lime, gypsum, and plaster

Some of the oldest chemistry humans ever mastered is the chemistry of calcium compounds you can dig out of the ground. Limestone, chalk, and marble are all calcium carbonate; gypsum is calcium sulfate. By heating and rehydrating these cheap rocks, people have made mortar, plaster, cement, and casts for thousands of years — long before anyone knew what an atom was. This is the everyday face of Group 2 chemistry.

The lime cycle is a chain of simple reactions. Roasting limestone, CaCO3, in a kiln drives off carbon dioxide and gives quicklime, calcium oxide CaO, a caustic white solid. Adding water to quicklime is the violently exothermic slaking step that gives slaked lime, calcium hydroxide Ca(OH)2 (a base; its solution is limewater). Slaked lime then slowly reabsorbs CO2 from the air and sets back into hard calcium carbonate — which is exactly how traditional lime mortar hardens, closing the cycle. Gypsum tells a parallel story with water rather than CO2: heating gypsum, CaSO4 dot 2H2O, drives off most of its water to give plaster of Paris, CaSO4 dot half-H2O; mix that powder with water and it rapidly re-crystallizes back into interlocking gypsum needles that set into a rigid solid — the basis of wall plaster and the casts that hold broken bones.

These materials matter because they are the bulk chemistry of the built world: lime and gypsum underpin cement, plaster, mortar, blackboard chalk (now often gypsum), agricultural soil treatment, and surgical casts. The honest note is that the names are easy to muddle — quicklime (CaO), slaked lime (Ca(OH)2), limestone (CaCO3), and limewater (dissolved Ca(OH)2) are four different things, and plaster of Paris is gypsum that has lost, then regains, its water of crystallization, not a calcium carbonate at all.