Microstructure evaluation method of hydration products of composite solid waste-based alkali-activated cementitious materials

Abstract

The present invention belongs to the technical field of green and environmentally friendly road materials and relates to a method for evaluating the microstructure of hydration products of composite solid waste-based alkali-activated cementitious materials. The method comprises: 1) preparing a composite solid waste-based alkali-activated cementitious material specimen; 2) curing the composite solid waste-based alkali-activated cementitious material specimen; 3) subjecting the composite solid waste-based alkali-activated cementitious material specimen to SEM-EDS, nanoindentation, FTIR, and MIP testing, respectively, and obtaining test results, the test results including SEM-EDS, nanoindentation, FTIR, and MIP; and 4) selecting any two of the test results for cross-reference analysis, ultimately performing a multi-angle and comprehensive evaluation of the microstructure of the hydration products of the composite solid waste-based alkali-activated cementitious material. The present invention has the advantages of improving evaluation quality and speed.

Description

Technical Field

[0001] The present invention belongs to the technical field of green and environmentally friendly road materials, and relates to a method for evaluating the microstructure of hydration products of alkali-activated cementitious materials, and in particular to a method for evaluating the microstructure of hydration products of composite solid waste-based alkali-activated cementitious materials. Background Art

[0002] With the increasing global attention to sustainable development and environmental protection, the production and use of traditional building materials such as silicate cement face problems such as high resource consumption, high carbon emissions and difficult waste disposal. Therefore, the development of new environmentally friendly and high-performance building materials has become a research hotspot. Alkali-activated materials, as an emerging green building material, have gradually attracted widespread attention from academia and industry due to their advantages such as low preparation cost, high strength and good durability. Alkali-activated materials are mainly composed of two parts: aluminosilicate minerals and alkali activators. Aluminosilicate minerals are usually derived from industrial waste residues, such as slag, fly ash, metakaolin, silica fume, steel slag, etc. These waste residues are not only widely available, but also can effectively realize the resource utilization of solid waste. Alkali activators mainly include caustic soda (such as NaOH), alkali-containing silicates, aluminates, etc., which promote the hydration reaction of aluminosilicate minerals through catalysis.

[0003] The reaction mechanism of alkali-activated materials primarily involves the dissolution of SiO₂ and Al₂O₃ on the surface of aluminosilicate minerals by alkali activators. Under the action of the alkali activator, Si-O-Si and Al-O-Al bonds on the raw material surface break, forming intermediate products such as orthosilicic acid and aluminosilicate. These products further diffuse in the liquid phase and undergo polycondensation, forming a geopolymer gel with a spatial network structure. With extended reaction time, the geopolymer gel may further develop into zeolite-like microcrystalline or semicrystalline structures, imparting high strength and durability to the material. The hydration products of alkali-activated materials primarily include hydrated aluminosilicate gel (CASH) and aluminosilicate gel. These gels are composed of multiple [SiO₄] and [AlO₄] tetrahedra connected by bridging oxygens, forming a complex spatial network structure. Among these, CSH gel, cross-linked gel (CNASH), calcium-rich alkali-activated gel (CASH), and sodium-rich alkali-activated gel (NASH) are the primary hydration products of alkali-activated solid waste materials, and their structure and properties directly influence the overall performance of the material. In addition, depending on the characteristics of the solid waste material and the type of activator, other types of hydration products may also be formed, such as hydrotalcite, AFt phase minerals, etc.

[0004] Currently, the main techniques used to evaluate the microstructure of hydration products of cementitious materials include scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), low-field nuclear magnetic resonance (LNMR), nanoindentation, thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP). However, these techniques have the following advantages and disadvantages: Scanning electron microscopy (SEM) is primarily used for high-resolution microtopography analysis, providing magnified topographic images and suitable for analyzing the surface microstructure of hydration products of solid waste cementitious materials. Its advantages lie in its intuitiveness and high resolution, but its disadvantage is that it cannot observe the internal microstructure of the material. Transmission electron microscopy (TEM) can observe the internal microstructure of the sample, including the morphology and distribution of the hydration products. It has deeper penetration than SEM and can reveal fine structures smaller than 0.2 microns, namely submicroscopic and ultramicroscopic structures. Its advantage lies in providing more comprehensive structural information, but its disadvantage is that the sample preparation process is relatively complex, which can easily lead to errors in the experimental data, and its use is generally not recommended. X-ray diffraction (XRD) is primarily used to analyze the crystal structure of materials and can determine the chemical composition and crystal structure of hydration products. Its advantages lie in its high analytical precision and ability to provide quantitative chemical composition information, but its disadvantage is its poor analytical performance for amorphous materials. Fourier transform infrared (FTIR) spectroscopy is used to analyze chemical bonds and functional groups in materials and infer the chemical structure of hydration products. Its advantages lie in its sensitivity to changes in the chemical environment and its ability to provide rich chemical structural information, but its disadvantage is that it cannot directly provide morphological and structural information. Low-field nuclear magnetic resonance (LNMR) can clearly observe the structure and dynamics of hydrogen atoms, improving high-resolution images. However, its disadvantages are that the signal-to-noise ratio and resolution may be slightly lower than those of high-field NMR. Furthermore, certain specific studies require NMR equipment with higher field strengths, which can affect the accuracy of observation and analysis. Nanoindentation is considered an effective technique for measuring the nano- and micromechanical properties of cementitious materials. It is used to measure the nanoscale hardness and elastic modulus of cementitious materials, revealing the mechanical properties of materials at the microscale. However, this technique requires specialized equipment and operating techniques. Thermogravimetric analysis (TGA) is used to assess the thermal stability of cementitious materials, studying their mass changes at different temperatures and thereby analyzing the thermal decomposition characteristics and composition of the materials. However, for some complex materials, this method may not provide accurate results. Mercury intrusion porosimetry (MIP) is used to determine the pore structure and pore size distribution of cementitious materials, understanding their density and permeability. This is important for optimizing the preparation process and performance of cementitious materials, such as controlling the pore structure to improve their strength and durability. However, this technique is destructive to the sample and the operation process is relatively complex.

[0005] Solid waste cementitious materials are currently widely used in building materials, showing great potential in particular for pavement base materials, but their microstructure is still not well understood. Solid waste cementitious materials, as a new type of cementitious material made from industrial solid waste, are gradually becoming a focus of the building materials industry. They not only achieve resource recycling but also demonstrate excellent performance and greater application potential. In civil engineering, road engineering and other fields, bases and concrete formulated with solid waste cementitious materials can meet the requirements of various strength grades and significantly improve the corrosion resistance and crack resistance of medium and low strength grade concrete. However, as a pavement base material, the microstructure of solid waste cementitious materials is still not well understood, which hinders the further optimization of their performance and the expansion of their application range. Therefore, strengthening the research on the microstructure of solid waste cementitious materials is of great significance for promoting their application in pavement base materials. Summary of the Invention

[0006] In order to solve the above technical problems existing in the background technology, the present invention provides a microstructure evaluation method for hydration products of composite solid waste-based alkali-activated cementitious materials, which can improve the evaluation quality and evaluation speed.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for evaluating the microstructure of hydration products of composite solid waste-based alkali-activated cementitious materials, characterized in that the method comprises the following steps:

[0009] 1) Preparation of composite solid waste-based alkali-induced gelling specimens;

[0010] 2) curing the composite solid waste-based alkali-activated gelling specimen prepared in step 1);

[0011] 3) performing SEM-EDS testing, nanoindentation testing, FTIR testing, and MIP testing on the composite solid waste-based alkali-induced gelling specimen after step 2), and obtaining test results, wherein the test results include SEM-EDS test results, nanoindentation test results, FTIR test results, and MIP test results;

[0012] 4) Select any two of the test results obtained in step 3) for cross-reference analysis, and ultimately conduct a multi-angle and comprehensive evaluation of the microstructure of the hydration product of the composite solid waste-based alkali-activated cementitious material.

[0013] Preferably, the step 1) composite solid waste based alkali-activated gelling specimen includes a precursor and an alkali activator; the alkali-activated material in the composite solid waste based alkali-activated gelling specimen is carbide slag; when preparing the composite solid waste based alkali-activated gelling specimen, sodium silicate or sodium hydroxide solid is used to replace 5%, 10%, 15% and 20% of the mass of the carbide slag; the modulus and dosage of the alkali activator are selected based on the principle that the compressive strength is maximized after the alkali activator is mixed with the precursor and cured for 28 days; the precursor is red mud, fly ash and / or circulating fluidized bed fly ash; the water-cement ratio of the composite solid waste based alkali-activated gelling is calculated based on the optimal moisture content measured by the static pressure molding method.

[0014] Preferably, the curing time in step 2) is 28 days, the curing temperature is 20° C.±1° C., and the curing relative humidity is 95%.

[0015] Preferably, the specific implementation method of performing SEM-EDS detection in step 3) is:

[0016] a.1) Determine the non-lateral compressive strength of the composite solid waste-based alkali-induced gelled specimens after step 2) using a MATEST 3000 kN compression testing machine at a loading rate of 0.6 MPa / s, obtaining the non-lateral compressive strength results and the non-lateral compressive strength of a portion of the composite solid waste-based alkali-induced gelled specimens after destruction;

[0017] a.2) immersing a portion of the composite solid waste-based alkali-stimulated gelling specimen obtained in step a.1) in anhydrous ethanol to stop the hydration reaction inside the composite solid waste-based alkali-stimulated gelling specimen;

[0018] a.3) drying the composite solid waste-based alkali-activated gelling specimen obtained in step a.2) in a vacuum oven at 60°C for 24 hours;

[0019] a.4) polishing the surface of the composite solid waste-based alkali-activated gelling specimen obtained in step a.3) to obtain a smooth surface;

[0020] a.5) performing a conductive treatment on the composite solid waste-based alkali-activated gelling specimen obtained in step a.4), performing SEM-EDS testing, and obtaining a BSE image;

[0021] a.6) Analyze the BSE image obtained in step a.5) using ImageJ software to obtain SEM-EDS detection results.

[0022] Preferably, the specific implementation method of performing nanoindentation detection in step 3) is:

[0023] b.1) measuring the non-lateral compressive strength of the composite solid waste-based alkali-induced gelled specimen after step 2) using a MATEST 3000 kN compression testing machine at a loading rate of 0.6 MPa / s, obtaining the non-lateral compressive strength results and the partial composite solid waste-based alkali-induced gelled specimen after non-lateral compressive strength failure;

[0024] b.2) immersing a portion of the composite solid waste-based alkali-stimulated gelling specimen obtained in step b.1) in anhydrous ethanol to stop the hydration reaction inside the composite solid waste-based alkali-stimulated gelling specimen;

[0025] b.3) drying the composite solid waste-based alkali-activated gelling specimen obtained in step b.2) in a vacuum oven at 60°C for 24 hours;

[0026] b.4) polishing the composite solid waste-based alkali-activated gelling specimen obtained in step b.3) to obtain a smooth surface;

[0027] b.5) preparing an indentation grid having an M×N matrix structure, wherein the gaps between the indentation grids are no greater than 5 μm;

[0028] b.6) Using a ladder-mode loading and unloading procedure, the indentation grid obtained in step b.5) is pressed onto the smooth surface obtained in step b.4) for nanoindentation testing, and finally the nanoindentation test results are obtained.

[0029] Preferably, in step b.6), the loading rate is no more than 400 μN / s, the maximum loading amount is no more than 2000 μN / s, and after reaching the maximum loading amount, the loading time is 2 s; the unloading process does not exceed 5 s.

[0030] Preferably, the specific implementation method of performing FTIR detection in step 3) is:

[0031] c.1) taking part of the powder of the composite solid waste alkali-activated gelling test piece after step 2) as a test piece sample;

[0032] c.2) preparing MS residue, HCl residue, and water residue based on the test piece sample obtained in step c.1);

[0033] c.3) Use OMNIC software to obtain spectra of HCl and MS selective solubilization hydration products.

[0034] Preferably, the MS residue in step c.2) is prepared by dissolving the test piece sample obtained in step c.1) in an MS solution, stirring the mixture using a magnetic stirrer, vacuum filtering the suspension, washing the insoluble residue with methanol, drying, and weighing to obtain the MS residue; the MS solution is a solution formed by dissolving 6.25 g of salicylic acid in 100 ml of methanol;

[0035] The HCl residue in step c.2) is prepared by adding the test piece sample obtained in step c.1) into a 1.76 wt% HCl solution for extraction to obtain an insoluble residue, which is the HCl residue;

[0036] The water residue in step c.2) is prepared by dissolving the paste of the test sample obtained in step c.1) in distilled water, dissolving in an ultrasonic bath at 25° C. for 5 hours, centrifuging at 10,000 rpm for not less than 10 minutes, and retaining the residue, which is the water residue;

[0037] The specific implementation method of c.3) is: using OMNIC software to subtract the spectrum of HCl residue and MS residue from the spectrum of the original gelled sample powder to obtain the spectrum of HCl and MS selective dissolution hydration products.

[0038] Preferably, the specific implementation method of the step of performing FTIR detection further includes, after step c.3), the following steps:

[0039] c.4) further analyzing and confirming the spectra of the HCl and MS selective dissolution hydration products obtained in step c.3) using a TGA method to obtain FTIR detection results; in the TGA method, the heating rate of the hydration product is 10°C / min, and the heating temperature is increased from 20°C to 1200°C.

[0040] Preferably, the specific implementation of the MIP detection in step 3) is:

[0041] d.1) taking the composite solid waste-based alkali-induced gelling specimen after undergoing no lateral compressive strength damage in step a.1);

[0042] d.2) crushing the composite solid waste-based alkali-activated gelation specimen obtained in step d.1) into specimen particles of 5 nm to 10 nm;

[0043] d.3) drying the sample particles obtained in step d.2) in a freeze dryer at -90°C under vacuum conditions for 36 hours;

[0044] d.4) taking out the dried product from step d.3) and placing it in an oven at 25°C to obtain a MIP test sample;

[0045] d.5) Performing a MIP test on the MIP test sample obtained in step d.4) to obtain a MIP test result.

[0046] Compared with the prior art, the present invention has the following advantages and effective results:

[0047] The present invention provides a method for evaluating the microstructure of hydration products of a composite solid waste-based alkali-activated cementitious material, comprising the following steps: 1) preparing a composite solid waste-based alkali-activated cementitious material specimen; 2) curing the composite solid waste-based alkali-activated cementitious material specimen prepared in step 1); 3) performing SEM-EDS testing, nanoindentation testing, FTIR testing, and MIP testing on the composite solid waste-based alkali-activated cementitious material specimen after step 2), and obtaining test results at the same time, wherein the test results include SEM-EDS test results, nanoindentation test results, FTIR test results, and MIP test results; 4) selecting any two of the test results obtained in step 3) for cross-reference analysis, and finally performing a multi-angle and comprehensive evaluation of the microstructure of the hydration products of the composite solid waste-based alkali-activated cementitious material. The present invention aims to solve the defect of the existing technical means of single microstructure analysis of complex hydration products of composite solid waste based alkali excited materials, and adopts a variety of technical means, including scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS), ImageJ software image analysis, nanoindentation test, infrared spectrometer (FTIR) and OMNIC software spectrum subtraction analysis, thermogravimetric analysis (TGA) and MIP mercury intrusion instrument, etc., to carry out multi-angle and comprehensive evaluation of the hydration products of composite solid waste based alkali excited cementitious materials. Through the comprehensive use of a variety of technical means, the present invention can more comprehensively understand the microstructure of the hydration products, realize the multi-angle and comprehensive evaluation of the microstructure of the hydration products of the composite solid waste based alkali excited cementitious materials, thereby improving the quality and speed of the evaluation, and providing more accurate data support for the research and development and application of composite solid waste based alkali excited materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The composite solid waste-based alkali-activated gelling specimen prepared by the present invention;

[0049] Figure 2 This is the SEM scanning electron microscope image of red mud;

[0050] Figure 3 This is the SEM image of circulating fluidized bed fly ash;

[0051] Figure 4 This is the SEM image of carbide slag;

[0052] Figure 5 This is the SEM image of fly ash;

[0053] Figure 6This is the phase image after processing by ImageJ. DETAILED DESCRIPTION

[0054] The present invention provides a method for evaluating the microstructure of hydration products of composite solid waste-based alkali-activated cementitious materials, which is a comprehensive and systematic evaluation method, specifically comprising the following steps:

[0055] 1) Preparation of composite solid waste-based alkali-induced gelling test specimens. At least 4 test specimens of each type are prepared. The prepared composite solid waste-based alkali-induced gelling test specimens are as follows: Figure 1 As shown;

[0056] The alkali-activated material is carbide slag (CS), and then sodium silicate (Na2SiO3) or sodium hydroxide (NaOH) solid is used to replace 5%, 10%, 15% and 20% of the mass of the carbide slag (CS). The modulus and dosage of the alkali activator are selected based on the principle that the compressive strength is maximized after the alkali activator is mixed with a precursor (red mud (RM), fly ash (FA), circulating fluidized bed fly ash (CFBFA) etc.) and cured for 28 days. The composite solid waste-based alkali-activated gelled specimen is prepared by uniformly mixing the precursor to be studied and analyzed (which can be one or more), the composite alternative alkali-activated material and water. The water-to-binder ratio of the composite solid waste-based alkali-activated gelled specimen is calculated based on the optimal moisture content measured by the static pressure molding method.

[0057] 2) Curing of specimens: The curing time is 28 days; the specimens need to be wrapped with plastic film and cured for 28 days under standard curing conditions of 20℃±1℃ and relative humidity of 95%.

[0058] 3) After the specimens were subjected to the non-lateral compressive strength test, the hydration products of the cementitious material were analyzed using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS);

[0059] For the specimens after curing, the non-lateral compressive strength of each specimen should be measured using a MATEST 3000kN compression testing machine at a loading rate of 0.6MPa / s;

[0060] The SEM-EDS test in step (3) uses a 1-2g sample from the center of the specimen after lateral compressive strength failure. The sample is then immersed in anhydrous ethanol for 7 days to stop the internal hydration reaction. The sample is then dried in a vacuum oven at 60°C for 24 hours.

[0061] To ensure the preservation of the sample, it was wrapped in epoxy resin under vacuum conditions before the SEM-EDS experiment. During the SEM-EDS test, the sample was removed from the epoxy resin and polished until a smooth surface was obtained. Finally, the sample was immersed in anhydrous ethanol and placed in an ultrasonic bath to remove various particles adhering to the sample surface. After polishing, the sample was dried in a vacuum oven at 60°C for 12 hours and subjected to conductive treatment before SEM-EDS analysis. The specific test method is as follows:

[0062] Analysis conditions: high vacuum environment, accelerating voltage 25 keV, working distance 15.0 mm;

[0063] Imaging method: Backscattered electron (BSE) imaging is used to improve the signal-to-noise ratio and image contrast, and to distinguish between gelled areas and other areas such as unreacted precursors;

[0064] Point analysis strategy: Through EDS point analysis, collect no less than 80 points from the sample focusing only on the gelled area, remembering to exclude the precursors that did not participate in the reaction, to study the composition of the gel;

[0065] Research objective: To study and analyze the distribution composition of different elements in the gelled area.

[0066] 4) Image analysis of BSE images obtained from SEM-EDS analysis was performed using ImageJ software;

[0067] The application requirements of ImageJ software in BSE image analysis in step (4) are as follows:

[0068] Image preprocessing: Use ImageJ software to open the BSE image obtained by SEM-EDS analysis, and use the color threshold function of the software to preprocess the image.

[0069] Phase separation and color assignment: By setting a specific color threshold, the different reaction phases and unreacted phases of the gel are effectively separated, and each phase is assigned a different color for easy distinction and identification.

[0070] Image combination and percentage calculation: The segmented images were combined to form a composite map of gel-pasted images, and the percentage of each phase was calculated and determined using the define scale function of ImageJ software.

[0071] Multi-site analysis: To ensure the accuracy and reliability of the analysis, more than 5 different sites were selected for each sample for image analysis, and the average value was used as the final result.

[0072] 5) Performing nanoindentation testing on the gelled sample material;

[0073] The sample used for nanoindentation testing was prepared in a similar process to that for SEM-EDS in step (3), but the polishing of the sample required a detailed grinding process lasting up to 25 minutes to obtain an extremely smooth surface;

[0074] To achieve precise indentation testing, a square grid layout consisting of 144 points, arranged in 12 rows by 12 columns, was carefully selected to ensure comprehensiveness and accuracy. The grid spacing was precisely set to 5μm, ensuring high resolution of the test data.

[0075] In the selection of loading and unloading procedures, a trapezoidal mode was adopted, which is widely recognized in materials testing for its stability and reliability. The loading rate is strictly controlled to 400μN / s to ensure a smooth test process while avoiding unnecessary impact or damage to the sample. The maximum load is set to 2000μN / s. This value not only ensures the adequacy of the test, but also avoids deformation or damage to the sample due to overload. After reaching the maximum load, this state is maintained for 2 seconds to ensure the stability and reliability of the test results. Subsequently, the unloading process is completed smoothly within 5 seconds. This operation not only ensures the continuity of the test, but also avoids additional stress or damage to the sample due to unloading too quickly.

[0076] 6) The composition and reaction degree of the hydrated gel products of the precursor and the alkali activator were studied by selective dissolution process with infrared spectroscopy (FTIR) and OMNIC software for spectral subtraction analysis;

[0077] The samples used for Fourier transform infrared (FTIR) analysis in step (6) include: 1-2 g of powder from the center of the gelled specimen, water residue, HCl residue, and MS residue. The preparation of the three residues is as follows:

[0078] 1. Preparation of MS residues:

[0079] The gelled sample powder and raw materials were dissolved in MS solution (specifically, a mixture of 100 ml of methanol and 6.25 g of salicylic acid). The mixture was stirred using a magnetic stirrer, and the suspension was vacuum filtered. The insoluble residue was washed with methanol, dried, weighed, and stored in a vacuum desiccator.

[0080] 2. Preparation of HCl residue:

[0081] 2 g of gelled sample paste was added to 300 ml of HCl solution (HCl solution was a mixture of 37 wt % concentrated hydrochloric acid and purified water in a volume ratio of 1:20) for extraction; during the extraction process, unreacted CFBFA and silica gel were left as insoluble residues.

[0082] 3. Preparation of water residue:

[0083] 2 g of the gelled sample paste was dissolved in 300 g of distilled water in an ultrasonic bath at 25° C. for 5 hours; after dissolution, the solution was centrifuged to separate the residue from the liquid, obtaining a water residue.

[0084] OMNIC software was used to study the dissolution of hydration products from the gelled samples. Spectra of the HCl and MS residues were subtracted from the original gelled sample powder spectrum to obtain spectra of the hydration products selectively dissolved by HCl and MS.

[0085] 7) The hydration products of the powder samples were further analyzed and confirmed using thermogravimetric analysis (TGA);

[0086] In step (7), when further analyzing and confirming the hydration products of the gelled sample powder by thermogravimetric analysis (TGA), it is ensured that each powder sample is about 5-10 mg, and the heating rate of all samples is 10°C / min, and the temperature is increased from 20°C to 1200°C.

[0087] 8) Use MIP mercury porosimetry to study the pore structure of powder samples;

[0088] In step (8), the void structure of the gelled powder sample is studied using a mercury porosimeter (MIP) instrument. The powder sample is crushed into particles of 5 nm to 10 nm using a ball mill or other crushing instrument after the gelled powder sample has been crushed in step (3) without lateral compressive strength. The powder sample is then dried in a freeze dryer at -90°C under a low vacuum pressure for 36 hours. After drying, the sample is removed from the freeze dryer and placed in a 25°C oven until the MIP test is performed.

[0089] 9) In combination with steps 3) to 8), a multi-angle and comprehensive evaluation is conducted on the microstructure of the hydration product of the composite solid waste-based alkali-activated cementitious material.

[0090] Any two microscopic analyses in steps (3)-(8) can be subjected to cross-analysis. By cross-analyzing the microstructure of the hydration products of composite cementitious materials using six technical means, the hydration products of composite solid waste-based alkali-activated cementitious materials can be evaluated from multiple angles and in an all-round manner.

[0091] The technical solution provided by the present invention is described in detail below with reference to the accompanying drawings:

[0092] Example 1:

[0093] A method for evaluating the microstructure of a hydration product of a composite solid waste-based alkali-activated cementitious material comprises the following steps:

[0094] (1) Preparation of composite solid waste-based alkali-induced gelling specimens

[0095] In this example, red mud (RM) and circulating fluidized bed fly ash (CFBFA) were used as solid waste precursors, and carbide slag (CS) was used as the alkaline activator. X-ray fluorescence (XRF) analysis of these three materials was performed, and the results are shown in Table 1. The precursors' main components are SiO2, Al2O3, CaO, and Fe2O3, with the combined content of SiO2 and Al2O3 exceeding 50%, making them suitable for use as geopolymer precursors. The alkaline activator, carbide slag (CS), contains as much as 83.39% CaO. This reacts with water to form Ca(OH)2, creating an alkaline environment that increases the reaction rate between red mud (RM) and circulating fluidized bed fly ash (CFBFA).

[0096] Table 1 Chemical composition test table of raw materials

[0097]

[0098] Red mud (RM) comes from the Bayer red mud produced by an aluminum plant in Shanxi Province. It was used in this experiment after being piled up for about a year. Because the piled red mud is in blocky particles, it needs to be crushed into powder using a jaw crusher. Because the red mud is relatively wet at this time, it needs to be placed in a drying blower for drying. After that, a screening machine is used to pass the dried powdered red mud through a 0.30mm square hole sieve for testing. The SEM electron microscope image of the red mud is as follows: Figure 2 As shown in the figure, it can be seen from the microscopic picture that the red mud particles are agglomerated. On its surface, it can be seen that the structure is loose and porous, and the pores between the particles are relatively dense. These pores are disordered and irregular, and most of the pores are between the particles. The contact area between the soil particles is relatively small. There are many shapes as a whole, such as block, strip, elliptical and other particle forms. Under a high-power scanning electron microscope, the surface of the red mud agglomerate may be honeycomb-shaped, and the walls of the large and small pores are closely connected; the circulating fluidized bed fly ash (CFBFA) comes from a thermal power plant in Shanxi Province, and its SEM electron microscope image is shown below. Figure 3 As shown in the figure, the appearance of CFB ash under a scanning electron microscope is mainly irregular, loosely structured, slag-like particles with a generally large particle size. The relatively large specific surface area is conducive to the formation of hydration products. The SEM electron microscope image of calcium carbide slag (CS) from a steel plant in Shanxi Province is shown in the figure. Figure 4 As shown in the figure, it can be observed that carbide slag particles have various morphologies, such as flakes, blocks, and a few strips, and there are obvious pore structures between the particles. These pore structures contribute to the material transport and adsorption properties of carbide slag in chemical reactions.

[0099] The addition ratios of RM, CFBFA and CS were 40%, 50% and 10% respectively. After adding 20g of carbide slag to 80g of red mud and 100g of circulating fluidized bed fly ash, the mixture was fully stirred in a small mixer at a speed of 800r / min. Then, 36g, 38g, 40g, 42g, 44g and 46g of distilled water were added respectively. Six groups of 50mm×50mm×50mm cylindrical specimens with moisture contents of 18%, 19%, 20%, 21%, 22% and 23% were prepared by static pressure molding test (such as Figure 1 After forming, the mass and height of the molded pieces were weighed and measured. The dry density was calculated and then a quadratic curve of moisture content versus dry density was fitted using Excel or other graphics software. The optimal moisture content was determined to be 20.5% using the capture tool. Based on this optimal moisture content, 41 g of water was added each time. Four parallel specimens were prepared according to the above steps.

[0100] (2) Maintenance of specimens

[0101] The four parallel specimens prepared in step (1) were wrapped and sealed with at least three layers of plastic film, and then placed in a curing box for 28 days under standard curing conditions of 20°C ± 1°C and a relative humidity of 95%.

[0102] (3) No lateral compressive strength and SEM-EDS test

[0103] After the specimens after curing were subjected to a non-lateral compression test at a loading rate of 0.6 MPa / s, four values were obtained, namely: 14.793 MPa, 15.322 MPa, 15.611 MPa, and 14.677 MPa. The average value of 15.101 MPa was taken as the final result of the test.

[0104] After the non-lateral compressive strength test, 2g of gelled sample from the center of each crushed specimen was taken and immersed in anhydrous ethanol for 7 days to prevent the solid waste base from inducing the gelling hydration reaction. The sample was then placed in a vacuum oven at 60°C and dried for 24 hours. Before SEM-EDS testing, the dried sample was wrapped with epoxy resin. 14 hours before the test, the sample was polished to a smooth surface and various particles on the sample surface were removed in an ultrasonic bath filled with anhydrous ethanol. The sample was then placed in a 60°C oven and dried for 12 hours. After drying, the sample was sprayed with gold for conductive treatment before the SEM-DES test was officially carried out. The specific test process is as follows:

[0105] In a high vacuum environment, an acceleration voltage of 25keV and a working distance of 15.0mm, backscattered electron (BSE) imaging is used to improve the signal-to-noise ratio and image contrast. After the magnification ratio is determined to be clear, the gelled area and unreacted precursor areas are distinguished (the precursor needs to be subjected to SEM in advance to determine its microscopic appearance structure so that the various phase areas can be distinguished in this step); then, the gelled area obtained by differentiation is concentrated and no less than 80 points are collected to study the elements in the microregion of the gelled material for detailed analysis, and the distribution composition of different elements in the gelled area is further determined.

[0106] (4) ImageJ processing to quantify gel composition

[0107] The BSE image obtained in step (3) was preprocessed using the color threshold function of ImageJ software; various reaction phases and unreacted phases of the gel were effectively separated by setting a specific color threshold, and each phase was assigned a specific color for easy distinction; the segmented images were then combined with the gel for composite mapping. The specific steps of this step are as follows:

[0108] 1) Image segmentation: Splitting the original image into multiple parts, perhaps for more detailed processing or analysis of specific areas of the image.

[0109] 2) Gel image processing: Perform necessary processing on the gel image, such as brightness adjustment, noise reduction, etc., to obtain a clear gel image.

[0110] 3) Image combination and pasting: The segmented image parts are combined as needed and pasted with the processed gel image.

[0111] 4) Composite Mapping: Finally, these images are composite mapped to generate a complete image containing all necessary information, such as Figure 6 shown.

[0112] Using the grayscale and scale functions of ImageJ software, four different points were selected for each sample for image analysis, the percentage of each phase was calculated, and the average value of each phase was taken as the final result.

[0113] Table 2 Phase percentage test table

[0114]

[0115] (5) Nanoindentation test

[0116] The samples used for nanoindentation testing were prepared in a similar manner to those used in the SEM-EDS test. It should be noted that during this treatment, the samples were polished for 25 minutes until an extremely smooth surface was obtained.

[0117] To achieve precise indentation testing, a representative square grid layout consisting of 144 points, arranged in 12 rows by 12 columns, was selected to ensure comprehensiveness and accuracy. The grid spacing was precisely set to 5 μm, ensuring high-resolution test data. Furthermore, a trapezoidal pattern was employed for loading and unloading, widely recognized in materials testing for its stability and reliability. The loading rate was strictly controlled to 400 μN / s to ensure smooth testing and avoid unnecessary impact or damage to the sample. The maximum load was set to 2000 μN / s, ensuring sufficient testing while preventing sample deformation or damage due to overloading. After reaching the maximum load, this value was maintained for 2 seconds to ensure stable and reliable test results. Unloading then proceeded smoothly within 5 seconds, ensuring test continuity while avoiding excessive stress or damage to the sample due to rapid unloading.

[0118] (6) Infrared spectroscopy (FTIR) combined with OMNIC software to further analyze the hydration products

[0119] This study employed a selective dissolution process, combined with spectral subtraction analysis using a Fourier transform infrared (FTIR) spectrometer and OMNIC software, to further investigate the composition and degree of reaction between the precursor and the hydrated gel product of the alkali activator. The samples used in the FTIR experiment included 2 g of powder from the center of the specimen after the lateral compressive strength test in step (3) and three types of residue. The specific experimental process is as follows:

[0120] 1) MS Residue Preparation: Dissolve the gelled sample powder and raw materials in a specific ratio of MS solution (100 ml methanol + 6.25 g salicylic acid). Stir with a magnetic stirrer and vacuum filter the suspension. Wash the insoluble residue with methanol, dry, weigh, and store.

[0121] 2) Preparation of HCl residue: The gelled sample paste was added to an HCl solution (HCl solution was prepared by mixing 37 wt % concentrated hydrochloric acid and purified water in a volume ratio of 1:20) for extraction, and unreacted CFBFA and silica gel were left as insoluble residues.

[0122] 3) Preparation and Spectral Analysis of Water Residues: The gelled sample powder was dissolved in distilled water, sonicated, and then centrifuged to separate the residue from the liquid. Using OMNIC software, the spectra of the selectively dissolved hydration product were analyzed by subtracting the HCl and MS residue spectra from the original gelled sample powder spectrum.

[0123] (7) Thermogravimetric analysis (TGA)

[0124] Before the experiment, it is important to check the elemental composition of the SEM-EDS to see if there are any chemical substances that will react with the crucible used in thermogravimetric analysis, thereby damaging the crucible and contaminating the reactants. Therefore, before the experiment, a crucible made of a non-reactive material should be selected according to the sample. In this case, due to the presence of Si-containing oxides, a conventional alumina crucible cannot be used, and a platinum crucible must be used. First, 5-10 mg of the sieved powdered sample is evenly spread on the bottom of the crucible (note that the particle size of samples in the same series must be kept within a similar range to ensure the reliability of the test data).

[0125] During the test, the relationship between the mass of the substance and temperature is measured at a program-controlled temperature (heating rate of 10°C / min, from 20°C to 1200°C). The thermogravimetric curve (TG curve) is recorded, with the ordinate representing weight percentage and the abscissa representing temperature or time.

[0126] Finally, data analysis is performed to analyze the change in sample weight over temperature and time using a TG curve. The first-order derivative can be used to generate a DTG curve to analyze the rate of weight change. The area of the DTG curve is proportional to the weight loss and can be used for further analysis.

[0127] (8) MIP mercury intrusion test

[0128] In order to study and analyze the void microstructure of the hydration products of composite solid waste-based alkali-activated cementitious materials, it was decided to use the MIP mercury intrusion test. The powder sample preparation process used in this test is as follows: First, the gel blocks that have undergone no lateral compressive strength test are crushed using a ball mill or other crushing instrument until the particle size reaches 5nm-10nm. Subsequently, the crushed samples are dried using a freeze dryer at a temperature of -90°C and a small vacuum pressure for 36 hours. After drying, about 3g of the sample is taken out and placed in an oven at 25°C until the MIP (mercury intrusion porosimetry) test is carried out; during the test, the instrument is calibrated first, and then the engineer sets the appropriate pressure range and step size according to the sample type and expected pore range; when the sample is loaded into the sample chamber, it should be ensured that the sample is well sealed with the instrument, and then mercury injection is performed to start the test. Finally, the analysis of the test data should be carried out in the following two steps:

[0129] 1) Abnormal Data Exclusion: During data analysis, identify and exclude abnormal data points, such as sudden pressure drops or volume changes. 2) Curve Fitting: Use an appropriate model to fit the pressure-volume curve to accurately calculate the pore size distribution.

[0130] (9) Comprehensive analysis

[0131] Any two microscopic analysis techniques in steps (3)-(8) can be cross-analyzed. By cross-analyzing and comparing the six different microstructural techniques used in the hydration products of composite cementitious materials, the characteristics of the hydration products of composite solid waste-based alkali-activated cementitious materials can be deeply evaluated and analyzed from multiple angles and in all directions, thereby ensuring the comprehensiveness and accuracy of the analysis. In this example, the following cross-analysis and comparison are performed:

[0132] The SEM-EDS analysis results were compared with nanoindentation test results to analyze the relationship between microstructure and mechanical properties. FTIR spectroscopy results were compared with thermogravimetric analysis to verify the composition and reaction degree of the hydration products. The MIP mercury porosimetry results were correlated with those of other techniques to fully understand the impact of pore structure on cementitious material properties. Through these cross-analyses and comparisons, the characteristics of the hydration products of composite solid waste-based alkali-activated cementitious materials can be thoroughly evaluated and analyzed from multiple perspectives.

[0133] Example 2

[0134] A method for evaluating the microstructure of a hydration product of a composite solid waste-based alkali-activated cementitious material comprises the following steps:

[0135] (1) Preparation of composite solid waste-based alkali-induced gelling specimens

[0136] In this example, red mud (RM) and fly ash (FA) were used as solid waste-based precursors, and carbide slag (CS) and sodium silicate (Na2SiO3) solids were used as alkaline activators. X-ray fluorescence (XRF) analysis was performed on the above three materials, and the results are shown in Table 2.

[0137] Table 2 Chemical composition test table of raw materials

[0138]

[0139] Red mud (RM) and carbide slag (CS) are the same as those in Example 1. Fly ash (FA) comes from a power plant in Shanxi Province. Figure 5 As shown in the figure, fly ash (FA) exhibits irregular shapes under an electron microscope, with spherical particles predominating. The surface of these spherical particles is quite smooth, and under magnification, numerous tiny glass spheres can be seen adhering to them. Furthermore, fly ash also contains loose, spongy, glassy particles, which also have a nearly smooth surface and a honeycomb-like appearance, with spherical shapes dotted within.

[0140] RM, FA, and CS were added in proportions of 45%, 45%, and 10%, respectively. 20g of carbide slag was added to 90g of red mud and 90g of fly ash. After thorough mixing in a small mixer at 800 rpm, 32g, 34g, 36g, 38g, 40g, and 42g of distilled water were added. Six sets of 50mm×50mm×50mm cylindrical specimens with moisture contents of 16%, 17%, 18%, 19%, 20%, and 21% were prepared using static pressure molding. After molding, the specimens were weighed and their heights were measured. Dry densities were calculated, and a quadratic curve of moisture content versus dry density was fitted using Excel or other graphics software. The optimal moisture content was determined to be 18.5% using the capture tool. Based on this optimal moisture content, 37g of water was added each time. Four replicate specimens were prepared using the same procedure. Then, sodium silicate (Na2SiO3) solid was used to replace 5%, 10%, 15% and 20% of the mass of carbide slag (CS) to prepare four parallel test pieces according to the above method. The rest of the test steps refer to Example 1.

Claims

1. A method for evaluating the microstructure of hydration products of composite solid waste-based alkali-activated cementitious materials, characterized by: The method for evaluating the microstructure of hydration products of composite solid waste-based alkali-activated cementitious materials comprises the following steps: 1) Preparation of composite solid waste-based alkali-induced gelling specimens; 2) curing the composite solid waste-based alkali-activated gelling specimen prepared in step 1); 3) performing SEM-EDS testing, nanoindentation testing, FTIR testing, and MIP testing on the composite solid waste-based alkali-induced gelling specimen after step 2), and obtaining test results, including SEM-EDS testing results, nanoindentation testing results, FTIR testing results, and MIP testing results; 4) Select any two of the test results obtained in step 3) for cross-reference analysis, and ultimately conduct a multi-angle and comprehensive evaluation of the microstructure of the hydration product of the composite solid waste-based alkali-activated cementitious material; In step 1), a composite solid waste-based alkali-activated gelling specimen comprises a precursor and an alkali activator; the alkali-activated material in the composite solid waste-based alkali-activated gelling specimen is carbide slag; sodium silicate or sodium hydroxide solid is used to replace 5%, 10%, 15%, and 20% of the carbide slag by mass when preparing the composite solid waste-based alkali-activated gelling specimen; the modulus and dosage of the alkali activator are selected based on the principle that the compressive strength is maximized after the alkali activator and the precursor are mixed and cured for 28 days; the precursor is red mud, fly ash, and / or circulating fluidized bed fly ash; and the water-binder ratio of the composite solid waste-based alkali-activated gelling is calculated based on the optimal moisture content measured by a static pressure molding method; The curing time in step 2) is 28 days, the curing temperature is 20°C ± 1°C, and the curing relative humidity is 95%; The specific implementation method of performing SEM-EDS detection in step 3) is: a.1) Determine the non-lateral compressive strength of the composite solid waste-based alkali-induced gelled specimens after step 2) using a MATEST 3000kN compression testing machine at a loading rate of 0.6MPa / s. Obtain the non-lateral compressive strength results and the non-lateral compressive strength of a portion of the composite solid waste-based alkali-induced gelled specimens after failure. a.2) immersing a portion of the composite solid waste-based alkali-stimulated gelling specimen obtained in step a.1) in anhydrous ethanol to stop the hydration reaction inside the composite solid waste-based alkali-stimulated gelling specimen; a.3) Dry the composite solid waste-based alkali-activated gelling specimen obtained in step a.2) in a vacuum oven at 60°C for 24 hours; a.4) polishing the surface of the composite solid waste-based alkali-activated gelling specimen obtained in step a.3) to obtain a smooth surface; a.5) Conductively treat the composite solid waste-based alkali-activated gelled specimen obtained in step a.4), perform SEM-EDS testing, and obtain a BSE image; a.6) Analyze the BSE image obtained in step a.5) using ImageJ software to obtain SEM-EDS results; The specific steps of analyzing the BSE image obtained in step a.5) using ImageJ software are as follows: 1) Image segmentation: Split the original image into multiple parts in order to process or analyze specific areas of the image more finely; 2) Gel image processing: Perform necessary processing on the gel image to obtain a clear gel picture; 3) Image combination and pasting: Combine the segmented image parts as needed and paste them with the processed gel image; 4) Composite mapping: Finally, these images are composited to generate a complete image containing all necessary information; Using the grayscale and scale functions of ImageJ software, four different points were selected for each sample for image analysis, and the percentage of each phase was calculated, and the average value of each phase was used as the final result; The specific implementation method of the nanoindentation test in step 3) is: b.1) Determine the non-lateral compressive strength of the composite solid waste-based alkali-induced gelled specimens after step 2) using a MATEST 3000kN compression testing machine at a loading rate of 0.6MPa / s. Obtain the non-lateral compressive strength results and the non-lateral compressive strength results of a portion of the composite solid waste-based alkali-induced gelled specimens after failure. b.2) immersing a portion of the composite solid waste-based alkali-stimulated gelling specimen obtained in step b.1) in anhydrous ethanol to stop the hydration reaction inside the composite solid waste-based alkali-stimulated gelling specimen; b.3) Dry the composite solid waste-based alkali-activated gelling specimen obtained in step b.2) in a vacuum oven at 60°C for 24 hours; b.4) polishing the composite solid waste-based alkali-activated gelling specimen obtained in step b.3) to obtain a smooth surface; b.5) Preparing an indentation grid having an M×N matrix structure, wherein the gaps between the indentations are no greater than 5 μm; b.6) Using a ladder-mode loading and unloading procedure, press the indentation grid obtained in step b.5) onto the smooth surface obtained in step b.4) for nanoindentation testing, ultimately obtaining nanoindentation test results; In step b.6), the loading rate is no more than 400 μN / s, the maximum loading amount is no more than 2000 μN / s, and after reaching the maximum loading amount, the loading time is 2 s; the unloading process is no more than 5 s; The specific implementation method of performing FTIR detection in step 3) is: c.1) taking a portion of the powder from the composite solid waste alkali-activated gelling specimen after step 2) as a specimen sample; c.2) preparing MS residue, HCl residue, and water residue based on the specimen sample obtained in step c.1); c.3) Acquire spectra of HCl and MS selective solubilization hydration products using OMNIC software; The MS residue in step c.2) is prepared by dissolving the test piece sample obtained in step c.1) in an MS solution, stirring the mixture using a magnetic stirrer, vacuum filtering the suspension, washing the insoluble residue with methanol, drying, and weighing to obtain the MS residue; the MS solution is a solution formed by dissolving salicylic acid in methanol at a solid-liquid ratio of 6.25 g:100 ml; The HCl residue in step c.2) is prepared by adding the test piece sample obtained in step c.1) into a 1.76 wt% HCl solution for extraction to obtain an insoluble residue, which is the HCl residue; The water residue in step c.2) is prepared by dissolving the paste of the test sample obtained in step c.1) in distilled water, dissolving in an ultrasonic bath at 25°C for 5 hours, centrifuging at 10,000 rpm for not less than 10 minutes, and retaining the residue, which is the water residue; The specific implementation method of c.3) is: using OMNIC software to subtract the spectrum of HCl residue and MS residue from the spectrum of the original gelled sample powder to obtain the spectrum of the HCl and MS selective dissolution hydration products; The specific implementation method of performing FTIR detection in step 3) further includes, after step c.3), the following steps: c.4) further analyzing and confirming the spectra of the HCl and MS selectively dissolved hydration products obtained in step c.3) using a TGA method to obtain FTIR detection results; in the TGA method, the heating rate of the hydration product is 10°C / min, and the heating temperature is increased from 20°C to 1200°C; The specific implementation method of performing MIP detection in step 3) is: d.1) taking the composite solid waste-based alkali-induced gelling specimen after undergoing no lateral compressive strength damage in step a.1); d.2) crushing the composite solid waste-based alkali-activated gelling specimen obtained in step d.1) into specimen particles of 5 nm to 10 nm; d.3) Drying the sample particles obtained in step d.2) in a freeze dryer at -90°C under vacuum conditions for 36 hours; d.4) The dried product from step d.3) was placed in an oven at 25°C to obtain a MIP test sample; d.5) performing a MIP test on the MIP test sample obtained in step d.4) to obtain a MIP test result; The SEM-EDS analysis results were compared with the nanoindentation test results to analyze the relationship between microstructure and mechanical properties; the FTIR spectral analysis results were compared with the thermogravimetric analysis results to verify the composition and reaction degree of the hydration products; the MIP mercury porosimeter analysis results were correlated with the results of other technical means to fully understand the influence of pore structure on the performance of cementitious materials; through cross-analysis and comparison of the above steps, the characteristics of the hydration products of composite solid waste-based alkali-activated cementitious materials were evaluated and analyzed.

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