Internal alkali transport in recycling concrete and its impact on alkali-silica reaction

Recycling concrete (RC) produced with recycled aggregates (RA) obtained from demolished concrete structures bears the risk of an alkali-silica reaction (ASR). In this study, the interplay between aggregate reactivity and alkalis contributed by the cement and the RA is investigated. Three sets of concrete mixtures are produced using RA and natural aggregates, both reactive and non-reactive ones. Caesium either present in the RA or in the mixing water of the RC is used to trace the transport of alkalis and their incorporation in ASR products. The result shows that alkali released by RA can intensify ASR. On the other hand, ASR in RA with a low alkali content but reactive aggregate particles can be triggered by the alkali provided by the new cement paste of the RC. The highest risk of ASR in RC is caused by RA from a source concrete already affected by ASR.