Add Slag more uses like...

Thje 1st of all, the Slag at high-end will be a problem because there is only 1 way to exchange it to a crushed one... or just dumping it on the island, something that is quite ugly.

There can be many ways to delete/exchange it, as for example, waste water/acid/brine/etc or diesel/h2/heavy, medium and light oil/etc. So in the same way there can be a building that use energy (or whatever) to "remove" or exchange it for something:

One way can be is that you can use it as fertilizer like in the real life (added link):

"The effective utilization of slag fertilizer in agriculture to neutralize soil acidity, improve crop productivity, mitigate greenhouse gas emissions, and stabilize heavy metals in contaminated soils turns it into a high value added product in sustainable agriculture."

Link: https://www.frontiersin.org/articles/10.3389/fmicb.2019.01320/full