Water retention and carbonation of concretes with supplementary cementitious materials after 21 years of natural exposure

Under unsaturated conditions, carbonation of the concrete cover is one of the main mechanisms responsible for reinforcement corrosion, resulting from the loss of alkalinity in the concrete due to reactions between CO₂ and cement hydrates in the pore solution. Carbonation kinetics are closely linked to CO₂ transport through the pore network, which depends on both pore structure and degree of saturation. This study investigates how the water retention curve (WRC) can contribute to explaining differences in natural carbonation rates of concretes with supplementary cementitious materials (SCMs - silica fume and blast furnace slag) exposed for 21 years under sheltered outdoor conditions. A novel approach for determining the WRCs of both concrete and mortar was developed, utilizing the chilled-mirror technique and specimen conditioning. The WRCs were analysed in conjunction with microstructural and physicochemical characterisation. The results indicate that the WRC provides a sensitive description of changes in the pore network induced by both SCMs and long-term natural carbonation. More generally, the internal hygrometric state of concrete, defined by the way in which water is retained and redistributed within the pore network under unsaturated conditions, is shown to be closely associated with the advancement of the carbonation front. These findings support the use of WRC-based parameters as a physically grounded link between pore structure, moisture transport, and carbonation kinetics, with direct implications for long-term durability assessment and performance-based service-life modelling of concrete structures.