Composite modified air entraining agent and high frost resistance roller compacted concrete suitable for low air pressure and large temperature difference environment and preparation method thereof

Abstract

The application provides a composite modified air entraining agent and high frost resistance roller compacted concrete suitable for low air pressure and large temperature difference environment and a preparation method thereof, and belongs to the technical field of building materials. The concrete is prepared from cement with a mass fraction of 60-76 parts, fly ash with a mass fraction of 85-119 parts, sand with a mass fraction of 720-736 parts, stone with a mass fraction of 1420-1426 parts, the composite modified air entraining agent with a mass fraction of 0.02-0.08 parts, the naphthalene-based high efficient water reducing agent with a mass fraction of 0.5-0.7 parts and water with a mass fraction of 80-96 parts through a specific roller compacting process. A special composite modified air entraining agent is used, the air entraining agent cooperatively and efficiently entrains air by sodium abietate and triterpenoid saponin, and gelatin and hydroxypropyl methyl cellulose are innovatively introduced to form a composite foam stabilizing system, the stability of the air bubbles under low air pressure and severe temperature difference is significantly enhanced, and an optimized pore structure with harmless pores and less harmful pores is formed in the concrete.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a composite modified air-entraining agent and a highly frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments, as well as its preparation method. Background Technology

[0002] Roller-compacted concrete (RCC) is a dry-hardened concrete constructed using vibratory compaction. It is widely used in large-volume concrete projects such as dams, roads, and airports. Its characteristics include low cement usage, high admixture content, low heat of hydration, and fast construction speed. However, in the unique environment of high-altitude regions, engineering structures often face multiple coupled effects, including low air pressure, drastic diurnal temperature variations, and frequent freeze-thaw cycles. This places extremely high demands on the durability of RCC, especially its frost resistance.

[0003] In low-pressure environments, the reduced atmospheric pressure causes dissolved air within the concrete mix to precipitate more easily. Simultaneously, the reduced external pressure on introduced air bubbles leads to a significant decrease in the stability of bubbles introduced by traditional air-entraining agents. These bubbles are prone to coalescing, coarsening, or bursting, resulting in rapid loss of air content in the concrete and making it difficult to form a stable, uniform, and closed microbubble system. Furthermore, drastic temperature differences not only exacerbate the freeze-thaw cycle stress of pore water within the concrete but also cause thermal expansion and contraction of the material itself, damaging the pore structure. These factors combined result in accelerated deterioration of the internal pore structure of roller-compacted concrete during service, leading to severely insufficient freeze-thaw resistance and making it highly susceptible to surface spalling, internal damage, and even structural failure.

[0004] Currently, techniques such as adding air-entraining agents, selecting superior raw materials, and optimizing mix proportions are commonly used to address the freeze-thaw resistance of concrete in high-altitude areas. Commonly used air-entraining agents, such as rosin and saponins, can introduce a certain amount of air bubbles under normal pressure, but their foam-stabilizing ability is limited under low pressure. The introduced bubble structure is often not fine or uniform enough, with a relatively large pore size distribution and a high proportion of harmful and multi-harmful pores, making it difficult to effectively block the penetration and migration of frost-swelling water. Furthermore, existing technologies for quality control of cement, admixtures, aggregates, and mixing water mainly focus on conventional strength and workability indicators, paying insufficient attention to the synergistic effect of these components with air-entraining agents under extreme conditions of low pressure and large temperature differences, and to the refined control of the final pore structure system. Simultaneously, improper selection of compaction process parameters (such as compaction thickness, number of passes, and speed) can easily lead to damage to the microbubble structure during compaction, resulting in air content loss and affecting the freeze-thaw resistance and durability of roller-compacted concrete. Therefore, there is an urgent need to develop a high-frost-resistant roller-compacted concrete that can adapt to harsh environments with low air pressure and large temperature differences, and has excellent bubble stability and pore structure, along with corresponding material control technology and preparation process. Summary of the Invention

[0005] The purpose of this invention is to provide a composite modified air-entraining agent and a high-freeze-resistant roller-compacted concrete suitable for low-pressure and large-temperature-difference environments, as well as a method for preparing the same, in order to solve the problems of difficulty in air entrainment, large air content loss, and insufficient freeze-thaw resistance of roller-compacted concrete in high-altitude environments in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a composite modified air-entraining agent, prepared from raw materials comprising the following parts by mass:

[0008] Sodium rosinate air-entraining agent 30-50 parts, triterpenoid saponin air-entraining agent 10-20 parts, gelatin 3-8 parts, hydroxypropyl methylcellulose 1-3 parts, sodium hydroxide 1.0-3.6 parts, methylisothiazolinone 0.1-0.3 parts, and deionized water 100-120 parts.

[0009] This invention also provides a method for preparing the above-mentioned composite modified air-entraining agent, comprising the following steps:

[0010] S1. Mix gelatin powder with some deionized water, let it stand to allow it to swell fully, then stir it in a water bath until it is completely dissolved, and cool it to obtain a gelatin solution.

[0011] S2. Mix hydroxypropyl methylcellulose powder with deionized water at 80-90°C until homogeneous, then add deionized water at 20-30°C during stirring to completely hydrate and dissolve it, thus obtaining a hydroxypropyl methylcellulose solution.

[0012] S3. Mix sodium rosinate air-entraining agent with deionized water until uniform, heat and then add triterpenoid saponin air-entraining agent, stir until completely dissolved to obtain a composite air-entraining agent solution.

[0013] S4. After diluting sodium hydroxide with deionized water, slowly add it dropwise to the composite air-entraining agent solution prepared in S3. After adjusting the pH value, add gelatin solution, hydroxypropyl methylcellulose solution, methylisothiazolinone and the remaining deionized water in sequence. Stir continuously for 30-60 minutes to mix evenly. After standing and aging, filter to obtain the composite modified air-entraining agent.

[0014] Furthermore, in step S1, the temperature of the water bath is 50~60°C;

[0015] In step S3, the heating temperature is 40~50°C.

[0016] Furthermore, in step S4, the pH is adjusted to 8-10, and the maturation time is 12-24 hours.

[0017] This invention also provides a highly frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments, prepared from raw materials comprising the following parts by weight:

[0018] 60-76 parts cement;

[0019] 85-119 parts of fly ash;

[0020] 720-736 parts of sand;

[0021] 1420-1426 portions of pebbles;

[0022] 0.02~0.08 parts of composite modified air-entraining agent;

[0023] 0.5-0.7 parts of naphthalene-based high-efficiency water-reducing agent;

[0024] 80-96 parts water.

[0025] Furthermore, the cement is low-heat silicate cement with a loss on ignition in the range of 0.5% ± 0.1%; the fly ash is Grade I fly ash with a loss on ignition in the range of 1.5% ± 0.3%.

[0026] Furthermore, the sand is artificial sand with a fineness modulus of 2.6 to 2.8 and a micro powder content controlled at 5% to 6%.

[0027] Cl in the water - SO4 2- Ca 2+ and Mg 2+ The content requirements are not to exceed 30 mg / L, 25 mg / L, 40 mg / L and 10 mg / L respectively.

[0028] Furthermore, the stones are three-graded artificial aggregates, and the combination of the three-graded artificial aggregates is based on a volume ratio of large stones: medium stones: small stones of 20~40:30~50:20~40; the particle size of the large stones is 40mm~80mm, the particle size of the medium stones is 20mm~40mm, and the particle size of the small stones is 5mm~20mm.

[0029] This invention also provides a method for preparing the above-mentioned high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments, comprising the following steps:

[0030] 1) Dilute the composite modified air-entraining agent and the naphthalene-based high-efficiency water-reducing agent separately with a portion of water;

[0031] 2) Mix cement, fly ash, sand, gravel and remaining water evenly. During the mixing process, add composite modified air-entraining agent dilution and naphthalene-based high-efficiency water-reducing agent dilution at a uniform speed. Then mix evenly with a forced mixer and compact using a specific rolling process to obtain high frost-resistant roller-compacted concrete suitable for low-pressure and large-temperature-difference environments.

[0032] Furthermore, the parameters of the specific compaction process are as follows: compaction thickness 20~40cm, number of compaction passes: 2 static compaction passes plus 6 vibratory compaction passes plus 2 static compaction passes, and compaction speed 1~3km / h.

[0033] The beneficial effects of this invention are:

[0034] (1) The core of this invention lies in the use of a specially formulated composite modified air-entraining agent. This air-entraining agent achieves a balance between efficient air entrainment and the generation of fine, uniform microbubbles through the synergistic effect of sodium rosinate and triterpenoid saponins. At the same time, gelatin and hydroxypropyl methylcellulose (HPMC) are innovatively introduced as a composite foam-stabilizing system, which greatly enhances the mechanical strength and stability of the bubble liquid film and effectively resists the problems of bubble rupture and rapid loss of air content caused by low air pressure and drastic temperature differences in high-altitude areas. Meanwhile, an optimized pore structure is formed in the concrete, mainly composed of "harmless pores" and "less harmful pores" with a significant reduction in harmful pores, thereby ensuring the high frost resistance of roller-compacted concrete.

[0035] (2) This invention strictly controls the quality of raw materials, including low-heat silicate cement with a loss on ignition within the range of 0.5% ± 0.1%, Grade I fly ash with a loss on ignition within the range of 1.5% ± 0.3%, and artificial sand with a fine powder content of 5% to 6%, to reduce their adsorption on the composite modified air-entraining agent and ensure that the composite modified air-entraining agent plays a full role in the concrete system. At the same time, the ion content in the mixing water is limited, Cl... - SO4 2- Ca 2+ and Mg 2+ The content of each component shall not exceed 30 mg / L, 25 mg / L, 40 mg / L and 10 mg / L respectively, so as to reduce the influence of water quality on the effect of composite modified air-entraining agent, thereby further improving the air-entraining capacity of composite modified air-entraining agent and improving the freeze-thaw resistance of roller-compacted concrete in low air pressure and large temperature difference environment.

[0036] (3) This invention details the preparation method of the composite modified air-entraining agent and concrete. By employing key steps such as step-by-step preparation of the air-entraining agent components, precise pH control, and static curing, the stability of the composite modified air-entraining agent's performance is ensured. During concrete mixing, the admixture is pre-diluted and then added at a uniform rate to ensure uniform dispersion in the system and avoid clumping or localized failure. Furthermore, the compaction process of roller-compacted concrete is optimized, employing a specific compaction technique to ensure sufficient compaction of the concrete while avoiding over-vibration that could lead to air content loss. Detailed Implementation

[0037] This invention provides a composite modified air-entraining agent, prepared from raw materials comprising the following parts by mass:

[0038] Sodium rosinate air-entraining agent 30-50 parts, triterpenoid saponin air-entraining agent 10-20 parts, gelatin 3-8 parts, hydroxypropyl methylcellulose 1-3 parts, sodium hydroxide 1.0-3.6 parts, methylisothiazolinone 0.1-0.3 parts, and deionized water 100-120 parts.

[0039] In this invention, the content of the sodium rosinate air-entraining agent is preferably 35-45 parts by weight, and more preferably 38-42 parts by weight.

[0040] In this invention, the content of the triterpenoid saponin gas-entraining agent is preferably 12-18 parts by mass, and more preferably 14-16 parts by mass.

[0041] In this invention, the content of gelatin is preferably 4 to 7 parts by weight, and more preferably 5 to 6 parts by weight.

[0042] In this invention, the content of hydroxypropyl methylcellulose is preferably 1.5 to 2.5 parts by weight, and more preferably 2 to 2.5 parts by weight.

[0043] In this invention, the content of sodium hydroxide is preferably 2.0 to 3.0 parts by mass, and more preferably 2.5 to 3.0 parts by mass.

[0044] In this invention, the content of the methylisothiazolinone is preferably 0.2 parts by weight.

[0045] In this invention, the content of deionized water is preferably 105-115 parts by mass, and more preferably 110-115 parts by mass.

[0046] This invention also provides a method for preparing the above-mentioned composite modified air-entraining agent, comprising the following steps:

[0047] S1. Mix gelatin powder with some deionized water, let it stand to allow it to swell fully, then stir it in a water bath until it is completely dissolved, and cool it to obtain a gelatin solution.

[0048] S2. Mix hydroxypropyl methylcellulose powder with deionized water at 80-90°C until homogeneous, then add deionized water at 20-30°C during stirring to completely hydrate and dissolve it, thus obtaining a hydroxypropyl methylcellulose solution.

[0049] S3. Mix sodium rosinate air-entraining agent with deionized water until uniform, heat and then add triterpenoid saponin air-entraining agent, stir until completely dissolved to obtain a composite air-entraining agent solution.

[0050] S4. After diluting sodium hydroxide with deionized water, slowly add it dropwise to the composite air-entraining agent solution prepared in S3. After adjusting the pH value, add gelatin solution, hydroxypropyl methylcellulose solution, methylisothiazolinone and the remaining deionized water in sequence. Stir continuously for 30-60 minutes to mix evenly. After standing and aging, filter to obtain the composite modified air-entraining agent.

[0051] In this invention, in step S1, the temperature of the water bath is 50~60°C, preferably 52~58°C, and more preferably 55°C;

[0052] In step S3, the heating temperature is 40~50°C, preferably 42~48°C, and more preferably 45°C.

[0053] In this invention, in step S4, the pH is adjusted to 8-10, preferably 9; the aging time is 12-24h, preferably 14-20h, and more preferably 15-18h.

[0054] This invention also provides a highly frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments, prepared from raw materials comprising the following parts by weight:

[0055] 60-76 parts cement;

[0056] 85-119 parts of fly ash;

[0057] 720-736 parts of sand;

[0058] 1420-1426 portions of pebbles;

[0059] 0.02~0.08 parts of composite modified air-entraining agent;

[0060] 0.5-0.7 parts of naphthalene-based high-efficiency water-reducing agent;

[0061] 80-96 parts water.

[0062] In this invention, the cement content is preferably 62-75 parts by weight, and more preferably 65-70 parts by weight.

[0063] In this invention, the content of fly ash is preferably 90-115 parts by weight, and more preferably 95-110 parts by weight.

[0064] In this invention, the content of sand is preferably 725 to 730 parts by mass.

[0065] In this invention, the content of the stones is preferably 1421 to 1425 parts by mass, and more preferably 1422 to 1424 parts by mass.

[0066] In this invention, the content of the composite modified air-entraining agent is preferably 0.03 to 0.07 parts by weight, and more preferably 0.04 to 0.06 parts by weight.

[0067] In this invention, the content of the naphthalene-based high-efficiency water-reducing agent is preferably 0.6 parts by weight.

[0068] In this invention, the water content is preferably 82-95 parts by mass, and more preferably 85-90 parts by mass.

[0069] In this invention, the cement is low-heat silicate cement with a loss on ignition in the range of 0.5% ± 0.1%; the fly ash is Grade I fly ash with a loss on ignition in the range of 1.5% ± 0.3%.

[0070] In this invention, the sand is artificial sand with a fineness modulus of 2.6 to 2.8, preferably 2.7; the content of fine powder is controlled at 5% to 6%, preferably 5.5%.

[0071] Cl in the water - SO4 2- Ca 2+ and Mg 2+ The content requirements are not to exceed 30 mg / L, 25 mg / L, 40 mg / L and 10 mg / L respectively.

[0072] In this invention, the stones are three-graded artificial aggregates. The combination of the three-graded artificial aggregates by volume ratio of large stones: medium stones: small stones is 20~40:30~50:20~40, preferably 22~38:32~45:22~38, and more preferably 25~35:35~40:25~35; the particle size of the large stones is 40mm~80mm, preferably 50~60mm; the particle size of the medium stones is 20mm~40mm, preferably 30~40mm; and the particle size of the small stones is 5mm~20mm, preferably 5~10mm.

[0073] This invention also provides a method for preparing the above-mentioned high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments, comprising the following steps:

[0074] 1) Dilute the composite modified air-entraining agent and the naphthalene-based high-efficiency water-reducing agent separately with a portion of water;

[0075] 2) Mix cement, fly ash, sand, gravel and remaining water evenly. During the mixing process, add composite modified air-entraining agent dilution and naphthalene-based high-efficiency water-reducing agent dilution at a uniform speed. Then mix evenly with a forced mixer and compact using a specific rolling process to obtain high frost-resistant roller-compacted concrete suitable for low-pressure and large-temperature-difference environments.

[0076] In this invention, the parameters of the specific rolling process are as follows: rolling thickness 20~40cm, preferably 30cm; number of rolling passes 2+6+2, that is, 2 static rolling passes plus 6 vibratory rolling passes plus 2 static rolling passes; rolling speed 1~3km / h, preferably 1.5km / h.

[0077] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0078] Example 1

[0079] A composite modified air-entraining agent is prepared by comprising 40 parts by weight of sodium rosinate air-entraining agent, 15 parts by weight of triterpenoid saponin air-entraining agent, 5.5 parts by weight of gelatin, 2 parts by weight of hydroxypropyl methylcellulose, 2.3 parts by weight of sodium hydroxide, 0.2 parts by weight of methylisothiazolinone, and 110 parts by weight of deionized water. The preparation method is as follows:

[0080] S1. First, mix the gelatin powder with deionized water evenly to fully disperse and wet the gelatin powder, preventing clumping. Let it stand for about 30 minutes to allow the gelatin particles to fully swell. Then, place it in a constant temperature water bath and stir continuously at 50°C until the gelatin is completely dissolved, forming a clear and transparent solution. Cool to room temperature to obtain a gelatin solution.

[0081] S2. Mix hydroxypropyl methylcellulose powder with deionized water at 85°C until homogeneous. Add room temperature deionized water during stirring. Once the hydroxypropyl methylcellulose is completely hydrated and dissolved, the solution gradually becomes clear and viscous, thus obtaining a hydroxypropyl methylcellulose solution.

[0082] S3. First, mix sodium rosinate air-entraining agent with deionized water evenly, then place it in a reaction vessel and heat it to 50°C. Then, slowly add triterpenoid saponin air-entraining agent and stir until completely dissolved to obtain a composite air-entraining agent solution.

[0083] S3. Dilute sodium hydroxide with deionized water and slowly add it dropwise to the composite air-entraining agent solution prepared in S3. Adjust the pH value to 8. Then add the gelatin solution prepared in S1, the hydroxypropyl methylcellulose solution prepared in S2, methylisothiazolinone and deionized water. Stir continuously for 30 min to ensure that all components are completely mixed and uniform. Stop heating and let stand for 12 h to mature. Finally, filter out insoluble impurities to obtain the composite modified air-entraining agent solution.

[0084] A highly frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments is composed of 68 parts by weight of cement, 102 parts by weight of fly ash, 728 parts by weight of sand, 1423 parts by weight of aggregate, 0.05 parts by weight of composite modified air-entraining agent, 0.6 parts by weight of naphthalene-based high-efficiency water-reducing agent, and 88 parts by weight of water. It is prepared through a specific roller compaction process, and the preparation method includes the following steps:

[0085] 1) Dilute the composite modified air-entraining agent and the naphthalene-based high-efficiency water-reducing agent separately with a portion of water;

[0086] 2) Mix cement, fly ash, sand, aggregate, and remaining water evenly. During the mixing process, add diluted composite modified air-entraining agent and diluted naphthalene-based high-efficiency water-reducing agent at a uniform speed. Then, use a forced mixer to mix evenly and compact the mixture. The compaction thickness is 30cm, with 2+6+2 compaction passes and a compaction speed of 1.5km / h to obtain high-frost-resistant roller-compacted concrete suitable for low-pressure and large-temperature-difference environments. The cement is low-heat silicate cement with a loss on ignition within the range of 0.5%±0.1%, the fly ash is Grade I fly ash with a loss on ignition within the range of 1.5%±0.3%, the sand is artificial sand with a fineness modulus of 2.6 and a micro-powder content controlled at 5%, and the aggregate is three-graded artificial aggregate. The three-graded aggregate is composed of large aggregate: medium aggregate: small aggregate in a volume ratio of 40:30:40; the particle size of large aggregate is 40mm~80mm, the particle size of medium aggregate is 20mm~40mm, and the particle size of small aggregate is 5mm~20mm. Cl in water - SO4 2- Ca 2+ and Mg 2+ The content requirements are not to exceed 30 mg / L, 25 mg / L, 40 mg / L and 10 mg / L respectively.

[0087] Example 2

[0088] The same raw materials and methods as in Example 1 were used, except that the proportions of the components in the composite modified air-entraining agent were different. The composite modified air-entraining agent was prepared by means of 30 parts by weight of sodium rosinate air-entraining agent, 10 parts by weight of triterpenoid saponin air-entraining agent, 3 parts by weight of gelatin, 1 part by weight of hydroxypropyl methylcellulose, 1 part by weight of sodium hydroxide, 0.1 parts by weight of methylisothiazolinone, and 100 parts by weight of deionized water.

[0089] Example 3

[0090] The same raw materials and methods as in Example 1 were used, except that the proportions of the components in the composite modified air-entraining agent were different. It was prepared from 50 parts sodium rosinate air-entraining agent, 20 parts triterpenoid saponin air-entraining agent, 8 parts gelatin, 3 parts hydroxypropyl methylcellulose, 3.6 parts sodium hydroxide, 0.3 parts methylisothiazolinone, and 120 parts deionized water.

[0091] Example 4

[0092] The same raw materials and methods as in Example 1 were used, except that the content of the composite modified air-entraining agent in the concrete was 0.02 parts.

[0093] Example 5

[0094] The same raw materials and methods as in Example 1 were used, except that the content of the composite modified air-entraining agent in the concrete was 0.08 parts.

[0095] Comparative Example 1

[0096] The same raw materials and methods as in Example 1 were used, but the difference from Example 1 was that no composite modified air-entraining agent was used. Instead, rosin-based air-entraining agents and triterpenoid saponin-based air-entraining agents were directly incorporated into the concrete mix.

[0097] Comparative Example 2

[0098] The same raw materials and methods as in Example 1 were used, except that the content of the composite modified air-entraining agent in the concrete was 0.1 parts.

[0099] Benchmark group:

[0100] The same raw materials and methods as in Example 1 were used, except that no composite modified air-entraining agent was used, and rosin-based air-entraining agents were directly added to the concrete mix.

[0101] Test example:

[0102] The concrete mixtures in the examples and comparative examples were molded and cured (90 days) under low pressure (67 kPa and 50 kPa) and large temperature difference (30℃) environments. The concrete specimens molded in the reference group were molded and cured under normal pressure (101 kPa) for 90 days. According to GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures", the air content, air content loss over time (1 h), pore structure and 90-day strength loss were tested (compared with the reference group). According to GB / T 50082-2024 "Standard for Test Methods of Long-term Performance and Durability of Concrete", 100 rapid freeze-thaw tests were conducted on the concrete specimens to test their mass loss and relative dynamic modulus of elasticity. The results are shown in Table 1.

[0103] Table 1 Test Results

[0104]

[0105] As can be seen from the test results in Table 1, the high frost-resistant roller-compacted concrete and its preparation method provided by the present invention, which are suitable for low-pressure and large-temperature-difference environments, can improve the air content and pore structure of roller-compacted concrete in high-altitude environments, and maintain the air content of roller-compacted concrete at a high level within 1 hour, while significantly reducing the 90-day strength loss of roller-compacted concrete; in addition, the frost resistance of roller-compacted concrete in low-pressure and large-temperature-difference environments is further improved, thereby improving the overall performance of roller-compacted concrete.

[0106] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A highly frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments, characterized in that, It is prepared from raw materials comprising the following parts by mass: 60-76 parts cement; 85-119 parts of fly ash; 720-736 parts of sand; 1420-1426 portions of pebbles; 0.02~0.08 parts of composite modified air-entraining agent; 0.5-0.7 parts of naphthalene-based high-efficiency water-reducing agent; 80-96 parts water; The composite modified air-entraining agent is prepared from raw materials comprising the following parts by mass: Sodium rosinate air-entraining agent 30-50 parts, triterpenoid saponin air-entraining agent 10-20 parts, gelatin 3-8 parts, hydroxypropyl methylcellulose 1-3 parts, sodium hydroxide 1.0-3.6 parts, methylisothiazolinone 0.1-0.3 parts, and deionized water 100-120 parts; The preparation method of the composite modified air-entraining agent includes the following steps: S1. Mix gelatin powder with some deionized water, let it stand to allow it to swell fully, then stir it in a water bath until it is completely dissolved, and cool it to obtain a gelatin solution. S2. Mix hydroxypropyl methylcellulose powder with deionized water at 80-90°C until homogeneous, then add deionized water at 20-30°C during stirring to completely hydrate and dissolve it, thus obtaining a hydroxypropyl methylcellulose solution. S3. Mix sodium rosinate air-entraining agent with deionized water until uniform, heat and then add triterpenoid saponin air-entraining agent, stir until completely dissolved to obtain a composite air-entraining agent solution. S4. After diluting sodium hydroxide with deionized water, slowly add it dropwise to the composite air-entraining agent solution prepared in S3. After adjusting the pH value, add gelatin solution, hydroxypropyl methylcellulose solution, methylisothiazolinone and the remaining deionized water in sequence. Stir continuously for 30-60 minutes to mix evenly. After standing and aging, filter to obtain the composite modified air-entraining agent.

2. The high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments according to claim 1, characterized in that, In step S1, the temperature of the water bath is 50~60°C; In step S3, the heating temperature is 40~50°C.

3. The high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments according to claim 1, characterized in that, In step S4, the pH is adjusted to 8-10, and the maturation time is 12-24 hours.

4. The high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments according to claim 1, characterized in that, The cement is low-heat silicate cement with a loss on ignition in the range of 0.5% ± 0.1%; the fly ash is Grade I fly ash with a loss on ignition in the range of 1.5% ± 0.3%.

5. The high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments according to claim 4, characterized in that, The sand is artificial sand with a fineness modulus of 2.6 to 2.8 and a micro powder content controlled at 5% to 6%. Cl in the water - SO4 2- Ca 2+ and Mg 2+ The content requirements are not to exceed 30 mg / L, 25 mg / L, 40 mg / L and 10 mg / L respectively.

6. The high frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments according to any one of claims 1 to 5, characterized in that, The stones are three-graded artificial aggregates, and the combination of the three-graded artificial aggregates is based on a volume ratio of large stones: medium stones: small stones of 20~40:30~50:20~40; the particle size of the large stones is 40mm~80mm, the particle size of the medium stones is 20mm~40mm, and the particle size of the small stones is 5mm~20mm.

7. The method for preparing highly frost-resistant roller-compacted concrete suitable for low-pressure, large-temperature-difference environments as described in any one of claims 1 to 5, characterized in that, Includes the following steps: 1) Dilute the composite modified air-entraining agent and the naphthalene-based high-efficiency water-reducing agent separately with a portion of water; 2) Mix cement, fly ash, sand, gravel and remaining water evenly. During the mixing process, add composite modified air-entraining agent dilution and naphthalene-based high-efficiency water-reducing agent dilution at a uniform speed. Then mix evenly with a forced mixer and compact using a specific rolling process to obtain high-frost-resistant roller-compacted concrete suitable for low-pressure and large-temperature-difference environments. The parameters of the specific rolling process are: rolling thickness 20~40cm, number of rolling passes: 2 static rolling passes plus 6 vibratory rolling passes plus 2 static rolling passes, and rolling speed 1~3km / h.

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