Concrete table DIY – fill air holes (nests) with black mass?

Hello everyone

I plan to make a concrete table myself.

I already have a concrete table, which I bought from a furniture store. The air holes in the concrete (called nests) were filled with a black compound. However, the compound seems to be a somewhat rubbery material.

I'd like to do the same with my own concrete table, but I don't know what material was used. (See photos)

Maybe someone has an idea?

I'm also happy to hear any tips and tricks you can use when making a DIY concrete coffee table – especially regarding aftercare 🙂

Many thanks and best regards!