1. A composition comprising at least, in % expressed by mass relative to the total mass of the composition, - from 0.01 to 3 % of polycarboxylic acid or salts thereof, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule; and from 97 to 99.99 % of a Belite-Calcium-Sulphoaluminate-Ferrite clinker (BCSAF clinker) comprising at least, in % expressed by mass relative to the total mass of BCSAF clinker ■ from 5 to 30 %, of a calcium aluminoferrite phase of a composition con-esponding to the general fomiula C2AxF(i.x), wherein X is from 0.2 to 0.8; " from 10 to 35 %, of a calcium sulfoaluminate phase « ye'elimite » (C4A3$), ■ from 40 to 75 % of belite (CzS), ■ from 0.01 to 10 % of one or more minor phases selected from calcium sulphates, alkali sulphates, perovskite, gehlenite, free lime and periclase, and/or a vitreous phase, and for which the total of the percentages of these phases is greater than or equal to 97%.

2. The composition according to claim 1 characterised in that it comprises from 0.01 to 3 % of polycarboxylic acid, more preferably 0.03 to 1 %, most preferably 0.1 to 0.5 %, % by mass relative to the total mass of corhposition.

3. The composition according to claim 1 or claim 2 characterised in that it comprises at least one polycarboxylic acid or salts thereof selected from citric acid, malonic acid, malic acid, glutaric acid, adipic acid, oxalic acid, maleic acid, tartric acid, succinic acid, ascortDic acid, glutamic acid or mixtures thereof.

4. The composition according to claim 3 characterised in that it comprises at least one citric acid or salts thereof.

5. The composition according to one of claims 1 to 4 characterised in that it further comprises an alkanolamine. ^^

6. The composition according to one of claim's 1 to 5 characterised in that the Belite-Calcium-Sulphoaiuminate-Ferrite clinker (BCSAF clinker) does not comprise a mineralogical phase C3S.

7. A cement comprising at least - from 30 to 99.9 % of a composition according to one of claims 1 to 6; and - from 0.1 to 40 % of calcium sulphate, % by mass relative to the total mass of cement; - from 0.1 to 70 % of mineral additions, % by mass relative to the total mass of cement.

8. The cement according to claim 7 characterised in that the mineral additions are selected from slags (as defined in the « Cement» NF EN 197-1 Standard, paragraph 5.2.2), steel industry slags, pozzolanic materials (as defined in the « Cement» NF EN 197-1 Standard, paragraph 5.2.3), fly ash (as defined in the « Cement » NF EN 197-1 Standard, paragraph 5.2.4), or limestones (as defined in the « Cement » NF EN 197-1 Standard, paragraph 5.2.6), or mixtures thereof.

9. A concrete or mortar comprising at least a BCSAF cement and at least one polycarboxylic acid or salts thereof, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule.

10. Use of at least one polycarboxylic acid or salts thereof to increase the mechanical strength of a BCSAF_cement after setting, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule. '

11. The use according to claim 10 characterised in that the said acid is present in the mixing water of the BCSAF cement.

12. A process for preparation of a concrete or of a mortar according to claim 9 comprising a step of mixing a BCSAF cement with aggregates, water, at least one polycarboxylic acid or salts thereof and optionally additives, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule.

13. Elements for the construction field made using the concrete or the mortar according to claim 9 or the cement according to one of claims 7 or 8.

The present invention relates to a composition comprising a Belite-Calcium-Sulphoaluminate-Ferrite clinl I Calcium sulphate I Limestone I Additive filler Control 1 3.5 % AH from le Pin 0% 0% Cement 1 3.5 % AH from le Pin 0% 0.3% succinic acid/cement Cement 2 3.5 % AH from le Pin 0% 0.3% ascorbic acid/cement Cement 3 3.5 % AH from le Pin 0% 0.3% oxalic acid/cement Cement 4 3.5 % AH from le Pin 0% 0.3% citric acid/cement "Cement 5 13.5 % AH from le Pin | 0% | 0.3% malic acid/cement The percentages of the additives (acids) are expressed by mass relative to the total mass of cement. Table III I Calcium sulphate I Limestone filler I Additive N°1 I Additive N°2 Control 2 3.5 % AH from le Pin 25% substitution of 0% 0% the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% oxalic acid 0% 6 the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% succinic acid 0% 7 the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% malic acid 0% 8 the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% ascorbic acid 0% 9 the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% citric acid 0% 10 the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% citric acid 0.1%DEA 11 the cement Cement 3.5 % AH from le Pin 25% substitution of 0.225% citric acid 0.1% 12 I I the cement | | IVIDEA The percentage of limestone filler, is expressed by mass relative to the total mass of BCSAF cement + limestone filler. The percentages of additives (acids and alkanolamines) are expressed by mass relative to the total mass of the Cement. Table IV Production of a mortar according to the invention: The mortar was made according to the EN 196-1 Standard. Quantities of materials used to make a mortar: - 450 g cement as detailed above -1350 g standardized sand ^ 17 5, - 225 g water For the preparation of the binder, all the materials were mixed using a Turbula mixer for 30 minutes; the mixing procedure respected the EN196-1 Standard. The mortar was then poured into steel moulds, then these moulds were placed in a cabinet at controlled hygrometry (> 97 %). After one day of hydration of the mortar, the prisms of mortar were demoulded and immersed in water at 20 °C until the breaking date. The compressive mechanical strengths measured at 28 days according to the EN 196-1 Standard are given in Tables V and VI herein below. For each mortar, a control mortar not comprising polycarboxylic acic| was made. Compressive strength at 28 days Control 1 50 MPa Cement 1 56.8 MPa Cement 2 59,3 MPa Cement 3 59,9 MPa Cement 4 62.3 MPa ~"Cement 5 | 63.5 MPa Table V Compressive strength at 28 days Control 2 21 MPa Cement 6 23.7 MPa Cement 7 26.1 MPa Cement 8 28.6 MPa Cement 9 28.8 MPa Cement 10 35.7 MPa Cement 11 43.2 MPa Cement 12 | V 44.7 MPa TableVI 18 WE CLAIM: 1. A composition comprising at least, in % expressed by mass relative to the total mass of the composition, - from 0.01 to 3 % of polycarboxylic acid or salts thereof, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule; and from 97 to 99.99 % of a Belite-Calcium-Sulphoaluminate-Ferrite clinker (BCSAF clinker) comprising at least, in % expressed by mass relative to the total mass of BCSAF clinker ■ from 5 to 30 %, of a calcium aluminoferrite phase of a composition con-esponding to the general fomiula C2AxF(i.x), wherein X is from 0.2 to 0.8; " from 10 to 35 %, of a calcium sulfoaluminate phase « ye'elimite » (C4A3$), ■ from 40 to 75 % of belite (CzS), ■ from 0.01 to 10 % of one or more minor phases selected from calcium sulphates, alkali sulphates, perovskite, gehlenite, free lime and periclase, and/or a vitreous phase, and for which the total of the percentages of these phases is greater than or equal to 97%. 2. The composition according to claim 1 characterised in that it comprises from 0.01 to 3 % of polycarboxylic acid, more preferably 0.03 to 1 %, most preferably 0.1 to 0.5 %, % by mass relative to the total mass of corhposition. 3. The composition according to claim 1 or claim 2 characterised in that it comprises at least one polycarboxylic acid or salts thereof selected from citric acid, malonic acid, malic acid, glutaric acid, adipic acid, oxalic acid, maleic acid, tartric acid, succinic acid, ascortDic acid, glutamic acid or mixtures thereof. 4. The composition according to claim 3 characterised in that it comprises at least one citric acid or salts thereof. 5. The composition according to one of claims 1 to 4 characterised in that it further comprises an alkanolamine. ^^ 6. The composition according to one of claim's 1 to 5 characterised in that the Belite-Calcium-Sulphoaiuminate-Ferrite clinker (BCSAF clinker) does not comprise a mineralogical phase C3S. 7. A cement comprising at least - from 30 to 99.9 % of a composition according to one of claims 1 to 6; and - from 0.1 to 40 % of calcium sulphate, % by mass relative to the total mass of cement; - from 0.1 to 70 % of mineral additions, % by mass relative to the total mass of cement. 8. The cement according to claim 7 characterised in that the mineral additions are selected from slags (as defined in the « Cement» NF EN 197-1 Standard, paragraph 5.2.2), steel industry slags, pozzolanic materials (as defined in the « Cement» NF EN 197-1 Standard, paragraph 5.2.3), fly ash (as defined in the « Cement » NF EN 197-1 Standard, paragraph 5.2.4), or limestones (as defined in the « Cement » NF EN 197-1 Standard, paragraph 5.2.6), or mixtures thereof. 9. A concrete or mortar comprising at least a BCSAF cement and at least one polycarboxylic acid or salts thereof, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule. 10. Use of at least one polycarboxylic acid or salts thereof to increase the mechanical strength of a BCSAF_cement after setting, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule. ' 11. The use according to claim 10 characterised in that the said acid is present in the mixing water of the BCSAF cement. 12. A process for preparation of a concrete or of a mortar according to claim 9 comprising a step of mixing a BCSAF cement with aggregates, water, at least one polycarboxylic acid or salts thereof and optionally additives, the said polycarboxylic acid comprising 2 to 4 carboxyl groups per molecule. 13. Elements for the construction field made using the concrete or the mortar according to claim 9 or the cement according to one of claims 7 or 8.