OA19968A - Use of a clay for producing a pozzolanic material. - Google Patents

Abstract

The present invention relates to the use of a clay comprising: - less than 25% of kaolinite; and - at least 20% of muscovite and/or illite; - the muscovite and/or illite / kaolinite weight ratio being greater than 1; for the preparation of a pozzolanic material

Description

The présent invention relates to the use of a clay for producing a pozzolanic material.

The manufacture of hydraulic binders, and in particular that of cements, essentially consists in calcining a mixture of carefully selected and dosed raw materials, also called «raw-mix». The cooking of this raw-mix gives an intermediate product, the clinker, which, ground with possible minerai additions, will give cernent. The type of manufactured cernent dépends on the nature and proportions of the raw materials as well as the cooking method. There are several types of cements: 10 Portiand cements (which represent most of cements produced in the world), aluminous cements (or calcium aluminate), natural quick setting cements, sulfo-aluminous cements, sulfo-belitic cements and other intermediate varieties.

The most common cements are the Portiand type cements. Portiand cements are obtained from 15 Portiand clinker, obtained after clinkering at a température in the range of 1450°C from a raw-mix rich in calcium carbonate in a furnace. The production of one ton of Portiand cernent is accompanied by the émission of very large amounts of CO₂ (about 0.8 to 0.9 tons of CO₂ per ton of cernent in the case of a CEM I).

2.0 Yet, in 2014, the amount of cernent sold around the world was around 4.2 billion tons (source: French Trade Union for the Cernent Industry - SFIC). This figure, which is constantly increasing, has more than doubled in 15 years. The cernent industry is therefore today looking for a valid alternative to Portiand cernent, that is to say cements having at least the same strength and quality features as Portiand cements, but which, during their production, émit less CO₂.

During the production of clinker, the main constituent of Portiand cernent, the release of CO₂ is linked to:

up to 40% for heating the cernent kiln, in grinding and in transport;

up to 60% of so-called Chemical, or of decarbonation CO₂.

Decarbonation is a Chemical reaction that takes place when limestone, the main raw material for making Portiand cernent, is heated at high température. The limestone is then transformed into quick lime and ΟΟ₂ according to the following Chemical reaction:

CaCO₃ CaO + CO₂

To reduce the CO₂ émissions related to the production of Portiand cernent, several approaches hâve been considered so far:

adapting or modernizing cernent methods in order to maximize the efficiency of heat exchanges;

developing new «low carbon» binders such as sulfo-aluminous cements prepared from raw materials less rich in limestone and at a lower cooking température, which enables a réduction in

CO₂ émissions of about 35%;

or even more (partial) substituting clinker in cements with materials allowing to limit CO₂ émissions.

Among the above approaches, that of the (partial) substitution of clinker in cements has been the subject of many developments. Two ways were mainly explored: the substitution of clinker by limestone filler and the substitution of clinker by so-called «pozzolanic» materials

The substitution of clinker by limestone filler (that is to say an inactive material) mainly has a diluting effect and is accompanied by a significant drop in résistance, which is very problematic.

However, the substitution of clinker by active or «pozzolanic» materials is accompanied by a much 15 lesser decrease in résistances and for some of them by an increase in these.

A pozzolanic material generally désignâtes any material having «pozzolanic properties», that is to say capable of combining at room température and in the presence of water with lime or Portlandite formed during the hydration of the cernent to give hydrates with very littie solubility likely to generate 20 additional résistance in the long term.

Portland cernent is mainly constituted by two types of anhydrous phases: calcium silicates (C3S and C₂S - in which C represents CaO and S represents SiO₂ ) and calcium aluminates (C₂A and C4AF in which C represents CaO, A represents AI₂O₃ and F represents Fe₂O₃). It also contains free lime in 25 small amounts.

It is the hydration of the silicate phases which generates the résistances through the formation of hydrates of the gel type: hydrated calcium silicates C-S-H according to the following équations (unbalanced):

C₃S + H C-S-H + CH C₂S + H C-S-H + CH in which C represents CaO, S represents SiO₂ and H represents H₂O.

Portlandite «CH» is a co-product of the hydration of calcium silicates. It represents between 15 and 20 weight% of completely hydrated cernent in the case of a CEM I and does not contribute to the résistances.

The pozzolanic material is a source of amorphous and highly reactive silica and/or alumina. When mixed with cernent, it will react with portlandite to form new hydrates:

A,S + CH C-A-S-H in which A represents AI₂O₃, C represents CaO, S represents SiO₂ and H represents H₂O.

The pozzolanic reaction, slower and later, therefore enables the consumption of portlandite which does not provide résistance to form secondary or late C-S-H generally richer in alumina than C-S-H derived from silicates, generally noted C-A-S-H for this reason. Like C-S-H, C-A-S-H are barely crystallized, or not at ail, hydrates which close the porosity and generate an increase in résistance in the longer term.

At the date ofthe présent invention, different pozzolanic materials are used:

the so-called «natural» pozzolans which are volcanic rocks rich in naturally amorphous silica and alumina;

fly ash derived from the production of electricity in coal-fired power plants and essentially constituted by silica, alumina and iron oxide;

silica smoke originating from the réduction of quartz by carbon during the production of Silicon and iron/silicon alloys;

blast-furnace slag, obtained in the steel industry during the production of cast iron, almost entirely amorphous, constituted by silica, alumina but also calcium and magnésium oxide; and calcined clays which are synthetic pozzolans obtained by the calcination at 600°C of kaolinic clays.

Nevertheless, the pozzolanicity of these materials remains variable, and the résistance of construction materials prepared from these pozzolanic materials is sometimes significantly lower than that of construction materials prepared from conventional Portland cements. It therefore remains interesting to identity new pozzolanic materials enabling the préparation of construction materials having a résistance in the medium and long term comparable to that of construction materials prepared from Portland cements, while significantly limiting CO₂ émissions during their préparation.

At the date of the présent invention, the use of clay for the préparation of synthetic pozzolan by calcination is probably the strategy for reducing the carbon footprint of cements and concrètes having the greatest potential. So far, the clays used for the production of pozzolanic material are socalled «kaolinic» clays.

Kaoiinite is a clay of the formula ASH₂ where A represents alumina AI₂O3, S represents silica SiO₂ and H represents water H₂O. Its crystallographic structure is organized into sheets of silica and alumina linked together by water molécules.

When kaolinite is brought to a température from 500°C to 700°C, its water molécules are eliminated in the form of water vapor, which has several conséquences in terms of structure and reactivity, among which:

- disappearance of the organization in sheets with the conséquence of the introduction of a structural disorder (we speak of amorphization);

very signifîcant increase in the blaine spécifie surface , which can reach several tens of times that of a conventional cernent; and appearance of induced pozzolanic activityi that is to say the possibility of reacting with 10 portlandite formed during the hydration of C₃S and C₂S to form new late hydrates.

The calcination of kaolin clays makes it possible to obtain a material classically called «metakaolin». Metakaolin is weil known to one skilled in the art and in particular constitutes an addition for concrète recognized by standards in the same way as silica fume.

In her publication «Options for the future of cernent», The Indien Concrète Journal, July 2014, Vol.88, Issue 7, pages 11 to 21, Karen L. Scrivener confirms that calcined clay is one of the rare pozzolanic materials présent in sufficient quantity to meet demand. The author further makes a direct link between the amount of kaolinite présent in the clay and the compressive strength of the 20 construction material finally prepared. According to the results presented by the author, a presence of kaolinite in an amount of 35% to 40% in the clay used to préparé the pozzolanic material (by calcination) appears to be necessary to obtain an acceptable compressive strength of the construction material finally prepared.

Similariy, in its publication « Investigation of the calcined kaolinite content on the hydration of Limestone Calcined Clay Cernent (LC3) », Cernent and Concrète Research, 2018, Vol.107, pages 124-135, F. Avet has tested clays of very variable nature and mineralogical composition and concludes that there is a direct corrélation between the performance of cernent (« LC3 » type therefore containing 30% calcined clay and 15% ground limestone) and high kaolinite contents.

However, the use of pure metakaolin as a pozzolanic material requires signifîcant additions of water, thus increasing the W/C ratio in the construction material and in fact reducing the performances, in particular the mechanical performances, thereof. Furthermore, the metakaolin can only be found in limited amounts and its price is high. It can therefore only be added in limited amounts in 35 construction materials, like the addition of silica fume.

In order to target a strong substitution of cements, in particular Portland, by pozzolans in construction materials, and consequently to obtain a substantial environmental impact, it is therefore still necessary to identify materials likely to be used for the préparation of pozzolanic materials which can themselves be used in high proportion in construction materials.

Yet, it has now been found, quite surprisingly, that clays rich in muscovite and/or illite but having kaolinite contents well below 35% could, once calcined, be used as pozzolanic material in construction materials, and this in large proportions, up to 50% or more of the construction material. The construction materials thus prepared hâve long-term strength comparable to that of construction materials prepared from conventional Portland cements, and can be prepared with significantly reduced CO₂ émissions.

Thus, the présent invention relates to the use of a clay comprising:

less than 25% of kaolinite; and at least 20% of muscovite and/or illite;

the muscovite and/or illite / kaolinite weight ratio being greater than 1 ; for the préparation of a pozzolanic material.

Against ail expectations, the clays having kaolinite contents well below 35% but rich in muscovite described above can, once calcined, be used as pozzolanic material in construction materials, and in proportions of up to 25% or even 50% of the construction material. The construction materials thus prepared hâve long-term strength comparable to that of construction materials prepared from conventional Portland cements, and can be prepared with significantly reduced CO₂ émissions.

In the context of the présent invention:

the term «clay» means any natural material rich in alumina and silica, essentially consisting of silicates and phylosilicates;

the term «muscovite» means the minerai of the phylosilicate family of formula KAI₂ (AISi₃Oio) (OH, F)₂;

the term «illite» means the minerai of the family of phylosilicates of formula (K, H₃O) (Al, Mg, Fe)₂ (Si, AI)₄O₁₀ [(OH),, (H₂O)];

the term «kaolinite» means the minerai of the phylosilicate family of formula AI₂Si₂O₅(OH)₄;

the term «calcite» means a polymorphous of calcium carbonate CaCO₃;

the term «dolomite» means magnésium carbonate MgCO₃;

the term «microdine» means the minerai of the family of tectosilicates of formula KAISi₃Oa;

the term «hématite» means iron (III) oxide Fe₂O₃;

the term «amorphous phase» means the non-or poorly diffracting fraction of the material. In X-ray diffraction, only diffracting species can be identified and quantified using the Rietveld method. The comparison of this quantification and ofthe corresponding chemistry with the actual chemistry of the material makes it possible to quantify the non-diffracting fraction of the material by «différence» by using the method of internai or external standards;

the term «pozzolanic material» means any material having pozzolanic properties within the meaning of European standard NF EN 197-1, that is to say adapted to be combined at room température and in the presence of water with lime or Portlandite formed during the hydration ofthe cernent to give very poorly soluble hydrates capable of generating additional long-term résistance;

and the term «construction material» means cernent, concrète or mortar.

In the context of the présent invention, the médian diameter or d50 corresponds to the diameter below which is located 50% of the total mass of the particles of the considered sample. This can be determined by any method known to one skilled in the art, in particular by dry or wet laser granulometry.

Finally, in the context of the présent invention, the proportions expressed in% correspond to percentages by weight relative to the total weight of the considered entity.

The présent invention therefore relates to the use of a clay having the mineralogical characteristics described above for the préparation of a pozzolanic material. Preferably, the présent invention relates to the use of a clay as defined above for the préparation of a pozzolanic material, said clay v·/, having the following characteristics, selected alone or in combination:

the clay contains less than 22% of kaolinite, preferably less than 20% of kaolinite, more preferably less than 18% of kaolinite, most preferably less than 15% of kaolinite;

clay contains at least 1 % of kaolinite;

the clay contains at least 25% of muscovite and/or illite, more preferably 25% to 50% of 20 muscovite and/or illite, quite preferably 25% to 40% of muscovite and/or illite;

the muscovite and/or illite/kaolinite weight ratio in the used clay is greater than 1, most preferably greater than 2;

the clay further contains at least 1% of calcite, preferably at least 2% of calcite, most preferably from 3% to 5% of calcite;

- the clay further contains an amorphous phase containing silica, alumina and/or calcium.

Preferably, the clay contains from 20% to 50% of said amorphous phase. More preferably, the clay contains from 30% to 40% of said amorphous phase;

the clay further contains chlorite, quartz, dolomite, microcline, hématite and/or smectite; and or

- the clay contains less than 25% smectite, preferably less than 20% smectite.

The clay described above can therefore be used to préparé a pozzolanic material by calcination. Thus, the présent invention also relates to a method for preparing a pozzolanic material from the . clay described above, said method comprising the following steps:

- possible drying and then possible grinding ofthe clay;

calcination of the obtained material at a température comprised between 650°C and 900°C; and possible disagglomeration of the obtained calcined clay, for example by grinding, until a médian diameter of 10 pm to 20 pm is reached.

During the possible grinding of the clay before calcination, this is preferably carried out with a view to obtain a powder of 100% passing at 2 mm.

The calcination step can be carried out using a rotary calciner, in which it lasts about 30 to 90 minutes. However, a «flash calciner» can also be used to calcine clay to obtain a pozzolanic 5 material, in which case the calcination step is very brief (1 to 2 seconds or less). The fact that a flash calciner can be used allows considerably reducing the energy required for calcination and préparation of the pozzolanic material.

During the possible grinding of the calcined clay, this is carried out until a médian diameter less than or equal to 25 pm is reached, more preferably less than or equal to 20 pm, most preferably less than 10 orequal to 15 pm.

The present invention can be illustrated without limitation by the following examples.

Example 1 - Calcination of clay

1.1- Composition of clay

A crude clay having the mineralogical composition reported in the following Table 1 is used.

Category	Phase	% (w/w)
Clays	Muscovite/lllite	39.8
Kaolinite	14.9
Chlorite	5.6
Smectite	7.9
Carbonates	Calcite	4.1
Dolomite	5.4
Others	Quartz	12.2
Hématite	1
Albite	0.4
Anatase	2.1
Microcline	2.3
Amorphous	4.3

Table 1 - Mineralogical composition ofthe clay before calcination

The above clay has the Chemical composition (in% (w/w)) reported in Table 2 below.

SiO2	ai203	Fe2O3	CaO	Mg O	SO3	K2O	Na2O	SrO	TiO2	P2O 5	Mn O	Loss on ignition
47.79	20.94	6.16	4.24	2.90	0.08	2.75	0.26	0.02	0.99	0.08	0.04	13.83

Table 2 - Chemical composition ofthe clay before calcination

The clay used also has the physical characteristics reported in Table 3 below.

Density (in g/cm3)	2.6
Spécifie surface	Blaine (in cm2/g)	2300
BET (in m2/g)	43.9

Table 3 - Physical characteristics ofthe clay before calcination

1.2- Calcination of day

1. 2.1 - In a laboratory furnace

The clay described above is dried for 12 hours at 105°C and then ground in a ring roll mill to a médian diameter of 30 to 40 pm. The powder thus prepared is cooked in a laboratory furnace in batches of 200 g at 800°C for 1 hour with hot charging and drawing. The calcined clay thus obtained (calcined clay AC-1) is again ground slightly in a in a planetary mill (15 seconds, 700 rpm) to deagglomerate it and obtain a médian diameter of 20 pm.

1. 2.2- In a flash calciner

The clay described above is dried for 72 hours at 105°C and then crushed in a jaw crusher until 100% passing to 2 mm is obtained. The powder thus prepared is then calcined in a flash calciner at 15 625°C (calcined clay ACF-1 ), 780°C (calcined ciay ACF-2), 870°C (calcined clay ACF-3) or 875°C under atmosphère reducing agent (calcined ciay ACF-4) with an average résidence time of 1 to 2 seconds. The calcined clay thus obtained is then ground again in a vertical mill to deagglomerate it and obtain a médian diameter of 10-11 pm.

The calcined clays thus obtained are analyzed. The mineralogical composition (in % (w/w)) thereof is reported in Table 4 below.

Category	Phase	ACF-1	ACF-2	ACF-3	ACF-4
Clays	Muscovite/lllite	26.4	24.8	17.6	17.2
Kaolinite	5.8	2.5		-
Chlorite	3.1	-		-
Carbonates	Calcite	3.3	3	1.7	2.1
Dolomite	1.1	0.2		-
Others	Quartz	10.7	11.8	12.1	11.9
Hématite	1.7	1.8	1.9	1.6
Microcline	4.2	3.5	2.3	2.3
Free lime		0.5	0.5	0.4
Periclase		0.5	0.4	0.4
Amorphous	43.8	51.3	63.4	64

Table 4 - Mineralogical composition of calcined clays ACF-1 to ACF-4

Example 2 - Mortar compositions

Préparation ofmortars 1 to 16

A reference mortar (hereinafter Mortar 1) is prepared from Portland cernent OEM I 52.5 R according to standard EN 196-1. The composition of mortar 1 is as follows:

450g of CEM I 52.5 R cernent;

1350g of standardized sand; and

225g of water.

Similarly, mortars 2 to 15 are respectively prepared from a mixture:

90% of CEM I 52.5 R /10% of AC-1 (mortar 2);

80% of CEM I 52.5 R / 20% of AC-1 (mortar 3);

70% of CEM I 52.5 R / 30% of AC-1 (mortar 4);

60% of CEM I 52.5 R / 40% of AC-1 (mortar 5);

83% of CEM I 52.5 R /17% of AC-1 (mortar 6);

83% of CEM I 52.5 R / (8.5% of AC-1 and 8.5% of limestone filler) (mortar 7);

83% CEM I 52.5 R /17% limestone filler (mortar 8);

75% of CEM I 52.5 R / 25% of AC-1 (mortar 9);

75% CEM I 52.5 R / 25% limestone filler (mortar 10);

50% of CEM I 52.5 R / 50% of AC-1 (mortar 11 );

50% of CEM I 52.5 R/(33% of AC-1 and 17% of limestone filler) (mortar 12);

50% of CEM I 52.5 R / (25% of AC-1 and 25% of limestone filler) (mortar 13);

50% of CEM I 52.5 R / (17% of AC-1 and 33% of limestone filler) (mortar 14); and

50% CEM I 52.5 R / 50% limestone filler (mortar 15);

the other ingrédients and their proportions remaining unchanged.

Finally, mortar 16 is prepared from a mixture of 75% CEM I 52.5 R / 25% commercial calcined clay (Argicem®), the other ingrédients and their proportions remaining unchanged.

Mechanical strength

The mechanical strength of the mortars is measured in accordance with standard EN 196-1 on prismatic mortar test specimens 4x4x16 cm³ prepared at 20°C.

The activity index characterizes the performance of the pozzolanic material when it is used at x% substitution. It is defined as the ratio of the compressive strengths (measured as indicated hereinabove) of a cernent mortar constituted by 100-x% of a reference cernent (CEM I) and x% of the considered pozzolanic addition, and of a mortar prepared with 100% of reference cernent.

AI (%) =

CS cernent substituted at x% CS Reference

The results of the compressive strength (CS) measurements are reported in the following Tables 5 and 6.

	Mortar 1 (ref.)	Mortar 2	Mortar 3	Mortar 4	Mortar 5	Mortar 6	Mortar 7	Mortar 8
Compressive strength (MPa)	2 days	46.6	42.5	36.5	32.6	27.8	38.7	39.5	40.1
7 days	55.9	51.6	46.2	42.7	38.0	50.1	52.4	49.1
28 days	60.5	57.6	56.6	51.2	46.4	58.0	59.1	57.6
Activity index (in %)	2 days	-	91.3	78.3	69.9	59.8	83.0	84.8	86.1
7 days	-	92.3	82.6	76.3	67.9	89.6	93.7	87.8
28 days	-	95.2	93.6	84.6	76.7	95.9	97.7	95.2

Table 5 - Compressive strengths

	Mortar 9	Mortar 10	Mortar 11	Mortar 12	Mortar 13	Mortar 14	Mortar 15	Mortar 16
Compressive strength (MPa)	2 days	34.6	35.8	21.1	22.4	22.3	21.0	19.4	31.5
7 days	44.7	45.0	32.7	36.1	33.6	31.3	25.4	45.9
28 days	54.6	50.4	40.0	41.1	39.5	36.9	27.8	54.6
Activity index (in %)	2 days	74.2	76.8	45.3	48.1	47.9	45.1	41.6	67.6
7 days	80.0	80.5	58.5	64.6	60.1	56.0	45.4	82.1
28 days	90.2	83.3	66.1	67.9	65.3	61.0	46.0	90.2

Table 6 - Compressive strengths

It appears that the mortar prepared from a binary mix Portland cernent/ calcined clay in a laboratory furnace (mortars 2 to 6, 9 and 11) hâve a mechanical strength at 28 days comparable to that of the mortar prepared from the Portland cernent alone (mortar 1) and comparable or even higher than that of the mortar prepared from a mixture of Portland cement/commercial calcined clay to 25% 15 substitution. In addition, it appears that the addition of calcined clay in laboratory furnace has a dilution effect by lowering the mechanical strength at 28 days.

The results obtained for the mortars prepared from a binary mix Portland cement/limestone filler (mortar 8; 10 and 15) or from a ternary mixture of Portland cement/(calcined clay in a laboratory' furnace + limestone filler) (mortars 7 and 12 to 14) highlight the positive effect to the presence of the calcined clay on the mechanical strength at 28 days, especially at the higher degree of substitution.

Example 3 - mortar compositions

Préparation of mortars 17 to 21

As in Example 2, mortars 17 to 21 are prepared from a mixture CEM I 52.5 R/clay in the following proportions:

75% CEM I 52.5 R / 25% ACF-1 (mortar 17);

75% CEM I 52.5 R / 25% ACF-2 (mortar 18);

75% CEM I 52.5 R / 25% ACF-3 (mortar 19);

75% CEM I 52.5 R / 25% ACF-4 (mortar 20); and

55% CEM I 52.5 R / (30% ACF-4 and 15% limestone filler) (mortar 21 );

the other ingrédients and their proportions remaining unchanged.

Mechanical strength

The mechanical strength of mortars is measured on prismatic mortar test specimens 4x4x16 cm³ prepared at 20°C in accordance with standard EN 196-1.

The activity index characterizes the performance of the pozzolanic material when it is used at x% substitution. It is defined as the ratio of the compressive strengths (measured as indicated hereinabove) of a cernent mortar constituted by 100-x% of a reference cernent (CEM I) and x% of the considered pozzolanic addition, and of a mortar prepared with 100% of reference cernent.

CS cernent substituted at x%

Al (%) =--——-------CS Reference

The results of the compressive strength (CS) measurements are reported in Table 7 below.

	Mortar 1 (ref.)	Mortar 16	Mortar 17	Mortar 18	Mortar 19	Mortar 20	Mortar 21
Compressive strength (MPa)	2 days	43.9	31.5	30.8	33.7	33.4	33.5	24.7
7 days	53.6	45.9	43.7	46.4	47.5	47.5	46.0

	28 days	61.8	54.6	52.5	56.2	62.0	64.7	56.0
Activity index (in %)	2 days	-	67.6	70.2	76.8	76.1	76.3	56.3
7 days	-	82.1	81.5	86.6	88.6	88.6	85.8
28 days	-	90.2	85.0	90.9	100.3	104.7	90.6

Table 7 - Compressive strengths

It appears that the mortars prepared from a binary mixture of 75% Portland cement/25% calcined clay in a flash calciner (mortars 17 to 20) hâve a mechanical strength at 28 days comparable or even higher (depending on the calcination température) than that of the mortar prepared from Portland cernent alone (Mortar 1) or from a Portland cement/commercial calcined clay mixture.

The results obtained for the mortar prepared from a ternary mixture of Portland cement/(clay calcined in a flash calciner + limestone filler) (mortar 21) show that it is possible to reduce the amount of Portland cernent used by 45% with less 10% loss of compressive strength at 28 days.

Claims (11)

1. A use of a clay comprising:

less than 25% of kaolinite; and at least 20% of muscovite and/or illite;

the muscovite and/or illite / kaolinite weight ratio being greater than 1 ;

for the préparation of a pozzolanic material.

2. The use according to claim 1, characterized in that the clay contains less than 22% of kaolinite.

3. The use according to claim 2, characterized in that the clay contains less than 20% of kaolinite.

4. The use according to any one of claims 1 to 3, characterized in that the clay contains at least 1% of kaolinite.

5. The use according to any one of claims 1 to 4, characterized in that the clay contains at least 25% of muscovite and/or illite.

6. The use according to claim 5, characterized in that the clay contains 25% to 50% of muscovite and/or illite.

7. The use according to any one of claims 1 to 6, characterized in that the muscovite and/or illite / kaolinite weight ratio in the used clay is greater than 2.

8. The use according to any one of claims 1 to 7, characterized in that the clay further contains at least 1% calcite.

9. The use according to any one of claims 1 to 8, characterized in that the clay further contains an amorphous phase containing silica, alumina and/or calcium.

10. The use according to any one of claims 1 to 9, characterized in that the clay further contains chlorite, quartz, dolomite, microcline, hématite and/or smectite.

11. A method for preparing a pozzolanic material comprising the following steps possible drying and then possible grinding of the clay according to any of claims 1 to 10;

calcination of the material obtained at a température comprised between 650°C and 900°C;and

5 - possible disagglomeration of the calcined clay obtained until a médian diameter of 10 pm to 20 pm is reached.