An Atomic Approach to Pore Refinement in Concrete: Carbon Dioxide to Carbonate Calcium

Cementitious binders are porous materials with significant portlandite content. Potentially, this portlandite content can react with CO2 and form carbonate compounds with binding ability. Using molecular dynamics simulations and density functional theory, in this paper the kinetics of CO2 inside the portlandite pores were studied. The results indicate that the larger dry pores within the portlandite-enriched binder matrix are the most effective at enabling the passage and adsorption of CO2 molecules. Also the pores smaller than 21 Å are capable of adsorbing CO2 on the surface of portlandite, leading to the formation of CaCO3. Two main attraction forces between Ca(portlandite)O(calcite) and O(portlandite)H(calcite) pair the newly formed Ca(OH)2 matrix and CaCO3 walls together, with the CaO providing the majority of binding energy. The existence of these attractions enhances the mechanical cohesion as the sliding resistance between portlandite, the main body of the pores, and the calcite, the carbonation product, reaches as high as 1.7 GPa in theory.