Wet Tailings stay wet and should be dealt with like other tailings. In the future they'll need to be contained in leak-proof pits, like with clay, but that is not the case yet. Slag should only pollute minorly.
The minor pollution is not on asphalt using treated and recycled slag, but on pure slag itself (and thats what I understood AnLambdy to want to use) that still contains minorly damaging substances. I don't know how stuff in Houston works, I'm from far over the pond - but here slag is treated before such usage. Asphalt is not polluting in the game.