Interfacial bonding of 3D printable concrete with chemically reactive coating for automatic repair

3D concrete printing can be used to automatically repair concrete structures, while the practical application is still hindered due to the lack of interfacial bonding. A concept using a chemically reactive coating at the interface is proposed to overcome this drawback. Specifically, calcium sulfoaluminate (CSA) cement paste is retarded by adding borax, and the retarded phase can be destroyed with increased pH. The retarded CSA cement paste is placed as coating on the substrate before the repair process, and 3D printable material is placed against the coating afterwards. After that, the hydration reaction of the retarded paste coating can be re -initiated after contact with high -pH 3D printable material, aiming for high interfacial bonding in a short period. We first characterized the structural build-up and early -age mechanical properties of the 3D printable mixtures. We further evaluated the interfacial bonding of samples in the fresh state using a modified pull-out test and investigated the effect of the fresh properties of the materials on the interfacial bonding. After that, we assessed the bond strength of hardened samples based on a modified pull -off test. Finally, we analyzed the microstructure at the interface region using scanning electron microscopy. Results indicate that the placement of chemically reactive coating improves interfacial bonding, which can be attributed to the fast setting and hardening of the material at the interface. Moreover, the enhancement in interfacial bonding is more effective for those 3D printable mixtures that could potentially provide more hydroxide ions to compensate for the retardation of paste coating.