Alkali release from aggregate and the effect on AAR expansion

In this study, two types of alkali-bearing aggregate, nepheline syenite and alaskite are investigated on their effects on AAR expansion using a new accelerate test method. Crushed nepheline syenite and alaskite with size between 0.15 mm and 0.65 mm are immersed in different alkaline solutions and cured at 150°C, the amounts of released alkalis from aggregates into solutions are determined. Results indicate that nepheline syenite and alaskite could increase AAR expansion when low alkali cements are used, but the enhance effect is weaken largely when high alkali cements are used. Nepheline mineral is stable in saturation Ca(OH)₂ solution, but decomposes when alkali is added into the solution and form new product. Na-feldspar and K-feldspar are stable in alkaline solution, only little dissolution might be found in the crystal interface. It is concluded that alkalis release from these alkali-bearing minerals mainly depend on ion change between surface layer of minerals and Ca²⁺ ion in solution.