Ceramics-Silikáty 56 (4) 374-382 (2012)

A WEAK ALKALI BOND IN (N, K)–A–S–H GELS: EVIDENCE FROM LEACHING AND MODELING

F. Škvára, V. Šmilauer, P. Hlaváček, L. Kopecký, Z. Cílová

The alkali bond in (N, K)–A–S–H gels presents an up-to-date insufficiently resolved issue with significant consequences for efflorescence in alkali-activated materials. A series of experiments shows nearly all alkalis are leachable from alkaliactivated fly-ash and metakaolin in excessive amounts of deionized water. A diffusion-based model describes well the alkali leaching process. Negligible changes of the (N, K)–A–S–H gel nanostructure indicate that Na,K do not form the gel backbone and H₃O⁺ is probably the easiest substitution for the leached alkalies. Small changes in the long-term compressive strength of leached specimens support this hypothesis.

Keywords: Aluminosilicate gel, N–A–S–H gel, Geopolymer, Leaching, Efflorescence