Secondary light concrete and preparation method thereof

Abstract

The invention relates to secondary light concrete and a preparation method thereof. Raw materials of the secondary light concrete comprise cement, fly ash, coarse aggregate, fine aggregate, an additive and water, the coarse aggregate comprises coarse light aggregate, the particle density of the coarse light aggregate is 1500-1540 kg / m < 3 >, the fine aggregate comprises fine light aggregate, the particle density of the fine light aggregate is 1500-1540 kg / m < 3 >, the secondary light concrete has 1800-2100 kg / m < 3 >, and the 28-day strength is greater than 50 MPa. The secondary light concrete adopts the light aggregate to replace part of common aggregate, so that not only can the overall density of the concrete be reduced, but also the light aggregate and the common aggregate can form dense skeleton accumulation, and further form continuous gradation with cement and fly ash, so that the closest accumulation is realized, and the effect of improving the compactness and strength of the concrete is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete, and in particular to a semi-light concrete and a preparation method thereof. Background Art

[0002] To accommodate the rapid growth in population, industrialization, and urbanization, construction projects are continuously evolving toward higher-rise structures and larger spans. For example, the use of these materials in bridge construction can significantly reduce static loads and improve bridge capacity. Furthermore, they can increase bridge spans, reduce structural cross-sections, conserve steel and prestressed tendons, and reduce foundation treatment costs. Analysis of concrete's development characteristics reveals that its primary development direction is toward high strength, lightweight, environmentally friendly, and durable properties. Therefore, employing modern concrete technology to develop lightweight, high-strength concrete with excellent durability, workability, physical and mechanical properties, applicability, volume stability, and economic rationality is crucial not only for the sustainable development of concrete and construction engineering, but also for the sustainable development of humanity, and is therefore of paramount importance.

[0003] Lightweight aggregate concrete has the advantage of being lightweight, which is undoubtedly of great significance for long-span structures such as bridges and high-rise buildings. However, its poor workability and lower strength than conventional aggregate concrete limit its application. Sub-lightweight concrete refers to ordinary aggregate mixed with an appropriate amount of lightweight aggregate, with a dry apparent density between 1800 and 2100 kg / m 3 The concrete between these two materials is also known as specified density concrete abroad. It can effectively overcome the shortcomings of lightweight aggregate concrete, has better working performance, higher compressive strength and good durability, and is widely used in certain specific environments.

[0004] At present, some colleges and universities are conducting theoretical research on sub-light concrete, but there is no summary of the application of sub-light concrete and an introduction to its preparation methods in the society. Summary of the Invention

[0005] The purpose of the present invention is to disclose a semi-light concrete and a preparation method thereof, so as to solve one or more technical problems existing in the prior art and provide at least one beneficial option or create conditions.

[0006] A first aspect of the present invention is to provide a semi-light concrete.

[0007] The second aspect of the present invention is to provide a method for preparing the semi-light concrete described in the first aspect of the present invention.

[0008] The raw materials of the semi-light concrete of the first aspect of the present invention include cement, fly ash, coarse aggregate, fine aggregate, admixture and water, wherein the coarse aggregate includes coarse light aggregate, and the particle density of the coarse light aggregate is 1500-1540 kg / m 3 The fine aggregate includes fine light aggregate, and the particle density of the fine light aggregate is 1500~1540 kg / m 3 The weight of the sub-light concrete is 1800~2100 kg / m 3 , 28-day strength > 50 MPa.

[0009] The present invention uses both common and lightweight aggregates, taking advantage of the low density of coarse and fine lightweight aggregates. This results in a concrete density intermediate between that of common and lightweight aggregates, meeting the requirements of sub-lightweight concrete while also providing high strength. This sub-lightweight concrete can be used in structural areas where concrete bulk density is relatively low and strength is required.

[0010] In a further embodiment, the coarse light aggregate is crushed stone shale ceramsite; preferably, the bulk density of the crushed stone shale ceramsite is 850-880 kg / m 3 , porosity 53~55%, water absorption rate 3.2% in 30 minutes, and water absorption rate 4.2% in 24 hours.

[0011] In a further embodiment, the fine light aggregate is fine-grained shale ceramsite; preferably, the bulk density of the fine-grained shale ceramsite is 870-900 kg / m 3 , water absorption rate is 13% in 1 hour.

[0012] In a further embodiment, the fine aggregate includes river sand in addition to the fine light aggregate; preferably, the river sand is continuously graded, with a fineness modulus of 2.7-2.8 and an apparent density of 2630-2650 kg / m 3 , bulk density is 1530~1550 kg / m 3 .

[0013] In a further embodiment, the coarse aggregate includes granite crushed stone in addition to the coarse and light aggregate; preferably, the granite crushed stone has a particle size range of 5 to 25 mm, continuous grading, a crushing value of 8.3%, and an apparent density of 2640 to 2660 kg / m 3 , bulk density is 1580~1600 kg / m 3 .

[0014] In a further application embodiment, the cement is ordinary Portland cement with a strength grade of ≥42.5.

[0015] In a further embodiment, the fly ash is Class II fly ash with an apparent density of 2100 kg / m 3 .

[0016] In a further application embodiment, the admixture is a polycarboxylate water reducer with a solid content of ≥35% and a water reduction rate of ≥25%.

[0017] In a further embodiment, the raw material ratio, calculated by weight, includes: 380-400 parts cement, 160-250 parts fly ash, 700-770 parts coarse aggregate, 510-550 parts fine aggregate, 4-5 parts admixture, and 160-260 parts water. This semi-lightweight concrete utilizes both conventional aggregate mixed with lightweight aggregate and an excess of fly ash, which has a lower density, to replace aggregate. This helps control concrete density while maintaining strength, thereby meeting the requirements of specific environments.

[0018] The preparation method according to the second aspect of the present invention comprises the steps of: 1) adding the coarse aggregate, the fine aggregate, the cement, and the fly ash into a mixing device in sequence and stirring and mixing them uniformly; 2) mixing water and the water reducer, adding the mixture into a stirring device and stirring to obtain a mixture; 3) The slump of the mixture is controlled at 120-160 mm, which is a semi-light concrete finished product.

[0019] In a further application embodiment, the coarse lightweight aggregate and the fine lightweight aggregate are soaked in water for at least 24 hours before being mixed and stirred.

[0020] The present invention provides a high-strength, lightweight concrete that uses lightweight aggregate to replace some conventional aggregate. This not only reduces the overall density of the concrete but also allows the lightweight aggregate to be mixed with conventional aggregate to form a dense skeleton, which in turn forms a continuous gradation with cement and fly ash, achieving the densest packing, thereby improving the density and strength of the concrete. The coarse and fine lightweight aggregates themselves have high water absorption rates. As the concrete ages, the moisture in the lightweight aggregate is continuously released, continuously reacting with the cement to further improve the concrete's strength.

[0021] The addition of fly ash creates a ball-bearing effect, improving the concrete's performance and later strength. The addition of polycarboxylic acid-based water-reducing agents can effectively reduce water consumption, improve concrete performance, and increase concrete viscosity, minimizing the separation of lightweight aggregates from standard aggregates. These technical measures can keep concrete density within a range of 1800-2100 kg / m 3The 7-day strength is 40~45 MPa, and the 28-day strength is 50~55 MPa, which meets the requirements of engineering parts for concrete density and higher strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a strength trend chart for different mix ratios at different ages in Example 1. DETAILED DESCRIPTION

[0023] The following examples further illustrate the present invention, but should not be construed as limiting the present invention. Without departing from the spirit and substance of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention are within the scope of the present invention.

[0024] Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art.

[0025] Example 1 (1) Determine the proportion of lightweight aggregate in the aggregate.

[0026] The top slab of an open-cut tunnel in a national highway project needs to be open to traffic, so the design requires that measures be taken to control the concrete density to 2100 kg / m 3 At the same time, the 28-day strength of concrete is controlled to be above 45 MPa. In this embodiment, the allowable density of concrete is determined according to the application scenario and conditions of concrete. The approximate ratio range of ordinary aggregate and lightweight aggregate can be obtained by calculation. Then, the exact ratio of lightweight aggregate to ordinary aggregate is determined through dense skeleton stacking test to ensure that its density is within the calculated ratio range. At the same time, it is ensured that there are fewer gaps between the aggregates and the stacking density is higher. The ordinary coarse aggregate used in this embodiment is granite crushed stone, and the ordinary fine aggregate is river sand. If the calculated density is less than 2100 kg / m 3 The mass ratio of ordinary coarse aggregate to coarse lightweight aggregate should be controlled within 1:(2-1), and the mass ratio of fine ordinary aggregate to fine lightweight aggregate should be controlled within 1:(1.3-1.8). The mixed stacking test yielded the stacking data shown in Tables 1 and 2.

[0027] Table 1. Stacking data of mixed coarse aggregate

[0028] Table 2. Stacking data of mixed fine aggregate

[0029] (2) Determine the water-cement ratio.

[0030] With the cement dosage constant, the lower the water-cement ratio, the thicker the cement paste and the lower the fluidity of the concrete mix. When the water-cement ratio is too low, the cement paste becomes dry and thick, and the fluidity of the concrete mix is too low, making construction difficult and failing to ensure concrete density. Increasing the water-cement ratio increases fluidity, but if the ratio is too high, the concrete mix loses cohesiveness and water retention, resulting in slurry flow and segregation, and seriously affecting the strength of the concrete.

[0031] Four different water-cement ratios were tested: 0.3, 0.32, 0.35, and 0.38. Analysis of the test results shows that, because the density of cement paste is lower than that of aggregate in concrete, concrete density decreases with increasing water-cement ratio. Simultaneously, the concrete's fluidity increases, as evidenced by increases in slump and spread. At a water-cement ratio of 0.3, the cement paste is dry and thick, with poor fluidity. This results in numerous voids in the concrete during molding, which compromises its strength. At water-cement ratios of 0.35 and 0.38, the concrete's cohesiveness and water retention are both poor, with significant flow and segregation, which compromise its compactness and, consequently, its strength. While the concrete's fluidity is better when the water-cement ratio is high, evaporation of water after molding results in more voids, impacting its compactness. Furthermore, due to its improved fluidity, lightweight aggregates in the concrete tend to float, resulting in poor homogeneity. The above two aspects lead to the decrease in the strength of sub-lightweight concrete at high water-cement ratio. Taking all factors into consideration, when the water-cement ratio is 0.32, the fluidity of concrete is moderate and the strength can reach an ideal level.

[0032] (3) Determine the water consumption.

[0033] According to the "Code for Design of Ordinary Concrete Mix Proportions" (JGJ55-2011) regarding concrete water consumption, combined with the design requirement of a concrete slump range of 120-160 mm, the water consumption is 210-230 parts per million. Furthermore, the admixture water reduction rate is calculated as 30%, resulting in a water consumption of 140-160 parts per million. The specific water consumption is determined through trial mixes. The water absorption of the coarse and fine lightweight aggregates should also be deducted during the concrete trial mix.

[0034] (4) Determine the amount of fly ash used.

[0035] When fly ash is added to concrete, it significantly affects the concrete's internal structure and physical and mechanical properties. The fly ash particle distribution is similar to that of cement, but its particle shape is spherical, resulting in a "morphological effect" in the concrete mixture. Most high-quality fly ash particles have a smooth surface. When added to concrete as a partial replacement for cement, they can increase the concrete's fluidity or reduce its water consumption. Fly ash particles have a lower density than cement particles. By replacing a portion of cement in concrete, they can reduce the density gap between the cement paste and coarse and lightweight aggregate, effectively alleviating the floating of lightweight aggregate in concrete and the resulting poor homogeneity, thereby improving the concrete's mechanical properties.

[0036] Fly ash participates less in the hydration reaction early on. Adding fly ash to concrete is equivalent to reducing the amount of cementitious materials while maintaining the same initial water usage. Consequently, the amount of hydration products per unit volume of concrete is low, the hardened cement paste structure is relatively loose, and the early strength decreases accordingly. As the concrete ages, the hydration reaction continues, and fly ash continuously participates in the hydration reaction to form hydrated calcium silicate and hydrated calcium aluminate. In the presence of gypsum, an AFt phase forms. Initially, the hydration products are primarily gel-like. With age, they gradually transform into fibrous crystals, increasing in number and forming interlocking structures that rapidly increase strength later in the concrete. Furthermore, the micro-aggregate effect of fly ash densifies the concrete structure, resulting in later strength and elastic modulus that rival or even exceed that of concrete without fly ash.

[0037] The sub-lightweight concrete was trial-mixed according to the four mix proportions provided in Table 3. Fly ash was added at a ratio of 18.6% to the cementitious materials. In addition, 5% to 20% of the coarse and fine aggregates were replaced by fly ash. The results were compared with those of ordinary concrete without excessive replacement, and the corresponding indicators were tested.

[0038] Table 3. Trial mix data of sub-light concrete with different fly ash replacement amounts

[0039] The test results are shown in Table 4 and Figure 1As shown in Figure 4, excessive fly ash replacement of fine aggregate can increase the paste volume and improve fluidity. Since fly ash has a lower density than aggregate, concrete density decreases with increasing excess replacement. Fly ash has low activity and is difficult to manifest in the early stages of concrete construction. Increasing the excess fly ash replacement ratio results in a decrease in early concrete strength. However, due to its potential activity, its later strength increases. Experimental data shows that when fly ash replacement exceeds 10%, the strength decreases rapidly, with the 7-day strength failing to reach 40 MPa. Given the rapid pace of modern construction projects, high early strength requirements are generally required. Incorporating large amounts of fly ash can seriously impact project quality or schedule. Table 4 shows that Groups 1 and 5 have similar compressive strengths at various stages. However, the 5% excess fly ash replacement in Group 1, combined with the use of lightweight aggregate, significantly reduces its wet density, meeting construction requirements. Therefore, this test can conclude that the amount of fly ash replacing aggregate should not be too high, and the replacement amount is determined to be 5%.

[0040] Table 4. Performance indicators of lightweight concrete with different fly ash replacement amounts

[0041] (5) Determine the dosage of admixture.

[0042] To reduce water usage, an admixture, polycarboxylate superplasticizer, is added to the raw materials for preparing the concrete. This polycarboxylate superplasticizer, with a solids content of ≥35%, offers excellent water-reducing properties despite minimal addition, resulting in concrete with improved workability and mechanical properties.

[0043] (6) Selection of concrete.

[0044] In order to ensure the mechanical strength of concrete, in the determined concrete mix ratio, the strength grade of the cement is ≥42.5.

[0045] (7) Comprehensively mix sub-light concrete.

[0046] The optimal mix ratio obtained through the above test steps includes, by mass, 385 parts cement, 169 parts fly ash, 512 parts ordinary coarse aggregate, 244 parts coarse lightweight aggregate, 340 parts ordinary fine aggregate, 201 parts fine lightweight aggregate, 5 parts admixture, and 177 parts water. The slump of the mixture is controlled between 120 and 160 mm.

[0047] In the present invention, ordinary materials are replaced by lightweight aggregates to reduce the mixed density of aggregates. It can be seen from the dense stacking test that when the volume of coarse lightweight aggregate accounts for 40% of the coarse aggregate, the volume of fine lightweight aggregate accounts for 50% of the fine aggregate, and the fly ash content is 11%, the density of the aggregate mixture reaches the maximum.

[0048] First, soak the coarse and fine light aggregates for 24 hours to allow them to absorb water and reduce internal porosity. Obtain the raw materials according to the above ratios, mix the coarse aggregate containing the coarse light aggregate and the fine aggregate containing the fine light aggregate. Then, add cement and fly ash to the mixer in sequence and mix thoroughly. Mix water and a water reducer, then add the mixture to the mixer and stir until the mixture becomes concrete, thus obtaining the finished semi-light concrete.

[0049] The finished sub-lightweight concrete material is poured into the desired construction site. As the concrete age increases, moisture in the coarse and fine light aggregates is gradually released and continuously hydrated with cement particles, further improving the mechanical strength of the concrete.

[0050] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A semi-light concrete, characterized in that: The raw materials include cement, fly ash, coarse aggregate, fine aggregate, admixture and water. The coarse aggregate includes coarse light aggregate, and the particle density of the coarse light aggregate is 1500~1540 kg / m 3 The fine aggregate includes fine light aggregate, and the particle density of the fine light aggregate is 1500~1540 kg / m 3 The density of the sub-light concrete is 1800-2100 kg / m 3 , 28-day strength > 50 MPa.

2. The semi-light concrete according to claim 1, characterized in that: The coarse and light aggregate is crushed stone shale ceramsite; preferably, the bulk density of the crushed stone shale ceramsite is 850-880 kg / m 3 , porosity 53~55%, water absorption rate 3.2% in 30 minutes, and water absorption rate 4.2% in 24 hours.

3. The semi-light concrete according to claim 1, characterized in that: The fine light aggregate is fine-grained shale ceramsite; preferably, the bulk density of the fine-grained shale ceramsite is 870-900 kg / m 3 , water absorption rate is 13% in 1 hour.

4. The semi-light concrete according to claim 1, characterized in that: The fine aggregate comprises river sand; preferably, the river sand has a fineness modulus of 2.7-2.8 and an apparent density of 2630-2650 kg / m 3 , bulk density is 1530~1550 kg / m 3 .

5. The semi-light concrete according to claim 1, characterized in that: The coarse aggregate includes granite crushed stone; preferably, the granite crushed stone has a particle size range of 5-25 mm, a crushing value of 8.3%, and an apparent density of 2640-2660 kg / m 3 , bulk density is 1580~1600 kg / m 3 .

6. The semi-light concrete according to claim 1, characterized in that: The cement is ordinary Portland cement with a strength grade of ≥42.

5.

7. The semi-light concrete according to claim 1, characterized in that: The fly ash is Grade II fly ash with an apparent density of 2100 kg / m 3 .

8. The semi-light concrete according to claim 4, characterized in that: The admixture is a polycarboxylate water reducer with a solid content of ≥35%.

9. The semi-light concrete according to any one of claims 1 to 8, characterized in that: Calculated by mass, the raw material ratio includes: 380~400 parts of cement, 160~250 parts of fly ash, 700~770 parts of coarse aggregate, 510~550 parts of fine aggregate, 4~5 parts of admixture, and 160~260 parts of water.

10. The method for preparing the semi-light concrete according to any one of claims 1 to 9, characterized in that: The coarse aggregate, the fine aggregate, the cement, and the fly ash are sequentially added to a mixing device and stirred until uniformly mixed; the water and the water reducer are mixed until uniformly mixed, and then added to the mixing device and stirred to obtain a mixture, wherein the slump of the mixture is controlled to be 120-160 mm, which is a finished semi-light concrete material.

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