Use of Class F Flyash to Mitigate Reactive aggregate

[H57]

In our concrete specification we specifically exclude the use of reactive aggregates. During the bid process of a job we have received the following question:

"Can a fine aggregate be used that is potentially reactive if it is used in conjunction with a Class F Flyash to mitigate the reactivity?"

In checking in appears that flyash will mitigate the reactive aggregate, but I would like to know if anyone has experience with this. Is flyash 100% effective? What potential problems can be encountered in allowing this? Should it be allowed in certain situations? (ie footings, floors to be covered)

Any help would be appreciated.

 

[Teguci]

Is this in regards to Alkali Silica Reaction? If so then a silicious aggregate can be used safely with a low alkali cement mix (choose to limit the alkali content of the cement). As for fly ashes mitigating effects, this is highly variable and should be accompanied with appropriate testing - (Mortar bar expansion).

If I recall correctly, ASTM C33 has an addendum that addresses ASR issues with Silicious aggregates.

 

[Ron]

I agree. I would not use Class F flyash in an attempt to completely mitigate reactive aggregate problems. ASR is caused by the alkali in cement reacting with certain silica components. The way the Class F flyash works is that it replaces some of the cement, thus fundamentally reducing the amount of available alkali.

To answer your question, it is not 100% effective.

Depending on the amount of reactive silica the result can range from surface popouts to extensive microcracking and deterioration of the concrete matrix. The quantity of either is not very predictable.