FR2669918A1 - Process for obtaining a geopolymer cement without emission of carbon dioxide CO2 and products obtained by this process - Google Patents

Abstract

The process described makes it possible to obtain a geopolymer cement whose manufacture takes place without release of CO2 gas discharged into the atmosphere and taking part in the greenhouse effect on the earth's climate. The process of preparation includes the following three reactive components: a) an aluminosilicate {9 [Si2O5,Al2O2], [Si2O5,Al2(OH)3 ]}, or, to simplify in what follows: (Si2O5.Al2O2) IV-V, having the Al cation in (IV-V) coordination as determined by the spectrum from analysis using nuclear magnetic resonance MAS-NMR for <27>Al; b) a sodium and/or potassium alkali metal melilite (Ca,Na,K)2[(Mg,Fe<2+>,Al,Si)3O7]; c) a calcium melilite Ca2[(Mg,Fe<2+>,AlSi)3O7]. During the alkaline activation of the said cement, a sodium and/or potassium alkali metal disilicate, (Na2,K2)(H3SiO4)2 and a calcium disilicate Ca(H3SiO4)2 are formed in situ, with the result that the molar ratios of the reactive components are equal to or included between and The curing of the geopolymer cement by alkali metal activation corresponds to the formation of a geopolymer of (Ca,Na,K)-poly(sialate-siloxo) type of formula varying between and

Description

Process for obtaining a geopolymer cement, without CO2 carbon dioxide emanation, and products obtained by this process.

The present invention relates to a process for obtaining a cement which, during its manufacture, has a very low release of carbon dioxide CO 2. All hydraulic cements are based on Portland cement and lime obtained by calcining calcium carbonate. For Portland cement, the reaction can be written as follows:
a) SCaC03 + 2SiO2 <(2CaO, SiO2) (3CaO, SiO2) + 5CO2.

b) C + O2 CO2
Thus the manufacture of 1 ton of clinker is accompanied by the production of 0.55 tons of C02 resulting from the chemical reaction and 0.35 to 0.40 tons of C02 resulting from the energy expenditure. To simplify, we can assume that when we manufacture 1 tonne of clinker, we produce 1 tonne of carbon dioxide C02. The world production of cement having been in 1987 of 1.035 billion tons, the cement activity was thus accompanied by the release into the atmosphere of more than 1 billion tons of C02 gas, that is to say practically 5% of the C02 produced by human activity, or the equivalent of all the CO2 gas generated by a country like Japan.

However, as everyone now knows, the constant increase in CO2 gas in the atmosphere is responsible for climate change and greenhouse warming. It is therefore important to offer cements whose production method is accompanied by a smaller or no quantity of CO2 gas.

However, the replacement of the manufacturing method must not change the characteristics of the products and their use in the preparation of concrete. These new cements must be able to compare favorably with the best portland cements. They must have the same mechanical strengths, or if possible better, identical setting and hardening times, or if possible faster. In short, these new cements must also bring innovation to the user, in order to be accepted more favorably.

Since CO2 gas comes mainly from the calcination of calcium carbonate, new cements should not be based on the hydration of Ca ++ ions, but rather on the reactivity of Na + and / or K + ions.

In the context of the present invention, the geopolymer cements obtained result from a reaction of mineral polycondensation by alkaline activation, called geopolymerization, as opposed to traditional hydraulic binders in which the hardening is the result of hydration of calcium aluminates and silicates. of calcium.The means of investigation used is the Resonance spectrum
Magnetic Nuclear, MAS-NMR for 27AI. The products resulting from the geopolymerization reaction, as recommended in the present invention, have a single peak at 55 ppm, characteristic of Al (IV) coordination, while the hydration compounds obtained in traditional hydraulic binders have a peak. at 0 ppm, characteristic of the Al (VI) coordination, that is to say of calcium hydroxyaluminate.

The MAS-NMR spectrum of 29Si also makes it possible to make a very clear differentiation between geopolymers and hydraulic binders. If we represent the degree of polymerization of the SiO4 tetrahedron by Qn (n = 0,1,2,3,4), we can distinguish between monosilicates (Qo), disilicates (Q1), silicate groups (Q2 ), grafted silicates (Q3) and silicates forming part of a three-dimensional network (Q4). These degrees of polymerization are characterized in MAS-NMR of 29Si by the following peaks: (Qo) from -68 to -76 ppm; (Q1) from -76 to -80; (Q2) from -80 to -85 ppm; (Q3) from -85 to -90 ppm; (Q4) from -91 to -130 ppm. The peaks characterizing the geopolymers are found in the -85 to -100 ppm zone and correspond to the three-dimensional network (Q4) characteristic of poly (sialates) and poly (sialate-siloxo). On the contrary, the results of the hydration of hydraulic binders leading to hydrated calcium silicate CSH (according to the terminology used in cement chemistry) produce peaks located in the region -68 to -85 ppm either the monosilicate (Qo) or the disilicate (Q1) (Q2); (see for example J. Hjorth,
Cement and Concrete Research, Vol. 18, nr. 4, 1988 and J. Skibsted, Geopolymer '88,
Session nr. 7, University of Compiègne, 1988).

According to the terminology in force for geopolymers (see for example
Geopolymer '88, Volume 1, Proceedings of the Geopolymer '88 Congress, University of
Technology, Compiègne France), the geopolymer cement of the present invention corresponds to a geopolymer of the type
(Ca, Na, K) -poly (sialate-siloxo) of formula varying between [0.6 (Na, K) + 0.2Ca] (- Si-O-Al-O-Si-O -), H2O
and
, 4 (Na, K) + 0.3Cai (-Si-O-Al-O-Si-O-), H2O.

The geopolymeric mineral compositions according to the present invention, making it possible to obtain a geopolymer cement, essentially comprise three reactive elements.

The first reagent is an alumino-silicate oxide of formula 19 [5i2O5, Al2O21, [5i2O5.Al2 (OH) 3]}, obtained by calcining a kaolinitic material, the said calcination is carried out so that the said aluminum oxide -silicate has a Resonance analysis spectrum
Nuclear Magnetic MAS-NMR for 27AI having in addition the two main resonances at 20 + 5ppm, Al (V) coordination, and 50 + 5ppm, Al (IV) coordination, a secondary resonance at 0 # 5ppm of much lower intensity, coordination A1 (VI).

We will adopt, in what follows, the notion of mixed coordination A1 (IV-V) for this oxide {9 [Si2O5, Ai2O2], iSi2O5, Ai2 (OH) 3]}, or to simplify in what follows si2o5 A1202) IV-V.

The second reagent is an alkaline melilite (Ca, Na, K) 2 [(Mg, Fe2 +, Al, Si) 307]. It will be essentially characterized, under the action of an alkaline attack, by its ability to generate the formation of a sodium and / or potassium disilicate, soluble in water, (Na2, K2) (H3SiO4) 2.

The third reagent is a calcium melilite Ca2 [(Mg, Fe +, Al, Si) 3O7]. It will be essentially characterized, under the action of an alkaline attack, by its ability to generate the formation of weakly basic calcium silicate, that is to say having a Ca / Si atomic ratio of less than 1, preferably close to 0.5. This characterization will be established using X-ray Photoelectron Spectrometry (X.p.s.) and analysis of Ca2p / Si2p ratios as reported by M. Regourd, Phil. Trans.

Royal Society London, A.310, pages 85-92 (1983).

In the past, binders and cements have been proposed which have rapid hardening and are based on geopolymeric reactions, and therefore also poor in CO2 gas released.

For example, the Davidovits / Sawyer patent US 4,509,985 and its European equivalent
EP 153,097 describe geopolymeric compositions allowing the production of rapid hardening mortar. As is also described in French patent application FR 90.10958 relating to a process for obtaining a geopolymer binder allowing the stabilization, solidification and consolidation of toxic waste, filed by the applicant hereof, the most alkali silicate reactive is that in the form of potassium disilicate K2 (H3SiO4) 2 powder.

In the cost price of the geopolymeric mineral compositions described in the previous formulations, the most expensive part is that allocated to potassium disilicate K2 (H3SiO4) 2. It was therefore very important to be able to significantly reduce the cost price of this very expensive product, in order to be able to produce a geopolymer cement whose price could be comparable to that of Portland cement, without losing More fundamental advantage resulting from a process that did not generate no CO2 gas.

This is the main objective of the present invention which describes a process for obtaining a geopolymer cement, without CO2 gas emanation, said cement being obtained from a geopolymeric mineral composition, in powder, containing:
a) an alumino-silicate oxide {9iSi2O5, Al2O2i, [Si2O5, Al2 (OH) 3]}, or for
simplify in the following: (Si2O5.Al2O2) IV-V, having the cation Ai in
coordination (IV-V) as determined by the analysis spectrum in Resonance
Magnetic Nuclear MAS-NMR for 27Ai;
b) an alkaline glassy melilite of sodium and / or potassium (Ca, Na, K) 2 [(Mg, Fe2 +, Al, Si) 307];
c) a vitreous calcium melilite Ca2 [(Mg, Fe2 +, Al, Si) 3O7]
During the alkaline activation of said geopolymer cement, an alkali disilicate, sodium and / or potassium, (Na2, K2) (H3SiO4) 2 and a calcium disilicate are formed in situ.
Ca (H3SiO4) 2, such that the molar ratios between the reactive elements, (Si2O5, Al2O2) 1V-V, (Na2, K2) (H3SiO4) 2, Ca (H3SiO4) 2, are equal or between
{(Na2, K2) (H3SiO4) 2} / (Si2O5, Al2O2) IV-V 0.40 and 0.60
Ca (H3SiO4) 2 / (Si2O5, Al2O2) IV-V 0.60 and 0.40
and
{(Na2, K2) (H3SiO4) 2 + Ca (H3SiO4) 2} / (Si2O5, Al2O2) IV-V = 1.0
In the case of calcium disilicate Ca (H3SiO4) 2 and potassium disilicate
K2 (H3SiO4) 2 we have the following reports:
K2 (H3SiO4) 2i (5i2O5, Ai2O2) 1V-V between 0.40 and 0.60
Ca (H3SiO4) 2 / (Si2O5, Al2O2) IV-V between 0.60 and 0.40.

In other words the sum of the number of moles of calcium disilicate,
Ca (H3SiO4) 2, and the number of moles of potassium disilicate, K2 (H3SiO4) 2, is equal to the number of moles of alumino-silicate oxide (Si2Os, A1202). This aluminosilicate oxide (Si2Os, A1202) determines all the reaction conditions of geopolymeric mineral compositions. It will react with an alkali or alkaline earth disilicate to form, after geopolymerization, a compound {Si2O5, Al2O2, Si2O5, (K2O, CaO)}, or (K, Ca) -poly (sialate-siloxo), it is i.e. of formula between
(0.6K + 0.2Ca) (-Si-O-Al-O-Si-O-) and
(0.4K + 0.3Ca) (-Si-O-Al-O-Si-O-).

The oxide (Si2O5, Al2O2) IV-V first reacts with the most soluble disilicate which is always the alkali disilicate (Na2, K2) (H3SiO4) 2. The amount of calcium disilicate involved in the geopolymeric reaction is determined essentially by the amount of alkali disilicate. If the sum of the moles of these disilicates is greater than 1, the unreacting part will be that which is the least soluble, that is to say the calcium disilicate.

However, it is the calcium Ca ++ ions that determine the rate of hardening, making alkaline geopolymeric gels less soluble, the optimum hardening speed being reached when these Ca ++ ions are integrated into the geopolymer structure. If the amount of Na + and / or K + alkali is large, more Ca ++ ion will be needed to achieve the same rate of hardening.

On the other hand, if the amount of alkali disilicate (Na2, K2) (H3SiO4) 2 is too low, the dissolution of calcium disilicate Ca (H3SiO4) 2 will be reduced, and rapid hardening at 20 "C will not take place. more, and the mechanical resistances will be lower.

Powdered mineral compositions are also found in the prior state of the art. Thus, Heitzmann US Pat. No. 4,642,137 claims mineral compositions comprising, among other parts, a mixture containing potassium silicate and potassium hydroxide. For raptors, the term “potassium silicate” used in this Heitzmann US patent 4,642,137 means industrial potassium silicate, in powder, corresponding to the formula K20.3Si02.3H20, soluble in water and making it possible to produce binders and glues having the same properties as “soluble glasses” or alkali metal silicate in solution.

It has also been recommended to replace all of the potassium silicate in solution or in powder, with a mixture of amorphous silica (for example silica fume) and potassium hydroxide as in the second Heitzmann patent US 4,640,715 .; silica fume and other amorphous silicas can easily turn into potassium or sodium silicate at moderate or even ambient temperature. On the contrary, within the framework of the invention, a vitreous alkaline melilite (Ca, Na, K) 2 [(Mg, Fe2 +, Al, Si) 307] having an average particle size of 10 microns is first of all produced. The alkaline activation then releases, in situ, the hydrated alkali disilicate phase (Na2, K2) (H3SiO4) 2.

In order to ensure optimum reactivity of this alkaline melilite (Ca, Na, K) 2 [Mg, Fe2 +, Al, Si) 307, it is quenched, in air or in water, according to known techniques. in similar industries, such as for example the granulation of blast furnace slag, or the quenching of alkaline chips used in the manufacture of enamels.

The raw materials used to manufacture the alkaline vitreous melilite are natural alkali alumino-silicates, such as, for example, clays, micas (Muscovite, biotite), pozzolans, as well as feldspars, feldspathoids and zeolites.

Any combination of siliceous, aluminous, natural or artificial materials can also be used with industrial waste rich in alkaline products.

The process according to the present invention should not be confused with the manufacture of a compound cement in which the products mentioned above are simply added to a cement, usually Portland cement or blast furnace slag. In fact, in the context of the present invention, the natural or synthetic alkaline alumino-silicates, referred to by cement manufacturers under the terms of fly ash, pozzolans and materials with pozzolanic characters, are subjected to a heat treatment between 900au and 1300au, in order to to make alkaline vitreous melilite.

When said alkaline melilite is of the potassium type, said alkali disilicate is potassium disilicate K2 (H3SiO4) 2; geopolymer cement corresponds to the formation of a geopolymer of (Ca, K) -poly (sialate-siloxo) type of formula varying between (0.6K + 0.2Ca) (-Si-O-Al-O-Si- O-), H2O
and
(0.4K + 0.3Ca) (-Si-O-Al-O-Si-O-), H2O
When said alkaline melilite is of the sodium type, said alkali disilicate is sodium disilicate Na2 (H3SiO4) 2; geopolymer cement corresponds to the formation of a geopolymer of the (Ca, Na) -poly (sialate-siloxo) type of formula varying between (0.6Na + 0.2Ca) (- Si-O-Al-O-Si- O-), H20
and
(0.4Na + 0.3Ca) (-SiO-Al-O-Si-O-), H20
The calcium disilicate Ca (H3SiO4) 2 will be produced, in its nascent state, in the binder, after the addition of water necessary for the solubilization of the various powdered reagents.

The starting material is a basic calcium melilite, solid solution containing wollastonite Ca (SiO3), gehlenite (2CaO.A1203.SiO2), akermanite (2CaO. MgO. SiO2).

When the grains of these materials are brought into contact with an alkaline solution (NaOH or KOH), a desorption of CaO very quickly occurs such that the Ca / Si atomic ratio becomes less than 1 and tends towards 0.5 for them. basic silicates initially with a Ca / Si ratio equal to or less than 2, such as wollastonite, gehlenite, rakermanite.

Blast furnace slag mainly contains calcium melilite and is therefore very suitable. In addition, as can be seen by X-ray photoelectron spectrometry (Xps), as indicated above, this alkaline attack of the basic silicate produces a weakly basic silicate of atomic ratio.
Ca / Si = 0.5, or precisely calcium disilicate Ca (H3SiO4) 2. This process is very smooth and can be completed in 30 minutes at room temperature.

Blast furnace slag is a by-product of the metallurgical industry. Its use in geopolymer cements does not generate CO2. In 1984, in France, the total production of slag was 10 million tonnes, a figure that should be compared to the production of portland clinker for the same year in France, or 22 million tonnes. There is therefore sufficient slag to replace all or part of the requirements for portland cement.

The geopolymeric reaction used in the present invention should not be confused with the simple alkaline activation of hydraulic binders, or the action of accelerating alkali setting on portland cements and other hydraulic binders.

Indeed, the simple action of alkalis, NaOH or KOH, on portlands cements or blast furnace slag, results in the production of hydrated calcium silicates, as mentioned above. Contrary to what happens in the present invention, these hydrated silicates crystallize to form CSH, the main constituent of hydraulic calcium cements. The CSH is a monosilicate and / or a bisilicate, that is to say that the SiO4 tetrahedra which constitute it belong to the category (Qo), (Q1) and possibly (Q2). On the contrary, geopolymerization leads to the formation of SiO4 tetrahedra of type (Q4), as determined by the analysis spectrum in
MAS-NMR Nuclear Magnetic Resonance for 29Si.

In the case of blast furnace slags, the alkalis are not setting accelerators, but develop the latent hydraulicity of the slags. The hardening accelerator is in general the temperature, or the addition of portland cement, as for example in the case of the Forss patent US 4,306,912 which describes the use of a cement based on slag, portland cement or lime. and an alkaline accelerator such as NaOH and sodium and / or potassium carbonates. No mention is made of the addition of alkaline vitreous melilite producing, in situ, the alkali disilicate (Na2, K2) (H3SiO) 2.

Scientific analysis using the MAS-NMR Nuclear Magnetic Resonance for 27AI shows that hydraulic slag cements result from the hydration of calcium aluminates, silicates and silico-aluminates, with the formation of either hydrated gehlenite 2CaO.A1203 .SiO2.8H20 or hydrated calcium aluminate 4CaO.A1203.10H20 in which the Ai cation is in Ai (VI) coordination. Spectrum
MAS-NMR for 29Si show that the SiO4 tetrahedra are mostly of the (Qo), (Q 1), (Q2) type, characterizing the CSH.

Likewise, a mixture comprising the alumino-silicate oxide (Si205, A1202) IV-V, slag, and alkalis KOH, NaOH, does not constitute a geopolymeric inorganic binder according to the present invention. A mortar made with this mixture and water does not harden at 20 "C for 24 hours. This type of reaction is described in the previous patents of the applicant. Thus the Davidovits patents FR 2,464,227 and FR 2,489,290 indicate that if the oxide (S1205, Al202) 1V-V is not masked by a polysilicate solution, against the attack of strong bases KOH, NaOH, a simple poly (sialate) of the type of rHydroxysodalite is formed which precipitates without binder function.Hydroxysodalite only constitutes a binder in ceramic pastes, with very little water and when the material is compressed, as claimed in Davidovits patents FR 2,346,121 and 2,341,522.

In the context of the invention, the part of calcium melilite not transformed into calcium disilicate during the alkaline attack, or that which could not participate in the geopolymerization reaction once the K2 (H3SiO4) ratio 2 + Ca (H3SiO4) 2 on (Si2Os, A1202) 1V-V has reached the value of 1, these silicates and aluminosilicates will hydrate according to the known mechanism in force for calcium silicates constituting hydraulic cements. In addition to the geopolymer (K, Ca) (Si-O-Al-Si-O-), the formation of hydrated gehlenite, C-S-H, hydrated calcium aluminate, and other silicates of magnesia is then obtained.

Analysis by Nuclear Magnetic Resonance spectrometry will indicate, for MAS-NMR of 27Ai, the presence of both peaks corresponding to the A1 (IV) and Al (VI) coordination. In general, in the context of the present invention, the concentration of Al (IV) is 4 to 6 times greater than that of Al (VI). It can go down if in the mixture other silico-aluminous or aluminous fillers are added, but, even in this case, the ratio between the concentration of A1 (IV) on the concentration of Al (Vl) will be Al (IV) / Al (VI) 2 1.

In the MAS-NMR spectrum of 29Si, these same basic calcium silicates will lead to the presence of both SiO4 (Q4), (Qo), (Q 1), (Q2) tetrahedra. In general.

the concentration of SiO4 (Q4) tetrahedra is 4 to 6 times greater than the sum of the SiO4 (Qo) + (Ql) + (Q2) tetrahedra concentrations, and depending on the nature of the charges we will have
(Q4) / (Qo) + (Ql) + (Q2)> 1.

Depending on the technological conditions of the calcination, the synthetic alumino-silicate (Si205, A1202) IV-V may fulfill its role as described in the present invention, or it may not. Thus, if the calcination is carried out at a temperature between 550 C and 650 C, the main resonance of the MAS-NMR 27A1 spectrum is O * Sppm, indicating a deficit in coordination Ai (IV-V). If the calcination is done at a temperature above 900 C, the main resonance of the MAS-NMR 27Al spectrum is also OrSppm, also indicating a deficit in coordination Ai (IV-V). 1100 C) and higher, the calcined material contains mullite, the main resonance of the MAS-NMR 27Al spectrum of which is also 0 + 5ppm.

The ideal calcination temperatures are between 700 C and 800 C. However, even at these temperatures, the type of furnace employed will be favorable or unfavorable to the production of silico-aluminate oxide (9 [Si2O5, Al2O2], [Si2O5 , Al2 (OH) 3]} recommended in this invention Thus, the fixed furnace calcination, making it possible to maintain a sufficient water vapor pressure throughout the calcination, delivers the reactive material having the desired MAS-NMR spectrum, whereas the rapid calcination technique, as it is usually employed in cement works using a rotary kiln, leads to a coordination deficit (IV-V).

In general, the industrial products known under the names of low temperature or high temperature calcined kaolins, intended for the paper industry, and commonly called metakaolin, are not suitable in the context of the invention, for the reasons explained above. The same is true of metakaolins produced by cement manufacturers, in a rotary kiln, by rapid cooking. This is for example the case for the Heitzmannn et al. US 4,842,649, the characteristic and manufacturing method of which are identified by the US ASTM C618-85 standard on calcined natural pozzolanic materials.

The manufacture of alumino-silicate oxide (Si2Os, A1202) 1V-V is carried out by treating kaolinitic clays between 650 ° C. and 800 ° C.. Calcination at this temperature requires less than 30% energy, compared to the energy expended for the calcination of portland clinker. It generates very little CO2 if you use fossil fuels, and no CO2 at all if you use electricity. Kaolinitic sands can be used as raw material, as well as some clays containing both kaolinite, montmorionite and illite; the same applies to lateritic soils and laterites containing kaolinite. Tests carried out on pyrophilites show that these materials are suitable for geopolymerization.

Those skilled in the art will understand the advantage of having at their disposal a method making it possible to very appreciably reduce the quantity of CO 2 gas generated by the manufacture of cements. Geopolymer cements, which do not generate C02 gas, can be used alone, or with the addition of Portland cement. Thus, initially, it is very easy to conceive of the use of mixtures comprising 50 to 90 parts of portland cement and 10 to 50 parts of geopolymer cement according to the present invention.

These mixtures already reduce the quantity of CO 2 gas resulting from the calcination reaction of calcium carbonate by 10 to 50%. In these cases, since the portland cement reacts by flash-set with the alkalinity of the geopolymer cement, it will be necessary to add a retarding agent such as potassium citrate or citric acid, or any other agent used in the activation. alkaline portland cement.

Of course, various modifications can be made by those skilled in the art to the geopolymer cements and to the process which have just been described by way of example only, without departing from the scope of the invention.

Claims (8)

1) Process for obtaining a geopolymer cement, without gas emanation CO2, the said cement being obtained from a geopolymeric mineral composition, in powder form, containing the following three reactive elements: a) an alumino-silicate oxide {9 [Si2O5, Al2O2], [Si2O5, Al2 (OH) 3]}, or for simplify in the following, (Si2Os, A1202) 1V-V having the cation Ai in coordination (IV-V) as determined by the Magnetic Resonance analysis spectrum Nuclear MAS-NMR for 27AI; b) an alkaline glassy melilite of sodium and / or potassium (Ca, Na, K) 2 [(Mg, Fe2 +, Al, Si) 307] ;; c) a vitreous calcium melilite Ca2 [(Mg, Fe +, Al, Si) 3O7] 2) Method according to claim 1) characterized in that during the alkaline activation of said cement, an alkali disilicate, sodium and / or potassium, (Na2, K2) (H3SiO4) 2 and a calcium disilicate Ca (H3SiO4) 2 such that the molar ratios between the reactive elements (Si2O5 Ai2 O2) 1V-V, (Na2, K2) (H3SiO4) 2, Ca (H3SiO4) 2, are equal to or between {(Na2, K2) (H3SiO4) 2} / (Si2O5, Al2O2) IV-V 0.40 and 0.60 Ca (H3SiO4) 2i (Si2O5, Ai2O2) 1V-V 0.60 and 0.40 and {(Na2, K2) (H3SiO4) 2 + Ca (H3SiO4) 2} / (Si2O5, Al2O2) IV-V V 1.0 the hardening by alkaline activation of said geopolymer cement corresponds to the formation of a geopolymer of (Ca, Na, K) -poly (sialate-siloxo) type of formula varying between [0.6 (Na, K) + O, 2Ca] (- Si-O-Al-O-Si-O -), H2O and [0.4 (Na, K) + 0.3Ca] (- Si-O-Al-O-Si-O -), H2O 3) Method according to claim 2) characterized in that when said alkaline melilite is of the sodium type, said alkali disilicate is sodium disilicate Na2 (H3SiO4) 2; the hardening by alkaline activation of said geopolymer cement corresponds to the formation of a geopolymer of the (Ca, Na) -poly (sialate-siloxo) type of formula varying between (0.6Na + 0.2Ca) (-Si-O-AI-O-Si-O-), H2O and (0.4Na + 0.3Ca) (-Si-O-AI-O-Si-O-), H2O 4) Method according to claim 2) characterized in that when said alkaline melilite is of the potassium type, said alkali disilicate is potassium disilicate K2 (H3SiO4) 2; the hardening by alkaline activation of said geopolymer cement corresponds to the formation of a geopolymer of (Ca, K) -poly (sialate-siloxo) type of formula varying between (0.6K + 0.2Ca) (- Si-O-Al-O-Si-O -), H2O and (0.4K + 0.3Ca) (-Si-O-AI-O-Si-O-), H2O 5) Geopolymer cement, powder, obtained according to any one of claims 1) to 4) containing: a) 100 parts by weight of alumino-silicate oxide {9 [Si2O5, Ai2O2], ISi2O5, Ai2 (OH) 3]), or to simplify, (5i2O5.Ai2O2) 1V-V, having the cation Ai in coordination (IV-V) as determined by the analysis spectrum in MAS-NMR Nuclear Magnetic Resonance for 27AI, and b) 30 to 100 parts by weight of alkaline vitreous melilite (Ca, Na, K) 2 [(Mg, Fe +, Al, Si) 3O7] with an average particle size of 10 microns and c) 30 to 100 parts by weight of vitreous calcium melilite Ca2 [(Mg, Fe +, Al, Si) 3O7] with an average particle size of 10 microns. 6) Geopolymer cement obtained according to claim 5), characterized in that, after alkaline activation and hardening, the cation Ai is entirely in coordination (IV) as determined by the MAS Nuclear Magnetic Resonance analysis spectrum NMR for 27A1, and the degree of polymerization of the SiO4 tetrahedron is (Q4) as determined by the MAS-NMR spectrum for 29Si. 7) Geopolymer cement according to claim 6), characterized in that, after alkaline activation and hardening, the ratio between the concentration of coordinating Al cation (rv) and the concentration of coordinating Al cation (VI) is: Al ( IV) / Al (VI) # 1. (Q4) / (Qo) + (Q1) + (Q2) 2 1. and the ratio between the concentration of tetrahedron SiO4 (Q4) and the concentration of tetrahedron SiO4 (Qo) + (Q1) + (Q2) is: MAS-NMR for 27Ai, as determined by the Nuclear Magnetic Resonance analysis spectrum MAS-NMR for 29Si. as determined by the Nuclear Magnetic Resonance analysis spectrum 8) Compound cement, powder, containing: a) 50 to 90 parts by weight of portland cement and b) 10 to 50 parts by weight of geopolymer cement according to any one of claims 5) to 7).