Mechanical properties and cracking behaviour of lightweight engineered geopolymer composites with fly ash cenospheres

Engineered geopolymer composites (EGCs) are known for their superior tensile performance and multiple cracking characteristics. The development of lightweight EGCs is crucial for applications in structural strengthening and repair. However, the addition of lightweight aggregates often compromises the mechanical properties of these composites. Fly ash cenosphere (FAC) has demonstrated its effectiveness as an aluminosilicate precursor in geopolymers and as a crack width controller in fibre-reinforced cementitious composites, thus showcasing significant potential for the advancement of high-performance lightweight EGCs. This study investigates the effects of FAC incorporation on the properties of lightweight EGCs. The results of this study confirmed the benefits of using FACs in terms of reducing the density of the composites while maintaining satisfactory compressive strength. With increasing the FAC replacement ratio, the tensile strength decreases; however, increasing the FACs/binder ratio preserves the tensile strength of the EGCs. Moreover, the utilisation of FACs preserves the high tensile strain capacity of lightweight EGCs, reaching approximately 9 %. Furthermore, the presence of fractured unreacted FACs, partially hollow reacted FACs, and a weak FAC/geopolymer interface act as artificial flaws, thereby enhancing the crack control ability of EGCs with higher FAC replacement or FACs/binder ratios. Moreover, the inclusion of FACs does not alter the main reaction products of EGCs, providing an attractive avenue for achieving lightweight EGCs with exceptional tensile strain capacity.