Method for measuring pore of cement-based material in situ based on low-field magnetic resonance longitudinal relaxation technology

Abstract

The application is based on a cement-based material original state pore measurement method based on low-field magnetic resonance longitudinal relaxation technology, which overcomes the technical bottlenecks of traditional magnetic resonance test methods in signal acquisition integrity, rapid relaxation signal capture and longitudinal surface relaxation strength determination by optimizing the acquisition and processing technology of longitudinal relaxation signals, and realizes the accurate detection of the pore structure of cement-based materials from the nanometer scale to the micron scale. Compared with the traditional transverse relaxation method, the method of the application is not disturbed by paramagnetic substances when testing ferrosilicate cement-based materials, and has higher applicability and accuracy. Overall, the application can non-destructively, accurately and accurately characterize the pore distribution characteristics of cement-based materials without drying pretreatment of the sample, 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 test technology in the fields of building engineering and material science.

Description

Technical Field

[0001] This invention relates to the field of pore structure testing technology for cement-based materials, specifically a method for undisturbed pore testing of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology. Background Technology

[0002] Cement-based materials, as the material foundation of the civil engineering industry, require precise design and control of their performance to ensure project quality and extend structural service life. The key properties of cement-based materials, such as mechanical properties, volume stability, and durability, are closely related to their complex pore structure. The size and distribution of multi-scale pores significantly influence, and even to a large extent directly determine, the key properties of cement-based materials, such as strength, crack resistance, impermeability, and durability. Traditional techniques such as mercury intrusion porosimetry and gas adsorption are commonly used for testing these properties. However, when using these traditional techniques to test the pore structure of porous materials, sample drying pretreatment is required. Because the hydrated calcium silicate gel, an important component of cement-based materials, is particularly fragile, its nanoporous structure dynamically evolves with changes in water content. In a completely dry state, the pore structure of cement-based materials has changed significantly compared to high-saturation conditions, with permeable pore sizes potentially differing by 1-2 orders of magnitude. In most service environments, cement-based materials maintain a high water content, and the pore structure obtained using traditional techniques such as mercury intrusion porosimetry may differ significantly from the actual state, potentially leading to erroneous conclusions.

[0003] Currently, non-destructive characterization methods for in-situ testing of the pore structure of porous media without drying pretreatment mainly include small-angle scattering (SAS) of neutrons or X-rays, thermal porosimetry, X-ray computed tomography (CT), and low-field magnetic resonance imaging (MRMRI). SAS utilizes X-rays or neutron beams to detect microstructural features at the nanometer to micrometer scale, theoretically allowing testing of undried samples. However, it only obtains overall characteristics such as the specific surface area of ​​nanopores, making it difficult to directly characterize pore size distribution. Thermal porosimetry detects heat changes during the freezing and thawing of water, theoretically suitable for testing nanoscale pores. However, temperature changes also significantly alter the structure of hydrated calcium silicate gel and its nanopores, and the effect of temperature changes on pore structure cannot be decoupled and subtracted, raising questions about the accuracy of pore structure information obtained by thermal porosimetry. X-ray computed tomography (CT) can achieve three-dimensional visualization of pore structures, but its spatial resolution is only at the sub-micrometer level, making it difficult to characterize the abundant and crucial nanoscale pores in cement-based materials.

[0004] Low-field magnetic resonance (LDMR) uses hydrogen nuclei in water as probes to perform non-destructive and rapid testing of the water content and distribution of water in pores of different scales (i.e., pore structure) in saturated specimens. It possesses unique technical advantages and promising applications, and has attracted widespread attention in the testing and analysis of cement-based materials. LDMR detects the transverse or longitudinal relaxation signals of pore water in saturated specimens, inverts and calculates the relaxation time spectrum (including transverse and longitudinal relaxation time spectra), and after rigorously calibrating the unit signal quantity and reasonably determining the transverse or longitudinal surface relaxation intensity, the porosity and pore size distribution curves of the saturated specimen can be obtained. However, transverse relaxation testing is susceptible to the influence of paramagnetic substances (such as iron-containing minerals) in cement-based materials, leading to significant deviations in the test results. Summary of the Invention

[0005] The purpose of this invention is to address the problem in the prior art where transverse relaxation testing is easily affected by paramagnetic substances in cement-based materials, leading to significant deviations in test results. This invention provides a method for undisturbed hole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology.

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

[0007] A method for undisturbed borehole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology includes the following steps:

[0008] Step 1: Obtain the corrected free induction attenuation signal of the cement-based material sample. The corrected free induction attenuation signal is the free induction attenuation signal after removing the background noise signal.

[0009] Step 2: Fit the corrected free induction decay signal using the inverse Laplace algorithm to obtain the recovery time T for each step. SR The corresponding longitudinal relaxation signal M z0 And based on the longitudinal relaxation signal M z0 The recovery curve M of the longitudinal relaxation signal as a function of recovery time was obtained. z (T SR );

[0010] Step 3: Use the inverse Laplace algorithm to analyze the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR By fitting the data, the total relaxation signal M0 and the longitudinal relaxation time spectrum f(T) are obtained. 1i );

[0011] Step 4: Obtain the longitudinal surface relaxation strength ρ1 of the cement-based material sample;

[0012] Step 5: Based on the total relaxation signal M0, and combined with the relaxation signal k per unit mass of water unitThe pore water content m of the saturated sample was obtained. pw ;

[0013] Step Six: Utilize the pore water content m of the saturated sample pw The total pore volume was obtained by combining the density of water. Then, the total volume V0 of the cement-based material sample was acquired, and the porosity of the cement-based material sample was obtained by calculating the ratio of the total pore volume to V0.

[0014] Step 7: Utilize the longitudinal surface relaxation intensity ρ1 and longitudinal relaxation time spectrum f(T) of the cement-based material sample 1i ), thus obtaining the longitudinal relaxation time spectrum f(T) 1i The equivalent aperture r corresponding to each longitudinal relaxation time in ) 1i ;

[0015] Step 8: Based on the longitudinal relaxation time spectrum f(T) 1i ), thus obtaining the longitudinal relaxation time spectrum f(T) 1i The semaphore f corresponding to each longitudinal relaxation time in ) i And based on semaphore f i Porosity of cement-based material samples The density of water ρ w And the total volume V0 of the cement-based material sample, to obtain the pore volume fraction θ corresponding to each equivalent pore size. i ;

[0016] Step 9: Utilize the porosity of the cement-based material sample Equivalent aperture r 1i and semaphore f i The specific surface area S was obtained. v ;

[0017] 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 and specific surface area S of the cement-based material sample v and the pore water content m of the saturated sample pw The specific surface area S was obtained. m .

[0018] Furthermore, the specific steps of step one are as follows:

[0019] Step 11: Use the saturation recovery-free induction decay sequence to acquire signals from the empty NMR sample chamber to obtain the background noise signal;

[0020] Steps 1 and 2: Obtain cement-based material samples and perform water saturation treatment on the cement-based material samples;

[0021] Step 13: Wrap the cement-based material sample with plastic wrap and place the plastic-wrapped cement-based material sample in the sample chamber of the nuclear magnetic resonance spectrometer to collect the free induction decay signal corresponding to each recovery time;

[0022] Step 14: Subtract the free induction attenuation signal of the cement-based material sample from the background noise signal to obtain the corrected free induction attenuation signal.

[0023] Furthermore, in step two, the corrected free induction attenuation signal is fitted using the inverse Laplace algorithm and expressed as follows:

[0024]

[0025] Among them, M xy (t) represents the freely inductively decaying signal, t represents time, and M represents the time interval. z0 To recover the initial intensity at time TSR, T 2i Let J be the transverse relaxation time of the pore water in the i-th group, where i = 1, 2, ..., J, and J is the number of groups of pores inside the cement-based material sample.

[0026] Furthermore, in step three, the inverse Laplace algorithm is used to analyze the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR The fitting expression is as follows:

[0027]

[0028] Among them, T 1i T is the longitudinal relaxation time of the i-th component. SR This refers to the recovery time.

[0029] Furthermore, the specific steps of step four are as follows:

[0030] Step 41: Obtain a sample identical to the cement-based material sample in Step 1, and place the sample in a constant humidity environment of 20±2℃ and relative humidity below 23% for moisture desorption treatment until the sample mass reaches a constant state. The constant state of sample mass means that the relative mass loss of the cement-based material sample is less than 1% within 7 days.

[0031] Step 42: Based on the cement-based material sample that has reached a constant mass, repeat steps one and two to obtain the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR );

[0032] Step 43: Based on the recovery curve M of the longitudinal relaxation signal over recovery time z (T SR), obtain the minimum time value t among them. min The corresponding longitudinal relaxation signal M z (t min Combined with M0 obtained in step three, the initial longitudinal relaxation time T is obtained. 1,Init ,

[0033] Represented as:

[0034] T 1,Init =M0t min / M z (t min );

[0035] Step 44: T 1,Init That is, the longitudinal surface relaxation time T 1S ;

[0036] Steps four and five: Obtain the thickness λ of the single water layer, and combine it with the longitudinal surface relaxation time T. 1S The longitudinal surface relaxation intensity ρ1 is obtained and expressed as:

[0037] ρ1=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 1, and place the sample in a constant humidity environment of 20±2℃ and relative humidity below 23% for moisture desorption treatment until the sample mass reaches a constant state. The constant state of sample mass means that the relative mass loss of the cement-based material sample is less than 1% within 7 days.

[0040] Step B: Based on the cement-based material sample that has reached a constant mass, repeat steps one and two to obtain the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR );

[0041] Step C: On the longitudinal relaxation recovery curve M z (T SR Seven recovery time points were selected at equal intervals, with an interval of 5-10 microseconds. Then, a linear fitting method was used to perform linear fitting on the seven recovery time points to obtain the initial longitudinal relaxation time T. 1,Init The fitted expression is as follows:

[0042]

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

[0044] Step E: Obtain the thickness λ of the monolayer water and combine it with the longitudinal surface relaxation time T. 1S The longitudinal surface relaxation intensity ρ1 is obtained and expressed as:

[0045] ρ1=λ / T 1S .

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

[0047] m pw =M0 / k unit

[0048] Where M0 is the initial intensity of the longitudinal relaxation signal at time zero, and k unit Let m be the relaxation signal quantity corresponding to 1g of water. pw The pore water content of the saturated sample.

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

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

[0051] Step 2: Measure the saturation mass m of the sample using a high-precision electronic balance. sat ;

[0052] Step 3: Place the saturated sample in a 40℃ oven for gentle drying. Record the current mass when the sample loses 0.2g of water. Use the current sample as the cement-based material sample in Step 1. Repeat Step 1 to Step 3 to obtain the total relaxation signal M0 of the sample.

[0053] Step 4: Repeat step 3 5-10 times, and finally place the sample in a 105℃ oven to dry for 72 hours until the mass no longer changes. Measure the completely dried mass m of the sample. dry Thus, the mass of evaporable water in the sample, m, is obtained. w , is represented as:

[0054] m w =m sat -m dry

[0055] Step 5: Fit M0 and m w The linear relationship was used to obtain the signal quantity k per unit mass of water. un i, the fitted expression is:

[0056] M0 = 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 The porosity of the sample is obtained by combining the total sample volume V0. Represented as:

[0058]

[0059] Furthermore, the longitudinal relaxation time spectrum f(T) 1i The equivalent aperture r for each longitudinal relaxation time in ) 1i Represented as:

[0060] r 1i =αρ1T 1i

[0061] Where α is a shape parameter, which is 1 for flat holes, 2 for cylindrical holes, and 3 for spherical holes.

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

[0063]

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

[0065]

[0066] The specific surface area is expressed as:

[0067] S m =V0S v / (m sat -m pw ).

[0068] The beneficial effects of this invention are:

[0069] This application overcomes the technical bottlenecks of traditional magnetic resonance imaging (MRI) testing methods in signal acquisition integrity, rapid relaxation signal capture, and longitudinal surface relaxation intensity measurement by optimizing the acquisition and processing techniques of longitudinal relaxation signals. This enables precise detection of the pore structure of cement-based materials from the nanoscale to the microscale. Compared with traditional transverse relaxation methods, the method presented in this application is not affected by paramagnetic substances when testing iron-containing silicate cement-based materials, exhibiting higher applicability and accuracy. Overall, this application can non-destructively, undisturbedly, and accurately characterize the pore distribution characteristics of cement-based materials without requiring sample drying pretreatment. This provides a scientific basis for optimizing the performance of cement-based materials and improving engineering quality control, effectively promoting the further development of pore structure testing technology in the fields of building engineering and materials science. Attached Figure Description

[0070] Figure 1 This is a schematic diagram of the saturation recovery-free induction attenuation test method and a typical free induction attenuation signal of this application.

[0071] Figure 2 This is a schematic diagram of the saturation recovery-free induction attenuation longitudinal relaxation signal of typical white cement mortar and silicate cement mortar in this application.

[0072] Figure 3 This is a schematic diagram showing the relationship between the measured moisture content and the actual moisture content of the mortar sample calibrated in this application;

[0073] Figure 4 This illustration shows the porosity of white cement mortar and silicate cement mortar obtained by various methods in this application. Figure 1 ;

[0074] Figure 5 This illustration shows the porosity of white cement mortar and silicate cement mortar obtained by various methods in this application. Figure 2 ;

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

[0076] Figure 7 This is a schematic diagram of the pore size distribution of white cement mortar and silicate cement mortar obtained by various methods in this invention. Figure 2 . Detailed Implementation

[0077] It should be noted that, where there is no conflict, the various embodiments disclosed in this application can be combined with each other.

[0078] Specific Implementation Method 1: The original borehole measurement method for cement-based materials based on low-field magnetic resonance longitudinal relaxation technology described in this implementation method includes:

[0079] S1, Test the longitudinal relaxation signal of the sample to be tested;

[0080] Setting test parameters includes configuring the basic parameters of the MRI scanner, as well as key parameters for the saturation recovery sequence and free induction decay sequence. The basic parameters of the MRI scanner include the width and frequency of the 90° and 180° pulses, and the center frequency of the MRI scanner. Saturation recovery sequence parameters include the number of 90° pulses, recovery time, and number of scans. Free induction decay signal acquisition parameters include the discharge time, acquisition delay time, sampling time interval, and number of data points acquired.

[0081] According to the test requirements, the above test parameters were set, and then longitudinal relaxation signal acquisition was performed, including sample signal acquisition, background noise signal acquisition, and signal data storage. First, sample signal acquisition was performed, and the sample saturation mass m was measured using a high-precision electronic balance. sat It was then tightly wrapped with plastic wrap and placed in the sample chamber. Using a saturation recovery-free induction decay sequence, the sample was processed at a set recovery time T. SR Longitudinal relaxation signals were acquired sequentially. To improve the signal-to-noise ratio, multiple scans were performed at each recovery time T. SR The sample signal is repeatedly acquired; after the sample signal acquisition is completed, the background noise signal is acquired, with the same test parameters set and the empty sample chamber is tested using a saturation recovery-free induction decay sequence. The acquired background noise signal can be used as a calibration benchmark for subsequent signal data analysis; finally, the signal data is stored, and the acquired longitudinal relaxation signal is stored according to the recovery time T. SR The signals are stored sequentially, and a preliminary check is performed on the longitudinal relaxation signals to ensure that the longitudinal relaxation signals are complete and free from obvious abnormal interference.

[0082] S2. Analysis and inversion of relaxation signals;

[0083] First, background noise correction is performed on the acquired sample signals to remove background noise and prevent it from interfering with subsequent analysis. Then, the inverse Laplace algorithm is used to fit the corrected free-induction decay signal to obtain the signal at different recovery times T. SR The initial intensity M at time t z0 From this, the recovery curve M of the longitudinal relaxation signal over recovery time can be obtained. z (T SR This curve reflects the relaxation characteristics of water content within the sample. Then, inversion analysis is performed, using the inverse Laplace algorithm to invert the longitudinal relaxation signal, calculating the total relaxation signal M0 and the longitudinal relaxation time spectrum f. i (T 1i Longitudinal relaxation time spectra can accurately characterize the multi-scale structural properties of pores inside saturated samples, and comprehensively reveal pore structure information from the nanoscale to the microscale.

[0084] S3. Determine the longitudinal surface relaxation strength ρ1 and equivalent aperture r. 1i ;

[0085] Another set of test samples was prepared, with raw materials and proportions consistent with the porous structure specimen. This set of samples was placed in a constant humidity environment (20±2℃, relative humidity below 23%) for water desorption treatment until its mass reached a constant state (relative mass loss less than 1% over 7 days) to ensure a stable monomolecular water film adsorption state on the pore surface. The saturated recovery pulse sequence was then used to test the signal and background noise of this set of samples. Based on the longitudinal relaxation theory and the data on the change of the longitudinal relaxation signal with recovery time, a linear fitting method was used to analyze the relationship between the longitudinal relaxation signal and recovery time. The longitudinal surface relaxation time T was determined by the slope of the fitted line. 1S The calculation of this surface relaxation time is crucial for further pore structure analysis; regarding the longitudinal surface relaxation time T... 1S To address the difficulty in experimentally determining the longitudinal surface relaxation strength ρ1, the longitudinal surface relaxation strength ρ1 was finally calculated, and the longitudinal surface relaxation time T was determined. 1S Then, the longitudinal surface relaxation intensity ρ1 can be calculated. This parameter provides important support for subsequent pore size distribution analysis, and can convert the longitudinal relaxation time into the equivalent pore size r. 1i .

[0086] S4. Determine pore structure information;

[0087] Porosity was determined by combining the total relaxation signal M0 of the saturated sample with the relaxation signal k per unit mass of water. unit It can calculate the pore water content (m) of a saturated sample. pw The total pore water volume (i.e., total pore volume) can be calculated by combining the density of water. The ratio of the total pore volume to the total sample volume is the porosity of the sample. This application also provides the relaxation signal k corresponding to a unit weight of water. unit The method of determination;

[0088] Determining the aperture distribution specifically includes determining the equivalent aperture r. 1i and the volume fraction θ of pores at each level i To determine the pore size, the longitudinal relaxation time spectrum and the longitudinal surface relaxation intensity ρ1 are used to convert different longitudinal relaxation times into corresponding pore sizes. Based on the signal intensity ratio of pores with different pore sizes, the volume fraction of the pore can be determined, and the pore size distribution curve of the sample can be plotted.

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

[0090] This application addresses the problem that existing pore structure characterization methods cannot accurately and non-destructively characterize the multi-scale pore structure of iron-containing silicate cementitious materials in a saturated state. A new pore size measurement method is proposed. This method eliminates the need for sample drying pretreatment and can accurately characterize the multi-scale pore size distribution characteristics of the cementitious material in a saturated state without damaging its pore structure. This overcomes the limitations of existing pore size measurement methods and breaks through several technical bottlenecks.

[0091] Example:

[0092] In-situ borehole measurement methods for cement-based materials based on low-field magnetic resonance longitudinal relaxation technology include:

[0093] The S1 test measures the longitudinal relaxation signal of the sample under test. 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 spectrometer. The basic parameters of the nuclear magnetic resonance spectrometer include the pulse width and frequency of the 90° and 180° pulses to ensure that the equipment operates in the best working condition.

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

[0097] The magnetic field strength of the nuclear magnetic resonance spectrometer is set at 0.047T, and the center frequency is set at 2MHz. The center frequency needs to be determined based on the equipment specifications.

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

[0099] In the saturation recovery-free induction decay pulse sequence test, a pulse width t of 90° is set. 90 The s value is 13 μs, ensuring that all spins of the sample are excited, forcing the longitudinal magnetization vector M to... z Saturation occurs when all spins are fully perturbed, and the longitudinal magnetization vector disappears, ensuring that M is in its initial state. z =0;

[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 This allows the longitudinal magnetization of the sample to be partially recovered by increasing the recovery time T. SR The value range is set to microseconds to seconds, which can collect longitudinal relaxation signals of pore water with different pore sizes. For different samples, the recovery time T SR The range can be set based on the estimated value of the longitudinal relaxation time of pore water;

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

[0102] Recovery time T SR The number of scans needs to be adjusted according to different samples. For cement mortar, the recovery time T... SR It 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 6000ms; recovery time T SR When the range 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 parameters for acquiring the free induction attenuation signal;

[0104] In the free induction decay test, a 90° pulse t is first applied to the sample. 90 The system waits for the residual pulse radio frequency energy to dissipate, and the time for this dissipation is called the damping time. After the pulse radio frequency energy is dissipated, the system waits for the acquisition delay time before starting to acquire signals. The longitudinal relaxation signal is acquired once every certain sampling time interval until the number of acquired data reaches the number of data points (samples). Finally, a series of longitudinal relaxation signals that decay with the acquisition time are obtained.

[0105] The required parameters for the free-induction attenuation sequence and their values ​​are as follows: 90° pulse width t 90The sampling interval is set to 13μs, the damping time to 25μs, the acquisition delay time to 50μs, the sampling time interval to [0.5μs, 4μs], and the number of data points to be collected to [10000, 200000].

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

[0107] The acquired free induction decay signal is used to calculate the longitudinal magnetization M of the sample. z0 ;

[0108] Change recovery time T SR By repeating the above test steps, a series of recovery times T can be collected. SR The free-induction attenuation signal under the condition;

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

[0110] Cement-based material samples are typically neat cement paste or mortar specimens made from cement and cementitious materials such as slag. The samples are cylindrical, with a maximum diameter of 25 mm and a height not exceeding 60 mm. Precise measurements of the sample dimensions are performed using vernier calipers to ensure they meet the size limitations required by the nuclear magnetic resonance spectrometer. The samples are then placed in a vacuum saturation device to ensure complete water saturation of the sample pores. Finally, the saturation mass m of the sample is measured using a high-precision electronic balance. sat Then, tightly wrap the cement-based material sample to be tested with plastic wrap to ensure that the sample will not exchange moisture with the environment during the test. Place the plastic-wrapped sample in the sample chamber of the nuclear magnetic resonance spectrometer to ensure that the sample is placed stably and will not move during the test.

[0111] S1.3 Acquires relaxation signals, the specific process is as follows:

[0112] S1.3.1 Acquire sample relaxation signals;

[0113] After placing the sample into the sample chamber, begin testing for different recovery times T. SR Signal acquisition was performed on the sample, with each acquisition consisting of a 90° pulse excitation followed by a recovery time T. SR Then, free induction attenuation signal acquisition is performed. Multiple scans should be performed during the acquisition process to improve the signal-to-noise ratio and ensure the reliability of the test data.

[0114] S1.3.2 Acquire background noise signals;

[0115] After the sample test is completed, background noise signal is collected. This process uses a saturation recovery-free induction decay sequence to test the empty sample chamber. 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 used during sample testing to ensure consistent signal comparison.

[0117] S1.3.3 Signal data storage;

[0118] The data obtained from each signal acquisition is stored, and the data is stored according to the recovery time T. SR Arrange in order;

[0119] A preliminary check should be performed when storing data to ensure that the signal is intact and free from obvious noise interference, so as to avoid erroneous 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, it is necessary to correct the background noise signal;

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

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

[0125] The initial intensity M is obtained by fitting the corrected signal (the sample signal minus the background noise signal) using the inverse Laplacian algorithm. z0

[0126] Acquire signal at the corresponding recovery time T SR Initial intensity M at time z0 The fitting formula is as follows:

[0127]

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

[0129] The inverse Laplace algorithm is used to perform inversion analysis on the corrected free-induction attenuated signal to obtain the signal at the corresponding recovery time T. SR Longitudinal relaxation signal M at time 1 z0 By analyzing different recovery times T SR The longitudinal relaxation recovery curve M of the sample is generated by inverting the free induction decay signals corresponding to different time points. z (T SR );

[0130] S2.3 Inversion Analysis;

[0131] The inverse Laplace algorithm is used to process the longitudinal relaxation signal matrix M. z (T SR Inversion analysis was performed to calculate the total relaxation signal M0 and the longitudinal relaxation time spectrum f(T). 1i This method characterizes the multi-scale structural properties of the pores within the sample, revealing the pore features from the nanometer to the micrometer scale. The inversion formula is as follows:

[0132]

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

[0134] S3 determines the longitudinal surface relaxation strength ρ1, and the specific process includes:

[0135] S3.1 Preparation of samples for longitudinal surface relaxation strength ρ1 measurement;

[0136] Another set of test samples was prepared, with raw materials and proportions consistent with the pore structure specimen. This set of test samples was placed in a constant humidity environment of 20±2℃ and relative humidity below 23% for water desorption treatment until the sample mass reached a constant state (relative mass loss of less than 1% over 7 days) to ensure the formation of a stable monomolecular water film adsorption state on the pore surface;

[0137] S3.2 Acquire relaxation signals;

[0138] The saturation recovery pulse sequence was used to test the signal and background noise of this group of samples, and the obtained signal data was used to calculate the longitudinal surface relaxation intensity ρ1.

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

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

[0141] The longitudinal relaxation recovery curve M of the sample was obtained using a linear fitting method. z (T SRThe initial longitudinal relaxation time T is obtained by fitting the data. 1,Init ;

[0142] The longitudinal surface relaxation time T can be determined by fitting the slope of the straight line. 1S The calculation of the surface relaxation time is crucial for further pore structure analysis;

[0143] Regarding the longitudinal surface relaxation time T 1S To address the difficulty in experimentally determining the longitudinal surface relaxation strength ρ1, this embodiment provides a method for deriving the longitudinal surface relaxation time T using relaxation theory. 1S The method, and then through the longitudinal surface relaxation time T 1S The longitudinal surface relaxation strength ρ1 is calculated.

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

[0145] Using Taylor expansion, the magnetization M z (t) at T SR Expanding around 0, when time T SR When the magnetization is close to zero, the magnetization M z (T SR The complex expression of ) can be simplified into an approximate form, thus facilitating further calculations.

[0146]

[0147] Among them, T 1,Init (s) represents the initial longitudinal relaxation time. Based on the above equation, a linear fitting method is used to analyze the relationship between the longitudinal relaxation signal and the recovery time. The surface relaxation time T is determined by the slope of the fitted line. 1S The minimum permissible time value t of the NMR spectrometer can also be measured during saturation recovery-free induction decay testing. min M below z (t min T can then be calculated using the following formula. 1,Init :

[0148] T 1,Init =M0t min / M z (t min )

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

[0150] S3.4 Calculate the longitudinal surface relaxation strength ρ1;

[0151] Longitudinal surface relaxation time T 1S Based on the established parameters and the known diameter of water molecules, the longitudinal surface relaxation intensity ρ1 can be calculated using the following formula. This parameter provides fundamental support for subsequent pore size distribution analysis.

[0152] ρ1=λ / T 1S

[0153] Where ρ1 (m / s) represents the longitudinal surface relaxation intensity; λ = 0.3 nm represents the thickness of the monolayer water;

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

[0155] S4.1 Determine the porosity;

[0156] The total relaxation signal M0 of the saturated sample is combined with the relaxation signal k per unit mass of water. unit It can calculate the pore water content (m) of a saturated sample. pw The calculation formula is as follows:

[0157] m pw =M0 / k unit

[0158] Where M0 is the initial intensity of the longitudinal relaxation signal at time zero; k unit Let m be the relaxation signal quantity corresponding to 1g of water. pw (g) represents the pore water content of the saturated sample;

[0159] Furthermore, determine k unit The specific process is as follows:

[0160] The first step involves placing a separately prepared set of test samples in a vacuum saturation apparatus to ensure complete water saturation in the pores of the samples. Then, a high-precision electronic balance is used to measure the saturation mass m of the samples. sat (Accurate to 0.001g);

[0161] The second step involves placing the saturated sample in a 40°C oven for gentle drying. Record the current mass for every 0.2g of water lost. Repeat steps S1.1-S2.3 to obtain the total relaxation signal M0.

[0162] Samples are taken out at regular intervals and their mass is measured to monitor the moisture loss process of the samples; when the sample loses about 0.2g of water, the current mass is recorded, and nuclear magnetic resonance is used for testing. The total relaxation signal M0 is obtained according to the method described in S2.3.

[0163] The third step involves testing the sample multiple times (5-10 times) (each time resulting in a loss of 0.2g), then finally drying the sample in a 105℃ oven for 72 hours until its mass no longer changes. The completely dried mass, m, is then measured. dry The mass of evaporable water in the sample, m, is calculated using the following formula. w The calculation formula is as follows:

[0164] m w =m sat -m dry

[0165] The fourth step is to fit M0 to m. w The linear relationship is used to determine the signal quantity k per unit mass of water using the following formula. unit The calculation formula is as follows:

[0166] M0 = k unit ·m w

[0167] The pore water content m of the saturated sample was calculated. pw Then, by combining the density of water, the total pore volume is calculated. By calculating the ratio of the total pore volume to the total sample volume, the porosity of the sample can be determined. The calculation formula is as follows:

[0168]

[0169] in, The porosity of the sample is represented by m. pw (g) represents the pore water content of the saturated sample; ρ w (g / cm 3 Let ρ be the density of water, taken as 1 g / cm³. 3 V0 represents the total volume of the sample, measured using the buoyancy method or vernier calipers.

[0170] S4.2 Determine the aperture distribution, the specific process is as follows:

[0171] S4.2.1 Determine the aperture;

[0172] The longitudinal surface relaxation intensity ρ1 obtained through S3.4 and the longitudinal relaxation time spectrum f(T) obtained through S2.3 are shown. 1i The longitudinal relaxation time spectrum f(T) is calculated using mathematical formulas. 1i The equivalent aperture r for each longitudinal relaxation time in ) 1i； The calculation formula is as follows:

[0173] r 1i =αρ1T 1i

[0174] Where, r 1i(m) represents the equivalent aperture obtained in S3.4; α represents the shape parameter, which is 1 for flat holes, 2 for cylindrical holes, and 3 for spherical holes; the longitudinal relaxation time spectrum f(T) 1i (This is obtained from S2.3;)

[0175] S4.2.2 Determine the volume fraction;

[0176] Based on the signal strength volume fraction (fi), the pore volume fraction corresponding to each equivalent pore size can be determined by the following formula. Based on this, a pore size distribution curve can be plotted. The calculation formula is as follows:

[0177]

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

[0179] The pore size distribution curve is determined by plotting pore size on the x-axis and volume fraction on the y-axis.

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

[0181] Based on the above test and calculation results, and combined with the sample's mass and volume, the volumetric surface area S is calculated using the following formula. v and specific surface area S m ;

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

[0183]

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

[0185] S m =V0S v / (m sat -m pw )

[0186] It should be noted that the specific embodiments are merely explanations and illustrations of the technical solution of the present invention and should not be used to limit the scope of protection. Any modifications made in accordance with the claims and specification of the present invention that are only partial should still fall within the protection scope of the present invention.

Claims

1. A method for undisturbed borehole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology, characterized in that... Includes the following steps: Step 1: Obtain the corrected free induction attenuation signal of the cement-based material sample. The corrected free induction attenuation signal is the free induction attenuation signal after removing the background noise signal. Step 2: Fit the corrected free induction decay signal using the inverse Laplace algorithm to obtain the recovery time T for each step. SR The corresponding longitudinal relaxation signal M z0 And based on the longitudinal relaxation signal M z0 The recovery curve M of the longitudinal relaxation signal as a function of recovery time was obtained. z (T SR ); Step 3: Use the inverse Laplace algorithm to analyze the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR By fitting the data, the total relaxation signal M0 and the longitudinal relaxation time spectrum f(T) are obtained. 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, and combined with the relaxation signal k per unit mass of water unit The pore water content m of the saturated sample was obtained. pw ; Step Six: Utilize the pore water content m of the saturated sample pw The total pore volume was obtained by combining the density of water. Then, the total volume V0 of the cement-based material sample was acquired, and the porosity of the cement-based material sample was obtained by calculating the ratio of the total pore volume to V0. Step 7: Utilize the longitudinal surface relaxation intensity ρ1 and longitudinal relaxation time spectrum f(T) of the cement-based material sample 1i ), thus obtaining the longitudinal relaxation time spectrum f(T) 1i The equivalent aperture r corresponding to each longitudinal relaxation time in ) 1i ; Step 8: Based on the longitudinal relaxation time spectrum f(T) 1i ), thus obtaining the longitudinal relaxation time spectrum f(T) 1i The semaphore f corresponding to each longitudinal relaxation time in ) i And based on semaphore f i Porosity of cement-based material samples The density of water ρ w And the total volume V0 of the cement-based material sample, to obtain the pore volume fraction θ corresponding to each equivalent pore size. i ; Step 9: Utilize the porosity of the cement-based material sample Equivalent aperture r 1i and semaphore f i The specific surface area S was 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 and specific surface area S of the cement-based material sample v and the pore water content m of the saturated sample pw The specific surface area S was obtained. m .

2. The method for undisturbed borehole measurement 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 as follows: Step 11: Use the saturation recovery-free induction decay sequence to acquire signals from the empty NMR sample chamber to obtain the background noise signal; Steps 1 and 2: Obtain cement-based material samples and perform water saturation treatment on the cement-based material samples; Step 13: Wrap the cement-based material sample with plastic wrap and place the plastic-wrapped cement-based material sample in the sample chamber of the nuclear magnetic resonance spectrometer to collect the free induction decay signal corresponding to each recovery time; Step 14: Subtract the free induction attenuation signal of the cement-based material sample from the background noise signal to obtain the corrected free induction attenuation signal.

3. The method for undisturbed borehole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 2, characterized in that... In step two, the corrected free induction attenuation signal is fitted using the inverse Laplace algorithm and expressed as follows: Among them, M xy (t) represents the freely inductively decaying signal, t represents time, and M represents the time interval. z0 To recover the initial intensity at time TSR, T 2i Let J be the transverse relaxation time of the pore water in the i-th group, where i = 1, 2, ..., J, and J is the number of groups of pores inside the cement-based material sample.

4. The method for undisturbed borehole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 3, characterized in that... In step three, the inverse Laplace algorithm is used to analyze the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR The fitting expression is as follows: Among them, T 1i T is the longitudinal relaxation time of the i-th component. SR This refers to the recovery time.

5. The method for undisturbed borehole measurement 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 four are as follows: Step 41: Obtain a sample identical to the cement-based material sample in Step 1, and place the sample in a constant humidity environment of 20±2℃ and relative humidity below 23% for moisture desorption treatment until the sample mass reaches a constant state. The constant state of sample mass means that the relative mass loss of the cement-based material sample is less than 1% within 7 days. Step 42: Based on the cement-based material sample that has reached a constant mass, repeat steps one and two to obtain the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR ); Step 43: Based on the recovery curve M of the longitudinal relaxation signal over recovery time z (T SR ), obtain the minimum time value t among them. min The corresponding longitudinal relaxation signal M z (t min Combined with M0 obtained in step three, the initial longitudinal relaxation time T is obtained. 1,Init , represented as: T 1,Init =M0t min / M z (t min ); Step 44: T 1,Init That is, the longitudinal surface relaxation time T 1S ; Steps four and five: Obtain the thickness λ of the single water layer, and combine it with the longitudinal surface relaxation time T. 1S The longitudinal surface relaxation intensity ρ1 is obtained and expressed as: ρ1=λ / T 1S 。 6. The method for undisturbed borehole measurement 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 four are as follows: Step A: Obtain a sample identical to the cement-based material sample in Step 1, and place the sample in a constant humidity environment of 20±2℃ and relative humidity below 23% for moisture desorption treatment until the sample mass reaches a constant state. The constant state of sample mass means that the relative mass loss of the cement-based material sample is less than 1% within 7 days. Step B: Based on the cement-based material sample that has reached a constant mass, repeat steps one and two to obtain the recovery curve M of the longitudinal relaxation signal over recovery time. z (T SR ); Step C: On the longitudinal relaxation recovery curve M z (T SR Seven recovery time points were selected at equal intervals, with an interval of 5-10 microseconds. Then, a linear fitting method was used to perform linear fitting on the seven recovery time points to obtain the initial longitudinal relaxation time T. 1,Init The fitted expression is as follows: Step D: T 1,Init That is, the longitudinal surface relaxation time T 1S ; Step E: Obtain the thickness λ of the monolayer water and combine it with the longitudinal surface relaxation time T. 1S The longitudinal surface relaxation intensity ρ1 is obtained and expressed as: ρ1=λ / T 1S 。 7. The method for undisturbed borehole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 6, characterized in that... The pore water content of the sample is m pw Represented as: m pw =M0 / k unit Where M0 is the initial intensity of the longitudinal relaxation signal at time zero, and k unit Let m be the relaxation signal quantity corresponding to 1g of water. pw The pore water content of the saturated sample, The relaxation signal quantity k corresponding to 1g of water unit It is obtained through the following steps: Step 1: Obtain a sample identical to the cement-based material sample in Step 1, and place the sample in a vacuum saturation device for saturation treatment to obtain a saturated sample; Step 2: Measure the saturation mass m of the sample using a high-precision electronic balance. sat ; Step 3: Place the saturated sample in a 40℃ oven for gentle drying. Record the current mass when the sample loses 0.2g of water. Use the current sample as the cement-based material sample in Step 1. Repeat Step 1 to Step 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 to dry for 72 hours until the mass no longer changes. Measure the completely dried mass m of the sample. dry Thus, the mass of evaporable water in the sample, m, is obtained. w , represented as: m w =m sat -m dry Step 5: Fit M0 and m w The linear relationship was used to obtain the signal quantity k per unit mass of water. un i, the fitted expression is: M0=k unit ·m w Step 6: Based on the pore water content m of the saturated sample pw And combined with the density ρ of water w The porosity of the sample is obtained by combining the total sample volume V0. Represented as:

8. The method for undisturbed borehole measurement 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 The equivalent aperture r for each longitudinal relaxation time in ) 1i Represented as: r 1i =number 1T 1i Where α is a shape parameter, which is 1 for flat holes, 2 for cylindrical holes, and 3 for spherical holes.

9. The method for undisturbed borehole measurement of cement-based materials based on low-field magnetic resonance longitudinal relaxation technology according to claim 8, characterized in that... The volume fraction θ of the i-th pore i Represented as:

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

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