Sealing composition and its use

1. A method for sealing rock or soil comprising inserting a sealing composition obtained by mixing a silica sol and at least one gelling agent into said rock or soil, wherein the silica sol has a S-value higher than about 72% and a silica content in the range from about 20 to about 70 wt %.

2. A method according to claim 1 , wherein the S-value is higher than about 75%.

3. A method according to claim 1 , wherein the silica sol is an aluminium modified silica sol.

4. A sealing composition according to claim 3 , wherein the composition has a weight ratio of silica particles to gelling agent from about 400:1 to about 10:1.

5. A method according to claim 1 , wherein the composition has a silica content from about 35 to about 50 wt %.

6. A method according to claim 1 , wherein the gelling agent is selected from the group consisting of potassium chloride, sodium chloride, calcium chloride, magnesium chloride, magnesium nitrate, magnesium sulfate and mixtures thereof.

7. A method according to claim 1 , wherein the composition has a relative standard deviation of the particle size distribution lower than about 15% by numbers.

8. A method according to claim 1 , wherein the composition has a gelling agent content from about 0.2 to about 5 wt %.

9. A method according to claim 1 , wherein the silica sol comprises silica particles having a specific surface area from about 40 to about 230 m 2 /g.

10. A method according to claim 1 , wherein the composition has a weight ratio of silica particles to gelling agent from about 400:1 to about 10:1.

11. A method according to claim 1 , wherein the composition contains silica particles having a size within the range of from about to about 12-50 nm.

12. A method according to claim 1 for sealing rock.

13. A method of sealing a leaking crack or cavity in rock or soil comprising

inserting into the crack or cavity at least one gelling agent; and

separately inserting into the crack or cavity a silica sol having an S-value higher than about 72% and a silica content in the range from about 20 to about 70 wt %; wherein the gelling agent and the silica sol mix in situ to form a seal in the crack or cavity.

14. A method of sealing a leaking crack or cavity in soil comprising inserting into the crack or cavity a silica sol having an S-value higher than about 72% and a silica content in the range from about 20 to about 70 wt %, wherein at least one gelling agent is present in the soil in a gelling effective amount and the gelling agent mixes with the silica sol to form a seal in the crack or cavity.

15. A method for sealing rock or soil comprising inserting a sealing composition at an injection pressure from about 2 to about 25 bar obtained by mixing a silica sol and at least one gelling agent into the rock or soil, wherein the silica sol has a S-value higher than about 72% and a silica content in the range from about 20 wt % to about 70 wt %.