Influence mechanisms of aeolian sand and basalt fiber incorporation on the mechanical properties and microscopic pore structure of concrete

Abstract To elucidate the influence and underlying mechanisms of double-doped basalt fiber (BF) and aeolian sand (AS) on the mechanical properties and microscopic pore structure of concrete, concrete specimens with varying BF lengths, volume fractions, and AS replacement ratios were prepared. The mechanical properties of the concrete, including compressive, splitting tensile, flexural, and uniaxial compression strengths, were evaluated. In parallel, nuclear magnetic resonance (NMR) was used to characterize the pore structure, while scanning electron microscopy (SEM) provided microstructural insights, enabling a multiscale elucidation of the underlying mechanisms. The results reveal that the strength of basalt fiber aeolian sand concrete (BF-ASC) increases with age, showing rapid development at early ages followed by a gradual deceleration. An exponential function-based model accurately captures the early-age strength development. Compressive, splitting tensile, and flexural strengths, as well as uniaxial compressive peak stress and elastic modulus, exhibit a non-monotonic behavior, initially increasing and then decreasing with increasing BF length, BF volume fraction, and AS replacement ratio. In addition, the failure mode of concrete changes from brittle to ductile with fiber incorporation. The toughness index exhibits a trend of first increasing and then decreasing with increasing BF length and AS replacement ratio. Notably, it increases progressively with BF volume fraction, reaching an increase of 90.96% at a volume fraction of 0.3% compared with the reference group. The BF-ASC constitutive model, constructed by describing the stress–strain relationship piecewise, provides a more accurate representation of its deformation and failure characteristics. NMR analysis indicated that appropriate incorporation of BF and AS effectively optimized the pore structure and enhanced the compactness of BF-ASC, whereas excessive incorporation led to pore-structure deterioration. Under the optimal BF condition, the harmless pore fraction increased from 62.51 to 68.41%, while the multiple harmful pore fraction decreased from 16.42 to 12.72%. Furthermore, the pore structure was further optimized when the aeolian sand replacement ratio was 25%. SEM observations revealed fiber pull-out, breakage, and effective fiber-matrix bonding. In the double-doped system, BF and AS interact synergistically within the concrete matrix, modifying both the microstructure and macroscopic mechanical properties of the material. When incorporated at appropriate dosages, BF effectively suppresses microcrack initiation and propagation through its bridging effect, while AS enhances matrix compactness and fiber-matrix interfacial bonding through its filling effect. This synergistic interaction significantly improves both the mechanical properties and pore structure of concrete. Based on the parameter range investigated in this study, the optimal combination is a BF length of 18 mm, a volume fraction of 0.1%, and an AS replacement ratio of 25%.