Calcium carbonate or calcium sulfate- also called chalk, marble dust, calcite and gypsum depending on its source, use, particle size (which effects both the texture (gritty or silky smooth) and the absorbency) and the purity level. (Talc is not included in this group and is too soft for use as a painting ground unless it is used in a blend with one of the above minerals).
It is a white powder ground from fossilized shell deposits (limestone), marble or made synthetically. It is made synthetically by precipitating fine particles of calcium carbonate making the most uniform, smoothest variety that is used for the preparation of panels for egg tempera for example.
Its use for artists is primarily as an ingredient in genuine gesso and acrylic primer.
It is also the opacifier that makes gouache an opaque form of watercolour paint. It is used in some soft and hard pastels. It is used as a filler for some paints. It is used to enhance the brightness of some paint colours, viridian for example. It is the inert base onto which lake pigments are precipitated. Some forms are used in modelling paste in acrylics and mixing with oil makes oil painting putty.
It is not strong enough to be used as a white pigment.
It is added to paper pulp as a buffer, to counteract reactions with pollution that would cause acidity in paper.
It is also used as a polishing powder and in ceramics as a flux.