Can I apply microcement on Ytong?

Ytong is a very absorbent and softer substrate, so it should not be considered a finished surface for microcement. It usually needs to be levelled and reinforced with a layer of adhesive and mesh. It is not just about the material of the substrate, but mainly its condition. Before application, check the strength, flatness, grease, dust, cracks and possible movement. If the surface is risky, it is better to reinforce it or first create a test patch. Typically, it is treated with mesh and flexible adhesive.