Effect of limestone fillers on microstructure and permeability due to carbonation of cement pastes under controlled CO2 pressure conditions

In underground concrete structures such as radioactive waste disposal facilities, the concrete is subjected to higher CO2 partial pressure than in the atmosphere and/or higher pressure gradient of liquid in which CO2 is dissolved. Under these conditions CO2 transport, which governs the carbonation, occurs both by diffusion and advection. This study aims at developing a new experimental method to carbonate cement-based materials by applying an elevated pressure gradient of pure CO2 to the specimens at 65% relative humidity. The proposed method was used to investigate the effects of limestone fillers on changes in microstructure and permeability due to carbonation. The experiments were performed on two hardened cement paste samples with different water/powder and limestone filler replacement ratios. Samples were subjected to an elevated CO2 pressure of 6 bar at the upstream side. The CO2 uptake was measured in the up-and downstream sides by precise mass flow meters. Carbonated samples were analyzed by a series of methods including SEM, MIP and N2-adsorption to characterize the microstructural changes; phenolphthalein spraying, XRD, TGA and water permeability measurements to study alterations in chemical compositions and transport properties.