Admixture for hydraulic binder

1. A method comprising: adding at least one copolymer or a salt thereof acting as a plasticizer and a blocking agent to a calcium sulphate-based hydraulic binder,

said copolymer comprising at least one unit of formula (I)

in which:

R1, R2 and R3 independently represent a hydrogen atom, a linear or branched C 1 to C 20 alkyl radical, or a —(CH 2 ) p —COOR radical or an ionized form thereof, with p being equal to 0 or 1, and R is a hydrogen atom or R is a C 1 to C 6 alkyl radical;

R4 represents a hydrogen atom, a monovalent, divalent or trivalent ion, or a quaternary ammonium radical;

and said copolymer comprising at least one unit of formula (II)

where

R5, R6 and R7 independently represent a hydrogen atom, a linear or branched C 1 to C 20 alkyl radical or a —(CH 2 ) n —(C═O) m —W—(R8-O) q —R9 radical;

R8 represents a linear or branched C 2 to C 20 alkyl radical;

R9 represents a hydrogen atom, or a linear or branched C 1 to C 20 alkyl radical;

W represents an oxygen or nitrogen atom;

n is an integer from 0 to 2;

m is an integer equal to 0 or 1;

q is an integer from 2 to 30;

and in which the molar mass of said copolymer is from 500 to 18 000 daltons,

wherein a relative proportion of unit of formula (I) with respect to unit of formula (II) is from 70/30 to 99/1.

2. The method of claim 1 , wherein said copolymer or a salt thereof is added to the calcium sulphate-based hydraulic binder chosen from a plaster based on calcium sulphate hemihydrate (α or β) or a binder based on anhydrous calcium sulphate.

3. The method of claim 1 , wherein said copolymer or a salt thereof has a block, random, alternating or gradient structure.

4. The method of claim 1 , wherein said copolymer or a salt thereof has a molar mass that is from 1000 to 16 000 daltons.

5. The method of claim 1 , wherein said copolymer has an R1 radical representing a hydrogen atom.

6. The method of claim 1 , wherein said copolymer has an R2 radical representing a hydrogen atom.

7. The method of claim 1 , wherein said copolymer has an R3 radical representing a hydrogen atom.

8. The method of claim 1 , wherein said copolymer has an R4 radical representing a hydrogen atom.

9. The method of claim 1 , wherein said copolymer has an R5 radical representing a hydrogen atom.

10. The method of claim 1 , wherein said copolymer has an R6 radical representing a hydrogen atom.

11. The method of claim 1 , wherein said copolymer has an R7 radical representing a hydrogen atom.

12. The method of claim 1 , wherein W represents an oxygen atom.

13. The method of claim 1 , wherein said copolymer has an R8 radical representing a C 2 alkyl radical.

14. The method of claim 1 , wherein said copolymer has an R9 radical representing a C 1 alkyl radical or a hydrogen atom.

15. The method of claim 1 , wherein q is an integer from 4 to 25.

16. The method of claim 1 , wherein a concentration of the at least one copolymer or a salt thereof is from 0.01% to 10% by dry weight of the copolymer of salt thereof relative to a dry weight of the calcium sulphate-based hydraulic binder.

17. The method of claim 1 , wherein the at least one copolymer or a salt thereof is added to the calcium sulphate-based hydraulic binder with one or more admixtures selected from the group consisting of an accelerator, a retarder, a thickener, a rheological agent, a dispersant and any combination thereof.