Cement-based material undisturbed hole measuring method based on low-field magnetic resonance longitudinal relaxation technology

Abstract

The invention discloses a cement-based material undisturbed pore measuring method based on a low-field magnetic resonance longitudinal relaxation technology, and the method overcomes the technical bottlenecks of a conventional magnetic resonance testing method in signal collection integrity, rapid relaxation signal capture and longitudinal surface relaxation intensity measurement through optimizing the collection and processing technology of longitudinal relaxation signals. The precise detection of the pore structure of the cement-based material from the nanoscale to the micron scale is realized. Compared with a traditional transverse relaxation method, the method disclosed by the invention is not interfered by paramagnetic substances when being used for testing the iron-containing Portland cement-based material, and has higher applicability and accuracy. Generally speaking, under the condition that a sample is not subjected to drying pretreatment, the pore distribution characteristics of the cement-based material can be accurately represented in a lossless and undisturbed manner, a scientific basis is provided for optimizing the performance of the cement-based material and improving the engineering quality control level, and further development of a pore structure testing technology in the fields of constructional engineering and material science is effectively promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of pore structure testing of cement-based materials, and specifically to a method for in-situ pore measurement of cement-based materials based on low-field nuclear magnetic resonance longitudinal relaxation technology. Background Art

[0002] As the material basis of the civil engineering industry, the precise design and regulation of the properties of cement-based materials are crucial for ensuring the engineering quality and extending the service life of structures. The key properties of cement-based materials, such as mechanical properties, volume stability, and durability, are closely related to their complex pore structures. The size and distribution of multi-scale pores significantly affect and even largely directly determine the key properties of cement-based materials, such as strength, crack resistance, impermeability, and durability. Conventional techniques such as mercury intrusion porosimetry and gas adsorption method are often used for testing. However, when using these conventional techniques to test the pore structure of porous materials, it is required to perform dry pretreatment on the samples. Since the important component hydrated calcium silicate gel inside the cement-based materials is particularly fragile, its nano-pore structure evolves dynamically with the change of water content. In the completely dry state, the pore structure of the cement-based materials has changed significantly compared with the high saturation condition, and its percolation pore diameter may even differ by 1-2 orders of magnitude. In most service environments, the cement-based materials maintain a high water content state, and there may be an obvious difference between the pore structure tested by conventional techniques such as mercury intrusion porosimetry and the actual state, thus possibly leading to incorrect conclusions.

[0003] Currently, non-destructive characterization methods for in-situ testing of the pore structure of porous media without dry pretreatment mainly include small-angle neutron or X-ray scattering method, thermoporometry, X-ray computed tomography, and low-field nuclear magnetic resonance method. The small-angle scattering method uses X-ray or neutron beam to detect microstructural features at the nano- to micro-scale. In theory, it can test the undried samples, but it can only obtain the overall features such as the specific surface area of nano-pores and is difficult to directly characterize the pore size distribution. Thermoporometry uses the detection of heat changes during the freezing and melting process of water for testing. In theory, it is applicable to the testing of nano-scale pores, but the temperature change will also cause significant evolution of the hydrated calcium silicate gel and its nano-pore structure, and the influence of temperature change on the pore structure cannot be decoupled and deducted. The accuracy of the pore structure information obtained by thermoporometry is questionable. X-ray computed tomography can achieve three-dimensional visualization imaging of the pore structure, but its spatial resolution can only reach the sub-micron level, and it is difficult to characterize the nano-scale pores that are rich and extremely important in cement-based materials.

[0004] Low-field nuclear magnetic resonance method uses hydrogen nuclei in water as probes, which can non-destructively and rapidly test the water content of water-saturated specimens and the distribution of water in pores of different scales (i.e., pore structure), having special technical advantages and application prospects, and has received extensive attention in the testing and analysis of cement-based materials. The low-field nuclear magnetic resonance method detects the transverse or longitudinal relaxation signals of pore water in saturated specimens, and inversely calculates the relaxation time spectrum (including transverse relaxation time spectrum and longitudinal relaxation time spectrum). After strictly calibrating the unit signal amount and reasonably determining the transverse or longitudinal surface relaxation intensity, the porosity and pore size distribution curve of the water-saturated specimen can be obtained by conversion. However, the transverse relaxation test is easily affected by paramagnetic substances (such as iron-containing minerals) in cement-based materials, which will cause significant deviation in the test results. Summary of the Invention

[0005] The object of the present invention is to provide a method for measuring the original pores of cement-based materials based on low-field nuclear magnetic resonance longitudinal relaxation technology for the problem that in the prior art, due to the transverse relaxation test being easily affected by paramagnetic substances in cement-based materials, the test results will have significant deviation.

[0006] The technical solution adopted by the present invention to solve the above technical problems is:

[0007] A method for measuring the original pores of cement-based materials based on low-field nuclear magnetic resonance longitudinal relaxation technology includes the following steps:

[0008] Step 1: Obtain the corrected free induction decay signal of the cement-based material sample, and the corrected free induction decay signal is the free induction decay signal after removing the background noise signal;

[0009] Step 2: Use the inverse Laplace algorithm to fit the corrected free induction decay signal to obtain the longitudinal relaxation signal amount M corresponding to each recovery time T, and based on the longitudinal relaxation signal amount M, obtain the recovery curve M(T) of the longitudinal relaxation signal with the recovery time; SR z0 z0 z SR

[0010] Step 3: Use the inverse Laplace algorithm to fit the recovery curve M(T) of the longitudinal relaxation signal with the recovery time to obtain the total relaxation signal amount M and the longitudinal relaxation time spectrum f(T); z SR 0 1i

[0011] Step 4: Obtain the longitudinal surface relaxation intensity ρ of the cement-based material sample; 1

[0012] Step 5: Based on the total relaxation signal amount M 0, and combined with the relaxation signal quantity k of unit mass of water unit , the pore water content m of the saturated sample is obtained pw ;

[0013] Step Six: Using the pore water content m of the saturated sample pw , and combined with the density of water, the total pore volume is obtained. After that, the total volume V of the cement-based material sample is obtained 0 , and by calculating the ratio of the total pore volume to the total volume V of the cement-based material sample 0 , the porosity of the cement-based material sample is obtained

[0014] Step Seven: Using the longitudinal surface relaxation strength ρ of the cement-based material sample 1 and the longitudinal relaxation time spectrum f(T 1i ), the equivalent pore diameter r corresponding to each longitudinal relaxation time in the longitudinal relaxation time spectrum f(T 1i ) is obtained 1i ;

[0015] Step Eight: According to the longitudinal relaxation time spectrum f(T 1i ), the signal quantity f corresponding to each longitudinal relaxation time in the longitudinal relaxation time spectrum f(T 1i ) is obtained i , and based on the signal quantity f i , the porosity of the cement-based material sample the density ρ of water w and the total volume V of the cement-based material sample 0 , the pore volume fraction θ corresponding to each equivalent pore diameter is obtained i ;

[0016] Step Nine: Using the porosity of the cement-based material sample the equivalent pore diameter r 1i and the signal quantity f i , the volume specific surface area S is obtained v ;

[0017] Step Ten: Obtain the saturated mass m of the cement-based material sample sat , and use the saturated mass m of the cement-based material sample sat , the total volume V of the cement-based material sample 0 , the volume specific surface area S v and the pore water content m of the saturated sample pw , the mass specific surface area S is obtained m .

[0018] Furthermore, the specific steps of the said Step One are:

[0019] Step 1: Use the saturation recovery-free induction decay sequence to collect signals from the empty sample chamber of the nuclear magnetic resonance instrument to obtain the background noise signal;

[0020] Step 2: Obtain a cement-based material sample and perform water saturation treatment on the cement-based material sample;

[0021] Step 3: Wrap the cement-based material sample with plastic wrap and place the cement-based material sample wrapped with plastic wrap in the sample chamber of the nuclear magnetic resonance instrument to collect the free induction decay signal corresponding to each recovery time;

[0022] Step 4: Subtract the free induction decay signal of the cement-based material sample from the background noise signal to obtain the corrected free induction decay signal.

[0023] Furthermore, in Step 2, the corrected free induction decay signal is fitted using the inverse Laplace algorithm and expressed as:

[0024]

[0025] where M xy (t) is the free induction decay signal, t is the time, M z0 is the initial intensity at the recovery time TSR, T 2i is the transverse relaxation time of the i-th group of pore water, i = 1, 2,..., J, and J is the number of groups of pores inside the cement-based material sample.

[0026] Furthermore, in Step 3, the inverse Laplace algorithm is used to fit the recovery curve M z (T SR ) of the longitudinal relaxation signal with respect to the recovery time and expressed as:

[0027]

[0028] where T 1i is the longitudinal relaxation time of the i-th component, and T SR is the recovery time.

[0029] Furthermore, the specific steps of Step 4 are as follows:

[0030] Step 4.1: Obtain a sample identical to the cement-based material sample in Step 1 and place the sample in a constant humidity environment at 20 ± 2 °C and a relative humidity lower than 23% for moisture desorption treatment until the sample mass reaches a constant state, where the sample mass reaching a constant state means that the relative mass loss of the cement-based material sample within 7 days is less than 1%;

[0031] Step 4.2: Based on the cement-based material sample with a constant mass, repeat Steps 1 to 2 to obtain the recovery curve M of the longitudinal relaxation signal with respect to the recovery timez (T SR );

[0032] Step Four Three: According to the recovery curve M of the longitudinal relaxation signal with the recovery time z (T SR ), obtain the minimum time value t min corresponding to the longitudinal relaxation signal M z (t min ), and combine with the M obtained in Step Three 0 to obtain the initial longitudinal relaxation time T 1,Init .

[0033] It is expressed as:

[0034] T 1,Init = M 0 t min / M z (t min );

[0035] Step Four Four: T 1,Init is the longitudinal surface relaxation time T 1S ;

[0036] Step Four Five: Obtain the thickness λ of the single-layer water, and combine with the longitudinal surface relaxation time T 1S to obtain the longitudinal surface relaxation intensity ρ 1 , which is expressed as:

[0037] ρ 1 = λ / T 1S .

[0038] Furthermore, the specific steps of Step Four are as follows:

[0039] Step A: Obtain a sample identical to the cement-based material sample in Step One, and place the sample in a constant humidity environment of 20 ± 2 °C and a relative humidity lower than 23% for moisture desorption treatment until the sample mass reaches a constant state, where the sample mass reaching a constant state means that the relative mass loss of the cement-based material sample within 7 days is less than 1%;

[0040] Step B: Based on the cement-based material sample with a constant mass, repeat Steps One to Two to obtain the recovery curve M of the longitudinal relaxation signal with the recovery time z (T SR );

[0041] Step C: Select 7 recovery time points at equal intervals on the longitudinal relaxation recovery curve M z (T SR ), with an interval of 5 - 10 microseconds, and then use the linear fitting method to perform linear fitting on the 7 recovery time points to obtain the initial longitudinal relaxation time T1,Init , the fitting is expressed as:

[0042]

[0043] Step D: T 1,Init is the longitudinal surface relaxation time T 1S ;

[0044] Step E: Obtain the thickness λ of the single-layer water, and combine it with the longitudinal surface relaxation time T 1S to obtain the longitudinal surface relaxation intensity ρ 1 , which is expressed as:

[0045] ρ 1 = λ / T 1S .

[0046] Furthermore, the pore water content m of the sample pw is expressed as:

[0047] m pw = M 0 / k unit

[0048] where M 0 is the initial intensity of the longitudinal relaxation signal at zero time, k unit is the relaxation signal amount corresponding to 1 g of water, and m pw is the saturated sample pore water content,

[0049] The relaxation signal amount k corresponding to 1 g of water unit is obtained through the following steps:

[0050] Step 1: Obtain a sample identical to the cement-based material sample in Step 1, and place this sample in a vacuum water saturation device for water saturation treatment to obtain a water-saturated sample;

[0051] Step 2: Use a high-precision electronic balance to measure the saturated mass m of the sample sat ;

[0052] Step 3: Place the water-saturated sample in an oven at 40°C for gentle drying. When the sample loses 0.2 g of water each time, record the current mass, and use the current sample as the cement-based material sample in Step 1. Repeat Steps 1 to 3 to obtain the total relaxation signal amount M of this sample 0 ;

[0053] Step 4: Repeat Step 3, 5 - 10 times. Finally, place the sample in an oven at 105°C and dry it for 72 hours until the mass no longer changes. Measure the completely dry mass m of the sample dry , and then obtain the mass m of the evaporable water in the sample w , which is expressed as:

[0054] m w = m sat -m dry

[0055] Step 5: Fit M 0 and m w 's linear relationship to obtain the signal quantity k of water per unit mass un i, and the fitting is expressed as:

[0056] M 0 = k unit ·m w

[0057] Step 6: Based on the pore water content m of the saturated sample pw , and combined with the density ρ of water w and the total volume V of the sample 0 , obtain the porosity of the sample which is expressed as:

[0058]

[0059] Furthermore, the equivalent pore diameter r of each longitudinal relaxation time in the longitudinal relaxation time spectrum f(T 1i ) is expressed as: 1i

[0060] r 1i = αρ 1 T 1i

[0061] where α is a shape parameter, taking 1 for flat plate-shaped pores, 2 for cylindrical pores, and 3 for spherical pores.

[0062] Furthermore, the volume fraction θ of the i-th pore i is expressed as:

[0063]

[0064] Furthermore, the volume specific surface area is expressed as:

[0065]

[0066] The mass specific surface area is expressed as:

[0067] S m = V 0 S v / (m sat -m pw ).

[0068] The beneficial effects of the present invention are:

[0069] ​By optimizing the acquisition and processing technology of longitudinal relaxation signals, this application overcomes the technical bottlenecks in the integrity of signal acquisition, the capture of fast relaxation signals, and the determination of longitudinal surface relaxation intensity in traditional magnetic resonance testing methods, and realizes the precise detection of the pore structure of cement-based materials from the nanoscale to the microscale. Compared with the traditional transverse relaxation method, the method of this application is not interfered by paramagnetic substances when testing iron-containing silicate cement-based materials, and has higher applicability and accuracy. Generally speaking, this application can nondestructively, in-situ, and accurately characterize the pore distribution characteristics of cement-based materials without drying and preprocessing the samples, providing a scientific basis for optimizing the performance of cement-based materials and improving the level of engineering quality control, and effectively promoting the further development of pore structure testing technology in the fields of construction engineering and materials science. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 It is a schematic diagram of the saturation recovery-free induction decay test method and typical free induction decay signals of this application.

[0071] Figure 2 It is a schematic diagram of the saturation recovery-free induction decay longitudinal relaxation signals of typical white cement mortar and portland cement mortar of this application;

[0072] Figure 3 It is a schematic diagram of the relationship between the measured water content and the actual water content of the calibrated mortar sample of this application;

[0073] Figure 4 It is a schematic diagram of the porosity of white cement mortar and portland cement mortar obtained by various methods of this application Figure 1 ;

[0074] Figure 5 It is a schematic diagram of the porosity of white cement mortar and portland cement mortar obtained by various methods of this application Figure 2 ;

[0075] Figure 6 It is a schematic diagram of the pore size distribution of white cement mortar and portland cement mortar obtained by various methods of this invention Figure 1 ;

[0076] Figure 7 It is a schematic diagram of the pore size distribution of white cement mortar and portland cement mortar obtained by various methods of this invention Figure 2 。 DETAILED DESCRIPTION OF THE EMBODIMENTS

[0077] It should be noted in particular that, without conflict, the various embodiments disclosed in this application can be combined with each other.

[0078] Embodiment 1: The method for measuring pores in the original state of cement-based materials based on low-field nuclear magnetic resonance longitudinal relaxation technology described in this embodiment includes:

[0079] S1. Measure the longitudinal relaxation signal of the sample to be tested;

[0080] Set the test parameters, including setting the basic parameters of the nuclear magnetic resonance instrument, and the key parameters of the saturation recovery sequence and the free induction decay sequence. The basic parameters of the nuclear magnetic resonance instrument include the widths and frequencies of the 90° and 180° pulses, and the center frequency of the nuclear magnetic resonance instrument; the parameters of the saturation recovery sequence include the number of 90° pulses, the recovery time, and the number of scans; the parameters for collecting the free induction decay signal include the relaxation time, the acquisition delay time, the sampling time interval, and the number of data points collected.

[0081] According to the test requirements, set the above test parameters, and then collect the longitudinal relaxation signal, including sample signal collection, background noise signal collection, and signal data storage. First, perform sample signal collection. Use a high-precision electronic balance to measure the saturated mass m of the sample sat and wrap it tightly with plastic wrap, then put it into the sample chamber, and use the saturation recovery-free induction decay sequence to perform longitudinal relaxation signal collection in sequence according to the set recovery time T SR To improve the signal-to-noise ratio, use the method of multiple scans to repeatedly collect the sample signal at each recovery time T SR After the sample signal collection is completed, perform background noise signal collection. Set the same test parameters and use the saturation recovery-free induction decay sequence to test the empty sample chamber. The collected background noise signal can be used as a calibration reference for subsequent signal data analysis; finally, perform signal data storage. Store the collected longitudinal relaxation signals in the order of the recovery time T SR and perform a preliminary check on the longitudinal relaxation signals to ensure that the longitudinal relaxation signals are complete and there is no obvious abnormal interference;

[0082] S2. Analyze and invert the relaxation signal;

[0083] First, perform background noise correction on the collected sample signal, subtract the background noise signal to avoid its interference affecting subsequent analysis. Then, use the inverse Laplace algorithm to fit the corrected free induction decay signal to obtain the initial intensity M of the signal at different recovery times T SR at each moment, and thus obtain the recovery curve M of the longitudinal relaxation signal with the recovery time z0 (T z ), which can reflect the relaxation characteristics of the moisture inside the sample. Then perform inversion analysis. Use the inverse Laplace algorithm to perform inversion analysis on the longitudinal relaxation signal to calculate the total relaxation signal amount M SR and the longitudinal relaxation time spectrum f 0 i ​(T 1i ). The longitudinal relaxation time spectrum can accurately characterize the multi-scale structural characteristics of pores inside the saturated sample, and comprehensively reveal the pore structure information with pore diameters ranging from the nanometer scale to the micrometer scale.

[0084] S3. Determine the longitudinal surface relaxation strength ρ 1 and the equivalent pore diameter r 1i ;

[0085] In addition, prepare another set of test samples with the same raw materials and ratios as the pore structure specimens. Place this set of samples in a constant humidity environment at 20 ± 2 °C and a relative humidity below 23% for moisture desorption treatment until their mass reaches a constant state (the relative mass loss in 7 days is less than 1%) to ensure the formation of a stable monolayer water film adsorption state on the pore surface; then use the saturation recovery pulse sequence to measure the signal and background noise signal of this set of samples. Combining with the longitudinal relaxation theory, based on the data of the change process of the longitudinal relaxation signal of the sample with the recovery time, use the linear fitting method to perform fitting analysis on the relationship between the longitudinal relaxation signal and the recovery time. Through the slope of the fitting line, the longitudinal surface relaxation time T 1S can be determined. The calculation of this surface relaxation time is crucial for further pore structure analysis; for the problem that it is difficult to experimentally determine the longitudinal surface relaxation time T 1S and the longitudinal surface relaxation strength ρ 1 , finally, calculate the longitudinal surface relaxation strength ρ 1 , determine the longitudinal surface relaxation time T 1S . After that, the longitudinal surface relaxation strength ρ 1 can be calculated. This parameter provides important support for subsequent pore size distribution analysis. The longitudinal relaxation time can be converted into the equivalent pore diameter r 1i .

[0086] S4. Determine the pore structure information;

[0087] Determine the porosity. By the total relaxation signal amount M 0 of the saturated sample, combined with the relaxation signal amount k unit of water per unit mass, the pore water content m pw of the saturated sample can be calculated. Combining with the density of water, the total pore water volume (i.e., the total pore volume) can be calculated. The ratio of the total pore volume to the total volume of the sample is the porosity of the sample This application also provides a method for determining the relaxation signal amount k unit corresponding to water per unit weight;

[0088] Determine the pore size distribution, specifically including determining the equivalent pore diameter r 1i and the volume fraction θ i of pores at all levels. To determine the pore size, it is necessary to rely on the longitudinal relaxation time spectrum and the longitudinal surface relaxation strength ρ 1, convert different longitudinal relaxation times into corresponding pore sizes; according to the signal intensity ratio of pores with different pore sizes, the volume fraction of the pores can be determined, and thus the pore size distribution curve of the sample can be plotted;

[0089] Using the above test and calculation results, combined with the mass and volume of the sample, the volume specific surface area S of the sample can also be calculated v and the mass specific surface area S m , and these parameters are of great significance for evaluating the pore structure and its properties of cement-based materials;

[0090] In view of the problem that the existing pore structure characterization methods cannot non-destructively, in-situ and accurately characterize the multi-scale pore structure of iron-containing silicate cement-based materials in the saturated state, this application proposes a pore measurement method. This method does not require dry pretreatment of the sample, and can accurately characterize the multi-scale pore size distribution characteristics of its saturated state without damaging the pore structure of the cement-based material, overcoming the limitations of the existing pore measurement methods and breaking through a number of technical bottlenecks.

[0091] Example:

[0092] An in-situ pore measurement method for cement-based materials based on low-field nuclear magnetic resonance longitudinal relaxation technology includes:

[0093] S1 Test the longitudinal relaxation signal of the sample to be tested. The specific process includes:

[0094] S1.1 Set the test parameters. The specific process is as follows:

[0095] S1.1.1 Set the basic parameters of the nuclear magnetic resonance instrument. The basic parameters of the nuclear magnetic resonance instrument include the pulse widths and frequencies of the 90° and 180° pulses to ensure that the equipment operates in the best working state;

[0096] Set the 90° pulse width t 90 to 13 μs, and the 180° pulse width t 180 is also set to 13 μs to ensure that the pulse can fully excite the nuclear magnetic resonance signal in the sample;

[0097] Determine that the magnetic field strength of the nuclear magnetic resonance instrument is 0.047 T and the center frequency is 2 MHz. The center frequency needs to be determined according to the equipment situation;

[0098] S1.1.2 Set the saturation recovery sequence parameters;

[0099] In the saturation recovery-free induction decay pulse sequence test, set the 90° pulse width t 90 to 13 μs to ensure that all spins of the sample are excited, forcing the longitudinal magnetization vector M z to be saturated, all spins are completely perturbed, and the longitudinal magnetization vector disappears to ensure that M z = 0 at the initial state;

[0100] Set multiple value ranges, and within each value range, set the recovery time T at equal intervals SR， Then set multiple recovery times T SR , allowing the longitudinal magnetization of the sample to partially recover. By setting the value range of the recovery time T SR from microseconds to seconds, the longitudinal relaxation signals of pore water with different pore diameters can be collected. For different samples, the range of the recovery time T SR can be set according to the estimated value of the longitudinal relaxation time of the pore water;

[0101] The saturation recovery sequence parameters to be set and their values are as follows. The 90° pulse width t 90 takes 13 μs, the number of 90° pulses ranges from [3, 15], the recovery time T SR ranges from [50 μs, 15 s], and the number of scans ranges from [64, 8192].

[0102] Recovery time T SR and the number of scans need to be adjusted according to different samples. For cement mortar, the recovery time T SR can be set to: 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 15, 20, 25, 30, 40, 50, 60, 130, 280, 600, 1200, 1800, 2400, 3000, and 6000 ms; When the range of the recovery time T SR is [0.05, 0.35] ms, [0.4, 2.9] ms, [3, 9.5] ms, [10, 130] ms, and [280, 6000] ms, the number of scans is 4096, 2048, 1024, 512, and 256 respectively.

[0103] S1.1.3 Set the free induction decay signal acquisition parameters;

[0104] In the free induction decay test, first apply a 90° pulse t to the sample 90, and wait for the residual pulsed radiofrequency energy to dissipate. The time for waiting for the pulsed radiofrequency energy to dissipate is the damping time. After the pulsed radiofrequency energy dissipates, wait for the acquisition delay time and then start collecting signals. Collect the longitudinal relaxation signals at regular sampling time intervals (dwell time) until the number of collected data reaches the number of acquired data points (samples), and finally obtain a series of longitudinal relaxation signals that decay with the acquisition time;

[0105] The parameters of the free induction decay sequence that need to be set and their values are as follows. The 90° pulse width t 90 takes 13 μs, the damping time takes 25 μs, the acquisition delay time takes 50 μs, the sampling time interval value range is [0.5 μs, 4 μs], and the range of the number of acquired data points is [10,000, 200,000];

[0106] If the sampling interval is set short enough (usually in the order of microseconds), fast relaxation signals can be captured;

[0107] The collected free induction decay signals are used to calculate the longitudinal magnetization M of the sample z0 ;

[0108] Change the recovery time T SR , repeat the above test steps, and a series of free induction decay signals at the recovery time T SR can be collected;

[0109] S1.2 Place the sample. The specific process is as follows:

[0110] Cement-based material samples are usually neat cement pastes or mortar specimens made of cement and slag and other cementitious materials. The samples are cylindrical with a maximum diameter of 25 mm and a height not exceeding 60 mm. Use a vernier caliper to accurately measure the sample size to ensure that it meets the size limitations required by the nuclear magnetic resonance instrument. Place the sample in a vacuum saturation device for saturation treatment to ensure that the pores of the sample are completely saturated with water. Then, use a high-precision electronic balance to measure the saturated mass m of the sample sat , and then tightly wrap the cement-based material sample to be tested with plastic wrap to ensure that the sample does not exchange moisture with the environment during the test. Place the sample wrapped with plastic wrap in the sample chamber of the nuclear magnetic resonance instrument to ensure that the sample is stably placed and does not move during the test;

[0111] S1.3 Collect relaxation signals. The specific process is as follows:

[0112] S1.3.1 Collect the relaxation signals of the sample;

[0113] After placing the sample in the sample chamber, signal acquisition of the sample at different recovery times T SR is started. Each acquisition excites the sample with a 90° pulse. After a certain recovery time T SR free induction decay signal acquisition is carried out. During the acquisition process, multiple scans should be performed to improve the signal-to-noise ratio and ensure the reliability of the test data;

[0114] S1.3.2 Collect background noise signals;

[0115] After the sample test is completed, background noise signal acquisition is carried out. This process uses a saturation recovery - free induction decay sequence to test the empty sample chamber, and the collected signal will be used as the reference for subsequent noise correction;

[0116] When collecting background noise, all test parameters are kept consistent with those during the sample test to ensure the consistency of signal comparison;

[0117] S1.3.3 Store signal data;

[0118] The data obtained from each signal acquisition is stored and sorted in the order of recovery time T SR ;

[0119] During data storage, preliminary checks are carried out to ensure that the signal is complete and there is no obvious noise interference, and to avoid incorrect data caused by external interference;

[0120] S2 Relaxation signal analysis and inversion. The specific process is as follows:

[0121] S2.1 Background noise correction;

[0122] Before analyzing the sample signal, background noise signal correction is required;

[0123] The collected sample signal is subtracted by the background noise signal (i.e., the signal obtained from the empty sample chamber test) to ensure the accuracy of the sample signal. The corrected data will be used as the basis for subsequent inversion analysis;

[0124] S2.2 Signal fitting and calculation of longitudinal relaxation signal;

[0125] The corrected signal (the signal obtained by subtracting the background noise signal from the sample signal) is fitted using the inverse Laplace algorithm to obtain the initial intensity M z0

[0126] The initial intensity M SR at the corresponding recovery time T z0 is obtained. The fitting formula is as follows:

[0127]

[0128] Assume that the internal pores of the cement mortar can be subdivided into J groups. The longitudinal relaxation time and volume fraction of the pore water in the i-th (i = 1, 2,..., J) group are T 2i (s) and f i . M z0 (a.u.) represents the initial strength at the recovery time T SR .

[0129] Use the inverse Laplace algorithm to perform inversion analysis on the corrected free induction decay signal to obtain the longitudinal relaxation signal amount M SR at the corresponding recovery time T z0 . By performing inversion on different free induction decay signals corresponding to different recovery times T SR , generate the longitudinal relaxation recovery curve M z (T SR ) of the sample;

[0130] S2.3 Inversion analysis;

[0131] Adopt the inverse Laplace algorithm to perform inversion analysis on the longitudinal relaxation signal matrix M z (T SR ) to calculate the total relaxation signal amount M 0 and the longitudinal relaxation time spectrum f(T 1i ), which characterize the multi-scale structural characteristics of the internal pores of the sample, reveal the pore characteristics of the sample from the nanoscale to the microscale, and the inversion formula is as follows:

[0132]

[0133] where T 1i (s) represents the longitudinal relaxation time of the i-th component.

[0134] S3 Determine the longitudinal surface relaxation strength ρ 1 , and the specific process includes:

[0135] S3.1 Prepare the sample for measuring the longitudinal surface relaxation strength ρ 1 ;

[0136] In addition, prepare a group of test samples with the same raw materials and ratios as the pore structure specimens. Place this group of samples to be measured in a constant humidity environment of 20 ± 2 °C and a relative humidity lower than 23%, and perform moisture desorption treatment until the sample mass reaches a constant state (the relative mass loss in 7 days is less than 1%) to ensure a stable single-molecular water film adsorption state is formed on the pore surface;

[0137] S3.2 Collect relaxation signals;

[0138] The saturation recovery pulse sequence is used to test the signal of this group of samples and the background noise signal, and the obtained signal data is used for the calculation of the longitudinal surface relaxation intensity ρ 1 ;

[0139] S3.3 Determine the longitudinal surface relaxation time T 1S ;

[0140] According to the data of the change of the longitudinal relaxation signal of the sample with the recovery time in S1.3.2,

[0141] Using the linear fitting method to fit the longitudinal relaxation recovery curve M z (T SR ), the initial longitudinal relaxation time T 1,Init ;

[0142] Through the slope of the fitting line, the longitudinal surface relaxation time T 1S can be determined, and the calculation of this surface relaxation time is crucial for further pore structure analysis;

[0143] For the problem that it is difficult to experimentally determine the longitudinal surface relaxation time T 1S and the longitudinal surface relaxation intensity ρ 1 , this embodiment provides a method for deriving the longitudinal surface relaxation time T 1S through relaxation theory, and then calculating the longitudinal surface relaxation intensity ρ 1S through the longitudinal surface relaxation time T 1 ;

[0144] The specific process is as follows. Perform mathematical processing on the basic equation of longitudinal relaxation,

[0145] Adopt the Taylor expansion method to expand the magnetization intensity M z (t) near T SR = 0. When the time T SR is close to zero, the complex expression of the magnetization intensity M z (T SR ) can be simplified to an approximate form, which is convenient for further calculation,

[0146]

[0147] where T 1,Init (s) is the initial longitudinal relaxation time. According to the above formula, use the linear fitting method to fit and analyze the relationship between the longitudinal relaxation signal and the recovery time, and determine the surface relaxation time T 1S . It is also possible to measure M min at the minimum time value t z (t min when performing saturation recovery - free induction decay testing.) Subsequently, T can be calculated through the following formula 1,Init :

[0148] T 1,Init = M 0 t min / M z (t min )

[0149] When there is almost no other evaporable water except for the monolayer water adsorbed on the pore wall, the longitudinal surface relaxation time T 1S ≈ T 1,Init .

[0150] S3.4 Calculate the longitudinal surface relaxation strength ρ 1 ;

[0151] Based on the determined longitudinal surface relaxation time T 1S , combined with the known water molecule diameter, the longitudinal surface relaxation strength ρ can be calculated through the following calculation formula, 1 This parameter provides basic support for subsequent pore size distribution analysis.

[0152] ρ 1 = λ / T 1S

[0153] Among them, ρ 1 (m / s) represents the longitudinal surface relaxation strength; λ = 0.3 nm represents the thickness of the monolayer water;

[0154] S4 Determine the pore structure information, and the specific process includes:

[0155] S4.1 Determine the porosity;

[0156] Through the total relaxation signal amount M 0 of the saturated sample, combined with the relaxation signal amount k unit per unit mass of water, the pore water content m pw of the saturated sample can be calculated. The calculation formula is as follows:

[0157] m pw = M 0 / k unit

[0158] Among them, M 0 is the initial intensity of the longitudinal relaxation signal at zero time; k unit is the relaxation signal amount corresponding to 1 g of water, and m pw (g) represents the pore water content of the saturated sample;

[0159] Furthermore, the specific process of determining k unit is as follows:

[0160] First step: Based on another set of prepared test samples, place these samples in a vacuum water saturation device for water saturation treatment. After ensuring that the moisture in the sample pores is completely saturated, use a high-precision electronic balance to measure the saturated mass m of the sample sat (accurate to 0.001 g);

[0161] Second step: Put the water-saturated samples into an oven at 40 °C for gentle drying. When the sample loses 0.2 g of water each time, record the current mass. Repeat S1.1 - S2.3 to obtain the total relaxation signal amount M 0

[0162] Take out the sample at regular intervals and measure its mass to monitor the water loss process of the sample; when the sample loses about 0.2 g of water each time, record the current mass, and at the same time use a nuclear magnetic resonance instrument for testing. Obtain the total relaxation signal amount M according to the method described in S2.3 0 ;

[0163] Third step: After the sample has been tested multiple times (5 - 10 times) (losing 0.2 g each time), finally place the sample in an oven at 105 °C for 72 hours until the mass no longer changes, and measure its completely dry mass m dry ; Calculate the mass of evaporable water m of the sample through the following formula w , and the calculation formula is as follows:

[0164] m w = m sat - m dry

[0165] Fourth step: Fit the linear relationship between M 0 and m w , and use the following formula to determine the signal amount k of water per unit mass unit , and the calculation formula is as follows:

[0166] M 0 = k unit · m w

[0167] After calculating the pore water content m pw of the saturated sample, further combine with the density of water to calculate the total pore volume. By calculating the ratio of the total pore volume to the total volume of the sample, the porosity of the sample can be determined The calculation formula is as follows:

[0168]

[0169] Among them, represents the porosity of the sample, m pw (g) represents the pore water content of the saturated sample; ρ w (g / cm 3) is the density of water, taken as 1 g / cm 3 ; V 0 represents the total volume of the sample, measured by the buoyancy method or vernier caliper;

[0170] S4.2 Determine the pore size distribution. The specific process is as follows:

[0171] S4.2.1 Determine the pore size;

[0172] The longitudinal surface relaxation intensity ρ 1 obtained through S3.4 and the longitudinal relaxation time spectrum f(T 1i ), calculate the equivalent pore size r of each longitudinal relaxation time in the longitudinal relaxation time spectrum f(T 1i ) through a mathematical formula. The calculation formula is as follows: 1i； The formula is as follows:

[0173] r 1i = αρ 1 T 1i

[0174] Among them, r 1i (m) represents the equivalent pore size obtained in S3.4; α represents the shape parameter, taking 1 for flat pores, 2 for cylindrical pores, and 3 for spherical pores; the longitudinal relaxation time spectrum f(T 1i ) is obtained through S2.3;

[0175] S4.2.2 Determine the volume fraction;

[0176] According to the signal intensity volume fraction (fi), the pore volume fraction corresponding to each equivalent pore size can be determined through the following formula. On this basis, a pore size distribution curve is drawn. The calculation formula is as follows:

[0177]

[0178] Among them, θ i (m 3 / g) is the volume fraction of the i-th pore;

[0179] Taking the pore size as the abscissa and the volume fraction as the ordinate, determine the pore size distribution curve.

[0180] S4.3 Calculate the specific surface area;

[0181] Using the above test and calculation results, combined with the mass and volume of the sample, calculate the volume specific surface area S v and the mass specific surface area S m ;

[0182] Volume specific surface area S v (m -1 ):

[0183]

[0184] Mass specific surface area S m (m 2 / g):

[0185] S m = V 0 S v / (m sat - m pw )

[0186] It should be noted that the specific implementation manners are only explanations and illustrations of the technical solutions of the present invention, and the scope of the patent protection cannot be limited thereby. Those that are only partial changes made according to the claims and the specification of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for measuring the pore size of cement-based materials in situ based on low-field magnetic resonance longitudinal relaxation technology, characterized in that The following steps are involved: Step 1: obtaining a corrected free induction decay signal of a cement-based material sample, wherein the corrected free induction decay signal is a free induction decay signal from which a background noise signal is removed; Step 2: Use the inverse Laplace algorithm to fit the corrected free induction decay signal to obtain each recovery time T SR The corresponding longitudinal relaxation signal M z0 , and based on the longitudinal relaxation signal M z0 , and the recovery curve M of the longitudinal relaxation signal versus recovery time is obtained z (T SR ); Step 3: Use the inverse Laplace algorithm to calculate the recovery curve M of the longitudinal relaxation signal over the recovery time z (T SR ) is fitted to obtain the total relaxation signal M0 and the longitudinal relaxation time spectrum f(T 1i ); Step 4: Obtain the longitudinal surface relaxation strength ρ1 of the cement-based material sample; Step 5: Based on the total relaxation signal M0, combined with the relaxation signal k per unit mass of water unit , and the pore water content m of the saturated sample is obtained pw ; Step 6: Use the pore water content m of the saturated sample pw , and combined with the density of water, the total pore volume is obtained. Then, the total volume V0 of the cement-based material sample is obtained, and the porosity of the cement-based material sample is obtained by calculating the ratio of the total pore volume to the total volume V0 of the cement-based material sample. Step 7: Using the longitudinal surface relaxation strength ρ1 and longitudinal relaxation time spectrum f(T 1i ), and the longitudinal relaxation time spectrum f(T 1i ) corresponds to the equivalent aperture r for each longitudinal relaxation time 1i ; Step 8: According to the longitudinal relaxation time spectrum f(T 1i ), and the longitudinal relaxation time spectrum f(T 1i ) corresponds to the signal f for each longitudinal relaxation time i , and based on the semaphore f i , Porosity of cement-based material samples Density of water ρ w And the total volume V0 of the cement-based material sample, the pore volume fraction θ corresponding to each equivalent pore size is obtained i ; Step 9: Using the porosity of cement-based material samples Equivalent aperture r 1i And the semaphore f i , and the volume specific surface area S is obtained v ; Step 10: Obtain the saturated mass m of the cement-based material sample sat , and using the saturated mass m of the cement-based material sample sat , the total volume V0 of cement-based material samples, volume specific surface area S v And the pore water content of the saturated sample m pw , and obtain the mass specific surface area S m .

2. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 1, characterized in that The specific steps of step one are: Step 1: Use the saturation recovery-free induction decay sequence to collect signals from an empty NMR sample chamber to obtain a background noise signal; Step 1 and 2: obtaining cement-based material samples and subjecting the cement-based material samples to saturation treatment; Step 13: Wrap the cement-based material sample with plastic wrap, and place the cement-based material sample wrapped with plastic wrap in the sample chamber of the nuclear magnetic resonance instrument to collect the free induction decay signal corresponding to each recovery time; Step 14: Subtract the free induction decay signal of the cement-based material sample from the background noise signal to obtain a corrected free induction decay signal.

3. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 2, characterized in that In the step 2, the inverse Laplace algorithm is used to fit the corrected free induction decay signal as follows: Among them, M xy (t) is the free induction decay signal, t is time, M z0 is the initial strength at the recovery time TSR, T 2i is the transverse relaxation time of the i-th group of pore water, i = 1, 2, ..., J, and J is the number of pore groups inside the cement-based material sample.

4. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 3, characterized in that In the step 3, the inverse Laplace algorithm is used to calculate the recovery curve M of the longitudinal relaxation signal along the recovery time. z (T SR ) is fitted as: Among them, T 1i is the longitudinal relaxation time of the ith component, T SR For recovery time.

5. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 4, characterized in that The specific steps of step 4 are: Step 41: Obtain a sample identical to the cement-based material sample in step 1, and place the sample in a constant humidity environment at 20±2°C and a relative humidity lower than 23% for water desorption treatment until the sample mass reaches a constant state, wherein the sample mass reaches a constant state when the relative mass loss of the cement-based material sample within 7 days is less than 1%; Step 42: Based on the cement-based material sample whose mass reaches a constant state, repeat steps 1 to 2 to obtain the recovery curve M of the longitudinal relaxation signal versus recovery time. z (T SR ); Step 43: Based on the recovery curve M of the longitudinal relaxation signal over the recovery time z (T SR ), get the minimum time value t min The corresponding longitudinal relaxation signal M z (t min ), and combined with M0 obtained in step 3, the initial longitudinal relaxation time T 1,Init , expressed as: T 1,Init =M0t min / M z (t min ); Step 44: T 1,Init is the longitudinal surface relaxation time T 1S ; Step 45: Obtain the thickness of the monolayer of water λ and combine it with the longitudinal surface relaxation time T 1S , the longitudinal surface relaxation strength ρ1 is obtained, which is expressed as: ρ1=λ / T 1S 。 6. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 4, characterized in that The specific steps of step 4 are: Step A: obtaining a sample identical to the cement-based material sample in step 1, and placing the sample in a constant humidity environment at 20±2° C. and a relative humidity lower than 23%, and performing a water desorption treatment until the sample mass reaches a constant state, wherein the sample mass reaches a constant state when the relative mass loss of the cement-based material sample within 7 days is less than 1%; Step B: Based on the cement-based material sample whose mass reaches a constant state, repeat steps 1 to 2 to obtain the recovery curve M of the longitudinal relaxation signal versus recovery time. z (T SR ); Step C: Longitudinal relaxation recovery curve M z (T SR ) , 7 recovery time points were selected at equal intervals, with an interval of 5-10 microseconds, and then the 7 recovery time points were linearly fitted using a linear fitting method to obtain the initial longitudinal relaxation time T 1,Init , the fitting expression is: Step D: T 1,Init is the longitudinal surface relaxation time T 1S ; Step E: Obtain the thickness of the water monolayer λ and combine it with the longitudinal surface relaxation time T 1S , the longitudinal surface relaxation strength ρ1 is obtained, which is expressed as: ρ1=λ / T 1S 。 7. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 6, characterized in that The sample pore water content m pw It is expressed as: m pw =M0 / k unit Where M0 is the initial intensity of the longitudinal relaxation signal at time zero, k unit is the relaxation signal corresponding to 1g of water, m pw is the pore water content of the saturated sample, The relaxation signal k corresponding to 1g of water unit Obtained through the following steps: Step 1: Obtain a sample that is the same as the cement-based material sample in step 1, and place the sample in a vacuum water-saturation device for water-saturation treatment to obtain a water-saturated sample; Step 2: Use a high-precision electronic balance to measure the saturated mass m of the sample sat ; Step 3: Place the saturated sample in a 40°C oven for gentle drying. When the sample loses 0.2 g of water, record the current mass and use the current sample as the cement-based material sample in step 1. Repeat steps 1 to 3 to obtain the total relaxation signal M0 of the sample. Step 4: Repeat step 3 5-10 times, and finally place the sample in a 105℃ oven for 72 hours until the mass no longer changes. The completely dry mass of the sample is measured as m dry , and then get the evaporable water mass m of the sample w , expressed as: m w =m sat -m dry Step 5: Fitting M0 and m w The linear relationship is obtained to obtain the signal k per unit mass of water. un i, the fitting expression is: M0=k unit ·m w Step 6: Based on the saturated sample pore water content m pw , and combined with the water density ρ w And the total volume of the sample V0, the porosity of the sample is obtained It is expressed as:

8. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 7, characterized in that The longitudinal relaxation time spectrum f(T 1i ) is the equivalent aperture r for each longitudinal relaxation time 1i It is expressed as: r 1i =number 1T 1i Where α is the shape parameter, which is 1 for a flat hole, 2 for a cylindrical hole, and 3 for a spherical hole.

9. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 8, characterized in that The volume fraction of the i-th pore θ i It is expressed as:

10. The method for measuring pores in situ of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 9, characterized in that The volume specific surface area is expressed as: The mass specific surface area is expressed as: S m =V0S v / (m sat -m pw )。

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