CN110887860A - Method for detecting water content of sand based on low-field nuclear magnetic resonance - Google Patents

Abstract

The invention relates to a method for detecting water content of sand based on low-field nuclear magnetic resonance, which comprises the following steps: taking a dry sand sample, and adding water to prepare a reference sample; collecting reference sample data by using a low-field nuclear magnetic resonance instrument; drawing a corresponding transverse relaxation time attenuation curve according to the reference sample data and carrying out inversion calculation to obtain a corresponding transverse relaxation time distribution curve and a relaxation peak area; obtaining a fitting equation of the relaxation peak area and the water content through curve fitting; taking sand to be measured in a natural wet state, collecting the data of the sand to be measured by using a low-field nuclear magnetic resonance instrument, drawing a corresponding transverse relaxation time attenuation curve, performing inversion calculation to obtain a relaxation peak area of the sand to be measured, and substituting the relaxation peak area into a fitting equation of the relaxation peak area and water content to obtain the water content of the sand to be measured; and calculating to obtain the water content of the sand to be detected according to the quality and the water content of the sand to be detected. Compared with the prior art, the method has the advantages of rapid detection process, no damage and high detection result accuracy.

Description

A method for detecting sand moisture content based on low-field nuclear magnetic resonance

technical field

The invention relates to the technical field of road engineering applications, in particular to a method for detecting sand moisture content based on low-field nuclear magnetic resonance.

Background technique

Concrete water consumption has an important influence on the working performance of fresh concrete, mechanical properties and durability of hardened concrete. The moisture content of sand is a key link in controlling the water consumption of concrete, which directly determines the actual amount of mixing water. The moisture content of sand is easily affected by factors such as species, place of origin, storage method, weather and environment, resulting in large fluctuations, which will increase the difficulty of construction personnel in controlling the quality of concrete production. Therefore, it is of great engineering significance to determine the moisture content of sand quickly and accurately and to correct the concrete mix ratio in time to ensure the quality of concrete production.

At present, the water content determination method of sand is tested according to GB/T14684-2011 "Sand for Construction". The specific methods are as follows:

1) Divide the sample in the natural wet state to about 1100g by quartering method, and divide it into two roughly equal parts for use after mixing well.

2) Weigh the mass of a sample, accurate to 0.1g. Pour the sample into a beaker of known mass, and bake it in a drying oven at (105±5)℃ to a constant weight, accurate to 0.1g.

3) The moisture content is calculated as follows:

In the formula: Z is the moisture content, G ₂ is the mass of the sample before drying, and G ₁ is the mass of the sample after drying.

This method takes a long time and easily affects the construction progress of the project. In addition, the commonly used methods for determining the water content of sand also include: alcohol burning method, microwave method, resistance method, capacitance method, etc., but these methods all have certain shortcomings: alcohol burning method and microwave method need to strictly control the drying Drying time and sample dosage, otherwise it will cause damage to the sample; the resistance method and the capacitance method are easily affected by the ambient temperature and the concentration of salt ions in the water, resulting in low accuracy of the measurement results. Therefore, establishing a non-destructive, rapid and accurate method for determining the water content of sand will be of great significance and value to engineering construction.

In recent years, low-field nuclear magnetic resonance technology has been widely used in the field of cement-based materials due to its simplicity, rapidity, and non-destructiveness. Determination of body secretion, etc. The basic principle of low-field nuclear magnetic resonance is to make the protons in water molecules transition from a low-energy state to a high-energy state by absorbing the energy provided by the radio frequency field without destroying the sample. Then from the non-equilibrium state of high energy level to the equilibrium state of low energy level, and generate the corresponding nuclear magnetic resonance signal, this process is called relaxation. One of the main parameters of nuclear magnetic resonance technology is the relaxation time, which can essentially reflect the interaction between the spin nucleus system and the environment or the interior of the system, that is, the same atomic nucleus, when the chemical and physical environments are different, its relaxation time. Time will change. The water contained in the sand is usually free water and physically bound water, and its transverse relaxation time value is usually 0.1-10ms. Therefore, the present invention considers the use of low-field nuclear magnetic resonance technology to detect the water content of the sand, through the relaxation time To distinguish water molecules in different states, after searching, the application of low-field nuclear magnetic resonance technology to detect the water content of sand is still rare in China.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for detecting the water content of sand based on low-field nuclear magnetic resonance in order to overcome the defects existing in the above-mentioned prior art, and to obtain the relaxation time by using low-field nuclear magnetic resonance technology, according to the relationship between relaxation and water content, The water content of the sand is calculated to achieve the purpose of fast, non-destructive and accurate detection of the water content of the sand.

The object of the present invention can be achieved by the following technical solutions: a method for detecting the water content of sand based on low-field nuclear magnetic resonance, comprising the following steps:

S1. Take a plurality of dry sand samples, add water to make a plurality of reference samples, and record the water content of each reference sample at the same time;

S2. Use a low-field nuclear magnetic resonance instrument to collect the data of each reference sample respectively;

S3. According to the collected data of each reference sample, draw a transverse relaxation time decay curve corresponding to each reference sample;

S4. Invert and calculate the transverse relaxation time decay curve of each reference sample to obtain the transverse relaxation time distribution curve and corresponding relaxation peak area of each reference sample;

S5. Combined with the relaxation peak area and water content of each reference sample, through curve fitting, the fitting equation of the relaxation peak area and water content is calculated;

S6. Take the sand to be tested in a natural wet state, record the quality of the sand to be tested, and use the same method as step S2 to collect the data of the sand to be tested by using a low-field nuclear magnetic resonance instrument;

S7, according to the collected sand data to be tested, in the same manner as in step S3, draw the transverse relaxation time decay curve of the sand to be tested;

S8. In the same way as in step S4, invert and calculate the transverse relaxation time decay curve of the sand to be measured, and obtain the transverse relaxation time distribution curve of the sand to be measured and the corresponding relaxation peak area;

S9. Substitute the relaxation peak area of the sand to be measured into the fitting equation of the relaxation peak area and the water content in step S5 to obtain the water content of the sand to be measured;

S10. According to the quality and water content of the sand to be tested, the water content of the sand to be tested can be calculated.

Further, the step S1 is specifically to take a plurality of dry sand samples with a mass of 5-12 g, add water to prepare a plurality of wet sand samples with a moisture content of 0.5% to 10%, and record the wet sand samples at the same time. The amount of water added, the amount of water added to the wet sand sample is equal to the product of the mass of the dry sand sample and the moisture content of the wet sand sample. The reference sample, wherein the water content of the wet sand sample is the water content of the reference sample.

Further, the anti-interference test container is specifically a tetrafuran product container or a quartz container.

Further, in the step S2, each reference sample is vertically placed into the sample chamber of the low-field NMR instrument, and the number of echoes, the cumulative sampling times and the repeated sampling interval are adjusted, and the samples are collected separately based on the CPMG sequence. Each reference sample data.

Further, the number of echoes is 10000-20000, the cumulative sampling times are 4-16, and the repeated sampling interval time is 4000-8000s.

Further, the low-field nuclear magnetic resonance instrument is specifically a MICRO-MR20 nuclear magnetic resonance analyzer.

Further, the step S4 specifically uses InvFit software to invert and calculate the transverse relaxation time decay curve.

Further, the step S5 specifically includes the following steps:

S51. Combine the relaxation peak area and water content of each reference sample, take the relaxation peak area as the abscissa and the water content as the ordinate, and draw a scatter diagram;

S52. Perform curve fitting on the scatter plot to obtain the fitting curve of the relaxation peak area and the water content, and then obtain the equation corresponding to the fitting curve according to the univariate linear regression equation, which is the fitting of the relaxation peak area and the water content equation.

Further, the step S6 is to take 5-12 g of sand to be tested in a natural wet state, record the quality of the sand to be tested, put the sand to be tested into a plastic test tube with anti-interference signal, and then put it vertically into a low field. In the sample chamber of the nuclear magnetic resonance instrument, the same method as in step S2 is used to collect the sand data to be measured by using a low-field nuclear magnetic resonance instrument.

Further, the water content calculation formula of the sand to be tested in the step S11 is:

Among them, Z is the water content of the sand, m ₁ is the quality of the sand to be tested, and m ₂ is the water content of the sand to be tested.

Compared with the prior art, the present invention detects the water content of the sand based on the low-field nuclear magnetic resonance technology, and specifically uses the low-field nuclear magnetic resonance instrument to test the transverse relaxation time (T ₂ ) decay curve of the sand sample; The T ₂ decay curve of , was calculated by inversion, and the distribution curve of the T ₂ value at different relaxation times and the peak area of the relaxation peak were obtained. According to the relationship between the peak area of the relaxation peak and the corresponding water content, the water content of the sand is calculated. The detection process is fast and the detection results are reliable. In addition, the anti-interference test instrument is used to ensure that the detection process is not disturbed by the environment, which further guarantees The non-destructiveness of the sand to be tested and the accuracy of the test results.

Description of drawings

Fig. 1 is the method flow schematic diagram of the present invention;

Fig. 2 is the transverse relaxation time distribution curve of each reference sample in the embodiment;

Fig. 3 is the fitting curve of relaxation peak area and water content in the embodiment;

FIG. 4 is a lateral relaxation time distribution curve of the sand to be tested in the embodiment.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

As shown in Figure 1, a method for detecting sand moisture content based on low-field nuclear magnetic resonance includes the following steps:

S1. Take a plurality of dry sand samples, add water to make a plurality of reference samples, and record the water content of each reference sample at the same time;

S2. Use a low-field nuclear magnetic resonance instrument to collect the data of each reference sample respectively;

S3. According to the collected data of each reference sample, draw a transverse relaxation time decay curve corresponding to each reference sample;

S4. Invert and calculate the transverse relaxation time decay curve of each reference sample to obtain the transverse relaxation time distribution curve and corresponding relaxation peak area of each reference sample;

S5. Combined with the relaxation peak area and water content of each reference sample, through curve fitting, the fitting equation of the relaxation peak area and water content is calculated;

S6. Take the sand to be tested in a natural wet state, record the quality of the sand to be tested, and use the same method as step S2 to collect the data of the sand to be tested by using a low-field nuclear magnetic resonance instrument;

S7, according to the collected sand data to be tested, in the same manner as in step S3, draw the transverse relaxation time decay curve of the sand to be tested;

S8. In the same way as in step S4, invert and calculate the transverse relaxation time decay curve of the sand to be measured, and obtain the transverse relaxation time distribution curve of the sand to be measured and the corresponding relaxation peak area;

S9. Substitute the relaxation peak area of the sand to be measured into the fitting equation of the relaxation peak area and the water content in step S5 to obtain the water content of the sand to be measured;

S10. According to the quality and water content of the sand to be tested, the water content of the sand to be tested can be calculated.

The above method is applied to this embodiment, and the specific process is as follows:

(1) Weigh a plurality of dry sand samples with a mass of 5-12 g, add water respectively, configure each dry sand sample into a wet sand sample with a moisture content of 0.5% to 10%, and put each wet sand sample into the Shake well in an anti-interference test container to make multiple reference samples, the mass is recorded as m ₀ , and the water content of each reference sample is recorded at the same time;

(2) Place the reference sample vertically in the sample chamber of the low-field NMR instrument, collect data, adjust the parameters of the number of echoes, the cumulative sampling times and the repeated sampling interval, and plot the T ₂ (transverse relaxation time) decay time) decay curve.

(3) Use InvFit software to invert the drawn T ₂ decay curve, and obtain the T ₂ value distribution curve and relaxation peak area at different relaxation times. The T ₂ value distribution curve and relaxation peak area can reflect different sand moisture content at relaxation time;

(4) Taking the relaxation peak area as the abscissa and the water content as the ordinate, and combining the relaxation peak area and water content of each reference sample, draw a scatter diagram, and make a fitting curve between the relaxation peak area and the water content, and then According to the univariate linear regression equation, the equation expression of the fitting curve is obtained, that is, the fitting equation of the relaxation peak area and the water content is obtained;

(5) Weigh 5~12g of sand to be tested in a natural wet state, the mass is recorded as m ₁ , put it into a plastic test tube with anti-interference signal, and then place it vertically in the sample chamber of the low-field NMR instrument to conduct data analysis. acquisition, the steps are the same as those described in steps (2) and (3), to obtain the relaxation peak area of the sand to be measured;

(6) Substitute the relaxation peak area of the sand to be tested into the fitting equation obtained in step (4), and calculate the water content corresponding to the relaxation peak area of the sand to be tested, which is recorded as m ₂ ;

(7) The water content of sand can be calculated according to the following formula

In the formula: Z—water content; m ₂ —the water content of the sand to be tested calculated according to the fitting formula; m ₁ —the quality of the sand to be tested in the natural state.

In the present invention, the water content of the reference samples described in step (1) is different, and the number of reference samples is two or more, and the test container is a tetrafuran product or a special quartz container.

In step (2), the CPMG sequence is used to collect data, and there are certain requirements for the parameter setting of the nuclear magnetic resonance instrument. The main parameter ranges are as follows:

Number of echoes NECHECH: 10000-20000;

Cumulative sampling times NS: 4-16;

The interval time of repeated sampling TW(s): 4000-8000.

In the embodiment, firstly, according to steps (3) and (4), the fitting curve and equation expression of the relationship between the relaxation peak area and the water content are obtained, which can be directly used after comparing and verifying with the actual situation. The method proposed by the invention can directly perform steps (5)-(7) for detecting the water content of sand, without repeating the operations of steps (1)-(4).

Specifically, for the reference sample: Weigh 5 parts of dry sand with a mass of 5-12g, add water to configure it into wet sand with a moisture content of 0.5%-10%, put it into an anti-interference test container and shake well , as the benchmark sample. The quality and actual moisture content of the benchmark samples are shown in Table 1. The reference sample is placed vertically in the sample chamber of the low-field NMR instrument to collect data, and the parameters that affect the data collection, such as the number of echoes, echo time, and repeated sampling time, are _adjusted respectively, and the T2 attenuation curve is drawn. The test equipment and parameters are: MICRO-MR20 nuclear magnetic resonance analyzer is used for the test, CPMG pulse sequence is used, the number of echoes is NECH=18000, the cumulative sampling times of each CPMG signal is NS=4 times, and repeated attenuation, repeated sampling interval time TW is 5000ms. InvFit software was used to invert the drawn T ₂ decay curve, and the distribution curve of T ₂ value at different relaxation times and the peak area of the relaxation peak were obtained, as shown in Figure 2. Taking the relaxation peak area as the abscissa and the water content as the ordinate, draw a scatter diagram to make a fitting curve between the relaxation peak area and the water content, and then find the equation expression of the fitting curve according to the univariate linear regression equation. The fitting curve is shown in Figure 3, and the equation expression is: y=0.0004x-0.01748, R ² >0.9955. The corresponding water content m ₂ can be obtained by substituting the relaxation peak area of each reference sample into the equation expression, and the water content can be calculated by formula (A) (Note: m ₁ in the formula here is the reference sample mass m ₀ ), and the results are shown in Table 1. After inspection, the error between the calculated moisture content and the actual moisture content is less than or equal to 5%, which shows that the error between the detection result of the method of the present invention and the reality is small, and the detection result is reliable and accurate.

Table 1

For the sand to be tested: MICRO-MR20 nuclear magnetic resonance analyzer is used for the test, CPMG pulse sequence is used, the number of echoes is NECH=18000, the cumulative sampling times of each CPMG signal is NS=4 times, and the attenuation is repeated, and the repeated sampling interval is TW is 5000ms. According to the method described above, weigh three parts of the sand to be tested with a mass of 5-12 g and in a natural wet state, the mass is recorded as m ₁ , put it into a plastic test tube with anti-interference signal, and then place it vertically on a low In the sample chamber of the field NMR instrument, data was collected, and the T ₂ decay curve drawn was inverted and calculated using the InvFit software, and the distribution curve and relaxation peak area of the T ₂ value at different relaxation times were obtained, as shown in Figure 4. shown. Substitute the relaxation peak area into the fitting equation y=0.0004x-0.01748 obtained in the example to obtain the water content m ₂ of the sand to be measured. The water content of the sand to be measured can be calculated by formula (A). The results are shown in Table 2. .

Table 2

Claims (10)

1. a method for detecting sand moisture content based on low-field nuclear magnetic resonance, is characterized in that, comprises the following steps: S1. Take a plurality of dry sand samples, add water to make a plurality of reference samples, and record the water content of each reference sample at the same time; S2. Use a low-field nuclear magnetic resonance instrument to collect the data of each reference sample respectively; S3. According to the collected data of each reference sample, draw a transverse relaxation time decay curve corresponding to each reference sample; S4. Invert and calculate the transverse relaxation time decay curve of each reference sample to obtain the transverse relaxation time distribution curve and corresponding relaxation peak area of each reference sample; S5. Combined with the relaxation peak area and water content of each reference sample, through curve fitting, the fitting equation of the relaxation peak area and water content is calculated; S6. Take the sand to be tested in a natural wet state, and record the quality of the sand to be tested. In the same way as in step S2, use a low-field nuclear magnetic resonance instrument to collect the data of the sand to be tested; S7, according to the collected sand data to be tested, in the same manner as in step S3, draw the transverse relaxation time decay curve of the sand to be tested; S8. In the same way as in step S4, invert and calculate the transverse relaxation time decay curve of the sand to be measured, and obtain the transverse relaxation time distribution curve of the sand to be measured and the corresponding relaxation peak area; S9. Substitute the relaxation peak area of the sand to be measured into the fitting equation of the relaxation peak area and the water content in step S5 to obtain the water content of the sand to be measured; S10. According to the quality and water content of the sand to be tested, the water content of the sand to be tested can be calculated. 2 . The method for detecting sand water content based on low-field nuclear magnetic resonance according to claim 1 , wherein the step S1 is to take a plurality of dry sand samples with a mass of 5 to 12 g, and add water to configure them respectively. 3 . A plurality of wet sand samples with a moisture content of 0.5% to 10% are formed, and the amount of water added to the wet sand samples is recorded at the same time. The amount of water added to the wet sand samples is equal to the product of the mass of the dry sand samples and the moisture content of the wet sand samples. , and then put each wet sand sample into the anti-interference test container and shake well to make a plurality of reference samples, wherein the water content of the wet sand sample is the water content of the reference sample. 3 . The method for detecting sand moisture content based on low-field nuclear magnetic resonance according to claim 2 , wherein the anti-interference test container is specifically a tetrafuran product container or a quartz container. 4 . 4. a kind of method for detecting sand water content based on low-field nuclear magnetic resonance according to claim 1, it is characterized in that, described step S2 specifically is to put each reference sample vertically into the low-field nuclear magnetic resonance instrument sample chamber respectively Inside, by adjusting the number of echoes, the cumulative sampling times and the repeated sampling interval, the data of each reference sample is collected based on the CPMG sequence. 5 . The method for detecting water content of sand based on low-field nuclear magnetic resonance according to claim 4 , wherein the number of echoes ranges from 10,000 to 20,000, the cumulative sampling times ranges from 4 to 16, and the The repeated sampling interval is 4000-8000s. 6 . The method for detecting sand water content based on low-field nuclear magnetic resonance according to claim 1 , wherein the low-field nuclear magnetic resonance instrument is specifically a MICRO-MR20 nuclear magnetic resonance analyzer. 7 . 7 . The method for detecting the water content of sand based on low-field nuclear magnetic resonance according to claim 1 , wherein the step S4 specifically adopts InvFit software to invert and calculate the transverse relaxation time decay curve. 8 . 8. a kind of method for detecting sand water content based on low-field nuclear magnetic resonance according to claim 1, is characterized in that, described step S5 specifically comprises the following steps: S51. Combine the relaxation peak area and water content of each reference sample, take the relaxation peak area as the abscissa and the water content as the ordinate, and draw a scatter diagram; S52. Perform curve fitting on the scatter plot to obtain the fitting curve of the relaxation peak area and the water content, and then obtain the equation corresponding to the fitting curve according to the univariate linear regression equation, which is the fitting of the relaxation peak area and the water content equation. 9 . The method for detecting sand moisture content based on low-field nuclear magnetic resonance according to claim 1 , wherein the step S6 is specifically taking 5-12 g of sand to be tested in a natural wet state, and recording the sand to be tested. 10 . The quality of the sand, put the sand to be tested into a plastic test tube with anti-interference signal, and then put it vertically into the sample chamber of the low-field nuclear magnetic resonance instrument. In the same way as step S2, use the low-field nuclear magnetic resonance instrument to collect the data of the sand to be tested. . 10. A method for detecting water content of sand based on low-field nuclear magnetic resonance according to claim 1, wherein the calculation formula of the water content of the sand to be measured in the step S11 is:

Among them, Z is the water content of the sand, m ₁ is the quality of the sand to be tested, and m ₂ is the water content of the sand to be tested.

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