Investigating coconut shell biochar as a sustainable solution for removing iron residues in recycled concrete aggregates

The advantages of Recycled Concrete Aggregate (RCA), a sustainable construction material derived from construction and demolition waste (CDW), are undermined by certain characteristic issues. One significant issue is the presence of iron residues in RCA, resulting from the corrosion process associated with the reinforcements in the parent concrete. These residual iron particles affect various aspects of RCA and Recycled Aggregate Concrete (RAC), including physical properties, structural integrity, and durability. This study aimed to provide new research findings to the industry, addressing the limited research on iron residues in RCA. The research involved petrographic analyses and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM–EDS) to examine cement matrices through thin sections derived from cement mortar bars produced with RCA and NCA blended with Coconut Shell Biochar (CSB) particles. Residual iron particles were observed in the cement matrices through petrographic images, while Fe²⁺ cations were detected as precipitates adsorbed onto biochar particles. The researchers concluded that iron residues in RCA could be removed using CSB as an additive in RAC. Additionally, it was found that finer coconut shell biochar particles act as effective fillers, occupying a significant portion of crack pore space in the cement matrices.