Geopolymer slurry suitable for shield synchronous grouting and preparation method thereof

Abstract

The invention relates to a grouting material, in particular to geopolymer slurry suitable for shield synchronous grouting and a preparation method of the geopolymer slurry. According to the invention, under the optimal performance ratio of RM / CMK = 1.32, SiO / NaO = 1.56, NaO% = 3.97%, and w / b = 0.52, the compressive strength in 28 days reaches 9.12 MPa; the optimal cost ratio RM / CMK is equal to 1.25, the optimal cost ratio SiO / NaO is equal to 1.27, the optimal cost ratio NaO is equal to 2.11%, and the optimal cost ratio w / b is equal to 0.45, so that the cost of each ton of material is reduced to 20.76 dollars, and meanwhile, various properties meet the engineering requirements. The geopolymer slurry provided by the invention has excellent performance and engineering adaptability, and has great popularization and application potential in shield construction.

Description

Technical Field

[0001] The invention relates to grouting materials, in particular to a geopolymer slurry suitable for shield synchronous grouting and a preparation method thereof. Background Art

[0002] With intensifying climate change and increasing environmental pollution, building a resource-saving and environmentally friendly society has become a shared goal for global sustainable development. The construction industry, a major contributor to carbon emissions, faces pressure to transition to a greener future. Traditional cement building materials, due to their high energy consumption and emissions, urgently require green alternatives. Against this backdrop, the resource utilization of industrial solid waste is considered a key path forward in the transition to green building materials. Red mud and coal gangue, typical major solid wastes, originate from alumina production and coal mining, respectively. In China, red mud production exceeds 70 million tons annually, requiring extensive land use and posing risks of alkaline pollution and heavy metal leakage. Approximately 800 million tons of coal gangue are produced annually, and improper disposal can lead to land waste and leachate pollution. Therefore, promoting their high-value utilization in engineering projects not only alleviates environmental pressures but also provides new opportunities for the development of green, low-carbon materials.

[0003] In recent years, the resource utilization potential of red mud and coal gangue has gradually attracted attention. Red mud is rich in active mineral components such as SiO2, Al2O3, and Fe2O3. After modification and alkaline activation, it can be converted into inorganic building materials with excellent mechanical properties and environmental durability. This not only helps to mitigate carbon emissions from cement production, but also provides a technical approach for its safe disposal. High-temperature calcination of coal gangue can produce coal-bearing metakaolin with high volcanic ash activity, a high Si / Al ratio, and a stable structure, making it an ideal silicon-aluminum resource material. Although it has been used locally as a cement admixture, its overall utilization level remains limited. There is an urgent need to promote its in-depth utilization in new green, low-carbon building materials to achieve high-value conversion of solid waste and serve the goals of energy conservation, emission reduction, and sustainable development. In this context, geopolymers are inorganic polymers with a three-dimensional network structure of NASH gel formed by dehydration and polycondensation under alkaline conditions using solid waste rich in SiO2 and Al2O3 as precursors. They are different from traditional cement in hydration reaction mechanism and have advantages such as high early strength, volume stability, chemical corrosion resistance and low carbon footprint, making them an important direction for replacing cement cementitious materials. Studies have shown that both red mud and coal-based metakaolin have good reactivity, and their combined use can significantly improve the reactivity and structural density of the geopolymer system. At present, geopolymers have been preliminarily applied in the fields of soil solidification, road reinforcement and building materials, showing good engineering adaptability and ecological and environmental benefits. However, the performance evolution characteristics of their slurries in shield synchronous grouting and engineering feasibility still lack systematic research. There is an urgent need to fill the relevant gaps and expand the application boundaries of geopolymers in underground engineering.

[0004] In actual urban tunnel construction, the shield method has been widely used in subway, highway, and railway tunnels due to its high excavation efficiency and excellent construction safety. During shield construction, synchronous grouting at the shield tail is an important measure to control ground deformation, reduce shield tail gaps, and improve lining structural stability. It is of critical importance to the safety and quality assurance of tunnel projects. Currently, cement-based two-liquid slurries or composite slurries are mostly used as grouting materials. However, these materials present a series of problems. On the one hand, the volume shrinkage caused by the hydration reaction of cement-based materials can easily lead to microcracks within the solidified slurry, reducing the overall density and durability of the structure and affecting the long-term service performance of the shield tunnel lining. On the other hand, cement production is energy-intensive and emits large carbon emissions, which does not meet the strategic requirements of green and low-carbon development. Therefore, there is an urgent need to develop green grouting materials with low-carbon, stable, and sustainable properties to meet the needs of improving the quality and efficiency of shield construction. Summary of the Invention

[0005] The present invention aims to solve the technical problems that cement-based grouting materials used in shield tunneling cause microcracks inside the slurry solidification body, resulting in poor long-term service performance of the shield tunnel lining, as well as high energy consumption and large carbon emissions in cement production. The present invention provides a geopolymer slurry suitable for shield synchronous grouting and a preparation method thereof.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a geopolymer slurry suitable for shield synchronous grouting, comprising red mud accounting for 34.2% to 44.2% of the total mass percentage of the slurry, coal-based metakaolin accounting for 22.0% to 34.2% of the total mass percentage of the slurry; industrial-grade water glass accounting for 9.3% to 18.8% of the total mass percentage of the slurry, analytical pure sodium hydroxide accounting for 1.4% to 3.2% of the total mass percentage of the slurry, water accounting for 10.1% to 27.8% of the total mass percentage of the slurry, the molar ratio of silicon dioxide to sodium oxide in the water glass being 1.2 to 1.6, and sodium oxide accounting for 2% to 4% of the total mass percentage of the slurry.

[0007] As a further limitation of the technical solution of the present invention, the mass ratio of red mud to coal-based metakaolin is 1.32, the mass ratio of silicon dioxide to sodium oxide is 1.56, the mass percentage of sodium oxide in the total slurry is 3.97%, and the water-solid ratio is 0.52.

[0008] As a further limitation of the technical solution of the present invention, the mass ratio of red mud to coal-based metakaolin is 1.25, the mass ratio of silicon dioxide to sodium oxide is 1.27, the mass percentage of sodium oxide in the total slurry is 2.11%, and the water-solid ratio is 0.45.

[0009] A geopolymer slurry suitable for shield synchronous grouting comprises the following steps: (1) Preparation of a composite alkali activator: dissolving sodium hydroxide in water and cooling to room temperature, and then mixing with water glass to obtain a composite alkali activator; (2) The pretreated red mud is mixed with coal-based metakaolin, and the composite alkali activator prepared in step (1) is added and stirred in a staged stirring process to obtain a geopolymer slurry.

[0010] As a further limitation of the technical solution of the present invention, the pretreatment method of the red mud in step (2) is to dry the red mud at 105°C and then grind and sieve it to obtain a powder material with uniform particle size.

[0011] As a further limitation of the technical solution of the present invention, the staged stirring process in step (2) is stirring at a low speed for 2 minutes, standing for 15 seconds to defoam, and then stirring at a high speed for 2 minutes.

[0012] Compared with the prior art, the present invention has the following beneficial effects: This study developed a novel geopolymer shield grouting material using Bayer red mud and coal-bearing metakaolin as the primary raw materials, and water glass and sodium hydroxide as activators. A three-level, four-factor orthogonal experiment was designed to systematically analyze the influence of mix parameters (red mud / coal-bearing metakaolin mass ratio, silica / sodium oxide molar ratio in the alkaline activator, sodium oxide mass fraction in the slurry, and water-solid ratio) on the slurry's basic properties. Using multivariate regression, performance influencing factor analysis, electrical property testing, and microstructural characterization, the performance evolution mechanism and material optimization path were revealed. A multi-objective optimization model was then used to identify the mix solution with optimal slurry performance and cost. This research finding will help promote the green application of geopolymer materials in tunnel grouting projects and provide theoretical support and technical reference for the resource utilization of solid wastes such as red mud and coal gangue.

[0013] Na2O% and w / b are the primary factors affecting slurry fluidity, consistency, stability, and setting time. SiO2 / Na2O and Na2O% exhibit a significant synergistic effect, jointly promoting polymerization and significantly improving both early and late slurry strength. RM / CMK influences gel formation efficiency and the density of its spatial structure by adjusting the Si / Al ratio.

[0014] The solidified body has a dense microstructure, demonstrating excellent long-term durability and corrosion resistance. Resistivity and EIS test results show that the slurry solidified body exhibits higher resistance characteristics at high Na2O% and high SiO2 / Na2O ratios, indicating that its internal ion migration channels are restricted and the pore structure is well closed, which helps prevent the intrusion of moisture and corrosive ions and improves the material's stability in complex underground environments.

[0015] The solidified material is rich in polymer products and has low porosity. XRD analysis detected a large amount of NASH gel and some crystalline minerals. SEM images show that the material has a dense structure, fine pores, and unreacted components in some areas.

[0016] Multi-objective optimization analysis results show that the optimal performance ratio (RM / CMK=1.32, SiO2 / Na2O=1.56, Na2O%=3.97%, w / b=0.52) achieves a 28-day compressive strength of 9.12 MPa. The optimal cost ratio (RM / CMK=1.25, SiO2 / Na2O=1.27, Na2O%=2.11%, w / b=0.45) reduces the material cost per ton to 20.76 yuan, while all performance characteristics meet engineering requirements. This demonstrates that the proposed geopolymer slurry for shield simultaneous grouting combines excellent performance with engineering adaptability, and has great potential for widespread application in shield construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is an analysis diagram of the influence of slurry density.

[0018] Figure 2 This is an analysis diagram of the influence of slurry fluidity.

[0019] Figure 3 This is an analysis diagram of the influence of slurry consistency.

[0020] Figure 4 This is an analysis chart of the impact of serous stone rate.

[0021] Figure 5 This is an analysis diagram of the influence of slurry bleeding rate.

[0022] Figure 6 This is an analysis diagram of the influence of slurry coagulation time.

[0023] Figure 7 This is the impact analysis diagram of slurry 3d-UCS.

[0024] Figure 8 This is the impact analysis diagram of slurry 7d-UCS.

[0025] Figure 9 This is the impact analysis diagram of slurry 28d-UCS.

[0026] Figure 10 This is the resistivity diagram of the slurry solidified body at the 28d curing age.

[0027] Figure 11 This is the electrochemical impedance spectroscopy of the slurry solidified body at the 28d curing age.

[0028] Figure 12 This is the SEM test image of the slurry solidification body.

[0029] Figure 13 This is the XRD test pattern of the slurry solidification body.

[0030] Figure 14 This is a structural model diagram of geopolymerization reaction. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to specific embodiments.

[0032] The raw materials used in the following examples are: Bayer red mud (RM) and coal-series metakaolin (CMK) were selected as the primary cementitious raw materials. The red mud was sourced from Bayer process solid waste from an aluminum plant in Shanxi, China, stored in a natural dump. To ensure experimental consistency, the red mud was dried at 105°C and then ground and sieved to produce a powder with uniform particle size. The coal-series metakaolin was sourced from a calcined kaolin company in Shanxi, China. It was produced by calcining and activating coal gangue at high temperatures, then dried and ground. The basic physical properties of the raw materials are listed in Tables 1 and 2. The main chemical compositions were determined by X-ray fluorescence (XRF) analysis, with the test results shown in Table 3. The results showed that the red mud contained high contents of Fe₂O₃, Al₂O₃, and SiO₂, demonstrating good alkali activation potential. The coal-series metakaolin, rich in active SiO₂ and Al₂O₃, exhibited excellent pozzolanic activity, making it an ideal silica-alumina resource for geopolymer reactions.

[0033] Table 1 Basic physical properties of red mud

[0034] Table 2 Test indicators of coal-bearing metakaolin

[0035] Table 3 Main chemical components of test materials (mass fraction, %)

[0036] The alkaline activator consists of industrial-grade sodium silicate and analytical-grade sodium hydroxide (purity ≥99.1%). The modulus of the sodium silicate is 3.1, and the mass fractions of its main components are SiO₂: 24.73% and Na₂O: 8.17%. During the preparation process, the sodium hydroxide is dissolved, cooled, and then mixed with the sodium silicate to prepare a composite alkaline solution for initiating the geopolymerization reaction. The mass fractions of the main components of the sodium silicate are shown in Table 4.

[0037] Table 4 Water glass composition Example 1

[0038] A three-level, four-factor orthogonal experimental design was used to investigate the effects of various mix parameters on the performance of geopolymer slurry used for shield tunneling grouting. The design variables included the red mud / coal-bearing metakaolin mass ratio (RM / CMK), the molar ratio of SiO₂ to Na₂O in the alkaline activator, the mass fraction of sodium oxide (Na₂O%, as a percentage of the total slurry mass), and the water-to-solid ratio (w / b). The factor level settings are shown in Table 5.

[0039] This example constructs L9(3 4 ) orthogonal table, with a total of 9 typical ratio schemes, see Table 6 for details. All ratios are calculated based on the absolute dry weight of raw materials to ensure the accuracy and comparability of the data.

[0040] Table 5 Horizontal factor design table

[0041] Table 6 Orthogonal test table

[0042] The preparation method of geopolymer slurry for shield synchronous grouting is as follows: A two-step mixing method was used for preparation. First, a composite alkaline activator was prepared according to the ratios listed in Table 6. NaOH was dissolved in water, cooled to room temperature, and then mixed with water glass in appropriate proportions. Subsequently, pretreated red mud and coal-bearing metakaolin were mixed according to the mass ratio. The alkaline activator solution was added and stirred using a staged mixing process. After preparation, a portion of the slurry was used for fresh performance testing, and the remaining portion was poured into a 40 mm cubic mold, allowed to stand for 24 hours, and then demolded. The mold was then cured under standard curing conditions for 3, 7, and 28 days, respectively, before various performance tests were performed.

[0043] The test indicators are as follows: 1. Comprehensive Analysis of Performance Impact Mechanisms In order to comprehensively evaluate the engineering adaptability of red mud-coal-based metakaolin polymer slurry, a number of performance test indicators were designed. From the perspective of pumping performance, the focus was on the constructability and flow behavior of the slurry in a fresh state. The test indicators included fluidity and consistency to evaluate the slurry's transportation efficiency and filling capacity in the pipeline; the density test was used to reflect the slurry's solid phase concentration and material utilization efficiency, which has an important impact on the construction pumping resistance and reinforcement performance. From the perspective of service performance, attention was paid to the stability and structural forming ability of the slurry during the static and solidification process after injection. The water seepage rate test is used to measure the degree of precipitation of free water in the slurry, reflecting the uniformity of the system and the risk of pore formation; the stone rate characterizes the sedimentation stability of the solid phase particles and is an important indicator for evaluating the slurry structure forming effect; the initial setting time is used to determine the time required for the slurry to transform from a fluid state to a solid state, which directly affects the on-site operation time and the rhythm of the grouting process. The bleeding rate is calculated using the static separation method as the ratio of the exudate volume to the initial slurry volume. The stone formation rate is calculated using the height difference before and after settling. The initial setting time is measured using a standard setting time meter. Relevant tests are conducted in accordance with ASTM C1437-20, ASTM C1586-20, and ASTM C1329 / C1329M-23, using a volumetric cylinder, an electric jumping table, and a consistency meter.

[0044] The test results are as follows: To further explore the mechanisms by which various mix parameters influence multiple slurry properties, this section combines single-factor and interaction analyses to explore the dominant effects of these variables and their synergistic relationships. Using the control variable approach, we analyze the impact of individual variables on slurry properties under a baseline mix ratio (RM / CMK = 1.5, SiO2 / Na2O = 1.4, Na2O% = 3%, w / b = 0.5). Furthermore, using interaction heat maps, we examine the coupled effects of different parameter combinations on performance, identifying potential synergistic enhancement or inhibition mechanisms.

[0045] a. Analysis of the mechanism affecting density performance Slurry density is a key parameter that reflects its solid phase concentration and structural density, and has an important influence on pumping performance and solidification strength. Regression analysis shows that Na2O% and w / b are the main variables affecting density, and there is a significant interaction between the two. Figure 1The results show that, with all other variables held constant, increasing the Na2O% from 2% to 4% increases the density from 1760 kg / m³ to 1840 kg / m³, an increase of approximately 4.5%. This increase is primarily due to the introduction of more high-density components (such as NaOH) into the slurry at a higher Na2O% content, which increases the total mass per unit volume. In contrast, increasing the w / b ratio from 0.4 to 0.6 results in a decrease in density of approximately 7.3%, primarily due to an increase in the water content and a decrease in the solids content per unit volume, which results in an overall decrease in density. Further interaction analysis reveals that the density enhancement effect of increasing the Na2O% is more pronounced at higher w / b ratios (reaching a 5.6% increase at w / b = 0.55). Furthermore, at a fixed Na2O% content, the decrease in density with increasing w / b ratio decreases with increasing alkalinity. For example, when the Na2O% increases from 2.5% to 3.5%, the density decrease decreases from 151 kg / m³ to 117 kg / m³ (a 22.5% decrease).

[0046] b. Analysis of the impact mechanism of fluidity performance Slurry fluidity is a key indicator for measuring its pumping performance and construction filling properties, and is controlled by a variety of ratio factors. Regression results show that w / b is the dominant variable affecting fluidity, and Na2O% and SiO2 / Na2O also have a synergistic regulatory effect under certain conditions. Figure 2 As shown, when w / b increases from 0.4 to 0.6, with other variables held constant, the fluidity significantly increases from 192.4 mm to 313.1 mm, a 62.7% increase. This increase is primarily attributed to the improved lubricity of the particles due to the increase in free water. In contrast, increasing Na2O% leads to a slight decrease in fluidity, while changes in SiO2 / Na2O have no significant effect on fluidity. Both factors indirectly regulate the rheological properties of the system primarily through their interaction with w / b. Further analysis reveals a synergistic thickening effect between SiO2 / Na2O and Na2O%. At low SiO2 / Na2O and Na2O%, the slurry exhibits excellent fluidity; however, when both increase simultaneously, the fluidity decreases significantly. In particular, at 3.5% Na2O, an increase in SiO2 / Na2O enhances particle cohesion and inhibits fluidity (Fig. 5a). Figure 2 b shows that under different SiO2 / Na2O levels, the increase of w / b always significantly improves fluidity, further confirming its dominant regulatory role. Figure 2 Figure c shows that under different Na2O% conditions, increasing w / b also continuously enhances flow properties, and the effect of Na2O% tends to weaken at high w / b. This indicates that in actual mixing ratios, w / b controls the basic flow level of the system, while SiO2 / Na2O and Na2O% play a secondary role in regulating flowability by affecting the system viscosity and particle structure distribution.

[0047] c. Analysis of the mechanism affecting consistency performance Slurry consistency is a key parameter to measure its pumpability and structural stability, and is regulated by multiple factors. Regression analysis shows that w / b has the most significant effect on consistency, while Na2O% and SiO2 / Na2O show a certain interactive regulation effect. Figure 3 As shown, when w / b increases from 0.4 to 0.6, with other variables held constant, the consistency increases from 61.9 mm to 134.3 mm, a 117% increase. This indicates that the increase in free water weakens the solid phase structure, reducing the interparticle support capacity, leading to a loose structure and enhanced rheological properties. In contrast, individual changes in Na2O% and SiO2 / Na2O have limited effects on consistency, but their combined effects demonstrate a synergistic effect. Figure 3 (a) shows that under fixed conditions of Na2O% or SiO2 / Na2O, increasing the other variable can slightly reduce the viscosity, indicating that both can optimize the dispersion of slurry particles and inhibit the tendency of structural bonding to a certain extent. Further analysis shows that w / b is the key variable that dominates the change in consistency. Figure 3 (b) shows that at different SiO2 / Na2O levels, the increase in w / b significantly increases the consistency, reflecting the trend of decreasing particle structure stability under high w / b conditions. Figure 3 Figure (c) reveals the regulatory effect of Na2O% on this trend. Although an increase in w / b consistently leads to an increase in consistency, the magnitude of this increase decreases with increasing Na2O%. For example, the increase in consistency is 74.2 mm at 2.5% Na2O, but decreases to 68.2 mm at 3.5%. This phenomenon suggests that higher alkalinity can promote NASH gel formation and improve skeleton density, thereby alleviating the increasing consistency trend caused by high w / b. Overall, w / b dominates the consistency evolution, and Na2O% and SiO2 / Na2O exhibit a certain synergistic inhibitory effect under high w / b conditions by regulating particle dispersibility and microstructural density.

[0048] d. Analysis of the mechanism affecting stone formation rate performance The stone rate reflects the ability of the slurry to stably control the solid phase particles during the static and solidification process, and is an important indicator for measuring its structural formation and system stability. Regression analysis showed that Na2O% and w / b were the main influencing factors, and there was a significant interaction between the two ( Figure 4Holding other variables constant, an increase in Na2O% from 2% to 4% increased the stone formation rate from 98.0% to 99.2%, indicating that increased alkalinity promoted gel formation and stabilized the skeleton structure, inhibiting particle settling. Increasing w / b slightly decreased the stone formation rate, reaching a minimum of 97.8%, reflecting the weakening of stability caused by slurry dilution and structural relaxation at high w / b. Further analysis revealed that the effect of Na2O% on increasing the stone formation rate was most significant at w / b = 0.55, indicating that the high free water content allows for more complete alkali-promoted reactions and a stronger enhancement of structural stability. When Na2O% was fixed, the stone formation rate exhibited a nonlinear trend of "first decreasing, then increasing" with w / b: When w / b increased from 0.40 to 0.50, the stone formation rate decreased due to enhanced particle dispersion and weakened structural support. However, when w / b further increased to 0.55, enhanced gel formation at higher Na2O% inhibited particle settling, leading to a slight increase in the stone formation rate. Taking Na2O% = 3.5% as an example, the fluctuation in stone formation rate caused by changes in w / b is controlled within 1.3%, and the system stability is good. This shows that increasing Na2O% not only enhances structural forming ability but also combats the risk of sedimentation caused by dilution under high w / b conditions.

[0049] e. Analysis of the mechanism affecting water bleeding rate performance Bleeding rate is an important indicator to measure the degree of free water precipitation during the static solidification process of slurry, which directly affects its uniformity and filling quality. The regression analysis results show that Na2O% and w / b are the key variables affecting the bleeding rate, and there is a significant interaction between the two ( Figure 5 With other variables held constant, increasing the Na2O% from 2% to 4% decreased the bleeding rate from 2.0% to 0.8%. This is primarily due to the accelerated NASH gel formation and denser structure under high alkalinity, which limits free water precipitation. In contrast, the effect of increasing the w / b ratio on the bleeding rate showed an initial upward and then downward trend, reaching a maximum of 2.5%. This indicates that during the initial increase in w / b, the stability of the adsorbed water layer on the particles decreases, increasing water precipitation; however, further increases to higher levels can partially alleviate the bleeding risk. Further analysis showed that at high w / b levels (e.g., 0.55), increasing the Na2O% ratio significantly inhibited the bleeding rate, with the bleeding rate decreasing by over 1.5% when the Na2O% ratio increased from 2% to 4%. Furthermore, as the Na2O% ratio increased, the sensitivity of the bleeding rate to w / b changes gradually decreased, significantly enhancing the stability of the system. For example, at a Na2O% ratio of 3.5%, the overall fluctuation of the bleeding rate was kept to a low level, significantly mitigating the risk of water precipitation associated with increased w / b. It can be seen that increasing the Na2O% can not only effectively enhance the density of the slurry structure, but also has the ability to compensate and adjust to counter the water precipitation problem caused by high w / b, which is beneficial to improve the stability of the system while ensuring fluidity.

[0050] f. Analysis of the mechanism affecting setting time performance Setting time is a key indicator for measuring the speed at which slurry changes from a fluid state to a solid state, directly affecting the operating window and process rhythm of grouting construction. Regression analysis shows that Na2O% and w / b are the dominant factors in regulating setting time, and SiO2 / Na2O also has a certain regulatory effect in some parameter combinations ( Figure 6 With other conditions unchanged, increasing the Na2O% from 2% to 4% significantly shortened the initial setting time from 2994 min to 391 min (a decrease of 86.9%), indicating that increased alkalinity accelerated the dissolution of Si and Al components and the formation of NASH gel, promoting rapid structural formation. Increasing the w / b ratio significantly delayed setting, exceeding 3000 min at w / b = 0.6. This was attributed to the increased free water diluting the reactant concentration and inhibiting ion migration, thereby reducing the polymerization reaction rate. Interaction analysis further revealed the coupling effects between the three variables: Figure 6 (a) shows that under low Na2O% (such as 2.5%), the increase of SiO2 / Na2O can accelerate the structure formation, but under high Na2O% (such as 3.5%) conditions, it manifests as delayed coagulation. This may be because the high modulus water glass increases the solution viscosity and inhibits ion migration and polymerization reaction. Figure 6 (b) shows that the regulating effect of SiO2 / Na2O on the coagulation time is significantly affected by w / b - the coagulation time can be shortened at low w / b, but prolonged at high w / b. Figure 6 Figure (c) shows that even though the overall setting time is shorter at high Na2O%, an increase in w / b ratio still delays setting, and this trend is more pronounced at low Na2O%. This suggests that increasing Na2O% can partially offset the reaction-slowing effect of high w / b ratio, while the effect of SiO2 / Na2O exhibits a condition-dependent "promotion-inhibition" transition. A balanced combination of alkalinity, water-to-solid ratio, and modulus should be considered in mix design to ensure the slurry has an appropriate setting rate and workability.

[0051] g. Analysis of UCS performance impact mechanism Unconfined compressive strength (UCS) is a key indicator for evaluating the mechanical properties of slurry and the bearing capacity of the solidified body, which directly affects its long-term stability during shield grouting. Regression analysis shows that Na2O%, w / b, RM / CMK and SiO2 / Na2O all have a significant impact on UCS ( Figure 7-9Taking the 28-day curing age as an example, under conditions where other variables remain constant, increasing the Na₂O₀% from 2% to 4% increases the UCS from 1.8 MPa to 5.6 MPa (a 211% increase). This increase is primarily attributed to the enhanced alkalinity promoting the dissolution of Si and Al components and the formation of NASH gels, which improves structural density and skeletal strength. However, an increase in w / b reduces solid phase concentration and increases porosity, significantly weakening strength performance. Increasing RM / CMK reduces the Si / Al ratio in the system and inhibits reaction activity, resulting in a decrease in UCS. Conversely, increasing the SiO₂ / Na₂O ratio increases the Si / Al ratio, enhances gel network formation, and correspondingly improves strength.

[0052] Further interaction analysis revealed a synergistic regulatory mechanism among the variables: under high SiO2 / Na2O (1.5) conditions, the increase of RM / CMK significantly inhibited the growth of UCS, while the trend was slower under low SiO2 / Na2O levels, indicating that the high RM content had a stronger inhibitory effect on the polymerization reaction under high modulus conditions ( Figure 9 The increase of Na2O% can enhance UCS at different RM / CMK levels, and shows a stronger compensatory effect when RM / CMK is higher ( Figure 9 b). Figure 9 (c) shows that when the Na2O% is high, the SiO2 / Na2O enhancement effect on UCS is more pronounced. The synergistic enhancement of the two significantly accelerates the gel structure formation rate and improves the structural density. Under the condition of fixed SiO2 / Na2O, the strengthening effect of Na2O% also increases, reflecting its core role in regulating reaction kinetics. Figure 9 (d) Further demonstrates the significant interaction between Na2O% and w / b. At a low w / b ratio (0.40), increasing Na2O% significantly enhances UCS. However, when w / b rises to 0.50 and above, despite the high Na2O%, the strength gain is weakened by the loose structure, and the magnitude of the increase decreases significantly. Increasing Na2O% and SiO2 / Na2O effectively stimulates system reactivity and improves UCS, while increasing RM / CMK and w / b generally weakens strength performance. Prioritizing alkalinity and Si / Al ratio optimization in mix design, while rationally controlling red mud content and w / b ratio, is crucial to achieving high strength and structural stability in the slurry system.

[0053] These results indicate that Na2O% and w / b are the primary factors influencing the fluidity, consistency, stability, and setting time of the slurry. SiO2 / Na2O and Na2O% exhibit a significant synergistic effect, jointly promoting polymerization and significantly improving both the early and late strength of the slurry. RM / CMK, by adjusting the Si / Al ratio of the system, influences the efficiency of gel formation and the density of its spatial structure.

[0054] 2. Electrical Performance Analysis In order to comprehensively evaluate the mechanical properties and electrical characteristics of the slurry after hardening, unconfined compressive strength (UCS), resistivity and electrochemical impedance spectroscopy (EIS) tests were carried out. UCS is used to reflect the bearing capacity of the solidified slurry. It is carried out according to ASTM C1329 / C1329M-23 and is measured at 3d, 7d and 28d using a microcomputer-controlled electronic universal testing machine. The electrical performance test includes resistivity and EIS tests. The former adopts the two-phase electrode method and is tested on 28-day cured cube specimens using a TH2828A LCR digital bridge (frequency range 50 Hz~1 MHz). The resistivity is calculated by the electrical impedance modulus to characterize its ion migration ability and pore structure characteristics; the latter uses a CS350 electrochemical workstation with a frequency range of 10⁻² Hz~10 6 Hz, also using 28-day-old samples as the object, the electrochemical impedance spectroscopy behavior was analyzed by Nyquist plot and Bode plot, so as to evaluate the influence of the interfacial resistance characteristics and micropore structure of the samples under different ratios on ion migration.

[0055] The test results are as follows: a. Resistivity analysis Figure 10 The results show that different samples have different curing conditions after 28 days at 10¹–10 6Resistivity response results in the Hz frequency range. All samples exhibit typical frequency-dependent characteristics, that is, the resistivity decreases with increasing frequency, especially in the low-frequency region (<10²Hz), indicating that ion migration in this section is mainly controlled by pore structure and interface polarization behavior. In this frequency band, samples No. 5 and No. 9 reach 485 Ω·m and 472 Ω·m, respectively, which are significantly higher than the other samples, indicating that their gel structure is denser and has fewer conductive channels. In contrast, samples No. 4, No. 6, and No. 8 have lower resistivities, indicating higher pore connectivity and loose structure. As the frequency increases further to the medium and high frequency regions, the resistivities between samples gradually converge, but No. 5 and No. 9 still maintain relatively high levels, further verifying their excellent microstructural density. Combined with the analysis of the mix parameters, it can be seen that lower w / b and RM / CMK ratios, and higher Na2O% and SiO2 / Na2O ratios help enhance the system's alkali excitation capacity, promote the dissolution of active Si and Al components and the formation of NASH gels, thereby building a dense skeleton and improving the material's resistivity and strength. This dense microstructure helps improve the slurry's impermeability and corrosion resistance, slowing the penetration and diffusion of corrosive media. Especially in shield grouting applications such as subways and tunnels, the high resistivity of the solidified body effectively blocks the ingress of moisture and corrosive ions, thereby enhancing the service life and safety of the lining structure.

[0056] b. Electrochemical impedance spectroscopy analysis Figure 11 All samples showed semicircular arc characteristics, indicating a typical interfacial charge transfer process. The semicircular arc diameters varied significantly between samples, reflecting differences in microstructural density. Samples No. 5 and No. 9 had larger semicircular arcs and higher interfacial resistance, indicating sufficient gel formation and low porosity. Samples No. 4, No. 6, and No. 8 had smaller arc diameters, indicating a loose structure and smoother charge transfer. Figure 11Further analysis revealed the impedance variations of the samples at different frequencies. In the low-frequency region (f ≤ 10²Hz), the impedance is primarily controlled by the pore structure, while in the high-frequency region, it is dominated by ionic conduction. Sample No. 5 exhibited the highest impedance in the low-frequency region, indicating optimal density, while Sample No. 8 exhibited the lowest impedance, indicating high pore connectivity and unobstructed ion migration pathways. Combined with analysis of the mixing parameters, a high Na₂O�% and a high SiO₂ / Na₂O ratio enhanced the reaction activity and gel formation efficiency, resulting in a dense structure, as evidenced by an increase in the semicircle diameter in the Nyquist plot, an increase in the low-frequency impedance in the Bode plot, and a correspondingly higher UCS. Conversely, higher RM / CMK and w / b weakened the gelation reaction, increased porosity, reduced interfacial resistance and overall impedance, and decreased material density and strength. Comprehensive EIS results indicate that samples with greater interfacial resistance and stronger phase shifts generally exhibit denser internal structures and lower pore connectivity, echoing the results of resistivity measurements for high-resistance samples. In engineering, this structural feature means that the material's shielding ability against corrosive media such as moisture and chloride ions is enhanced, which helps to improve the durability and service stability of the tunnel lining system.

[0057] These results demonstrate the dense microstructure of the solidified material, demonstrating excellent long-term durability and corrosion resistance. Resistivity and EIS testing indicate that the slurry solidified material exhibits higher resistance at high Na2O% and high SiO2 / Na2O ratios, indicating that its internal ion migration channels are restricted and the pore structure is well closed, which helps prevent the intrusion of moisture and corrosive ions and improves the material's stability in complex underground environments.

[0058] 3. Micro-Analysis To investigate the microstructure and mineralogical composition of the slurry after 28 days of hardening, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used for microstructural analysis. SEM analysis, performed using a field-emission scanning electron microscope (Zeiss Sigma 500), focused on the polymer gel density, pore structure, and distribution of unreacted particles in the cured slurry. XRD analysis, performed using an X-ray diffractometer (Panalytical Empyrean) using the powder pellet method and a scanning range of 10°–90°, analyzed the mineralogical composition of the samples.

[0059] The test results are as follows: The system of the present invention has the most complete polymerization reaction under alkali excitation, rich reaction products, well-developed gel network, and typical microscopic characteristics, making it suitable as a representative sample for analyzing the structural density and product type of geopolymers. Figure 12As shown, the solidified body exhibits a dense structure as a whole, forming a large amount of typical NASH gel phase, indicating that the Si and Al components in the system undergo a significant polymerization reaction under alkaline excitation conditions. In addition, some porous structures are visible in the image, indicating that a certain porosity still exists during the solidification process, which may affect the final mechanical properties of the slurry. The uniform distribution and density of NASH gel play a key role in improving the mechanical properties of the solidified body and inhibiting ion migration. Its density directly determines the long-term stability and durability of the material.

[0060] XRD test spectrum Figure 13 As shown. The diffraction analysis results show that there are obvious crystal diffraction peaks such as Na(AlSi3O8) and NaFe(Si2O6) in the solidified body, indicating that the silicon-aluminum skeleton structure in the system has been reorganized under an alkaline environment to generate stable reaction products. In addition, residual aluminum silicate minerals such as Al2O3 and Al2(SiO4)O can still be detected, corresponding to the unreacted particles observed in the SEM image. This shows that although water glass and sodium oxide activators promote the polymerization reaction of the system, the degree of reaction is still limited by factors such as the activity of raw materials, ratio design and curing conditions. It is worth noting that Ca3Fe2Si3O 12 The appearance of iso-calcium iron silicate phase may come from the residual calcium components in the raw materials and may have a certain promoting effect on the early strength development of the system.

[0061] like Figure 14 As shown, the resulting NASH gel spatially constructs a continuous, dense, three-dimensional gelling framework, effectively filling the pores within the slurry and significantly improving the density and mechanical strength of the cured structure. This microstructural reconstruction process constitutes the fundamental mechanism for the improved slurry performance. In particular, under conditions of high Na2O content and a moderate w / b ratio, the system's reaction kinetics are significantly enhanced, promoting early strength development and structural stability.

[0062] These results indicate that the solidified material is rich in polymer products and has low porosity. XRD analysis detected the formation of a large amount of NASH gel and some crystalline minerals. SEM images show that the material has a dense structure, fine pores, and the presence of unreacted components in some areas. Example 2

[0063] To achieve multi-objective coordinated optimization of slurry performance and cost, the present invention solved the constructed model using the NSGA-II genetic algorithm in the MATLAB software platform. The optimization process was performed using the following parameters: a maximum number of iterations of 400, a population size of 100, a crossover probability of 0.9, and a mutation probability of 0.03. Ultimately, two representative optimal mixes were obtained: a performance-optimal combination and a cost-optimal combination. The corresponding mix parameters (decision variables) are shown in Table 7, and the corresponding performance indicators and cost per ton are shown in Table 8.

[0064] Table 7 Target variable values

[0065] Table 8 Objective function values

[0066] The optimal performance mix, while maintaining a cost comparable to conventional cement slurry, demonstrated superior slurry properties, including density, fluidity, consistency, stone formation rate, and water bleeding rate. In particular, it significantly outperformed the control group in 3-day and 28-day compressive strength, demonstrating superior mechanical properties. In comparison, the optimal cost mix achieved comparable performance across all eight performance indicators, but with a lower unit cost, reducing the cost per ton by $8.41 USD. This achieved significant economic advantages while still meeting performance requirements. These results demonstrate that the red mud-coal-bearing metakaolin polymer slurry meets the performance requirements of shield grouting and possesses promising alternative potential and promotional value.

[0067] Multi-objective optimization analysis results show that the optimal performance ratio (RM / CMK=1.32, SiO2 / Na2O=1.56, Na2O%=3.97%, w / b=0.52) achieves a 28-day compressive strength of 9.12 MPa. The optimal cost ratio (RM / CMK=1.25, SiO2 / Na2O=1.27, Na2O%=2.11%, w / b=0.45) reduces the material cost per ton to $20.76, while meeting all performance requirements. This demonstrates that the red mud-coal-bearing metakaolin slurry combines excellent performance with engineering adaptability, and has great potential for widespread application in shield tunneling.

Claims

1. A geopolymer slurry suitable for shield synchronous grouting, characterized in that: The red mud accounts for 34.2% to 44.2% of the total mass percentage of the slurry, the coal-based metakaolin accounts for 22.0% to 34.2% of the total mass percentage of the slurry; the industrial-grade water glass accounts for 9.3% to 18.8% of the total mass percentage of the slurry, the analytical-grade sodium hydroxide accounts for 1.4% to 3.2% of the total mass percentage of the slurry, the water accounts for 10.1% to 27.8% of the total mass percentage of the slurry, the molar ratio of silicon dioxide to sodium oxide in the water glass is 1.2 to 1.6, and the sodium oxide accounts for 2% to 4% of the total mass percentage of the slurry.

2. The geopolymer slurry suitable for shield synchronous grouting according to claim 1, characterized in that: The mass ratio of the red mud to coal-based metakaolin is 1.32, the mass ratio of silicon dioxide to sodium oxide is 1.56, the mass percentage of sodium oxide in the total slurry is 3.97%, and the water-solid ratio is 0.

52.

3. The geopolymer slurry suitable for shield synchronous grouting according to claim 1, characterized in that: The mass ratio of the red mud to coal-based metakaolin is 1.25, the mass ratio of silicon dioxide to sodium oxide is 1.27, the mass percentage of sodium oxide in the total slurry is 2.11%, and the water-solid ratio is 0.

45.

4. A method for preparing a geopolymer slurry suitable for shield synchronous grouting according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) Preparation of a composite alkali activator: dissolving sodium hydroxide in water and cooling to room temperature, and then mixing with water glass to obtain a composite alkali activator; (2) The pretreated red mud is mixed with coal-based metakaolin, and the composite alkali activator prepared in step (1) is added and stirred in a staged stirring process to obtain a geopolymer slurry.

5. The method for preparing a geopolymer slurry suitable for shield synchronous grouting according to claim 4, characterized in that: The red mud pretreatment method in step (2) is to dry the red mud at 105°C and then grind and sieve it to obtain a powder material with uniform particle size.

6. The method for preparing a geopolymer slurry suitable for shield synchronous grouting according to claim 4, characterized in that: The staged stirring process in step (2) is to first stir for 2 minutes at a stirring blade rotation speed of 140 r / min and an orbital speed of 62 r / min, let it stand for 15 seconds to defoam, and then stir for 2 minutes at a stirring blade rotation speed of 285 r / min and an orbital speed of 125 r / min.

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