Comparative Study of Porous Recycled Concrete Aggregates Treated with Pozzolanic Slurry or Carbonation and Resulting Recycled Concrete Properties

The optimal treatment method for enhancing the properties of recycled concrete aggregates (RCAs) remains uncertain, and it is still unclear whether RCA treatments can improve the characteristics of the resulting recycled concrete. A comparison of various treatments utilizing the same RCA, coupled with an evaluation of the recycled concretes produced, is necessary to determine the efficacy of RCA treatments properties. In this study, recycled low-strength (porous) concrete aggregates (RCAs) were treated via carbonation or immersion in a brick waste powder pozzolan slurry. The treated aggregates were characterized via density and water absorption measurements, mercury intrusion porosimetry, scanning electron microscopy, petrography, and thermogravimetric analysis, and their results were compared. Recycled concretes were produced and assessed for compressive strength and capillary absorption tests using natural aggregates and a 25% (v/v) substitution of both treated and untreated RCAs, and results compared. Furthermore, a simplified analysis of the CO2 emissions resulting from the production of recycled concretes was conducted. The treatment with red brick pozzolan did not improve the RCA properties, such as the porosity and density. Nonetheless, the concrete produced with pozzolanic-treated RCA (with a water-to-cement ratio below 0.5) presented compressive strength 30% higher than concrete with untreated RCA, and only 5% lower than the reference concrete made with natural aggregate. Owing to the capillary absorption, the recycled concrete cannot be used externally in polluted areas. The carbonation method increased the density (by 8%) and reduced the porosity of the RCA (by 30%). However, only a 10% increase was observed in the compressive strength of the resulting recycled concrete relative to concrete made with untreated RCA. The CO2 emissions of the recycled concrete made with carbonated RCA were reduced by 7% relative to concrete made with untreated RCA, and the capillary absorption (a durability requirement) of the recycled concrete was reduced by almost 30%. Therefore, there is no single optimal method for RCA treatment as the effectiveness of each method varies depending on the intended purpose. Different methods may be more suitable for enhancing; concrete strength or improving durability and CO2 uptake.