Wait, fly ash actually makes concrete stronger? Nobody told me this for 8 years

I was reading an ACI spec packet on a commercial job in Phoenix and stumbled on a section saying fly ash can boost long term compressive strength past 56 days. My whole career I thought it was just a cheap Portland substitute that slowed set time. I asked my foreman and he said yeah, 20% replacement is common for durability. But I've also seen fly ash batches that never reached design strength because the mix design was off. That's a big swing between saving money and failing a core test. Anyone else have a fly ash job that surprised them, good or bad? Or am I the last guy learning this?

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