DETERMINATION OF ACIDIFYING POTENTIAL BY INFRARED SPECTROMETRY IN NATURAL SOILS AND EXCAVATED MATERIALS

Abstract

The present invention is a rapid method for determining the acidifying potential of an excavated material for the purpose of neutralizing it. Abstract figure: Figure 1

Description

Title of the invention: DETERMINATION OF ACIDIFYING POTENTIAL BY INFRARED SPECTROMETRY IN NATURAL SOILS AND EXCAVATION MATERIALS technical field

[0001] The present disclosure falls within the field of characterizing natural soils and excavated materials, particularly those extracted by tunnel boring machine, to determine their acidifying potential and neutralize them. Prior art

[0002] During any construction on or in the ground, particularly during tunnel excavation, ground preparation is carried out by excavation. Quantities of excavated material are then extracted, depending on the scale and type of work. Typically, a tunnel boring machine used to excavate a tunnel during the construction of a subway line produces approximately 800 tonnes of excavated material per day.

[0003] The excavated materials thus extracted contain various chemical species. Some chemical species originate from the composition of the rock or sand; these are referred to as the matrix. Other chemical species are present in smaller quantities; these are referred to as trace elements. When these trace elements exhibit toxicity, they are called pollutants. There is endogenous pollution, originating from the geological environment of the extraction site, and pollution linked to local human activity at the surface. These chemical species can pose a risk to the environment when the excavated materials are stored after extraction. Before storing these excavated materials, it is therefore necessary to determine their contamination level. Determining this contamination level allows the materials to be directed to one of the three existing treatment streams. These three treatment streams are as follows: 1) Inert materials are stored in order to be recycled, 2) Slightly contaminated materials are stored in specialized landfills (whose subsoil does not allow runoff into groundwater), 3) Contaminated materials are sent to a recovery facility to recover pollutants.

[0004] The selection of the appropriate treatment process depends on the mass concentration of the pollutant in the excavated materials and the pH of the leachate obtained from the excavated material. The maximum mass concentration thresholds for pollutants to be respected are set by legislation. In France, this is Decision No. 2003 / 33 / EC of 19 December 2002 establishing criteria and procedures for the acceptance of waste materials. waste in landfills and the decrees of 30 December 2002 concerning the storage of hazardous waste and of 12 December 2014 concerning the conditions for the acceptance of inert waste [...]. According to this Decision and these decrees, the limit values ​​for the mass concentration of each of the inorganic pollutants to be detected are among the most stringent in Europe. As shown in Table 1 below, these limit values ​​are very low and highly dispersed (from 0.01 to 800 mg of inorganic pollutants per kg of dry matter).

[0005] [Tables 1] Summary of pollutants to be detected in leaching and their mass concentration limit values. Inorganic pollutants to be detected. ISDI* limit value. ISDND** limit value. ISDD*** limit value. Mass concentration in mg / kg dry matter. Antimony (Sb) 0.06 0.7 5. Arsenic (As) 0.5 2 25. Barium (Ba) 20 100 300. Cadmium (Cd) 0.04 1 5. Chromium (Cr) total 0.5 10 70. Copper (Cu) 2 50 100. Mercury (Hg) 0.01 0.2 2. Molybdenum (Mo) 0.5 10 30. Nickel (Ni) 0.4 10 40. Lead (Pb) 0.5 10 50. Selenium (Se) 0.1 0.5 7. Zinc (Zn) 4 50 200 Chlorine (as chlorides Cl-) 800 1500 25000 Fluorine (as fluoride F-) 10 150 500 Sulfur (as sulfate SO42-) 1000 20000 50000 ISDI: Inert Waste Storage Facility ISDND: Non-Hazardous Waste Storage Facility ISDD: Hazardous Waste Storage Facility * Annex II of the Order of 12.12.2014 ** Decision No. 2003 / 33 / EC of 19.12.2002 *** Annex I of the Order of 30.12.2002

[0006] The pH values ​​are presented in Table 2 below.

[0007] [Tables2] pH value to be detected in leaching ISDI limit value* ISDND limit value** ISDD limit value*** pH 7.5 < pH < 8.0 7.5 < pH < 8.0 7.5 < pH < 9.5 ISDI: Inert Waste Storage Facility ISDND: Non-Hazardous Waste Storage Facility ISDD: Hazardous Waste Storage Facility * Annex II of the Order of 12.12.2014 ** Decision No. 2003 / 33 / EC of 19.12.2002 *** Annex I of the Order of 30.12.2002

[0008] If the pH range between 7.5 and 8.0 is maintained and the mass concentration of each inorganic / organic pollutant in the excavated material is below the ISDI limit value indicated in Table 1, then the excavated material is considered inert. It can then be stored in inert waste storage facilities for recovery, for example as a construction material or for landscaping.

[0009] If the pH range between 7.5 and 8.0 is maintained and the mass concentration of at least one of the inorganic pollutants contained in the excavated material is within the ISDI and ISDND limit values ​​indicated in Table 1, then the excavated material is considered to be slightly contaminated. It can then be stored in non-hazardous waste storage facilities.

[0010] If the pH range between 7.5 and 9.5, in the case of alkaline neutralization, is maintained and the mass concentration of at least one of the inorganic pollutants contained in the excavated material is within the ISDND and ISDD limit values ​​indicated in Table 1, then the excavated material is considered contaminated. It is then stored in hazardous waste storage facilities. There, it can be decontaminated to recover and reuse the inorganic pollutants.

[0011] If the mass concentration of at least one of the inorganic pollutants contained in the excavated material exceeds the ISDD limit value indicated in Table 1, then the excavated material is considered highly contaminated. It is then stored in facilities specifically dedicated to its decontamination, recovery, and reuse of the inorganic pollutants.

[0012] The procedure for determining the mass concentration of inorganic pollutants in excavation sludge is defined by national standards. In France, these are the French standards NF EN 12457-2 (December 1, 2002) and NF EN 16192 (March 1, 2020). According to these standards, the quantity of pollutant in excavation sludge is determined by a physicochemical analysis of the composition of the leachate obtained after simulated leaching of said excavation sludge. According to these French standards, leaching (a well-known water treatment that leads to the dissolution of soluble species) of the excavation sludge is simulated for 24 hours at ambient temperature (20°C ± 5°C). Then the leachate (residual liquid from the leaching) is analyzed to determine the mass concentration of inorganic pollutants in said leachate.This mass concentration is representative of the mass concentration of inorganic pollutants present in excavation sludge.

[0013] Iron pyrite, with the chemical formula FeS2, is a naturally occurring chemical compound found in soils. During the excavation of materials from tunnel boring machines, iron pyrite can oxidize upon contact with air and water to form sulfuric acid. This formation of sulfuric acid is problematic because it lowers the pH of the leachate, potentially changing the classification of an excavated material from "inert" to "slightly contaminated" or even "contaminated".

[0014] To correct this problem, it is possible to neutralize the excavated material by adding a weak base buffer, such as a carbonate, which are commonly and naturally present in excavated materials.

[0015] For this neutralization to be effective, it is necessary to precisely quantify the mass concentration of iron pyrite and the mass concentration of weak base buffer of the excavation material in order to then determine the quantity of weak base buffer to add to the excavation material.

[0016] Currently, the NF EN 15875 standard (December 1, 2011) defines the following potentials: AP: Acid generation potential (expressed in H+ mol / kg), NP: Neutralization potential (expressed in H+ mol / kg), and NPR: Neutralization potential ratio.

[0017] These potentials can be calculated according to the following formulas: = XX WCQ3, and NPR = where Mpyr is the molar mass of iron pyrite (119 g / mol), wpyr is the mass percentage content of iron pyrite in the excavated material. n is the number of mol of H+ that reacts with the carbonate ion, i.e. n = 2, MCo3 is the molar mass of the carbonate ion CO32, i.e. MCcb = 60 g / mol, and wCO3 is the mass percentage content of carbonate ion in the excavated material.

[0018] Only iron pyrite is considered for calculating the acid generation potential, AP, in the formula above. This assumption stems from the fact that iron pyrite is commonly considered the main acidifying species in geology due to its predominance compared to other acidifying species and its extremely acidifying effect.

[0019] Similarly, only the carbonate ion is considered for calculating the neutralization potential, NP, in the formula above. This assumption stems from the fact that carbonates, minerals characterized by the carbonate ion CO32, are commonly considered the principal basic species in geology due to their predominance compared to other basic species.

[0020] Currently, the mass content of iron pyrite in the excavated material is determined by X-ray fluorescence spectrometry of leachates obtained by rapid EDGE extraction of the excavated material. X-ray fluorescence spectrometry allows the sulfate content in the leachate to be determined. From these concentrations, it is possible to determine the iron pyrite content by considering that all the sulfur in the sulfate is present as iron pyrite in the excavated material.

[0021] However, the Applicant noted that this method of determination is not precise enough because the results are obtained with a large standard deviation. The Applicant is of the opinion that the imprecision of this method is due to the fact that: - that sulfur may be in the form of gypsum, anhydride, sulfurous organic matter, thiosulfates, sulfites and / or sulfates in the excavated material, and - that these forms cannot be neglected even if they are largely in the minority. The X-ray fluorescence spectrometry method for determining leachate can lead to underestimating the iron pyrite content and therefore to treating the excavated material with an incorrect amount of low-level buffer. This poses an environmental problem because iron pyrite can form sulfuric acid if the amount of buffer incorporated into the excavated material is too low. This poses an economic problem if the amount of buffer incorporated into the excavated material is too high.

[0022] Furthermore, this analytical method requires sensitive and complex equipment that cannot be located directly on the construction site because it is incompatible with construction site activities (vibration, dust). The laboratory housing this sensitive equipment is therefore generally located far from the construction site. This method of analysis is therefore lengthy, taking about ten days, and can be complicated to carry out.

[0023] There is therefore a need for a method of determining the mass content of iron pyrite and carbonate ion CO32 in an excavation material which is accurate, fast and simple to carry out.

[0024] It is to the Applicant's credit that they have developed such a method of determination. Summary

[0025] A method for analyzing an excavated material is proposed, comprising the following steps: a) analysis by infrared spectroscopy of a sample of excavated material in a range of wavenumbers between 4000 cm1 and 350 cm1, in particular between 1600 cm1 and 370 cm1, more particularly between 500 cm1 and 400 cm1 to obtain an infrared spectrum, b) determination of the mass content of iron pyrite in said sample from the infrared spectrum obtained in step a).

[0026] Advantageously, step a) of the method of the present invention makes it possible to obtain an infrared spectrum that may include an absorption band around 440 cm⁻¹ which is specific to iron pyrite. By determining the extent of this absorption band and comparing it to a calibration curve previously obtained from the spectroscopic analysis of samples whose mass content of iron pyrite is known, it is then possible to determine precisely, and with a low or even zero standard deviation, the mass content of iron pyrite in the sample of the excavated material.

[0027] The analytical method of the present invention therefore makes it advantageous to determine the mass content of iron pyrite with a precision superior to the X-ray fluorescence spectrometry method of leachates obtained by rapid EDGE extraction of the excavation material.

[0028] Moreover, this analysis is advantageously quick and easy to perform. Indeed, infrared spectrometers are compact and robust devices that can be used on a construction site and can perform step a) of the analysis in a few seconds.

[0029] In addition, the analysis method of the present invention makes it possible to determine quickly, simply and accurately the mass content of carbonate ion present in the excavation material.

[0030] It is then possible to neutralize, if necessary, the excavation material by adding a precise quantity of weak base buffer to the excavation material.

[0031] Thus, a method for neutralizing a material is also proposed. excavation including the following step: d) adding a small amount of base buffer to an excavated material to be neutralized to obtain a neutralized excavated material, in which The mass content of iron pyrite in the excavated material to be neutralized was determined by the analytical method as defined above. The mass content of carbonate ions in the excavated material to be neutralized was determined by the analytical method as defined above, and the amount of weak base buffer added to the excavation material to be neutralized is adjusted - in relation to the mass content of iron pyrite in the excavated material to be neutralized, - in relation to the mass content of carbonate ions in the excavated material to be neutralized, and - such that the neutralization potential ratio, NPR, of the neutralized excavation material is equal to a predetermined value, for example 4.

[0032] Advantageously, this treatment method is ecological and economical because it uses the appropriate amount of low base buffer. Brief description of the drawings

[0033] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analysis of the accompanying drawings, on which: Fig. 1

[0034] [Fig.1] shows an infrared spectrum of a sample of excavated material undergoing the analysis method according to the invention. Fig. 2

[0035] [Fig.2] shows part of the infrared spectrum of [Fig.1]. Description of the implementation methods

[0036] According to a first object of the invention, a method for analyzing an excavation material is proposed, comprising the following steps: a) analysis by infrared spectroscopy of a sample of excavated material in a range of wavenumbers between 4000 cm1 and 350 cm1, in particular between 1600 cm1 and 370 cm1, more particularly between 500 cm1 and 400 cm1 to obtain an infrared spectrum, b) determination of the mass content of iron pyrite in said sample from the infrared spectrum obtained in step a).

[0037] For the purposes of this application, "infrared spectroscopy" means a chemical analysis technique based on the absorption of light by most molecules in the infrared region of the electromagnetic spectrum, and by converting this absorption into molecular vibration. This absorption corresponds specifically to the bonds present in the molecule. With a spectrometer, this absorption of infrared radiation by the sample material is measured as a function of wavenumbers. The result is an infrared spectrum comprising one or more absorption bands specific to the molecules present in the analyzed sample.

[0038] For the purposes of this application, "excavated material" means any material excavated during civil engineering or construction work, whether on the Earth's surface, for example during excavations or foundation work, or underground, for example during the digging of tunnels, caverns, and galleries. Typically, excavated material includes: - loose rocks such as gravel, sand, silt, clay and mixtures thereof; - crushed rocks; - materials originating from previous buildings or polluted sites such as landfills; or - excavation sludge.

[0039] According to one embodiment, the excavation material is an excavation mud.

[0040] Typically, excavation mud can be produced by a tunnel boring machine excavating a tunnel during the construction of a subway line, a train line, a road.

[0041] According to a particular embodiment, the excavation mud is extracted from the Parisian subsoil.

[0042] The excavated material used in step a) may have a dry matter content, or dryness, of 70% to 100%, in particular 75% to 90%, especially 78% to 82%.

[0043] For the purposes of this application, the "dry matter content" is the ratio between the dry mass of excavated material and the mass of the excavated material before drying, the dry mass of the excavated material being measured after drying approximately 30 grams of excavated material for 30 minutes at 145°C.

[0044] Advantageously, using an excavation material with such a dry matter content makes it possible to obtain an infrared spectrum whose resolution allows for the rapid and easy identification of the specific absorption band of iron pyrite.

[0045] Drying excavated material is a standard step known to those skilled in the art. They will therefore be able to implement a drying step for the excavated material to obtain the dry matter content described above.

[0046] The excavated material used in step a) may have a particle size distribution such that 35% by mass of the excavated material has a diameter of less than 63 µm, in particular 95% by mass of the excavated material has a diameter of less than 125 µm, in particular 100% by mass of the material the excavation has a diameter of less than 200°pm.

[0047] Advantageously, using an excavation material with such a grain size makes it possible to obtain an infrared spectrum whose resolution allows for the rapid and easy identification of the specific absorption band of iron pyrite.

[0048] The particle size distribution of the excavated material can be determined by sieving.

[0049] This particle size distribution can be obtained by manual or automatic grinding, in particular manual grinding, more specifically manual grinding in an agate mold. Manual grinding is advantageously suited to the varying hardness of the components of the excavated material.

[0050] Step b) of the analysis method may include the following substeps: 1) detection between 450 cm⁻¹ and 435 cm⁻¹ of a specific absorption band of iron pyrite in the infrared spectrum obtained in step a), 2) integration of the absorption band detected in step 1) to calculate an area, and b3) comparison obtained in step b2) with a calibration curve to determine the mass content of iron pyrite in the excavated material.

[0051] Advantageously, the wavenumber range of substep b1) allows for the reproducible and precise calculation of the specific absorption band of iron pyrite during step b2). From this area, it is then possible to determine, reproducibly and precisely, the mass content of iron pyrite in the sample of excavated material during step b3).

[0052] These substeps bl) to b3) can allow the mass content of iron pyrite in the excavated material to be determined quickly, precisely and reproducibly.

[0053] The analysis method of the present invention also makes it possible to determine precisely the mass content of carbonate ion present in the excavation material.

[0054] Thus, the analytical method may include a step c) of determining the mass content of carbonate ion in said sample from the infrared spectrum obtained in step a).

[0055] For example, step c) of the method of the present invention may include the following substeps: cl) detection between 1550 cm 1 and 1180 cm 1 of a specific absorption band of the carbonate ion in the infrared spectrum obtained in step a), c2) integration of the absorption band detected in step c1) to calculate an area, and c3) comparison to be made obtained in step c2) with a calibration curve to determine the mass content of carbonate ion of the excavated material.

[0056] Advantageously, the range of wavenumbers in substep cl) allows for the reproducible and precise calculation of the specific absorption band of the carbonate ion during step c2). From this area, it is then possible to determine, in a reproducible and precise manner, the mass content of the carbonate ion of the excavated material during step c3).

[0057] These substeps cl) to c3) can therefore allow the mass content of carbonate ions in the excavated material to be determined quickly, precisely and reproducibly.

[0058] Thanks to the precise determination of the mass content of iron pyrite and the mass content of carbonate ions in the excavated material by the analytical method of the present invention, it is possible to: - accurately calculate the acid generation potential of the excavated material, AP, - accurately calculate the neutralization potential of the excavated material, NP, then - determine precisely the amount of weak base buffer to add to the excavation material to be neutralized so that the neutralization potential ratio, NPR, is equal to a predetermined value.

[0059] Thus, according to another object of the invention, a method for neutralizing an excavation material is proposed, comprising the following step: d) adding a small amount of base buffer to an excavated material to be neutralized in order to obtain a neutralized excavated material, in which The mass content of iron pyrite in the excavated material to be neutralized was determined by the analytical method as defined above. The mass content of carbonate ions in the excavated material to be neutralized was determined by the analytical method as defined above, and the amount of weak base buffer added to the excavation material to be neutralized is adjusted - in relation to the mass content of iron pyrite in the excavated material to be neutralized, - in relation to the mass content of carbonate ions in the excavated material to be neutralized, and - such that the neutralization potential ratio, NPR, of the neutralized excavation material is equal to a predetermined value.

[0060] The predetermined value of the NPR is determined empirically and imposed by national regulations. For example, in France, the value of 4 is recommended by standard NF EN 15875 (December 1, 2011).

[0061] Typically, the weak base buffer added to the excavation material to be neutralized can be a hydroxide, a carbonate, a carboxylate or mixtures thereof.

[0062] For the purposes of this application, the term "hydroxide" means a compound comprising one or more hydroxide ions OH. For example, hydroxide could be sodium hydroxide or potassium hydroxide.

[0063] For the purposes of this application, the term "carboxylate" means a compound comprising one or more carboxylate ions HCOO.

[0064] For the purposes of this application, the term "carbonate" means a compound comprising one or more carbonate ions, CO32. For example, carbonate may be calcium carbonate, magnesium carbonate, potassium carbonate, and mixtures thereof; calcium carbonate is particularly common. Advantageously, the carbonates in this list, especially calcium carbonate, are predominantly found in excavated materials. Furthermore, the carbonates in this list are natural and therefore accepted from a regulatory standpoint for neutralizing excavated material.

[0065] According to the neutralization method of the invention, the quantity of weak base buffer to be added to the excavation material to be neutralized satisfies the following formula: mTBF = Wtbf* mmatériauàneutraiiser, with w where I Bl- \ / n*10 AP and NP are as defined above, NPR is the ratio of the neutralization potential of the excavated material to be neutralized, mmaterial to be neutralized is the mass of the material to be neutralized. wTbf is the mass content of low-level base buffer to be added to the excavated material to be neutralized. n is the number of moles of H+ that react with the weak base buffer, for example n = 2 for calcium carbonate and n = 1 for potassium hydroxide and sodium hydroxide, and MTBf is the molar mass of the weak base buffer.

[0066] According to a very particular embodiment, the weak base buffer is calcium carbonate.

[0067] According to this very particular embodiment, the quantity of calcium carbonate to be added to the excavated material to be neutralized complies with the following formula: mCaco3 = wCaco* mmatériauaneutraiiser, with _ (àP *NPR - NP^^^ ' Oij AP and NP are as defined above, NPR is the ratio of the neutralization potential of the excavated material to be neutralized, mmaterial to be neutralized is the mass of the material to be neutralized. Wcaccn is the mass content of calcium carbonate to be added to the excavated material to be neutralized, and MCaco3 is the molar mass of calcium carbonate, i.e. MCaco3 = 100 g / mol.

[0068] The neutralized excavation material obtained from the neutralization method described above can be recovered.

[0069] Thus, according to another object of the invention, a method of valorization of a neutralized excavation material is proposed according to the neutralization method as described above, comprising a step of valorizing said neutralized excavation material as a construction material.

[0070] Typically, the construction material can be an embankment or an aggregate, in particular an embankment, an aggregate for concrete or an aggregate for asphalt.

[0071] For the purposes of this application, "fill" means a construction material intended to raise land, fill a hollow or fill voids in mining operations.

[0072] For the purposes of this application, "aggregate" means a construction material used for the construction of civil engineering works, road works and buildings.

[0073] Concrete aggregate and asphalt aggregate are examples of aggregate.

[0074] For the purposes of this application, "aggregate for asphalt" means an aggregate used for the production of bituminous asphalt.

[0075] For the purposes of this application, "concrete aggregate" means aggregate used in the production of concrete.

[0076] The valorization step may include a step of producing a construction material from the inert material.

[0077] Typically the production step may include one or more sub-steps for shaping the inert material, the sub-step or sub-steps being adapted to the construction material, in particular to the backfill or aggregate, more particularly to the backfill, aggregate for concrete or aggregate for asphalt. Examples

[0078] The following examples illustrate the invention without, however, limiting it.

[0079] Example 1: Excavated material from the Grand Paris Express construction site

[0080] Example L1: Determination of the mass content of iron pyrite and ion carbonate from a sample of excavated material.

[0081] A known mass (30 g) of a sample of the excavation material is dried in an oven at 145°C for 30 minutes to obtain a dried sample having a dry matter content of 100%.

[0082] 5 g of this dried sample are ground by hand in an agate mortar, until The aim is to obtain a visually homogeneous powder that cannot be further reduced. This powder is then sieved to recover a ground sample with a particle size distribution such that 100% by mass of the sample has a diameter of less than 100 µm.

[0083] The ground sample is then divided into two samples, Echl and Ech2. Each Sample Echl and Ech2 then undergo infrared spectroscopy analysis between 4000 cm1 and 370 cm1. The instrument used to perform this analysis is a PerkinElmer SpectrumTwo FT-IR, and the parameters are the analysis range between 4000 cm1 and 400 cm1.

[0084] Figures [Fig. 1] and [Fig. 2] show the infrared spectrum obtained for sample Ech1. The spectrum obtained for sample Ech2 is similar.

[0085] The infrared spectrum of [Fig. 1] and [Fig. 2] includes an absorption band, in the form of a trough, around 440 cm⁻¹. This absorption band is characteristic of iron pyrite. An area is obtained by integrating the absorption band between 449 cm⁻¹ and 435 cm⁻¹. Comparing this area with a calibration curve, previously established but not shown, allows the mass content of iron pyrite in sample Echl to be determined. This content is given in Table 3 below. The mass content of iron pyrite in sample Ech2 is determined in the same way and is given in Table 3 below.

[0086] This infrared spectrum does not include the absorption band of the carbonate ion around 1450 cm1. The mass concentration of carbonate ions in samples Ech1 and Ech2 is therefore zero. This concentration is shown in Table 3 below.

[0087] The acid generation potential (AP), the neutralization potential (NP), and the neutralization potential ratio (NPR) of the two samples Echl and Ech2 are also shown in Table 3 below.

[0088] [Tables3] Sample n Mass content of iron pyrite (%) Mass content of CO32 (%) AP NP NPR Sample 7 0 2.35 0 0 Sample 2 7 0 2.35 0 0

[0089] Table 3 highlights that the method of the present invention, which implements spectroscopic analysis, makes it possible to determine in a few minutes the iron pyrite content in the excavation material with a standard deviation of zero.

[0090] Example L2: Neutralization of the excavation material of Example LL

[0091] The excavated material is neutralized by adding calcium carbonate.

[0092] The quantity of calcium carbonate (CaCO3) to be added to the excavated material is determined from the AP and NP potentials indicated in Table 3 above such that the NPR potential, i.e. the NP / AP ratio, of the neutralized excavated material is equal to 4 in accordance with standard NF EN 15875 (December 1, 2011). Thus, the quantity of calcium carbonate to be added to the excavated material to be neutralized, Is CaCO3 equal to W(2aCO3)? Material to be neutralized, aVCC «'ccm = (2.35 x 4 - 0) x ( ^ ) ,^.47 %■ Example 2: Excavated material from the Grand Paris Express construction site

[0093] Example 2.1: Determination of the mass content of iron pyrite and carbonate ion of a sample of excavated material.

[0094] A sample of the excavation material undergoes the protocol of example 1.1, the difference being that it is not separated into 2 samples Echl and Ech2.

[0095] The mass contents of iron pyrite and carbonate ion, acid generation potential (AP), neutralization potential (NP), and neutralization potential ratio (NPR) of the sample are shown in Table 4 below.

[0096] [Tables4] Mass content of iron pyrite (%) Mass content of CO32 (%) AP NP NPR 4.3 11.3 1.45 3.77 2.60

[0097] Example 2.2: Neutralization of the excavated material from Example 2.1,

[0098] The excavated material is neutralized by adding calcium carbonate.

[0099] The quantity of calcium carbonate (CaCO3) to be added to the excavated material is determined from the AP and NP potentials indicated in Table 4 above such that the NPR potential, i.e., the NP / AP ratio, of the neutralized excavated material is equal to 4 in accordance with standard NF EN 15875 (December 1, 2011). Thus, the quantity of calcium carbonate to be added to the excavated material to be neutralized, aCaCO3, is equal to aCaCO3^material to be neutralized, aVCC wCaCO3 = (1.45 x 4 - 3.77) x (^) Le. 10.15%-

Claims

Demands

1. Method of analyzing an excavation material comprising the following steps: a) analyzing by infrared spectroscopy a sample of an excavation material in a wavenumber range between 4000 cm1 and 350 cm' to obtain an infrared spectrum, b) determining the mass content of iron pyrite in said sample from the infrared spectrum obtained in step a), wherein step b) comprises the following substeps: b1) detecting between 450 cm1 and 435 cm1 a specific absorption band of iron pyrite in the infrared spectrum obtained in step a), b2) integrating the absorption band detected in step b1) to calculate an area, and b3) comparing the area obtained in step b2) with a calibration curve to determine the mass content of iron pyrite of the excavation material.

2. Method according to claim 1 wherein the excavated material has a dry matter content of 70% to 100%.

3. Method according to claim 1 or claim 2, further comprising the following step: c) determination of the mass content of carbonate ion in said sample from the infrared spectrum obtained in step a).

4. Method according to claim 3 wherein step c) comprises the following substeps: c1) detection between 1550 cm1 and 1180 cm1 of a specific absorption band of the carbonate ion in the infrared spectrum obtained in step a), c2) integration of the absorption band detected in step c1) to calculate an area, and c3) comparison of the area obtained in step c2) with a calibration curve to determine the mass content of carbonate ion of the excavated material.

5. A method for neutralizing an excavated material comprising the following step: d) adding a small amount of base buffer to an excavated material to be neutralized to obtain an excavated material

6.

7.

8.

9.

10. neutralized, wherein the mass content of iron pyrite in the excavated material to be neutralized was determined by the analytical method as defined in any one of claims 1 to 4, the mass content of carbonate ion in the excavated material to be neutralized was determined by the analytical method as defined in claim 4 or claim 4, and the amount of weak base buffer added to the excavation material to be neutralized is adjusted - in relation to the mass content of iron pyrite in the excavated material to be neutralized, - in relation to the mass content of carbonate ions in the excavated material to be neutralized, and - such that the neutralization potential ratio, NPR, of the neutralized excavated material is equal to a predetermined value. Method according to claim 5 wherein the predetermined value is equal to 4. Method according to claim 5 or claim 6 wherein the weak base buffer added to the excavation material to be neutralized is a hydroxide, a carboxylate, a carbonate or mixtures thereof. Method according to any one of claims 5 to 7 wherein the weak base buffer added to the excavation material to be neutralized is calcium carbonate. A method for valorizing neutralized excavated material according to the neutralization method as defined in any one of claims 5 to 8, comprising a step of valorizing said neutralized excavated material as a construction material. A method according to claim 9, wherein the construction material is backfill or aggregate.