Accelerated carbonation: Changes in water transport, porosity and phases of mortar due to different CO₂ pressures

Currently, the carbonation resistance of concrete is assessed on the basis of accelerated tests performed with high (unrealistic) CO2 concentrations. It remains unclear whether these high concentrations reflect the processes occurring under natural conditions and enable the accurate prediction of field behaviour. To develop future test procedures with higher reliability, it is necessary to deepen the knowledge on the mechanisms of carbonation. Thin slices of mortar were stored in a gaseous mixture of 2 % CO₂ and 98 % N2 at atmospheric pressure or 5 bar. 'H-NMR equipment was used to determine moisture profiles with a resolution of 0.2 mm. At 5 bar a drying front at the near-surface region of the mortar discs severely inhibited the progression of the carbonation reactions. This effect was prevented by exposure to altemating cycles of pressure and storage at atmospheric pressure (65%RH) which significantly increased the rate of carbonation, but was too severe to resolve the effect of cement type on carbonation resistance. Furthermore, the evolution of water beyond the carbonation front of mortar samples under accelerated concentration and at atmospheric pressure was clearly visible. This may lead to an overestimation of carbonation resistance of samples tested under accelerated conditions at the optimal relative humidity for natural carbonation (50 - 70% RH).