Method for detecting content of soluble salt in fly ash improved soil based on Na2CO3 selective precipitation

Abstract

The invention relates to a method for detecting the content of soluble salts in fly ash improved soil based on Na2CO3 selective precipitation, a composite correction detection method comprises the steps of pH adjustment and composite precipitation of a leach liquor of the fly ash improved soil, and pH adjustment can reduce the solubility of Ca (OH) 2 and eliminate the interference of Ca (OH) 2; ca < 2 + > and false salt can be removed in sequence through selective precipitation, only real soluble salt ions in the improved soil are reserved, and the salinized soil treatment cost caused by misjudgment is avoided; through detection, the recovery rate of Ca < 2 + > in the composite correction detection method for the content of the primary salt in the fly ash improved soil is 98.0-98.5%.

Description

Technical Field

[0001] The invention belongs to the technical field of soil sample detection, and in particular relates to a method for detecting the soluble salt content of fly ash-improved soil based on Na2CO3 selective precipitation. Background Art

[0002] Fly ash, a finely powdered solid waste generated in coal-fired power plants, is primarily composed of the incombustible mineral components remaining after coal combustion. It plays an increasingly important role in modern industry and environmental protection, transforming waste into treasure. Due to its lightweight, gelling activity, and cost-effectiveness, it is widely used in highway subgrade improvement.

[0003] Soluble salt testing in highway engineering involves quantitatively analyzing the content and composition of soluble salts in soil. These salts are readily soluble in water and can repeatedly dissolve and crystallize under temperature and humidity fluctuations, causing structural damage to the roadbed. Fly ash-amended soil is a mixture of undisturbed soil and fly ash. However, the calcium hydroxide (Ca(OH)2) and calcium sulfate (CaSO4) present in fly ash are readily dissolved and released during water extraction. This results in the total salt content measured using the 5:1 water-soil ratio extraction method specified in the current "Highway Soil Test Code" (JTG 3430-2020) being significantly higher than the actual effective salt content. When fly ash content in fly ash-amended soil was 5%, 10%, 15%, 20%, 25%, and 30%, respectively, the soluble salt contents measured using the current soluble salt testing method were 5.9, 6.8, 7.4, 8.5, 9.2, 10.2, and 5.2 g / kg. This error may lead to engineering misjudgments, such as overestimating the risk of salt expansion and increasing unnecessary treatment costs. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the present invention proposes a method for detecting the soluble salt content of fly ash-improved soil based on Na2CO3 selective precipitation.

[0005] The technical solution of the present invention is: A method for detecting the soluble salt content of fly ash-improved soil based on Na2CO3 selective precipitation comprises the following steps: S1. Selective precipitation: The original soil and fly ash were mixed to obtain fly ash-improved soil, the fly ash-improved soil was pretreated and mixed with the extract to obtain a mixed solution A, a carbonate solution was added to the mixed solution A, stirred, centrifuged, and filtered to obtain an intermediate supernatant B; S2. pH adjustment: adjust the pH of the supernatant B to 7.0±0.5 to obtain the final extract, measure the ion content of the final extract, and remove the fly ash soluble salt content to finally obtain the corrected primary salt content of the fly ash-amended soil.

[0006] Furthermore, the mass ratio of undisturbed soil to fly ash in the fly ash-improved soil in step S1 is (95-70):(5-30).

[0007] Furthermore, the specific steps of pre-treating the fly ash-improved soil in step S1 are: air-drying, grinding, and sieving the fly ash-improved soil.

[0008] Furthermore, the technical parameters of the air drying are: air drying the fly ash at room temperature for 24-48 hours.

[0009] Furthermore, the pretreated fly ash improved soil described in step S1: extract = 1g: 5ml.

[0010] Furthermore, the carbonate solution in step S1 is a sodium carbonate solution.

[0011] Furthermore, the concentration of the carbonate solution in step S1 is 4-7%.

[0012] Furthermore, in step S1, after adding the carbonate solution to the mixed solution A, the pH value is 8.5-9.0.

[0013] Furthermore, the method for adjusting the pH of the supernatant B in step S2 is to use dilute hydrochloric acid to adjust the pH to 7.0±0.5.

[0014] Compared with the prior art, the present invention has at least the following advantages: The present invention relates to a method for detecting the soluble salt content of fly ash-improved soil based on Na2CO3 selective precipitation. The composite correction detection method comprises the steps of pH adjustment and composite precipitation of the extract of fly ash-improved soil. The pH adjustment can reduce the solubility of Ca(OH)2 and eliminate the interference of Ca(OH)2; the selective precipitation can sequentially remove Ca(OH)2. 2+ , exclude false salt, retain only the real soluble salt ions in the improved soil, and avoid the cost of saline soil disposal caused by misjudgment; after testing, the composite correction detection method Ca of the original salt content of fly ash improved soil 2+ The recovery rate is 98.0-98.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the specific embodiments of the present invention, the following briefly introduces the drawings required for the specific embodiments or the description of the prior art.

[0016] Figure 1 This is a SEM image of fly ash, the raw material used in the present invention; Figure 2 The XRD pattern of fly ash, the raw material used in the present invention; Figure 3This is the XRD analysis diagram of the crystals of the improved soil of the present invention after drying in the soluble salt detection test in the "Highway Geotechnical Test Code" (JTG 3430-2020). DETAILED DESCRIPTION

[0017] The present invention is described in further detail below. It is necessary to point out that the following specific implementation methods are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above application content.

[0018] This document provides general and / or specific descriptions of the materials and experimental methods used in the experiments. Unless otherwise specified, all experimental or testing methods are conventional methods. All reagents and instruments used, unless the manufacturer is specified, are commercially available, conventional products and were prepared or used using conventional methods.

[0019] The current "Highway Geotechnical Test Code" (JTG3430-2020) soil soluble salt detection method is as follows: first prepare a fly ash-amended soil sample, take the original soil, add fly ash to the original soil at a mass ratio of 5-30%, and stir evenly to obtain fly ash-amended soil; then, according to the JTG 3430-2020 code, extract the soluble salt content in the fly ash at a water-soil ratio of 5:1, and the soluble salt content in the extracted improved soil. The original soil salt content is calculated using the following formula: , The meaning of each parameter in the formula: S original : the original salt content per unit mass of undisturbed soil; C mix : Salt concentration measured after leaching of fly ash-amended soil (unit: mg / kg or mg / L); m soil : Mass of the original soil of the raw material (unit: kg or g); m flyash : Mass of raw material fly ash (unit: kg or g); C flyash : Soluble salt content of fly ash extracted from raw material (unit: C mix consistent); C mix ×(m soil + m flyash ): total salt mass in fly ash-amended soil; C flyash ×m flyash : Salt mass in fly ash.

[0020] The XRD analysis of the dried crystals was tested by the current soil soluble salt detection method. Figure 1 As shown in the figure, it can be seen that the calcium salt content in the dried crystals is relatively high. The calcium hydroxide (Ca(OH)2) and calcium sulfate (CaSO4) rich in fly ash are easily dissolved and released during the water extraction process, resulting in the total salt content determined by the above-mentioned soluble salt detection method being significantly higher than the actual effective salt.

[0021] Reagent Source: Na2CO3 solution: analytical grade, purchased from Sinopharm Group; Ion chromatography calibration solution: mixed anion / cation standard; Dionex six kinds of anion / cation mixed standard solution; Original soil: taken from the saline soil area of ​​Zhundong coal power industry base in Qitai County, Changji, Xinjiang; Fly ash: F-class II fly ash from Wanneng Jiangbu Power Plant; SEM image of fly ash is as follows Figure 2 As shown in the figure, it can be seen that the fly ash particles are irregular in shape and poorly dispersed; the XRD pattern of fly ash is as follows Figure 3 The main chemical components of the fly ash are shown in Table 1. Figure 3 As can be seen from Table 1, the CaO content in fly ash is relatively high.

[0022] Table 1 Main chemical components of fly ash Ion chromatography-related reagents and equipment: Chromatographic column models: Dionex lonPac AS11-HC (anion analysis), CS12A (cation analysis); Mobile phase: Anion: 30 mmol / L KOH (gradient elution); Cation: 20 mmol / L methanesulfonic acid (MSA); Flow rate: 1.0 mL / min; Detector: Conductivity detector (suppressed type).

[0023] Example 1 Method for detecting the original salt content of fly ash-improved soil of the present invention S1. Preparation of fly ash-amended soil samples: 9.5 g of undisturbed soil was mixed with 0.5 g of fly ash at a 5% mass ratio to obtain fly ash-amended soil. The fly ash-amended soil was air-dried at 25°C for 48 h, protected from direct sunlight, ground in an agate mortar, and passed through a 2 mm standard sieve. The soil samples were cured for 7 days to obtain the amended soil samples. The extractability of soluble salts in fly ash was previously determined. S2. Prepare an extract and selectively precipitate: The extract is deionized water with a conductivity of ≤0.2 μS / cm. 50 ml of the extract is added to the modified soil sample prepared in S1 at a water-to-soil ratio of 5:1, and the mixture is stirred to obtain a mixed solution A. A 5% Na2CO3 solution is added dropwise to the mixed solution A at 25°C with stirring until the mixture is no longer turbid. The mixture is allowed to stand for 20 minutes, centrifuged at 4500 rpm for 15 minutes, and filtered through a 0.45 μm filter membrane to obtain a supernatant B. S3. pH fine adjustment: add 0.1 mol / L dilute hydrochloric acid to the supernatant B prepared in S2 to adjust the pH to 7.0 to obtain the final extract. The Cl content in the final extract was determined by ion chromatography (equipment ICS-5000+). - 、SO4 2- , Ca 2+ 、Na + ion content, and removing the fly ash soluble salt extraction value measured in step S1, the final improved soil soluble salt content is 5.2g / kg.

[0024] The present invention takes the final extract obtained in Example 1 as an example to test the Ca content in the extract. 2+ The recovery rate was 98.1%.

[0025] Example 2 The steps of this embodiment are basically the same as those of the first embodiment, except that in step S1, 9 g of original soil is taken, 1 g of fly ash is added to the original soil at a mass ratio of 10%, and the content of soluble salt in the final improved soil is 5.4 g / kg.

[0026] Example 3 The steps of this embodiment are basically the same as those of the first embodiment, except that in step S1, 8.5 g of original soil is taken, 1.5 g of fly ash is added to the original soil at a mass ratio of 15%, and the content of soluble salt in the final improved soil is 5.7 g / kg.

[0027] Example 4 The steps of this embodiment are basically the same as those of the first embodiment, except that in step S1, 8.0 g of original soil is taken, 2.0 g of fly ash is added to the original soil at a mass ratio of 20%, and the content of soluble salt in the final improved soil is 5.9 g / kg.

[0028] Example 5 The steps of this embodiment are basically the same as those of the first embodiment, except that in step S1, 7.5 g of original soil is taken, and 2.5 g of fly ash is added to the original soil at a mass ratio of 25%, and the content of soluble salt in the final improved soil is 6.2 g / kg.

[0029] Example 6 The steps of this embodiment are basically the same as those of the first embodiment, except that in step S1, 7.0 g of original soil is taken, 3.0 g of fly ash is added to the original soil at a mass ratio of 30%, and the content of soluble salt in the final improved soil is 6.4 g / kg.

[0030] Example 7 The steps of this embodiment are basically the same as those of the first embodiment, except that fly ash is not added in step S1, and the content of soluble salt in the final improved soil is 4.9 g / kg.

[0031] Test Case This test example uses the method described in the prior art as a control, and fly ash is added to the original soil at a mass ratio of 0%, 5%, 10%, 15%, 20%, 25%, and 30% to obtain fly ash-improved soil. The soluble salt content of the improved soil is tested, and the soluble salt content of the improved soil measured by the prior art method is compared with that of Examples 1 to 7. The results are shown in the following table: Table 2: Soluble salt content of improved soil measured by prior art methods in Examples 1-7 As can be seen from Table 2, when the fly ash content is between 0-30%, the higher the fly ash content, the greater the relative deviation between the soluble salt content obtained by the detection method of the present invention and the soluble salt content obtained by the method of the comparative example, and there is a significant difference between the two data.

[0032] The present invention relates to a method for detecting the native salt content of fly ash-improved soil. Traditional methods for detecting soluble salts in improved soil include a leaching step. However, fly ash is rich in CaO. When fly ash is dissolved in water, CaO reacts with water to form Ca(OH)2, resulting in an alkaline extract (pH > 10). The solubility of Ca(OH)2 increases with increasing pH. Traditional methods for detecting soluble salts in improved soil can misidentify these as soluble salts, leading to deviations in measured values, misjudgments in engineering projects, and increased saline soil disposal costs. The present method for detecting the native salt content of fly ash-improved soil combines pH adjustment with selective precipitation to remove interfering factors and restore the true soluble salt content of the improved soil, thus promising promising applications.

[0033] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and description of the present invention.

Claims

1. A method for detecting the soluble salt content of fly ash-improved soil based on Na2CO3 selective precipitation, characterized in that: The following steps are involved: S1. Selective precipitation: The original soil and fly ash were mixed to obtain fly ash-improved soil, the fly ash-improved soil was pretreated and mixed with the extract to obtain a mixed solution A, a carbonate solution was added to the mixed solution A, stirred, centrifuged, and filtered to obtain an intermediate supernatant B; S2. pH adjustment: adjust the pH of the supernatant B to 7.0±0.5 to obtain the final extract, measure the ion content of the final extract, and remove the fly ash soluble salt content to finally obtain the corrected primary salt content of the fly ash-amended soil.

2. The detection method according to claim 1, wherein The mass ratio of undisturbed soil to fly ash in the fly ash-improved soil in step S1 is (95-70):(5-30).

3. The detection method according to claim 2, characterized in that The specific steps of pre-treating the fly ash-improved soil in step S1 are: air-drying, grinding, and sieving the fly ash-improved soil.

4. The detection method according to claim 3, characterized in that The technical parameters of the air drying are: air drying the fly ash at room temperature for 24-48 hours.

5. The detection method according to claim 1, wherein The fly ash-improved soil after the pretreatment described in step S1: the extract = 1g: 5ml.

6. The detection method according to claim 1, characterized in that The carbonate solution in step S1 is a sodium carbonate solution.

7. The detection method according to claim 6, characterized in that The concentration of the carbonate solution in step S1 is 4-7%.

8. The detection method according to claim 1, wherein The method for adjusting the pH of the supernatant B in step S2 is to use dilute hydrochloric acid to adjust the pH to 7.0±0.5.

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