How does Gypsum Concrete work?

When mixed with water how does gypsum act as a binder in creating concrete?

• Do you mean using Plaster of Paris as the binder to make a concrete ? Jan 30 '20 at 17:25

In the absence of gypsum, the reaction of C3A with water is very rapid. The C3A is converted to calcium aluminate hydrate ($$\ce{4CaO.Al2O3.19H2O}$$) which forms interlynking bridges of solid material between the cement particles. These interlinking bridges reduce the mobility of the cement particles and produce a firm structure, which results in setting of cement mixture. The rapid setting is called a flash set and its irreversible.
The presence of gypsum in cement slows the reaction of the C3A. When mixed with water gypsum dissolves and reacts with C3A to form ettringite ($$\ce{3CaO.Al2O3.3CaSO4.32H2O}$$). This ettringite is initially formed as very fine-grained crystals, which form a coating on the surface of the C3A particles.