This paper addresses the effects of relative humidity (RH) and carbon dioxide (CO2) concentration on the rate and effects of accelerated carbonation in alkali-activated slag/metakaolin (MK) concretes. Strength and water absorption are used alongside phenolphthalein measurements to monitor carbonation, and the effects of drying at different RHs are particularly significant in controlling carbonation rates. Different trends in the carbonation rate as a function of MK content are observed when varying the CO2 concentration, further revealing that the carbonation rates of these materials under accelerated conditions are influenced strongly by the testing protocol. The standard phenolphthalein method for testing carbonation depth appears only to be capturing the change in alkalinity with pore solution carbonation, meaning that it does not correlate well with other performance parameters at high CO2 concentrations.
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