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If you are doing High Strength Concrete R&D, look for “Silica Fume”. I participated (and won) the regional ACI High Strength Concrete competition for my school years and years ago...

Aside from the a Coarse Aggregate and Fine Aggregate (sand) you want Silica Fume, and the most critical thing is a low Water/Cement ratio... which can only be accomplished by adding a Superplasticizer.

The Fly Ash/ Silica Fume component is included with the Portland Type II as Cement in the W/C ratio... and what it does is imparts a charge to the cement particles so when the water is added, the particles space themselves out in the matrix.

Look for your base strength to be 8ksi... anything more is bonus.

It's tough to nominate a generic fly ash because each coal seam has distinct chemistry. A low ash fusion coal will produce an entirely different ash than a high ash fusion coal. Clinkers don't always come in small pea or nut sized shapes; a local power plant had to shut down for several weeks while the pick-up sized clinker cooled down enough so a crew could enter the combustion chamber and drill, shoot, and load it out. Obviously, some of the minerals in the coal were no longer evident in the fly ash.

As an aside. several years back the epa ruled that solid components - the ash and fly ash - after combustion were to be considered emissions. It doesn't take long for a high quality steam coal, with 10% ash content, to produce a boat load of HAZMAT emissions.

As for Silica Fume, yes, it absolutely increases the strength of concrete, irrespective of the fly ash component. I'm sure it effects the compressive strength, but I don't have any idea how to plug in the numbers to determine if the rebar components should be adjusted to reflect a change in the tensile strength. Our engineers always added the Silica Fume just before the pour to manage the final product. Myself, I would add a bit if I were pouring a patio or foundation for a house. Just be darn sure to wear a well fitting respirator with an NBC filter while handling the bags.