This paper presents the consequences of the carbonation phenomenon in the case of a mortar with cellulose ether as admixture on its mechanical properties, microstructure and length variations. Carbonation was found to improve mechanical strengths and decrease the global porosity with modifying the pore size distribution. The latter is beneficial regarding durability. However, carbonation also led to an increase of shrinkage, and thus to a probable increase of cracking. Carbonation and shrinkage kinetics could be slowed down by sheltering the material from carbonation during hardening.