@pastanaga - One of the problems with evaluating the quality and risks of using fly ash in concrete is that the quality of the ash varies depending on where they get it from. Not just with how they made it, but also from what kind of coal they made it from.
So, it's difficult to know whether observations that it is less durable and possibly dangerous to health is true of all kinds of fly ash for concrete, or only the bad quality kinds. More research must be done to evaluate this.
Although, frankly, I'd prefer it if we just didn't have to burn so much coal in the first place.