What is a requirement for fly ash when used in conjunction with potentially reactive aggregates?

When fly ash is used in conjunction with potentially reactive aggregates, controlling the alkali content is crucial to minimize the risk of alkali-silica reaction (ASR), which can lead to expansion and cracking in concrete.

Having fly ash with alkali levels that do not exceed 0.6% is significant because it helps mitigate the harmful effects that reactive aggregates might otherwise induce. If alkali levels in the fly ash are higher than this threshold, the potential for ASR increases, especially when the fly ash is used with aggregates that are known to be reactive. Therefore, ensuring that the alkali content of the fly ash remains at or below this level is an essential requirement for maintaining the durability and performance of the concrete.

In contrast, the other options do not adequately address the primary concern regarding the use of fly ash with reactive aggregates. Limiting the percentage of fly ash to less than 5% or not allowing alkalis at all does not consider the balancing act between the benefits of fly ash in concrete and the potential risk of ASR. Additionally, while fine grinding can improve the pozzolanic reactivity of fly ash, it is not a regulatory requirement specific to its use with reactive aggregates. Thus, option C accurately reflects