Accelerated carbonation of recycled concrete aggregates: Investigation on the microstructure and transport properties at cement paste and mortar scales

•Accelerated carbonation produced several polymorphs of calcium carbonate.•Portlandite formed first then aragonite and vaterite.•Degree of carbonation correlated with microstructural changes and macroscopic behaviour.•Significant stiffness increase associated to vaterite formation.•Both carbonation shrinkage induced damage and pore clogging influenced permeability. The accelerated carbonation of recycled concrete aggregates (RA) has been suggested to improve their properties. Several physical and mechanical phenomena occur during the carbonation of adhered cement mortar, but their effects are scarcely distinguished and correlated. In this study cement paste and mortar specimens have been exposed to carbonation after extended curing, and then characterized at different scales to monitor the evolution of their microstructure and macroscopic properties as a function of their carbonation degree. First the carbonation of portlandite and calcite precipitation dominate, then the carbonation of C-S-H develops with the precipitation of vaterite and the development of material stiffness. The increase in elastic modulus and carbonation shrinkage both favor microcraking: an increase in macroporosity and permeability is actually observed. Clogging effect due to the formation of calcium carbonate in the microstructure finally results in the reduction of the porosity, pore sizes, and permeability.