The addition of low-cost few layers graphene (FLG) to improve flexural strength of coal fly ash based-geopolymer

The flexural strength of geopolymer has been improved by the addition of few layers graphene (FLG). The composites were made by mixing coal fly ash, alkaline activator, fine aggregate and FLG. FLG was obtained from a simple, low-cost and environmentally friendly liquid shear exfoliation method using a kitchen blender. Flexural strength and morphological properties were characterized using hydraulic concrete beam testing, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The interaction between the geopolymer matrix and the FLG were also discussed through the density functional theory (DFT) approach. The flexural strength of composites increased with the increasing FLG content, and reached a maximum increase of 14.9% when 12% of FLG was added. SEM micrographs showed that the FLG addition changed the surface profile of geopolymers, from a grain-containing surface to a grain-less surface. TEM figures confirmed the existence of FLG, where various sizes of FLG were distributed and embedded in the matrix. The optimized structure through the DFT calculations revealed the emergence of several types of secondary bonds between carbon atoms on the FLG surface and the nearest oxygen (O), sodium (Na) and silicon (Si) atoms that strengthen the composite. Unexpectedly, this geopolymer composite could maintain its high flexural strength after exposure to a temperature of 300 °C. Overall, low-cost FLG is a promising additive for manufacturing high flexural strength geopolymers for industrial scale and for expanding their catalog of contemporary applications.