Effects of silicas and aluminosilicate synthesized by sol-gel process on the structural properties of metakaolin-based geopolymers

This study synthesized two silicas and an aluminosilicate (AlS, Al₂O₃.2SiO₂) via the sol-gel process to evaluate their effects on metakaolin-based geopolymers. Two geopolymer dosages were prepared with metakaolin and 0, 1, 2, and 10 %wt. sol-gel materials, using metakaolin and silica-rich byproducts (rice husk ash and silica fume) as references. The results revealed that sol-gel silicas above 2 %wt. led to particle aggregation and alkalis entrapment, worsening efflorescence. Geopolymers with 10 % sol-gel silicas exhibited 3-day compressive strengths of approximately 10 MPa, compared to ∼35 MPa for geopolymers with 10 % sol-gel aluminosilicate or pure metakaolin. This indicates that AlS particles dissolved more readily during geopolymerization, forming binder structures at early ages. Furthermore, AlS-based structures inhibited Na⁺ diffusion in the hardened matrix, reducing efflorescence compared to reference samples. However, this effect was less noticeable at the more porous dosage of geopolymers. AlS-derived structures were more porous than the metakaolin-based structures, limiting strength gain up to 28 days.