Building slab based on gypsum comprises a cover made of glass threads on both main surfaces, where: the glass threads comprise a coating on at least one of the two main surfaces and the coating contains optimized photocatalytically active titanium dioxide. An independent claim is included for preparing the building slab comprising providing the glass threads with the coating, loading a coated/uncoated glass threads with a hardenable calcium sulfate mixture, subsequently covering with a coated/uncoated glass threads and cutting and drying, after initiation of the hardening is carried out, where at least one of the main surface of the building slab comprises a coated glass thread.
