[NicknamedBob]

"The arsenic and mercury and stuff are still a problem, though. Put it on the roads, and it runs off into the water supply."

Hence, concrete. It pretty effectively wraps it up.

The advantage for the concrete is that the microstructure of the ash provides a better high-strength bond. Volcanic ash works in a similar fashion.

[Tax-chick]

Very interesting. I wonder how the costs compare, coal ash vs. volcanic ash for the same industrial use.