I researched Waste Rock Liquefaction but the drones don't seem to be doing anything with the waste rock piles. Do I have to command them to retrieve the concrete, or does it work in some other way? Thanks!
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Place a concrete depot beside the waste rock piles. Idle drones will head to the rock piles and convert them into concrete.
Drones require a free concrete depot and a pile containing at least 5-10 waste rocks in order to create concrete. This means having a dumping site is usually best since it gathers waste rock in a spot where it's guaranteed to have enough to convert.
I had about 25 hexes of waste rock scattered about the Colony site. Finished research on Waste Rock Liquefaction and the Drones had cleared the lot within 1 Sol... 
Waste Rock Liquefaction is to Surviving Mars what a massive number of new recycling centers in a city with a few landfills is to City Skylines; the moment it's done, all the useless trash vanishes.
The drones convert 5 rock into a single concrete, with the artificial muscles, the drones want to carry two concrete, so they need 10 rocks, so in that case you need a single rock dumping site to gather the rocks. The dumping site is helpful anyway to gather the rocks from multiple place into a single pile, also, if you had multiple dumping site, you should remove all but one, otherwise the drones will leave a few rocks in each dumping site.