"SPRAYING MACHINE FOR THE BUILDING INDUSTRY" The present invention relates to machines for transporting and spraying concretes, refractories and building materials in general, and particularly to spraying machines with pneumatic transport of materials. Various embodiments of spraying machines of this type are known, in particular for the building industry. These known machines are generally provided with a rotor wherein the material to be sprayed is conveyed. Inside the rotor a flow of compressed air carries the material toward a spray nozzle through a piping of variable length according to the needs. In these known machines, however, obstruction of the pipings may occur. As a matter of fact, if the flow of the compressed air decreases or the diameter of the pipings is not adequate for the amount and/or for the particle size of the mixtures to be subjected to the process, the pipings are obstructed causing the stop of the whole process. This brings about great inconveniences, especially when there are very long pipings, because the restoration of the operative conditions requires interventions which imply complex and expensive -operations, in particular from the labour point of view. The object of the present invention is therefore to provide a spraying machine for the building industry substantially free from said drawbacks. This object is achieved with a spraying machine whose main features are specified in the first claim , while the other features are specified in the remaining claims. The spraying machine for the building industry according to the present invention allows to prevent the pipes from being obstructed and thus the stop of the machine itself. It is furthermore characterized by a simple and completely automatic functioning, besides the low costs of investment and working. Further advantages and features of the machine according to the present invention will be clear to those skilled in the art from the following detailed and non limiting description of one embodiment thereof with reference to the attached drawings wherein: figure 1 shows a sectional view of a fraction of the spraying machine according to the present invention; and figure 2 shows a perspective view of the whole spraying machine according to the present invention. With reference to the drawingss there is shown that the spraying machine comprises a loading hopper 1 wherein the material to be processed is fed. This hopper 1, frustoconically shaped, is connected to a rotor 2, substantially cylindrically shaped, wherein several chambers are obtained (one of which is indicated in the drawing with the reference number 2'), wherein the inlet material is allowed to fall by gravity. The chambers of said rotor 2 are connected to an inlet 3 for compressed air. This latter supplies the power required to empty the single chambers of the rotor, continuously rotating around its axis, and to feed the material to the outlet 4 and thus to a pipe 5 (in case, but not necessarily, flexible) until a nozzle 6, that is the point to which it must be conveyed. Said nozzle 6, suitable for spaying the material outside, is a spray nozzle of known type and therefore it does not need to be further described. Rotor 2 is driven to rotate by a preferably electric motor, not shown in the drawings. The speed of rotation of the electric motor and thus of the rotor 2 may be controlled by an inverter (frequency converter) which thus also controls the amount of material introduced in the piping in a time unit. The spraying machine is connected to a compressor 7 fitted for introducing therein compressed air to supply the power necessary to transport the material. According to the present invention, the spraying machine is provided with a thrust meter 8 fitted to give an electric pulse to a PLC (programmable logic controller) to which said inverter (frequency converter) is connected, so as to control the speed of rotation of rotor 2 and the flow of the fed material. In regular working conditions, the material to be sprayed is conveyed into the chambers of rotor 2 and thus carried by air, which enters in the machine through the inlet 3, towards the outlet 4. The continuous rotation of rotor 2 around its axis 9 ensures the continuity of the flow of the outlet material. This material is then conveyed in the nozzle 6 through pipe 5. When the pipings tend to become obstructed, a pressure increase of the compressed air takes place. The thrust meter 8 detects this pressure increase and transmits a signal to the PLC which, suitably programmed, manages the inverter to decrease the speed of rotation of the electric motor and thus of the rotor, highly decreasing therefore the amount of material introduced in the piping in a time unit. This allows pipe 5 to reduce its , content up to become almost empty; this condition causes the pressure to return to the values set before the block took place. In the meantime, the thrust meter 8 transmits another signal to the PLC which, through the inverter, restores the foreset rotational speed. This occurs in a completely automatic way without requiring the machine operator to intervene. The spraying machine according to the present invention may be used both with dry mixtures, as for example dry concrete, in powder or granular form, and with previously hydrated mixtures, as for example humid concrete. In the case in which the spraying machine is fed with dry material which needs to be diluted with water or other liquids, a liquid stream is introduced immediately before the discharge of the material. This stream is mixed to the material so as to obtain the product ready to be used. In the preferred embodiment, the spraying machine is provided with wheels 10 or analogous means for the transport thereof. It is obvious that the embodiment here described and illustrated is only a non limiting example of the invention. Changes and/or additions may be made by those skilled in the art to the embodiment of the invention here described and illustrated without departing from the scope of the invention itself.