It's a mixture of sand and cement, dry mixed onsite and pushed through a three inch hose using air generated by large compressors. Water is added inside the nozzle tip with a special water-ring, the amount controlled by the nozzleman. The material exits the nozzle as a wet mixture under pressure which adheres to vertical walls.
All the inconsistencies of gunite are avoided by using a computer controlled mix design from a concrete batch plant. Where each truckload of shotcrete(concrete) is manufactured using regulated and exacting standards. The wet material (concrete) is pumped through a two inch hose to a nozzle with an air-ring, allowing it to adhere to vertical walls.