Vertical buried pipe backfill material with phase change temperature regulation function and preparation method

Abstract

The invention discloses a vertical buried pipe backfill material with a phase change temperature regulation function and a preparation method, and belongs to the technical field of ground source heat pump system engineering materials. The material comprises the following components in percentage by weight: 30-45% of a cement-based cementing material, 10-20% of a multifunctional phase change microcapsule, 25-40% of aggregate, 2-8% of a functional additive and 8-20% of water, the multifunctional phase change microcapsule is composed of a paraffin and CaCl2 particle inner core, a bacillus pasteurii spore middle layer and a temperature-sensitive polymer shell. The temperature fluctuation is stabilized through paraffin phase change, graded repair of cracks is triggered through temperature-sensitive response, and the temperature-sensitive self-repairing material has the characteristics of efficient temperature adjustment, active self-repairing, high stability and excellent comprehensive performance, is simple and controllable in preparation process, is suitable for long-term stable operation of a vertical buried pipe system, and has wide application prospects. The problems that a traditional material is single in temperature adjustment, prone to cracking and ineffective in sealing are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of engineering materials technology for ground source heat pump systems, and in particular to a backfill material for vertical buried pipes with phase change temperature regulation function and its preparation method. Background Technology

[0002] Ground source heat pump systems, as a highly efficient and environmentally friendly renewable energy utilization technology, have been widely applied in building heating, cooling, and domestic hot water supply due to their advantages of low energy consumption and low emissions. Vertical buried pipes, as the core heat exchange component of the ground source heat pump system, transfer energy through heat exchange with the underground soil. Their heat exchange efficiency directly determines the overall operating performance and energy-saving effect of the system. Backfill material, as the key medium filling the space between the vertical buried pipes and the borehole wall, not only undertakes the basic functions of fixing the buried pipes and sealing the borehole, but also needs to possess excellent heat transfer performance, sufficient mechanical strength, and play a crucial role in mitigating soil temperature fluctuations around the buried pipes and preventing heat accumulation during long-term operation. It is a vital foundation for ensuring the long-term stable and efficient operation of the system.

[0003] In practical engineering applications, traditional backfill materials for vertical buried pipes mainly consist of cement-based composite materials, sand-soil mixtures, and bentonite. However, these traditional materials have gradually revealed numerous technical defects during service, making it difficult to meet the high-quality operation requirements of ground source heat pump systems. On the one hand, traditional backfill materials lack temperature regulation capabilities and cannot effectively mitigate soil temperature fluctuations. During long-term operation of a ground source heat pump system, there is often an imbalance between the heat discharged into the ground in summer and the heat extracted from the ground in winter, leading to hot or cold accumulation in the soil around the buried pipes. As the years of operation increase, the soil temperature deviates from the original equilibrium temperature, reducing the heat exchange temperature difference between the buried pipes and the soil, significantly decreasing the system's heat exchange efficiency, and even causing compressor overload and a sharp increase in energy consumption, severely restricting the application of ground source heat pump systems in large-scale buildings or long-term service scenarios.

[0004] To address temperature regulation issues, the industry has explored incorporating phase change materials (PCMs) into backfill material systems. PCMs can absorb, store, and release significant amounts of latent heat through solid-liquid or liquid-solid phase change processes, effectively mitigating environmental temperature fluctuations and theoretically significantly improving the thermal stability of backfill materials. However, existing PCM backfill materials still face insurmountable technical bottlenecks in practical applications: First, PCMs are prone to leakage in their softened and molten state, leading not only to latent heat loss and continuous degradation of temperature regulation performance but also potential pollution of the surrounding soil environment, affecting the long-term service stability of the material; second, backfill materials are highly susceptible to micro-cracks and even macro-cracks during the curing process due to heat release and volume shrinkage, or during service due to factors such as temperature stress and mechanical vibration. These cracks can compromise the sealing of the backfill material, allowing groundwater to seep into the borehole, increasing the thermal resistance between the buried pipe and the soil, and further reducing heat exchange efficiency. More importantly, existing phase change backfill materials lack the ability to actively repair cracks. Once cracks appear, they continue to expand, eventually causing the backfill layer to fail. This requires excavation and repair, which is not only costly to maintain but also affects the normal operation of the system.

[0005] Traditional backfill materials also have significant shortcomings in balancing overall performance. Some materials, in pursuit of higher mechanical strength, often sacrifice thermal transfer properties, resulting in low thermal conductivity; while others that prioritize heat transfer suffer from insufficient mechanical strength and are prone to weathering and cracking. Furthermore, traditional materials struggle to balance construction fluidity and volumetric stability. Poor fluidity leads to difficulties in deep-hole grouting, resulting in loose backfill and voids; excessive fluidity, on the other hand, easily causes segregation, affecting the overall uniformity of the material's performance. These problems combine to make it impossible for traditional backfill materials and existing phase change modified materials to meet the comprehensive requirements of vertical buried pipe systems for efficient heat transfer, stable temperature regulation, mechanical reliability, and crack resistance and self-healing properties in backfill materials.

[0006] With the continuous improvement of building energy efficiency standards and the ongoing expansion of ground source heat pump system applications, the market demands increasingly stringent performance requirements for backfill materials used in vertical buried pipes. Developing a new type of backfill material that combines efficient phase change temperature regulation, active self-healing capabilities for cracks, and meets engineering requirements in terms of mechanical strength, heat transfer performance, workability, and long-term stability has become crucial for overcoming existing technological bottlenecks and improving the operating efficiency and service life of ground source heat pump systems. This has significant practical implications and engineering value for promoting the further application of ground source heat pump technology and fostering the development of the renewable energy utilization industry. Summary of the Invention

[0007] The purpose of this invention is to provide a backfill material for vertical buried pipes with phase change temperature regulation function and its preparation method, so as to solve the problems existing in the prior art. The backfill material for vertical buried pipes provided by this invention can smooth temperature fluctuations through a phase change process, and at the same time achieve active sealing and repair when cracks occur.

[0008] To achieve the above objectives, the present invention provides the following solution: This invention provides a backfill material for vertical buried pipes with phase change temperature regulation function. The raw materials, by weight percentage, comprise the following components: Cement-based cementitious materials 30-45%, multifunctional phase change microcapsules 10-20%, aggregates 25-40%, functional additives 2-8%, water 8-20%; The multifunctional phase change microcapsule consists of a core, a middle layer, and a shell. The core is a mixture of paraffin and CaCl2 particles, the middle layer is Bacillus pasteurellii spores, and the shell is a thermosensitive polymer.

[0009] Furthermore, in the multifunctional phase change microcapsule, the weight ratio of paraffin to CaCl2 particles is 5-8:1; the weight of the Bacillus pasteurellii spores is 5-10% of the core weight; and the thermosensitive polymer is selected from at least one of poly(N-isopropylacrylamide), polyethylene glycol derivatives, and ethylene-vinyl acetate copolymers.

[0010] Furthermore, the thickness of the outer shell is 5-20 μm, and the thermal softening temperature is 55-65℃.

[0011] Furthermore, taking the total amount of cement-based cementitious materials as 100%, the cement-based cementitious materials are composed of the following components in the following weight ratios: 70-85% ordinary Portland cement, 10-20% fly ash, and 5-10% coal gangue.

[0012] Furthermore, the aggregate is composed of quartz sand and slag in a weight ratio of 3-5:1, wherein the particle size of the quartz sand is 0.15-2.36 mm.

[0013] Furthermore, based on the total amount of functional additives being 100%, the functional additives include the following components in the following weight ratios: 30-50% water-reducing agent, 5-15% air-entraining agent, 20-40% thermal conductivity enhancer, and 5-15% water-retaining agent.

[0014] Furthermore, the thermal conductivity enhancer is selected from graphite powder or aluminate cement.

[0015] This invention also provides a method for preparing the above-mentioned vertical buried pipe backfill material with phase change temperature regulation function, comprising the following steps: Preparation of multifunctional phase change microcapsules: Paraffin wax was melted and mixed with CaCl2 to obtain a core mixture. Then, a suspension of Bacillus pasteurellis spores was sprayed on the surface of the core mixture to form a middle layer coating structure. Subsequently, a temperature-sensitive polymer shell was coated on the middle layer using interfacial polymerization. After drying, multifunctional phase change microcapsules were obtained. Preparation of dry mix: Cement-based cementitious materials and aggregates are mixed to obtain dry mix; Preparation of the mixture: The multifunctional phase change microcapsules and the functional additives are added to the dry mixture, and then water is added and stirred. The spread of the backfill material slurry is controlled to be 200-250mm to obtain the vertical buried pipe backfill material with phase change temperature regulation function.

[0016] Furthermore, when the paraffin wax is melted and mixed with CaCl2, the stirring speed is 300-500 r / min and the stirring time is 10-15 minutes.

[0017] Furthermore, the reaction temperature of the interfacial polymerization method is 40-60℃, and the reaction time is 1-2h.

[0018] Furthermore, the stirring rate used in the preparation by interfacial polymerization is preferably 300-500 r / min.

[0019] Furthermore, the concentration of the *Bacillus pasteurellii* spore suspension is 10. 8 -10 9 per mL.

[0020] The present invention further provides the application of the above-mentioned vertical buried pipe backfill material with phase change temperature regulation function in vertical buried pipe backfilling.

[0021] The vertical buried pipe backfill material of this invention has phase change temperature regulation and crack self-healing properties. Through the precise structural design of multifunctional phase change microcapsules and the synergistic effect of each component, it systematically solves the defects of traditional backfill materials, such as insufficient temperature regulation, easy cracking, and inability to achieve self-healing. The phase change temperature regulation mechanism of this invention achieves a dynamic balance between heat energy storage and release through the phase change process of paraffin wax in the core of multifunctional phase change microcapsules, thus mitigating soil temperature fluctuations around buried pipes. The paraffin wax in the microcapsule core has a phase change temperature of 20-40℃ and a latent heat of phase change of 150-200 J / g, making it a key carrier for the temperature regulation function. Meanwhile, the temperature-sensitive polymer shell (thermotropic softening temperature 55-65℃) remains intact within the normal temperature regulation range, effectively preventing paraffin wax leakage. When the local source heat pump system cools in summer or the ambient temperature rises, and the soil temperature reaches the paraffin wax phase change temperature, the paraffin wax melts from a solid to a liquid state, absorbing a large amount of surrounding heat and storing it as latent heat, preventing heat accumulation that could reduce the heat exchange temperature difference. When the system heats in winter or the ambient temperature drops, and the soil temperature falls below the paraffin wax's freezing point, the liquid paraffin wax solidifies, releasing the stored latent heat, slowing the rate of temperature drop, and preventing cold accumulation. Even after 500 phase change cycles, it still retains an extremely high latent heat of paraffin, which, combined with the protective effect of the temperature-sensitive polymer shell, ensures that the temperature regulation function remains stable and effective over a long period of time.

[0022] This invention employs a temperature-sensitive response and tiered repair mode for crack self-healing. Through the synergy of temporary sealing and permanent plugging, it actively repairs cracks generated during material curing or service life. When backfill material cracks due to hydration shrinkage, temperature stress, or mechanical vibration, stress concentration and localized temperature rise occur at the crack tip. On one hand, under early hydration heat release or localized heat accumulation conditions, the temperature at the crack can rise above the thermo-softening temperature (55-65°C) of the temperature-sensitive polymer shell, causing the shell to soften or melt and rupture, releasing the core repair agent and triggering the repair process. On the other hand, under normal or even low-temperature conditions during service life, even if the temperature at the crack does not reach the shell softening temperature, some microcapsules located on the crack surface can still mechanically rupture under tensile deformation and cracking strain, similarly releasing repair components such as paraffin and CaCl2. After the crack is connected, groundwater or pore water seeps into the crack, activating the Bacillus pasteurellii spores in the middle layer within an ambient temperature range of 5-40°C. This induces calcium carbonate precipitation, which, combined with the self-healing properties of cement-based materials, seals the crack, thus achieving active self-healing from low to high temperatures. First, the paraffin wax in the core rapidly flows out, filling the cracks with its fluidity to form a continuous liquid sealing layer, temporarily sealing the cracks and preventing groundwater infiltration and heat loss, thus preventing further crack expansion. Simultaneously, CaCl2 particles mixed with the paraffin wax seep into the cracks, dissolving in the water to create a high-salinity environment that activates the Pasteurella spores in the middle layer. The activated Pasteurella spores secrete highly active urease, catalyzing the hydrolysis of urea to produce ammonia and carbonate ions. These carbonate ions combine with calcium ions produced during the hydration of cement-based materials, forming water-insoluble calcium carbonate precipitate, which gradually fills the crack pores, forming a dense, permanently bonded sealing layer. In the example, after 28 days of indoor curing, for specimens with an initial crack width of no more than 0.40 mm, the crack width closure rate and / or impermeability recovery rate can reach over 80%.

[0023] In this invention, auxiliary components such as cement-based binders, aggregates, and functional additives provide crucial support for phase change temperature regulation and crack self-healing, as detailed below: Cement-based cementitious materials (30-45%) serve as the matrix: ordinary silicate cement provides basic mechanical strength, fly ash fills micropores to improve density, and coal gangue improves volume stability while providing a source of calcium ions for calcium carbonate precipitation; in aggregates (25-40%), quartz sand (particle size 0.15-2.36mm) forms a skeleton to support and improve overall rigidity, and slag serves as a lightweight porous aggregate to optimize particle size distribution and help improve heat transfer uniformity. Functional additives (2-8%) specifically optimize performance: water-reducing agents (30-50%) improve slurry fluidity and ensure uniform dispersion of microcapsules; air-entraining agents (5-15%) introduce microbubbles to alleviate hydration thermal stress and reduce crack initiation; thermal conductivity enhancers (20-40%) increase the overall thermal conductivity of the material (1.2-1.8 W / (m•K)) to ensure heat transfer efficiency and mechanism triggering sensitivity; and water-retaining agents (5-15%) reduce moisture evaporation and provide a continuous humidity environment for cement hydration and bacterial repair.

[0024] This invention reduces temperature fluctuations and lowers the probability of cracks caused by thermal stress through phase change temperature regulation. A self-healing mechanism repairs existing cracks, preventing increased thermal resistance from affecting heat transfer efficiency and ensuring the uniformity of temperature regulation. The cement-based matrix and aggregates provide stable structural support, while functional additives optimize workability and overall performance, laying the foundation for long-term operation of phase change temperature regulation and crack self-healing. This multi-dimensional synergy ultimately achieves a virtuous cycle of temperature stability, crack self-healing, and long-term performance maintenance, overcoming the technical shortcomings of traditional vertical ground pipe backfill materials.

[0025] The present invention discloses the following technical effects: This invention provides a backfill material for vertical buried pipes with phase change temperature regulation function, achieving breakthroughs and optimizations over traditional backfill materials in multiple dimensions. Through the core function of multifunctional phase change microcapsules, a highly efficient phase change temperature regulation function is achieved. The paraffin in its core has a high latent heat of phase change of 150-200 J / g, which can effectively absorb or release heat when the ambient temperature changes, reducing the temperature fluctuation range of the soil around the buried pipe by 30-50%, significantly improving the thermal stability of the ground source heat pump system, and avoiding the problems of heat or cold accumulation caused by long-term operation.

[0026] Based on the structural design of temperature-sensitive microcapsules, this invention also has an active self-healing function. When cracks occur in the backfill material, paraffin can quickly flow out for temporary sealing. Subsequently, calcium carbonate precipitate is generated by Bacillus pasteurellium catalysis to achieve permanent sealing. The repair efficiency is over 80%, which greatly extends the service life of the backfill material and solves the pain point of sealing failure after cracking of traditional materials.

[0027] The material of this invention exhibits excellent stability. The temperature-sensitive polymer shell effectively prevents leakage at the normal phase transition temperature of paraffin and retains an extremely high latent heat retention rate after 500 phase transition cycles, ensuring long-term stability of the temperature regulation function. Simultaneously, the microcapsules demonstrate good compatibility with cement-based materials, exhibiting no significant interface delamination during service, thus guaranteeing the integrity of the overall material structure.

[0028] The backfill material of this invention also exhibits excellent comprehensive performance, with a 28-day compressive strength of 12-18 MPa and a thermal conductivity of 1.2-1.8 W / (m•K), fully meeting the core requirements for mechanical strength and heat transfer efficiency in vertical buried pipe backfilling projects. Furthermore, the slurry spread is controlled at 200-250 mm, exhibiting good fluidity and adaptability to drilling and grouting operations at different depths, making operation convenient and efficient.

[0029] The preparation process of this invention is simple and controllable. It adopts conventional stirring equipment and mature microcapsule preparation technology, without the need for additional special equipment. The production cost is easy to control and it is suitable for large-scale industrial production, providing an economical and feasible material solution for the engineering application of ground source heat pump systems. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the vertical buried pipe heat exchange system and its backfill structure according to an embodiment of the present invention; Among them: 1: surface energy storage station; 2: U-shaped underground pipe; 3: filling material; 4: stratum; 5: microcapsules before phase change; 6: microcapsules after phase change; 7: functional additives. Detailed Implementation

[0032] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0033] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0034] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0035] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0036] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0037] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0038] Unless otherwise specified, all figures in the following embodiments of the present invention are based on weight percentages.

[0039] Comparative Example 1 This embodiment provides a backfill material for vertical buried pipes. The components and their proportions by weight percentage are as follows: cement-based cementitious material 45%, aggregate 30%, functional additives 7%, and water 18%.

[0040] The cement-based cementitious material consists of 33% ordinary Portland cement, 8% fly ash, and 4% coal gangue (the proportions of the three are calculated based on the total weight of the materials, totaling 45%); the aggregate consists of 25% quartz sand and 5% slag (totaling 30% of the total weight of the materials); and the functional additives consist of 4% polycarboxylate-based high-efficiency water-reducing agent, 0.5% alkyl sulfonate air-entraining agent, 2% aluminate cement, and 0.5% hydroxypropyl methylcellulose water-retaining agent (totaling 7% of the total weight of the materials).

[0041] The preparation steps for this backfill material are as follows: Step S1: Prepare dry mix: Add ordinary silicate cement, fly ash, coal gangue, quartz sand and slag together to a forced mixing equipment and dry mix for 4 minutes until all components are evenly dispersed.

[0042] Step S2: Prepare the mixture: Add water-reducing agent, air-entraining agent, aluminate cement and water-retaining agent to the above dry mixture, stir for 6 minutes to make the components evenly distributed, then add the metered water, continue stirring for 12 minutes, control the slurry expansion to 220mm, and you will get the vertical buried pipe backfill material with phase change temperature regulation function.

[0043] Example 1 This embodiment provides a backfill material for vertical buried pipes with phase change temperature regulation function. The raw material composition is as follows by weight percentage: 40% cement-based cementitious material, 12% multifunctional phase change microcapsules, 30% aggregate, 7% functional additives, and 11% water.

[0044] The cement-based cementitious material consists of 28% ordinary silicate cement, 8% fly ash, and 4% coal gangue (the proportions of these three components are calculated based on the total weight of the materials, totaling 40%). The multifunctional phase change microcapsules are configured according to their own weight as follows: the core is paraffin wax 9.6% and CaCl2 particles 1.6%, the middle layer is Bacillus pasteurellosis spores 0.6%, and the outer shell is poly(N-isopropylacrylamide) 0.2% (these four components total 100% of the total weight of the multifunctional phase change microcapsules, corresponding to 12% of the total weight of the materials). The aggregate consists of 25% quartz sand and 5% slag (totaling 30% of the total weight of the materials). The functional additives consist of 3% polycarboxylate superplasticizer, 1% alkyl sulfonate air-entraining agent, 2% aluminate cement, and 1% hydroxypropyl methylcellulose water-retaining agent (totaling 7% of the total weight of the materials).

[0045] The preparation steps for this backfill material are as follows: Step S1: Preparation of multifunctional phase change microcapsules: (1) Heat the paraffin to 70°C to soften and melt it, add CaCl2 particles, and stir at 400 r / min for 12 minutes to form a uniform core mixture; (2) A spray drying method was used to spray a coating with a concentration of 5×10 on the surface of the above-mentioned core mixture. 8 A suspension of Bacillus pasteurellium spores per mL forms a middle layer coating structure; (3) Using interfacial polymerization, the reaction was stirred at 400 r / min for 1.5 hours at 50℃. The middle layer structure was coated with a poly(N-isopropylacrylamide) shell (shell thickness was 10 μm, thermal softening temperature was 60℃), and then dried for later use.

[0046] Step S2: Prepare dry mix: Add ordinary silicate cement, fly ash, coal gangue, quartz sand and slag together to a forced mixing equipment and dry mix for 4 minutes until all components are evenly dispersed.

[0047] Step S3: Preparation of the mixture: Add the multifunctional phase change microcapsules, water-reducing agent, air-entraining agent, aluminate cement and water-retaining agent prepared in step S1 to the above dry mixture, stir for 6 minutes to make the microcapsules and each additive evenly distributed, then add the metered water, continue stirring for 12 minutes, control the slurry expansion to 220mm, and thus obtain the vertical buried pipe backfill material with phase change temperature regulation function.

[0048] Example 2 This embodiment provides a backfill material for vertical buried pipes with phase change temperature regulation function. The components and their proportions by weight percentage are as follows: cement-based cementitious material 30%, aggregate 30%, multifunctional phase change microcapsules 15%, functional additives 7%, and water 18%.

[0049] The cement-based cementitious material consists of 20% ordinary silicate cement, 6% fly ash, and 4% coal gangue (the proportions of these three components are calculated based on the total weight of the materials, totaling 30%). The multifunctional phase change microcapsules are configured according to their own weight as follows: the core is paraffin wax 12% and CaCl2 particles 2%, the middle layer is Bacillus pasteurellosis spores 0.75%, and the outer shell is poly(N-isopropylacrylamide) 0.25% (these four components total 100% of the total weight of the multifunctional phase change microcapsules, corresponding to 15% of the total weight of the materials). The aggregate consists of 25% quartz sand and 5% slag (totaling 30% of the total weight of the materials). The functional additives consist of 4% polycarboxylate superplasticizer, 0.5% alkyl sulfonate air-entraining agent, 2% aluminate cement, and 0.5% hydroxypropyl methylcellulose water-retaining agent (totaling 7% of the total weight of the materials).

[0050] The preparation steps for this backfill material are as follows: Step S1: Preparation of multifunctional phase change microcapsules: (1) Heat the paraffin to 70°C to soften and melt it, add CaCl2 particles, and stir at 400 r / min for 12 minutes to form a uniform core mixture; (2) A spray drying method was used to spray a coating with a concentration of 5×10 on the surface of the above-mentioned core mixture. 8 A suspension of Bacillus pasteurellium spores per mL forms a middle layer coating structure; (3) Using the interfacial polymerization method, the reaction was stirred at 400 r / min for 1.5 hours at 50℃. The middle layer structure was coated with a poly(N-isopropylacrylamide) shell (shell thickness was 10 μm, and the thermal softening temperature was 60℃). After drying, it was ready for use.

[0051] Step S2: Prepare dry mix: Add ordinary silicate cement, fly ash, coal gangue, quartz sand and slag together to a forced mixing equipment and dry mix for 4 minutes until all components are evenly dispersed.

[0052] Step S3: Preparation of the mixture: Add the multifunctional phase change microcapsules, water-reducing agent, air-entraining agent, aluminate cement and water-retaining agent prepared in step S1 to the above dry mixture, stir for 6 minutes to make the microcapsules and each additive evenly distributed, then add the metered water, continue stirring for 12 minutes, control the slurry expansion to 220mm, and thus obtain the vertical buried pipe backfill material with phase change temperature regulation function.

[0053] Example 3 This embodiment provides a backfill material for vertical buried pipes with phase change temperature regulation function. The raw material composition is as follows by weight percentage: cement-based cementitious material 34%, multifunctional phase change microcapsules 18%, aggregate 30%, functional additives 7%, and water 11%.

[0054] The cement-based cementitious material consists of 25% ordinary silicate cement, 6% fly ash, and 3% coal gangue (the proportions of these three components are calculated based on the total weight of the materials, totaling 34%). The multifunctional phase change microcapsules are configured according to their own weight as follows: the core is paraffin wax 14.4% and CaCl2 particles 2.4%, the middle layer is Bacillus pasteurellosis spores 0.9%, and the outer shell is poly(N-isopropylacrylamide) 0.3% (these four components total 100% of the total weight of the multifunctional phase change microcapsules, corresponding to 18% of the total weight of the materials). The aggregate consists of 25% quartz sand and 5% slag (totaling 30% of the total weight of the materials). The functional additives consist of 3% polycarboxylate superplasticizer, 1% alkyl sulfonate air-entraining agent, 2% aluminate cement, and 1% hydroxypropyl methylcellulose water-retaining agent (totaling 7% of the total weight of the materials).

[0055] The preparation steps for this backfill material are as follows: Step S1: Preparation of multifunctional phase change microcapsules: (1) Heat the paraffin to 70°C to soften and melt it, add CaCl2 particles, and stir at 400 r / min for 12 minutes to form a uniform core mixture; (2) A spray drying method was used to spray a coating with a concentration of 5×10 on the surface of the above-mentioned core mixture. 8 A suspension of Bacillus pasteurellium spores per mL forms a middle layer coating structure; (3) Using interfacial polymerization, the reaction was stirred at 400 r / min for 1.5 hours at 50℃. The middle layer structure was coated with a poly(N-isopropylacrylamide) shell (shell thickness was 10 μm, thermal softening temperature was 60℃), and then dried for later use.

[0056] Step S2: Prepare dry mix: Add ordinary silicate cement, fly ash, coal gangue, quartz sand and slag together to a forced mixing equipment and dry mix for 4 minutes until all components are evenly dispersed.

[0057] Step S3: Preparation of the mixture: Add the multifunctional phase change microcapsules, water-reducing agent, air-entraining agent, aluminate cement and water-retaining agent prepared in step S1 to the above dry mixture, stir for 6 minutes to make the microcapsules and each additive evenly distributed, then add the metered water, continue stirring for 12 minutes, control the slurry expansion to 220mm, and thus obtain the vertical buried pipe backfill material with phase change temperature regulation function.

[0058] The backfill material slurry for vertical buried pipes was prepared according to the above proportions and poured into cubic molds with sides of 70mm×70mm×70mm and 40mm×40mm×160mm. After compaction, it was cured at 20±2℃ and relative humidity ≥90% for 24 hours before demolding. Subsequently, the 70mm×70mm×70mm specimens were placed in a standard curing room (20±2℃, relative humidity ≥90%) for further curing to 28 days, and the 40mm×40mm×160mm specimens were placed in a standard curing room (20±2℃, relative humidity ≥90%) for further curing to 7 days. The 70mm×70mm×70mm specimens were used for compressive strength and thermal conductivity testing. A single crack with a width of approximately 0.30~0.35mm was pre-induced in the middle of a 40mm×40mm×160mm specimen by a three-point bending loading method. The specimen was then cured at 20±2℃ for 28 days. The crack width closure rate was calculated by microscopic measurement of the crack width to characterize the self-healing effect.

[0059] The material cured for 28 days was crushed and sieved to a particle size of less than 2 mm to prepare DSC test samples. A temperature range of 20~70℃ was set, and 500 heating-cooling cycles were performed at a heating and cooling rate of 5℃ / min. The endothermic and exothermic peak areas of the first and 500th phase change processes were recorded, and the latent heat retention rate of the phase change was calculated.

[0060] Table 1. Performance Comparison of Backfill Materials for Vertical Buried Pipes with Different Mixture Ratios Effect verification example To further verify the applicability of the backfill material in actual buried pipe engineering, the backfill material prepared according to the experimental example of this invention was applied to a grouting test simulating a vertical buried pipe borehole section. The grouting hole was 20m deep and 130mm in diameter, and the inlet temperature of the buried pipe was approximately 30°C. The filling was performed using a bottom-up pumping method. The grout exhibited good fluidity and anti-segregation properties; no pipe blockage, stratification, or segregation occurred during the grouting process. The material uniformly filled the annular space between the buried pipe and the borehole wall, forming a continuous and dense backfill layer.

[0061] Compared with conventional cement-based backfill materials that do not employ phase change and self-healing functions, the self-made backfill material enables the 28-day compressive strength of a heat exchange single well to reach 12-18 MPa, the thermal conductivity to be 1.2-1.8 W / (m•K), and the heat exchange capacity to be increased by 30%~40%, fully meeting the core requirements of mechanical strength and heat transfer efficiency for vertical buried pipe backfilling projects. At the same time, after multiple hot and cold cycles, no voids or significant increases in thermal resistance due to cracks were observed at the interface between the backfill layer and the buried pipe, proving that the temperature-sensitive triggering self-healing mechanism of the material can effectively ensure the long-term compactness and heat transfer performance of the backfill layer.

[0062] In summary, the backfill material provided by this invention exhibits excellent construction adaptability, structural stability, and long-term heat exchange performance in practical engineering applications. It can significantly improve the operating efficiency and stability of vertical buried pipe systems and meet the comprehensive requirements of ground source heat pump projects for high-performance backfill materials.

[0063] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A backfill material for vertical buried pipes with phase change temperature regulation function, characterized in that, The raw materials comprise the following components by weight percentage: The composition includes 30-45% cementitious materials, 10-20% multifunctional phase change microcapsules, 25-40% aggregates, 2-8% functional additives, and 8-20% water. The multifunctional phase change microcapsule consists of a core, a middle layer, and a shell. The core is a mixture of paraffin and CaCl2 particles, the middle layer is Bacillus pasteurellii spores, and the shell is a thermosensitive polymer.

2. The backfill material for vertical buried pipes with phase change temperature regulation function according to claim 1, characterized in that, In the multifunctional phase change microcapsule, the weight ratio of paraffin to CaCl2 particles is 5-8:1; the weight of the Bacillus pasteurellii spores is 5-10% of the core weight; the thermosensitive polymer is selected from at least one of poly(N-isopropylacrylamide), polyethylene glycol derivatives, and ethylene-vinyl acetate copolymers.

3. The backfill material for vertical buried pipes with phase change temperature regulation function according to claim 1, characterized in that, The thickness of the outer shell is 5-20 μm, and the thermal softening temperature is 55-65℃.

4. The backfill material for vertical buried pipes with phase change temperature regulation function according to claim 1, characterized in that, Based on a total amount of 100% cement-based cementitious materials, the cement-based cementitious materials are composed of the following components in the following weight ratios: 70-85% ordinary Portland cement, 10-20% fly ash, and 5-10% coal gangue.

5. The backfill material for vertical buried pipes with phase change temperature regulation function according to claim 1, characterized in that, The aggregate consists of quartz sand and slag in a weight ratio of 3-5:

1.

6. The backfill material for vertical buried pipes with phase change temperature regulation function according to claim 1, characterized in that, Based on a total amount of 100% functional additives, the functional additives include the following components in the following weight ratios: 30-50% water-reducing agent, 5-15% air-entraining agent, 20-40% thermal conductivity enhancer, and 5-15% water-retaining agent.

7. The backfill material for vertical buried pipes with phase change temperature regulation function according to claim 6, characterized in that, The thermal conductivity enhancer is selected from graphite powder or aluminate cement.

8. The method for preparing the backfill material for vertical buried pipes with phase change temperature regulation function as described in any one of claims 1-7, characterized in that, Includes the following steps: Preparation of multifunctional phase change microcapsules: Paraffin wax was melted and mixed with CaCl2 to obtain a core mixture. Then, a suspension of Bacillus pasteurellis spores was sprayed on the surface of the core mixture to form a middle layer coating structure. Subsequently, a temperature-sensitive polymer shell was coated on the middle layer using interfacial polymerization. After drying, multifunctional phase change microcapsules were obtained. Preparation of dry mix: Cement-based cementitious materials and aggregates are mixed to obtain dry mix; Preparation of the mixture: The multifunctional phase change microcapsules and the functional additives are added to the dry mixture, and then water is added and stirred to obtain the vertical buried pipe backfill material with phase change temperature regulation function.

9. The preparation method according to claim 8, characterized in that, The reaction temperature for the interfacial polymerization method is 40-60℃, and the reaction time is 1-2h.

10. The application of the vertical buried pipe backfill material with phase change temperature regulation function as described in any one of claims 1-7 in the backfilling of vertical buried pipes.

Images