Report of RILEM TC 281-CCC: phase assemblage alterations and carbonation potential of mortar with blended cements induced by long duration carbonation exposure

Replacement of Portland cement is a practical strategy to reduce concrete manufacturing CO₂ emissions. However, this approach typically results in a diminished portlandite content in the hardened mix, elevating the risk of carbonation-induced corrosion in steel-reinforced concrete. Carbonation is frequently studied by exposing the samples to elevated CO₂ levels (1% and 20%). However, the carbonation process and its by-products might differ markedly under natural conditions. In the context of RILEM TC 281-CCC ‘Carbonation of Concrete with SCMs’, a comprehensive three-year natural carbonation study on mortar samples was carried out across three laboratories. Samples were made with commercially available cement (CEM I, CEM II/B-V, CEM III/B). This study examined two natural carbonation scenarios: one in a regulated climate chamber and the other outdoors, protected from direct rainfall. The progression of carbonation was determined using a phenolphthalein indicator and compared to optical pH measurements. The phase composition was analysed by X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy, and thermogravimetric analysis. Additionally, the CO₂ capture in three-year-old naturally carbonated samples was assessed and contrasted against the reactive CaO content. The thermogravimetric analysis data revealed a non-linear relationship between the portlandite content in the uncarbonated zone and the carbonation rate. A reduced clinker content leads to lower pH values in carbonated and uncarbonated zones. Notably, samples containing CEM II displayed the largest formation of CaCO₃ which, divided by the theoretical maximum amount of CaCO₃ from reactive CaO, signifies the highest degree of carbonation among the cement types studied.