SCMs were conducted in Chennai, India (hot-humid, tropical climate). This study has 21 concrete mixes with various SCMs (ordinary Portland cement, fly ash, and limestone calcined clay systems), water-to-binder ratios, and compressive strength grades. The specimens were exposed to natural atmospheric conditions for about 10 years in sheltered and open exposure. The sheltered specimens showed higher carbonation depths than the open-exposure specimens. It was observed that the type and replacement levels of SCMs, water-to-binder ratio, paste content, etc., could influence the carbonation resistance. The long-term natural carbonation study concluded that concretes with SCMs offer higher resistance to carbonation at a later stage of carbonation—hence, necessitating the decision-making based on models made using long-term natural carbonation.