The CO2 volume fraction for accelerated carbonation test according to Standard for Test Methods of Long Term Performance and Durability of Ordinary Concrete(GB/T 50082—2009) is 20%. However, it usually lies between 2%5% in similar international specifications. In order to compare the impact of CO2 concentration on the test results, the concrete samples with different mineral admixtures that are widely used in many infrastructures in China were prepared and accelerated carbonation test were conducted in chambers with CO2 volume fraction being 3% and 20%. Mineral admixtures used in this investigation included fly ash, ground granulated blast furnace slag and the combination of both. The carbonation depth and carbonation rate coefficient of the concrete samples were compared. It is demonstrated that the carbonation test at 3% CO2 volume fraction follows the diffusion law of CO2 in natural environment, and the carbonation resistance of the concrete with similar water binder ratio but different mineral admixtures can be clearly distinguished, though carbonation test takes longer time. It is found that accelerated carbonation test at 20% CO2 volume fraction fails to differentiate the carbonation resistance of the concrete with similar water binder ratio but different mineral admixtures.