PROCESS FOR THE PREPARATION OF CALCINED NATURAL POZZOLANA

Abstract

PROCESS FOR PREPARING CALCINED NATURAL POZZOLANA The present invention relates to a process for preparing calcined natural pozzolan, comprising a step of flash calcination of a mixture of clay schists and clay-silico-calcareous fillers, said flash calcination step being carried out at a temperature of between 650°C and 850°C. Figure for abstract: none

Description

Title of the invention: PROCESS FOR THE PREPARATION OF CALCINED NATURAL POZZOLANA

[0001] The present invention relates to a process for preparing calcined natural pozzolan.

[0002] The construction sector represents a significant part of France's economic activity. The significant role of this sector and the construction / renovation of the building stock leads to a continuous and significant consumption of natural resources, a significant release of CO2 and a significant consumption of energy. The public authorities' awareness of the need to reduce greenhouse gas emissions tends to improve the environmental and energy performance of the construction sector, both in terms of materials and the design of structures. In the construction sector, the increase in the price of raw materials, the depletion of natural resources and the administrative difficulties of opening new quarry sites tend to move towards alternative resources such as recycled aggregates or industrial co-products.In the field of industrial co-products, quarry washing fines can be considered as a resource with high potential for recovery in the field of eco-materials, due to their large volumes and their clayey nature.

[0003] Most of the aggregates used in the construction industry come from alluvial or massive quarries. To meet the quality and performance criteria of the materials and products used (concrete, treated gravel, etc.), the aggregates must only contain a small quantity of clayey fines so as not to cause adhesion problems, particularly between the cement matrix and the aggregates for concrete. Therefore, washing units clean the aggregates and remove the fines.

[0004] For alluvial quarries, the washing fines are discharged into old excavations as soon as they leave the facilities. The exploitation of a massive quarry requires mining and reworking of the natural terrain. The production of aggregates leads to the production of by-products which can be unexploited and stored like quarry washing fines.

[0005] However, these fines require significant storage capacities and can be a disadvantage in terms of the exploitation of the deposit. Indeed, the aggregate washing fines, at the outlet of the process, have a more or less significant concentration in the water (150 g / l to 750 g / l). The washing water cannot be discharged into the water table due to these significant concentrations. Consequently, settling basins make it possible to store this sludge in order to separate the water from the fines. washing. Currently, washing fines are not highly valued. The increase in aggregate production is generating significant volumes of these fines.

[0006] It is therefore necessary today, in order to preserve natural material resources, to find ways of exploiting these fine clay-silico-limestones.

[0007] The present invention aims to reduce the storage volumes of the washing sludge fines mentioned above.

[0008] The aim of the invention is therefore to propose a process allowing the recycling of quarry waste, in particular wash water containing fine clay-silico-limestone.

[0009] To this end, the invention relates to a process for preparing calcined natural pozzolan, comprising a step of flash calcination of a mixture of clay schists and clay-silico-calcareous fillers, said flash calcination step being carried out at a temperature of from 650°C to 850°C.

[0010] The process of the invention is therefore based on a heat treatment of a mixture of clay schists and clay-silico-limestone fillers, namely a mineral filler comprising in particular clay, silica and limestone, as explained below.

[0011] The temperature mentioned here corresponds to a set temperature.

[0012] The method of the invention is advantageous in particular in that it makes it possible to obtain a CO2 gain on the calcined product, and this in particular thanks to the implementation of the flash calcination step, and therefore the use of a flash kiln. In fact, the consumption to produce a calcined clay is 139 kg CO2 eq / t for the flash kiln vs 239 to 250 kg CO2 eq / t for the rotary kiln. Indeed, the use of a flash calciner makes it possible to significantly reduce the energy required for calcination. In addition, the material obtained does not require any grinding after flash calcination.

[0013] Pozzolanic materials are naturally occurring siliceous or silico-aluminous substances, or a combination thereof. Pozzolanic materials do not harden by themselves when mixed with water but, when finely ground, they react at room temperature, in the presence of water, with dissolved calcium hydroxide [Ca(OH)2] to form strength-generating calcium silicate and calcium aluminate compounds. These compounds are comparable to those formed during the hardening of hydraulic materials. Pozzolans are composed essentially of reactive silicon dioxide (SiO2) and aluminum oxide (Al2O3). The remaining part contains iron oxide (Fe2O3) and other oxides. The proportion of reactive CaO is negligible for hardening. The content of reactive SiO2 must be at least 25.0% by mass.

[0014] By "calcined natural pozzolan" is meant a material of volcanic origin, clays, schists or sedimentary rocks, thermally activated. These materials must comply with the above-mentioned specifications which correspond to standard NF EN 197-1. Calcined natural pozzolan can also be called constituent Q.

[0015] As indicated above, the constituent Q (or calcined natural pozzolan) contains more than 25% reactive silica.

[0016] According to standard NF EN 197-1, the content of reactive silicon dioxide (SiO2) corresponds to the proportion of soluble silica dioxide after treatment in hydrochloric acid (HCl) and in a boiling solution of potassium hydroxide (KOH). The quantity of reactive silica dioxide is determined by subtracting from the total content of silica dioxide (see EN 196-2) the proportion contained in the residue insoluble in hydrochloric acid and in potassium hydroxide (see EN 196-2), both being determined on a dry sample.

[0017] Thus, the present invention relates to a process which surprisingly allows, under specific conditions, the obtaining of a calcined natural pozzolan which corresponds well to the standard. It is the conditions of the heat treatment by flash calcination as mentioned above as well as the intrinsic properties of the mixture which allow a suitable product to be obtained.

[0018] The specific heat treatment implemented according to the method of the invention is a calcination with a very short residence time, namely a flash calcination. Flash calcination technology is a technology well known to those skilled in the art, which makes it possible to calcine fine particles in hot gases for a very short residence time, more particularly of the order of a second.

[0019] As indicated above, the specificity of the process of the invention is linked to a set temperature of between 650°C and 850°C. This specific range makes it possible to obtain a constituent Q corresponding to the standard.

[0020] The combination of a suitable intrinsic composition of the mixture and the flash calcination temperature makes it possible to produce a suitable Q constituent.

[0021] Preferably, the flash calcination step is carried out at a temperature of from 700°C to 820°C, and preferably from 720°C to 790°C, or from 750°C to 790°C.

[0022] The method of the invention also makes it possible to “recycle” products obtained from washing sludge, in particular loaded with clay and limestone, and which were stored in basins, as explained above.

[0023] As indicated above, the method of the invention is based on a heat treatment of a mixture of two main elements as defined above.

[0024] One of these elements is formed by clay shales.

[0025] By "clay shales" we mean rocks with a laminated structure that can easily divide (cleave) into blades or sheets. These rocks are composed predominantly of clay and silt. The essential minerals are kaolinite, illite, micas, montmorillonite and quartz, typically with a small grain size. It is notably a quarry by-product.

[0026] According to one embodiment, the mixture used in the method of the invention comprises from 30% to 90%, in particular from 60% to 80%, and preferably from 65% to 75%, by mass of clay schists relative to the total mass of said mixture.

[0027] Preferably, the clay shales used according to the invention are in the form of a powder whose particle size is less than 200 μm.

[0028] By particle size is meant in particular the average diameter of said particles.

[0029] Preferably, the particle size of this shale powder is characterized by d50 and / or by d90.

[0030] The values ​​of d50 and / or d90 are notably measured by laser granulometry in the dry and wet process.

[0031] The d50 of the particles of the aforementioned shale powder is preferably less than 50 pm, or even less than 30 pm, or even less than 20 pm. Preferably, the d50 is between 5 and 15 pm. These d50 values ​​indicate that 50% of the particles have a size less than these d50 values.

[0032] The d90 of the particles of the aforementioned shale powder is preferably less than 100 pm, preferably less than 80 pm, and preferably greater than 20 pm, or even greater than 25 pm, and preferably greater than 30 pm. Preferably, the d90 is from 25 pm to 100 pm, preferably from 30 pm to 80 pm, in particular from 31 pm to 75 pm. These d90 values ​​indicate that 90% of the particles have a size less than these d90 values.

[0033] The clay shales used in the process of the invention may comprise kaolinite.

[0034] According to one embodiment, the clay shales used according to the invention contain from 8% to 45%, in particular from 15% to 38%, for example from 15% to 35%, and preferably from 20% to 38%, preferentially from 25% to 38%, by mass of kaolinite relative to the total mass of clay shales.

[0035] The kaolinite content (Al2(Si2O5)(OH)4) can be estimated from chemical analysis by X-ray fluorescence spectrometry taking into account the presence of chlorite (Mgn i48Feo.852)((Si4.99Al3 oi) O20(OH)i6) and illite / muscovite (K2 (Al3.74Fe0.26)(Al2 Si6O20)(OH)4).

[0036] The clay shales used in the process of the invention may comprise muscovite / illite, namely a mixture of muscovite and illite.

[0037] According to one embodiment, the clay shales contain from 10% to 40%, in particular from 15% to 35%, and preferably from 15% to 30%, preferentially from 20% to 25%, by mass of muscovite / illite relative to the total mass of clay shales.

[0038] The muscovite / illite content is estimated in particular from the chemical analysis by X-ray fluorescence spectrometry taking into account the presence of chlorite and kaolinite.

[0039] The clay shales used in the process of the invention may comprise silica. Preferably, the silica is in the form of clays (kaolinite, muscovite / illite, etc.) but also in the form of quartz and chalcedony.

[0040] According to one embodiment, the clay shales contain from 45% to 65%, in particular from 47% to 62%, preferably from 50% to 62%, by mass of SiO2 relative to the total mass of clay shales.

[0041] The silica content is notably measured by chemical analysis using X-ray fluorescence spectrometry.

[0042] The clay shales used in the process of the invention may comprise sulfur in the form of pyrite or gypsum.

[0043] According to one embodiment, the clay shales contain from 0.2% to 7%, in particular from 0.8% to 6.8% by mass of SO3 relative to the total mass of clay shales, preferably from 0.5% to 1.5%.

[0044] The SO3 content is notably measured by X-ray fluorescence spectrometry.

[0045] This specific SO3 content is particularly advantageous for increasing the reactivity of clay shales at a young age.

[0046] As mentioned above, the mixture used in the process of the invention also comprises clay-silico-limestone fillers.

[0047] According to one embodiment, the mixture used in the context of the process of the invention comprises from 10% to 70%, in particular from 15% to 50%, and preferably from 25% to 40%, by mass of clay-silico-calcareous fillers relative to the total mass of said mixture.

[0048] By “clay-silico-limestone filler” is meant a mineral filler comprising in particular clay, silica and limestone.

[0049] According to one embodiment, the clay-silico-calcareous fillers are obtained from wash water from mineral material washing processes, in particular from washing calcareous materials.

[0050] These washing waters (or sludges) are loaded with “undesirable” compounds for conventional applications, also called washing fines, and correspond to the waters obtained and stored after washing the limestone aggregates.

[0051] These wash waters are therefore in the form of a liquid solution, in which different minerals are present, such as clay, limestone or silica.

[0052] According to one embodiment, these wash waters comprise:

[0053] - from 8% to 45%, in particular from 12% to 40%, and preferably from 15% to 36%, by mass of SiO2 relative to the total mass of said aqueous composition,

[0054] - from 1% to 20%, in particular from 3% to 15%, and preferably from 6.5% to 15%, by mass of A12O3 relative to the total mass of said aqueous composition, and

[0055] - from 25% to 85%, in particular from 30% to 81%, in particular from 34% to 70%, and preferably from 35% to 62%, by mass of limestone relative to the total mass of said aqueous composition.

[0056] For example, results obtained by X-ray fluorescence spectrometry and X-ray diffraction have shown that the solid fraction of these wash waters contained 36 to 78% calcium carbonate, 4 to 23% kaolinite, 4 to 17% illite / muscovite and 5% to 21% quartz. Other minerals may be present in small quantities (dolomite, goethite, anorthite, smectite, chlorite IA, interstratified illite / smectite and illite / chlorite).

[0057] According to one embodiment, the aforementioned clay-silico-calcareous fillers are obtained by a process comprising a step of filtration of the washing waters as defined above, making it possible to obtain a filter cake, followed by a step of lumping and then drying said filter cake.

[0058] At the end of this process, the charges obtained are therefore in particular in solid form, and more particularly in powder form.

[0059] The clay-silico-calcareous fillers thus obtained therefore include in particular clay in various forms, quartz, chalcedony, and limestone (or calcium carbonate).

[0060] According to one embodiment, the clay-silico-calcareous fillers are in the form of a powder whose particle size is less than 200 μm.

[0061] By particle size is meant in particular the average diameter of said particles.

[0062] Preferably, the particle size of this powder is characterized by d50 and / or by d90.

[0063] The values ​​of d50 and / or d90 are notably measured by laser diffraction granulometry which is based on the interactions between light and matter. In a laser diffraction measurement, a laser beam passes through a sample of dispersed particles and the intensity of the scattered light is measured as a function of the angle. Large particles diffract light at small angles and small particles diffract low intensities at large angles. The diffracted intensities are then analyzed to calculate the particle size using an appropriate optical model. The particle size represents the diameter of the equivalent sphere having the same volume as the particle.

[0064] The d50 of the particles of the aforementioned powder is preferably less than 50 pm, or even less than 40 pm, or even less than 35 pm. Preferably, the d50 is between 5 and 15 pm. These d50 values ​​indicate that 50% of the particles have a size less than these d50 values.

[0065] The d90 of the particles of the aforementioned powder is preferably less than 200 pm, preferably less than 180 pm, and preferably greater than 20 pm, or even greater than 30 pm, and preferably greater than 35 pm. Preferably, the d90 is from 20 pm to 75 pm, in particular from 20 pm to 50 pm. These d90 values ​​indicate that 90% of the particles have a size less than these d90 values.

[0066] According to one embodiment, the clay-silico-calcareous fillers used in the process of the invention comprise kaolinite.

[0067] According to one embodiment, the clay-silico-calcareous fillers comprise more than 8% by mass of kaolinite relative to the total mass of said fillers.

[0068] According to one embodiment, the kaolinite content in the clay-silico-limestone fillers used in the process of the invention is less than 25%, or even less than 22%, by mass relative to the total mass of said fillers.

[0069] Preferably, the clay-silico-calcareous fillers used in the process of the invention comprise from 3% to 30%, preferably from 8% to 25%, preferably from 10% to 22%, by mass of kaolinite relative to the total mass of said fillers.

[0070] According to one embodiment, the method of the invention comprises a step of measuring the kaolinite content, for example by near infrared spectrometry.

[0071] The clay-silico-calcareous fillers used in the process of the invention may also comprise silica. Preferably, the silica is in the form of clays.

[0072] The silica content is notably measured by X-ray fluorescence spectrometry.

[0073] Preferably, the clay-silico-calcareous fillers comprise more than 10% by mass of silica relative to the total mass of said fillers.

[0074] Preferably, the clay-silico-calcareous fillers comprise from 5% to 40%, in particular from 8% to 36%, and preferably from 15% to 30%, preferentially from 15% to 25%, by mass of silica relative to the total mass of said fillers.

[0075] The clay-silico-calcareous fillers used in the process of the invention may also comprise alumina or Al2O3.

[0076] The alumina content is notably measured by X-ray fluorescence spectrometry.

[0077] Preferably, the clay-silico-calcareous fillers comprise more than 3%, preferably more than 6.5%, by mass of A12O3 relative to the total mass of said fillers.

[0078] According to one embodiment, the clay-silico-calcareous fillers comprise from 1% to 20%, in particular from 3% to 15%, preferably from 6.5% to 15%, or even from 6.5% to 10%, by mass of A12O3 relative to the total mass of said fillers.

[0079] According to one embodiment, the clay-silico-calcareous fillers used in the method of the invention also includes limestone.

[0080] The limestone content is measured in particular either by X-ray diffraction (XRD) (with rutile as internal standard, Rietveld method, TOPAS software) or by calculation from chemical analysis by X-ray fluorescence spectrometry according to the equation:

[0081] • % Calcite CaCO3 = (% CaO - % Dolomite * 56 / 184) * 100 / 56

[0082] With: M(CaO) = 56 g / mol

[0083] M(CaCO3) = 100 g / mol

[0084] % Dolomite CaMg(CO3)2 = % MgO * 184 / 40

[0085] M(MgO) = 40g / mol

[0086] M(CaMg(CO3)2) = 184g / mol

[0087] Preferably, the clay-silico-calcareous fillers comprise more than 30%, preferably more than 34%, in particular more than 45% or even more than 50%, preferably more than 55%, and preferentially more than 56%, by mass of limestone (or calcium carbonate) relative to the total mass of said fillers.

[0088] According to one embodiment, the clay-silico-limestone fillers comprise from 25% to 85%, in particular from 34% to 70%, preferably from 35% to 65%, and in particular from 45% to 65%, or even from 35% to 60%, by mass of limestone relative to the total mass of said fillers.

[0089] In particular, the clay-silico-limestone fillers used in the process of the invention comprise, for example, from 48% to 62% by mass of limestone relative to the total mass of said fillers. EXAMPLES

[0090] Example 1: Preparation of clay-silico-limestone fillers

[0091] Here we use an average clay-silico-calcareous composition from the washing water of limestone aggregates from the Boulonnais Quarries.

[0092] Table 1 below indicates the chemical composition of this starting product.

[0093] [Tables 1] SiO2 18.2% A12O3 7.5% Fe2O3 2.3% CaO 35.8% MgO 1.2% Na2O 0.08% k2o 0.94% TiO2 0.38% MnO 0.03% SO3 0.14% PAF at 1000°C 32.9%

[0094] A dryer / deagglomerator was used to obtain a powdery and dry material. Its chemical composition was determined by X-ray fluorescence spectrometry, which is an analytical technique for obtaining elementary quantitative analyses. Pelleting with a hydraulic press was used for sample preparation. A specific calibration was implemented.

[0095] The mineralogical composition of the powder obtained was estimated from chemical analysis by X-ray fluorescence spectrometry (see Table 2).

[0096] [Tables2] Kaolinite (A12O3) 2(SiO2) 2(H2O) 11% Illite 3(A12O3) 6(SiO2) (K2O) 2(H2O) 8% Quartz SiO2 9% goethite FeO(OH) 3% limestone CaCO3 61% dolomite 6% rutile + anatase 0.4%

[0097] The methylene blue (MBF) value of 14 g / kg was determined in accordance with EN 933-9:2022.

[0098] The particle size distribution was determined by air jet sieving according to NF EN 933-10.

[0099] [Tables3] % of cumulative passers-by 40 pm 63 pm 125 pm 250 pm 90.0 97.2 99.6 99.81 Example 2: Preparation of clay shales

[0100] The cuttings are first carried out in order to estimate the chemical composition of the deposit. The extraction of the rock is done by blasting. When identifying a clay shale deposit, the materials are set aside and do not follow the usual process for treating limestone materials. In order to be able to use these clay shale for flash calcination, the undesirable elements (sandstone and limestone) are isolated with a shovel before crushing the blasted materials into pieces < 40 mm. The clay shale is then crushed using a grinding machine (e.g. hammer mill, etc.) in order to obtain a powder less than 125 pm.

[0101] Its chemical composition was determined by X-ray fluorescence spectrometry (bead) (see Table 4).

[0102] [Tables4] SiO2 59.8% A12O3 15.9% Fe2O3 6.1% CaO 3.6% MgO 1.2% Na2O 0.15% k2o 2.38% TiO2 0.85% p2o5 0.12% M112O3 0.05% so3 4.4% TOC 1.55% PAF at 1000°C 9.3%

[0103] Table 5 below shows the mineralogical composition of the schists estimated from chemical analysis by X-ray fluorescence spectrometry and DRX analysis (for phase identification)

[0104] [Tables5] schist Kaolinite Al2(Si2O5)(OH)4 20.1% Muscovite / illite K2(A13.74 Fe0.26) (Al2Si6O20)(OH)4 19.6% Chlorite (Mgn.us Feo.852)((Si4.99 Al3.oi)02o(OH)i6) 3.0% Anorthosite (Na0.75K0.25) (AlSi3O8) 1.9% Quartz SiO2 39.5% Calcite CaCO3 6.5% Hematite Fe2O3 6.5% Pyrite FeS2 3.3% rutile + anatase TiO2 0.9%

[0105] The methylene blue (MBF) value of 13 g / kg was determined in accordance with EN 933-9:2022.

[0106] The diameters D50 and D90 were obtained by dry laser granulometry.

[0107] [Tableauxô] Diameter D50, pm 15 Diameter D90, pm 72

[0108] Example 3: Preparation of calcined natural pozzolan

[0109] This example concerns the flash calcination of a mixture of 85% shale (from example 2) and 15% of a clay-silico-limestone charge (from example 1) at 760°C with the introduction of ultrafines from the preheating stage into the calciner (Gas temperature = 1060°C, set temperature = 760°C)

[0110] The material does not require any grinding after flash calcination. It has the following properties:

[0111] [Table 7] The diameters D50 and D90 were obtained by dry laser granulometry. Diameter D50, pm 13 Diameter D90, pm 62

[0112] Table 8 below indicates the mineralogical analysis (DRX) of the material obtained.

[0113] [Tables8] Product calcined at 760°C kaolinite 0.1% muscovite / illite 16.1% chlorite 0% orthoclase 1.3% quartz 40.1% calcite 6.3% hematite 2.7% pyrite 0% anhydrite 0.6% lime 0.8% amorphous phase 32.1%

[0114] The content of reactive SiO2 is 30%.

[0115] The activity index characterizes the performance of calcined natural pozzolan when used at x% substitution. It is expressed as the ratio of the compressive strengths of a mortar consisting of 100-x% test cement and x% calcined natural pozzolan, and a mortar prepared with test cement only.

[0116] [T ableaux9] at 2 days at 7 days at 28 days Activity index 15% substitution 0.85 0.90 0.94 Activity index 25% substitution 0.71 0.88 0.92

[0117] Example 4: Preparation of another calcined natural pozzolan

[0118] This example concerns the preparation of a calcined pozzolan from the flash calcination of a mixture of 70% shale (from example 2) and 30% of a clay-silico-limestone charge (from example 1) at 790°C with the introduction of the ultrafines from the preheating stage into the calciner (Gas temperature = 1100°C, set temperature = 790°C).

[0119] The material does not require any grinding after flash calcination. It has the following properties:

[0120] The diameters D50 and D90 were obtained by dry laser granulometry.

[0121] [TableauxlO] Diameter D50, pm 11 Diameter D90, pm 52

[0122] Table 11 below indicates the mineralogical analysis (DRX) of the material obtained.

[0123] [Tableauxll] Product calcined at 790°C kaolinite 0% muscovite / illite 15.5% chlorite 0% orthoclase 2.4% quartz 33.6% calcite 8.1% hematite 2.4% pyrite 0% anhydrite 0.7% lime 4.3% belite 0.2% amorphous phase 33.0%

[0124] The content of reactive SiO2 is 26.5%.

[0125] The activity index characterizes the performance of calcined natural pozzolan when used at x% substitution. It is expressed as the ratio of the compressive strengths of a mortar consisting of 100-x% test cement and x% calcined natural pozzolan, and a mortar prepared with test cement only.

[0126] [Tables 12] at 2 days at 7 days at 28 days Activity index 15% substitution 0.83 0.92 0.96 Activity index 25% substitution 0.76 0.86 0.92

[0127] Example 5: Preparation of other calcined natural pozzolans

[0128] This example concerns the preparation of a calcined pozzolan from the flash calcination of a mixture of 70% shale and 30% of a clay-silico-calcareous filler at two different temperatures:

[0129] - 760°C (Gas temperature = 1070°C, set temperature = 760°C)

[0130] - 790°C (Gas temperature = 1100°C, set temperature = 790°C)

[0131] None of the materials require grinding after flash calcination. They have the following properties:

[0132] The diameters D50 and D90 were obtained by dry laser granulometry.

[0133] [Tables 13] Product calcined at 760°C Product calcined at 790°C Diameter D50, pm 12 11 Diameter D90, pm 60 52

[0134] Table 14 below indicates the mineralogical analysis (DRX) of the material obtained.

[0135] [Tablesl4] Product calcined at 760°C Product calcined at 790°C kaolinite 0% 0% muscovite / illite 16.3% 15.5% chlorite 0% 0% orthoclase 3.4% 2.4% quartz 31.6% 33.6% calcite 13.3% 8.1% hematite 2.2% 2.4% pyrite 0% 0% anhydrite 0.7% 0.7% lime 3.3% 4.3% belite 0% 0.2% amorphous phase 29.2% 33.0%

[0136] The content of reactive SiO2 is 31.4% and 26.5% respectively for 760°C and 790°C.

[0137] The activity index characterizes the performance of calcined natural pozzolan when used at x% substitution. It is expressed as the ratio of the compressive strengths of a mortar consisting of 100-x% test cement and x% calcined natural pozzolan, and a mortar prepared with test cement uniquely.

[0138] [Tablesl5] at 2 days at 7 days at 28 days 760°C 790°C 760°C 790°C 760°C 790°C Activity index 15% substitution 0.79 0.83 0.89 0.92 0.97 0.96 Activity index 25% substitution 0.74 0.76 0.84 0.86 0.87 0.92

Claims

Claims

1. Process for the preparation of calcined natural pozzolan, comprising a step of flash calcination of a mixture of clay schists and clay-silico-calcareous fillers, said flash calcination step being carried out at a temperature of from 650°C to 850°C, in particular from 720°C to 820°C, and preferably from 750°C to 790°C.

2. Preparation process according to claim 1, in which the mixture comprises from 30% to 90%, in particular from 60% to 80%, and preferably from 65% to 75%, by mass of clay shale relative to the total mass of said mixture.

3. Preparation process according to claim 1 or 2, in which the clay shales contain from 8% to 45%, in particular from 15% to 38%, and preferably from 20% to 38%, preferentially from 25% to 38%, by mass of kaolinite relative to the total mass of clay shales.

4. Preparation process according to any one of claims 1 to 3, in which the clay shales contain from 10% to 40%, in particular from 15% to 35%, and preferably from 15% to 30%, preferentially from 20% to 25%, by mass of muscovite / illite relative to the total mass of clay shales.

5. Preparation process according to any one of claims 1 to 4, in which the clay shales contain from 45% to 65%, in particular from 47% to 62%, preferably from 50% to 62%, by mass of SiO2 relative to the total mass of clay shales.

6. A preparation process according to any one of claims 1 to 5, wherein the clay shales contain from 0.2% to 7% by mass of SO3 relative to the total mass of clay shales.

7. Preparation process according to any one of claims 1 to 6, in which the mixture comprises from 10% to 70%, in particular from 15% to 50%, and preferably from 25% to 40%, by mass of clay-silico-calcareous fillers relative to the total mass of said mixture.

8. Preparation process according to any one of claims 1 to 7, in which the clay-silico-calcareous fillers comprise from 3% to 30%, in particular from 8% to 25%, preferably from 10% to 22%, by mass of kaolinite relative to the total mass of said fillers.

9. Preparation process according to any one of claims 1 to 8, in which the clay-silico-calcareous fillers comprise from 5% to 40%, in particular from 8% to 36%, and preferably from 15% to 30%, mass of SiO2 relative to the total mass of said charges.

10. Preparation process according to any one of claims 1 to 9, in which the clay-silico-calcareous fillers comprise from 1% to 20%, in particular from 3% to 15%, preferably from 6.5% to 15%, or even from 6.5% to 10%, by mass of A12O3 relative to the total mass of said fillers.

11. Preparation process according to any one of claims 1 to 10, in which the clay-silico-limestone fillers comprise from 25% to 85%, in particular from 34% to 70%, and preferably from 45% to 65%, by mass of limestone relative to the total mass of said fillers.