Study on Multi-Scale Strength Formation Mechanism of Fly Ash-Based Geopolymer Concrete Based on Statistical Damage Theory

Uniaxial compression tests were conducted on fly ash-based geopolymer concrete (FAG) with varying alkali-binder ratios (0.25, 0.35, 0.45, 0.55, 0.65) and curing ages (7 d, 28 d) to ascertain its mechanical performance parameters and stress–strain relationship curves. The formation mechanism of FAG multiscale strength is revealed through a systematic process that integrates statistical damage theory with microscopic testing techniques. This process involves the progression of microstructural state and the evolution of mesoscopic damage, providing a comprehensive understanding of the multiscale strength formation process. The results indicate that as the alkali-binder ratio increased, there was an initial rise and subsequent decline in microstructure density. At an alkali-binder ratio of 0.45, the alkaline activator can fully stimulate the fly ash to undergo depolymerization and polycondensation reactions. The result of this process is the formation of a continuous and dense cementitious matrix, thereby achieving the optimal improvement in macroscopic initial mechanical properties. Concurrent microstructural alterations further modify the morphology and path of microcrack initiation and propagation during uniaxial compression, as well as the effective force skeleton adjustment process. The characteristic parameters that are indicative of the evolution of microfracture and yield damage demonstrate regular changes in accordance with the alkali-binder ratio. The joint effect of these two factors determines the evolution characteristics of the macroscopic nonlinear stress–strain behavior of FAG, ultimately resulting in an increasing and then decreasing trend of FAG strength with the increase of alkali-binder ratio, while ductility shows a trend of decreasing first and then increasing. At an equivalent alkali-binder ratio, the porosity of the 7 d sample exhibited a decrease of 1.13% to 17.13%. Conversely, the strength of the 7 d sample increased by 39% to 312%. However, the deformation capacity of the 7 d sample decreased, with a peak strain reduction of 21% to 52% at 28 d. This research achievement has the potential to provide significant theoretical support for the practical engineering promotion and application of FAG.