Preparation method of quick-hardening dry powder supporting material for underground coal mine

Abstract

This invention relates to the field of support materials and discloses a method for preparing a rapid-hardening dry powder support material for underground coal mines. The method includes: uniformly mixing 40%-50% sulfoaluminate cement, 10% slag powder, and 40%-50% hemihydrate gypsum to form a ternary mixed dry powder; mixing 0.5%-2% of a composite early-strength agent (by mass of sulfoaluminate cement) with 4% of a quick-setting agent (by mass of total dry powder), and adding this mixture along with water to the dry powder while stirring for 30 seconds to obtain an initial slurry; then adding 2.5%-4% of a composite expansion agent (by mass of sulfoaluminate cement) and 0.5%-2% of a composite activator (by mass of slag powder) to the slurry, and continuing high-speed stirring for 3 minutes; finally, pouring the mixture into a mold, allowing it to stand, demolding, and curing. This invention, by constructing a ternary cementitious system and combining it with specific additives and a segmented mixing process, balances the early-stage setting, volume compensation, and later-stage strength development of the material, enabling the support material to possess both rapid setting and long-term structural stability.

Description

Technical Field

[0001] This invention relates to the field of support materials, specifically a method for preparing a fast-hardening dry powder support material for underground coal mines. Background Technology

[0002] Support in underground coal mine roadways is a fundamental aspect of maintaining the stability of the surrounding rock. Due to the complex geological conditions and rapid deformation of rock strata underground, support materials need to have the ability to set quickly and establish early strength in order to provide the necessary support resistance in a short period of time.

[0003] Traditional support materials often use ordinary Portland cement, which has a long setting time and slow early strength development, making it difficult to meet the engineering requirements of rapid support. To shorten the setting time, current technologies often use fast-setting materials such as sulfoaluminate cement as the main cementitious system. Although these fast-setting materials hydrate quickly in the initial stage, they are prone to problems such as later strength stagnation or shrinkage in practical applications. At the same time, fast-setting materials are usually accompanied by volume shrinkage during the hydration stage, and the resulting shrinkage stress can cause microcracks inside the support structure, thereby weakening the overall load-bearing capacity and impermeability of the structure.

[0004] Furthermore, to improve the construction and mechanical properties of materials, admixtures such as accelerators, quick-setting agents, and expanding agents are often added to the formulation. Current preparation processes mostly involve mixing all dry powders and admixtures with water in a single step. The simultaneous dissolution and hydration of multiple admixtures can easily cause excessively rapid local reactions, leading to premature loss of fluidity in the slurry before pouring into the mold, thus affecting the actual casting operation. This compounding and preparation process makes it difficult to simultaneously balance the relationship between setting acceleration, volume compensation, and subsequent strength development, making it difficult for the support material to maintain long-term structural stability in the downhole stress environment. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing a fast-hardening dry powder support material for underground coal mines, which solves the problem that existing fast-hardening support materials for coal mines cannot simultaneously achieve rapid early solidification and hardening, continuous strength development in the later stages, and volume shrinkage and crack prevention.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a fast-hardening dry powder support material for underground coal mines, comprising the following steps: At room temperature, 40%-50% of sulfoaluminate cement, 10% of slag powder and 40%-50% of hemihydrate gypsum are mixed in proportion to form a uniform ternary mixed dry powder. Mix 0.5%-2% of the composite early strength agent by weight of the sulfoaluminate cement with 4% of the quick-setting agent by weight of the total dry powder. Add the mixed dry material together with water weighed at a water-cement ratio of 0.45 to the ternary mixed dry powder and stir with a mixer for 30 seconds to form an initial slurry. Add 2.5%-4% of the composite expansion agent (by mass of the sulfoaluminate cement) and 0.5%-2% of the composite activator (by mass of the slag powder) to the initial slurry, and then continue high-speed stirring for 3 minutes. After the slurry forms a stable and uniform consistency, it is poured into a mold; after standing, it is demolded, cured, and finally a fast-hardening dry powder support material is prepared.

[0007] By adopting the above technical solution, the following beneficial effects are achieved: This invention, through the construction of a ternary cementitious system of sulfoaluminate cement, slag powder, and hemihydrate gypsum, and in conjunction with specific types and amounts of admixtures and a stepwise mixing process, realizes rapid setting, hardening, and volume stability of the material. The specific hydration reaction process and innovative mechanism are as follows: Upon addition of water, the hemihydrate gypsum in the system dissolves rapidly. At this point, the anhydrous calcium sulfoaluminate mineral phase in the sulfoaluminate cement undergoes a hydration reaction with the gypsum, promoting the formation of a large amount of ettringite. Because ettringite exhibits a needle-like or rod-shaped crystal morphology, these crystals interlock and overlap within the slurry, forming an initial spatial framework structure, which endows the material with the ability to set rapidly and establish early strength.

[0008] Subsequently, the dissolution of the composite early-strength agent increases the concentration of free ions in the liquid phase. Calcium ions present in the liquid phase can accelerate the hydration rate of silicate minerals, while sulfate ions participate in the nucleation and crystal growth process of ettringite. These combined effects improve the early mechanical properties of the support material.

[0009] The development of the material's later-stage strength mainly relies on the alkaline environment provided by the composite activator. Under this environment, the glassy structure on the surface of the slag powder is disrupted, releasing free silica and alumino groups. These groups undergo secondary hydration under the action of calcium ions, thereby generating hydrated calcium silicate and hydrated calcium aluminate gels. The increase in later-stage strength is mainly related to the gel formed after slag activation filling the pores, and the material's density increases with age.

[0010] To address the shrinkage issue, the composite expansive agent component in the formulation generates an expansive product through a hydration reaction. The volume expansion accompanying the reaction compensates for the chemical and drying shrinkage caused by moisture consumption and temperature changes during the setting and hardening stage of the slurry, thereby preventing the formation of microcracks within the material.

[0011] At the process level, the preparation process employs a step-by-step feeding combined with segmented stirring. Initially, an early-strength agent and a quick-setting agent are added to form an initial slurry, allowing the coagulation system to be uniformly dispersed beforehand. After the initial slurry is formed, a composite expanding agent and a composite activator are added and stirred at high speed. This step-by-step operation prevents all additives from being added at once, which could cause excessively rapid localized reactions, avoids premature loss of slurry fluidity, and ensures the uniformity and workability of the mixture before it is poured into the mold.

[0012] Preferably, when preparing the ternary mixed dry powder, the sulfoaluminate cement is P.O42.5 sulfoaluminate cement, and the slag powder is S105 slag powder; the amounts of the sulfoaluminate cement, the slag powder, and the hemihydrate gypsum are respectively: the sulfoaluminate cement accounts for 50% of the total dry powder mass, the slag powder accounts for 10% of the total dry powder mass, and the hemihydrate gypsum accounts for 40% of the total dry powder mass.

[0013] By adopting the above technical solution, the ratio of the early skeleton provided by sulfoaluminate cement to the gel filler generated by the later hydration of slag powder is balanced under this specific ratio. In addition, the set amount of hemihydrate gypsum can just meet the sulfate requirements of sulfoaluminate cement for the formation of ettringite, preventing insufficient strength due to gypsum deficiency.

[0014] Preferably, the amount of the composite early strength agent added is 2% of the mass of the sulfoaluminate cement; the amount of the composite expansion agent added is 4% of the mass of the sulfoaluminate cement; and the amount of the composite activator added is 0.5% of the mass of the slag powder.

[0015] By adopting the above technical solution and limiting the specific dosage of these admixtures, the setting acceleration, early strength, and expansion compensation rate of the material system can be matched with each other. During this period, the expansion stress generated by the expansion agent will be limited by the structural strength established in the early stage, and then transformed into internal stress within the material, thereby improving the overall density and impermeability of the material.

[0016] Preferably, the composite early strength agent is composed of calcium formate and sodium sulfate.

[0017] Preferably, the preparation method of the composite early strength agent is as follows: at room temperature, the calcium formate and the sodium sulfate are weighed at a mass ratio of 1:2, and the calcium formate and the sodium sulfate are placed in a mixing device and stirred and ground thoroughly until they are mixed evenly to obtain the composite early strength agent.

[0018] By employing the above-mentioned technical solution, the combination of calcium formate and sodium sulfate can jointly promote the early reaction of the system. The former helps to increase the calcium ion concentration, while the latter can replenish sulfate ions and improve the reaction environment. Therefore, this composite early-strength agent has a positive effect on shortening the setting time.

[0019] Preferably, the composite expanding agent is composed of AEA sulfoaluminate expanding agent and TD-P1 calcium oxide expanding agent.

[0020] Preferably, the composite expanding agent is prepared by weighing the AEA sulfoaluminate expanding agent and the TD-P1 calcium oxide expanding agent at a mass ratio of 1:2 at room temperature, placing the AEA sulfoaluminate expanding agent and the TD-P1 calcium oxide expanding agent in a mixing device and stirring thoroughly until they are mixed evenly to obtain the composite expanding agent.

[0021] By employing the above technical solution, the AEA sulfoaluminate-type expansive agent hydrates to form ettringite, which tends to provide volume expansion in the early stages of hydration; in contrast, the TD-P1 calcium oxide-type expansive agent hydrates to form calcium hydroxide crystals, mainly responsible for generating volume expansion in the middle and later stages of hydration. Using this ratio of dual expansion source system allows the material to obtain continuous volume compensation throughout the entire hardening cycle, thus avoiding expansion cracking or later shrinkage caused by excessively concentrated action time of a single expansive agent.

[0022] Preferably, the composite activator is composed of sodium hydroxide and calcium hydroxide.

[0023] Preferably, the composite activator is prepared by weighing sodium hydroxide and calcium hydroxide at a mass ratio of 1.5:1 at room temperature, placing the sodium hydroxide and calcium hydroxide in a mixing device and stirring thoroughly until they are mixed evenly to obtain the composite activator.

[0024] By employing the above technical solution, the dissolved sodium hydroxide in the system provides a high concentration of hydroxide ions, which rapidly establish a strongly alkaline environment to disrupt the active inert layer on the surface of the slag powder. Simultaneously, the dissolution of calcium hydroxide replenishes the required calcium ions. This formulation combines alkali activation with calcium salt activation, accelerating the disintegration and recombination process of the active silica-alumina layer inside the slag powder, thereby improving the efficiency of gel product formation.

[0025] Preferably, the settling time is 2 hours; the post-demolding curing steps include: post-demolding curing for 3 days, 7 days, and 28 days.

[0026] By adopting the above technical solution, the 2-hour static setting ensures that the material has the initial strength required for demolding. The subsequent 3-day, 7-day, and 28-day curing cycles help maintain the internal moisture distribution of the material, allowing the secondary hydration reaction of the slag powder to continue, thereby ensuring the stable development of the long-term mechanical properties of the support material.

[0027] This invention provides a method for preparing a fast-hardening dry powder support material for underground coal mines. It has the following beneficial effects: 1. A ternary cementitious system composed of sulfoaluminate cement, slag powder, and hemihydrate gypsum is used, along with an early-strength agent formulated with calcium formate and sodium sulfate. When water is added to the material, the hemihydrate gypsum dissolves, promoting the formation of needle-like ettringite crystals from the calcium sulfoaluminate mineral phase. These crystals interlock to form the initial framework. Simultaneously, the early-strength agent increases the concentration of calcium and sulfate ions in the liquid phase, further accelerating mineral hydration and crystal growth. This combination helps the support material to set quickly and establish early strength.

[0028] 2. During the later hydration stage of the material, an activator composed of sodium hydroxide and calcium hydroxide was added to the formula. After dissolving, this activator creates an alkaline environment, disrupting the glassy structure on the surface of the slag powder. This releases silica and alumino groups, which then undergo secondary hydration with calcium ions. The resulting gel-like substance gradually fills the pores of the ettringite framework, increasing structural density and maintaining stable strength development in the later stages.

[0029] 3. The material also incorporates a dual expansion source system of sulfoaluminate and calcium oxide compounds, coupled with a stepwise feeding and mixing process. The composite expansion agent generates expansion products at different stages of hydration, continuously compensating for volume shrinkage throughout the material's hardening cycle and reducing the formation of microcracks. Operationally, the components of the coagulation accelerator are first thoroughly mixed before adding the expansion and activation systems. This avoids premature loss of slurry fluidity due to simultaneous mixing of multiple additives, ensuring the workability of the material before it is poured into the mold. Attached Figure Description

[0030] Figure 1 The graph shows the test results of initial fluidity and setting time for various embodiments of the present invention; Figure 2 The figures show the uniaxial compressive strength test results at different ages for various embodiments of the present invention; Figure 3 The figures show the test results of micro-expansion rate at different ages for various embodiments of the present invention; Figure 4 This is a comparison diagram of the uniaxial compressive strength of the samples in the very early stage (1h-24h) of this invention; Figure 5 This is a comparison chart of the uniaxial compressive strength of the samples in the later stages (3d-28d) of this invention; Figure 6 The figure shows the test results of the interfacial bond strength of the samples at different ages according to the present invention; Figure 7 The figures shown are short-time flow performance test results of embodiments and comparative examples of the present invention. Sub-figure (a) is a curve showing the change in slurry flowability over time; sub-figure (b) is a curve showing the change in slurry flowability retention rate over time. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a composite early strength agent, including the following steps: At room temperature, calcium formate and sodium sulfate were weighed out in a mass ratio of 1:2, and then placed in a mixing device for thorough stirring and grinding until they were mixed evenly to obtain a composite early strength agent.

[0033] Preparation Example 2: This preparation example provides a method for preparing a composite expanding agent, including the following steps: At room temperature, AEA sulfoaluminate type expansion agent and TD-P1 calcium oxide type expansion agent were weighed out at a mass ratio of 1:2, and the two were placed in a mixing device and stirred thoroughly until they were mixed evenly to obtain a composite expansion agent.

[0034] Preparation Example 3: This preparation example provides a method for preparing a composite activator, including the following steps: At room temperature, sodium hydroxide and calcium hydroxide were weighed out in a mass ratio of 1.5:1, and then placed in a mixing device and stirred thoroughly until they were mixed evenly to obtain a composite activator.

[0035] Examples 1-5: Example 1: This embodiment provides a method for preparing a fast-hardening dry powder support material for underground coal mines, including the following steps: At room temperature, 40% of the total dry powder mass of P.O42.5 sulfoaluminate cement, 10% of S105 slag powder and 50% of hemihydrate gypsum are mixed in proportion to form a uniform ternary mixed dry powder. The composite early strength agent prepared in Preparation Example 1, which accounts for 2% of the mass of the above P.O42.5 sulfoaluminate cement, is mixed evenly with the quick-setting agent accounting for 4%. The mixed dry material is added to the ternary mixed dry powder along with water weighed at a water-cement ratio of 0.45, and stirred with a mixer for 30 seconds to form an initial slurry. The composite expansive agent prepared in Preparation Example 2, which accounts for 4% of the mass of the above-mentioned P.O42.5 sulfoaluminate cement, and the composite activator prepared in Preparation Example 3, which accounts for 0.5% of the mass of the above-mentioned S105 slag powder, were added to the initial slurry. Then, the mixture was stirred at high speed for 3 minutes. After a stable and uniform slurry was formed, it was poured into a mold with a diameter of 50 mm × 50 mm × 50 mm. After standing for 2 hours, the material was demolded and then cured for 3 days, 7 days, and 28 days to finally produce a fast-hardening dry powder support material.

[0036] Example 2: This embodiment provides a method for preparing a fast-hardening dry powder support material for underground coal mines, including the following steps: At room temperature, 50% of P.O42.5 sulfoaluminate cement, 10% of S105 slag powder and 40% of hemihydrate gypsum were mixed in proportion to form a uniform ternary mixed dry powder. The composite early strength agent prepared in Preparation Example 1, which accounts for 2% of the mass of the above P.O42.5 sulfoaluminate cement, is mixed evenly with the quick-setting agent accounting for 4%. The mixed dry material is added to the ternary mixed dry powder along with water weighed at a water-cement ratio of 0.45, and stirred with a mixer for 30 seconds to form an initial slurry. The composite expansive agent prepared in Preparation Example 2, which accounts for 4% of the mass of the above-mentioned P.O42.5 sulfoaluminate cement, and the composite activator prepared in Preparation Example 3, which accounts for 0.5% of the mass of the above-mentioned S105 slag powder, were added to the initial slurry. Then, the mixture was stirred at high speed for 3 minutes. After a stable and uniform slurry was formed, it was poured into a mold with a diameter of 50 mm × 50 mm × 50 mm. After standing for 2 hours, the material was demolded and then cured for 3 days, 7 days, and 28 days to finally produce a fast-hardening dry powder support material.

[0037] Example 3: This embodiment provides a method for preparing a fast-hardening dry powder support material for underground coal mines, including the following steps: At room temperature, 50% of P.O42.5 sulfoaluminate cement, 10% of S105 slag powder and 40% of hemihydrate gypsum were mixed in proportion to form a uniform ternary mixed dry powder. The composite early strength agent prepared in Preparation Example 1, which accounts for 0.5% of the mass of the above P.O42.5 sulfoaluminate cement, is mixed evenly with 4% of the quick-setting agent. The mixed dry material is then added to the ternary mixed dry powder along with water weighed at a water-cement ratio of 0.45, and stirred with a mixer for 30 seconds to form an initial slurry. The composite expansive agent prepared in Preparation Example 2, which accounts for 4% of the mass of the above-mentioned P.O42.5 sulfoaluminate cement, and the composite activator prepared in Preparation Example 3, which accounts for 0.5% of the mass of the above-mentioned S105 slag powder, were added to the initial slurry. Then, the mixture was stirred at high speed for 3 minutes. After a stable and uniform slurry was formed, it was poured into a mold with a diameter of 50 mm × 50 mm × 50 mm. After standing for 2 hours, the material was demolded and then cured for 3 days, 7 days, and 28 days to finally produce a fast-hardening dry powder support material.

[0038] Example 4: This embodiment provides a method for preparing a fast-hardening dry powder support material for underground coal mines, including the following steps: At room temperature, 50% of P.O42.5 sulfoaluminate cement, 10% of S105 slag powder and 40% of hemihydrate gypsum were mixed in proportion to form a uniform ternary mixed dry powder. The composite early strength agent prepared in Preparation Example 1, which accounts for 2% of the mass of the above P.O42.5 sulfoaluminate cement, is mixed evenly with the quick-setting agent accounting for 4%. The mixed dry material is added to the ternary mixed dry powder along with water weighed at a water-cement ratio of 0.45, and stirred with a mixer for 30 seconds to form an initial slurry. The composite expansive agent prepared in Preparation Example 2, which accounts for 2.5% of the mass of the above P.O42.5 sulfoaluminate cement, and the composite activator prepared in Preparation Example 3, which accounts for 0.5% of the mass of the above S105 slag powder, were added to the initial slurry. Then, the mixture was stirred at high speed for 3 minutes until a stable and uniform slurry was formed. The mixture was then poured into a mold with a diameter of 50 mm × 50 mm × 50 mm. After standing for 2 hours, the material was demolded and then cured for 3 days, 7 days, and 28 days to finally produce a fast-hardening dry powder support material.

[0039] Example 5: This embodiment provides a method for preparing a fast-hardening dry powder support material for underground coal mines, including the following steps: At room temperature, 50% of P.O42.5 sulfoaluminate cement, 10% of S105 slag powder and 40% of hemihydrate gypsum were mixed in proportion to form a uniform ternary mixed dry powder. The composite early strength agent prepared in Preparation Example 1, which accounts for 0.5% of the mass of the above P.O42.5 sulfoaluminate cement, is mixed evenly with 4% of the quick-setting agent. The mixed dry material is then added to the ternary mixed dry powder along with water weighed at a water-cement ratio of 0.45, and stirred with a mixer for 30 seconds to form an initial slurry. The composite expansive agent prepared in Preparation Example 2, which accounts for 4% of the mass of the above-mentioned P.O42.5 sulfoaluminate cement, and the composite activator prepared in Preparation Example 3, which accounts for 2% of the mass of the above-mentioned S105 slag powder, are added to the initial slurry. Then, the mixture is stirred at high speed for 3 minutes. After a stable and uniform slurry is formed, it is poured into a mold of 50mm×50mm×50mm. After standing for 2 hours, the material was demolded and then cured for 3 days, 7 days, and 28 days to finally produce a fast-hardening dry powder support material.

[0040] Comparative Examples 1-8: Comparative Example 1: Compared with Example 2, the difference is that no slag powder was added, and the original proportion of slag powder was replaced with an equal amount of sulfoaluminate cement, while the rest are the same.

[0041] Comparative Example 2: Compared with Example 2, the difference is that the hemihydrate gypsum is replaced with an equal amount of conventional dihydrate gypsum, and all other aspects are the same.

[0042] Comparative Example 3: The difference from Example 2 is that no composite expanding agent was added; otherwise, they are the same.

[0043] Comparative Example 4: The difference from Example 2 is that no composite activator was added; otherwise, they are the same.

[0044] Comparative Example 5: Compared with Example 2, the difference lies in the change of the feeding and stirring process. The ternary mixed dry powder, composite early strength agent, quick-setting agent, composite expansion agent, composite activator and water are added to the mixer at the same time for mixing. All other aspects are the same.

[0045] Comparative Example 6: Compared with Example 2, the difference is that the mass ratio of sodium hydroxide to calcium hydroxide in the composite activator was adjusted, and the composite activator was prepared according to a mass ratio of 1:3. All other aspects are the same.

[0046] Comparative Example 7: Compared with Example 2, the difference is that no composite early strength agent was added, but instead it was replaced with an equal amount of single calcium formate additive, while the rest are the same.

[0047] Comparative Example 8: Compared with Example 2, the difference is that the composite activator of Preparation Example 3 was not used, but was directly replaced with an equal amount of single sodium silicate activator; otherwise, they are the same.

[0048] Test Examples 1-7: Test Example 1: The experimental subjects were the slurries prepared in Examples 1 to 5 that were in the initial stirring stage.

[0049] The fluid slurry after stirring according to the predetermined parameters in each embodiment was taken as the test sample.

[0050] The initial flowability test was conducted using a standard truncated cone mold. The truncated cone mold was placed on a horizontal glass plate, and the freshly mixed slurry was quickly poured into the mold and the surface was smoothed with a scraper. Then, the mold was lifted vertically upwards, and the slurry collapsed and flowed freely on the glass plate under its own weight. After the slurry completely stopped flowing, the maximum expansion diameter in two mutually perpendicular directions was measured using vernier calipers, and the average value was calculated and recorded as the initial flowability.

[0051] Fill the Vicat apparatus with the remaining mixed slurry and smooth the surface. Place it in a standard curing chamber for setting time testing. Adjust the Vicat apparatus probe to contact the slurry surface. Periodically release the probe to observe its sinking depth and record the time points when the probe sinks to the corresponding distance in the bottom plate. Measure the initial setting time and final setting time respectively. Each penetration test should avoid the previous test hole points.

[0052] The experimental data for the above tests are summarized below.

[0053] Table 1. Test data on working performance and setting time of Examples 1 to 5 Figure 1 The graph shows the initial mobility data line corresponding to the left vertical axis, and the initial setting time and final setting time data line corresponding to the right vertical axis. The horizontal axis represents different implementation examples. Specifically, the solid lines marked with gray solid circles represent the initial mobility data, the dark gray dashed lines marked with white hollow squares represent the initial setting time data, and the dotted lines marked with black solid triangles represent the final setting time data.

[0054] Summary: Based on Table 1 and Figure 1 While the performance and setting time of each embodiment differed, all met the engineering requirements. The initial flowability of each embodiment ranged from 198.3 mm to 241.1 mm. Specifically, Example 2 exhibited an initial flowability of 236.4 mm, with continuous slurry expansion without clumping or bleeding. Adding the composite activator and composite expanding agent 30 seconds after initial stirring avoided false setting caused by intense hydration and alkali activation of the dry powder upon contact with water.

[0055] Figure 1As shown in Table 1, the initial setting time of each embodiment ranges from 12.3 to 18.5 minutes, and the final setting time is controlled between 18.9 and 27.3 minutes, all within 30 minutes. When in a static state, the substrate can form a gypsum dihydrate and ettringite crystal framework. Some embodiments, such as Example 4, adjusted the amount of admixture, increasing the flowability to 241.1 mm while extending the setting time from initial setting to 18.5 min and final setting to 27.3 min; while Example 5 had the lowest flowability at 198.3 mm and the shortest setting time, indicating that the admixture ratio has a regulatory relationship on the setting and flow of the material.

[0056] Test Example 2: The experimental subjects were 50mm×50mm×50mm fast-hardening dry powder support material test blocks prepared in Examples 1 to 5.

[0057] Select the test blocks from each embodiment that were demolded after standing in the mold for 2 hours, wipe off the surface scum and loose material, and immediately transfer some of the test blocks to the test bench of the microcomputer-controlled electro-hydraulic servo universal testing machine. Set the loading rate to 2.4 kN / s, perform uniaxial compressive strength test and record the maximum load at failure, and calculate the 2-hour compressive strength.

[0058] The remaining demolded test blocks were transferred to a standard constant temperature and humidity curing chamber, and the curing conditions were set as follows: temperature 20±2℃ and relative humidity not less than 95%.

[0059] When the curing age of the test blocks reached 3 days, 7 days and 28 days respectively, the corresponding test blocks were taken out in batches, and after wiping the surface free moisture with a damp cloth, the compressive strength test was carried out using the same equipment and loading regime as for 2 hours.

[0060] The uniaxial compressive strength at each age was taken as the average value of three test blocks from the same batch as the final data to reduce the discrete error in the experimental process.

[0061] The experimental data for the above tests are summarized below.

[0062] Table 2. Compressive strength test data at different ages in Examples 1 to 5 Figure 2 The graph shows the uniaxial compressive strength trends of the test blocks from each embodiment at four key curing periods: 2h, 3d, 7d, and 28d. The horizontal axis represents the curing period, and the vertical axis represents the compressive strength. In the graph, the solid line marked with a black asterisk represents Embodiment 1, the dashed line marked with a white hollow rhombus represents Embodiment 2, the dotted line marked with a dark gray solid pentagram represents Embodiment 3, the dashed line marked with a black multiplication sign represents Embodiment 4, and the solid line marked with a light gray solid downward triangle represents Embodiment 5.

[0063] Summary: Based on Table 2 and Figure 2 The data shows that the compressive strength of each embodiment increases with the extension of curing age. In the initial curing stage, the 2-hour compressive strength of each embodiment ranged from 14.8 MPa to 18.9 MPa, with Example 2 showing the highest at 18.9 MPa and Example 5 showing the lowest at 14.8 MPa. The initial mechanical properties are attributed to the synergistic hydration reaction between hemihydrate gypsum and sulfoaluminate cement. In this scheme, the hemihydrate gypsum combines with the mixing water in a short time to form a dihydrate gypsum crystal network, which, combined with the ettringite formed by the sulfoaluminate cement, fills the pores and solidifies the matrix, providing initial support.

[0064] As the curing period progresses, the internal hydration reaction gradually shifts to the alkali activation stage of the slag. Figure 2 The compressive strength of each embodiment showed an increasing trend from 3 days to 28 days, with the compressive strength at 3 days and 7 days reaching over 24.3 MPa and over 32.5 MPa, respectively. At 28 days, the final compressive strength of each embodiment ranged from 42.7 MPa to 54.6 MPa, with Example 2 exhibiting the highest strength at 28 days, reaching 54.6 MPa. Early hydration consumed free water and released heat of hydration, maintaining the internal temperature, humidity, and high pH of the system. The alkaline substances in the composite activator disrupted the glassy network structure on the surface of the slag powder, and the depolymerized active components underwent a pozzolanic reaction, generating hydrated calcium silicate gel. The gel product filled the pores between early crystals, increasing the matrix density and ensuring the load-bearing capacity of the specimen in the later stages.

[0065] Test Example 3: The experimental subjects were the mixed slurries prepared in Examples 1 to 5 in their initial flow state.

[0066] Pour the well-stirred initial slurry into a standard triple shrinkage and expansion mold of 40mm×40mm×160mm that has been pre-coated with a thin layer of release agent. Stainless steel measuring copper heads have been fixed at the center of both ends of the mold in advance. After the slurry is poured into the mold, it is gently tamped to remove internal air bubbles and smooth the surface.

[0067] The mold filled with slurry was placed in a standard curing chamber with a temperature of 20±2℃ and a relative humidity of not less than 95% and left to stand. After the slurry had completed its final setting and had a certain early strength, the mold side plate was removed, the formed prism specimen was carefully taken out, and the floating slurry and impurities on the measuring copper heads at both ends were wiped clean with a damp cotton cloth.

[0068] Then immediately place it vertically on the measuring bracket of the dial gauge length comparator, rotate the specimen to stabilize its position, and read the initial value of the dial gauge. Record this data as the initial reference length of the specimen.

[0069] After completing the initial readings, the specimens were returned to the standard curing environment for continued curing. They were then removed in batches at curing ages of 1 day, 3 days, 7 days, 14 days, and 28 days. The free water on the surface of the specimens was wiped dry with a dry cloth, and the length at the specific age was measured using the same operating procedure on a length comparator. The micro-expansion rate at the corresponding age was obtained by calculating the percentage of the difference between the length at each age and the initial reference length relative to the measured length. The arithmetic mean of three specimens under the same conditions in each group was taken as the final result.

[0070] The experimental data for the above tests are summarized below.

[0071] Table 3. Test data of micro-expansion rate at different ages in Examples 1 to 5 Figure 3 The graph illustrates the evolution of linear expansion rates of specimens from each embodiment over an age period of 1 to 28 days. The horizontal axis represents the set test age, and the vertical axis represents the percentage of micro-expansion of the specimen relative to its initial length. In the figure, a black solid line with a white-filled hexagon represents Embodiment 1; a dark gray dashed line with a dark gray plus sign represents Embodiment 2; a gray dotted line with a white-filled circle represents Embodiment 3; a black dotted line with a light gray solid square represents Embodiment 4; and a light gray solid line with a black solid rhombus represents Embodiment 5.

[0072] Summary: Table 3 and Figure 3 The results show that all embodiments maintained a state of slight volume expansion within 28 days, exhibiting a two-stage expansion characteristic. From 1 to 3 days, the embodiments showed initial expansion, with the slight expansion rate at 1 day ranging from 0.011% to 0.021%; by 3 days, the slight expansion rate increased to between 0.018% and 0.036%, with Embodiment 2 showing the highest slight expansion rate at 0.036% at 3 days. This is because the AEA sulfoaluminate-type expanding agent generates ettringite crystals in an alkaline environment, and the crystallization pressure compensates for the early self-shrinkage.

[0073] From day 7 to day 28, the micro-expansion rate of each embodiment continued to increase. After 14 days of continuous hydration (micro-expansion rate reaching 0.033% to 0.058%), the micro-expansion rate of each embodiment was finally distributed between 0.041% and 0.064% at day 28, with Example 2 reaching the highest value of 0.064% at day 28. The later expansion was due to the delayed hydration of the TD-P1 calcium oxide-type expanding agent. Free calcium oxide particles combined with water in the later stages of curing to form calcium hydroxide crystals, increasing in volume. Secondary expansion filled the microscopic gel pores, creating a compressive effect on the surrounding environment. The combined effect of early ettringite and later calcium hydroxide compensated for the later shrinkage.

[0074] Test Example 4: The experimental subjects were the early standard test blocks prepared during the molding process of Example 2, Comparative Example 2, and Comparative Example 5.

[0075] After the slurry fills the mold and is formed, it is placed in an indoor environment at a temperature of 20±2℃ and left to stand with the mold on. When the curing age is close to 1h, 2h, 4h, 8h and 24h respectively, the demolding operation is carried out in sequence. For the test blocks with low early strength and difficult demolding, the air pump is used to slightly assist in demolding to avoid edge damage.

[0076] The actual pressure area of ​​the demolded specimen was measured using vernier calipers. Free moisture and release agent residue on the surface of the specimen were wiped away, and then it was moved to the center of the lower pressure plate of the computer-controlled universal testing machine.

[0077] The pressurization rate of the testing machine loading system was set to 2.4 kN / s. The equipment was started to continuously and uniformly load the specimen until it underwent penetrating failure. The peak failure load captured by the control system was recorded.

[0078] The load value read is divided by the actual compressive area of ​​the specimen to calculate the ultra-early compressive strength at each time point. The arithmetic mean of the test results of three independent specimens of the same age and the same formula is calculated.

[0079] The experimental data for the above tests are summarized below.

[0080] Table 4. Comparison of early compressive strength test data between Example 2 and the comparative example. Figure 4 The graph illustrates the evolution of uniaxial compressive strength of Example 2 and two comparative specimens during the early period from 1 hour to 24 hours. The horizontal axis represents the test age, and the vertical axis represents the ultra-early compressive strength value. In the figure, the black solid line marked with a black upper triangle represents Example 2, the dark gray dashed line marked with a white lower triangle represents Comparative Example 2, and the light gray dotted line marked with a light gray square represents Comparative Example 5.

[0081] Summarize: Figure 4 As shown in Table 4, Example 2 exhibited a compressive strength of 9.4 MPa at 1 hour and 18.9 MPa at 2 hours. The strength gradually increased thereafter, reaching 23.5 MPa at 4 hours and 27.2 MPa at 8 hours, peaking at 31.4 MPa at 24 hours. Comparative Example 2, using conventional dihydrate gypsum instead of hemihydrate gypsum, showed a strength of 1.2 MPa at 1 hour, with a slower early strength increase. At 24 hours, the compressive strength was 18.3 MPa, lower than that of Example 2. Conventional dihydrate gypsum primarily functions as a filler or slowly participates in later reactions, failing to achieve significant hydration in the initial stages. In this scheme, the dihydrate gypsum crystals generated by the hemihydrate gypsum formed the initial framework and provided an adhesion substrate for ettringite; both synergistically contributed to the early strength.

[0082] Comparative Example 5, which employed a one-time mixing and stirring process for all dry powders and the liquid phase, exhibited low strength at all ages, with a 1-hour strength of 4.5 MPa and a 24-hour strength of 21.2 MPa. This one-time stirring introduced the strong alkaline activator into the system at the initial stage of water addition, causing localized flocculation of the slurry before it was uniformly dispersed, resulting in internal defects and pores that hindered the bonding of hydration products. Example 2, employing a step-by-step feeding and stirring process, delayed the introduction of the strong alkaline component, ensuring dispersibility and providing a uniform reaction space for crystal growth.

[0083] Test Example 5: The experimental subjects were standard test blocks of 50mm×50mm×50mm prepared in Example 2, Comparative Example 1 and Comparative Example 4 and which entered the mid-to-late stage of curing.

[0084] Example and comparative specimens that were cured in a standard constant temperature and humidity curing chamber (temperature 20±2℃, relative humidity not less than 95%) for a specific age period (3d, 7d, 14d, 28d) were selected and removed from the curing environment.

[0085] Wipe away any free moisture from the surface of the test block with a slightly damp cotton cloth, ensuring that the bearing surface is flat and free of debris. Then place it in the center area of ​​the lower bearing plate of the microcomputer-controlled electro-hydraulic servo universal testing machine.

[0086] Adjust the pressure plate on the testing machine to make it evenly contact the surface of the test block, set the pressure rate of the loading system to 2.4 kN / s, apply a continuous and uniform axial load to the specimen until the specimen undergoes macroscopic fracture failure, and record the peak failure load captured by the system.

[0087] The failure load was converted into a uniaxial compressive strength value based on the pressure area. Three independent test blocks were tested in parallel for each formula at each age. After removing outliers, the arithmetic mean was calculated as the final compressive strength at that age.

[0088] The experimental data for the above tests are summarized below.

[0089] Table 5. Comparison of mid-to-late stage compressive strength test data between Example 2 and the comparative example. Figure 5 The graph illustrates the evolution of uniaxial compressive strength of Example 2 and two comparative specimens during the mid-to-late curing stage, from 3 days to 28 days. The horizontal axis represents the corresponding curing age, and the vertical axis represents the mid-to-late curing compressive strength values. In the figure, the black solid line marked with a solid black pentagram represents Example 2, the dark gray dashed line marked with a white-filled circle represents Comparative Example 1, and the gray dotted line marked with a light gray-filled diamond represents Comparative Example 4.

[0090] Summary: Table 5 and Figure 5 The results show that the compressive strength of Example 2 increased from 3 to 28 days. Its initial compressive strength at 3 days was 32.4 MPa, subsequently reaching 43.1 MPa and 49.3 MPa at 7 days and 14 days respectively, and finally reaching 54.6 MPa at 28 days. Comparative Example 1, lacking slag powder, showed a stagnation and decrease in strength in the later stages. Its 3-day strength was 28.1 MPa, increasing to 31.8 MPa at 14 days, but decreasing to 30.2 MPa at 28 days. In this example, slag powder was introduced, utilizing the initial alkalinity and heat of hydration to generate hydrated calcium silicate gel, which encapsulates the initial crystal framework and fills the pores.

[0091] Comparative Example 4, lacking the composite activator, exhibited a gradual increase in later-stage strength, with a 3-day strength of 29.5 MPa, increasing to 35.8 MPa at 14 days and 37.1 MPa at 28 days. Under weakly alkaline or neutral conditions, the glassy structure on the surface of slag powder is difficult to depolymerize. The composite activator provides a strongly alkaline environment, promoting the breaking of chemical bonds on the surface of slag particles, releasing ions to participate in secondary hydration, and forming a later-stage gel-filled structure.

[0092] Test Example 6: The experimental subjects were the initial flowable mixed slurry prepared in Example 2, Comparative Example 3 and Comparative Example 6, and a standard sandstone block of 50mm×50mm×50mm that had been pre-polished and cut at a 45-degree angle.

[0093] Take a half-piece of sandstone with a 45-degree bevel, wash off the surface dust with clean water and let it air dry in a cool place until it is surface dry. Then place it with the bevel facing up into a standard 50mm×50mm×50mm cube mold, occupying half of the mold space.

[0094] The test slurry, which is in an initial flow state, is slowly poured into the remaining space of the test mold. Micro-vibration is performed on the vibration table for about 15 seconds to ensure that the slurry is fully wetted and conforms to the rough inclined section of the sandstone. Excess slurry on the surface is scraped off and smoothed.

[0095] The mold containing the composite specimen was transferred into a standard constant temperature and humidity curing chamber at a temperature of 20±2℃ and a relative humidity of not less than 95% and left to stand for 24 hours. After the slurry hardened and had sufficient strength, the outer mold was removed and the sandstone support material composite specimen was put back into the curing chamber for further curing.

[0096] When the composite specimens have been cured for 1 day, 3 days, 7 days and 28 days, they are taken out in batches and placed on the bearing pad of the universal testing machine. Continuous axial load is applied to the specimens at a loading rate of 0.5 MPa / s until shear slip failure occurs along the interface.

[0097] The peak load at the moment of failure was recorded and divided by the actual contact area of ​​the 45-degree oblique section to calculate the shear bond strength at each age. Three composite specimens of each formulation were tested in parallel at each test age, and the arithmetic mean was taken.

[0098] The experimental data for the above tests are summarized below.

[0099] Table 6. Test data of interfacial bond strength at different ages for Example 2 and Comparative Example. Figure 6 The graph illustrates the evolution of the shear bond strength between the slurry prepared in Example 2 and the sandstone interface during the curing period from 1 day to 28 days. The horizontal axis represents the corresponding curing age, and the vertical axis represents the calculated interfacial bond strength value. In the figure, the solid black line marked with a right-facing black triangle represents Example 2, the dark gray dashed line marked with a left-facing white triangle represents Comparative Example 3, and the black dotted line marked with a down-facing light gray triangle represents Comparative Example 6.

[0100] Summary: Combining Table 6 and... Figure 6 Analysis shows that the interfacial bond strength in Example 2 increases with age, reaching 1.28 MPa at 1 day, then increasing to 2.36 MPa and 3.14 MPa at 3 and 7 days respectively, finally reaching 4.47 MPa at 28 days. The slurry initially exhibits low yield stress and wettability. After penetrating the surface fissures of the sandstone, the hydrated ettringite and dihydrate gypsum crystals form a physical anchor. The micro-expansion characteristics compensate for chemical shrinkage, and the resulting compressive stress causes the matrix to compress against the rock surface, increasing the mechanical interlocking force at the interface.

[0101] Comparative Example 3, without the addition of a composite expanding agent, had a 1-day bond strength of 0.94 MPa. Due to the lack of subsequent extrusion pressure, the strength growth was slow, with a 28-day bond strength of 1.83 MPa. The loss of volume compensation led to internal moisture loss, causing self-shrinkage and resulting in shrinkage stress and microcracks at the contact surface, affecting the interfacial structure. Comparative Example 6, with an adjusted activator ratio, showed a decrease in early water retention, a 1-day strength of 0.52 MPa, and a final 28-day strength of 2.08 MPa, lower than Example 2. The failure to uniformly wet the rock surface in the early stages altered the local water-cement ratio, making it difficult for the gel to compensate for the initial voids.

[0102] Test Example 7: This test primarily evaluates the retention of the material's working performance in the short time before initial setting. The experimental subjects were the mixed slurries prepared in the initial flow state according to Example 2, Comparative Example 7, and Comparative Example 8. Since the aforementioned test examples verified that the material would initially set within 15 to 20 minutes, this test examines the loss of flowability over time within this time window.

[0103] Place a dry and clean 600mm×600mm square glass plate on a flat, level experimental platform. Place a standard truncated cone mold with an upper diameter of 36mm, a lower diameter of 60mm, and a height of 60mm in the center of the glass plate. Apply a very small amount of lubricating oil to the inner wall of the mold to reduce the frictional resistance of the side walls.

[0104] Quickly and continuously pour the freshly mixed slurry into the truncated cone mold until the slurry is level with the upper edge of the mold. Smooth the surface with a ruler, let it stand for about 5 seconds, and then lift the mold vertically and steadily upwards, allowing the slurry to fall freely onto the glass plate and spread outwards under its own weight.

[0105] After the slurry stops flowing, measure the maximum diameter of the slurry paving body in two mutually perpendicular directions, take the arithmetic mean as the initial flowability, and record this moment as 0min.

[0106] The remaining mixed slurry was kept moist in a covered mixing pot. After standing for 3 min, 6 min, 9 min, 12 min and 15 min respectively, the mixer was restarted and slowly stirred for 10 seconds to restore the rheological state. Then the aforementioned truncated cone mold paving test steps were repeated to measure and record the flowability data at each time point.

[0107] The experimental data for the above tests are summarized below.

[0108] Table 7. Test data on the short-term flowability of slurries in Example 2 and the comparative example over time. Figure 7 The decay process of flowability of each group of slurries within a time window of 0 min to 15 min is shown. Subfigure (a) shows the decay curve of the absolute value of slurry flowability over time, and subfigure (b) shows the evolution of flowability retention rate calculated based on the initial flowability over time. A 40% construction warning line is set in subfigure (b) to visually assess whether the material meets the minimum workability requirements for casting. In the attached figures, the black solid line marked with a black solid hexagon represents Example 2, the dark gray dashed line marked with a white filled square represents Comparative Example 7, and the gray dotted line marked with a light gray filled diamond represents Comparative Example 8.

[0109] Summary: Combining Table 7 and... Figure 7Analysis shows that the initial (0 min) fluidity of each group of slurries is relatively similar, ranging from 225.3 mm to 236.4 mm. The decay patterns differ over time. In Example 2, the fluidity remained at 228.1 mm and 215.3 mm at 3 min and 6 min, respectively, and still maintained a fluidity of 190.5 mm at 9 min, with a fluidity retention rate of 80.6%. At 12 min, the fluidity was 142.6 mm, and its retention rate remained above the construction warning line; at 15 min, it decreased to 85.2 mm and approached initial setting. This operating window benefited from the matching of the step-by-step feeding process and the composite admixture. In the initial stage of mixing dry powder with water, sodium sulfate and hemihydrate gypsum preferentially dissolved, establishing a preliminary suspension system and inhibiting local flocculation.

[0110] Comparative Example 7 used calcium formate as an early-strength agent, but lacked the buffering effect of sulfate ions, leading to an increase in the concentration of calcium ions in the liquid phase. The fluidity of its slurry decreased to 195.2 mm at 3 minutes and further to 140.6 mm after 6 minutes. Its fluidity retention rate decreased to 37.0% at 9 minutes (absolute fluidity of 85.4 mm), below the construction warning line. It lost its fluidity at 12 minutes, reducing the construction tolerance. Comparative Example 8 directly used sodium silicate as an activator. Its initial fluidity was 225.3 mm, which decreased to 182.4 mm and 115.8 mm at 3 and 6 minutes, respectively. Because the strongly alkaline free silicate ions reacted with the hydrated calcium ions to form amorphous flocculent calcium silicate, it caused false setting of the slurry, resulting in a loss of fluidity at 9 minutes, making casting tests impossible.

Claims

1. A method for preparing a fast-hardening dry powder support material for underground coal mines, characterized in that, include: At room temperature, 40%-50% of the total dry powder mass of sulfoaluminate cement, 10% of the total dry powder mass of slag powder, and 40%-50% of the total dry powder mass of hemihydrate gypsum are mixed in proportion to form a uniform ternary mixed dry powder. A composite early strength agent comprising 0.5%-2% of the mass of the sulfoaluminate cement is mixed with a quick-setting agent comprising 4% of the mass of the total dry powder to obtain a mixed dry material. The mixed dry material is then added to the ternary mixed dry powder along with water weighed at a water-cement ratio of 0.45, and stirred with a mixer for 30 seconds to form an initial slurry. Add 2.5%-4% of the composite expansion agent (by mass of the sulfoaluminate cement) and 0.5%-2% of the composite activator (by mass of the slag powder) to the initial slurry, and then continue stirring for 3 minutes to form a stable and uniform slurry. Pour the stable and uniform slurry into the mold; After the slurry poured into the mold is allowed to stand, it is demolded. The demolded slurry is then cured to finally produce a fast-hardening dry powder support material for underground coal mines.

2. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 1, characterized in that, When preparing the ternary mixed dry powder, the sulfoaluminate cement is P.O42.5 sulfoaluminate cement, and the slag powder is S105 slag powder; The amounts of the sulfoaluminate cement, the slag powder, and the hemihydrate gypsum are respectively: The sulfoaluminate cement accounts for 50% of the total dry powder mass, the slag powder accounts for 10% of the total dry powder mass, and the hemihydrate gypsum accounts for 40% of the total dry powder mass.

3. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 1, characterized in that: The amount of the composite early strength agent added is 2% of the mass of the sulfoaluminate cement; The amount of the composite expansive agent added is 4% of the mass of the sulfoaluminate cement; The amount of the composite activator added is 0.5% of the mass of the slag powder.

4. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 1, characterized in that, The composite early strength agent is composed of calcium formate and sodium sulfate.

5. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 4, characterized in that, The preparation method of the composite early strength agent is as follows: At room temperature, the calcium formate and sodium sulfate are weighed in a mass ratio of 1:

2. The calcium formate and sodium sulfate are placed in a mixing device and stirred and ground thoroughly until they are mixed evenly to obtain the composite early strength agent.

6. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 1, characterized in that, The composite expanding agent consists of AEA sulfoaluminate type expanding agent and TD-P1 calcium oxide type expanding agent.

7. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 6, characterized in that, The preparation method of the composite expanding agent is as follows: At room temperature, the AEA sulfoaluminate type expanding agent and the TD-P1 calcium oxide type expanding agent are weighed in a mass ratio of 1:

2. The AEA sulfoaluminate type expanding agent and the TD-P1 calcium oxide type expanding agent are placed in a mixing device and stirred thoroughly until they are mixed evenly to obtain the composite expanding agent.

8. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 1, characterized in that, The composite activator is composed of sodium hydroxide and calcium hydroxide.

9. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 8, characterized in that, The preparation method of the composite activator is as follows: At room temperature, the sodium hydroxide and calcium hydroxide are weighed in a mass ratio of 1.5:1, and the sodium hydroxide and calcium hydroxide are placed in a mixing device and stirred thoroughly until they are mixed evenly to obtain the composite activator.

10. The method for preparing the rapid-hardening dry powder support material for underground coal mines according to claim 1, characterized in that, The slurry poured into the mold was allowed to stand for 2 hours. The post-demolding curing steps include: Curing time after demolding is 3 days, 7 days, and 28 days.

Images