Method of producing a pozzolanic material

Abstract

A method of producing a pozzolanic material for use in a low-carbon cement, comprising: providing a raw material comprising - a hydrous aluminum phyllosilicate that comprises at least 5 wt.% montmorillonite and - at least 1 wt.% of at least one high-temperature polymorph of silica, wherein the raw material is substantially free from kaolinite, calcining the raw material at a temperature of 500-900°C in order to obtain the pozzolanic material.

Description

[0001] Method of producing a pozzolanic material

[0002] The invention refers to a method of producing a poz zolanic material for use in a low-carbon cement , a poz zolanic material for use in a low-carbon cement , as well as to a hydraulic cement composition comprising the poz zolanic material .

[0003] Cement production is one of the most signi ficant sources of anthropogenic carbon dioxide ( CO2 ) emissions , accounting for approximately 7- 8 % of global CO2 emissions . This is primarily due to the fact that the production of traditional Portland cement involves the calcination of limestone ( calcium carbonate ) , a process that releases a signi ficant amount of CO2 • Furthermore , the energy-intensive nature of cement production, which often involves burning fossil fuels , contributes additional CO2 emissions .

[0004] Low-carbon cements , also known as green or sustainable cements , have been developed to address these environmental concerns . These types of cements aim to signi ficantly reduce the carbon footprint associated with cement production . This is achieved by altering the composition and manufacturing process of the cement , in order to decrease the amount of CO2 produced per unit of cement produced .

[0005] One of the main ways in which low-carbon cements achieve this reduction is by replacing some of the clinker with other materials . Examples of such substitutions include the use of industrial byproducts such as fly ash and slag, as well as naturally occurring materials like poz zolana, or calcined clays . An example of a low-carbon cement is the Limestone Calcined Clay Cement ( LC3 ) , a composite binder comprising clinker, calcined clay, limestone , and gypsum .

[0006] However, the practical implementation of low-carbon cement production based on calcined clay presents several challenges . Foremost among these is the requirement for the raw clay used to produce the calcined clay component of the cement to have a high content of kaolinite , typically no less than 40% . Unfortunately, it is not uncommon to encounter clay materials with kaolinite contents below this threshold, typically not exceeding 25% . This low percentage makes them unsuitable for the production of low-carbon cement under conventional conditions .

[0007] Kaolinite has a layered silicate structure , composed of alternating layers of tetrahedral sheets of silica and octahedral sheets of alumina linked with each other by oxygen atoms . When kaolinite is heated during the calcination process , a sequence of reactions occurs leading to the dehydroxylation ( loss of water ) and trans formation of the kaolinite into an amorphous material known as metakaolin .

[0008] Metakaolin is highly reactive in a cementitious environment . It reacts with calcium hydroxide (produced during the hydration of Portland cement ) in a process known as poz zolanic reaction, forming calcium silicate hydrates ( C-S- H) , calcium aluminate hydrates and calcium aluminate silicate hydrates . All these products contribute signi ficantly to the compressive strength of the cement . They are the main binders in concrete and responsible for the strength development . The greater the volume of binders produced, the higher the compressive strength of the resulting cementitious matrix . Therefore , a higher kaolinite content , which leads to more metakaolin, allows for more binder formation and results in higher compressive strength .

[0009] Thus , the requirement for high kaolinite content in the clay used for low-carbon cement production is tied directly to the contribution of the resulting metakaolin to the mechanical performance of the cement , particularly its compressive strength . Using clay with a lower kaolinite content without compensating for the decrease in reactivity could lead to a drop in the cement ' s mechanical performance .

[0010] Therefore , the instant invention aims at enabling the production of a low-carbon cement using clay with low kaolinite content without compromising compressive strength performance .

[0011] To solve this obj ect , the invention, according to a first aspect , provides a method of producing a poz zolanic material for use in a low-carbon cement , comprising : providing a raw material comprising

[0012] - a hydrous aluminum phyllosilicate that comprises at least 5 wt . % montmorillonite and

[0013] - at least 1 wt . % of at least one high-temperature polymorph of silica, wherein the raw material is substantially free from kaolinite , calcining the raw material at a temperature of 500- 900 ° C in order to obtain the poz zolanic material .

[0014] The invention is based on the surprising finding that by calcining a montmorillonite clay, which is substantially free from kaolinite together with at least one high-temperature polymorph of silica it is possible to obtain a cementitious material with poz zolanic reactivity that is comparable to a calcined raw clay having a kaolinite content of 35-40 wt . % . In this way, the invention expands the range of raw clays that can be used as potential raw materials for obtaining cementitious materials with high poz zolanic reactivity .

[0015] A raw material , which may be a raw material mixture , that is substantially free from kaolinite is understood to refer to a material that has a kaolinite content of less than 5 wt . % , preferably less than 4 wt . % , preferably less than 3 wt . % , preferably less than 2 wt . % , preferably less than 1 wt . % .

[0016] Montmorillonite , a member of the smectite group, is a 2 : 1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina .

[0017] The at least one high-temperature polymorph of silica preferably comprises cristobalite and / or tridymite . Preferably, the at least one high-temperature polymorph of silica comprises cristobalite and tridymite .

[0018] During calcination, the montmorillonite clay undergoes a trans formative process known as dehydroxylation . As the temperature increases , the clay loses its chemically bound water, leading to the collapse of its layered structure and the formation of an amorphous silica-alumina phase . This amorphous phase possesses a high surface area and a disordered structure , which are essential characteristics that contribute to its increased reactivity .

[0019] It has been found that a combination of a montmorillonite clay with at least one high-temperature polymorph of silica, such as cristobalite and / or tridymite , increases the poz zolanic reactivity of the material by increasing the content of reactive silica .

[0020] Reactivity is considered here as the availability of silica from the clay in a cementitious environment and the contribution of these to the formation of strength-building phases .

[0021] The reactivity can be determined by a test described in standard EN 196-2 based on ion solubility of the calcined material .

[0022] This increased reactive silica content contributes to the poz zolanic reactivity of the material . When the calcined material is used as a partial replacement for clinker in cement production, the reactive silica readily reacts with the calcium hydroxide produced during cement hydration . This poz zolanic reaction leads to the formation of additional calcium silicate hydrates ( C-S-H) , which are the primary strength-contributing compounds in the cement matrix . The higher the content of reactive silica, the more extensive the poz zolanic reaction, and consequently, the greater the contribution to the mechanical performance of the resulting low-carbon cement .

[0023] The calcination of montmorillonite can also lead to an increase of the content of reactive alumina in the calcined material . The increase in reactive alumina content can enhance the poz zolanic reactivity of the calcined material by allowing the reactive alumina to react with the calcium hydroxide produced during cement hydration, forming calcium aluminate hydrates ( C-A-H) and calcium aluminate silicate hydrates ( C-A-S-H) . These hydrates contribute to the strength development and durability of the cement matrix, complementing the role of calcium silicate hydrates (C-S-H) formed by the pozzolanic reaction of reactive silica.

[0024] According to the invention, the calcination of the raw material is performed at a temperature range of 500-900°C. This temperature range is specifically selected to achieve optimal dehydroxylation of the montmorillonite clay while preventing the recrystallization of amorphous phases into less reactive crystalline structures. The calcination process within this temperature range maximizes the formation of reactive silica and alumina phases. However, in some other embodiments of the invention, the calcination may also be carried out at a temperature of below 500°C, such as at 350- 500°C.

[0025] The raw material that is subjected to the calcination step may comprise other components in addition to the hydrous aluminum phyllosilicate and the at least one high-temperature polymorph of silica. However, in another embodiment, the raw material consists of only the hydrous aluminum phyllosilicate that comprises at least 5 wt . % montmorillonite and at least 1 wt . % of the at least one high-temperature polymorph of silica .

[0026] In a preferred embodiment of the present invention, the hydrous aluminum phyllosilicate further comprises muscovite.

[0027] Preferably, montmorillonite is present in an amount of > 20 wt.%, preferably > 30 wt.%, preferably > 40 wt.%, preferably 40-60 wt.%, based on the combined weight of the hydrous aluminum phyllosilicate and the at least one high- temperature polymorph of silica. Preferably, the at least one high-temperature polymorph of silica is present in an amount of > 10 wt . % , preferably > 20 wt . % , preferably 30-50 wt . % , based on the combined weight of the hydrous aluminum phyllosilicate and the at least one high-temperature polymorph of silica . I f the raw material comprises only one type of high-temperature polymorph of silica, said range refers to the amount of said single type of high-temperature polymorph of silica in the raw material . I f two or more types of high-temperature polymorphs of silica are present in the raw material , such as cristobalite and tridymite , said range refers to the combined amounts of the two or more high-temperature polymorphs of silica in the raw material .

[0028] In a preferred embodiment of the present invention, the raw material has a SiCy / A^Oa weight ratio greater than 4 , and more preferably greater than 5 . A higher SiOa / AlaOa ratio in the raw material favors the formation of reactive silica phases during the calcination process . As the ratio increases , there is a greater abundance of silica available for reaction, leading to the formation of a more homogeneous and highly reactive silica phase .

[0029] In a preferred embodiment of the present invention, the raw material has a SiCy content greater than 65 wt . % , and more preferably greater than 70 wt . % , as determined by X-ray fluorescence (XRF) analysis . This high SiCR content in the raw material enhances the performance of the resulting poz zolanic material and the low-carbon cement produced from it . A high SiCh content in the raw material ensures a suf ficient amount of silica available for the formation of reactive silica phases during the calcination process . As the SiCR content increases, there is a greater potential for the formation of a highly reactive silica phase.

[0030] When reference is made to X-ray fluorescence (XRF) analysis for determining the chemical (i.e. stoichiometric or elemental) composition, the following method is used. First, the powdered samples are transformed into fused bead by fusion using a Phoenix II Fluxer (XRF Scientific) equipment. Subsequently, the chemical analysis is carried out on the pellets by wavelength dispersive X-ray fluorescence analysis (WDX-XRF) using S8 TIGER XRF Wavelength Dispersive (WDX) Spectrometer by Bruker. The results of the chemical analysis for the individual elements are presented in terms of the amounts of the corresponding oxides.

[0031] The raw material may comprise any of the following mineral components, in addition to montmorillonite and cristobalite and / or tridymite: Feldspar (including microcline, anorthite, albite) , smectite clays, quartz, mica (including muscovite) , illite, oxides of iron, and other minerals (including epistilbite, clinoptilolite) .

[0032] In a preferred embodiment of the present invention, the raw material has a BET surface area greater than 35 m2 / g. This high surface area of the raw material enhances the performance of the resulting pozzolanic material and the low- carbon cement produced from it. A high BET surface area of the raw material indicates a large available surface area for chemical reactions to occur during the calcination process and subsequent cement hydration. The increased surface area promotes the formation of reactive silica and alumina phases during calcination, as there is a greater interface between the solid particles and the gas phase. This enhanced reactivity of the silica and alumina phases is beneficial for the poz zolanic reactivity of the calcined material , as they readily react with calcium hydroxide during cement hydration to form additional strength-contributing compounds , such as calcium silicate hydrates ( C-S-H) , calcium aluminate hydrates ( C-A-H) , and calcium aluminate silicate hydrates ( C-A-S-H) .

[0033] The BET (Brunauer-Emmett-Teller ) surface area is measured according to the standard ISO 9277 : 2022 .

[0034] In a preferred embodiment of the present invention, the raw material is calcined over a time period of 15-240 minutes . The calcination time period of 15-240 minutes provides suf ficient time for the raw material to undergo the necessary chemical and physical trans formations , while avoiding excessive energy consumption and potential degradation of the reactive phases .

[0035] Several methods may be used for calcining the raw material , with the choice of the method being determined by factors such as the scale of production, the speci fic nature of the raw materials , and the desired characteristics of the final product .

[0036] According to one alternative a rotary kiln calcination is carried out . This involves feeding the raw material into a rotating cylindrical furnace , where it is heated to the desired temperature range of 500- 900 ° C . The rotation of the kiln ensures thorough mixing and even heating of the material , promoting consistent and ef ficient dehydroxylation of the clay minerals . Alternatively, a vertical shaft kiln may be used for calcination . The raw material is loaded at the top of a tall , vertical furnace and heated as it descends through the kiln, undergoing the necessary chemical trans formations .

[0037] Alternatively, a fluidi zed bed calcination may be used, which is a method wherein the raw material is suspended in a rising flow of hot gas . This ensures very high heat and mass trans fer rates , providing rapid, uni form calcination .

[0038] According to a second aspect , the invention provides a poz zolanic material for use in a low-carbon cement , comprising at least 30 wt . % of an alumino-silicate mineral material and at least 50 wt . % of a SiCy mineral material and having a weight ratio of reactive SiCy to reactive AI2O3 of at least 4 . 0 , wherein the poz zolanic material is substantially free from metakaolin .

[0039] The at least 30 wt . % of an alumino-silicate mineral material and the at least 50 wt . % of a SiCy mineral material refers to the mineralogical composition of the poz zolanic material as measured by the XRD Rietveld method .

[0040] The poz zolanic material of the present invention is designed to enhance the properties of low-carbon cement by providing a highly reactive silica and alumina source that can readily react with calcium hydroxide during cement hydration . The high content of at least 50 wt . % of a SiCy mineral material promotes the formation of calcium silicate hydrates ( C-S-H) , the primary strength-contributing phase in cement . The increased formation of C-S-H leads to enhanced mechanical properties , such as compressive strength, and improved durability of the low-carbon cement even in the absence of metakaolin . In this way, the invention expands the range of suitable alumino-silicate mineral materials to those substantially free from metakaolin without compromising the performance of the cement .

[0041] Due to its content of at least 30 wt . % of an alumino-silicate mineral material , the poz zolanic material of the present invention can be produced on a wide variety of clay materials naturally occurring in various parts of the world . Preferably, the poz zolanic material of the invention is obtained by calcining a raw material comprising a hydrous aluminum phyllosilicate , such as a clay material , at an elevated temperature . During calcination, the hydrous aluminum phyllosilicate undergoes an at least partial dehydroxylation .

[0042] When the calcined material is used as a partial replacement for clinker in cement production, the reactive silica it comprises readily reacts with the calcium hydroxide produced during cement hydration . The higher the content of reactive silica, the more extensive the poz zolanic reaction, and consequently, the greater the contribution to the mechanical performance of the resulting low-carbon cement .

[0043] The calcination of a clay material can also lead to an increase of the content of reactive alumina in the calcined material . The increase in reactive alumina content can enhance the poz zolanic reactivity of the calcined material by allowing the reactive alumina to react with the calcium hydroxide produced during cement hydration, forming calcium aluminate hydrates ( C-A-H) and calcium aluminate silicate hydrates ( C-A-S-H) . These hydrates contribute to the strength development and durability of the cement matrix, complementing the role of calcium silicate hydrates (C-S-H) formed by the pozzolanic reaction of reactive silica.

[0044] Preferably, the calcination of the raw material is performed at a temperature range of 500-900°C. This temperature range is specifically selected to achieve optimal dehydroxylation of the clay while preventing the recrystallization of amorphous phases into less reactive crystalline phases. The calcination process within this temperature range maximizes the formation of reactive silica and alumina phases.

[0045] The invention is based on the surprising finding that a clay material, which is substantially free from kaolinite can be processed to obtain a cementitious material with pozzolanic reactivity that is comparable to a calcined raw clay having a kaolinite content of 35-40 wt.%, if the calcined material has a content of at least 50 wt.% of a SiCy mineral material and a weight ratio of reactive SiCy / reactive AI2O3 of at least 4.0.

[0046] A pozzolanic material that is substantially free from metakaolin is understood to refer to a material that has a metakaolin content of less than 5 wt.%, preferably less than 4 wt.%, preferably less than 3 wt.%, preferably less than 2 wt.%, preferably less than 1 wt.%.

[0047] According to a preferred embodiment of the invention, the pozzolanic material comprises at least > 60 wt.% reactive silica, which ensures that a sufficient portion of the silica is available for reacting with the calcium hydroxide produced during cement hydration, resulting in the formation of additional calcium silicate hydrates (C-S-H) . Preferably, the poz zolanic material comprises at least 1 . 0 wt . % reactive alumina, which ensures that a suf ficient portion of the alumina is available for reacting with the calcium hydroxide produced during cement hydration, forming calcium aluminate hydrates ( C-A-H) and calcium aluminate silicate hydrates ( C-A-S-H) .

[0048] Reactivity is considered here as the availability of silica and alumina, respectively, from the poz zolanic material in a cementitious environment and the contribution of these to the formation of strength-building phases .

[0049] The proportion of the reactive silica (SiO2react) based on the total amount of the poz zolanic material can be calculated using the following formula :

[0050] %Ins x %SiO2ins %SiO2react = %SiO2tot - — - -

[0051] 100 wherein,

[0052] • SiO2tot is the total silica content in the poz zolanic material determined by XRF;

[0053] • Ins is the insoluble residues determined in hydrochloric acid and potassium hydroxide following the method described in the standard EN 196-2 : 2005 ; and

[0054] • SiO2ins is the content of insoluble silica in the Ins ( insoluble residues ) determined using XRF .

[0055] The proportion of reactive alumina based on the total amount of the poz zolanic material corresponds to the % of alumina dissolved by a nitric acid and can be performed according to following method . Approximately 1 . 0010 . 01g of the test sample is accurately weighed in a Teflon crucible exactly to the nearest 0 . 0001g (mO) . The sample is moistened with a few mL of ultrapure water and dispersed using a glass rod . Under a fume hood, l OmL of concentrated 69% nitric acid is gently added to the mixture . The crucible is covered with a watch glass and heated in a water bath or on a hot plate at 100 ° C for 4 hours . After heating, the crucible is allowed to cool to room temperature . The solution is then filtered through rapid filtration filter paper, and the filtrate is collected in a l O OmL volumetric flask (Vfiask> in liter) . The flask is rinsed and volumi zed with ultrapure water . The aluminium content Almg / Lin the solution is measured using ICP-OES ( inductively coupled plasma optical emission spectroscopy) and expressed asAl2°3mg / L according to the following equation :

[0056] The proportion of reactive alumina based on the amount of the poz zolanic material is then calculated using the following equation : %Al2O3 reactive= - - - - x 100. reacnve1000 x mO

[0057] As to the nature of the alumino-silicate mineral material present in the poz zolanic material of the invention, a preferred embodiment provides that the alumino-silicate mineral material comprises at least 20 wt . -% , preferably at least 50 wt . % calcined montmorillonite , based on the total weight of said alumino-silicate mineral material .

[0058] Preferably, the SiCy mineral material comprises at least one high-temperature polymorph of silica, which is present in an amount of at least 10 wt . % based on the total weight of the pozzolanic material.

[0059] The pozzolanic material of the invention may be produced by a method according to the first aspect of the invention.

[0060] In a preferred embodiment of the present invention, the alumino-silicate mineral material further comprises calcined muscovite .

[0061] Preferably, the calcined montmorillonite is present in an amount of > 20 wt.%, preferably > 30 wt.%, preferably

[0062] > 40 wt.%, preferably 40-60 wt.%, based on the combined weight of the alumino-silicate mineral material and the at least one high-temperature polymorph of silica.

[0063] Preferably, the at least one high-temperature polymorph of silica is present in an amount of > 10 wt.%, preferably

[0064] > 20 wt.%, preferably 30-50 wt.%, based on the combined weight of the alumino-silicate mineral material and the at least one high-temperature polymorph of silica. If the pozzolanic material comprises only one type of high- temperature polymorph of silica, only said single type of high-temperature polymorph of silica is taken into consideration when calculating said combined weight. If two or more types of high-temperature polymorphs of silica are present in the pozzolanic material, such as cristobalite and tridymite, the two or more high-temperature polymorphs of silica are taken into consideration when calculating said combined weight.

[0065] In a preferred embodiment of the present invention, the pozzolanic material has a weight ratio of reactive SiC reactive AI2O3 greater than 5.0. A higher reactive SiCg / reactive AI2O3 ratio in the material favors the formation of reactive silica phases. As the ratio increases, there is a greater abundance of silica available for reaction, leading to the formation of a more homogeneous and highly reactive silica phase.

[0066] In a preferred embodiment of the present invention, the pozzolanic material has a SiCt content greater than 65 wt.%, and more preferably greater than 70 wt.%, as determined by X- ray fluorescence (XRF) analysis. This high SiCR content in the pozzolanic material enhances the performance of the material and the low-carbon cement produced from it. A high SiCR content in the pozzolanic material ensures a sufficient amount of silica available for the formation of reactive silica phases. As the SiCy content increases, there is a greater potential for the formation of a highly reactive silica phase.

[0067] In a preferred embodiment of the present invention, the pozzolanic material has a BET surface area greater than 15 m2 / g. This high surface area of the material enhances the performance of the material and the low-carbon cement produced from it. A high BET surface area of the material indicates a large available surface area for chemical reactions to occur during the cement hydration. The BET (Brunauer-Emmett-Teller ) surface area is measured according to the standard ISO 9277:2022.

[0068] According to a third aspect, the invention provides a hydraulic cement composition comprising:

[0069] - 20-80 wt . % Portland cement, - at least 10 wt . % of a poz zolanic material obtained by a method according to the first aspect of the invention or of a poz zolanic material according to the second aspect of the invention,

[0070] - optionally an additional material selected from the group consisting of ground granulated blast- furnace slag, silica fume , natural poz zolana, siliceous fly ash, calcareous fly ash, burnt shale and limestone , and recycled concrete fines .

[0071] Hence , a low-carbon cement is provided that has a reduced clinker content and that , due to the presence of the poz zolanic material of the invention, still has an acceptable compressive strength . In particular, the hydraulic cement of the invention has a compressive strength that is comparable to the compressive strength achieved in an equivalent cement composition, where the poz zolanic material has been obtained by calcining a clay having a kaolinite content of 35-40 wt . % .

[0072] The term "Portland cement" , also referred to as "Ordinary Portland Cement ( OPC ) " , refers to a type of hydraulic cement that sets , hardens , and adheres to other materials to bind them together . The primary components of Portland cement are calcium, silicon, aluminum, and iron, which are typically derived from limestone , clay, and other natural materials . These materials are crushed, combined, and heated in a kiln to a high temperature to produce clinker . This clinker is then ground into a fine powder, often with a small amount of gypsum added to control the setting time of the final product .

[0073] Therefore , the Portland cement preferably is a cement of the type "CEM I" , as defined in European standard EN 197 - 1 : 2011 . It consists of 90-97% Portland cement clinker and up to 5% of a minor additional constituent such as limestone, and contains a source of calcium sulfate such as gypsum that enhances the properties or the workability of the cement.

[0074] Preferably, the hydraulic cement composition of the invention is a cement of the type CEM II / C-M, as defined in European standard NF EN 197-5:2021. This type of cement is defined by a Portland cement content of 50-64 wt.%, a combined content of the additional material and the pozzolanic material of 36- 50 wt.% and an optional content of minor additional constituents (0-5 wt.%) .

[0075] According to a preferred embodiment of the invention, the additional material is limestone.

[0076] Preferably, the hydraulic cement of the invention is a cement of the type CEM II / C-M (Q-LL) , as defined in European standard EN 197-5:2021. This type of cement is defined by a Portland cement content of 50-64 wt . % and a combined content of limestone and the pozzolanic material of 36-50 wt . % .

[0077] Preferably, the hydraulic cement of the invention comprises 15-30 wt . % of the additional material, in particular limestone .

[0078] Preferably, the hydraulic cement of the invention comprises

[0079] 15-30 wt . % of the pozzolanic material.

[0080] Preferably, the combined amount of the additional material and the pozzolanic material is < 50 wt.%, preferably 30- 50 wt.%. Preferably, the hydraulic cement composition has the following composition:

[0081] - 40-60 wt . % of the Portland cement

[0082] - 20-30 wt . % of the pozzolanic material

[0083] - 20-30 wt . % of the additional material, preferably limestone .

[0084] As used herein, any content of Portland cement, the additional material and the pozzolanic material given in wt . % is based on the total weight of the hydraulic cement composition .

[0085] In the following, the invention will be described in more detail with reference to the following examples.

[0086] Five different raw materials were used for preparing a pozzolanic material. Table 1 indicates the mineral composition of the raw materials according to Examples 1-5. The mineralogical composition of the materials was measured by the XRD Rietveld method.

[0087] The materials "B-l", "B-2" and "B-3" are non-kaolinite clays according to the invention. The materials "K-l" and "K-2" are reference clays for comparative examples containing kaolinite. Clays according to the invention (B-l and B-2) do not contain kaolinite or muscovite in the composition, but do contain cristobalite, tridymite and montmorillonite.

[0088] Table 1

[0089] The XRD spectrum of the material "B-l" is shown in Fig. 1.

[0090] The XRD spectrum of the material "B-2" is shown in Fig. 2.

[0091] The XRD spectrum of the material "B-3" is shown in Fig. 3.

[0092] The XRD spectrum of the material "K-l" is shown in Fig. 4.

[0093] The XRD spectrum of the material "K-2" is shown in Fig. 5.

[0094] The quantitative content of kaolin in the reference clays determined by XRD using the Rietveld method is summarized in table 2 :

[0095] Table 2

[0096] The chemical composition determined using XRF for the raw clay samples is summari zed in table 3 .

[0097] Table 3

[0098] As can be seen in table 3, the clays according to the invention are characterized by a higher SiCt / A^Oa weight ratio .

[0099] The montmorillonite content of the samples according to the invention was quantified from the TGA thermogravimetric analysis (TGA) and mass balance and is summarized in table 4.

[0100] Table 4

[0101] Comments on the quantification of montmorillonite by TGA: Among the mineral phases identified in the Bentonite samples

[0102] (XRD) , only Montmorillonite is likely to lose mass by TGA: f ree-interlayer water before 220°C, structural OH in the range 350 -750°C. Due to the uncertainty of the water content of montmorillonite the % calculation is based on the quantity of OH in the molecular-water free structure (4.9%) . (Handbook of thermogravimetric system of minerals and its use in geological practice, Maria Fdldvari, Budapest 2011) .

[0103] The physical properties of raw clays are given in table 5.

[0104] Table 5

[0105]

[0106] The proportion of the reactive silica (SiO2react) based on the total amount of the raw clay is disclosed in Table 6 and was calculated using the following formula :

[0107] %Ins x %SiO2ins

[0108] %SiO2react = %SiO2tot - — - -

[0109] 100 wherein,

[0110] • SiO2tot is the total silica content in the raw clay determined by XRF;

[0111] • Ins is the insoluble residues determined in hydrochloric acid and potassium hydroxide following the method described in the standard EN 196-2 : 2005 ; and

[0112] • SiO2ins is the content of insoluble silica in the Ins ( insoluble residues ) determined using XRF .

[0113] Table 6 The raw clays were calcined at 800°C for 3h. The calcination was performed in a lab muffle furnace.

[0114] The chemical composition determined using XRF for the calcined clays is summarized in table 7.

[0115] Table 7

[0116] The XRD qualitative analysis of the calcined clays is given in table 8 .

[0117] Table 8 overlapping with peaks )

[0118] The proportion of reactive silica based on the total amount of calcined clay is disclosed in Table 9 and was determined using the aforementioned test method for the determination of the reactive silica in the raw clay . The proportion of reactive alumina based on the total amount of the calcined clay and disclosed in table 9 corresponds to the % of alumina dissolved by a nitric acid test performed according to the following method . Approximately 1 . 0010 . 01g of the test sample is accurately weighed in a Teflon crucible exactly to the nearest 0 . 0001g (mO) . The sample is moistened with a few mL of ultrapure water and dispersed using a glass rod . Under a fume hood, l OmL of concentrated 69% nitric acid is gently added to the mixture . The crucible is covered with a watch glass and heated in a water bath or on a hot plate at 100 ° C for 4 hours . After heating, the crucible is allowed to cool to room temperature . The solution is then filtered through rapid filtration filter paper, and the filtrate is collected in a l O OmL volumetric flask (Vfiask> in liter) . The flask is rinsed and volumi zed with ultrapure water . The aluminium content Almg / Lin the solution is measured using ICP-OES ( inductively coupled plasma optical emission spectroscopy) and expressed asAl2°3mg / L according to the following equation :

[0119] The proportion of reactive alumina based on the amount of calcined clay is then calculated using the following equation :

[0120] A^2 ^3ma / LXVflask

[0121] %Al2O reactive - i - x 100.

[0122] 1000 x mO

[0123] Table 9

[0124] By comparing table 6 with table 9, it can be seen that the calcination results in an increase of the proportion of reactive silica.

[0125] The physical properties of calcined clay are indicated in table 10

[0126] Table 10

[0127] Experiment 1 :

[0128] Different mortars were prepared using the calcined materials B-l, B-2, B-3, K-l and K-2 described above.

[0129] • KQL / 50:25:25 (50 wt . % CEM I 52.5 N, 25 wt . % clay,

[0130] 25 wt . % limestone) using B-l, B-2, B-3, K-l and K-2 Materials :

[0131] • CEM I 52.5 N, Usine Saint Pierre La Cour, France

[0132] • Limestone: BL200 supplied by Omya

[0133] The results of the compressive strength measurements after 28 days (28D) of the mortars described above are indicated in Table 11 and shown in Fig. 6 measured according to EN-196-1 of 2016.

[0134] Table 11

[0135] The results demonstrate that much higher compressive strength values can be achieved for mortars produced using a pozzolanic material according to the invention. The compressive strength for KQL / 50 : 25 : 25 / B-l , KQL / 50 : 25 : 25 / B-2 , and KQL / 50 : 25 : 25 / B-3 is higher than for the KQL / 50 : 25 : 25 / K-l and KQL / 50 : 25 : 25 / K-2 samples.

[0136] Experiment 2 :

[0137] Different mortars were prepared using the calcined materials B-l, B-2, B-3, K-l and K-2 described above. 75 wt.% CEM I / 25 wt.% limestone

[0138] 75 wt.% CEM I / 25 wt.% B-l

[0139] 75 wt.% CEM I / 25 wt.% B-2

[0140] 75 wt.% CEM I / 25 wt.% B-3

[0141] 75 wt.% CEM I / 25 wt.% K-l

[0142] 75 wt.% CEM I / 25 wt.% K-2

[0143] The results of the compressive strength measurements after 28 days of the mortars described above are indicated in Table 12 and shown in Fig 7.

[0144] Table 12

[0145] Again, the results demonstrate that much higher compressive strength values can be achieved for mortars produced using a pozzolanic material according to the invention. The compressive strength for CEM I / B-l, CEM I / B-2 and CEM I / B-3 is higher than for the CEM I / K-l and CEM I / K-2 samples.

Claims

Claims :

1. A method of producing a pozzolanic material for use in a low-carbon cement, comprising: providing a raw material comprising- a hydrous aluminum phyllosilicate that comprises at least 5 wt.% montmorillonite and- at least 1 wt.% of at least one high-temperature polymorph of silica, wherein the raw material is substantially free from kaolinite, calcining the raw material at a temperature of 500-900°C in order to obtain the pozzolanic material.

2. Method according to claim 1, wherein the at least one high-temperature polymorph of silica comprises cristobalite and / or tridymite.

3. Method according to claim 1 or 2, wherein the hydrous aluminum phyllosilicate further comprises muscovite.

4. Method according to claim 1, 2 or 3, wherein montmorillonite is present in an amount of > 20 wt.%, preferably > 30 wt.%, preferably > 40 wt.%, preferably 40- 60 wt.%, based on the combined weight of the hydrous aluminum phyllosilicate and the at least one high-temperature polymorph of silica.

5. Method according to any one of claims 1 to 4, wherein the at least one high-temperature polymorph of silica is present in an amount of > 10 wt.%, preferably > 20 wt.%, preferably 30-50 wt.%, based on the combined weight of thehydrous aluminum phyllosilicate and the at least one high- temperature polymorph of silica.

6. Method according to any one of claims 1 to 5, wherein the raw material has a SiCt / A^Oa weight ratio of > 4, preferably > 5.

7. Method according to any one of claims 1 to 6, wherein the raw material has a SiCy content of > 65 wt.%, preferably > 70 wt.%, determined using XRF, wherein preferably the raw material has a content of reactive silica based on the total amount of the raw material of > 50 wt.%.

8. Method according to any one of claims 1 to 7, wherein raw material has a BET surface area of > 35 m2 / g.

9. Method according to any one of claims 1 to 8, wherein the raw material is calcined over a time period of 15-240 min .

10. A pozzolanic material for use in a low-carbon cement, comprising at least 30 wt.% of an alumino-silicate mineral material and at least 50 wt.% of a SiCR mineral material and having a weight ratio of reactive SiCy / reactive AI2O3 of at least 4.0, wherein the pozzolanic material is substantially free from metakaolin.

11. Pozzolanic material according to claim 10, wherein the pozzolanic material comprises at least 60 wt.% reactive silica .

12. Pozzolanic material according to claim 10 or 11, wherein the pozzolanic material comprises at least 1.0 wt . % reactive alumina .

13. Pozzolanic material according to claim 10, 11 or 12, wherein the alumino-silicate mineral material comprises at least 20 wt.-%, preferably at least 50 wt.% calcined montmorillonite, based on the total weight of said aluminosilicate mineral material.

14. Pozzolanic material according to any one of claims 10 to 13, wherein the SiCy mineral material comprises at least one high-temperature polymorph of silica, which is present in an amount of at least 10 wt . % based on the total weight of the pozzolanic material.

15. Pozzolanic material according to claim 14, wherein the at least one high-temperature polymorph of silica comprises cristobalite and / or tridymite.

16. Pozzolanic material according to any one of claims 10 to15, wherein the alumino-silicate mineral material further comprises muscovite.

17. Pozzolanic material according to any one of claims 14 to16, wherein the calcined montmorillonite is present in an amount of > 20 wt.%, preferably > 30 wt.%, preferably> 40 wt.%, preferably 40-60 wt.%, based on the combined weight of the alumino-silicate mineral material and the at least one high-temperature polymorph of silica.

18. Pozzolanic material according to any one of claims 14 to17, wherein the at least one high-temperature polymorph ofsilica is present in an amount of > 10 wt.%, preferably> 20 wt.%, preferably 30-50 wt.%, based on the combined weight of the alumino-silicate mineral material and the at least one high-temperature polymorph of silica.

19. Pozzolanic material according to any one of claims 10 to18, wherein the pozzolanic material has a weight ratio of reactive SiCy / reactive AI2O3 of > 5.0.

20. Pozzolanic material according to any one of claims 10 to19, wherein the pozzolanic material has a SiCR content of> 65 wt.%, preferably > 70 wt.%, determined using XRF.

21. Pozzolanic material according to any one of claims 10 to20, wherein the pozzolanic material has a BET surface area of> 15 m2 / g .

22. A hydraulic cement composition comprising:- 20-80 wt.% Portland cement,- at least 10 wt.% of a pozzolanic material according to any one of claims 10 to 21, or obtained by a method according to any one of claims 1 to 9,- optionally an additional material selected from the group consisting of ground granulated blast-furnace slag, silica fume, natural pozzolana, siliceous fly ash, calcareous fly ash, burnt shale and limestone, and recycled concrete fines.

23. Hydraulic cement composition according to claim 22 having the following composition:- 40-60 wt.% of the Portland cement- 20-30 wt.% of the pozzolanic material- 20-30 wt . % of the additional material , preferably limestone .

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