Second-Generation Recycled Concrete Aggregates: Comprehensive Characterization of Physical, Mechanical, and Microstructural Properties

The rapid expansion of concrete production has intensified the depletion of natural aggregate (NA) resources, necessitating sustainable alternatives in the construction industry. Recycling construction and demolition (C&D) waste offers a solution to enhance environmental sustainability and resource efficiency. Most existing studies have mainly focused on first-generation RCAs (RCA1), with little work on second-generation RCAs (RCA2), especially fine fractions. This study examined the properties of recycled concrete aggregates (RCAs) across first and second recycling cycles, focusing on their upcycling potential. Therefore, commercially sourced NAs and RCA1 were compared with lab-produced RCA2, both coarse and fine, derived from further recycling of first-generation recycled aggregate concrete (RAC1). Comprehensive tests assessed morphology and physical, mechanical, and microstructural properties to provide a clear insight into how RCA2 differs from RCA1. Average sphericity for coarse RCA1 was 0.81, an 8% decrease from NA’s 0.88, while RCA2 had an average sphericity of 0.76, a 14% decrease. The results revealed a progressive decline in aggregate quality with each cycle. RCA1 exhibited water absorption of 9.53% (fine) and 5.55% (coarse), while RCA2 showed higher absorption at 13.16% (fine) and 6.88% (coarse). RCA1’s crushing value was 25.9%, a 41% rise over NA’s 18.09%, while RCA2’s reached 29.2%, a 61% increase. Coarse RCA2 contained 51.03% attached old mortar, 50% more than the 33.95% in RCA1. Fine RCA2 showed significant performance reductions, limiting these aggregates to non-structural downcycling applications. Microstructure analyses confirmed RCA2’s porous structure, attributed to increased adhered old mortar, including multiple weak interfaces, and numerous microcracks compared to RCA1, necessitating careful consideration when using coarse RCA2 for upcycling in sustainable construction.