Global Concrete and Cement Association[GCCA], page-3

How much more fly ash? Can we see the results of the studies? You always make broad statements without much substance by way of references.

I read one reference which said, "The reason why limited to use fly ash up to 20% because it has a beneficial effect on the workability and cost of concrete but it may not be enough to sufficiently improve against sulfate attack and durability of concrete."
Then it goes on to say.
"However, in commercial practice the dosage of fly ash is limited to 15%- 20% by mass of the total cementitious material ... This research shows that using fly ash by 35% and 50% has a negative impact on early strength and longer initial setting time due to the low reactivity of fly ash.

The reason why limited to use fly ash up to 20% because it has a beneficial effect on the workability and cost of concrete but it may not be enough to sufficiently improve against sulfate attack and durability of concrete."

Another paper state:
"High volume fly ash concrete (HVFAC) is a feasible solution not only for reducing greenhouse gas emissions produced by cement but also minimizes the waste deposited by dumping fly ash in many parts of the world. Low strength development is a major drawback of high volume fly ash system and it worsens with cement replacements more than 65%."

So as not to cherry pick.

"The mechanisms by which the incorporation of high volume of the ash in concrete reduces the water demand, improves the workability, minimizes cracking due to thermal and drying shrinkage, and enhances durability to reinforcement corrosion, sulfate attack, and alkalile-silica expansion are discussed."

Seems to contradict other research. It's a shame that we don't seem to get a consensus.