Process for the production of ballast

1. A process for the production of ballast for railway track laying, road construction, dam construction or bank protection, comprising

(1) spreading ballast stones to form ballast and

(2) applying between the spread ballast stones a reaction mixture for the preparation of a polyurethane foam, produced from components by a low-pressure process, wherein the reaction mixture is prepared in a pressure range of from 2 to 20 bar from components present in at least two component streams, and wherein the polyurethane foam has a compression hardness at a 10% compression set of at least 1.0 kPa and a tensile strength of at least 0.1 MPa; and

wherein the polyurethane foam is obtained from

a) one or more isocyanate compounds selected from the group consisting of polyisocyanates with an NCO content of 28 to 50 wt. % and NCO prepolymers with an NCO content of 10 to 48 wt. %, consisting of polyisocyanates with an NCO content of 28 to 50 wt. % and polyetherpolyols with a hydroxyl number of 6 to 112, polyoxyalkylenediols with a hydroxyl number of 113 to 1100 or alkylenediols with a hydroxyl number of 645 to 1850, and mixtures thereof, and

b) a polyol component consisting of one or more polyetherpolyols with a hydroxyl number of 6 to 112 and a functionality of 1.8 to 8, in the presence of

c) 0 to 26 wt. %, based on reaction components b) to g), of one or more chain extender with a hydroxyl or amine number of 245 to 1850 and a functionality of 1.8 to 8,

d) 0.05 to 5 wt. %, based on reaction components b) to g), of one or more blowing agent,

e) 0 to 5 wt. %, based on reaction components b) to g), of one or more catalyst,

f) 0 to 50 wt. %, based on reaction components b) to g), of one or more filler, and

g) 0 to 25 wt. %, based on reaction components b) to g), of one or more auxiliary substance and/or additive, wherein the index of the reaction mixture is from 70 to 130.

2. The process of claim 1 wherein the reaction mixture is applied by a low-pressure mixing head.

3. The process of claim 1 wherein the reaction mixture is prepared from an isocyanate component, a polyol component and a catalyst component in a low-pressure mixer, wherein the isocyanate component, the polyol component and the catalyst component are being fed into the low-pressure mixer in the form of a component stream.

4. The process of claim 1 , wherein the polyurethane foam has a free foam density of 20 to 800 kg/m 3 .

5. The process of claim 4 , wherein the polyurethane foam has a free foam density of 50 to 300 kg/m 3 .