Evolution of structural and mechanical properties of concrete exposed to high concentration CO2

There are many natural high concentration CO2 reservoirs in the deep subsurface worldwide, and there is a potential for CO2 leakage from deep CO2 reservoirs to shallower underground spaces. As a result, underground mining and tunnel construction may encounter zones with high concentration CO2, which raises concern of concrete carbonation. Concrete carbonation under low CO2 concentration may increase the strength of concrete, but concrete carbonation under high concentration CO2 may cause concrete damage. High concentration CO2 may significantly increase the carbonation rate of concrete, which causes the strength and service life of concrete to decrease. In order to investigate the effect of CO2 concentration on concrete carbonation, this paper studied the carbonation process of concrete samples in dry gas, pure water and 70% relative humidity (RH) environments with 100 kPa, 500 kPa and 1000 kPa CO2 partial pressures. The chemical compositions, pore structure and mechanical properties of the concrete samples were also studied. The results showed that the carbonation environment affected the crystallization process of carbonation products. The existence of water increased the level of concrete carbonation in pure water environment, and some C-S-H also reacted with CO2 to produce aragonite. Therefore, after 28 d carbonation of concrete in 100 kPa CO2, aragonite existed in pure water environment but did not exist in dry gas environment. For all carbonation environments, the compressive strength of concrete increased after carbonation in 100 kPa and 500 kPa CO2, but the compressive strength of concrete first increased and then decreased in pure water and 70% RH environment with 1000 kPa CO2. In summary, if the concentration of CO2 reaches a certain level (i.e., 1000 kPa partial pressure), the CO2-induced carbonation can eventually lead to concrete damage.