Characterization of the Interfacial Transition Zone Between Carbonated Recycled Aggregates and New Cementitious Matrix: The Cement Type Influence on Carbonation

One sustainable destination for demolished concrete is the extraction of aggregates for subsequent reuse. The performance of recycled concrete, produced with 100% recycled aggregates (RA), is limited by the existence of the attached mortar, resulting in multiple, weaker and larger interfacial transition zones (ITZ). One promising improvement approach is the accelerated carbonation of RA. In this technique, CO₂ reacts with the adhered cementitious phase of RA, changing its chemical and physical properties. Many studies reported beneficial developments in mechanical and durability performances of recycled concrete produced using RA from Ordinary Portland Cement demolished concrete (N-RA). Therefore, in this study, chemical and physical changes caused by carbonation of RA from Ground Granulated Blast Furnace Slag blended cement demolished concrete (BB-RA) were also investigated, and the influence of demolished concrete’s cement type on accelerated carbonation treatment of RA was discussed. Then, the new ITZ, between non-carbonated and carbonated RA and the new cement paste of modeled recycled concretes, was characterized by microhardness test. From experimental results, an increase in dry density and a decrease in Calcium ions re-leased from RA after carbonation evidenced that CaCO₃ formation occurred effectively for both aggregates. However, while in N-RA’s carbonation, the filling effect was predominant, reducing RA’s water absorption and evaporation ratios and strengthening the ITZ of the modeled recycled concrete, carbonation treatment applied to BB-RA increased water absorption and evaporation ratios, resulting in a wider new ITZ and indicating significant microstructural changes, where the filling effect was surpassed by porosity coarsening caused by carbonation shrinkage.