We Claim:
1. A method for preparing a slag having hydraulically binding properties on the basis of a mixture comprising limestone, fly ash and any additives comprising the following steps: mixing the starting materials, supplying the mixture at the upper side of a cylindrical, vertical heat exchanger, supplying the fired product obtained at the bottom side of said cylindrical vertical heat exchanger to a melting furnace so as to obtain a melted product, quenching the melted product, and pulverizing and mixing it with fly ash and said additives so as to obtain a binder, characterized in that the starting materials are mixed in the desired weight ratio whereafter the mixture thus obtained is granulated whilst adding a solid fuel, whereafter the granules comprising said solid fuel and starting materials are supplied to the vertical heat exchanger, wherein the said mixture comprises 33-52 wt.% calcium oxide, 9-25 wt.% aluminum oxide and 23-45 wt.% silicon oxide and the carbon content is between 0-15 wt.%.
2. A method as claimed in claim 1, wherein the hot gases from the hot gases from the melting furnace are supplied to the bottom side of said cylindrical vertical heat exchanger, which hot gases move through the heat exchanger in counterflow with the package of granules moving in downward direction.
3. A method as claimed in any one of the preceding claims, wherein the temperature in the cylindrical vertical heat exchanger is controlled so that sintering together of the granules supplied is prevented and complete decarbonation is achieved.
4. A method as claimed in any one or more of the preceding claims, wherein the temperature of the melting furnace is higher than the temperature of the cylindrical vertical heat exchanger.
5. A method as claimed in claim 4, wherein the temperature at the bottom side of the cylindrical vertical heat exchanger is 1100-13 00°C and the temperature of the melting furnace is 1400-1550°C.
6. A method as claimed in any one or more of the preceding claims, wherein a shaft furnace is used as said cylindrical vertical heat exchanger.

7. A method as claimed in any one or more of the preceding claims, wherein a melting
furnace for the preparation of rock wool or glass wool is used as said melting furnace.
8. A method as claimed in any one or more of the preceding claims, wherein pulverized
starting materials are used.
9. A method as claimed in any one or more of the preceding claims, wherein the additive
as used in mixing with slag comprises an activator.
10. A method as claimed in any one of the preceding claims, wherein the additive as used
in mixing with starting materials comprises a correction agent.
11. A method as claimed in one of the preceding claims, wherein the additive as used in
mixing with slag is dried natural pozzolan, which has been pulverized to the
dimension of cement.
12. A method as claimed in any one or more of the preceding claims, wherein the granules vary only to a slight degree as regards their grain diameter, the diameter in particular being 1-2 cm.