Method for measuring content of chloride ions in concrete powder

Abstract

The invention discloses a method for determining the content of chloride ions in concrete powder, and belongs to the technical field of detection of chloride ions in building materials, the determination method comprises the following steps: S1, adding 10-15g of concrete powder and 80-100ml of distilled water into a sample bottle, dropwise adding nitric acid until the pH value of the solution is 5-6, and oscillating and dispersing to obtain a concrete powder suspension; s2, dissolving 30-40 mmol of sodium bicarbonate and 0.5-2 mmol of amino alcohol in 20-50 ml of distilled water, then quickly adding into the concrete powder suspension, and tightening a bottle cap; and S3, vibrating the sample bottle for at least 2 minutes, standing, filtering, and detecting the chloride ion concentration of the filtrate through detection equipment. According to the method, sodium bicarbonate and amino alcohol are added into a slightly acidic concrete powder solution to consume Ca < 2 + > and OH <-> in the solution, so that water-insoluble chloride ions in Friedel's salt are changed into a free state, the water-soluble chloride ions in the concrete powder can be detected without heating and boiling, and the method has the advantages of simplicity, rapidness and accuracy.

Description

Technical Field

[0001] The invention belongs to the technical field of chloride ion detection in building materials, and particularly relates to a method for determining the chloride ion content in concrete powder. Background Art

[0002] Since the introduction of concrete into civil engineering, numerous reinforced concrete structures have failed prematurely due to various reasons, failing before their intended service life. Statistics show that the majority of these premature failures are due to insufficient structural durability. In coastal and offshore areas of my country, the primary cause of concrete durability issues is chloride ion corrosion, which can be categorized into two types: one in which chloride ions from the ocean, in the form of seawater and sea mist, seep into concrete, affecting the performance and service life of concrete structures; the other in the form of seawater and sea sand, which are incorporated into concrete during mixing. The first type primarily occurs in ports, docks, and coastal structures, while the second can be widespread over a wider geographical area. Over time, chloride ion corrosion caused by seawater and sea sand in coastal and offshore areas has become increasingly apparent, garnering widespread attention. Based on practical engineering experience, the best approach for concrete structures incorporating chloride ions is to conduct an inspection of existing structures by measuring the chloride ion content in the concrete, enabling early detection and resolution of issues.

[0003] The method for leaching water-soluble chloride ions from concrete generally follows the "Determination of Water-Soluble Chloride Ion Content in Mortar of Concrete" in JTJ 270-1998, "Testing Procedure for Concrete for Water Transport Engineering." The chloride ion leaching procedure involves grinding the concrete, sieving it, drying it, and immersing it in distilled water. After vigorous shaking for 1-2 minutes, the mixture is immersed for 24 hours. However, studies have found that the chloride ion content detected by the immersion method is only 50% of the total chloride ion content, compared to the 80% reported by the American Standard method. This result is too low, potentially affecting technicians' ability to accurately assess the quality of sea sand or concrete. Therefore, JGJ / T 322-2013, "Technical Specification for the Determination of Chloride Ion Content in Concrete," references ASTM C1218 and adds boiling and filtration to the chloride ion leaching procedure. It also explicitly states that boiling for 5 minutes can effectively promote the dissolution of water-soluble chloride ions. However, the boiling method requires heating, and in practice, concrete core samples must be drilled on-site and then sent to a laboratory for processing and testing, which is time-consuming and costly.

[0004] Therefore, a simple, fast and accurate chloride ion detection method still needs to be developed to meet the increasing demand for on-site detection of existing concrete structures. Summary of the Invention

[0005] In order to overcome the above-mentioned shortcomings of the prior art, the present invention provides a method for determining the chloride ion content in concrete powder, by adding trace amounts of nitric acid, sodium bicarbonate and amino alcohol, and adjusting the pH in the system to consume the Ca in the solution. 2+ With OH - , in order to achieve the effect of releasing chloride ions in Friedel's salt, so as to achieve the purpose of simply, quickly and accurately detecting chloride ions in concrete powder without heating. Compared with the results of the boiling method, it can meet the requirements of engineering accuracy.

[0006] The technical solutions for achieving the purpose of the present invention are as follows:

[0007] A method for determining the chloride ion content in concrete powder comprises the following steps:

[0008] S1. Add 10 to 15 g of concrete powder and 80 to 100 ml of distilled water to a sample bottle, add nitric acid dropwise to a pH of 5 to 6, and oscillate to obtain a concrete powder suspension.

[0009] S2. Dissolve 30-40 mmol of sodium bicarbonate and 0.5-2 mmol of amino alcohol in 20-50 ml of distilled water, then quickly add to the concrete powder suspension and tighten the cap;

[0010] S3. Shake the sample bottle for at least 2 minutes, let it stand, and then filter. The filtrate is tested for chloride ion concentration using a detection device.

[0011] Generally speaking, the main forms of chloride ions in concrete are free chloride ions and bound chloride ions, of which bound chloride ions include chloride ions solidified by chemical bonding through hydration product reactions and chloride ions adsorbed by positively charged hydrates of cement. These states of chloride ions can be converted into each other. For example, chemically bound Friedel's salt is a compound that may be formed during the cement hydration process. Its main component is calcium chloride aluminate, which can release chloride ions in both aqueous solution and acidic conditions, and participate in the corrosion reaction of steel bars. However, due to the instability of the Friedel's salt structure, it can both fix chloride ions and decompose to release chloride ions. This is a reversible reaction. Therefore, this property of reversible reaction can be used to promote the precipitation of calcium ions, so that the reaction continues to proceed to the right and the leaching rate of chloride ions is increased. The reaction formula is shown in Formula 1:

[0012]

[0013] The present invention first adds a small amount of nitric acid to the concrete powder to adjust the pH value of the concrete powder to 5-6, appropriately dissolves the Friedel salt in the concrete powder, and then quickly adds sodium bicarbonate and amino alcohol solution. When the solutions first come into contact, the system has not yet formed a homogeneous solution. Sodium bicarbonate will first decompose to produce carbon dioxide when it comes into contact with an acidic environment, and the amino alcohol can act as a carbon dioxide absorber to capture carbon dioxide to generate carbamate and H + , H + Can be used with sodium bicarbonate The reaction continues to produce carbon dioxide, but this reaction is relatively weak. Carbamate can also react with water to produce and amino alcohol, which is a dynamic equilibrium process. At the same time, sodium bicarbonate has the function of buffering pH, so that the pH value of the solution is finally stabilized at 9-10, thereby avoiding the re-precipitation of Friedel's salt. In this pH environment, the sodium bicarbonate Produced with amino alcohols Consumes OH separated from Friedel's salt - Convert to Ca separated from Friedel's salt in formula 1 2+ The reaction forms calcium carbonate precipitate, which promotes the reaction in formula 1 to the right, causing Friedel's salt to gradually decompose and release Cl - ;

[0014] Preferably, the amino alcohol is selected from at least one of aminoethanol and 2-amino-2-methyl-1-propanol.

[0015] Preferably, the nitric acid is a nitric acid solution with a concentration of 1 to 2 mol / L, and the solvent is distilled water.

[0016] Preferably, the sample bottle is a thick-walled pressure-resistant bottle.

[0017] Preferably, the method for preparing the concrete powder in step S1 is: using an electric drill to drill a hole in a concrete component whose chloride ion content needs to be tested, and collecting the dust generated by the drilling, which is the concrete powder.

[0018] Preferably, the number of drilling positions is at least 3, the hole spacing is ≥100 mm, the hole depth is 5-7 mm, and the weight of collected dust is ≥20 g.

[0019] Preferably, dust at the same depth is poured into a plastic bag and mixed, and a portion is put into a sample bottle and compacted to the mark line, which represents the volume mark of 10 to 15 g of dust. The volume mark is determined by weighing the dust in the laboratory or on-site.

[0020] Preferably, the dust is dry dust with a particle size of 0.02 to 0.08 mm.

[0021] Preferably, the shaking time in step S2 is 3 to 5 minutes.

[0022] Preferably, the chloride ion concentration is detected using a chloride ion selective electrode. Before detection, the chloride ion selective electrode is activated in a sodium chloride solution with a concentration of 0.001 mol / L for 2 h, and then calibrated in sodium chloride calibration solutions with concentrations of 0.005 mol / L, 0.02 mol / L, 0.05 mol / L, and 0.5 mol / L, respectively.

[0023] Beneficial effects

[0024] The present invention provides a method for determining the chloride ion content in concrete powder, which is simple, rapid, and accurate. The method utilizes a simple reaction apparatus and steps, requiring no heating or large-scale equipment, making it suitable for on-site testing of the chloride ion content of existing concrete structures or components. Sodium bicarbonate and amino alcohols are used as carbonate ion feeders. Under conditions of first weak acidity and then weak alkalinity, the Friedel salt structure dissociates and forms calcium carbonate precipitation with the carbonate ions, facilitating the release of fixed chloride ions. Comparative testing has shown that this method can achieve the same accuracy as the boiling method without heating or consuming large amounts of nitric acid, making it particularly suitable for on-site testing of the chloride ion content of existing concrete structures or components. DETAILED DESCRIPTION

[0025] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0026] In the examples, the experimental methods used are conventional methods unless otherwise specified, and the materials, reagents, etc. used are all commercially available unless otherwise specified.

[0027] The raw materials and equipment used in the embodiments and comparative examples are described below:

[0028] Cement: Portland cement, PII42.5R, produced by Zhujiang Cement Plant;

[0029] Sand: comes from purified sea sand from Zhanjiang;

[0030] Crushed stone: limestone, particle size 5-8 mm;

[0031] concrete powder

[0032] Mix 120kg of gravel, 70kg of sea sand, 30kg of cement and 15kg of tap water, cast into a 1m*1m*0.1m mold, vibrate and level it, and then perform standard curing for 28 days. Use an impact drill to drill 5 6mm deep holes on the concrete surface with a hole spacing of 100mm and a drill bit size of 18mm. Collect 20g of concrete powder, mix it in a plastic bag, weigh 15g of it, and compact it in a sample bottle with a capacity of 20ml. Mark the volume of 15g of concrete powder. Weigh 15g of concrete powder samples three times, compact them in the same sample bottle, and draw a marking line in the sample bottle.

[0033] Example 1

[0034] A method for determining the chloride ion content in concrete powder comprises the following steps:

[0035] S1. At room temperature, 15g of concrete powder and 100ml of distilled water were added to a thick-walled pressure bottle, shaken to form a suspension, and a 2M nitric acid solution was added dropwise to a pH of 5. The amount of nitric acid solution added was recorded as 5ml.

[0036] S2. Dissolve 30 mmol of sodium bicarbonate and 2 mmol of aminoethanol in 45 ml of distilled water and then quickly add it to the concrete powder suspension and tighten the cap;

[0037] S3. Shake the sample bottle for 5 minutes, let it stand and then filter. The filtrate is activated in a 0.001 mol / L sodium chloride solution for 2 hours through a chloride ion selective electrode before detection. It is then calibrated in sodium chloride ion calibration solutions with concentrations of 0.005 mol / L, 0.02 mol / L, 0.05 mol / L, and 0.5 mol / L, respectively. The determination coefficient of the standard solution calibration curve is greater than 99.9%.

[0038] Example 2

[0039] Compared with Example 1, the difference is that aminoethanol is replaced by 2-amino-2-methyl-1-propanol, and the other contents remain unchanged.

[0040] Example 3

[0041] Compared with Example 1, the difference is that 40 mmol of sodium bicarbonate and 0.5 mmol of aminoethanol are added in step S2.

[0042] Comparative Example 1

[0043] Compared with Example 1, the difference is that aminoethanol is not added.

[0044] Comparative Example 20.0

[0045] Compared with Example 1, the difference is that sodium bicarbonate is not added.

[0046] Comparative Example 3

[0047] Weigh 15g of concrete powder and disperse it in 150ml of water. The chloride ion leaching method specified in JGJ / T 322-2013 "Technical Specification for the Determination of Chloride Ion Content in Concrete" was used for determination, i.e., the concrete powder suspension was boiled for 5min.

[0048] Comparative Example 4

[0049] Weigh 15g of concrete powder and disperse it in 150ml of water. Use the chloride ion leaching method of "Determination of water-soluble chloride ion content of mortar in concrete" in JTJ270-1998 "Test Procedure for Concrete in Water Transport Engineering" to determine the chloride ion content. That is, shake the concrete powder suspension for 1 to 2 minutes and then soak it for 24 hours.

[0050] Comparative Example 5

[0051] Weigh 15g of concrete powder and disperse it in 150ml of dilute nitric acid (volume ratio of concentrated nitric acid: water = 15:85). The total chloride ion leaching method of "Determination of total chloride ion content of mortar in concrete" in JTJ270-1998 "Test Procedures for Concrete in Water Transport Engineering" was used for determination.

[0052] The following are the test methods for the performance parameters involved in the present invention:

[0053] Chloride ion concentration: A chloride ion selective electrode (Oleron 9617BNWP) was used to test the leachate of concrete powder. Prior to testing, the chloride ion selective electrode was activated in a 0.001 mol / L sodium chloride solution for 2 h. The electrode was then calibrated in sodium chloride ion calibration solutions with concentrations of 0.005 mol / L, 0.02 mol / L, 0.05 mol / L, and 0.5 mol / L, respectively. The coefficient of determination for the standard solution calibration curve was greater than 99.9%. When determining the chloride ion concentration of a sample, the solution only needed to flow 2 cm below the electrode. The chloride ion concentration c (mol / L) of the test solution was ultimately measured. Each sample group was tested three times before and after testing. The average of the three tests was calculated according to formula (3) to obtain the sample's chloride ion percentage w, as follows:

[0054]

[0055] Where,

[0056] w——chloride ion content, %;

[0057] c——The concentration of the filtrate measured by the chloride ion rapid analyzer, in mol / L;

[0058] v——the volume of distilled water added to the sample, in milliliters (ml);

[0059] m——mass of concrete powder sample, in grams (g);

[0060] Table 1 Chloride ion content determination results of Examples and Comparative Examples

[0061]

[0062] Taking the water-soluble chloride ion content measured by the boiling method in Comparative Example 3 as the standard value, and comparing it with the total chloride ion content measured by the nitric acid method in Comparative Example 5, it can be seen from the data in Table 1 that the chloride ion fraction measured by the method for determining the chloride ion content in concrete powder of the present invention relative to the boiling method test is in the range of 97.34% to 99.82%, and the ratio relative to the nitric acid method test is between 77.76% and 79.75%, which are very close to the water-soluble chloride ion results tested by the boiling method, and can meet the requirements of engineering accuracy. In addition, the determination method used in the present invention does not require heating, has a simple source of raw materials, and is particularly suitable for use as a supplement to the on-site testing method of existing concrete components, reducing the burden of laboratory testing and greatly improving testing efficiency.

[0063] From the test results of Comparative Examples 1 and 2, it can be seen that in Comparative Example 1, only sodium bicarbonate was added, and the pH value of the solution was measured to be 8.3, which did not meet the alkaline conditions required for the precipitation of calcium carbonate. Therefore, the chloride ions were only partially precipitated due to the nitric acid treatment. In Comparative Example 2, only aminoethanol was added, and the pH value of the solution was measured to be 10.9, which is a strong alkalinity. At this time, Friedel's salt can exist stably, and the water-insoluble chloride ions are basically not precipitated. In addition, the calcium ions precipitated during the nitric acid treatment also lack carbonate groups to precipitate with it.

[0064] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for determining the chloride ion content in concrete powder, characterized in that: The following steps are involved: S1. Add 10 to 15 g of concrete powder and 80 to 100 ml of distilled water to a sample bottle, add nitric acid dropwise to a pH of 5 to 6, and oscillate to obtain a concrete powder suspension. S2. Dissolve 30-40 mmol of sodium bicarbonate and 0.5-2 mmol of amino alcohol in 20-50 ml of distilled water, then quickly add to the concrete powder suspension and tighten the cap; S3. Shake the sample bottle for at least 2 minutes, let it stand, and then filter. The filtrate is tested for chloride ion concentration using a detection device.

2. The method for determining the chloride ion content in concrete powder according to claim 1, wherein: The amino alcohol is selected from at least one of aminoethanol and 2-amino-2-methyl-1-propanol.

3. The method for determining the chloride ion content in concrete powder according to claim 1, wherein: The nitric acid is a nitric acid solution with a concentration of 1 to 2 mol / L, and the solvent is distilled water.

4. The method for determining the chloride ion content in concrete powder according to claim 1, wherein: The sample bottle is a thick-walled pressure-resistant bottle.

5. The method for determining the chloride ion content in concrete powder according to claim 1, wherein: The method for preparing the concrete powder in step S1 is as follows: using an electric drill to drill a hole in a concrete component whose chloride ion content needs to be tested, and collecting the dust generated by the drilling, which is the concrete powder.

6. The method for determining the chloride ion content in concrete powder according to claim 5, wherein: There should be at least 3 drilling positions, the hole spacing should be ≥100mm, the hole depth should be 5-7mm, and the collected dust weight should be ≥20g.

7. The method for determining the chloride ion content in concrete powder according to claim 6, wherein: Pour the dust at the same depth into a plastic bag and mix it. Take a portion and put it into a sample bottle and compact it to the mark line. The mark line represents the volume mark of 10 to 15 grams of dust. The volume mark is determined by weighing the dust in the laboratory or on-site.

8. The method for determining the chloride ion content in concrete powder according to claim 5, wherein: The dust is dry dust with a particle size of 0.02 to 0.08 mm.

9. The method for determining the chloride ion content in concrete powder according to claim 1, wherein: The shaking time of step S3 is 3 to 5 minutes.

10. The method for determining the chloride ion content in concrete powder according to claim 1, wherein: The chloride ion concentration was detected using a chloride ion selective electrode. Before detection, the chloride ion selective electrode was activated in a sodium chloride solution with a concentration of 0.001 mol / L for 2 h, and then calibrated in sodium chloride calibration solutions with concentrations of 0.005 mol / L, 0.02 mol / L, 0.05 mol / L, and 0.5 mol / L, respectively.

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