Carbonation Performance of Low‐Clinker Binders Incorporating Calcined Clay and Natural Pozzolan: Role of w/CaO _reactive and Mortar‐to‐Concrete Scale Transition

Abstract

This study investigates the carbonation resistance of very low‐clinker binders containing 35–45 wt.% clinker, limestone, and either calcined clay or natural pozzolan. Mortar prisms and selected concrete beams were exposed to accelerated carbonation at 1% CO ₂ after 28 days of water curing and 14 days of preconditioning at 20 ± 2 °C and 65 ± 5% RH. The carbonation coefficient, Kacc, increased with increasing w/b ratio and w/CaOreactive, confirming the importance of reactive calcium availability for carbonation buffering in low‐clinker systems. However, the observed scatter indicates that pore structure, SCM reactivity, degree of hydration, moisture state, and mixture scale also influence carbonation resistance. Mortar‐scale testing reproduced the relative ranking of carbonation resistance observed at concrete scale, but the absolute carbonation coefficients differed. Concrete Kacc values were approximately 0.76 times the corresponding mortar values. The results show that mortar testing is useful for binder screening, whereas application‐oriented carbonation assessment requires concrete‐scale testing to obtain realistic coefficients for service‐life prediction.