Is that the final one on the right? Just wasn't sure.
The one on the right is from someone who dd this before. I used it as an example of what I have in mind. If you look up DIY"aquarium, foam rocks and cement" you will get an abundance of examples in various stages of the process.
Ok so all the foam is silicone together with 100% silicon, I've read for (In the USA) DAP makes some and GE has a some.
I let them set over night in a clamp, lightly clamped just enough for a little pressure and not to crush it. Then another day unclamped while chipping and carving caves and roughing up the surfaces.
I've taken two of the three spurs and mixed up some Quickrete cement and acrylic additive called "Concrete Bonding Adhesive and Acrylic fortifier." mixed thin to get in little cracks. I've noticed the cement doesn't seem to cover well on those places that I did not get roughed up. This maybe a problem later we'll see. It's only the first coat so I can still rough those spots up tomorrow without fighting the cement too much.
I used for these two pieces about 1-1/2 cups of cement and enough liquid additive to mix it thin; I had to add more additive a couple times as the cement settled on the bottom while I used the weaker mix on top.
Getting there. After a couple or more in some spots of thinner coats covered all the foam at least I thought I covered it all. Every now and then I found a spot that showed the Blue foam.
Now I've put on a 3 or so heavier coats, but not as heavy as you would for cementing brick or concrete patches. Have it just thin enough to use your paint / utility brush.
The smaller sections are looking just like a granite or shale shelf (repeat shale shelf many times fast).
To this point I have used both my 10lb bags of cement and about 3/4 of a gallon acrylic fortifier.