Silane-modified graphene oxide in geopolymer: reaction kinetics, microstructure, and mechanical performance

Graphene oxide (GO) and 3-aminopropyltriethoxy silane (APTS) have been successfully used in geopolymers to solve specific problems. Nevertheless, the dispersion of GO to strengthen the matrix remains a challenge owing to the characteristic high alkalinity of geopolymer binders. Concurrently, independent addition of APTS to improve the workability severely retards geopolymer dissolution and condensation reactions. In this paper, an attempt is made to mutually exploit benefits of these two additives through a surface functionalisation approach whereby surface functional groups of GO were modified by APTS to provide repulsive effects that could better isolate the GO nanosheets from each other, while the accelerating effects of GO on the geopolymerisation process compensated for the retardation effects of APTS. The structure of GO-APTS was observed to be stable in the highly alkaline sodium silicate environment and significantly improved the dispersion of the nanosheets in the geopolymer composite according to the UV–Vis and SEM/TEM observations. Furthermore, the nucleation and reinforcing effects of GO were enabled by APTS. The GO-APTS–cement samples not only demonstrated better workability and transport properties compared with the reference and pure GO samples, but also superior mechanical strength, with improvements in tensile strength of ∼110% and ∼52% at 7 and 28 days, respectively. These findings evidence the effectiveness and potential of silane-functionalised GO in overcoming the dispersion issues experienced by GO in the highly alkaline media of geopolymers while simultaneously enhancing the mechanical and durability performance of geopolymer systems.