My understanding is that more than direct exposure to radioactive materials in fly ash, the real risk is Radon. As Thorium and Uranium decay they will produce R

My understanding is that more than direct exposure to radioactive materials in fly ash, the real risk is Radon. As Thorium and Uranium decay they will produce Radon, which can accumulate especially in low-lying areas with poor ventilation (e.g. basements, which tend to rely on concrete as a key building material). Since Radon can be inhaled, it has the potential to cause far more harm than anything "baked into" the concrete itself.