Can I apply microcement over anhydrite?

Anhydrite is suitable if it is completely dry and free from surface laitance. In practice, this means sanding, thorough vacuuming and ensuring flatness as well as correct priming. It is therefore not only about the substrate material but mainly about its condition. Before application, check for strength, flatness, grease, dust, cracks and any movement. If the surface is risky, it is better to reinforce it or first create a test area. And ensure it is level.