Self-compacting concrete viscosity modified material for foundation pit engineering and application of self-compacting concrete viscosity modified material

Abstract

The invention belongs to the technical field of building materials and civil engineering, and particularly relates to a self-compacting concrete viscosity modified material for foundation pit engineering and application of the self-compacting concrete viscosity modified material. The composite viscosity modified material obtained by the invention is prepared from the following components in percentage by mass: 45%-55% of I-grade fly ash, 25%-35% of metakaolin, 15%-20% of silica fume, 0.3%-1.5% of hydrophobically modified hydroxyethyl cellulose (HMHEC) and 0.2%-1% of non-ionic polyacrylamide (NPAM). The concrete has segregation and bleeding resistance and excellent slurry pollution resistance, the 3-day compressive strength of the concrete can reach 60% or above of 28-day strength, a foundation pit supporting structure can quickly form bearing capacity, the construction period is shortened, and the long-term service performance of an underground structure is improved. And the material cost is effectively reduced while the performance is improved.

Description

Technical Field

[0001] This invention belongs to the field of building materials and civil engineering technology, specifically relating to a viscosity-modifying material for self-compacting concrete used in foundation pit engineering and its application. Background Technology

[0002] Self-compacting concrete is increasingly widely used in foundation pit engineering, especially in scenarios such as backfilling narrow trenches, pouring water-stop curtains between support piles, and treating joints in diaphragm walls, due to its significant advantages of not requiring vibration and being easy to construct. However, at the same time, groundwater seepage or residual mud from construction often exists within the foundation pit. Ordinary self-compacting concrete is prone to problems such as loss of cementitious materials and aggregate settlement during the pouring process. In addition, the foundation pit backfill or water-stop structure needs to withstand water and soil pressure for a long time, and the concrete needs to bond tightly with the damp and rough foundation pit soil or the surface of the support piles. Therefore, self-compacting concrete used in foundation pit engineering needs to have a high impermeability grade, a low shrinkage rate, and sufficient early and late strength to provide support force.

[0003] Currently, commonly used viscosity modifiers (VMAs) on the market, such as cellulose ethers, styrene, or some polymeric thickeners, while offering some improvement to the segregation and bleeding problems of self-compacting concrete, have significant shortcomings in addressing the aforementioned comprehensive challenges. For example, cellulose ethers cause severe air entrainment and weaken strength. Therefore, developing a viscosity modifier that can specifically solve the problems in foundation pit engineering has engineering value. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a viscosity-modifying material for self-compacting concrete used in foundation pit engineering and its application. This material, through the compounding and synergistic effect of specific functional components, enables concrete to simultaneously meet the requirements of high stability, high resistance to disturbances, high early strength, and high durability in complex foundation pit environments.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A self-compacting concrete composite viscosity modifier for foundation pit engineering is composed of the following components by mass percentage: 45%-55% Grade I fly ash, 25%-35% metakaolin, 15%-20% silica fume, 0.3%-1.5% hydrophobically modified hydroxyethyl cellulose (HMHEC), and 0.2%-1% nonionic polyacrylamide (NPAM).

[0007] As a preferred embodiment of the present invention, the composite viscosity-modifying material is composed of the following components in mass percentage: 45%-50% Grade I fly ash, 30%-33% metakaolin, 15%-20% silica fume, 0.4%-1.5% hydrophobically modified hydroxyethyl cellulose, and 0.4%-0.8% nonionic polyacrylamide.

[0008] Preferably, the water requirement ratio of the Class I fly ash is ≤95%.

[0009] Preferably, the metakaolin activity index is ≥105%.

[0010] Preferably, the silica fume has an SiO2 content of ≥92% and a specific surface area of ​​≥15000 m². 2 / kg of silica fume.

[0011] Preferably, the nonionic polyacrylamide has a molecular weight of 6 million to 10 million.

[0012] The above-mentioned method for preparing composite viscosity-modified materials includes the following steps: (1) Raw material pretreatment: Air separation and carbon removal of fly ash: Class I fly ash is treated by air classifier to effectively reduce its unburned carbon content; Temperature-controlled calcination activation of metakaolin: Kaolin raw materials are calcined at a constant temperature of 750℃-850℃, followed by rapid cooling, to obtain metakaolin with a stable activity index ≥105%. Salt-resistant pretreatment of HMHEC and NPAM mixture: Hydrophobic modified hydroxyethyl cellulose (HMHEC) is mixed with nonionic polyacrylamide (NPAM), and 0.5%-1.0% sodium sulfate is added as a salt-resistant agent. The mixture is then placed in dry hot air at 40℃-50℃ and stirred for 20-30 minutes to enhance its solubility stability in high alkalinity and high salinity pore liquids in concrete. (2) Segmented multi-stage dry mixing: Primary mixing: Mix all the pretreated fly ash, 50% by weight of silica fume, and all the HMHEC and NPAM mixture for 15-20 minutes to ensure that the three materials are mixed evenly and without agglomeration, thus obtaining the primary mixture. Secondary mixing: Add all the metakaolin and the remaining 50% of silica fume to the primary mixture, and continue mixing for 20-25 minutes until the mixture is uniform in color and free of lumps; Packaging: After mixing, the mixture is transferred to a homogenization chamber and left to stand for 45 minutes to 2 hours to allow the particles to reach a humidity balance. Finally, it is sealed and moisture-proof packaged and placed in a dry and ventilated place to obtain the composite viscosity modified material.

[0013] The present invention also provides the application of the above-mentioned composite viscosity modified material, namely, its use in the preparation of self-compacting concrete for foundation pit engineering.

[0014] Preferably, the amount of the composite viscosity modifier added is 8%-15% of the total mass of the cementitious materials in the self-compacting concrete, and more preferably 10%-12%. The cementitious materials include cement, mineral admixtures, and the composite viscosity modifier.

[0015] The functions and synergistic effects of each component are explained below: Grade I fly ash: As a major micro-aggregate and low-activity admixture, its spherical particle effect can effectively improve the rheological properties of the paste, reduce the heat of hydration and early shrinkage of concrete, and at the same time significantly reduce material costs.

[0016] Metakaolin: As a highly active volcanic ash material, its core function is to react rapidly with calcium hydroxide, a cement hydration product, to generate a large amount of additional CSH gel, which significantly improves the mid-to-late stage strength, density, impermeability and chemical erosion resistance of concrete.

[0017] Silica fume: It exerts a dual effect of ultrafine filling and high pozzolanic activity. Its nano-sized particles can effectively fill pores, significantly improving the early strength, segregation resistance, and resistance to chloride ion penetration of concrete.

[0018] Hydrophobically modified hydroxyethyl cellulose: the core rheology modifier. Its unique hydrophobic association mechanism endows concrete with excellent "shear thinning" behavior: the viscosity decreases under construction shear force, ensuring high fluidity and pumpability; the viscosity recovers rapidly when at rest, providing extremely strong resistance to aggregate settlement and paste segregation, and its performance is minimally affected by temperature.

[0019] Nonionic polyacrylamide: an interface protection component. Its long polymer chains form a protective film on the surface of cement particles through physical adsorption, effectively blocking the intrusion of clay particles and harmful ions in the foundation pit slurry, and significantly reducing the deteriorating effects of slurry contamination on concrete workability and interfacial bond strength.

[0020] The beneficial effects of this invention are as follows: The composite viscosity-modified material obtained by this invention has the following properties: 1. Anti-segregation and anti-bleeding performance: Under high flow conditions with a slump expansion greater than 650 mm, the static segregation rate of concrete can be controlled below 10%, and the bleeding rate is close to zero, which meets the absolute stability requirements for deep foundation pit pouring.

[0021] 2. Excellent resistance to mud contamination: Concrete incorporating the material of this invention can reduce its spread loss rate by more than 50% after simulating mud contamination. It can effectively prevent the concrete from becoming loose in layers, experiencing a sudden drop in strength, and suffering from "mud inclusion" disease caused by mud intrusion, thus ensuring the integrity of the pile or wall interface.

[0022] 3. Ideal early strength development: Based on the synergistic effect of silica fume and metakaolin, the 3-day compressive strength of concrete can reach more than 60% of the 28-day strength, which is conducive to the rapid formation of bearing capacity of the foundation pit support structure and shortens the construction period.

[0023] 4. Excellent and long-lasting workability: The shear-thinning properties provided by hydrophobic modified hydroxyethyl cellulose ensure good pumpability of concrete and good passage through the gaps between the steel bars. At the same time, it has excellent slump retention and minimal workability loss within 1-2 hours.

[0024] 5. Enhanced durability: The composite mineral admixture system significantly optimizes the pore structure of concrete, and the chloride ion migration coefficient after 28 days can be lower than 3.0 × 10⁻⁶. -12 m 2 / s, with a seepage resistance grade of P12 or higher, improving the long-term service performance of underground structures.

[0025] 6. Good economic efficiency: By using fly ash to partially replace cement, the material cost is effectively reduced while improving performance.

[0026] 7. This invention ensures the homogeneity of the composite viscosity-modifying material: a segmented multi-stage dry mixing process is adopted, first using fly ash and part of silica fume as a "carrier" to disperse the easily agglomerated polymer, and then adding highly active metakaolin, which ensures that the components, especially trace organic components, are highly uniformly distributed in the final product, avoiding uncertainty in the results caused by the unevenness of the mixture during use. Detailed Implementation

[0027] The present invention will be further illustrated below with reference to specific embodiments. These examples are merely illustrative and not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0028] Example A self-compacting concrete composite viscosity modifier for foundation pit engineering The composite modified materials of Examples 1-4 (S1-S4) and Comparative Examples 1-5 (D1-D5) of the present invention were prepared according to the component formulations in Table 1.

[0029] Table 1: Composite Modified Material Formulation (%)

[0030] Raw materials: PO 52.5 ordinary Portland cement; Grade I fly ash (water requirement ≤ 95%); metakaolin (activity index ≥ 105%); silica fume (SiO2 content ≥ 92%, specific surface area ≥ 15000 m²). 2 / kg); hydrophobically modified hydroxyethyl cellulose (HMHEC); nonionic polyacrylamide (NPAM, molecular weight approximately 8 million); polycarboxylate high-performance water-reducing agent (solid content 40%); simulated wall protection slurry prepared using local loess (density 1.15 g / cm³).

[0031] The preparation method of the composite modified material includes the following steps: (1) Grade I fly ash is treated by air classifier.

[0032] (2) The metakaolin raw material was calcined at 800℃ and then rapidly cooled to obtain metakaolin with a stable activity index ≥105%.

[0033] (3) Mix hydrophobically modified hydroxyethyl cellulose (HMHEC) with nonionic polyacrylamide (NPAM) and add sodium sulfate at 0.5% of their total mass. Stir in dry hot air at 40°C for 20 minutes.

[0034] (4) Put all the pretreated fly ash, 50% by weight of silica fume and all the HMHEC and NPAM mixture into a high-efficiency three-dimensional motion mixer and mix for 15 minutes.

[0035] (5) Add all the metakaolin and the remaining 50% of silica fume to the primary mixture, and continue mixing for 20 minutes until the mixture is uniform in color and free of lumps; (6) Transfer the mixed material into the homogenization chamber and let it stand for 1 hour to achieve humidity balance between particles and obtain the composite modified material.

[0036] Experimental Example 1 Freshly mixed material performance testing: including slump spread, T0 500 Time, static separation rate (sieve analysis method, according to GB / T 50080) and bleeding rate.

[0037] Test method: The concrete reference mix is ​​shown in Table 2 below. The modified materials in Table 1 are used to prepare concrete at a dosage of 10% of the total mass of cementitious materials. Water-reducing agent is added to make the initial spread 680±20mm.

[0038] Simulated mud contamination test: Simulated mud equivalent to 1.5% of the total mass of cementitious materials was added to fresh concrete, and after being mixed evenly, its spread loss rate was tested.

[0039] Hardening performance tests include 3-day and 28-day compressive strength (GB / T 50081) and 28-day electrical flux (GB / T 50082).

[0040] Table 2 Mix proportions of self-compacting concrete (kg / m³) 3 )

[0041] 3. Concrete performance test results The test results are shown in Table 3.

[0042] Table 3 Comparison of the performance of self-compacting concrete

[0043] 4. Results Analysis As shown in Table 3, the test results indicate that: All embodiments of the present invention (S1-S4) exhibit excellent overall performance: high fluidity (expansion > 690 mm), good passability (T 500 <5s, high stability (segregation rate <9%), strong anti-fouling ability (expansion loss rate <18%), high early strength (3d strength >38MPa) and high durability (electrical flux <900C).

[0044] Comparative Example D1 (Excessive Fly Ash): Workability (T) 500 The increased fly ash content (due to prolonged time) and decreased stability (increased segregation rate), along with the deterioration of early strength and impermeability (increased electrical flux), demonstrate that an excessively high fly ash ratio is detrimental to performance balance.

[0045] Comparative Example D2 (insufficient silica fume, excessive kaolin): Although the early strength was high, the workability, stability, anti-pollution ability and later strength development were poor, proving that there was a lack of sufficient silica fume to fill and thicken, and the proportion of mineral active components was unbalanced.

[0046] Comparative Example D3 (low silica fume content): its segregation resistance, contamination resistance and early strength are inferior to those of the preferred Example S1, proving that silica fume content is one of the key factors to achieve optimal performance.

[0047] Comparative Example D4 (Missing HMHEC): Severe concrete segregation (segregation rate 32%), T 500 Over an extremely long period, it almost completely loses its self-compacting ability and exhibits the worst impermeability. This strongly demonstrates the indispensability of HMHEC as a core rheology modifier in ensuring the stability of self-compacting concrete under high flow conditions.

[0048] Comparative Example D5 (lacking NPAM): Although its fresh mix stability was acceptable, its resistance to mud contamination was extremely poor, with a spread loss rate as high as 55%, far worse than the example. This clearly demonstrates that NPAM plays a decisive role in resisting mud contamination in foundation pits and protecting the interfacial properties of concrete.

[0049] Conclusion: This invention, through the compounding of specific types and proportions of Grade I fly ash, metakaolin, silica fume, hydrophobically modified hydroxyethyl cellulose (HMHEC), and nonionic polyacrylamide (NPAM), produces a significant synergistic effect. This composite viscosity-modified material can systematically solve multiple challenges faced by self-compacting concrete in foundation pit engineering, including high stability, high disturbance resistance, high early strength, and high durability. The technical effects are significant, demonstrating outstanding creativity and practicality.

Claims

1. A self-compacting concrete composite viscosity modifier for foundation pit engineering, characterized in that, The composite viscosity-modifying material is composed of the following components by mass percentage: 45%-55% Grade I fly ash, 25%-35% metakaolin, 15%-20% silica fume, 0.3%-1.5% hydrophobically modified hydroxyethyl cellulose (HMHEC), and 0.2%-1% nonionic polyacrylamide (NPAM).

2. The composite viscosity-modified material according to claim 1, characterized in that, The composite viscosity-modifying material is composed of the following components by mass percentage: 45%-50% Grade I fly ash, 30%-33% metakaolin, 15%-20% silica fume, 0.4%-1.5% hydrophobically modified hydroxyethyl cellulose, and 0.4%-0.8% nonionic polyacrylamide.

3. The composite viscosity-modified material according to claim 1, characterized in that, The composite viscosity-modified material contains Class I fly ash with a water requirement ratio ≤ 95%; the metakaolin has an activity index ≥ 105%; and the silica fume has a SiO2 content ≥ 92% and a specific surface area ≥ 15000 m². 2 / kg of silica fume; the nonionic polyacrylamide has a molecular weight of 6 million to 10 million.

4. The method for preparing the composite viscosity-modified material according to any one of claims 1-3, characterized in that, The preparation method includes the following steps: (1) Raw material pretreatment: Fly ash air separation and carbon removal: Grade I fly ash is treated by air classifier; Temperature-controlled calcination activation of metakaolin: Kaolin raw materials are calcined at a constant temperature of 750℃-850℃, followed by rapid cooling, to obtain metakaolin with a stable activity index ≥105%. Salt-resistant pretreatment of HMHEC and NPAM mixture: Hydrophobically modified hydroxyethyl cellulose (HMHEC) and nonionic polyacrylamide (NPAM) are mixed, and sodium sulfate is added as a salt-resistant agent. The mixture is then placed in dry hot air at 40℃-50℃ and stirred for 20-30 minutes. (2) Segmented multi-stage dry mixing: Primary mixing: Mix all the pretreated fly ash, 50% by weight of silica fume, and all the HMHEC and NPAM mixture for 15-20 minutes to ensure that the three materials are mixed evenly and without agglomeration, thus obtaining the primary mixture. Secondary mixing: Add all the metakaolin and the remaining 50% of silica fume to the primary mixture, and continue mixing for 20-25 minutes until the mixture is uniform in color and free of lumps; Packaging: The mixed material is transferred to a homogenization chamber and left to stand for aging to achieve humidity balance between particles. Finally, it is sealed and moisture-proof packaged and placed in a dry and ventilated place to obtain the composite viscosity modified material.

5. The preparation method according to claim 4, characterized in that, In step (1), the amount of sodium sulfate added is 0.5%-1.0% of the total mass of HMHEC and NPAM.

6. The preparation method according to claim 4, characterized in that, The settling and aging time in step (2) is 45 min to 2 h.

7. The application of the composite viscosity-modifying material as described in any one of claims 1-3 in the preparation of self-compacting concrete for foundation pit engineering.

8. The application of the composite viscosity-modified material obtained by the preparation method according to any one of claims 4-6 in the preparation of self-compacting concrete for foundation pit engineering.

9. The application according to claim 7 or 8, characterized in that, The amount of the composite viscosity modifier added is 8%-15% of the total mass of the cementitious materials in the self-compacting concrete. The cementitious materials include cement, mineral admixtures, and the composite viscosity modifier.

10. The application according to claim 9, characterized in that, The amount of the composite viscosity modifier added is 10% - 12% of the total mass of cementitious materials in the self-compacting concrete.

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