Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation

摘要

Binders formed through alkali-activation of slags and fly ashes, including 'fly ash geopolymers', provide appealing properties as binders for low-emissions concrete production. However, the changes in pH and pore solution chemistry induced during accelerated carbonation testing provide unrealistically low predictions of in-service carbonation resistance. The aluminosilicate gel remaining in an alkali-activated slag system after accelerated carbonation is highly polymerised, consistent with a decalcification mechanism, while fly ash-based binders mainly carbonate through precipitation of alkali salts (bicarbonates at elevated CO_2 concentrations, or carbonates under natural exposure) from the pore solution, with little change in the binder gel identifiable by nuclear magnetic resonance spectroscopy. In activated fly ash/slag blends, two distinct gels (C-A-S-H and N-A-S-H) are formed; under accelerated carbonation, the N-A-S-H gel behaves comparably to fly ash-based systems, while the C-A-S-H gel is decalcified similarly to alkali-activated slag. This provides new scope for durability optimisation, and for developing appropriate testing methodologies.