Method for detecting content of chloride ions in sea sand concrete

Abstract

The invention belongs to the field of material measurement, and particularly discloses a novel method for detecting the content of chloride ions in hardened sea sand concrete, which comprises the following steps: drilling a concrete sample and freezing and grinding with liquid nitrogen until the particle size is less than or equal to 0.15 mm; mixing the powder with a nitric acid solution, and synchronously carrying out ultrasonic oscillation (30-50 kHz) and microwave-assisted extraction (2400-2500 MHz, 300-500 W) in a water bath at 60-80 DEG C; filtering, adding excessive silver nitrate solution, centrifuging, and measuring the total amount of chloride ions by adopting potassium thiocyanate back titration; and secondarily calibrating the free chloride ions through a spectrophotometric method, and deducting the background value of the cementing material. According to the method, the C-S-H gel structure is synergistically destroyed by utilizing microwave-ultrasound, and a double-calibration mechanism is combined, so that the detection time is shortened to be less than 1 hour, the relative error is less than 5%, and the method is obviously superior to a traditional acid dissolution method and an artificial end point judgment method, and is suitable for rapid and accurate detection of concrete with complex components.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of material measurement, and specifically discloses a new method for detecting the content of chloride ions in hardened sea sand concrete. BACKGROUND

[0002] With the wide application of sea sand in concrete, the problem of steel bar corrosion caused by chloride ions is becoming increasingly serious. Existing detection technologies are mainly aimed at unhardened concrete or raw material sea sand, but there are significant defects in the detection of chloride ions in hardened sea sand concrete: Complex occurrence state of chloride ions: part of the chloride ions in hardened concrete are chemically combined or physically adsorbed by cementitious materials, and the traditional acid dissolution method cannot completely release them; Low detection efficiency: the boiling method of Appendix B of “Technical Specification for Detection of Chloride Ion Content in Concrete” (JGJ / T322-2013) needs more than 2 hours and the dissolution rate is less than 80%; High dependence on equipment: although the chloride ion resistant concrete proposed in patent CN116768576A optimizes the material ratio, it does not provide a detection method for hardened concrete; Many error sources: patent CN202011516162A improves the detection precision of chloride ions in sea sand by heating and stirring, but does not consider the interference of cementitious materials on detection.

[0003] In addition, patent CN202110307089A uses electromigration method to accelerate the extraction of chloride ions, but needs special equipment and has poor penetration for hardened concrete; patent CN202310598551A analyzes chloride ion diffusion through fractional order model, which belongs to theoretical simulation rather than actual measurement method; the traditional silver nitrate titration method relies on manual end point judgment and is easily interfered by sulfur ions, resulting in poor repeatability. SUMMARY

[0004] To solve the above technical problems, the application discloses a method for detecting the content of chloride ions in sea sand concrete. Through microwave-assisted acidolysis and ultrasonic oscillation, the wrapping effect of C-S-H gel on chloride ions is destroyed; combined with potential titration and spectrophotometric method double calibration, free and bound chloride ions are distinguished; the background value deduction mechanism of cementitious materials is introduced, which significantly improves the detection accuracy of hardened concrete (relative error <5%), and the detection time is shortened to within 1 hour, which is better than the existing technology.

[0005] The application includes the following technical solutions: A method for detecting the content of chloride ions in sea sand concrete, comprising the following steps: (1) drilling a sample from the hardened sea sand concrete to be tested, and crushing and grinding the sample into a powder with a particle size of ≤0.15 mm; (2) the powder is mixed with a nitric acid solution with a mass concentration of 5%-10% at a ratio of 1:5-1:10, and ultrasonic oscillation and microwave-assisted extraction treatment are simultaneously performed in a 60-80°C water bath, wherein: the ultrasonic oscillation frequency is 40 kHz, and the duration is 20-40 minutes; the microwave frequency is 2400-2500 MHz, the power is 300-500 W, and the treatment time is 5-10 minutes; (3) after filtration, an excess of a silver nitrate standard solution is added to the filtrate to generate a silver chloride precipitate, and after centrifugal separation, the residual silver nitrate concentration is determined by using a potentiometric titration method, and the total content of chloride ions is calculated by difference; (4) the free chloride ions in the filtrate are calibrated again by using a spectrophotometric method to eliminate the detection error of the combined chloride ions.

[0006] Further, in the above method for detecting the chloride ion content in sea sand concrete, the test sample in step (1) needs to avoid the area above 5 cm of the concrete surface, and liquid nitrogen freezing grinding is used to avoid the escape of chloride ions.

[0007] Further, in the above method for detecting the chloride ion content in sea sand concrete, the mass concentration of the nitric acid solution in step (2) is 8%, and the mixing ratio of the powder to the nitric acid solution is 1:8.

[0008] Further, in the above method for detecting the chloride ion content in sea sand concrete, the power of the microwave treatment in step (2) is 400 W, and the treatment time is 8 minutes.

[0009] Further, in the above method for detecting the chloride ion content in sea sand concrete, the potentiometric titration method in step (3) uses a potassium thiocyanate back titration method, ammonium ferric sulfate is used as an indicator, and the end point is determined by a combination of a potential jump and a color change.

[0010] Further, in the above method for detecting the chloride ion content in sea sand concrete, the spectrophotometric method in step (4) uses a mercury thiocyanate ferric sulfate color developing agent, the detection wavelength is 460 nm, and the calibration curve covers a chloride ion concentration range of 0.01%-0.1%.

[0011] Further, in the above method for detecting the chloride ion content in sea sand concrete, step (3) further includes deducting the chloride ion background value of the cementitious material, and specifically, the chloride ion release amount of cement and mineral powder alone is determined by a blank control group.

[0012] Further, in the above method for detecting the chloride ion content in sea sand concrete, the water bath temperature in step (2) is constant at 70°C.

[0013] Further, the method for detecting the content of chloride ions in sea sand concrete, the concentration of the silver nitrate standard solution in step (3) is 0.05 mol / L, and the amount of addition is 1.2 times the theoretical requirement.

[0014] Further, the method for detecting the content of chloride ions in sea sand concrete, the content of chloride ions in step (3) is calculated by the following formula:

[0015] In the formula, V 0 is the consumption amount (mL) of the silver nitrate solution in the blank test, V 1 is the consumption amount (mL) of the silver nitrate solution in the sample, C is the concentration (mol / L) of the silver nitrate solution, m is the mass (g) of the sample.

[0016] Compared with the prior art, the present application has the following outstanding beneficial effects: The application discloses a method for detecting the content of chloride ions in sea sand concrete, which significantly improves the dissolution efficiency of chloride ions in hardened sea sand concrete (more than 95%) through microwave-ultrasonic synergistic extraction technology, effectively distinguishes free-state and combined-state chloride ions by combining the double-calibration mechanism of potentiometric titration and spectrophotometry, and controls the detection error within ±5%. Through the background value deduction of cementitious materials and the double-wavelength anti-interference design, the problems of low dissolution rate, long time consumption and multiple error sources of the traditional method are solved. The detection time is shortened to within 1 hour, the method is suitable for concrete with high-concentration chloride ions and complex components such as mineral powder and fly ash, and has the advantages of high efficiency, high precision and strong universality. BRIEF DESCRIPTION OF DRAWINGS

[0017] Fig. 1 Comparison of dissolution rates (%) in test example 1; Fig. 2 Comparison of detection times (min) in test example 1; Fig. 3 Comparison of formula calculation errors (%) in test example 1. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the application will be described below in a clear and complete manner. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0019] Table 1 reagents used in the embodiments

[0020] Table 2 Equipment used in the examples

[0021] Example 1 A method for detecting the content of chloride ions in sea sand concrete, comprising the following steps: (1) Sample preparation: A Φ50mm core sample is drilled from the interior of a sea sand concrete structure (8cm from the surface), and after being frozen with liquid nitrogen, it is ground to a particle size of 0.10mm using a planetary ball mill (verified by a laser particle size analyzer).

[0022] (2) Acid leaching treatment: 5.00g of powder (accurate to 0.0001g) is mixed with 40.00mL of 8% nitric acid solution (mass concentration) and placed in a 70°C constant temperature water bath. A 40kHz ultrasonic wave (power 200W, treatment time 30 minutes) and a 400W microwave treatment (2450MHz, 8 minutes) are started simultaneously.

[0023] (3) Precipitation and titration: After filtration, 12.00mL of 0.05mol / L silver nitrate standard solution (theoretical requirement is 10mL, excess 20%) is added to the filtrate, and silver chloride precipitate is separated by centrifugation. The volume of potassium thiocyanate solution consumed V1=8.32mL is determined by back titration with 0.05mol / L potassium thiocyanate solution, with the end point determined by a potential jump (ΔE=58mV) and the solution changing from milky white to light red.

[0024] (4) Take 5.00mL of the filtrate, add 2mL of mercury thiocyanate color developing agent, and measure the absorbance A=0.358 at 460nm. According to the calibration curve equation C 游离 =0.0215A+0.0032, the free chloride ion concentration, C 游离 =0.0215×0.358+0.0032=0.0109% is calculated.

[0025] Background value deduction and total content calculation: The blank test measures the cement background chloride ion content C 本底 =0.007% The total chloride ion content is calculated according to the formula: .

[0026] Example 2 High chloride ion concentration sample Concrete chloride content 0.15%, step (2) nitric acid concentration increased to 10%, mixing ratio 1:5, microwave power 500W for 6 minutes. The rest is the same as example 1.

[0027] Calculation result: titration consumes potassium thiocyanate VI = 6.15 mL, spectrophotometrically measured free chlorine 0.132%, final content 0.143% (theoretical value 0.150%, error -0.007%).

[0028] Example 3 Low particle size grinding Step (1) ground to 0.08 mm, step (2) water bath temperature 60℃, ultrasonic time 40 minutes. The rest is the same as example 1.

[0029] Effect: dissolution rate increased to 97%, but need to control microwave power (300W) to avoid local overheating.

[0030] Example 4 Mineral admixture-containing concrete Concrete contains 30% mineral powder, step (7) blank test additional determination of mineral powder background value 0.012%, total background deduction amount = cement background + mineral powder background = 0.007% + 0.012% = 0.019% when calculating. The rest is the same as example 1.

[0031] Result: test value 0.068% (actual content 0.085%), error due to incomplete release of chloride ions combined with mineral powder.

[0032] Example 5 On-site rapid detection Cancel liquid nitrogen freezing grinding, use portable crusher to grind to 0.15 mm, microwave treatment is shortened to 5 minutes. The rest is the same as example 1.

[0033] Result: test value 0.051% (laboratory reference value 0.054%), relative error 5.6%, meet the needs of on-site rapid evaluation.

[0034] Comparative example 1 No microwave-ultrasonic cooperation Step (2) only use 80℃ water bath heating for 40 minutes, no ultrasonic and microwave treatment.

[0035] Defects: dissolution rate is only 65%, formula calculation value 0.038% (actual 0.054%), error up to -29.6%. Artificial end point determination Step (3) cancel potential titration, use potassium chromate indicator (Mol method) instead, determine brick red end point by naked eye.

[0036] Defect: The consumption of potassium thiocyanate V1 = 8.20 mL (8.32 mL by potentiometric method), the error is due to the end point lag, the detection value is 0.006% higher.

[0037] Comparative Example 3 Without background value deduction Cancel step (7) background deduction, directly use the formula to calculate the result.

[0038] Error: detection value 0.066% (actual 0.054%), positive deviation +22.2%.

[0039] Comparative Example 4 Traditional acid dissolution method According to the standard JGJ / T322-2013, after boiling for 2 hours, filter and titrate.

[0040] Defect: detection time 180 minutes, and high temperature destroys part of the chloride ion, detection value 0.049% (actual 0.054%).

[0041] Test Example 1 Comparison of dissolution efficiency Test principle: Microwave-ultrasonic synergy through cavitation effect (ultrasound) and dielectric heating (microwave) to destroy C-S-H gel structure, improve the release efficiency of chloride ion. Dissolution rate calculation formula: Dissolution rate = (measured total chloride ion amount / sample theoretical total chloride ion amount) x 100% Sample preparation: prepare sea sand concrete sample with theoretical chloride ion content of 0.054% (cement: sand: stone = 1:2:3, water-cement ratio 0.45, mixed with 0.1% NaCl).

[0042] Group processing: Example 1: ultrasonic (40 kHz, 30 min) + microwave (400 W, 8 min) synergy treatment; Comparative Example 1: 80°C water bath heating for 40 minutes; Comparative Example 2: manual end point judgment; Comparative Example 4: traditional acid dissolution method (boiling for 2 hours).

[0043] The results are shown in Table 3 and Figs. 1-3 .

[0044] Table 3 Comparison of dissolution efficiency results

[0045] From the data results in Table 3, the following conclusions can be drawn: Microwave-ultrasound synergistic effect (Example 1): dissolution rate increased to 95.3%, 46.6% higher than Comparative Example 1 (water bath only), proving that microwave dielectric heating can destroy the C-S-H gel structure.

[0046] Detection time optimization: Example 1 detection time 45 minutes, 75% shorter than Comparative Example 4, meeting the demand for rapid detection.

[0047] Error analysis: Comparative Example 2 has a positive deviation due to manual endpoint lag, proving the necessity of potentiometric titration.

[0048] Test Example 2 Spectrophotometric calibration effect verification Sample setup: Example 1: Detect total chloride and free chloride according to standard procedure; Comparative Example 3: Cancel spectrophotometric calibration, rely only on titration results; Example 3: Low-particle-size ground sample.

[0049] Calibration curve establishment: Prepare 0.01%-0.1% Cl⁻ standard solution, determine absorbance and fit equation.

[0050] C 游离 = 0.0215A + 0.0032 (R² = 0.998).

[0051] Error calculation: Uncalibrated error = Uncalibrated detection value - Actual value.

[0052] Results are shown in Table 4 Table 4 Spectrophotometric calibration effect verification results

[0053] From the data results in Table 4, the following conclusions can be drawn: Spectrophotometric calibration effect: Example 1 removes bound chloride (0.0481%) by calibration, with an error control of ±0.005%; Uncalibrated risk: Comparative Example 3 has an error of +22.2% due to not distinguishing between free and bound chloride, far exceeding the industry allowed error (±5%).

[0054] Test Example 3 Microwave power optimization test Experimental design: Fix ultrasonic parameters (40 kHz, 30 min), adjust microwave power (300W / 400W / 500W).

[0055] Temperature monitoring: Use FLIR T540 infrared thermal imager to monitor solution temperature fluctuations in real time.

[0056] Free chlorine ratio calculation: Free chlorine ratio = (free chlorine measured by spectrophotometry / total chlorine ion) x 100%.

[0057] Results are shown in Table 5 Table 5 Microwave power optimization test results

[0058] From the data in Table 5, the following conclusions can be drawn: Optimal power selection: The dissolution rate is highest (95.3%) at 400W, and the free chlorine ratio is 92.1%, indicating that the release of chlorine ions is sufficient; Power is too high: 500W leads to local boiling (temperature > 85℃), partial Cl⁻ volatilization, and free chlorine ratio decreases to 88.7%.

[0059] Test Example 4 Interfering ion influence test Test method: Interference simulation: Add 0.1% SO4²⁻ (Na2SO4) and 0.05% S²⁻ (Na2S) to the filtrate, respectively test the anti-interference ability of Example 1 (single wavelength) and Example 5 (double wavelength).

[0060] Dual-wavelength correction: Simultaneous detection at 460nm (Cl⁻ characteristic peak) and 620nm (S²⁻ / SO4²⁻ interference peak), and interference is eliminated by differential calculation.

[0061] Results are shown in Table 6.

[0062] Table 6 Interfering ion influence test results

[0063] From the data in Table 6, the following conclusions can be drawn: Interference mechanism: S²⁻ reacts with AgNO3 to form Ag2S precipitate, and SO4²⁻ competes with Ag⁺ to cause false high detection value; Dual-wavelength advantage: Example 5 reduces the combined interference error from +0.009% to +0.003% through 620nm correction, with a reduction of 66.7%.

[0064] Test Example 5 Different cementitious material background deduction verification 1 Material preparation: Use ordinary Portland cement, slag cement, and fly ash concrete to prepare samples, and measure the actual background chlorine ion content.

[0065] 2 Error calculation: Undeducted error = (undeducted detection value - actual dosage) / (actual dosage) x 100%.

[0066] Results are shown in Table 7 Table 7 Different cementitious material background deduction verification

[0067] From the data in Table 7, the following conclusions can be drawn: Background deduction necessity: the error of slag cement without deduction is up to +30%, and after deduction, it is reduced to +5%; Versatility verification: this method is suitable for concrete containing mineral powder, fly ash and other admixtures, and the error is controlled within ±5%.

[0068] From the above test examples, the method described in the present application has the following outstanding advantages: 1. Significantly improve the efficiency of chloride ion dissolution: the present application uses microwave-assisted acidolysis combined with ultrasonic oscillation technology, through the combined action of cavitation effect and dielectric heating, effectively destroys the wrapping effect of C-S-H gel on chloride ions, and makes the chloride ion dissolution rate increase to more than 95%, which is significantly higher than the dissolution rate of traditional methods.

[0069] 2. Shorten the detection time: by optimizing the detection process, the present application shortens the entire detection time to within 1 hour, which is significantly shorter than the traditional acid dissolution method (such as the boiling method of Appendix B of “Technical Specification for Detection of Chloride Ion Content in Concrete” (JGJ / T322-2013), which takes more than 2 hours), greatly shortens the detection time and improves the detection efficiency.

[0070] 3. Improve detection accuracy: the present application combines the dual calibration mechanism of potentiometric titration and spectrophotometry, effectively distinguishes between free and bound chloride ions, and controls the detection error within ±5%, which is significantly better than the detection accuracy of traditional methods.

[0071] 4. Introduce cementitious material background value deduction mechanism: by measuring the release amount of chloride ions in cement, mineral powder and other cementitious materials in the blank control group, and deducting the background value in the calculation of total chloride ion content, the accuracy of the detection result is further improved.

[0072] 5. Solve the problems existing in traditional methods: Solve the problem that traditional acid dissolution method cannot completely release chloride ions in hardened concrete, avoid the problems of low detection efficiency and insufficient dissolution rate.

[0073] Overcome the problems of high equipment dependency and multiple error sources, such as poor repeatability caused by manual end point judgment and interference of sulfur ions.

[0074] 6. Strong universality: this method is suitable for concrete with high concentration of chloride ions and complex components such as mineral powder and fly ash, and has wide applicability.

[0075] The above merely describes a few preferred embodiments of the present application in a specific and detailed manner, but should not be understood as a limitation to the patent scope of the present application. It should be noted that, for those of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application.

Claims

1. A method for detecting the content of chloride ions in sea sand concrete, characterized by, The method comprises the following steps: (1) drilling a sample from the hardened sea sand concrete to be tested, and crushing and grinding the sample into a powder with a particle size of ≤0.15 mm; (2) mixing the powder with a nitric acid solution with a mass concentration of 5%-10% at a ratio of 1:5-1:10, and simultaneously performing ultrasonic oscillation and microwave-assisted extraction treatment in a 60-80°C water bath, wherein: the ultrasonic oscillation frequency is 30-50 kHz, and the duration is 20-40 minutes; the microwave frequency is 2400-2500 MHz, the power is 300-500 W, and the treatment time is 5-10 minutes; (3) adding an excess of a silver nitrate standard solution to the filtrate to generate a silver chloride precipitate, centrifuging and separating the precipitate, and then measuring the residual silver nitrate concentration by using a potentiometric titration method, and calculating the total chlorine ion content by using a difference method; (4) using a spectrophotometric method to perform secondary calibration on the free chlorine ions in the filtrate to eliminate the detection error of the combined chlorine ions.

2. The method of claim 1, wherein, In step (1), the sample needs to avoid the area above 5 cm of the concrete surface, and liquid nitrogen is used for freezing and grinding to avoid the escape of chlorine ions.

3. The method of claim 1, wherein, In step (2), the mass concentration of the nitric acid solution is 8%, and the mixing ratio of the powder to the nitric acid solution is 1:

8.

4. The method of claim 1, wherein, In step (2), the microwave treatment power is 400 W, and the treatment time is 8 minutes.

5. The method of claim 1, wherein, In step (3), the potentiometric titration method uses a potassium thiocyanate back titration method, and ferric ammonium sulfate is used as an indicator, and the end point is determined by a combination of a potential jump and a color change.

6. The method of claim 1, wherein, In step (4), the spectrophotometric method uses a mercury thiocyanate color reagent, the detection wavelength is 460 nm, and the calibration curve covers a chlorine ion concentration range of 0.01%-0.1%.

7. The method of claim 1, wherein, Step (3) further comprises deducting the background value of the chloride ions in the cementitious material, and the chlorine ion release amount of the cement and the mineral powder is determined by using a blank control group.

8. The method of claim 1, wherein, In step (2), the water bath temperature is constant at 70°C.

9. The method of claim 1, wherein, In step (3), the concentration of the silver nitrate standard solution is 0.05 mol / L, and the addition amount is 1.2 times the theoretical requirement.

10. The method of claim 1, wherein, In step (3), the chlorine ion content is calculated by the following formula: wherein V 0 is the blank test silver nitrate solution consumption (mL), V 1 is the sample silver nitrate solution consumption (mL), C is the silver nitrate solution concentration (mol / L), m is the sample mass (g).

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