Structural Characteristics of Fire‐Exposed Fly Ash Based Geopolymer Concrete

ABSTRACT Fly ash‐based geopolymer Concrete has emerged as a sustainable alternative to Ordinary Portland Cement (OPC) concrete due to its lower carbon footprint and superior thermal stability. This study investigates the residual mechanical, bond‐slip, shear friction, and microstructural behavior of fly ash‐based geopolymer exposed to elevated temperatures ranging from 200°C to 800°C. Five fly ash replacement levels (0%, 20%, 30%, 40%, and 50%) were experimentally evaluated through compressive strength, splitting tensile strength, stress–strain response, pull‐out bond behavior, and shear friction testing. The experimental results revealed that all geopolymer mixes experienced progressive degradation with increasing temperature; however, mixes containing 20%–30% fly ash exhibited superior residual performance and thermal stability. At 400°C, FA30 retained approximately 80% of its original compressive strength, whereas significant deterioration was observed beyond 600°C. The modulus of elasticity decreased by nearly 85%–95% at 800°C, while peak strain increased considerably, indicating a transition from brittle to ductile behavior under thermal exposure. Bond strength degradation was more gradual in fly ash‐based geopolymer compared to conventional OPC concrete, with FA20 and FA30 exhibiting improved bond retention and lower slip values at elevated temperatures. SEM observations confirmed that the enhanced thermal resistance of geopolymer was primarily associated with the formation of a stable alumino‐silicate gel network, reduced microcracking, and controlled pore development compared to OPC concrete. Regression‐based degradation and bond‐slip models were proposed and validated against experimental data, demonstrating good agreement with existing literature. The study highlights the potential application of fly ash‐based geopolymer in fire‐resistant and sustainable structural systems, while also providing predictive tools for evaluating post‐fire structural performance. Unlike previous studies focusing primarily on compressive behavior, the present investigation integrates mechanical, bond‐slip, shear friction, and microstructural assessment with predictive degradation after exposure to elevated temperatures.