Influence of slag content on the bond strength, chloride penetration resistance, and interface phase evolution of concrete repaired with alkali activated slag/fly ash

The implement of old concrete repair with the more environmentally friendly slag/fly ash to replace the ordinary Portland concrete (OPC) is explored widely. The effect of slag content on the splitting tensile strength and chloride penetration resistance of the repaired concrete was considered. In this study, the slag and fly ash were used as binding material, and the sodium silicate to sodium hydroxide ratio was 1.3. Results showed that the interfacial bond strength of the repaired concrete increased significantly when the slag content was 50%, which was more than 62% higher than that of the repaired concrete with OPC. The repaired concrete showed the best chloride penetration resistance under 50% slag content, which was better than that of the repair using OPC. It was evident that some reactions happened between the alkali activated slag/fly ash and the cement from the old concrete substrate, and the C-S-H, C-A-S-H, and N-A-S-H gels were detected by X-ray diffraction (XRD) analysis. The amount of the gels and their interaction are the dominant factors to influence the mechanical properties and chloride penetration resistance of the repaired concrete.