Durability of alkali-activated fly ash concrete: Chloride penetration in pastes and mortars

• Coarse fly ash requires high amount of alkali activator, leading to high porosity.
• AAFA binder and mortar may have higher Cl− penetration rate than Portland cement.
• Reducing liquid/solid ratio and slag substitution can reduce Cl− penetration rate.
• Rethink about the durability of reinforced AAFA concrete.

Durability of alkalis-activated fly ash (AAFA) concretes is of vital concern for their commercial application. This study investigates the durability in the aspect of chloride penetration in AAFA pastes and mortars in comparison with Portland cement. A coarse fly ash with 0–40% substitution by a granulated blast furnace slag were used to prepared AAFA pastes and mortars at liquid/solid ratio of 0.6–0.8 (in mass). It was observed that at the same grade of compressive strength, even higher, the unsaturated AAFA paste and mortar had higher Cl− penetration rate than Portland cement paste and mortar. Only the mortars with the liquid/solid ratio of 0.6 and the mortar with 40% slag substitution exhibited similar Cl− penetration rate as the cement mortar at w/c of 0.5. The pore structure analysis showed that the porosity and tortuosity are the two most significant factors affecting the Cl− penetration. This study underlines the important roles of the natural properties of fly ash, liquid/solid ratio and slag substitution in the development of durable AAFA concrete.