The effects of phase changes on the bonding property of geopolymer to hydrated cement

Using geopolymers as bonding materials could be an alternative solution to the concrete repair. This study reports the effects of phase changes on the bonding property of geopolymer to hydrated cement. A geopolymer binder was prepared by activation of heated kaolin with sodium silicate solution, bonded with 28 days aged cement paste and cured under different conditions. The experimental results demonstrated that under the 20 °C air curing conditions, the bond strength achieved 1.3 MPa at 7 days and increased to 1.5 MPa at 28 days. While under the 80 °C steam and water curing conditions, the bond strength decreased by 31% and 37% respectively. The XRD, FTIR and SEM analysis of the geopolymers and hydrated cement pastes show that the strength loss is due to two factors: (1) the increased porosity in cement paste due to the water loss and/or crystallization of C–S–Hs; and (2) the mineralogical change (crystallization) in geopolymer binder which becomes more ordered structures. The increased porosity and the crystallization either in cement or geopolymer generate local stress and weak regions at the interface. This study suggests that the stability of metakaolin-based geopolymers should be taken into consideration when they are used as bonding or coating materials for concrete repair, particularly at the conditions with elevated temperatures.