Effect of fumed silica content on the microstructural evolution and mechanical properties of metakaolin-based basalt fiber-reinforced geopolymer matrix composites

Geopolymer matrix composites (GMCs) have emerged as promising heat-resistant alternatives to conventional composites, which often suffer from thermal degradation or excessive density. Fumed silica (F-Si) has recently gained attention as a key additive due to its ultrafine size, high surface area, and strong reactivity that enhances geopolymerization and matrix refinement. However, most studies have treated it as an inert filler, leaving its reactive role and structural effects insufficiently understood. This study investigates the effect of F-Si addition (0–25 wt.%) on the microstructure and mechanical performance of metakaolin-based, basalt-fiber-reinforced GMCs. F-Si acted as a reactive nanofiller, promoting gel-network reconstruction from Al-rich to Si-rich configurations, as confirmed by FTIR analysis. Complementary SEM and micro-CT observations revealed that F-Si significantly densified the matrix and improved structural homogeneity by reducing porosity. Consequently, the compressive, flexural, and tensile strengths increased by 220%, 245%, and 267%, respectively, at 25 wt.% F-Si. Notably, 25 wt.% was identified as the optimal F-Si content, at which maximum matrix densification and load-transfer efficiency were achieved while maintaining acceptable fiber integrity. Although the 10 M KOH activator inherently adversely affects fiber durability, F-Si effectively compensates for this through robust matrix densification. This highlights that optimizing F-Si content is essential for maximizing the structural integrity of GMCs.