Abstract: A process for spraying concrete or mortar, by a dry route or by a wet route, in order to produce on a surface a layer of concrete or of mortar. Just before or at the time of spraying, an aqueous suspension (A) of silica and of an aluminum compound chosen from aluminum sulphate, basic aluminum sulphates, alums and mixtures thereof is added to the concrete or mortar. The aqueous suspension (A) has a pH lower than 4, a solids content of between 10 and 50% by weight, and, after a period at rest of 48 hours, is in the form of a gel which is reversible under low shear. The aqueous suspension (A) is effective for increasing the thickness of the sprayed layer and reducing rebound of the concrete or mortar sprayed onto the surface.