Method for evaluating volcanic ash activity of Bayer process red mud

Abstract

The invention relates to the technical field of cement and concrete, in particular to a Bayer process red mud volcanic ash activity evaluation method which comprises the following steps: calcining Bayer process red mud to be detected to obtain calcined Bayer process red mud; the polymerization degree of the calcined Bayer process red mud is measured by adopting Fourier transform infrared spectroscopy, the polymerization degree of the Bayer process red mud is in negative correlation with the activity, the pozzolanic activity of the Bayer process red mud is evaluated by measuring the polymerization degree, and the pozzolanic activity of the Bayer process red mud is evaluated by adopting the Fourier transform infrared spectroscopy to measure the polymerization degree of the Bayer process red mud. The polymerization degree of the red mud is reduced along with the increase of the volcanic ash activity and is in negative correlation, and the compressive strength and the dissolution solubility of Si + Al are in positive correlation with the volcanic ash activity of the red mud.

Description

Technical Field

[0001] This invention relates to the field of cement and concrete technology, and more specifically to a method for evaluating the activity of Bayer process red mud pozzolanic. Background Technology

[0002] Cement concrete is a pillar of modern social development and a key building material for major national strategic projects. Global cement production has continued to grow, from 3.603 billion tons in 2010 to 4.163 billion tons in 2022. Looking at the cement production data of major cement-producing countries in 2022, China ranked first with 2.13 billion tons, accounting for 51.2% of global production. The cement industry accounts for 8% of global CO2 emissions, with approximately 40% of these emissions originating from the calcification process of limestone, a raw material for cement. These large amounts of CO2 emissions place enormous pressure on the ecological environment. Therefore, to adapt to the demands of green production, the cement industry needs continuous upgrading and innovation. A widely accepted method is to introduce industrial waste into concrete production to partially replace cement. This not only reduces the amount of cement used, but some industrial waste can also act as auxiliary cementitious materials, reacting with the hydration products of cement to form secondary hydration products, improving the density of cement paste, thereby improving certain properties of concrete and extending its service life.

[0003] Meanwhile, Bayer red mud (RM) is a polluting waste produced during the alumina refining process from bauxite. For every ton of alumina produced, 0.8 to 2 tons of red mud are generated. Statistics show that in 2022, China's annual alumina production was 79.76 million tons, accounting for 57.4% of global production. However, the annual emissions of RM exceed 100 million tons, with accumulated storage exceeding 1 billion tons, yet its comprehensive utilization rate is less than 10%. The speed of RM treatment and comprehensive utilization is far slower than the development speed of the alumina industry. RM has high alkalinity, complex composition, and contains radioactive heavy metal ions; large-scale stockpiling causes enormous environmental damage. To solve the RM pollution problem, the comprehensive utilization of RM has become a major focus of research.

[0004] The main chemical components of RM include elements such as CaO, SiO2, Al2O3, and Fe2O3, as well as some amorphous aluminosilicates. By mixing RM with appropriate amounts of limestone, sandstone, etc., various types of cement can be prepared during the sintering process. Furthermore, RM possesses certain cementitious activity and can be used as an active admixture in cement. Using RM as a concrete admixture can produce concrete materials with the strength and durability required for engineering applications. This approach not only effectively alleviates the various pollution problems caused by RM stockpiling and achieves efficient utilization of RM resources, but also significantly reduces energy consumption in cement and concrete production processes, demonstrating clear environmental and economic advantages.

[0005] However, due to its high alkalinity and low pozzolanic activity, red mud (RM) currently has a relatively low application rate in cement materials. The pozzolanic activity of RM is a hot research topic. The composition of RM is influenced by bauxite ore and production processes, resulting in significant differences in RM composition across different regions. Furthermore, there is currently no mature method to accurately and scientifically evaluate the activity of different types and properties of RM materials. How to more effectively and scientifically utilize RM with varying activities is of great significance in the field of building materials. Therefore, designing a scientifically sound method for evaluating RM activity will help achieve efficient utilization of red mud resources in cement-based materials and facilitate targeted research on activity activation technologies and hydration mechanisms.

[0006] Over the past few decades, scholars have proposed various methods for assessing the pozzolanic activity of red mud (RM). Some methods are based on the interaction principle between calcium hydroxide and red mud. In addition, some simple, rapid, and quantitative methods have been used to assess the pozzolanic activity of RM, including analyzing the chemical composition of RM, the crystallinity of the minerals, and its strength and electrochemical properties. While strength assessment methods can comprehensively and intuitively reflect the overall pozzolanic activity of RM, they typically require long testing cycles. Dissolution assessment methods require the separation of active and inert components under appropriate conditions. However, the implementation of this method is affected by several factors, such as the type and concentration of alkaline solution, dissolution time, and temperature, which may lead to significant errors in the experimental results. In recent years, some scholars have proposed new methods for testing the pozzolanic activity of RM. Martín et al. measured the pozzolanic activity of red mud using a modified Chapelle test method and compared it with the compressive strength of hardened cement paste samples, obtaining better results. Basto et al. used the conductivity method to study the pozzolanic activity of red mud and found a good correlation between mortar compressive strength testing and conductivity testing. Hasani et al. proposed a novel method using molecular dynamics simulations to simply compare the activity of volcanic ash. They simulated the adsorption process of water molecules on the surface of volcanic ash minerals and used the water / surface interaction energy as a standard for studying mineral activity. It is worth noting that the factors influencing the activity of RM volcanic ash are very complex; therefore, finding a rapid and effective method to determine the volcanic ash activity of RM is crucial.

[0007] In existing studies, MAS NMR is generally used to characterize the coordination structure of Si and Al in materials. However, the presence of iron components in RM gives it a certain degree of magnetism, so this method cannot be used to analyze the pozzolanic activity of RM. Currently, there is a lack of a simple, rapid, and direct effective method to evaluate the pozzolanic activity of red mud, making it difficult to quickly, intuitively, and accurately understand the performance of Bayer process red mud. Summary of the Invention

[0008] To address the shortcomings of the existing technology, the purpose of this invention is to provide a method for evaluating the pozzolanic activity of Bayer process red mud. This invention characterizes the degree of polymerization of red mud (RM) using FTIR and analyzes the changes in the chemical structure of RM at different temperatures. Furthermore, physical, chemical, and microscopic analytical methods are also used to evaluate the pozzolanic activity of RM calcined at different temperatures. Correlation between the degree of polymerization and other pozzolanic activity evaluation methods is established, providing a theoretical reference for the application of RM in the cement and concrete fields.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] A method for evaluating the activity of Bayer process red mud volcanic ash includes the following steps:

[0011] The Bayer red mud to be tested was calcined to obtain calcined Bayer red mud.

[0012] The degree of polymerization of Bayer red mud after calcination was determined by Fourier transform infrared spectroscopy to evaluate the pozzolanic activity of Bayer red mud. The results showed that the degree of polymerization of Bayer red mud decreased with the increase of pozzolanic activity, showing a negative correlation.

[0013] In a preferred embodiment of the present invention, the degree of polymerization refers to the higher the degree of polymerization of the [Si(Al)O4] tetrahedron in the pozzolanic activity of Bayer red mud.

[0014] In a preferred embodiment of the present invention, the method for calculating the degree of polymerization of the [Si(Al)O4] tetrahedron is as follows:

[0015] Where n = 1, 2, 3, 4 represents the number of coordination bridge oxygens around Si, and Q n RBO represents the degree of polymerization, corresponding to the relative area of ​​the resonance peak.

[0016] In a preferred embodiment of the present invention, the relative areas of the corresponding resonance peaks are determined by using Origin software to measure the area of ​​the FTIR spectrum from 800 to 1200 cm⁻¹. -1 Si(Al)Q within the range n The peak areas were separated and fitted to obtain

[0017] In a preferred embodiment of the present invention, the FTIR spectrum of Bayer process red mud is in the range of 800–1200 cm⁻¹. -1 Molecular bonds within the range were measured using Fourier transform infrared spectroscopy, with wavenumbers ranging from 400 to 4000 cm⁻¹. -1 .

[0018] In a preferred embodiment of the present invention, the main chemical components of the Bayer red mud are 26.39 wt.% CaO, 25.71 wt.% Fe2O3, 17.77 wt.% Al2O3, 15.06 wt.% SiO2, 6.83 wt.% TiO2 and 5.91 wt.% Na2O by mass fraction.

[0019] In a preferred embodiment of the present invention, the calcination temperature is 500–900°C.

[0020] In a preferred embodiment of the present invention, the heat preservation time is 2 hours.

[0021] In a preferred embodiment of the present invention, the heating rate is 5°C / min.

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

[0023] 1. The RM Fourier transform infrared spectroscopy (FTIR) method can analyze the Si-O-Si(Al) molecular bonds in materials. Therefore, this invention characterizes the degree of polymerization of RM by FTIR and analyzes the changes in the chemical structure of RM at different temperatures. In addition, physical testing, chemical testing and microscopic analysis methods are also used to evaluate the pozzolanic activity of RM calcined at different temperatures. The correlation between the degree of polymerization and other pozzolanic activity evaluation methods is established, providing a theoretical reference for the application of RM in the cement and concrete fields.

[0024] 2. Under certain temperature conditions, the crystalline phases of red mud undergo changes, particularly the breaking of bridging oxygen bonds in the [Si(Al)O4] tetrahedrons, leading to the generation of more active sites and thus increasing the content of active silicon-aluminum substances. When this RM is used as an auxiliary cementitious material, it can significantly improve the hydration degree of the cementitious material, thereby generating more hydration products and increasing the compressive strength of the prepared cementitious mortar. Furthermore, this invention proposes a novel method, the degree of polymerization method, for evaluating the pozzolanic activity of red mud, and studies the compressive strength, electrical conductivity, and Ca2+ activity. 2+ Al 3+ Si 4+ The relationship between leaching concentration, crystallinity, and degree of polymerization and the activity of red mud volcanic ash was investigated. The main findings are as follows:

[0025] The pozzolanic activity of red mud can be quantitatively calculated using indicators such as the degree of polymerization, compressive strength, and (Si+Al) dissolution concentration. A clear linear relationship exists between the degree of polymerization method and the pozzolanic activity index obtained through the compressive strength index and (Si+Al) dissolution concentration. The degree of polymerization method shows that the degree of polymerization of red mud decreases with increasing pozzolanic activity, exhibiting a negative correlation, while compressive strength and (Si+Al) dissolution concentration are positively correlated with the pozzolanic activity of red mud.

[0026] Compared to the 28-day compressive strength test and the 7-day (Si+Al) dissolution concentration test, the degree of polymerization method has advantages such as shorter testing cycle, convenient operation, and lower cost. It is closely related to the structure of silicon and aluminum in red mud and is less affected by the testing environment. Although this invention only relates to one type of industrial solid waste material, namely red mud, the results show that the degree of polymerization method may be applicable to evaluating the blending activity of many other industrial solid waste materials.

[0027] 3. This invention uses the degree of polymerization method to evaluate the pozzolanic activity of Bayer process red mud. The results show that this method is effective and can serve as a rapid, simple, and direct method for evaluating the pozzolanic activity of Bayer process red mud. The research in this application is expected to promote the green development of the cement industry, better utilize waste residue resources, reduce CO2 emissions, and improve the performance of building materials. Attached Figure Description

[0028] Figure 1 This is a flowchart of the experiment of the present invention;

[0029] Figure 2 This is a TG-DTG curve of the Bayer process red mud of this invention;

[0030] Figure 3 The graph shows the compressive strength results of red mud and cementitious mortar at different temperatures over 28 days according to the present invention.

[0031] Figure 4 This is a graph showing the change in conductivity of the RM in a saturated Ca(OH)2 solution over time.

[0032] Figure 5 This is a graph showing the relationship between the decrease in conductivity and the intensity activity index of this invention.

[0033] Figure 6 The present invention relates to Ca in red mud solutions at different calcination temperatures. 2+ Concentration and pH value changes graph;

[0034] Figure 7 This is a graph showing the relationship between the RM pozzolanic activity index of the present invention and its Si leaching concentration, Al leaching concentration, and (Si+Al) leaching concentration.

[0035] Figure 8 The XRD patterns of RM activated at different temperatures according to the present invention are shown below.

[0036] Figure 9 These are microscopic morphology images of red mud at different temperatures according to the present invention.

[0037] Figure 10 These are FT-IR images of the RMs after thermal activation treatment at different temperatures according to the present invention;

[0038] Figure 11 The FTIR spectra of RM at different temperatures in the range of 800-1200 cm⁻¹ are obtained according to the present invention. -1 Peak fitting results within the range;

[0039] Figure 12 This is a graph showing the relationship between the degree of RM polymerization (RBO) and its pozzolanic activity index at different temperatures according to the present invention. Detailed Implementation

[0040] The following detailed description, in conjunction with embodiments of the present invention and accompanying drawings, provides a clear and complete illustration of the technical solutions in these embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0041] It should be noted that all technical terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0042] Example 1

[0043] A method for evaluating the volcanic ash activity of Bayer process red mud includes the following steps:

[0044] 200g of Bayer red mud (RM) was weighed and placed in a muffle furnace. The muffle furnace was heated to 500℃ at a rate of 5℃ / min and held at that temperature for 2h. The red mud was then cooled to room temperature and ground to obtain red mud with pozzolanic activity, which was named RM500. The degree of polymerization of [Si(Al)O4] tetrahedra in RM500 was then determined by Fourier transform infrared spectroscopy to evaluate the pozzolanic activity of RM500.

[0045] Example 2

[0046] A method for evaluating the volcanic ash activity of Bayer process red mud includes the following steps:

[0047] 200g of Bayer red mud (RM) was weighed and placed in a muffle furnace. The muffle furnace was heated to 600℃ at a rate of 5℃ / min and held for 2h. The red mud was then cooled to room temperature and ground to obtain red mud with pozzolanic activity, named RM600. The degree of polymerization of [Si(Al)O4] tetrahedra in RM600 was then determined by Fourier transform infrared spectroscopy to evaluate the pozzolanic activity of RM600.

[0048] Example 3

[0049] A method for evaluating the volcanic ash activity of Bayer process red mud includes the following steps:

[0050] 200g of Bayer red mud (RM) was weighed and placed in a muffle furnace. The muffle furnace was heated to 700℃ at a rate of 5℃ / min and held for 2h. The red mud was then cooled to room temperature and ground to obtain red mud with pozzolanic activity, named RM700. The degree of polymerization of [Si(Al)O4] tetrahedra in RM700 was then determined by Fourier transform infrared spectroscopy to evaluate the pozzolanic activity of RM700.

[0051] Example 4

[0052] A method for evaluating the volcanic ash activity of Bayer process red mud includes the following steps:

[0053] 200g of Bayer red mud (RM) was weighed and placed in a muffle furnace. The muffle furnace was heated to 800℃ at a rate of 5℃ / min and held for 2h. The red mud was then cooled to room temperature and ground to obtain red mud with pozzolanic activity, named RM800. The degree of polymerization of [Si(Al)O4] tetrahedra in RM800 was then determined by Fourier transform infrared spectroscopy to evaluate the pozzolanic activity of RM800.

[0054] Example 5

[0055] A method for evaluating the volcanic ash activity of Bayer process red mud includes the following steps:

[0056] 200g of Bayer red mud (RM) was weighed and placed in a muffle furnace. The muffle furnace was heated to 900℃ at a rate of 5℃ / min and held at that temperature for 2h. The red mud was then cooled to room temperature and ground to obtain red mud with pozzolanic activity, which was named RM900. The degree of polymerization of [Si(Al)O4] tetrahedra in RM900 was then determined by Fourier transform infrared spectroscopy to evaluate the pozzolanic activity of RM900.

[0057] Comparative Example 1

[0058] Evaluation of the pozzolanic activity of Bayer process red mud using the intensity activity index method

[0059] The pozzolanic activity index of RM was calculated according to the Chinese standard GB / T 2847-2005 (Pozzolanic Materials for Cement Production). A material is considered to have pozzolanic cementitious activity when this ratio is greater than 0.65 (as specified in GB / T 12,597-2005) or 0.75 (as specified in ASTM C618). In the experiment, the masses of RM, cement, and standard sand with different pozzolanic activities were 135g, 315g, and 1350g, respectively. These materials were stirred and mixed at a water-cement ratio of 0.5. The resulting slurry was poured into 40mm×40mm×40mm molds, and the prepared mortar samples were placed in a curing chamber. After curing for 24 hours, the molds were removed, and curing continued for up to 28 days. The curing temperature was 20±1℃, and the curing humidity was 95%. Unconfined compressive strength (UCS) tests were conducted using a YHS-229WJ-50kN microcomputer-controlled electronic universal testing machine manufactured by Shanghai Yihuan Instrument Technology Co., Ltd., at a testing speed of 1 mm·min. -1 .

[0060] Comparative Example 2

[0061] Electrical conductivity method for evaluating the pozzolanic activity of Bayer process red mud

[0062] First, the prepared saturated Ca(OH)₂ solution was placed in a constant temperature chamber at 40±1℃. Then, 200mL of the saturated Ca(OH)₂ solution was measured. The initial conductivity was measured using a DDSJ-318T conductivity meter. Then, 5g of RM500, RM600, RM700, RM800, and RM900 powder materials were weighed and added to the saturated Ca(OH)₂ solution, and the change in conductivity of the mixed solution was measured. By measuring the change in conductivity of the cement-pozzolant slurry, the ability of the pozzolant material to consume Ca(OH)₂ was reflected, thereby indirectly measuring the content of active SiO₂ and Al₂O₃ in the material, and thus determining its pozzolant activity.

[0063] Comparative Example 3

[0064] Evaluation of the pozzolanic activity of Bayer process red mud using lime absorption method

[0065] Bayer process red mud can react with Ca(OH)2 formed during cement hydration to form hydrated calcium silicate or hydrated calcium aluminate, thereby reducing the concentration of Ca(OH)2 in the liquid phase. The remaining Ca in the solution was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, model ICPOES730, Agilent Technologies) and a PHSJ-6L laboratory pH meter. 2+ The pozzolanic activity of Bayer red mud was quantitatively evaluated using the concentration of OH-.

[0066] Comparative Example 4

[0067] Evaluation of pozzolanic activity of Bayer process red mud using alkaline leaching (Si+Al) method

[0068] Take 1g of RM500, RM600, RM700, RM800, and RM900 and place them in a 100mL plastic bottle. Add 1mol / L NaOH solution, seal the bottle, and incubate it in a 20℃ curing room for 7 days. Then filter the solution, seal the filtrate in a plastic bottle, and use ICP-OES to test the Si content in the filtrate. 4+ And Al 3+ content.

[0069] Comparative Example 5

[0070] Phase and microstructure evaluation of the volcanic ash activity of Bayer process red mud

[0071] The phase composition of RM500, RM600, RM700, RM800 and RM900 after activation at different temperatures was determined using an Empyrean X-ray diffractometer (200mA, 40kV, Cu target, scanning speed 2° / min, scanning range 5°~90°). In addition, to further calculate its crystallinity, the obtained XRD patterns were peak-fitted using Jade software (6.5), and the amorphous phase content of RM was determined by peak area fitting and calculation. On the other hand, the microstructure of RM at different temperatures was analyzed by a Gemini SEM 300 scanning electron microscope (SEM).

[0072] Materials and methods

[0073] The Bayer process red mud used in this invention was sampled on-site at Xiaoyi Aluminum Plant in Shanxi Province, China. Cement (42.5) was purchased from Xishan Huatong Cement Co., Ltd. in Shanxi Province, China. Standard sand was purchased from Yuanheng Water Purification Materials Factory in Gongyi City, Henan Province, China. High-purity sodium hydroxide was supplied by Gongyi Longze Purification Materials Co., Ltd. in Zhengzhou City, Henan Province, China. Deionized water was prepared in the laboratory. The main chemical components of RM were analyzed using a PANalyticalAxios X-ray fluorescence spectrometer (XRF). The main chemical components of RM are shown in Table 1. The main chemical components of RM include CaO, Fe2O3, Al2O3, SiO2, TiO2, and Na2O, with the total amount of the six components accounting for more than 97%. RM has a high content of calcium, silicon, and aluminum, and has the potential for gelling activity in terms of composition. Differential thermal analysis of virgin RM was performed using a Swiss Mettler thermogravimetric analyzer. The test conditions were: alumina crucible, air atmosphere, heating to 1000℃, heating rate 5℃ / min. The flowchart of the experiment is shown in [link to flowchart]. Figure 1 .

[0074] Table 1. Main chemical components of RM

[0075] Oxides SiO2 Al2O3 <![CDATA[Fe2O3]]> CaO MgO <![CDATA[Na2O]]> <![CDATA[SO3]]> <![CDATA[TiO2]]> RM 15.06 17.77 25.71 26.39 0.40 5.91 0.81 6.83

[0076] Physical Testing - Strength Activity Index Method: The pozzolanic activity index of RM was calculated according to the Chinese standard GB / T 2847-2005 (Pozzolanic Materials for Cement Production). A ratio greater than 0.65 (as specified in GB / T 12,597-2005) or 0.75 (as specified in ASTM C618) indicates that the material possesses pozzolanic cementitious activity. In the experiment, the masses of RM, cement, and standard sand with different pozzolanic activities were 135g, 315g, and 1350g, respectively. These materials were stirred and mixed at a water-cement ratio of 0.5, and the resulting slurry was poured into 40mm×40mm×40mm molds. The prepared mortar samples were placed in a curing chamber, cured for 24 hours, then demolded, and cured for another 28 days. The curing temperature was 20±1℃, and the curing humidity was 95%. Unconfined compressive strength (UCS) tests were conducted using a YHS-229WJ-50kN microcomputer-controlled electronic universal testing machine manufactured by Shanghai Yihuan Instrument Technology Co., Ltd., at a testing speed of 1 mm·min. -1 No lateral constraints were imposed during the experiment. The final UCS value was the average of six parallel samples. The pozzolanic activity index (Kα) of the RM is expressed by the following formula:

[0077]

[0078] Where R is the 28-day compressive strength of the mixed mortar containing a certain amount of RM, in MPa; and R0 is the compressive strength of the blank cement mortar at the same age, in MPa.

[0079] Physical Testing - Conductivity Method: The conductivity method reflects the ability of volcanic ash materials to consume Ca(OH)2 by measuring the change in conductivity of cement-volcanic ash slurry, thereby indirectly measuring the content of active SiO2 and Al2O3 in the material and determining its activity. As ions in the slurry continuously react, its conductivity gradually decreases; generally, the greater the decrease in conductivity, the higher the activity of the volcanic ash. First, the prepared saturated Ca(OH)2 solution is placed in a constant temperature chamber at 40±1℃, and then 200mL of the saturated Ca(OH)2 solution is measured. The initial conductivity is measured using a DDSJ-318T conductivity meter. Then, 5g of powder material is weighed and added to the saturated Ca(OH)2 solution, and the change in conductivity of the mixed solution is measured. Continuous stirring is required throughout the process to maintain the homogeneity of the solution. The conductivity of the mixed solution at 120s is recorded as the final compensated conductivity. The difference (Δσ) between this and the initial conductivity can be used to classify the reactivity level of the volcanic ash material. When Δσ > 1.2 mS / cm, it means that the material has good pozzolanic activity; when Δσ < 0.4 mS / cm, it means that the material does not have pozzolanic activity; and when Δσ is in between, it means that the material has average pozzolanic activity.

[0080] Chemical testing – the lime absorption method: The principle of the lime absorption method is that pozzolanic materials can react with Ca(OH)₂ formed during cement hydration to produce hydrated calcium silicate or hydrated calcium aluminate, thereby reducing the concentration of Ca(OH)₂ in the liquid phase. The remaining Ca in the solution was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, model ICPOES730, Agilent Technologies) and a PHSJ-6L laboratory pH meter. 2+ and OH - The concentration of volcanic ash is used to quantitatively evaluate the activity of volcanic ash materials. The methods for determining the volcanic ash activity of materials in the Chinese standard "Volcanic Ash Blends for Cement" (GB / T2847-2005) and the European standard "Pozzolanicity test for pozzolanic cement" (EN196-5:2005) are both based on the lime absorption method.

[0081] Chemical testing – Alkali leaching of Si+Al: 1g of RM with different pozzolanic activities was placed in a 100ml plastic bottle, and 1mol / L NaOH solution was added. The bottle was then sealed and cured in a 20℃ curing room for 7 days, followed by filtration. The filtrate was stored in a sealed plastic bottle. The Si content in the filtrate was tested using ICP-OES. 4+ And Al 3+ Content. It is important to note that the more dissolved silicon and aluminum ions there are, the higher the pozzolanic activity of the RM.

[0082] Microscopic analysis – phase and morphology method: Phase composition has a significant impact on the pozzolanic activity of red mud. Numerous studies have shown that the amorphous phase is the most active phase in auxiliary cementitious materials. The phase types of red mud activated at different temperatures were determined using an Empyrean X-ray diffractometer (200mA, 40kV, Cu target, scanning speed 2° / min, scanning range 5°~90°). In addition, to further calculate its crystallinity, the obtained XRD patterns were peak-fitted using Jade software (6.5), and the amorphous phase content of the red mud was determined by peak area fitting and calculation. On the other hand, the micromorphology of the red mud at different temperatures was analyzed using a GeminiSEM 300 scanning electron microscope (SEM).

[0083] The microscopic analysis method—the degree of polymerization method—demonstrates that the volcanic ash reaction is essentially a repetitive process of self-polymerization of [Si(Al)O4] tetrahedra—isolated state—polymerized state. The depolymerization of polymerized [Si(Al)O4] tetrahedra (breaking of Si-O-Si(Al) bonds) increases activity. Therefore, the activity can be reflected by evaluating the degree of polymerization of tetrahedra in the material. The higher the degree of polymerization of the network, the more stable the network structure and the lower the activity. Based on the number of coordinated bridging oxygen atoms around Si atoms, it can be divided into SiQ... 0 SiQ 1 SiQ 2 SiQ 3 and SiQ 4 Structure. Among them, SiQ n In [SiO4], 'n' represents the number of coordinated bridging oxygen atoms around the Si atom. The breaking of the Si-O-Si bond in [SiO4] causes a change in the coordination structure of Si atoms near the bridging oxygen atoms, i.e., from SiQ... n To SiQ n-1 Conversely, if Si-O and Si-O polymerize to form Si-O-Si bonds, the coordination structure of Si atoms around the bridging oxygen will change from SQ. n Transformation into SiQ n+1 Furthermore, during the polymerization process, Al atoms in the system can enter [SiO4] to replace Si atoms and form [AlO4]. - However, this does not affect the change in the number of bridging oxygen bonds. Therefore, the change in the number of bridging oxygen bonds can be used to reflect the relative degree of polymerization or depolymerization of [Si(Al)O4] tetrahedra in the system. The molecular bonds of mud samples at different temperatures (800-1200 cm⁻¹) were measured using a Fourier transform infrared spectrometer (Thermo Scientific Nicolet iS20, USA), with wavenumbers ranging from 400 to 4000 cm⁻¹. -1 In the FTIR spectrum, SiQ 0 SiQ 1 SiQ 2 SiQ 3and SiQ 4 The characteristic peak positions are approximately 850 cm⁻¹. -1 950cm -1 1000cm -1 1050cm -1 and 1100cm -1 Origin software (2022) was used to analyze the 800-1200 cm⁻¹ region of the FTIR spectrum. -1 Si(Al)Q within the range n The peak areas were separated and fitted. The degree of polymerization of [SiO4] was evaluated using the concept of relative bridging oxygen bonds (RBO), and the calculation method is as follows:

[0084]

[0085] Where n (n = 1, 2, 3, 4) represents the number of coordination bridge oxygens around Si; Q n This represents the relative area of ​​the corresponding resonance peak.

[0086] Results Analysis

[0087] 1. Differential thermal analysis of Bayer red mud

[0088] The TG-DTG curve of red mud is as follows Figure 2 As shown. The total mass loss of Bayer red mud between room temperature and 1000℃ is 15.73%, and the main weight loss process can be divided into four stages: (I) room temperature to 220℃, (II) 220~500℃, (III) 500~620℃, (IV) 620~710℃; the weight loss before 220℃ is mainly caused by the evaporation of free water and adsorbed water in the red mud, with a weight loss of 1.73%. When the temperature is higher than 220℃, gibbsite begins to decompose (Equation (3)), corresponding to the weight loss in stage II of the TG curve, 220~500℃. The weight loss is the largest in this stage, reaching 7.72%, and the corresponding endothermic peak of the DTG curve is also strong. Kato stone undergoes a dehydroxylation reaction at 500~620℃ to form hydrogrossular, corresponding to the weight loss in stage III of the TG curve, with the smallest weight loss in this stage, 1.83%. Between 620 and 710℃, the main reactions are the dehydroxylation of dickite and hydrogrossular, as well as the decomposition of carbonates in nepheline, corresponding to the weight loss in stage IV of the TG curve, with a weight loss of 3.83%. A significant sudden mass loss occurs at 682.2℃, indicating that the material structure is unstable and the reactivity is high at this point. After 900℃, the red mud experiences virtually no further mass loss, and no endothermic or exothermic peaks appear, indicating that the substances within the red mud are in a relatively stable state after 900℃. In summary, between 0 and 500℃, red mud mainly undergoes decomposition of water and some organic compounds, resulting in significant mass loss; between 500 and 900℃, structural changes occur. Therefore, the calcination of red mud between 500 and 900℃ is the key research area for this experiment.

[0089] 2Al(OH)3→Al2O3+3H2O (3)

[0090] 2. Physical testing method

[0091] 2.1 Intensity Index Method

[0092] The activity assessment of red mud volcanic ash is typically based on the compressive strength of the mortar. This mortar is a cementitious material prepared by mixing red mud and cement. Its compressive strength reflects the secondary reactions of red mud during the hardening process of the cementitious material, as well as the filling and water absorption effects of the red mud particles. Therefore, the compressive strength test can comprehensively reflect the role of red mud as an auxiliary cementitious material in the entire system. Figure 3 This study demonstrates the compressive strength of red mud and cementitious mortar at different temperatures over 28 days. Figure 3 It can be observed that the activity of red mud is improved to varying degrees after calcination. This is because the calcination activation process increases the content and availability of active silica and alumina, thereby promoting the hydration reaction of the cementitious material and improving the compressive strength. The compressive strength of both red mud and cementitious mortar first increases and then decreases with increasing temperature. Among them, RM700 has the highest compressive strength, reaching 31.83 MPa. Table 2 shows the pozzolanic activity index of red mud at different temperatures (the compressive strength of standard cement mortar at 28 days is 42.5 MPa). As can be seen from Table 2, the trend of the pozzolanic activity index of red mud at different temperatures is consistent with the trend of its compressive strength, indicating that RM700 has the highest pozzolanic activity and contains a high amount of active silica and alumina. However, the compressive strength of RM900 decreases significantly, and the compressive strength at 28 days is lower than that of RM500. This indicates that the compressive strength and pozzolanic activity index of RM900 are lower, suggesting that its active silica and alumina content is the lowest, mainly playing a filling role rather than participating in the hydration reaction of the cementitious mortar. In summary, the volcanic ash activity of red mud varies significantly under different temperatures, with RM700 exhibiting the highest volcanic ash activity.

[0093] Table 2 Volcanic ash activity index of red mud at different temperatures

[0094] Samples RawRM RM500 RM600 RM700 RM800 RM900 Kα 40.21 43.84 68.09 74.89 53.15 41.86

[0095] 2.2 Conductivity Method

[0096] The initial conductivity is determined by the amount of Ca dissolved in a saturated Ca(OH)₂ solution. 2+ and OH - The amount produced gradually decreases as the volcanic ash reaction continuously consumes Ca(OH)₂. Therefore, the change in the conductivity of RM in a saturated Ca(OH)₂ solution over time (e.g., ...) Figure 4The activity of the volcanic ash was characterized by the following (as shown in the figure). As expected, the measured conductivity decreased rapidly within the test time; the higher the activity of the finely ground volcanic ash, the greater the decrease in conductivity over a given time. Figure 4 It can be seen that the conductivity of the mixed solution decreased rapidly in the first 5 minutes of testing, then continued to decrease slowly for the remaining testing time, and continued to decrease at a constant rate after 30 minutes. The sensitivity of conductivity to temperature may be the reason for the different initial conductivity. The order of pozzolanic activity determined by the decrease in conductivity in the first 30 minutes is RM700, RM600, RM800, RM500, RM900, and Raw RM. The corresponding decreases in conductivity are 1.30 mS / cm, 1.27 mS / cm, 1.25 mS / cm, 0.95 mS / cm, 0.85 mS / cm, and 0.71 mS / cm, respectively. Based on the classification criteria suggested by Luxan et al., RM700, RM600, and RM800 can be classified as good pozzolanic materials (Δσ > 1.2 mS / cm). RM500, RM900 and Raw RM can be classified as general volcanic ash active materials (0.4mS / cm≤Δσ≤1.2mS / cm).

[0097] Figure 5 The relationship between the decrease in electrical conductivity and the strength activity index was further clarified. It can be seen that with increasing calcination temperature, or more specifically with increasing amorphous phase content, both the decrease in electrical conductivity and the compressive strength ratio show a trend of first increasing and then decreasing. However, the correlation coefficient between the two is only 0.78. This means that the correlation between the decrease in electrical conductivity and the compressive strength ratio is poor. This is because the adsorption effect of pozzolanic materials is the main factor leading to the decrease in electrical conductivity, while its correlation with the amorphous phase content or chemical composition of the material is relatively small.

[0098] 3. Chemical testing method

[0099] 3.1 Lime Absorption Method

[0100] During different stages of the hydration reaction, Ca(OH)₂ exists as a solid phase. It disrupts the reaction equilibrium and dissolves into the solution until RM or Ca(OH)₂ is completely consumed. As the calcination temperature increases, the remaining Ca in the solution... 2+ The concentration exhibited a specific trend, first decreasing and then increasing, a trend consistent with the pH change of the solution. When the calcination temperature reached 700℃, the Ca concentration in the RM material was observed to decrease. 2+ The absorption reached its maximum value, and the Ca in the solution... 2+ Only 778.7 mg / L remained. Meanwhile, pH test results also showed that the OH- ions in the solution... -The concentration of ions reached its lowest point at this temperature. This finding suggests that at this specific temperature, the dissolution of active SiO2 and Al2O3 in RM reaches its maximum, while the concentration of Ca in the solution decreases. 2+ It is also consumed to the greatest extent by reaction mechanisms, mainly including ion exchange, coprecipitation, and surface adsorption. This observation is consistent with the aforementioned strength, activity, and conductivity test results, further highlighting the significant influence of calcination temperature on the properties of RM pozzolanic.

[0101] 3.2 Alkali leaching (Si+Al) method

[0102] The active silica-alumina substances produced by the decomposition of solid waste under alkaline conditions are considered the main source of pozzolanic activity. Under alkaline conditions, active Si and Al dissolve and participate in the hydration reaction of cement. Therefore, the higher the content of soluble Si and Al in red mud, the greater the degree of hydration reaction that can occur, and the more hydration products are produced. The amount of Si and Al dissolved in RM in alkaline solution can be used as an indicator to evaluate its pozzolanic activity. Table 3 shows the Si, Al, and (Si+Al) dissolution concentrations of RM at different temperatures in alkaline solution.

[0103] Table 3. Dissolution concentrations of Si, Al, and (Si+Al) of RM in alkaline solution at different temperatures.

[0104]

[0105] Table 3 shows that the leaching concentrations of active Si and Al in red mud first increase and then decrease with increasing temperature. The leaching concentrations of Si and Al in the original RM were 57.2 mg / L and 221.3 mg / L, respectively; in RM700, they were 94.7 mg / L and 848.6 mg / L, respectively; and in RM900, they were 60.5 mg / L and 269.7 mg / L, respectively. These results indicate that the trend of Si and Al leaching concentrations in RM with temperature is consistent with the pozzolanic activity index. The relationship between the RM pozzolanic activity index and its Si, Al, and (Si+Al) leaching concentrations is shown in the table below. Figure 7 .from Figure 7 As can be seen, there is a significant correlation between the leaching concentrations of Si, Al, and (Si+Al) in red mud and its pozzolanic activity index, and the relationship between them shows a positive linear trend. Higher Si, Al, and (Si+Al) dissolution rates in RM correspond to higher pozzolanic activity indices. The R² values ​​for the linear fit between RM pozzolanic activity and the leaching concentrations of Si, Al, and (Si+Al) are 0.98, 0.97, and 0.98, respectively. Therefore, the leaching concentrations of Si and Al in RM can be used as a method for evaluating its pozzolanic activity.

[0106] 4. Microscopic analysis method

[0107] 4.1 Phase and Microstructure

[0108] XRD patterns of RM after activation at different temperatures are shown below. Figure 8 As shown. From Figure 8 It can be observed that the original red mud (RM) exhibits numerous clear and sharp diffraction peaks, indicating a low content of amorphous material and high crystallinity of each phase. Phase analysis of RM reveals that its main phases include grossular (42.7%), nepheline (26.5%), hematite (13.3%), calcite (7.2%), gibbsite (5.7%), and clay minerals (4.6%). Furthermore, the crystallinity of each phase is quite high, reaching 98.87%. XRD analysis shows that the phases of RM500-700 are essentially consistent, while those of RM800-900 are relatively similar. This indicates that the phase composition of red mud gradually changes during temperature increases. When the thermal activation temperature is ≥500℃, the XRD pattern shows that the diffraction peaks of gibbsite in RM gradually weaken, but no alumina crystallization peaks are observed, indicating that after calcination, gibbsite decomposes into more reactive amorphous alumina. Meanwhile, as the temperature rises, calcite gradually decomposes, its characteristic peaks gradually weaken and eventually disappear, reaching only 2.8% at 700℃. In the 500-700℃ range, hydrogrossular and nepheline in RM begin to undergo dehydroxylation reactions, and their characteristic peak intensities gradually weaken. The phases of RM800-900 mainly include hematite, calcium aluminum feldspar, and nepheline, with the latter two being newly formed phases, indicating that the original materials in RM underwent a phase transition. After 700℃, with increasing temperature, the peak intensity of the crystalline phases in red mud gradually increases, indicating a relative increase in the content of crystalline phases in red mud. Furthermore, RM sintered at different temperatures exhibits different degrees of crystallinity. RM700 has the lowest crystallinity (94.51%), corresponding to the highest amorphous phase content, while RM900 has the lowest amorphous phase content (1.82%), indicating differences in the pozzolanic activity of red mud at different temperatures.

[0109] Microstructure of red mud at different temperatures Figure 9As shown, it can be observed that with increasing temperature, the pore structure inside the red mud particles changes. The particles are tightly bonded together, forming larger aggregates, the porosity decreases, and the density increases significantly. This is because during the high-temperature sintering process, the surface of the solid particles in the red mud begins to melt, forming a liquid phase that fills the pores. At the same time, it promotes the reaction of various mineral phases, leading to the expansion and migration of grain boundaries, and the particles are constantly rearranged to achieve densification. The microstructure of the original RM sample shows a relatively uniform crystalline granular shape. When the temperature reaches 700℃, due to the low melting point of alkali metal oxides, they begin to melt and form a liquid phase, causing the particles to bond together, forming larger aggregates with a small amount of porosity. After the temperature reaches 700℃, the small grains gradually grow, distributed in the range of 1-2 μm, with smooth particle surfaces and obvious interfaces, forming a coral-like pore framework structure.

[0110] 4.2 Degree of Aggregation Method

[0111] Figure 10 FT-IR plots of RMs after thermal activation treatment at different temperatures, by Figure 10 It can be known that 3656cm -1 The absorption peak at 3443 cm⁻¹ corresponds to the stretching vibration of the free -OH group. -1 The absorption peak at 1472 cm⁻¹ corresponds to the stretching vibration of associated -OH groups. With increasing temperature, gibbsite and hydrogrossular gradually lose water, and the free -OH groups in RM gradually transform into associated -OH groups, indicating the presence of a certain amount of hydrogen bonds in their molecular structure. Additionally, the absorption peak at 1472 cm⁻¹... -1 and 1576cm -1 The absorption peaks at these locations correspond to the stretching and antisymmetric stretching vibrations of CO, respectively. The peak areas gradually decrease with increasing temperature, indicating that calcite and nepheline disappear after activation and decomposition. (800–1200 cm⁻¹) -1 It is the stretching vibration of Si-O and Al-O bonds. After activation, the bottom of the red mud absorption zone narrows and the opening widens, and this occurs at 1200–1600 cm⁻¹. -1 The peak of the band shifts to the right, especially at 700℃, where this band changes significantly. This indicates that the Si-O and Al-O bonds in the red mud are destroyed after high-temperature activation, reducing the degree of polymerization of aluminosilicates and making the polymerization reaction more favorable.

[0112] On the other hand, the FTIR spectra of RM at different temperatures range from 800 to 1200 cm⁻¹. -1 The results of peak fitting within the range are shown below. Figure 11 The relevant peak parameters are shown in Table 4. As can be seen from Table 4, the degree of polymerization of RM first decreases and then increases with increasing temperature. This indicates that the highly polymerized [Si(Al)O4] tetrahedrons undergo depolymerization under the influence of temperature, that is, the Si-O-Si(Al) bonds are broken, which increases the content of active silicon-aluminum substances in RM.

[0113] Table 4. FTIR spectra of RM at different temperatures (800-1200 cm⁻¹) -1 Relevant peak parameters within the range

[0114]

[0115] The relationship between the degree of RM polymerization (RBO) and its pozzolanic activity index at different temperatures is shown in the figure. Figure 12 ,from Figure 12 It can be seen that there is a significant negative correlation between the volcanic ash activity index of red mud and its degree of polymerization (RBO); that is, the smaller the degree of polymerization (RBO) of red mud, the higher its volcanic ash activity.

[0116] In summary, under certain temperature conditions, the crystalline phases of red mud undergo changes, particularly the breaking of bridging oxygen bonds in the [Si(Al)O4] tetrahedrons, leading to the generation of more active sites and thus increasing the content of active silica-alumina. When this RM is used as an auxiliary cementitious material, it can significantly improve the hydration degree of the cementitious material, thereby generating more hydration products and increasing the compressive strength of the prepared cementitious mortar. Furthermore, this invention proposes a novel method, the degree of polymerization method, for evaluating the pozzolanic activity of red mud. This invention investigated compressive strength, electrical conductivity, and Ca2+. 2+ Al 3 + Si 4+ The relationship between leaching concentration, crystallinity, and degree of polymerization and the activity of red mud volcanic ash was investigated. The main findings are as follows:

[0117] The pozzolanic activity of red mud can be quantitatively calculated using indicators such as the degree of polymerization, compressive strength, and the leaching concentration of (Si+Al). A clear linear relationship exists between the degree of polymerization and the pozzolanic activity index obtained from the compressive strength index and the leaching concentration of (Si+Al). The degree of polymerization method shows that the degree of polymerization of red mud decreases with increasing pozzolanic activity, exhibiting a negative correlation. Compressive strength and the leaching concentration of (Si+Al) are positively correlated with the pozzolanic activity of red mud.

[0118] Compared to the 28-day compressive strength test and the 7-day (Si+Al) dissolution concentration test, the degree of polymerization method has advantages such as shorter testing cycle, convenient operation, and lower cost. It is closely related to the structure of silicon and aluminum in red mud and is less affected by the testing environment. Although this study only involves one type of industrial solid waste material, red mud, the results indicate that the degree of polymerization method may be applicable to evaluating the blending activity of many other industrial solid waste materials.

[0119] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for evaluating the activity of Bayer process red mud volcanic ash, characterized in that, Includes the following steps: The Bayer red mud to be tested was calcined to obtain calcined Bayer red mud. The degree of polymerization of calcined Bayer red mud was determined by Fourier transform infrared spectroscopy. The degree of polymerization of Bayer red mud was negatively correlated with its activity. The pozzolanic activity of Bayer red mud was evaluated by measuring the degree of polymerization.

2. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 1, characterized in that, The degree of polymerization refers to the degree of polymerization of [Si(Al)O4] tetrahedra in Bayer process red mud.

3. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 2, characterized in that, The method for calculating the degree of polymerization of the [Si(Al)O4] tetrahedron is as follows: Where n = 1, 2, 3, 4 represents the number of coordination bridge oxygens around Si, and Q n RBO represents the degree of polymerization, corresponding to the relative area of ​​the resonance peak.

4. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 3, characterized in that, The relative areas of the corresponding resonance peaks were determined using Origin software from the 800–1200 cm⁻¹ region of the FTIR spectrum. -1 The peak areas of the samples within the range are obtained by separation and fitting.

5. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 4, characterized in that, The FTIR spectrum of Bayer red mud is in the range of 800–1200 cm⁻¹. -1 Molecular bonds within the range were measured using Fourier transform infrared spectroscopy, with wavenumbers ranging from 400 to 4000 cm⁻¹. -1 .

6. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 1, characterized in that, The main chemical components of the Bayer process red mud are 26.39 wt.% CaO, 25.71 wt.% Fe2O3, 17.77 wt.% Al2O3, 15.06 wt.% SiO2, 6.83 wt.% TiO2 and 5.91 wt.% Na2O.

7. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 1, characterized in that, The calcination temperature is 500–900℃.

8. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 1, characterized in that, The heat preservation time is 2 hours.

9. The method for evaluating the activity of Bayer process red mud volcanic ash according to claim 1, characterized in that, The heating rate is 5°C / min.

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