High-activity targeted dense composite mineral admixture and concrete preparation method thereof

Abstract

The application discloses a high-activity targeted dense composite mineral admixture and a concrete preparation method thereof, and belongs to the technical field of building materials. The admixture is a multi-grade composite active mineral powder, the particle size of which covers 0.01-20 mu m. Based on the statistical characteristic curve of the target concrete pore size distribution, the dosage of each particle size component is determined according to the most compact packing theory and the pore volume proportion of each pore size interval, so as to realize the hierarchical matching filling of pores. The concrete is prepared by using cement and fly ash as cementitious materials, externally adding 0.5-1.5% of the admixture and 0.8-1.5% of a water reducing agent through a step-by-step stirring process. The application can precisely optimize the pore structure of the concrete, reduce the proportion of harmful pores and connected pores, significantly improve the sulfate corrosion resistance and freeze-thaw resistance of the concrete, and is suitable for the concrete structures of railway, highway and water conservancy projects in corrosion and freeze-thaw areas.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to a highly active targeted dense composite mineral admixture and its concrete preparation method. It is suitable for highly corrosive environments such as salt lakes, saline soils, and high sulfate groundwater areas, as well as for railway, highway, water conservancy, and industrial and civil building concrete projects under repeated freeze-thaw environments in frigid regions. It can effectively improve the durability of concrete under the coupled effects of corrosion and freeze-thaw. Background Technology

[0002] Salt lakes, saline soils, and areas with high sulfate levels are widely distributed in western my country, such as the Qarhan Salt Lake in Qinghai, Lop Nur in Xinjiang, and the Hexi Corridor in Gansu. In some areas, the sulfate concentration far exceeds the maximum corrosion level specified in current standards. At the same time, these areas have large diurnal temperature differences and severe winters, and the engineering concrete structures are subjected to the coupled effects of sulfate erosion, salt crystallization pressure, and freeze-thaw cycles for a long time.

[0003] Sulfate ions can penetrate into the concrete through interconnected capillaries, microcracks, and interfacial transition zones, reacting with calcium hydroxide and hydrated calcium aluminate in the cement hydration products to form ettringite and gypsum, leading to internal expansion and cracking of the concrete. Freeze-thaw cycles cause the solution in the pores to repeatedly freeze, expand, and thaw, generating osmotic pressure differences, which exacerbate the propagation of microcracks and the connection of pores, further accelerating the intrusion of corrosive media, forming a vicious cycle of "corrosion-freeze-thaw" interaction and deterioration, ultimately leading to a decrease in concrete strength and a significant reduction in service life.

[0004] Currently, the protective technologies against sulfate corrosion and freeze-thaw damage in concrete mainly fall into four categories: First, incorporating conventional mineral admixtures such as fly ash, slag, and silica fume to optimize pore structure and reduce permeability through pozzolanic reaction; second, using high-performance water-reducing agents to lower the water-cement ratio and reduce the formation of harmful pores; third, adding air-entraining agents to form a closed-pore system to improve freeze-thaw resistance; and fourth, using physical isolation measures such as surface coatings and penetrating crystallizing materials to block the intrusion of harmful media.

[0005] However, the above technologies all have obvious shortcomings: the particle size of conventional mineral admixtures is mostly single or wide gradation range, which cannot form a precise "particle size-pore size" match with the multi-scale pore structure of concrete. The filling capacity of nanoscale gel pores and transition pores is limited, the proportion of harmful capillary pores is difficult to reduce effectively, and the improvement of durability is limited; the addition of air-entraining agents will reduce the strength of concrete and is not conducive to the densification of pore structure; the surface protective coating is prone to aging and peeling, and the protective performance deteriorates rapidly in areas with fluctuating groundwater levels, freeze-thaw cycles and pressure seepage conditions, and is difficult to repair.

[0006] Furthermore, existing concrete mix design primarily focuses on strength and workability as core control indicators, failing to adequately consider the intrinsic relationship between pore structure characteristics and the transport behavior of harmful media. The particle size distribution of existing corrosion-resistant and freeze-thaw-resistant mineral admixtures is mostly selected based on experience, without establishing a quantitative correspondence with the actual pore distribution characteristics of the target concrete. This results in low material utilization efficiency, insufficient pore filling, and difficulty in meeting the long-term service requirements under highly corrosive and high-freeze-thaw environments. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by solving problems such as poor matching between the particle size of existing mineral admixtures and the pore size of concrete, insufficient precision in pore structure control, low material utilization efficiency, and limited improvement in corrosion resistance and freeze-thaw resistance under strong corrosion and high freeze-thaw coupling environments. It provides a highly active targeted dense composite mineral admixture that can accurately match the pore structure of concrete, as well as a corresponding concrete preparation method.

[0008] To achieve the above objectives, the specific technical solution of the present invention is as follows: First, this invention provides a highly active targeted dense composite mineral admixture. The admixture is a multi-graded composite active mineral powder with an overall particle size range of 0.01 μm to 20 μm, composed of active mineral components corresponding to different particle size ranges. These different particle size ranges correspond one-to-one with the pore size distribution range of the target concrete. The dosage ratio of the active mineral components in each particle size range is determined based on the pore volume ratio of the corresponding pore size range in the statistical characteristic curve of the target concrete pore size distribution, calculated according to the closest packing theory, achieving graded matching filling of pores from nanometer to micrometer scale.

[0009] Furthermore, the statistical characteristic curve of the target concrete pore size distribution is obtained by extracting, screening and statistically analyzing the measured data of the pore structure of concrete of the same strength grade and the same cementitious system, and then normalizing and fitting the mean. The sample covers the pore size distribution data of different water-cement ratios, curing ages and curing conditions, and can reflect the common laws of the pore structure of this type of concrete.

[0010] Furthermore, the active mineral components include at least two of nano-calcium carbonate, silica fume, and ultrafine fly ash. The particle size range of each component corresponds to the gel pore, transition pore, and capillary pore size range of concrete, and all of them have hydration activity and can participate in secondary hydration reactions in the later stage of hydration.

[0011] Secondly, this invention provides a corrosion-resistant and freeze-thaw-resistant concrete containing the above-mentioned admixtures. Based on the total mass of the cementitious materials (100%), the cementitious materials consist of 65%–70% cement and 30%–35% fly ash; the amount of natural sand is 1.5–2.0 times the total mass of the cementitious materials, and the amount of crushed stone is 2.0–2.3 times the total mass of the cementitious materials; the highly active targeted dense composite mineral admixture is added externally at a dosage of 0.5%–1.5% of the total mass of the cementitious materials; it also includes an externally added water-reducing agent at a dosage of 0.8%–1.5% of the total mass of the cementitious materials.

[0012] Furthermore, the fly ash is Class F, Grade II fly ash, with a loss on ignition of less than 8%, sulfur trioxide content of less than 3.0%, free calcium oxide content of less than 1%, and strength activity index of greater than 70%; the water-reducing agent is a polycarboxylate-based high-performance water-reducing agent with a water reduction rate of not less than 28%.

[0013] Finally, the present invention provides a method for preparing the above-mentioned corrosion-resistant and freeze-thaw-resistant concrete, comprising the following steps: (1) Dry mixing of powder: Cement, fly ash and highly active targeted dense composite mineral admixture are put into a forced concrete mixing equipment and dry mixed for 90 seconds to fully disperse the ultrafine powder components and avoid agglomeration; (2) Aggregate mixing: Add natural sand and crushed stone to the mixing equipment and continue mixing for 90 seconds to make the cementitious powder and aggregate initially mixed evenly; (3) Wet mixing molding: After the water and water-reducing agent are mixed evenly in advance, they are added to the mixing equipment and mixed for 180s~240s to obtain a homogeneous concrete mixture. The slump at the discharge point is controlled to be 180~220mm and the spread is ≥500mm.

[0014] Furthermore, the finished product of the highly active targeted dense composite mineral admixture has a particle size range of 0.01μm to 50μm and is prepared through a process of graded grinding, particle size sorting and proportional compounding.

[0015] Compared with the prior art, the present invention has the following significant advantages: 1. Targeted and precise filling with high pore optimization efficiency. This invention designs the admixture particle size distribution based on the statistical characteristic curve of the pore size distribution of the target concrete, and determines the dosage of each particle size component according to the pore volume ratio, achieving graded matching filling of "particle size-pore size". It can simultaneously cover nanoscale gel pores to micron-scale capillary pores, greatly improving pore filling efficiency, effectively reducing the proportion of harmful pores and interconnected pores, and blocking the transmission channels of corrosive ions and moisture.

[0016] 2. High performance with low dosage, and good economic efficiency. Relying on precise gradation design, only 0.5%~1.5% of the admixture is needed to achieve a significant densification effect, which is far lower than the dosage of conventional mineral admixtures, thus improving durability while controlling material costs.

[0017] 3. Dual mechanism of action for stable durability. The admixture exerts a microparticle filling effect in the early stage of hydration, improving the initial density; in the later stage of hydration, the active mineral components participate in the secondary hydration reaction, generating additional hydration products to further refine the pore structure. It can inhibit the degradation of pores caused by sulfate attack and freeze-thaw cycles for a long time, and maintain stable performance under coupled environment.

[0018] 4. Strong adaptability to construction. The step-by-step mixing preparation process effectively solves the engineering problem of easy agglomeration of ultrafine powder, ensuring that the admixture is uniformly dispersed in concrete, and the workability of the concrete meets the requirements of conventional construction. It can be directly applied to existing engineering construction systems. Attached Figure Description

[0019] Figure 1 The pore size distribution curves of each concrete specimen at 0 days of salt-freezing coupling effect are shown. Figure 2 The pore size distribution curves of each concrete specimen after 300 days of salt-freezing coupling effect are shown. Figure 3 A diagram showing the porosity of different pore sizes at 0 days of age for each concrete specimen subjected to salt-freezing coupling. Figure 4 The porosity diagram of different pore sizes in each concrete specimen at 300 days of salt-freezing coupling effect. Figure 5 Photo of the finished product of a highly active targeted dense composite mineral admixture. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are implemented based on the technical solution of the present invention and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following contents.

[0021] 1. Admixture Design and Preparation This embodiment focuses on the design and preparation of a highly active, targeted, dense composite mineral admixture for C50 strength grade fly ash-cement system concrete. The specific design process is as follows: First, measured data of the pore structure of C50 strength grade concrete with a fly ash content of about 30% were collected. The sample covered multiple sets of test results with water-cement ratio of 0.35~0.45 and curing age of 28d~180d. After removing outlier data, the data was normalized and fitted to obtain the statistical characteristic curve of pore size distribution of the concrete system. The concrete pore size is divided into three levels: 0.01~0.1μm (gel pore range), 0.1~1μm (transition pore range), and 1~20μm (capillary pore range). Based on the closest packing theory and the pore volume ratio of each range, the proportion of active mineral components for the corresponding particle size is determined. Nano-calcium carbonate corresponds to the gel pore range, silica fume corresponds to the transition pore range, and ultrafine fly ash corresponds to the capillary pore range. The raw materials are subjected to ultrafine grinding and air classification to obtain single-component powders with target particle sizes. Then, they are compounded and mixed with high precision according to the calculated ratio to finally obtain the finished additive. The overall particle size range of the finished product is 0.01μm~50μm.

[0022] 2. Concrete mix design This embodiment sets up one control group and four experimental groups. The concrete mix proportions for each group are shown in Table 1. The control group does not contain the highly active targeted dense composite mineral admixture, while the experimental groups contain different dosages of the admixture. The dosages of other components remain consistent, and the total mass of cementitious materials is 460 kg / m³. 3 .

[0023] The cement used is P·O 42.5 grade ordinary Portland cement; the fly ash is Class F II fly ash with a loss on ignition of 7.2%, sulfur trioxide content of 2.5%, free calcium oxide content of 0.8%, and strength activity index of 75%; the water-reducing agent is a polycarboxylate-based high-performance water-reducing agent with a water reduction rate of 30%; the natural sand is Zone II medium sand with a fineness modulus of 2.6; and the crushed stone is 5~20mm continuously graded granite crushed stone.

[0024] 3. Concrete preparation process All concrete groups were prepared using the following step-by-step process: (1) Dry mixing of powder: Cement and fly ash (highly active targeted dense composite mineral admixture was added at the same time in the experimental group) were put into a forced twin-shaft concrete mixer and dry mixed for 90s to fully disperse the ultrafine powder and avoid agglomeration; (2) Aggregate mixing: Add natural sand and crushed stone to the mixer and continue mixing for 90 seconds to make the cementitious powder evenly adhere to the surface of the aggregate and achieve initial uniform mixing of solid materials; (3) Wet mixing molding: After mixing the water and polycarboxylate superplasticizer evenly, add them to the mixer and continue mixing for 180s to obtain a uniform concrete mixture.

[0025] Testing revealed that the slump of each concrete batch was 190-210 mm and the spread was 520-560 mm, meeting the workability requirements for pumping construction. The mixture was cast into 100 mm × 100 mm × 100 mm cube specimens, and after standard curing for 28 days, salt-freeze coupling performance tests were conducted.

[0026] 4. Performance Verification and Result Analysis The experiment employed a semi-immersion environment with an 8% sodium sulfate solution, simultaneously applying temperature cycles ranging from -20℃ to 40℃, with a positive to negative temperature duration ratio of 2:1 and a cycle period of 1 day. The compressive strength, dynamic elastic modulus, and mass changes of the specimens were measured at 0 days and 300 days of the experiment. Simultaneously, nuclear magnetic resonance (NMR) was used to test the pore size distribution and verify the evolution characteristics of the pore structure.

[0027] Depend on Figure 1 , Figure 3 It is evident that at the initial stage of the experiment (0d), the pore size distribution of the concrete in the admixture-added experimental group was more concentrated in the small pore size range, with a higher proportion of harmless pores below 100nm, and the proportion of harmful macropores and interconnected pores was significantly lower than that in the control group. This indicates that the admixture exerted a good microparticle filling effect in the early stage of hydration, optimizing the initial pore structure.

[0028] Depend on Figure 2 , Figure 4 It is evident that after 300 days of salt-freezing coupling, the proportion of large pores in the control group concrete increased significantly, the pores became coarser and the connectivity was enhanced, showing a significant deterioration trend; while the pore size distribution curve of the experimental group concrete showed little change, the pore deterioration was effectively suppressed, and the overall pore structure remained stable.

[0029] As shown in Table 1, after 300 days of salt-freeze coupling, the compressive strength retention rate, relative dynamic elastic modulus, and relative mass of the experimental group were significantly higher than those of the control group. Among them, the performance improvement of Example 1 was the most significant, with the 300-day compressive strength increasing by 49.8% compared with the control group, the relative dynamic elastic modulus increasing by 9.16 percentage points, and the relative mass increasing by 1.93 percentage points. This proves that the admixture of the present invention can significantly improve the durability of concrete under the coupled corrosion and freeze-thaw environment.

Claims

1. A highly active targeted dense composite mineral admixture, applied to optimize the pore structure and improve the durability of concrete, characterized in that: The admixture is a multi-graded composite active mineral powder with an overall particle size range of 0.01μm to 20μm, composed of active mineral components corresponding to different particle size ranges. The different particle size ranges correspond one-to-one with the pore size distribution range of the target concrete. The dosage ratio of the active mineral components in each particle size range is determined by calculation based on the pore volume ratio of the corresponding pore size range in the statistical characteristic curve of the pore size distribution of the target concrete, according to the closest packing theory, to achieve graded matching filling of pores from nanometer to micrometer.

2. The highly active targeted dense composite mineral admixture according to claim 1, characterized in that: The statistical characteristic curve of the target concrete pore size distribution is obtained by extracting, screening and statistically analyzing the measured pore structure data of concrete with the same strength grade and cementitious system, and then normalizing and fitting the mean. The sample covers pore size distribution data under different water-cement ratios, curing ages and curing conditions.

3. The highly active targeted dense composite mineral admixture according to claim 1, characterized in that: The active mineral components include at least two of nano-calcium carbonate, silica fume, and ultrafine fly ash, and the particle size range of each component corresponds to the gel pore, transition pore, and capillary pore size range of concrete, respectively.

4. A corrosion-resistant and freeze-thaw resistant concrete, characterized in that: The mixture comprises the highly active targeted dense composite mineral admixture as described in any one of claims 1 to 3; the cementitious material, based on 100% of its total mass, consists of 65% to 70% cement and 30% to 35% fly ash; the amount of natural sand is 1.5 to 2.0 times the total mass of the cementitious material, and the amount of crushed stone is 2.0 to 2.3 times the total mass of the cementitious material; the highly active targeted dense composite mineral admixture is added by external admixture, with a dosage of 0.5% to 1.5% of the total mass of the cementitious material; it also includes an externally admixture water-reducing agent, with a dosage of 0.8% to 1.5% of the total mass of the cementitious material.

5. The corrosion-resistant and freeze-thaw-resistant concrete according to claim 4, characterized in that: The fly ash is Class F, Grade II fly ash, with a loss on ignition of less than 8%, sulfur trioxide content of less than 3.0%, free calcium oxide content of less than 1%, and strength activity index of greater than 70%; the water-reducing agent is a polycarboxylate-based high-performance water-reducing agent with a water reduction rate of not less than 28%.

6. A method for preparing corrosion-resistant and freeze-thaw-resistant concrete as described in claim 4 or 5, characterized in that, Includes the following steps: (1) Dry mixing of powder: Cement, fly ash and highly active targeted dense composite mineral admixture are put into a forced concrete mixing equipment and dry mixed for 90 seconds to make the ultrafine powder components evenly dispersed. (2) Aggregate mixing: Add natural sand and crushed stone to the mixing equipment and continue mixing for 90 seconds to make the cementitious powder and aggregate initially mixed evenly; (3) Wet mixing molding: After the water and water-reducing agent are mixed evenly in advance, they are added to the mixing equipment and mixed for 180s~240s to obtain a homogeneous concrete mixture. The slump at the discharge point is controlled to be 180~220mm and the spread is ≥500mm.

7. The method for preparing corrosion-resistant and freeze-thaw-resistant concrete according to claim 6, characterized in that: The high-activity targeted dense composite mineral admixture has a particle size range of 0.01μm to 50μm and is prepared by graded grinding, particle size sorting and proportional compounding processes.

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