Microstructure–Durability Relationships in Recycled Concrete: A Critical Review of Scanning Electron Microscopy Evidence on Pore Structure, Hydration Products and Interfacial Bonding

Abstract

Recycled aggregate concrete (RAC) offers a route to reducing the extraction of natural aggregate and the landfilling of construction and demolition waste, yet its uptake in durability-critical applications remains constrained by uncertainty over long-term performance. The behaviour of RAC is governed less by bulk mix proportions than by features resolved at the micrometre scale: residual adhered mortar, a heterogeneous pore network, an altered distribution of hydration products, and multiple interfacial transition zones (ITZ) of differing quality. Scanning electron microscopy (SEM), particularly in backscattered electron mode with energy-dispersive X-ray analysis, has become the principal tool for visualising these features, but the evidence it yields is frequently reported descriptively and interpreted in isolation from complementary porosimetry and transport measurements. This review critically synthesises SEM-based evidence on the pore structure, hydration assemblage and interfacial bonding of recycled concrete aggregate (RCA) and RAC, and examines how these microstructural attributes map onto durability under chloride ingress, carbonation, freezing and thawing, and sulfate exposure. The synthesis distinguishes robust and consistent findings, such as the porosity and weakness of the old adhered mortar and its associated interfaces, from claims that remain conditional, such as the quantitative extent of ITZ densification achieved by aggregate pre-treatment. Recurring methodological weaknesses are identified, including the two-dimensional and qualitative nature of much SEM reporting, the reconciliation of pore parameters obtained by different techniques, and the scarcity of long-term field data. Enhancement strategies, notably accelerated and wet carbonation, pozzolanic and nano-modified surface coatings, supplementary cementitious materials, and modified mixing, are evaluated for the strength of their microstructural rationale and the durability gains they deliver. The review concludes that microstructure–durability links in RAC are qualitatively well established but quantitatively under-specified, and it sets out prioritised research needs directed at quantitative, three-dimensional and time-resolved characterisation.