Use of a marl for preparing a pozzolanic material

Abstract

The present invention relates to the use of a marl, comprising: - more than 45% (w / w) calcite; and - at least 1% (w / w) kaolinite, for the preparation of a pozzolanic material by calcination of the marl at a temperature ranging from 400°C to 950°C.

Description

[0001] USE OF MARL FOR THE PREPARATION OF A POZZOLANIC MATERIAL

[0002] The present invention relates to the use of a marl for the preparation of a pozzolanic material.

[0003] The manufacture of hydraulic binders, and in particular cements, essentially consists of calcining a mixture of carefully selected and measured raw materials, also known as "raw material". The firing of this raw material produces an intermediate product, clinker, which, when ground with possible mineral additions, produces cement. The type of cement produced depends on the nature and proportions of the raw materials as well as the firing process. There are several types of cement: Portland cements (which represent the vast majority of cements produced in the world), aluminous cements (or calcium aluminate cements), natural quick-setting cements, sulfo-aluminous cements, sulfo-belitic cements and other intermediate varieties.

[0004] The most widely used cements are Portland cements. Portland cements are made from Portland clinker, obtained after clinkerization at a temperature of around 1450°C of a raw material rich in calcium carbonate in a kiln. The production of one tonne of Portland cement is accompanied by the emission of very large quantities of CO2 (approximately 0.8 to 0.9 tonnes of CO2 per tonne of cement in the case of a CEM I).

[0005] However, in 2019, the quantity of cement sold worldwide was around 4.1 billion tonnes (source: Syndicat Français de l'Industrie Cimentière - SFIC). This figure, which is constantly increasing, has more than doubled in 15 years. The cement industry is therefore today looking for a valid alternative to Portland cement, i.e. cements with at least the same strength and quality characteristics as Portland cements, but which, during their production, release less CO2.

[0006] During the production of clinker, the main constituent of Portland cement, the release of CO2 is linked to:

[0007] - up to 40% for heating the cement kiln, grinding and transport;

[0008] - up to 60% to so-called chemical CO2, or decarbonation. Decarbonation is a chemical reaction that takes place when limestone, the main raw material for the manufacture of Portland cement, is heated to a high temperature. The limestone is then transformed into quicklime and CO2 according to the following chemical reaction:

[0009] To reduce CO2 emissions linked to the production of Portland cement, several approaches have been considered so far:

[0010] - the adaptation or modernization of cement processes in order to maximize the efficiency of heat exchanges;

[0011] - the development of new “low carbon” binders such as sulfo-aluminous cements prepared from raw materials with less limestone and at a lower firing temperature, which allows a reduction in CO2 emissions of around 35%;

[0012] - or the (partial) substitution of clinker in cements with materials that limit CO2 emissions.

[0013] Among the above approaches, the (partial) substitution of clinker in cements has been the subject of numerous developments. Two main avenues have been explored: the substitution of clinker with limestone filler and the substitution of clinker with so-called "pozzolanic" materials.

[0014] The substitution of clinker with limestone filler is limited due to its low chemical reactivity. Such a substitution therefore leads to a dilution effect which is accompanied by a significant drop in resistance for high levels of substitution.

[0015] On the other hand, the substitution of clinker by active or "pozzolanic" materials is accompanied by a much smaller reduction in resistance and for some of them by an increase in resistance.

[0016] A pozzolanic material generally refers to any material with "pozzolanic properties", i.e. capable of combining at room temperature and in the presence of water with lime or portlandite formed during the hydration of cement to give very slightly soluble hydrates capable of generating additional long-term strength. Portland cement is in fact mainly made up of two types of anhydrous phases: calcium silicates (C3S and C2S - in which C represents CaO and S represents SiC>2) and calcium aluminates (C2A and C4AF - in which C represents CaO, A represents AI2O3 and F represents Fe20s). It also contains small amounts of free lime.

[0017] It is the hydration of the silicate phases which generates the resistances thanks to the formation of gel-type hydrates: hydrated calcium silicates CSH according to the following equations (unbalanced):

[0018] C3S + H -> CSH + CH

[0019] C2S + H -> CSH + CH in which C represents CaO, S represents SiO2 and H represents H2O.

[0020] Portlandite “CH” is a co-product of the hydration of calcium silicates. It represents between 15 and 20% by mass of the fully hydrated cement in the case of CEM I.

[0021] Pozzolanic material is a source of amorphous and highly reactive silica and / or alumina. When mixed with cement, it will react with portlandite to form new hydrates: in which A represents AI2O3, C represents CaO, S represents SiO2 and H represents H2O.

[0022] The pozzolanic reaction, slower and later, therefore allows the portlandite to be consumed to form secondary or late CSHs, which are generally richer in alumina than the CSHs from silicates; they are generally referred to as CASHs for this reason. Like CSHs, CASHs are hydrates with little or no crystallization which close the porosity and generate an increase in resistance in the longer term.

[0023] At the date of the present invention, different pozzolanic materials are used:

[0024] - so-called “natural” pozzolans which are volcanic rocks rich in naturally amorphous silica and alumina;

[0025] - fly ash from electricity production in coal-fired power stations and consisting mainly of silica, alumina and iron oxide;

[0026] - silica fume from the reduction of quartz by coal 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, consisting of silica, alumina but also calcium and magnesium oxide; and

[0027] - calcined clays which are synthetic pozzolans obtained by calcining at temperatures varying from 600°C to 900°C clays, in particular kaolinitic, illitic or montmorillonitic clays.

[0028] The pozzolanicity of these materials nevertheless remains variable, and the strength of construction materials prepared from pozzolanic materials is sometimes significantly lower than that of construction materials prepared from conventional Portland cements. It therefore remains interesting to identify new pozzolanic materials allowing the preparation of construction materials with medium and long-term strength comparable to that of construction materials prepared from Portland cements, while significantly limiting CO2 emissions during their preparation.

[0029] At the date of the present invention, the use of clays for the preparation of synthetic pozzolan by calcination is probably the strategy for reducing the carbon footprint of cements and concretes with the greatest potential. However, the use of large quantities of clay will pose problems of availability of the raw material.

[0030] Alternative solutions have thus been studied, such as the use of marl for the preparation of synthetic pozzolan by calcination.

[0031] Marl is a sedimentary rock, mainly composed of a mixture of calcite (CaCOs) and clay.

[0032] In their publication "Microstructure and ore composition of Roman cements produced at defined calcination conditions", Materials Characterization, 58, 2007, pages 1217-1228, J. Weber et al. study the pozzolanic properties of calcined marls. The marls studied contain 71% and 63% calcite by weight, respectively, and are free of kaolinite. The authors note that while the (very) long-term strengths obtained with these calcined marls are satisfactory, the short-term or medium-term strengths are insufficient. Similarly, in their publication "Calcareous smectite clay as a pozzolanic alternative to kaolin", European Journal of Environmental and Civil Engineering, 2019, D. Tanner et al. also study the pozzolanic properties of calcined marls. The first marl studied contains 47% kaolinite by weight and is free of calcite. The second marl studied comprises 8% by weight of kaolinite and 25% of calcite.Beyond 20% substitution of Portland cement by these calcined marls, the short-term resistances observed for the construction material thus prepared appear insufficient.

[0033] To aim for a strong substitution of cements, particularly Portland, by pozzolans in construction materials, and consequently obtain a substantial environmental impact, it is therefore still necessary to identify materials capable of being used for the preparation of pozzolanic materials which can themselves be used in high proportions in construction materials while maintaining mechanical properties, and in particular short-term resistances compatible with the implementation and use of said construction material.

[0034] Chinese patent application CN 102 674 722 describes the use of a marl containing 85% by weight of CaCOs and 15% of white clay calcined between 1000°C and 1400°C as a cementitious material that can be used to reinforce the ruins of an earthen building. In addition to the fact that the calcined marl is not used as a pozzolanic material but as a building material in its own right, the calcination temperature of the marl does not differ sufficiently from the clinkerization temperature of Portland cements to significantly limit the quantities of CO2 emitted during the production of this material.

[0035] Now, it has now been found, quite surprisingly, that calcite-rich marls containing kaolinite can, once calcined at temperatures below 1000°C, be used as a pozzolanic material in construction materials, and this in significant proportions, up to 50% by weight or even more of the construction material while maintaining a short-term resistance of the construction material thus prepared compatible with its use. The construction materials thus prepared also have medium and long-term resistances comparable to those of construction materials prepared from conventional Portland cements, and can be prepared while significantly limiting CO2 emissions.Thus, the subject of the present invention is the use of a marl comprising: more than 45% (w / w) of calcite; and at least 1% (w / w) of kaolinite; for the preparation of a pozzolanic material by calcination of said marl at a temperature varying from 400°C to 950°C.

[0036] Surprisingly, marls with high calcite contents and containing kaolinite can, once calcined, be used as pozzolanic material in construction materials, in proportions of up to 25% by weight or even 50% by weight of the construction material while maintaining a short-term resistance of the construction material thus prepared compatible with its use. The construction materials thus prepared also have medium and long-term resistances comparable to those of construction materials prepared from conventional Portland cements, and can be prepared while significantly limiting CO2 emissions.

[0037] In the context of the present invention:

[0038] - “calcite” means a polymorph of calcium carbonate CaCOs;

[0039] - “kaolinite” means the mineral of the phylosilicate family with the formula AI2Si2O5(OH)4;

[0040] - “marl” means any sedimentary rock composed mainly of polymorphs of calcium carbonate and / or magnesium carbonate, and clay phases, in particular kaolinite, illite and / or montmorillonite phases;

[0041] - “montmorillonite” means the mineral of the phylosilicate family with the formula (Na,Ca)0.3(AI,Mg)2Si4Ow(OH)2 ■ n H2O;

[0042] - “illite” means the mineral of the phyllosilicate family of type 2:1 with potassium as an interfoliar cation, also including muscovite-type micas, of formula (K,H30)(AI,Mg,Fe)2(Si,AI)4Oio;

[0043] - "pozzolanic material" means any material possessing pozzolanic properties within the meaning of European standard NF EN 197-1, i.e. capable of combining at room temperature and in the presence of water with lime or Portlandite formed during the hydration of cement to give very slightly soluble hydrates capable of generating additional resistance in the long term; and

[0044] - "construction material" means a cement, a concrete, a mortar. In the context of the present invention, the median diameter or d50 corresponds to the diameter below which 50% of the total volume of the particles of the sample considered is found. This can be determined by any method known to those skilled in the art, in particular by dry or wet laser granulometry.

[0045] Finally, in the context of the present invention, the proportions expressed in % correspond to mass percentages relative to the total weight of the entity considered (w / w).

[0046] The present invention therefore relates to the use of a marl having the mineralogical characteristics described above for the preparation of a pozzolanic material by calcination of said marl at a temperature varying from 400°C to 950°C. Preferably, the present invention relates to the use of a marl as defined above for the preparation of a pozzolanic material, said marl having the following characteristics, chosen alone or in combination: the marl contains at least 50% (w / w) of calcite, more preferably the marl contains at least 55% (w / w) of calcite, most preferably the marl contains at least 60% (w / w) of calcite; the marl contains at least 1.5% (w / w) of kaolinite, more preferably the marl contains at least 2% (w / w) of kaolinite, most preferably the marl contains at least 2.5% (w / w) of kaolinite;the marl further contains montmorillonite, more preferably the marl contains at least 3% (w / w) of montmorillonite, most preferably the marl contains at least 5% (w / w) of montmorillonite; the marl further contains illite, more preferably the marl contains at least 3% (w / w) of illite, most preferably the marl contains at least 5% (w / w) of illite; the marl further contains quartz, more preferably the marl contains at least 3% (w / w) of quartz, most preferably the marl contains at least 7% (w / w) of quartz; the marl further contains chlorite, dolomite, microcline, albite, ankerite, orthoclase, pyrite, siderite, anatase and / or celestine; and / or the calcination of the marl is carried out at a temperature ranging from 500°C to 900°C, more preferably at a temperature ranging from 650°C to 900°C, most preferably at a temperature ranging from 650°C to 800°C.;

[0047] The marl described above can therefore be used to prepare a pozzolanic material by calcination. Thus, the present invention also relates to a process for preparing a pozzolanic material from the marl described above by calcining said marl at a temperature ranging from 400°C to 950°C, preferably from 500°C to 900°C, more preferably from 650°C to 900°C, most preferably from 650°C to 800°C. Preferably, the process according to the present invention comprises the following steps: drying and / or grinding the marl; calcining the material obtained at a temperature ranging from 400°C to 950°C, preferably from 500°C to 900°C, more preferably from 650°C to 900°C, most preferably from 650°C to 800°C; and possible deagglomeration of the calcined marl obtained, for example by grinding, until reaching a median diameter of 5 pm to 20 pm.

[0048] When the marl is ground before calcination, this is preferably done to obtain a powder of 100% passing 2 mm.

[0049] The calcination step can be carried out using a rotary calciner, in which case it lasts approximately 30 to 90 minutes. However, a "flash calciner" can also be used to calcine marl to obtain pozzolanic 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 significantly reduces the energy required for calcination and preparation of the pozzolanic material.

[0050] When the calcined marl is possibly crushed, this is carried out until a median diameter of less than or equal to 25 pm is reached, preferably less than or equal to 20 pm, and most preferably less than or equal to 15 pm.

[0051] The present invention may be illustrated in a non-limiting manner by the following examples.

[0052] Example 1 – Calcination of marl

[0053] 1.1 - Composition of the marl

[0054] A raw marl having the mineralogical composition reported in the following Table 1 is used.

[0055] Table 1 - Mineralogical composition of the marl before calcination

[0056] The above marl has the chemical composition (in % (w / w)) reported in the following Table 2.

[0057] Table 2 - Chemical composition of marl before calcination 1.2 - Calcination of marl

[0058] The marl described above is dried for 12 hours at 105°C and then ground in a ring mill to a median diameter of 30 to 40 μm. The powder thus prepared is fired in a laboratory oven in batches of 200 g at 750°C for 1 hour with hot charging and removal. The calcined marl thus obtained (calcined marl AC-1) is then lightly ground again in a planetary mill (15 seconds, 700 rpm) to deagglomerate it and obtain a median diameter of 20 μm.

[0059] 1.3 - Mineralogical composition

[0060] The calcined marl thus obtained is analyzed. Its mineralogical composition (in % (w / w)) is reported in the following table 3.

[0061] Table 3 - Mineralogical composition of a calcined marl AC-1

[0062] Example 2 - Mortar composition according to the invention

[0063] Preparation

[0064] A reference mortar (hereinafter Mortar 1) is prepared from a Portland cement CEM I

[0065] 52.5 R according to standard EN 196-1. The composition of mortar 1 is as follows:

[0066] 450g of CEM I 52.5 R cement;

[0067] 1350g of standardized sand; and

[0068] 225g of water. Similarly, mortars 2 to 4 are respectively prepared from a mixture:

[0069] 88% CEM I 52.5 R / 12% limestone filler (mortar 2);

[0070] 79% CEM I 52.5 R / 21% limestone filler (mortar 3); and

[0071] CEM I 52.5 R / 15% limestone filler / 30% AC-1 (mortar 4).

[0072] Mechanical resistance

[0073] The mechanical resistance of mortars is measured in accordance with standard EN 196-1 on prismatic mortar specimens 4x4x16 cm3 prepared at 20°C.

[0074] The performance index characterizes the performance of the pozzolanic material when used at x% substitution. It is defined as the ratio of the compressive strengths (measured as indicated above) of a cement mortar consisting of 100-x% of a reference cement and x% of the pozzolanic addition considered, and a mortar prepared with 100% of reference cement.

[0075] RC cement substituted at x% IP (%) = - RC Re —fe -rence

[0076] The results of the compressive strength (Rc) measurements are reported in the following Table 4.

[0077] Table 4 - Compressive strengths The mortar according to the invention (mortar 4) has medium and long-term compressive strengths higher than expected and acceptable performances compared to those observed for the reference CEM I (mortar 1) and compatible with the intended uses.

[0078] Furthermore, the mortar according to the invention (mortar 4) has short, medium and long term compressive strengths equivalent to those of mortar 2 (which only contains 12% filler) and significantly higher than those of mortar 3 (containing 21% filler). The addition of calcined marl therefore allows a CEM I substitution rate significantly higher than the substitution rate obtained using limestone filler alone.

Claims

CLAIMS 1. Use of a marl comprising: more than 45% (w / w) of calcite; and at least 1% (w / w) of kaolinite; for the preparation of a pozzolanic material by calcining said marl at a temperature varying from 400°C to 950°C.

2. Use according to claim 1, characterized in that the marl contains at least 50% (w / w) of calcite.

3. Use according to claim 2, characterized in that the marl at least 55% (w / w) of calcite.

4. Use according to any one of claims 1 to 3, characterized in that the marl contains at least 2% (w / w) of kaolinite.

5. Use according to claim 4, characterized in that the marl contains at least 2.5% (w / w) of kaolinite.

6. Use according to any one of claims 1 to 5, characterized in that the marl also contains montmorillonite.

7. Use according to any one of claims 1 to 6, characterized in that the marl also contains illite.

8. Use according to any one of claims 1 to 7, characterized in that the marl also contains quartz.

9. Use according to any one of claims 1 to 8, characterized in that the marl additionally contains chlorite, dolomite, microcline, albite, ankerite, orthoclase, pyrite, siderite, anatase and / or celestine.

10. Use according to any one of claims 1 to 9, characterized in that the calcination of the marl is carried out at a temperature varying from 650°C to 900°C.

11. Process for preparing a pozzolanic material by calcining a marl as defined in any one of claims 1 to 9 at a temperature varying from 400°C to 950°C.

12. Process for preparing a pozzolanic material according to claim 11, said process comprising the following steps: - drying and / or grinding of the marl according to one of claims 1 to 9; calcination of the material obtained at a temperature ranging from 400°C to 950°C; and optional deagglomeration of the calcined marl obtained until it reaches a median diameter of 10 pm to 20 pm.

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