Set-accelerating admixture having improved stability

1. A set-accelerating admixture for hydraulic compositions comprising, in an aqueous solution:

the reaction product of an alkanolamine with a concentrated strong acid;

sulfate anions; and

alkali or alkaline earth cations;

having a pH comprised between 5 and 12.

2. The admixture according to claim 1 , further comprising an additional set accelerating agent.

3. The admixture according to claim 2 , wherein the set-accelerating agent is selected from the group consisting of sodium chloride, calcium chloride, sodium thiocyanate, calcium thiocyanate, sodium nitrate, calcium nitrate and mixtures thereof.

4. The admixture according to claim 1 , wherein the alkanolamine is selected from the group consisting of triethanolamine (TEA), triisopropanolamine (TIPA), diethanolamine (DEA), diethanolisopropanolamine (DEIPA), tetrahydroxylethylethylenediamine (THEED) and mixtures thereof.

5. The admixture according to claim 1 , having a pH comprised between 5.5 and 7.

6. A method for preparing a set-accelerating admixture for hydraulic compositions comprising the steps:

(1) adding a concentrated strong acid to an aqueous solution of alkanolamine;

(2) adding to the product obtained in step (1) an aqueous solution of an alkali metal or alkaline earth metal chloride and/or sulfate; and

(3) adjusting the pH to a value from 5 to 12 , before or after step (2).

7. The preparation method according to claim 6 , wherein step (1) is carried out by adding concentrated sulfuric acid and step (2) is carried out by adding a solution of an alkali metal chloride.

8. The preparation method according to claim 6 , wherein step (1) is carried out by adding concentrated hydrochloric acid and step (2) is carried out by adding a solution of an alkali metal sulfate.

9. The method according to claim 6 , wherein the molar ratio between the amine and the acid in step (1) is comprised between 0.2 to 0.6.

10. The preparation method according to claim 6 , wherein step (3) is carried out by adding a base selected from the group consisting of sodium hydroxide and potassium hydroxide.

11. A set-accelerating admixture for hydraulic compositions which may be obtainable by the method according to claim 6 .

12. A method for preparing a hydraulic composition, comprising the step for adding an admixture according to claim 1 to the hydraulic binder, before, during or after the grinding.

13. The preparation method according to claim 12 , wherein the admixture is added with a dosage from 500 to 10,000 ppm by weight based on the weight of the hydraulic binder.

14. The preparation method according to claim 12 wherein the hydraulic binder is a cement containing less than 75% by weight of clinker.

15. A hydraulic composition, containing a cement with a clinker content of less than 75% by weight and an admixture according to claim 1 .