1. A method for the production of a lastingly workable building material from contaminated mineral aquatic sediment- or soil material (3), where the material is dewatered to a water content not exceeding 130% of its dry weight and homogeneously comminuted, and the resulting comminuted material (3) is homogeneously mixed (10) with, in terms of its dry weight, a) 5 - 10% powdered clay, b) 0.1 - 0.5% organophilic bentonite or 1 - 5% urea and c) 2 - 5% hydraulic binder, characterised in that the hydraulic binder comprises 5 - 15 wt % and in particular 10 wt % Portland cement clinker, 60 - 80 wt % and in particular 70 wt % ground granulated blast furnace slag and 10 - 30 wt % and in particular 20 wt % fine ground high-calcium lime, and that the proportional admixture of urea constitutes 1 - 5% of the dry weight of the contaminated soil material.

2. A method in accordance with Claim 1, characterised in that, where soil material is involved, the water content of the material is reduced to 5 - 40 wt % of its dry weight.

3. A method in accordance with Claim 1, characterised in that the material is mixed with 7.5 - 8.5% and in particular 8% powdered clay, 0.15 - 0.25% and in particular 0.2% organophilic bentonite, 1 - 3% and in particular 2% urea and 3 - 5% and in particular 4% hydraulic binder.

4. A method in accordance with one of the preceding Claims, characterised in that aquatic sediments (3) are worked with a proportion of sand not exceeding 50 wt %.

5. A method in accordance with one of the preceding Claims, characterised in that the material (3) is aerated and homogenised both before and after mixing.

6. A method in accordance with one of the preceding Claims, characterised in that the powdered clay consists of 10 - 75% kaolinite and 5 - 25% quartzite.

7. A method in accordance with one of the preceding Claims, characterised in that the powdered clay has a grinding fineness of up to approximately 1.5 mm.

8. A method in accordance with one of the preceding Claims, characterised by the use of powdered clay in which not less than approximately 70% of the grain content has a particle size up to 0.002 mm.

9. A method in accordance with one of the preceding Claims, characterised in that the resulting mixture is held in temporary storage (24) for at least one full day prior to laying or further working as a building material.

10. A method in accordance with one of the preceding Claims, characterised in that the urea has an amide nitrogen content of 40 - 50% and in particular 46%.

11. A method in accordance with one of the preceding Claims, characterised in that the urea is prilled.

12. A method in accordance with one of the preceding Claims, characterised in that the urea has a biuret content not exceeding 1.2%.