Make Cement with Lots of Holes
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).