Carbonation resistance of calcined clay binders: influence of calcined clay reactivity, replacement level, and curing duration

Carbonation resistance of low-clinker calcined clay-containing binders is a major concern limiting their adoption in critical reinforced concrete infrastructure. This study evaluated the carbonation performance of binary calcined clay binders (30 % calcined clay) and ternary binders (30 % calcined clay + 15 % limestone) under accelerated carbonation (3 % CO2) exposure for 70 days, and natural carbonation for 1000 days. Three calcined clays with varying reactivity levels (300-800 J/g of SCM) were used to examine the role of calcined metakaolinite content on the carbonation performance after 2 day and 28 day of initial curing. Ternary binders with 45 wt. % clinker replacement showed significantly lower carbonation resistance, reporting carbonation coefficients 2 to 4 times higher under accelerated carbonation exposure than those identified under natural carbonation. Initial curing duration had a greater influence on carbonation performance in ternary binders than in binary binders. Both natural and accelerated carbonation coefficients showed a 30 % reduction with extended curing from 2 days to 28 days. A strong correlation was observed between the 70 day accelerated carbonation coefficient and the 1000 day natural carbonation coefficient across all binder formulations, irrespective of curing duration. Performance-based design approaches for concrete materials showed that a combination of interventions could improve carbonation resistance in calcined clay concrete, e.g., using a higher-reactivity calcined clays and/or extending the initial curing duration