Material to be used as a concrete additive

1. A cement admixture for use in a cementitious composition, wherein the admixture comprises:

a cementitious binder;

microfibrillar cellulose; and/or a derivative thereof; and/or labile chemically modified cellulose pulp or labile chemically modified cellulose raw material which forms microfibrillar cellulose during the use of the admixture;

wherein the amount of the microfibrillar cellulose is between 0.002% and 0.2% by weight of the cementitious binder in the cementitious composition; and

optionally water.

2. The admixture according to claim 1 , wherein the admixture is a mixture such as a solid mixture or a dispersion of microfibrillar cellulose and/or a derivative thereof.

3. The admixture according to claim 1 , wherein the admixture further comprises at least one plasticizer and/or dispersing agent.

4. The admixture according to claim 1 , wherein the admixture comprises microfibrillar cellulose, and wherein the diameter of the cellulose microfibril or microfibril bundles is less than 1 μm.

5. The admixture according to claim 1 , wherein the admixture comprises a chemically or physically modified derivative of a cellulose microfibril or microfibril bundles.

6. The admixture according to claim 1 , wherein the microfibrillar cellulose is obtained from raw material comprising plant material or derived from bacterial fermentation processes.

7. The admixture according to claim 1 , wherein the labile cellulose pulp or labile chemically modified cellulose raw material is obtained by N-oxyl mediated oxidation.

8. A method of manufacturing a cement admixture for use in a cementitious composition according to claim 1 comprising

providing microfibrillar cellulose and/or a derivative thereof,

mixing together said microfibrillar cellulose and/or a derivative thereof, and optionally water;

adding at least one plasticizer and/or dispersing agent prior to, during or after providing microfibrillar cellulose to obtain said admixture;

wherein the amount of the microfibrillar cellulose is between 0.002% and 0.2% by weight of a cementitious binder in the cementitious composition.

9. A method for modifying rheology or controlling segregation of a cementitious composition comprising providing an admixture according to claim 1 .

10. A cementitious composition comprising an admixture as defined in claim 1 .

11. The cementitious composition according to claim 10 , wherein the water to cement ratio is 1.0 or less.

12. The cementitious composition according to claim 10 , wherein the amount of microfibrillar cellulose is 2% or less by weight of water and wherein the water to cement ratio is 1.0 or more.

13. The cementitious composition according to claim 10 , wherein the composition further comprises at least one plasticizer and/or dispersing agent.

14. The cementitious composition according to claim 10 , wherein the cementitious composition is concrete.

15. A method of manufacturing a cementitious composition according to claim 10 , comprising

mixing together a cementitious binder, aggregate material, water and a cement admixture, wherein the admixture comprises microfibrillar cellulose; and/or a derivative thereof; and/or labile chemically modified cellulose pulp or labile chemically modified cellulose raw material which forms microfibrillar cellulose during the use of the admixture; and optionally water; and

optionally adding at least one plasticizer and/or dispersing agent.

16. The method of claim 15 , wherein the microfibrillar cellulose is obtained from labile chemically modified cellulose pulp or labile chemically modified cellulose raw material during the manufacture of a cementitious composition.

17. The method according to claim 15 wherein the water to cement ratio is 1.0 or less.

18. The method according to claim 15 , wherein the amount of microfibrillar cellulose in the admixture is 2% or less by weight of water and wherein the water to cement ratio is 1.0 or more.

19. A construction element made of the cementitious composition according to claim 10 .

20. The admixture according to claim 1 , wherein the admixture comprises microfibrillar cellulose, and wherein the diameter of the cellulose microfibril or microfibril bundles is less than 200 nm.

21. The admixture according to claim 1 , wherein the admixture comprises microfibrillar cellulose, and wherein the diameter of the cellulose microfibril or microfibril bundles is less than 100 nm.

22. A cement admixture for use in a cementitious composition, wherein the admixture comprises:

a cementitious binder;

microfibrillar cellulose; and/or a derivative thereof; and/or labile chemically modified cellulose pulp or labile chemically modified cellulose raw material which forms microfibrillar cellulose during the use of the admixture,

wherein a cationic and/or anionic substance is adsorbed on a surface of the microfibrillar cellulose, derivative thereof, and/or labile chemically modified cellulose pulp or cellulose raw material which forms microfibrillar cellulose during the use of the admixture; and

wherein the amount of the microfibrillar cellulose is between 0.002% and 0.2% by weight of the cementitious binder in the cementitious composition;

optionally water.

23. The cementitious composition according to claim 10 , wherein the amount of microfibrillar cellulose is 0.2% or less by weight of water and wherein the water to cement ratio is 1.0 or more.

24. The method according to claim 15 , wherein the at least one plasticizer and/or dispersing agent is added prior to or during the preparation of a cementitious composition.

25. The method according to claim 15 , wherein the amount of microfibrillar cellulose in the admixture is 0.2% or less by weight of water, and wherein the water to cement ratio is 1.0 or more.

26. The cementitious composition according to claim 10 , wherein the cementitious composition is self compacting concrete.

27. The cementitious composition according to claim 10 , wherein the cementitious composition is selected from the group consisting of mortar, grout and injection grout.