If you embed broken up styofoam peanuts all over the fresh surface, once it sets solid you can disolve them away with almost any solvent leaving the voids they occupied. Saw a "meteor" movie prop made that way by an old friend. Only difference was he used a lightweight plaster product instead of concrete. Just make sure you DO NOT get the water soluble, corn-starch type peanuts as they will disolve as soon as the get wet.

Make sure to post some pix. Sounds like a fun project.

All My Best ---Tango

Great idea! I've also had luck casting with a trash bag between the hypertufa and the mould. This gives me the idea of using the bag between the styrofoam and the plaster.

Just a quick question for Tango..what kind of solvent would you use to dissolve the styro? Turps, varsol? I've made small rocks for garden edging, but, would like to try making some big, heavily pitted 'boulders' for my rock garden.

Since there are a variety plastic formulations out there, you may have to experiment a little, but lacquer thinner will attack most of them. And you might try some of the "oderless" turpentine first just to see if you can dissolve it without having to deal with the smell of the more powerful solvents. Another good option is to try some aerosol solvents like automotive brake cleaner or electrical contact cleaners (most auto parts stores). That's what I used a few years ago. They are easy to use and get into nooks & crannies, but...the EPA has made the manufacturers change their formulas in the last couple of years, so I'm not sure if they still work the way they used to. Let us know how it turns out & what works best for you.
All my best --- Tango

You can pour crete with sand in the spots you want pitted, when crete is hardened you can wash the sand away.
bonnie

I can't say I've actually used this on styrofoam peanuts but it works on other plastics/adhesives: acetone nail polish remover! I use it to remove the excess goopey adhesive left from an old vehicle inspection sticker and other adhesive residue. I learned it's potency back when I was 13 and spilled some of it on my mother's new dining room table--it takes varnish off like a charm!! I've used it to take off some of the Great Stuff foam residue from my hands also. I did a Google search and I'm seeing that you can use it to dissolve styrofoam peanuts--just saying I haven't done that particular one.

Be sure though that you don't buy the non-acetone remover by accident.

What about ice chunks in the mix? You'd have to work fast, I guess, but getting them out would be painless -- just wait a while! I wonder if it would affect how the cement cures. Hmmm.

petrol/gasoline is a sure fire way to dissolve polystyrene. And it's cheap and easily available. And you can set the whole thing on fire to make red hot lava!

I would think it would also be a way to have the whole thing explode!!

Not to mention, it'd be expensive fuel for the fire!

Roger on the gasoline. It's the only thing I use anymore to remove the styro. I use an eye dropper and can get all of them without having to soak the whole piece; takes a lot less gas too. Nice thing is after it evaporates there is no residue to deal with. Just to CYA, remember what you are dealing with; outside only and away from open flames.
Packrat

I think you'll find that petrol is cheaper than pretty much anything else you can buy (in some places it is cheaper than drinking water!). I don't think petrol is any more dangerous than acetone or LPG (in aerosol sprays). Acetone has been the cause of many chem lab fires!

Most peanuts you get now a days is made from a corn solvent and is very quickly dissolved with water.... Smells like corn.... HAHA Anyhoo that would be my recommendation to use that type of packing peanuts.