You're confusing fly and bottom ash. Fly ash is comparatively safe, being comprised of non-toxic light particulates with insignificant heavy element composition. Strictly speaking, it's the least-toxic constituent of concrete and the least-toxic pozzolan available; limestones used in cement manufacture can and do have higher concentrations of heavy elements, radioisotopes, and other toxic chemicals.
Bottom ash is concentrated death. So naturally, we use fly ash for road construction and bottom ash for building construction. Hence the whole "radon in schools" thing: public service buildings made as cheaply as possible, using concrete foundations and cinder blocks manufactured with CBA.
I know what you're referring to, but its not quite so clear cut. In NA Fly Ash was very nearly labelled a hazardous material due to the combined factors of a very serious release in the States and the presence of water-soluble known hazardous materials like mercury. At the time I was working in a concrete design and testing facility as a researcher and it was the talk of the town - labeling that stuff as hazardous would have changed our entire industry. The EPA ended up backing down on that because the disaster was not necessarily related to the components of Fly Ash and the water soluble hazardous materials made up a relatively small portion of the overall material in most NA Fly Ash. We are finding though that there is a much larger portion of acid-soluble mercury which is concerning in areas where there is a carbonation risk, and there is talk of higher mercury contents being found in limited Fly Ash samples. China in particular seems to be reporting rising mercury content in their Fly Ash, but its hard to say if thats a source or a handling issue, and this material is generally not shipped globally so that's not as much of a local concern.
The fine-ness of the material is still an issue though, and has been under scrutiny and probably will continue to be so. Many of the concrete plants I work with have over the years instituted higher standards for PPE requirements for mixing concrete due to the fly ash apparently being able to slip through a lot of standard face masks. Cement may be more immediately toxic to breathe, although I question that in my neck of the woods and I'm gonna ask around a bit, but its also much less fine and way easier to filter out of the air.
The general point being, we really thought Fly Ash was perfectly safe in my industry and we've been getting steadily worse and worse news about how accurate that assumption might have been. I'm kind of afraid I'm gonna start hearing about people getting the Grey Lung or some shit in ten years and then I have to see a doctor every two months until I die. God knows people who do rehabilitative construction work on concrete have been finding some nasty shit at times - turns out some psychos mixed asbestos into concrete at times, and guess what happens when you use a grinder on that.
I'm not super worried about what name of which ash you're using, the basic idea is right. Except, I think, for calling it a replacement for cement. Fly ash (or gypsum in general) is not performing the role of cement, it is an additive that changes the way, particularly the rate, that the chemicals react. Without something like this, it isn't just "well, now we gotta use more cement", it's "well, now the concrete sets too fast and releases too much heat that we have to rewrite the logistics of how we mix and pour entirely and still end up with a lower quality structure."
We're gonna use some sort of gypsum, the only question is which. Do you want more mines and quarries to rip similarly radioactive chemicals out of the ground, or would you prefer recycling industrial byproducts we generate anyway?
You've got a few different ideas going on here, and some of it is headed in the right direction but I think you got your wires crossed at some stage. To start, it is absolutely a replacement for cement - in fact in the standard specifications for bridge construction used where I live we directly refer to Fly Ash as a supplementary cementitious material, because it both adjusts the properties of the overall mix and hydrates with a byproduct of cement hydration to add to the strength of the overall matrix. Its also completely capable of hydrating on its own without cement - I had to deal with a power plant that had been dumping fly ash on the ground instead of storing it properly and the stuff at the bottom hydrated with rain runoff over time. We had to bring in a coring machine to cut through it and the sample we got clocked in at over 80 MPa, which is nearly double what is referred to as "high performance" concrete.
Now, when you said it is used because concrete hydrating too fast can release too much heat you are correct that it can be used in this way, but that isn't always, or really even usually, the end goal of including Fly Ash in a mix. Excessive heat of hydration is an issue that arises in mass concrete placements, and won't drive most of the uses for concrete you come across day to day - the specifications I work with require a least dimension of 1m/3ft before a placement even counts as mass concrete and that's not happening in the vast majority of concrete work. In fact its primary use in high performance bridge concrete has to do with improving the workability of the overall mix rather than anything to do with the heat of hydration and target strength. Thanks to superplasticiser we can hit damn near any strength with not much more than cement powder and sand if we really want to.
While you can mix concrete with no Fly Ash the fact is that Fly Ash is a cheap as hell substitute with notable benefits and cement costs a lot and has only been increasing in cost over time. Everybody uses it in everything, because at 25% replacement that's a lot of saved money.
I'm not really sure why your getting into the radioactivity of gypsum though - the thrust of my original gist was that in the past structural engineers and researchers have believed things to be safe and chosen to dump it in concrete without really checking that, considering the material to be fully mitigated once cast. Now we are finding that this isn't necessarily the case, and working with the additives before casting and the concrete after casting is starting to be troubling as we look into the long term effects of having it present in almost all concrete, which is a tremendously widely used material. Thusly, I think the person suggesting we mitigate fertilizer byproducts by casting it into usable concrete has a good heart, but we still need people to use their heads for a while first before we jump in with both feet.