Migration of alkali ions in mortar due to several mechanisms

The migration and concentration of alkali ions in mortar caused by freezing/thawing cycles, wetting/drying cycles and electric current have been systematically studied to understand the magnitude of their effects and possible implications for alkali-aggregate reaction. Over 400 slices of samples exposed to one sided freezing/thawing and wetting/drying cycles, constant humidity gradients, and electric current were analysed for total K₂O and Na₂O contents by Neutron Activation Analysis (NAA). The experimental results have shown that alkali ions migrated from the wet or the warm ends to the dry or the freezing ends respectively, or from the anodic to the cathodic ends. The maximum alkali concentration was found after wetting/drying cycles being 1.99 eq.Na₂O after 270 cycles, while the original alkali content of the cement was only 0.89 eq.Na₂O. Therefore, migration and concentration of alkali ions may accelerate alkali-aggregate reactions when reactive aggregates are used, and initiate localized alkali-aggregate reaction even if a low alkali cement is used. The driving forces of ion migration were believed to be moisture gradients and electric potential differences. The former results in alkali migration from the wet parts of concrete to the dry parts and precipitates alkalies near the dry surface as moisture is evaporated. The latter causes an electrochemical effect.