Admixtures - Micro Silica / Fly Ash and Concrete Hydration

Micro Silica is reactive and contributes to the heat of hydration. I believe the initial heat of hydration is higher, resulting in a higher early strength. This is useful in some applications. Sorry, I don't know the actual values in heat per unit mass.

Fly ash however is reactive but contributes less heat per unit of mass than Portland cement. So yes the use of fly ash would reduce the heat of hydration of concrete if used to replace Portland cement in the mix design.

The following are two good references (Not Wikipedia) that will help you out:

Concrete: Scientific Principles

Influence of Supplementary cementing Materials on the Heat of Hydration of Concrete