Effects of hybrid fibers and nanosilica on mechanical and durability properties of lightweight engineered geopolymer composites subjected to cyclic loading and heating–cooling cycles

Abstract

This paper discusses the influence of nanosilica and hybrid fibers on the mechanical behavior and microstructure of fly ash and slag-based lightweight geopolymer composites exposed to 0, 10, 20, and 30 heating–cooling cycles. Nanosilica (NS), polypropylene (PP), and polyvinyl alcohol (PVA) fibers 2% by volume raction were used to enhance lightweight engineered geopolymer composites (EGC). Prior to and beyond heat cycles, the flowability, density, mass loss, scanning electron microscopy (SEM), and residual mechanical characteristics (static and cyclic loading responses, tensile stress, strain capacity, and load–deflection response) were evaluated. The experimental findings revealed that adding 2%NS, 2%PP fiber, and 2%PVA fiber into lightweight EGC composites enhanced compressive strength by 4.75%, 13.59%, and 27.14%, respectively, at room temperature. Thermal cycles increased the compressive strength of nanosilica and PVA fiber samples by 12.41% and 21.67%, respectively. After 30 heat cycles, the samples reinforced with PP fibers lost 20.26% of their compressive strength. Since tensile characteristics were much more sensitive to the development of internal microstructure deficiencies during thermal treatment, the tensile stress and strain capacity of all lightweight EGC specimens dramatically reduced and lost their strain hardening behavior after being exposed to new heating–cooling cycles. The microstructural analysis of the hybrid system indicated that the bonding characteristics between both the EGC matrix and the PP fibers were weak, and the PP fibers had a negligible influence on the ductility properties of the hybrid composite as compared to using just PVA and PP fibers. PVA fibers did not melt after being subjected to various heating–cooling cycles at 200 °C, but PP fibers melted entirely during thermal cycles, and the lightweight EGC microstructure revealed apparent faults such as micro-cracks and a significant number of capillaries.