Calcium sulfate an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth, an
Ant chalk Ant chalk Ant chalk, also known as Chinese chalk or 'Miraculous Insecticide Chalk', is an insecticide in the form of normal looking chalk. It contains the pesticides deltamethrin and cypermethrin. While the active ingredients are legal in the United States, the chalk is not legal there. Labeling often falsely claims the chalk is "harmless to human beings and animals" and "safe to use." Chalks have been found to cause serious health problems and deaths. Packaging, often containing lead-based inks, generally does not list ingredients. Despite its illegal status, "Chinese Chalk" is illegally imported from China and sold in corner stores