A method of calculating the degree of hydration of fly ash in a cementitious material

Abstract

The application discloses a method for calculating the hydration degree of fly ash in a cement-based material, and steps of the method comprise the following steps: 1, a spectrum analyzer equipped with a scanning electron microscope is used to obtain a characteristic element distribution map in a cement-based material slurry; 2, the characteristic element distribution map of fly ash is used to perform image recognition on unhydrated fly ash in the slurry; 3, a sufficient number of images is selected to obtain statistically reasonable area fraction and volume fraction of the unhydrated fly ash, the initial volume fraction of the fly ash is obtained according to the mixing ratio of the cement-based material and the density of raw materials, and finally the hydration degree of the fly ash is obtained. The application aims at the difficulty in image recognition of fly ash in the cement-based material, improves the recognizability of unhydrated fly ash by using a characteristic chemical element distribution image, and improves the accuracy of calculation of the hydration degree of fly ash in the cement-based material.

Description

Technical Field

[0001] This invention relates to the field of concrete materials technology, and in particular to a method for calculating the degree of fly ash hydration in cement-based materials. Background Technology

[0002] Fly ash, as a mineral admixture, is increasingly widely used in cement-based materials. It is well known that fly ash plays three main roles in cement-based materials: micro-aggregate effect, morphological effect, and pozzolanic effect. Among these, the pozzolanic effect of fly ash refers to the secondary hydration reaction between the active substances in fly ash and calcium hydroxide, a hydration product of cement, to generate hydrated calcium silicate gel or hydrated calcium aluminum silicate gel. This fills the pores, which is beneficial for the later-stage strength development and improves the durability of cement-based materials. Therefore, determining the degree of hydration of fly ash is of great significance for evaluating its reactivity and the contribution of the pozzolanic effect to the system structure formation.

[0003] Currently, the main methods for determining the degree of fly ash hydration in cement-based materials are selective solvent dissolution (SSD) and backscattered electron imaging (SEMI). The basic principle of SSD is that in cement-based materials, the hydration products of cement and fly ash, as well as unhydrated cement particles, dissolve in a specific acid (e.g., hydrochloric acid), while unhydrated fly ash particles do not. Therefore, this acid can be used to separate the cement and hydration products from the unhydrated fly ash particles, obtaining the mass fraction of unhydrated fly ash particles. Based on the initial mass fraction of fly ash in the mix proportion, the degree of fly ash hydration can then be determined. However, in the process of testing the degree of fly ash hydration using SSD, the selected acidic solution cannot actually completely dissolve the hydration products and unhydrated cement particles, leading to inaccurate test results. Backscattered electron image analysis mainly utilizes grayscale thresholds and the spherical morphology of fly ash particles to segment fly ash particles in the backscattered images of cement-based materials, obtaining the area and volume fractions of unhydrated fly ash particles. The degree of hydration reaction is then determined based on the initial volume fraction of the fly ash. However, this process is time-consuming and labor-intensive, and more importantly, it cannot identify internally hollow fly ash particles or irregularly shaped fly ash particles, ultimately failing to accurately test the degree of fly ash hydration. Summary of the Invention

[0004] To overcome the shortcomings of existing methods in accurately testing the degree of fly ash hydration in cement-based materials, this invention proposes a method for separating unhydrated fly ash particles in cement-based materials using characteristic elements in fly ash, thereby obtaining the volume fraction of unhydrated fly ash particles. Based on the initial volume fraction of fly ash in the mix proportion of the cement-based material, the degree of fly ash hydration is finally calculated.

[0005] This invention provides a method for calculating the degree of fly ash hydration in cement-based materials, comprising the following steps:

[0006] (1) Preparation of cement-based materials:

[0007] Cement-based materials are prepared from raw materials including aggregates and cementitious materials, with cementitious materials including fly ash and cement.

[0008] (2) Acquisition of microscopic images:

[0009] Samples were taken from the slurry of cement-based materials for observation, and surface scanning of characteristic elements was performed to obtain elemental distribution maps;

[0010] (3) Image recognition and separation of fly ash:

[0011] Analysis of the elemental distribution map revealed that unhydrated fly ash was separated based on the different contents of characteristic elements, including calcium, silicon, aluminum, sulfur, oxygen, iron, sodium, and potassium.

[0012] (4) Calculation of the degree of hydration α of fly ash:

[0013]

[0014] in, The average volume fraction of unhydrated fly ash in the slurry was obtained statistically by software.

[0015] This is the volume fraction of the paste in the cement-based material, calculated from the mix proportion of the cement-based material and the density of the raw materials;

[0016] This represents the initial volume fraction of fly ash in cement-based materials, calculated from the mix proportions and the density of the raw materials.

[0017] Preferably, in step (1), the aggregate includes silica sand, and the raw materials also include water-reducing agent.

[0018] Preferably, in step (2), the cement-based material undergoes molding curing, cutting into thin slices, soaking, polishing, ultrasonic cleaning, and carbon spraying before sampling to meet the requirements of microscopic testing.

[0019] Preferably, in step (3), the characteristic elements include silicon, aluminum, oxygen, iron, sodium and potassium.

[0020] Preferably, in step (3), the element distribution map is processed to obtain the total distribution map of the sum of characteristic elements, and the total distribution map is subjected to grayscale threshold processing to separate the unhydrated fly ash.

[0021] Preferably, in step (4), the statistics are performed by randomly selecting at least 30 regions in the cement-based material slurry at a magnification of 1000 to 2000.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] This invention utilizes the characteristic element distribution map of fly ash to perform image recognition of unhydrated fly ash, which can avoid the test deviations caused by selective solvent dissolution method and backscattered electron image analysis method, and more accurately test the content of unhydrated fly ash in cement-based materials, thereby giving a more accurate result on the degree of fly ash hydration. Attached Figure Description

[0024] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention, wherein:

[0025] Figure 1 This is an image showing the distribution of calcium, silicon, aluminum, sulfur, oxygen, iron, sodium, and potassium elements in the mortar of Example 2 provided by this invention; wherein, Figure 1 a is a calcium distribution image. Figure 1 b is the silicon distribution image. Figure 1 c is the aluminum distribution image. Figure 1 d is the sulfur distribution image. Figure 1 e is the iron distribution image. Figure 1 f is the potassium distribution image. Figure 1 g represents the sodium distribution image. Figure 1 h represents the oxygen distribution image;

[0026] Figure 2 This is an image showing the total distribution of silicon, aluminum, oxygen, iron, sodium, and potassium elements in the mortar of Example 2 provided by the present invention.

[0027] Figure 3 This is an image of unhydrated fly ash separated from the mortar in Example 2 of the present invention. Detailed Implementation

[0028] The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed.

[0029] Example

[0030] 1. Raw materials and test plan

[0031] 1.1 Raw materials

[0032] The cement-based material used in this embodiment of the invention is a high-volume fly ash mortar. The raw materials include cementitious materials, aggregates, water, and a water-reducing agent. The cementitious materials include P.II 42.5 type cement and Grade I fly ash, both with densities of 3.13 g / cm³. 3 and 2.56 g / cm 3 The chemical composition and loss on ignition of both are shown in Table 1. The aggregate in the mortar is silica sand with a maximum particle size of 4.75 mm, a fineness modulus of 2.7, and an apparent density of 2618 kg / m³. 3 The aggregate is brought to a saturated surface-dry state before use. The water-reducing agent used is a polycarboxylate-type high-efficiency water-reducing agent with a density of 1.08 g / cm³. 3 It has a pH value of 5.0 and a solid content of 34.4%.

[0033] Table 1. Chemical composition (mass fraction) of cement and fly ash

[0034]

[0035] 1.2 Mix Design

[0036] This invention uses a fixed total amount of cementitious materials and a fixed amount of aggregate to prepare high-volume fly ash mortars with different fly ash contents and water-cement ratios, and studies the degree of fly ash hydration under different mix proportions. In the mortar mix design, the fly ash contents are 66%, 75%, and 84%, and the water-cement ratios are 0.27, 0.30, and 0.33, respectively. A water-reducing agent is used to adjust the mortar fluidity to maintain consistency. The specific mix proportions of the high-volume fly ash mortars are shown in Table 2.

[0037] Table 2. Mix proportions of high-volume fly ash mortar

[0038]

[0039] High-volume fly ash mortar was prepared using a laboratory mortar mixer. Fly ash, cement, and sand were first dry-mixed for 5 minutes, then mixing water and a water-reducing agent were added and mixed for another 5 minutes to achieve better workability. After mixing, the freshly mixed mortar was poured into cylindrical molds with a diameter of 50.8 mm and a height of 101.6 mm. All samples were covered with a plastic film and then cured at room temperature for 24 hours. Afterward, the samples were demolded and placed in a curing chamber (temperature 22±2℃, relative humidity 98%) for curing for 28 days.

[0040] 1.3 Test Methods

[0041] To observe the microstructure of mortar samples after curing, a JEOL JXA-8500F scanning electron microscope was used. 1cm × 1cm thin test specimens were cut from the center of cylindrical mortar cured for 28 days. After drying, the specimens were immersed in low-viscosity epoxy resin and then polished with oil-based diamond powder, progressing from coarse to fine, until the surface was sufficiently smooth. After polishing, the specimens were ultrasonically cleaned in anhydrous ethanol to ensure surface cleanliness before testing. Finally, the specimen surface was carbonized. The operating voltage of the scanning electron microscope was set to 15kV, the scanning time for each specimen was 60s, and the mortar was selected for observation at a magnification of 1500. The wavelengths and intensities of characteristic X-rays corresponding to specific elements were captured and analyzed by the spectrometer equipped in the scanning electron microscope, allowing for quantitative analysis of the chemical elemental composition of the specimen surface. Using the calibration curve of a reference sample, the intensity of X-rays corresponding to the target element was converted into a mass fraction. Finally, an elemental distribution map of the sample can be obtained, where the concentration of elements is represented by color levels. In each selected region, eight elements are analyzed: calcium, silicon, aluminum, sulfur, oxygen, iron, sodium, and potassium, and distribution maps of these characteristic elements are obtained.

[0042] 2. Test Results

[0043] 2.1 Distribution of Feature Elements

[0044] The spectrometer equipped in an electron scanning microscope can be used to analyze and visualize the chemical element distribution of slurry. This invention utilizes spectroscopic analysis to analyze eight elements in the slurry: calcium, silicon, aluminum, sulfur, oxygen, iron, sodium, and potassium. These eight elements were selected primarily because the hydration products of cement and fly ash, as well as fly ash itself, are mainly composed of these chemical elements; analyzing their distribution helps determine the characteristic elements of fly ash. Figure 1 The figure shows the distribution of eight elements within the same mortar sample from Example 2. The color bars in the figure indicate the relative concentration of each element, and the numbers next to the color bars correspond to the X-ray counts measured for each color. Figure 1 As can be seen, the concentration of calcium in both fly ash particles and the surrounding hydration products is relatively high, making it difficult to separate unhydrated fly ash particles from the slurry. Meanwhile, compared to the slurry surrounding the fly ash particles, the concentrations of silicon, aluminum, oxygen, iron, sodium, and potassium in the fly ash particles are relatively high, while the sulfur content is relatively low. As a non-homogeneous material, the distribution of chemical elements between and within fly ash particles is not uniform. Therefore, it is advisable to use silicon, aluminum, oxygen, iron, sodium, and potassium as characteristic elements to separate unhydrated fly ash particles.

[0045] Image processing software was used to process the distribution map of silicon, aluminum, oxygen, iron, sodium, and potassium elements in the mortar of Example 2, resulting in a total distribution map of the sum of silicon, aluminum, oxygen, iron, sodium, and potassium elements in this region, as shown in the image. Figure 2 . Figure 2 The image is displayed as a grayscale image, with areas having higher grayscale values ​​(brighter images) representing areas with higher relative abundance of these six elements. It can be seen that there is a significant difference in grayscale between unhydrated fly ash particles and the surrounding hydration products, allowing for the segmentation of unhydrated fly ash particles using grayscale thresholding. Figure 3 This displays the unhydrated fly ash particles separated after grayscale thresholding.

[0046] 2.2 Degree of hydration of fly ash α

[0047] according to Figure 3 Image processing software was used to calculate that the area fraction of unhydrated fly ash particles in the mortar region of Example 2 (see observation regions 2-3 in Table 3) was 36.21%. According to stereochemical principles, in cement-based materials, the area fraction of unhydrated fly ash particles in the mortar is equivalent to their volume fraction. Based on the mix proportions of Example 2 in Table 2 and the densities of cement, fly ash, water, sand, and water-reducing agent, the volume fraction of the paste, including cement, fly ash, water, and water-reducing agent, in the mortar can be calculated. The percentage was 54.01%, thus yielding the volume fraction of unhydrated fly ash particles in the mortar. It is 19.56%.

[0048]

[0049] Meanwhile, based on the mix proportions of Example 2 in Table 2 and the density of fly ash, the initial volume fraction of fly ash in the mortar can be calculated. The hydration degree α of the fly ash was 23.55%. The final hydration degree α of the fly ash was 16.95%.

[0050]

[0051] At a magnification of 1500, 60 regions were randomly selected multiple times from the slurry portion of the mortar in Example 2 (two additional random regions are shown in observation regions 2-1 and 2-2 in Table 3) for microstructure observation and image processing to ensure statistical rationality. The average and standard deviation of the fly ash hydration degree were finally obtained from these 60 regions. Similarly, at a magnification of 1000, 30 regions were randomly selected multiple times from the slurry portion of the mortar in Example 1 (three random regions are shown in observation regions 1-1, 1-2, and 1-3 in Table 3) to calculate the fly ash hydration degree of the mortar in Example 1. Similarly, at a magnification of 2000, 80 regions were randomly selected multiple times from the slurry portion of the mortar in Example 3 (three random regions are shown in observation regions 3-1, 3-2, and 3-3 in Table 3) to calculate the fly ash hydration degree of the mortar in Example 3. Table 3 lists the representative values ​​of the fly ash hydration degree in the mortar of Examples 1-3 in three random areas, as well as the average value and standard deviation of the overall observation area in each example. As can be seen from Table 3, the standard deviation of the fly ash hydration degree calculated in the three different examples is less than 3.5%, indicating that even when observing in different randomly selected areas, the data results obtained by the test method provided by this invention are relatively stable and reliable, proving that this invention can more accurately calculate the hydration degree of fly ash in cement-based materials.

[0052] Table 3. Hydration degree of fly ash in three groups of mortars

[0053]

[0054] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the claims.

[0055] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for calculating the degree of fly ash hydration in cement-based materials, characterized in that, Includes the following steps: (1) Preparation of cement-based materials: The cement-based material is prepared from raw materials including aggregates and cementitious materials, wherein the cementitious materials include fly ash and cement. (2) Acquisition of microscopic images: Samples were taken from the slurry of the cement-based material for observation, and surface scanning of characteristic elements was performed to obtain an elemental distribution map; (3) Image recognition and separation of fly ash: The elemental distribution map was analyzed, and unhydrated fly ash was separated according to the different contents of the characteristic elements, including calcium, silicon, aluminum, sulfur, oxygen, iron, sodium and potassium. (4) Calculation of the degree of hydration α of fly ash: in, The average volume fraction of the unhydrated fly ash in the slurry is obtained by software statistics; The volume fraction of the slurry in the cementitious material is calculated from the mix proportion of the cementitious material and the density of the raw materials. The initial volume fraction of fly ash in the cement-based material is calculated from the mix proportion and the density of the raw materials.

2. The method for calculating the degree of fly ash hydration in cement-based materials according to claim 1, characterized in that, In step (1), the aggregate includes silica sand, and the raw materials also include a water-reducing agent.

3. The method for calculating the degree of fly ash hydration in cement-based materials according to claim 1, characterized in that, In step (2), the cement-based material undergoes molding curing, cutting into thin slices, soaking, polishing, ultrasonic cleaning, and carbon spraying before sampling to meet the requirements of microscopic testing.

4. The method for calculating the degree of fly ash hydration in cement-based materials according to claim 1, characterized in that, In step (3), the characteristic elements include silicon, aluminum, oxygen, iron, sodium and potassium.

5. The method for calculating the degree of fly ash hydration in cement-based materials according to claim 1, characterized in that, In step (3), the element distribution map is first processed to obtain the total distribution map of the sum of the feature elements, and then the total distribution map is subjected to grayscale threshold processing to separate the unhydrated fly ash.

6. The method for calculating the degree of fly ash hydration in cement-based materials according to claim 1, characterized in that, In step (4), the statistics are performed by randomly selecting at least 30 regions in the slurry of the cement-based material at a magnification of 1000 to 2000.

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