Belite-calcium aluminate as an additive

1. A method for accelerating the early stiffening/setting and/or the hardening of Portland cement comprising adding to the Portland cement a belite calcium aluminate accelerator produced according to the following steps:

(a) providing one or more raw materials selected from the group consisting of Ca-, Al-, Fe-, and Si-containing solid materials,

(b) mixing the raw materials to form a starting material mixture, wherein the starting material has a molar Ca/(Si+Al+Fe) ratio from 1 to 3.5 and a molar Al/Si ratio from 100 to 0.1,

(c) hydrothermally treating the starting material mixture produced in step (b) in an autoclave at a temperature from 100 to 300° C. and a residence time from 0.1 to 24 hours, wherein a water/solids ratio is 0.1 to 100, to form an intermediate product, and

(d) tempering the intermediate product obtained in step (c) at 350 to 600° C., wherein a heating rate is between 10-6000° C./min and a residence time is between 0.01-600 min to form the belite calcium aluminate accelerator.

2. The method according to claim 1 , further comprising adding calcium sulfate to the Portland cement.

3. The method according to claim 2 , wherein calcium sulfate is added in an amount from 10 to 40% by weight based on the accelerator.

4. The method according to claim 1 , wherein the belite calcium aluminate accelerator comprises at least one compound selected from the group consisting of calcium silicate, calcium aluminate, calcium aluminium silicate, calcium (aluminium, iron) silicate, and at least one X-ray amorphous phase, wherein the sum of calcium silicates, calcium aluminates, calcium aluminium silicates and calcium (aluminium, iron) silicates is at least 30% by weight.

5. The method according to claim 1 , wherein the belite calcium aluminate accelerator comprises the following components:

1-95% by weight reactive calcium aluminates, in the form of crystalline C 12 A 7 , or semi-crystalline or amorphous aluminate phases,

1-80% by weight calcium (aluminium, iron) silicates, in the form of crystalline, semi-crystalline or amorphous phases,

1-80% by weight C 2 S polymorphs, in the form of crystalline, semi-crystalline or amorphous phases,

1-80% by weight calcium aluminate silicates, in the form of crystalline, semi-crystalline or amorphous phases,

up to 30% by weight traces and minor components, and

0-30% by weight hydrates from the step (c) hydrothermal treatment.

6. The method according to claim 5 , wherein the traces and minor components are one or more compounds selected from the group consisting of C 5 A 3 , CA, calcium oxide, aluminas, quartz, limestone, CaO, calcium sulfate, FeO, Fe 2 O 3 , Fe 3 O 4 , iron silicates, Fe 2 SiO 4 , and amorphous iron-containing phases.

7. A binder containing Portland cement and an accelerator consisting of a belite calcium aluminate produced according to the following steps:

(a) providing one or more raw materials selected from the group consisting of Ca-, Al-, Fe-, and Si-containing solid materials,

(b) mixing the raw materials to form a starting material mixture, wherein the starting material has a molar Ca/(Si+Al+Fe) ratio from 1 to 3.5 and a molar Al/Si ratio from 100 to 0.1,

(c) hydrothermally treating the starting material mixture produced in step (b) in an autoclave at a temperature from 100 to 300° C. and a residence time from 0.1 to 24 hours, wherein a water/solids ratio is 0.1 to 100, to form an intermediate product, and

(d) tempering the intermediate product obtained in step (c) at 350 to 600° C., wherein a heating rate is between 10-6000° C./min and a residence time is between 0.01-600 min to form the accelerator.

8. The binder according to claim 7 , wherein a measured Brunauer-Emmett-Teller surface area of the binder ranges from 1 to 30 m 2 /g.

9. The binder according to claim 7 , wherein the binder further contains latent hydraulic materials and/or pozzolans.

10. The binder according to claim 7 , wherein the binder further contains metakaolin and/or limestone.

11. The binder according to claim 7 , wherein the belite calcium aluminate comprises at least one calcium silicate, calcium aluminate, calcium aluminium silicate, calcium (aluminium, iron) silicate, and at least one X-ray amorphous phase, wherein the sum of calcium silicates, calcium aluminates, calcium aluminium silicates and calcium (aluminium, iron) silicates is at least 30% by weight.

12. The binder according to claim 7 , wherein the belite calcium aluminate comprises the following components:

1-95% by weight reactive calcium aluminates, in the form of crystalline C 12 A 7 , or semi-crystalline, or amorphous aluminate phases,

1-80% by weight calcium (aluminium, iron) silicates, in the form of crystalline, semi-crystalline, or amorphous phases, which may contain foreign ions such as Ca, Fe and Al 1-80% by weight C 2 S polymorphs, in the form of crystalline, semi-crystalline, or amorphous phases,

1-80% by weight calcium aluminate silicates, in the form of crystalline, semi-crystalline or amorphous phases,

up to 30% by weight traces and minor components, and

0-30% by weight hydrates from the step (c) hydrothermal treatment.

13. The binder according to claim 12 , wherein the traces and minor components are one or more compounds selected from the group consisting of C 5 A 3 , CA, calcium oxide, aluminas, quartz, limestone, CaO, calcium sulfate, FeO, Fe 2 O 3 , Fe 3 O 4 , iron silicates, Fe 2 SiO 4 , and amorphous iron-containing phases.