Reinforcement interfaces in low-carbon concretes

Calcium sulfoaluminate cement (CSA) is a potential low-carbon binder that has a markedly less alkaline matrix environment compared to traditional Portland cement. Investigations on its long-term chemical compatibility with different reinforcements (i.e. glass fibre and steel rebar) are necessary before widespread adoption could occur. In this project, studies mainly focus on the microstructural evolution at interfacial zones over time, which has a significant impact on the durability properties of reinforced concretes. Glass fibre reinforced composites made with two matrix formulations (OPC, and OPC plus calcium sulfoaluminate based matrices) aged for 10 years at 25℃ and steel reinforced CSA concretes aged for 28 days and 1.5 years are studied. Optical transmitted microscopy and SEM/EDX on the thin section petrographic specimens of composites are employed to give a straightforward interfacial zone characterization. The results suggest that CSA cement is highly compatible with the embedded glass fibre and steel reinforcement in the long term. Aged GRC composite modified by calcium sulfoaluminate cement shows greatly retained toughness, accompanied by considerably flexible interfacial and interfilamentary areas around the glass fibres. This is in contrary to the aged OPC/GRC, which demonstrates significantly brittle behaviour with substantial densification of C-S-H/CH intermixture occurring around glass fibres. In the steel reinforced CSA concrete, elemental mappings clearly show that there is a non-continuous aluminium-rich layer at the steel/concrete interface; and the aluminium gel is slightly consumed over time by chemical reactions.