Influence of fly ash on the water and chloride permeability of alkali-activated slag mortars and concretes

Mechanical strength, water and chloride permeability of alkali activated slag mortar and concretes with partial fly ash substitution are investigated. Volume of permeable voids (VPV) and sorptivity testing show that alkali-activated materials exhibit higher water absorption than OPC-based samples, and increasing fly ash addition leads to reduced mechanical strength and increased water absorption. Conversely, chloride permeability testing by the NordTest NT Build 492 standard accelerated method, and by chloride ponding (ASTM C1543), shows that the diffusion of chloride in alkali-activated binders is much less than in OPC binders. This divergence between measured water uptake and chloride permeability in alkali-activated specimens is attributed largely due to the specified standard preconditioning (drying) of the samples prior to water absorption testing due to the difference in water environments as a function of slag/fly ash ratio. Drying is likely to be inducing desiccation and consequent microcracking of slag-based binding gels, while less disturbance occurs in a fly ash geopolymer gel. The chemistry of the binding gels in these alkali-activated systems significantly controls the mass and ionic transport in chloride-containing environments. Although higher porosity was measured by these standards, a denser Al-substituted calcium silicate hydrate (C–A–S–H) gel contributes to a higher mechanical strength, and durability under chloride exposure. The inclusion of fly ash promotes the formation of more porous sodium aluminosilicate (N–A–S–H) type gels, reducing the resistance to transport.