Magnesium oxide wallboard

Magnesium oxide, more commonly called magnesia, is a mineral that when used as part of a cement mixture and cast into thin cement panels under proper curing procedures and practices can be used in residential and commercial building construction. Some versions are suitable for general building uses and for applications that require fire resistance, mold and mildew control, as well as sound-control applications.

Source: Wikipedia — Magnesium oxide wallboard (CC BY-SA 4.0)