Graphene's effect and mechanism on the properties of alkali-activated slag coating

Using alkali-activated slag to prepare coatings is an alternative solution for concrete repair and protection. However, shrinkage cracking limits the application of alkali-activated slag, which also increases the risk of coating peeling. Graphene dispersions (content of 0.2%, 0.4%, 0.6% and 0.8%) were added to the alkali-activated slag coatings. The drying time, apparent circumstance (thicknesses of 1 mm, 2 mm, 3 mm and 5 mm) and the tensile bond strength with 1 mm thickness coating were determined. The mechanism of action was analyzed in terms of both product and interfacial properties. The experimental results showed that the graphene dispersion would shorten the drying time of the coatings. The drying time was reduced by 57.1% (from 116 min to 52 min). Moreover, the graphene dispersion improves the cracking of the coating. The larger the coating thickness is, the better it improves the cracking effect. The surface cracking of the coating with 5 mm thickness was reduced by 60.7% (from 1.6751% to 0.6583%). Graphene can significantly improve the bond strength of the coating. When the graphene content is 0.8%, the bonding strength with a thickness of 1 mm is 0.502 MPa at 7 days and 1.577 MPa at 28 days. The bond strength increased by 280.3% and 175.2%, respectively, XRD, FTIR and SEM analysis showed that the increase in bond strength was caused by two factors. (1) Graphene can promote the generation of zeolite phase in alkali-activated slag. This is conducive to enhancing the interlayer adhesion of the coating itself. (2) Graphene is beneficial to reduce the pores in the transition zone between the coating and the substrate and enhance the contact area between the coating and the substrate and makes the transition zone denser.