Alkali dissolution from aggregates and its effect on ASR expansion simulated by the use of alkali rock powder

Concrete expansion due to an alkali-silica reaction (ASR) occurs when alkali ions (OH⁻) in the pore water react with silica minerals in reactive aggregates. To mitigate ASR expansion, the concentration of alkali ions determined primarily by the amount of alkali metals derived from cement has to be controlled. In addition to silica minerals, minerals in the aggregates containing alkali metals may also undergo reactions, resulting in the dissolution of alkali metals. It is considered that this release increases the OH⁻ concentration. This alkali metal dissolution reaction is referred to as alkali release. This phenomenon was investigated through the concrete prism test using a reactive chert aggregate and nepheline syenite (NS) powder, an alkali-rich rock. The results indicated that the addition of NS contributed to increased expansion over the storage period. At an NS addition of 5 % by mass of fine aggregate (NS5 %), the alkali dissolution from NS, estimated from the expansion, was 9.2 kg/m³, which closely matched the measured water-soluble alkali content of the concrete, 8.2 kg/m³. However, when the NS addition was increased to 10 % (NS10 %), the water-soluble alkali content increased to 11.2 kg/m³, yet the expansion rate remained unchanged compared to NS5 %. Thus, alkali dissolution beyond a certain threshold did not contribute further to expansion. Additionally, the alkali dissolution from NS, as evaluated using RILEM AAR-8, was considerably underestimated, with values of 0.13 kg/m³ for NS5 % and 0.25 kg/m³ for NS10 %.