2.1. Materials

Portland cement (OPC)-CEM I 42.5R with a mineralogical composition of C3S = 56.7%, C2S = 16.6%, C3A = 10.0%, C4AF = 9.2% was used in this study. The OPC presented a specific surface (Blaine) of 452 m2/kg and a density of 3070 kg/m3, with a chemical composition of CaO = 62.76%, SiO₂ = 19.42%, Al₂O₃ = 5.39%, Fe₂O₃ = 3.00%, MgO= 1.74%, K₂O = 0.53%, Na₂O = 0.12%.

Lab-made CPP originating from cement paste (CPPp) was produced from a well-hydrated cement paste with a water/cement ratio of 0.45, which was cured in a moist chamber for 1 year. Additionally, CPP originating from concrete (CPPc) was produced and cured in similar conditions to the cement paste using a standard concrete composition, as described in Table 1.

Concrete composition.

The CPPp and CPPc specimens were then dried at 100 °C for 2 days, crushed in a jaw crusher, and submitted to an additional drying period of 2 days to ensure maximum free water removal, which is essential to minimizing the adhesion of material to the walls of the mill in the next step. Finally, the ensuing crushed material was grinded using a jaw crusher, a roller mill, and a ball mill followed by sieving through 250 µm mesh. After this process, the water content of CPP was 1.5 wt.%.

A specific procedure was developed to achieve CPP samples with different initial water content in order to study the influence of this parameter on carbonation. The process started by mixing CPP with water at a ratio of 1:1, obtaining a very fluid inert paste. This paste was then spread in a tray, ensuring a very small thickness of material, and placed in a furnace at 80 °C to promote the evaporation of free water. The CPP was removed from the furnace after the time required to achieve a given water content. These periods were determined from a previous experiment in which the mass loss evolution of CPP inside the furnace was measured in periodic and constant time intervals. Wet CPP presented a looser appearance and homogeneous water content across the CPP layer.