no matter

there are two more ways using slack
first you can convert them in violett blocks (don´t remember the name here on work) and convert these in iron ore, oil or everything else, which is possible
second in agriculture there are an Alien (the white one) eating slack and two other things and ...

With Frank & Pepe Smelting you get so much slack, you can lay down it in your whole base. and with the rest, you can build so much stonebricks, you never need to dig stones any more.