Physical, chemical and mineralogical properties of fine recycled and natural aggregates, and their potential influence on concrete properties

This study investigated the physical, chemical and mineralogical properties of fine recycled aggregate (FRA) and their influence on the properties of fresh and hardened concrete. Four FRA fractions, obtained from a single source of parent concrete, were investigated separately to determine the effect of particle size on FRA properties. These were compared to two common South African fine natural aggregates (FNA) - dune sand and greywacke crusher sand. The parent concrete was characterised by compressive strength and carbonation measurements, while water absorption, density, particle size analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were carried out on the fine aggregates and cement. While the FRA displayed less dense and porous adhered cement paste (ACP), as expected, the ACP consisted largely of hydrated cement phases which were more concentrated in the finer (< 0.15 mm) powder fractions. The results inferred that, for new concrete applications incorporating the FRA, phases in the ACP such as Ca(OH) 2 and C-S-H react with CO 2 during the carbonation of concrete to produce a buffering effect, while the C-S-H assists in chloride binding. This is in addition to the capacity of calcite, C-S-H and Aft in the ACP to provide nucleation sites, thereby promoting hydration reactions in new concrete. Furthermore, the properties of the ACP can be optimised through synergistic reactivity between the chemical phases in the binder and the ACP to improve the long-term mechanical and durability performance of new concrete.