WO1999058469A1

WO1999058469A1 - Cement material containing lithium with improved mechanical properties, useful for retaining cations, and methods for making same

Info

Publication number: WO1999058469A1
Application number: PCT/FR1999/001140
Authority: WO
Inventors: Pascal Faucon; Jean-Claude Petit; Arnaud Muret; Hélène VIALLIS
Original assignee: Commissariat A L'energie Atomique
Priority date: 1998-05-13
Filing date: 1999-05-12
Publication date: 1999-11-18
Also published as: FR2778652A1; FR2778652B1

Abstract

The invention concerns a cement material comprising at least a hydrated calcium silicate substituted by lithium, said substituted silicate having a mol ratio Ca: Si of 0.3 to 1.7 and a mol ratio Li: Si of 0.01 to 1. In said material, part of the lithium atoms are mobile and can easily be exchanged by other cations. Thus, said material can be used to retain cations such as cesium. Moreover, it has improved mechanical resistance.

Description

CEMENT MATERIAL CONTAINING LITHIUM PROPERTIES

IMPROVED MECHANICS FOR USE IN THE RETENTION OF

CATIONS, AND METHODS FOR THE PRODUCTION THEREOF.

DESCRIPTION

Technical area

The invention relates to a new cementitious material which has, on the one hand, the property of being able to adsorb water-soluble cations, in particular cesium, and on the other hand, improved mechanical properties. This material can be used for the retention of cations, in particular for the storage of nuclear waste.

It can also be used in other fields where one is looking for cation fixation and / or good mechanical properties, in particular in civil engineering.

State of the art

Cementitious materials are used in civil engineering for their "bonding" properties, ensuring the mechanical cohesion of the parts making up a civil engineering structure. The cement mainly used is Portland cement, which hydrated, consists mainly of hydrated calcium silicates which confer on hydrated cement its properties of assembly and bonding.

Cementitious materials are also used in the context of nuclear waste storage as a coating matrix for nuclear waste. low activity and medium activity. They are also used as an engineered barrier in low-level waste storage sites and in medium and high-level waste storage sites, as well as in possible deep storage sites.

The cementitious material used mainly for these storage is also Portland cement, which hydrated, consists mainly of hydrated calcium silicates.

In hydrated Portland cement, the molecular structure of hydrated calcium silicates has certain analogies with a clay structure of the smectite type. FIG. 1 schematically represents a sheet of the structure of a hydrated calcium silicate of this type which consists of a stack of several sheets.

In this figure, we see that the sheet comprises two tetrahedral layers (Te) constituted by chains of tetrahedrons of Si0 ₂ of variable lengths and not planes as in a smectite, between which is placed an octahedral layer (Oc) constituted by CaO plan. Figure 2 illustrates the nuclear magnetic resonance spectrum of silicon at the magic angle of these hydrated calcium silicates. It makes it possible to determine the length of the chains which consist of tetrahedra Si [Q ₂ ], Si [Q ₂ L] and Si [Ql] referenced in FIG. 1.

The mechanical properties conferred by hydrated calcium silicates are linked to interlayer charges of hydrated calcium silicates.

Unlike clay, hydrated calcium silicates do not have a “structural” charge deficit linked to cationic substitutions in the CaO plane or in the layers of silicon tetrahedra (Te). This absence of cationic substitutions prevents the specific adsorption of soluble cations by hydrated calcium silicates, which does not allow good cation retention when these cements are used for the storage of radioactive waste.

Other waste coating matrices such as glass and ceramics have also been used to confine radionuclides, but the use of such materials leads to very disadvantageous costs compared to cement.

Clay is known for its adsorption properties for soluble cations, but it can hardly replace cement for the creation of an engineered barrier. Furthermore, if the waste is itself cemented, this solution will present a risk of chemical incompatibility between the cemented waste and the clay barrier. In addition, it is very often desirable to use the concrete necessary for the construction of the storage site as a containment barrier.

However, the concrete and cementitious materials currently used do not have the property of specifically adsorbing soluble cations such as cesium.

To increase the mechanical resistance of cementitious materials based on calcium silicates hydrated, public works companies currently obtain gains in mechanical strength by reducing the porosity of these materials. This reduction is obtained by reducing the quantities of water in the water-cement mixture (making it possible to obtain hydrated calcium silicates), or even by adding silica smoke or aggregates. It is also possible to improve the dispersion of the initial mixtures or to play on the pressure and the temperature during hardening.

The present invention specifically relates to a new cementitious material which, thanks to its composition, has improved mechanical strength and the advantageous property of adsorbing cations. Therefore, the material can be used for the storage of waste, in particular radioactive waste containing cesium, and for the retention of soluble cations.

It can also be used in the construction of civil engineering works to increase the mechanical resistance of concrete.

Exhibition of the invention

According to the invention, the cementitious material comprises at least one hydrated calcium silicate substituted by lithium, said silicate having a Ca molar ratio. If from 0.3 to 1.7 and a molar ratio Li: If from 0.01 to 1

In this -atenau, the substitution of Ca ^{2 ^} ions for hydrated calcium silicate by Lι ⁺ ions makes it possible to create a structural load deficit, similar to that stored in a smectite-type clay. This deficit is comoense because of lithium ions which can be exchanged very easily with other cations, and in particular cesium.

Thus, this new material has a modified surface charge due to the partial substitution of Ca by Li modifying the nature of the forces brought into play between each particle of calcium silicate hydrate. This modification is at the origin of the increase in the mechanical resistance of the cementitious material containing lithium. Preferably, in this material, the Ca: Si molar ratio of said hydrated calcium silicate is from 0.6 to 1.7, and the Li: Si molar ratio of said hydrated calcium silicate is from 0.01 to 0.75.

Such a calcium substitution rate with lithium makes it possible to obtain the advantageous cation exchange properties described above, as well as an improvement in mechanical strength.

The use of alkali metals, in particular sodium, in cements has already been envisaged to activate their hydration, as described in "Avances in Cernent Researcn, 1995, 7, n ° 27, pages 93-102 [1] . However, the amounts added in this case are very small and do not make it possible to obtain substituted hydrated calcium silicates as in the invention.

According to the invention, the cementitious material can also comprise other constituents such as those which are usually present in hardened cements. These Deuvert constituents can be chosen, for example, from calcium aluminates, calcium ferro-aluminates, silica, carboalummates, calcite, all the hydrates covering precititate in a cement, and their mixtures. The cementitious material of the invention can be obtained from a cementitious material comprising at least one hydrated calcium silicate, by subjecting it to a complementary treatment of substitution of part of the calcium atoms by lithium atoms .

Also, according to a first embodiment, the process for preparing the cementitious material of the invention consists in bringing a cementitious material based on hydrated calcium silicate into contact with an aqueous solution of lithium hydroxide or salt to replace a part of the calcium atoms of the calcium silicate hydrated by lithium atoms, the lithium concentration of the aqueous solution being such that it corresponds to a Li: Si molar ratio of at least 0.01.

This embodiment is easy to implement since it suffices to carry out an additional treatment on an existing cementitious material. Generally, the aqueous hydroxide or lithium salt solution has a lithium concentration of 0.5 mol / l.

In this embodiment, any hydrated cementitious material prepared from any cement capable of forming hydrated calcium silicates upon hardening can be used. For example, it can be Portland cement, blast furnace slag, silica smoke, pozzoianic material, fly ash and mixtures thereof. Preferably, any hydrated cementitious material based on Portland cement is used.

The lithium salt which can be used for this treatment is a lithium salt soluble in water, in particular a mineral acid salt such as a halide (fluoride, chloride, bromide and iodide), a nitrate, a sulfate, a carbonate or even a silicate. It can also be an organic salt.

Preferably, lithium chloride is used.

The starting cementitious material can be obtained from a composition comprising, in addition to the cement, one or more additives such as silica smoke (Sι0 ₂ amorphous), fly ash, inert fillers such as sand, limestone aggregates or siliceous, or additives making it possible to adapt the properties of the cementitious material obtained, as well as thinning agents and setting retarding agents.

In this case, the starting cementitious material can be obtained by adding the composition to an aqueous solution, and hardening of the cementitious material in this solution.

In this first embodiment, it is also possible to prepare the starting cementitious material based on hydrated calcium silicate, by mixing calcium and silicon compounds in water in quantities such that the Ca: Si molar ratio is greater. or equal to 0.3.

For this preparation, various calcium compounds can be used, for example calcium oxide CaO, calcium hydroxide, calcium silicates, any calcium-based compound which can hydrate, and mixtures thereof.

The silicon compound can be chosen from silica, silica fume, calcium, any silicon compound that can hydrate, and mixtures thereof.

For this preparation, it is important that the amounts of compounds added to the aqueous solution are such that the Ca: Si molar ratio is at least 0.3, preferably at least

0.66 to ensure the formation of calcium silicate.

The amount of water is variable and can be adjusted to obtain a suspension or a paste. It is preferred to obtain a suspension and, in this case, the weight ratio water: (compounds of Ca and Si) is greater than 1, preferably greater than 20, for example 50.

According to the invention, the cementitious material based on hydrated calcium silicate substituted by lithium can also be obtained directly by coprecipitation from a mixture of calcium, silicon and lithium compounds.

Also, according to a second embodiment, the process for preparing the cementitious material comprises the following steps:

- mix in an aqueous solution of calcium, silicon and lithium compounds in proportions such that the molar ratio Ca: Si in the solution is greater than or equal to 0.3 and that the molar ratio Li: Si in the solution is greater or equal to 0.01, and

- harden the cementitious material in this aqueous solution thus ootenu. For this preparation, it is possible to use the caxcium and silicon compounds mentioned above in the case of the preparation of a cementitious material without lithium. The lithium compound used can be lithium hydroxide or a lithium salt such as those mentioned above. Lithium chloride is preferred. As before, it is important that the amounts of calcium and silicon compounds are such that the Ca: Si molar ratio is at least 0.3, preferably at least 0.66, to ensure the formation of silicate calcium. Likewise, the amount of lithium compound must be such that the Li: Si ratio is at least 0.01.

The amount of water is variable and can be adjusted to obtain a suspension or a paste. In the case of a suspension, the water weight ratio: (compounds of Ca, Si and Li) is preferably greater than 20, for example 50. In the case of a paste, the water weight ratio: (compounds of Ca , Si and Li) is less than 1. The invention also relates to a process for the storage of waste comprising the use of a barrier made of a cementitious material described above to isolate the waste from the environment.

The waste is preferably waste containing water-soluble cations which are liable to migrate into the environment by water infiltration.

By way of example of such waste, mention may be made of radioactive waste containing cesium. It also relates to a process for retaining water-soluble cations such as cesium, by contacting an aqueous solution. said cations with a cementitious material according to the invention.

Finally, it relates to a process for improving the mechanical resistance of a cementitious material comprising at least one hydrated calcium silicate, which consists in bringing the cementitious material into contact with an aqueous solution of lithium salt or hydroxide to replace part of the calcium atoms of calcium silicate by lithium atoms.

In this process, the starting cementitious material can be a material obtained from Portland cement, slag, blast furnace, pozzolanic material and / or fly ash. Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of illustration and not limitation, with reference to the appended drawings.

Brief description of the drawings

FIG. 1 already described illustrates the structure of a hydrated calcium silicate in accordance with the prior art.

FIG. 2 illustrates the nuclear magnetic resonance spectrum of silicon at the magic angle, of the hydrated calcium silicate of FIG. 1.

FIG. 3 illustrates the X-ray diffractogram of unsubstituted hydrated calcium silicates in accordance with the prior art. FIG. 4 illustrates the X-ray diffractogram of lithium-substituted hydrated calcium silicates of Examples 1 to 4, in accordance with the invention.

Figures 5 and 6 illustrate the nuclear magnetic resonance spectra of silicon at the magic angle of unsubstituted hydrated calcium silicates (Figure 5) and substituted with lithium (Figure

6) examples 1 to 4.

FIG. 7 illustrates the static nuclear magnetic resonance spectra of lithium of hydrated calcium silicates substituted with lithium of examples 1 to 4.

Figure 8 illustrates the two lines identified in each of these spectra. FIG. 9 illustrates the structure of hydrated calcium silicates substituted with lithium, in accordance with the invention.

FIG. 10 illustrates the nuclear magnetic resonance spectra of silicon, at the magic angle, of hydrated calcium silicates unsubstituted by Li of examples 13 and 14 and substituted by Li of example 15.

Detailed description of the embodiments

The examples which follow illustrate the preparation and the mechanical and retention properties of cations of cementitious materials in accordance with the invention. Example 1: Preparation of a cementitious material based on calcium silicate substituted by lithium.

In this example, the first embodiment of the process of the invention is used, that is to say the carrying out of a complementary substitution treatment with Li on a cementitious material already prepared.

First of all, a cementitious material is prepared from calcium oxide CaO and silica Si0 ₂ by operating as follows.

A total of 1.8 g of CaO and Si0 _{2 is introduced} in proportions such that the Ca: Si molar ratio is 0.66, in 90 ml of water, which corresponds to a water weight ratio: (CaO + Si0 ₂ ) of 50, and the whole is left to stand for three weeks. A cementitious material based on hydrated calcium silicate is thus obtained in the medium.

Then added to this medium 1.89 g of lithium chloride and allowed to stand again for three weeks, at 25 ° C, in a nitrogen atmosphere to avoid carbonation.

A cementitious material substituted with lithium is thus obtained. In the appended Table 1, the starting Ca / Si molar ratio and the Ca: Si and Li: Si molar ratios are indicated in the product obtained.

Examples 2 to 4.

The same procedure is followed as in Example 1 to prepare other cementitious materials having different Ca: Si ratios, in using in each case 1.8 g of mixture of CaO and Si0 ₂ with different molar ratios, and the same amount of LiCl for the substitution treatment.

The starting and final Ca: Si molar ratios and the Li: Si molar ratio of cementitious materials obtained are also given in Table 1.

Comparative examples 1 to 4: preparation of cementitious materials based on unsubstituted calcium silicate.

In these examples, the same procedure is followed as in Examples 1 to 4, but the substitution treatment is not carried out using lithium chloride.

Cement materials unsubstituted by lithium are thus obtained having Ca: Si molar ratios of 0.66; 0.83; 1.2 and 1.7 as in Examples 1 to 4. The cementitious materials obtained in Examples 1 to 4 and in Comparative Examples 1 to 4 are analyzed by X-ray diffraction and by nuclear magnetic resonance from silicon to magic angle. Figure 3 illustrates the diffractogram

X unsubstituted hydrated calcium silicates obtained in Comparative Examples 1 to 4.

FIG. 4 illustrates the X-ray diffractograms of the lithium-substituted hydrated calcium silicates obtained in Examples 1 to 4.

The comparison of Figures 3 and 4 shows that the addition of lithium did not cause the precipitation of new crystallized phases. FIG. 5 illustrates the nuclear magnetic resonance spectra of silicon at the magic angle of hydrated calcium silicates not substituted by lithium obtained in Comparative Examples 1 to 4.

FIG. 6 illustrates the spectra of similar products substituted with lithium obtained in examples 1 to 4.

If we compare Figures 5 and 6, we note that we find the same sites Ql, Q2L and Q2 and in the same proportions in the silicates substituted by Li and in the unsubstituted silicates. This indicates that the tetrahedral layers are identical in products substituted by lithium and in unsubstituted products.

If we refer to Table 1, we note that when the Ca: Si molar ratio is 0.66 at the start, we obtain a final Ca: Si molar ratio of 0.579. Conventionally, this should correspond to the coexistence of a hydrated calcium silicate having a Ca: Si of 0.66 and silica gel. However, as shown in FIG. 6, no silica gel is detected by NMR of the silicon. In fact, part of the calcium in the octahedral sheet has been replaced by lithium, explaining that the Ca: Si ratio is less than 0.66. chemical analyzes confirm the presence of lithium replacing calcium.

The static nuclear magnetic resonance spectra of lithium of lithium-substituted hydrated calcium silicates of Examples 1 to 4 are shown in FIG. 7, the peaks corresponding to mobile lithium having been truncated. In FIG. 8, the two lines which appear on the spectra of FIG. 7 have been identified. This figure makes it possible to identify two lines, one broad or site 1 (non-average quadrupolar interaction) which corresponds to low-level lithium ions. or not mobile, and the other fine (site- 2) corresponding to mobile lithium ions.

In FIG. 9, the structure of lithium-substituted hydrated calcium silicates has been shown, as it emerges from the analyzes carried out previously.

In this figure, we find the tetrahedral layers (Te) made up of chains of tetrahedra of Si0 ₂ of figure 1 and the octahedral layer (Oc) of the CaO plane in which x calcium atoms are substituted by lithium atoms (xLiO ⁺ ). The substitution of Ca ²⁺ by Li ⁺ creates a charge deficit compensated by lithium ions in interlayer (x / 2 Li ⁺ ) on either side of the sheet. This structure is confirmed by the X-ray diffractograms of FIGS. 3 and 4 (no other crystallized phase) and by the spectra of FIGS. 5 and 6 which give the same tetrahedral sites and by the spectra of FIG. 7 and the results of table 2 which show that we have fixed lithium atoms and mobile lithium atoms.

The material of the invention is very interesting because these mobile lithium atoms in the interlayer can be exchanged easily with other cations, for example cesium ^as' will see below. Example 5: Preparation of a cementitious material based on calcium silicate substituted by lithium.

The second embodiment of the process is used for this preparation, namely the direct precipitation of the material from a mixture of calcium, silicon and lithium compounds.

The compounds Ca, Si and Li used are calcium oxide CaO, silica Si0 ₂ and lithium chloride LiCl.

In 90 ml of water is introduced an amount of lithium chloride, such that the lithium concentration of the ^s' olution is 0.5 mol / 1 and 1.8 g in total of CaO and Si0 _2, the proportions of CaO and Si0 ₂ corresponding to a Ca: Si molar ratio of 0.66. The reaction medium is left to stand for three weeks at 25 ° C. in a nitrogen atmosphere to avoid carbonation, and the cementitious material based on calcium silicate hydrate substituted with lithium is recovered from this medium.

In the annexed Table 2, the starting Ca: Si molar ratio and the Ca: Si and Li: Si molar ratios are indicated in the product obtained.

Examples 6 to 8.

The same procedure is followed as in Example 5 to prepare other cementitious materials having different Ca: Si ratios, using in each case 1.8 g of mixture of CaO and Si0 ₂ with different molar ratios, and the same amount of LiCl. The Ca: Si starting and final molar ratios and the Li: Si molar ratio of cementitious materials obtained are given in Table 2.

The results of Table 2 show that the second embodiment of the method is less favorable to the substitution of Ca by Li.

The X-ray diffraction analyzes of the materials obtained in Examples 5 to 8 give spectra identical to those of FIG. 4, and the same is true for the analysis by spectrometry by nuclear magnetic resonance of silicon at the magic angle. , which gives spectra identical to those of FIG. 5.

Thus, there is no lithium salt in the material, lithium therefore interacts with hydrated calcium silicates. It is incorporated into the structure and modifies the surface properties. As a result, the retention and interaction properties between two particles of hydrated calcium silicate are improved. This last property is at the origin of the increase in the mechanical resistance noted later (examples 13 to 15).

Examples 9 to 12

In these examples, the properties of the cementitious materials obtained in examples 5 to 8 are tested for the fixation of cesium. For this purpose, cesium salt (CsCl) is added to the mixtures of Examples 5 to 8. The concentration of cesium salt is adjusted to 0.5 mmol / l.

The concentration of cesium remaining in solution is then determined by determination by capillary electrophoresis after a preliminary calibration, the detection limit being 0.1 mmol / 1.

The results obtained are given in table 3. It is thus noted that the cementitious materials of the invention which have a very low rate of substitution by lithium, make it possible to obtain a cesium retention greater than 80%.

Comparative examples 5 to 8.

In these examples, the same procedure is followed as in Examples 9 to 12, but a solution of cesium chloride at 0.5 mol / 1 and the cementitious materials of Comparative Examples 1 to 4, not substituted by lithium, are used. .

The results obtained are given in Table 4 as a percentage of fixed Cs. In this table 4, the percentage of fixed cesium was also given for a solution with an initial cesium concentration of 0.5 mmol / l, based on the fact that the quantity of fixed cesium is proportional to the quantity of initial cesium, as we have seen. Thus, with a 0.5 mmol / l solution of cesium, cesium is practically not fixed by cementitious materials not substituted by lithium, the retention of cesium being at most 0.02%. The substitution by lithium of cementitious materials therefore makes it possible to obtain a very high cesium retention rate.

Therefore, the cement materials of the invention can be used for the storage of waste, either as a storage matrix or as an engineered barrier arranged around coated waste. This makes it possible to isolate the waste from the environment and to ensure that the cesium is fixed in the barrier in the event that the cesium could be dissolved by water which has migrated into the waste. Likewise, the cementitious materials of the invention can be used for the retention of water-soluble cations such as cesium, by bringing an aqueous solution of these cations into contact with the cementitious material of the invention.

Examples 13 to 15

In these examples, the second embodiment of the process is used to prepare a cementitious material from fume of silica carbosil (Si0 ₂ ) and calcium hydroxide Ca (OH) ₂ with or without the addition of lithium chloride.

The compositions used for the preparation of the material are given in Table 5 which follows.

In these examples, all of the constituents are kneaded for approximately 2 minutes, then each of the pastes is introduced into a mold of three test tubes (4 cm x 4 cm x 16 cm). The tightening of the pasta is obtained by introduction in two stages, applying 60 shocks to the mold each time.

The mold of Example 13 is stored in water at 25 ° C for 28 days. The molds of Examples 14 and 15 are placed in an airtight plastic container filled with water at 85 ° C. for 28 days. The resistance to compression of the cementitious materials obtained in each of the examples is then determined.

The results obtained are given in Table 6 which follows. The values in the table are the average of three measurements made on three different test pieces (dispersion <10%).

Thus, it is found that the mechanical strength of test pieces containing lithium (Ex 15) is significantly higher than that of test pieces without Li, whatever the hardening temperature.

The use of a temperature of 85 ° C activates the kinetics of hydration because the silica smoke is not very reactive.

In FIG. 10, the nuclear magnetic resonance spectra of the silicon of the cementitious materials obtained in examples 13 to 15 are shown. In this figure, it can be seen that the residual silica smoke is important when the hydration takes place at 25 °. vs. When the hardening temperature goes from 25 ° C to 85 ° C, the hydration of the silica smoke is accelerated. The system therefore contains more hydrated calcium silicate. The resulting hydrated calcium silicates have shorter chains than at 25 ° C. The proportion of Ql tetrahedra is indeed lower at 85 ° C than at 25 ° C. A collapse in mechanical strength is then observed (Table 7) which is therefore not linked to the quantity of hydrated silica smoke.

The addition of LiCl slightly activates this hydration (known effect of salts). This addition changes little or no structure of the chains of silicon tetrahedra for the same hardening temperature. Mechanical resistance is therefore not correlated in the present case to the length of tetrahedron chains in hydrated calcium silicates. The increase in mechanical strength can therefore be attributed to the addition of lithium, which is incorporated into the structure of hydrated calcium silicates by modifying their interface properties. Indeed, it does not result from a greater hydration effect of the anhydrous materials due to the lithium salt because one would then have observed on the spectrum a significant reduction in the contribution Q4 corresponding to the silica smoke not hydrated with the addition of LiCl.

Citation cited

[1]: Avances in Cernent Research, 1995, 7, n ° 27, pages 93-102.

Table 1

Table 2

Table 3

Table 4

Ex Ca / Si% Cs fixed for% Cs fixed for

[Cs] initial [Cs] initial

Comparative 0.5M 0.5 mmol / 1

5 0.66 10-20 0.01-0.02

(ex comp. D

6 0.83 10-20 0.01-0.02

(e.g. comp. 2)

7 1.20 <10 <0.01

(e.g. comp. 3)

8 1.70 <5 <0.005

(e.g. comp. 4) Table 5

Table 6

Claims

1. Cementary material comprising at least one hydrated calcium silicate substituted with lithium, said substituted silicate having a Ca: Si molar ratio of 0.3 to 1.7 and a Li: Si molar ratio of 0.01 to 1.

2. A cementitious material according to claim 1, in which the Ca: Si molar ratio of said silicate is from 0.6 to 1.7.

3. A cementitious material according to claim 1 or 2, in which the Li: Si molar ratio of said silicate is from 0.01 to 0.75.

4. cementitious material according to any one of claims 1 to 3, further comprising constituents of hardened cement chosen from calcium aluminates, calcium ferroaluminates, silica, carboaluminates, calcite, all hydrates which can precipitate in a cement, and their mixtures.

5. Method for preparing a cementitious material according to any one of claims 1 to 4, which consists in bringing a cementitious material based on hydrated calcium silicate into contact with an aqueous solution of hydroxide or lithium salt for substitute part of the calcium atoms of the hydrated calcium silicate with lithium atoms, the lithium concentration of the aqueous solution being such that it corresponds to a Li: Si molar ratio of at least 0.01.

6. The method of claim 5, wherein the aqueous solution of hydroxide or lithium salt has a lithium concentration of 0.5 mol / 1.

7. The method of claim 5 or 6, wherein the cementitious material is any cementitious material based on Portland cement.

8. Method according to any one of claims 5 to 7, in which the lithium salt is lithium chloride.

9. Method according to any one of claims 5 to 8, in which the cementitious material based on hydrated calcium silicate is first prepared, by mixing calcium and silicon compounds in water in such quantities that the Ca: Si molar ratio is greater than or equal to 0.3.

10. Process for preparing a cementitious material based on hydrated calcium silicate (s) substituted with lithium, comprising the following steps:

- mixing in an aqueous solution of calcium, silicon and lithium compounds in proportions such that the Ca: Si molar ratio is greater than or equal to 0.3 and that the Li: Si molar ratio is greater than or equal to 0.01 , and

- hardening of the cementitious material in this solution.

11. The method of claim 9, wherein the calcium compound is chosen from calcium oxide CaO, calcium hydroxide, calcium silicates, any calcium-based compound which can hydrate, and mixtures thereof.

12. Method according to claim 9 or 10, in which the silicon compound is chosen from silica, silica smoke, calcium silicates, any silicon compound that can hydrate, and mixtures thereof.

13. The method of claim 10, wherein the lithium compound is lithium chloride.

14. A waste storage method comprising the use of a barrier made of cementitious material according to any one of claims 1 to 4 to isolate the waste from the environment.

15. The method of claim 14, wherein the waste is radioactive waste containing cesium.

16. A method of retaining water-soluble cations by bringing an aqueous solution of said cations into contact with a cementitious material according to any one of claims 1 to 4.

17. The method of claim 16, wherein said cation is cesium.

18. A method for improving the mechanical strength of a cementitious material comprising at least one hydrated calcium silicate, which consists in bringing the cementitious material into contact with an aqueous solution of lithium salt or hydroxide to replace part of the atoms of calcium from calcium silicate by lithium atoms.

19. The method of claim 18, wherein the aqueous solution is a 0.5 M lithium chloride solution.

20. A method according to any one of claims 18 and 19, wherein the cementitious material is a material obtained from a cement chosen from Portland cements, slag from blast furnaces, pozzolanic materials, fly ash and mixtures thereof.