Study on silica fume-hybrid fiber composite modified alkali-activated slag concrete: mix ratio orthogonal experiment, crack propagation, and microstructure

Abstract

ABSTRACT Alkali-activated materials (AAM), which are derived from aluminosilicate substances and activators, represent a sustainable and low-carbon alternative to ordinary Portland cement (OPC). The incorporation of fibers helps mitigate brittleness and enhance toughness. An orthogonal experimental approach was used to systematically examine the impact of silica fume (SF) content, the volume fraction of basalt fiber (BF) and polypropylene fiber (PPF), and alkali contents on the mechanical properties of slag-based cementitious materials. The synergistic enhancement mechanism of SF modification and its combination with hybrid fibers were revealed through macroscopic mechanical testing, digital image correlation (DIC) for deformation field analysis, pore structure testing, and microstructural observation. The orthogonal experimental results indicated that the optimal mix ratio for comprehensive mechanical properties was SF of 6 wt.%, BF of 0.2 vol.%, PPF of 0.1 vol.%, and alkali of 9 wt.%. Under this optimal mix ratio, the uniaxial compressive and splitting tensile properties were significantly improved compared to the control group. Combined SF and hybrid fibers promoted crack propagation and multi-cracking. DIC results demonstrated that hybrid fibers effectively redistributed strain concentration and delayed crack initiation. Microstructural analysis shows that SF optimizes pore distribution through physical filling and participates in chemical reactions to form C-A-S-H gel. Fibers acted to hinder crack propagation and bridge cracks. The synergy not only reduced harmful pores in the matrix but also promoted microstructural densification, thereby significantly enhancing the mechanical properties of the material.