He is 100% right but…………

fly ash is a by product of coal fired furances. Fly ash as the same chemical properties has cement when it reacts with calicum hydroxide which is a by product of curing concrete.

fly ash can replace up to 45% of cement by weight. and fly ash is free.

the properties of a fly ash mix is. because less cement is used, the mix is not as hot, it will setup slower. Because the particles are more rounder it make a more worable mix with a lower water cement ratio.

the fly ash does not start to react till the calicum reacts which is usually around 10-14 days. so the mix starts off slow and doesnt gain maximum strength till last of 28 days. what this does is reducing cracking due to shrinkage.

in conclusion.
mix is cheaper
mix is more workable
mix controls cracking

mix is just as good as the high dollar mix but……………

if you need to set steel fast, you you a staight cement mix with super P.

Give me a three inch mix 6 sack straight with super P and I can gurantee you 3500 in 3 days. Pour on friday, hang steel on monday