There are 2 types of flyash C & F. I have played with both.
Toxicity? Everything has a toxicity, especially nothing. It is just a measure, like temperature. Water, air, lettuce, aspirin, radon..... everything has a toxicity number.
"Fairly" is a word like "very". It is gut feel and has no quantification. Since flyash is the byproduct of burning coal for energy, the toxicity is not created by using it in a concrete or PC mix. I don't think concrete is more toxic because flyash is added, perhaps less because less Portland is used. The environmental impact of using flyash in the mix is less than using all Portland.
Type F flyash is a bit more expensive, more pozzolonic, and harder to find than Type C. My regular cement product supplier process their own type F. I believe it is Type F from Type C. This apparently removes some of the reputed contaminants in Type C. Type F is a dark gray not the light brown like type C, hence it gives a differently colored finished product. Type C is cheaper than Portland, Type F is about the same price as Portland. I do not know how Type F is processed. It could be that the energy required to process it makes the environmental impact the same as using Portland.
I have heard that road repairs in the cold weather areas use flyash because it lessens the risk of freeze cracking on freshly poured mixes. I notice more plasticity in the mixes I use.
Gary
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