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
Chalk stream Chalk stream Chalk streams are streams that flow through chalk hills towards the sea. They are typically wide and shallow, and due to the filtering effect of the chalk their waters are alkaline and clear. There are at least 210 chalk streams in the world, and 160 of those are in England. Chalk streams are popular with fly fishermen who fish for trout on these rivers. Chalk is a highly porous and permeable rock and rain falling onto chalk topography percolates directly into the ground, where the chalk acts as an aquifer. The groundwater flows through the chalk bedrock, re-emerging