I'm not sure about refractory cement. I think the clay content and type is what allows it to be used in a fireplace, or high temperature variable space.
Heavy metals, if in the flyash would be bound up in the cured mix, wouldn't they? Isn't the risk with heavy metals getting them into your body?
I recall a recent patent for a very porous non-alkali cement filter that allowed 3 strains of bacteria to live in the cement after cure. It purified contaminated water making a brackish pond clear in a short time. The cement was not Portland but a geopolymer cement (not affordable for me).
Gary
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