Performance of concrete exposed to natural carbonation: use of the k-value concept

•The contribution of fly ash to the concrete carbonation resistance is very weak.•Slag contributes to the concrete carbonation resistance.•Compressive strength at 28 days is a reasonable indicator of carbonation resistance.•k-values from EN 206 are not conservative, when carbonation resistance is concerned.•There is a need to replace deemed-to-satisfy rules by performance testing approach.