What is Ytong nest made of you ask? Hmmm... let me tell you a secret: it's made out of Ytong!!!!!!
You seem to be puzzled by this material as everywhere on this forum you are asking about it. So let me give you some answers. Ytong is a building material. It is used for making walls and it has great insulating properties.
Ytong nest are usually built from a single block of Ytong. It's quite britle and it soaks up water too. Nevertheless water does not damage this material so it is perfect for antkeeping. And I've never seen mould grow on it although it was constatly wet to suit ants needs for moist. Just go to your local building material store and ask someone there about it. They will show you lots of different sizes of block to choose from. So make up your mind what kind of nest you want to build and buy something that suits your need. It's not that expensive. I'd say avarage price of one block (depending on the size of it) should be around 1 €.
I hope I answered your questions.
good luck