Best digging medium?

[Fulkol]

So currently I'm carving an Ytong nest of which I will be posting a tutorial video or so, and I want to have most of the Ytong carved out and fill the space with some sort of digging medium.
What do you think the best would be? I will be housing medium sized black ants so ideally the color of the sand would be a light color. Did look in a pet store for it but the regular aquarium sand is quartz which means it won't really get sticky when wet so the ants would have hard time building from it, I've seen this happen this year in my outworld.
However they had something like desert sand in red and black color no yellow/white tho, so I'll be looking for it in an another store tomorrow, but looking for you input until then.

[AntsSA]

I think that play sand that they use in sand pits would work, just make sure to sterilise it before by boiling it in water.