Make Cement with Lots of Holes What would be a good way to make cement with lots of hollow areas? I want it to resemble a sponge when cured. I’ve see a similar material used as siding in buildings.
You could try Orbeez / water beads.

Thoroughly water them to make them inflate as much as possible. Then mix them into the moist cement and let the cement set for several days. The moisture retained in the orbeez should slowly evaporate through the pores of the hardened cement, leaving behind a sponge-like structure (and the shriveled, dried out Orbeez of course).

Here's a video of someone mixing Orbeez into transparent epoxy resin (start watching at 11:00 minutes) and a closer look at the result in a second video (trigger warning for Trypophobia).