Gunite (also known as the “dry gun process,” the term gunite has become the unofficial standard in the industry, but was once a company trademark) is the mixture of one part cement to four parts sand. (The sand can also be replaced by refractory aggregates. This process makes the already heat tolerant surface even more so.) Gunite is loaded into specially designed machines that spay the mixture onto a variety of surfaces including brick and concrete. The mixture is dry until compressed air sends it through a rubber hose and nozzle where water is added. Adding the water at the end of the process allows the mixture to be sprayed with the highest velocity and least amount of waste.
There is a similar product known as shotcrete. The main difference between gunite and shotcrete is that the latter is wet when it is loaded into the dispensing machine. Conventional concrete, as well as shotcrete, tends to have excess water trapped inside. This excess water eventually evaporates leaving air pockets that weaken the concrete. The gunite process creates a dense surface that has minimal hydration and is therefore thought by many to be stronger.
Gunite is used in swimming pools, tunnels, rockscaping, refractory installation et cetera.