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You should be able to specify concrete with NO fly ash.
The railways here do not allow fly ash.
Fly ash is just a cement replacement. It does make the concrete more workable however, so it is not without its benefits. Or so they say.
Now silica fume concrete is a real bitch to work with. It's so fine that they deliver it to the redimix places in a slurry form. If I understand it correctly, it is used in the production of high performance concrete and helps reduce chloride penetration into the concrete. The problem is it is very tricky to cure and it cracks a lot.
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