A forming process for targeting improvement of the interfacial transition zone of high-volume fly ash concrete

Abstract

The application discloses a forming process for targeted improvement of an interface transition zone of a large-mix fly ash concrete and belongs to the technical field of concrete materials. The forming process for targeted improvement of the interface transition zone of the large-mix fly ash concrete comprises the following steps: 1, a slag modified cement slurry is weighed and used to stir and wrap coarse aggregates, and pretreated coarse aggregates covered with a layer of the slag modified cement slurry are obtained after drying; and 2, the pretreated coarse aggregates obtained in the step 1 are mixed with cement, fly ash, fine aggregates and water to obtain the large-mix fly ash concrete. The forming process can target the improvement of the interface transition zone, can reduce the porosity of the interface transition zone of the large-mix fly ash concrete to 23.5%, has high accuracy, and the method for targeting the improvement of the interface transition zone is simple.

Description

Technical Field

[0001] This invention belongs to the field of concrete materials technology, specifically relating to a method for improving the interfacial transition zone of high-volume fly ash concrete using a novel molding process. Background Technology

[0002] Compared to ordinary cement concrete, high-volume fly ash concrete, due to the large amount of fly ash in its cementitious materials, exhibits improved properties such as increased workability, reduced shrinkage, less alkali-aggregate reaction, and improved resistance to chloride ion penetration. However, high-volume fly ash concrete also typically suffers from problems such as decreased early mechanical strength and poor frost resistance. In concrete, the interfacial transition zone is considered the weakest area. The presence of a large number of unreacted fly ash particles in high-volume fly ash concrete makes this interfacial transition zone particularly weak. During the performance degradation process of high-volume fly ash concrete, the interfacial transition zone is the first to lose structural stability; conversely, improving the interfacial transition zone also enhances the performance of high-volume fly ash concrete.

[0003] In existing technologies, appropriate amounts of highly reactive mineral admixtures such as slag or silica fume are typically added to concrete. Their high pozzolanic activity promotes the pozzolanic reaction, thereby improving the interfacial transition zone. However, these mineral admixtures directly enter the matrix during the concrete incorporation process, weakening their effect on improving the interfacial transition zone and failing to provide targeted improvement.

[0004] In addition, a staged mixing process can be employed. This involves first partially mixing water to prepare cement paste or mineral admixture / nanomaterial-modified cement paste, then mixing it with aggregates to coat them, and finally adding the remaining water. This allows the cement paste surrounding the aggregates to form a new interfacial transition zone in the hardened concrete, which can improve its microstructure. However, research has found that the water added later in the staged mixing process can enter the aggregate coating layer and form pores in the interfacial transition zone after the concrete hardens, increasing the porosity of the interfacial transition zone. Simultaneously, there is a lack of research on the enrichment degree of mineral admixtures or nanomaterials in the interfacial transition zone of hardened concrete when using mineral admixture / nanomaterial-modified cement paste. This leads to doubts about the actual improvement effect on the interfacial transition zone, limiting the degree of performance improvement in the concrete. Summary of the Invention

[0005] In view of the deficiencies in the existing technology, and in order to overcome the difficulties in improving the interface transition zone of high-volume fly ash concrete using existing methods, the present invention aims to design and provide a molding process that uses slag-modified cement paste to coat coarse aggregate, thereby preparing high-volume fly ash concrete and ultimately targeting the improvement of the interface transition zone.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] On the one hand, the present invention provides a molding process for targeted improvement of the interfacial transition zone of high-volume fly ash concrete, comprising the following steps:

[0008] (1) Weigh out slag-modified cement paste and mix and coat the coarse aggregate. After drying, obtain pretreated coarse aggregate covered with a layer of slag-modified cement paste.

[0009] (2) The pretreated coarse aggregate obtained in step (1) is mixed with cement, fly ash, fine aggregate and water to obtain high-volume fly ash concrete.

[0010] The molding process for targeted improvement of the interface transition zone of high-volume fly ash concrete, wherein the water-cement ratio of the slag-modified cement paste in step (1) is 0.3-0.4;

[0011] The slag content in the slag-modified cement paste is 1-10%.

[0012] The mass ratio of coarse aggregate to slag-modified cement paste is 1.5:1 to 4.5:1.

[0013] The molding process for targeted improvement of the interface transition zone of high-volume fly ash concrete, wherein the stirring time in step (1) is 2-10 min and the drying time is 1-8 h.

[0014] The molding process for targeted improvement of the interface transition zone of high-volume fly ash concrete, in step (1), the water-cement ratio of the slag-modified cement paste is 0.35, the slag content in the slag-modified cement paste is 3.80%, and the mass ratio of coarse aggregate to slag-modified cement paste is 3.2:1.

[0015] The molding process for targeted improvement of the interface transition zone of high-volume fly ash concrete, wherein the stirring time in step (1) is 10 min and the drying time is 1 h.

[0016] The molding process described above is a targeted improvement process for the interfacial transition zone of high-volume fly ash concrete, wherein the coarse aggregate is continuously graded crushed stone in a saturated surface-dry state.

[0017] The molding process for targeting and improving the interface transition zone of high-volume fly ash concrete, wherein the fly ash in step (2) accounts for 50%-70% of the total mass of cement and fly ash, and the water-cement ratio of high-volume fly ash concrete is 0.3-0.4.

[0018] The molding process for targeted improvement of the interface transition zone of high-volume fly ash concrete, wherein the fine aggregate in step (2) is sand; preferably, the fineness modulus is 2.7.

[0019] Secondly, the present invention provides a high-volume fly ash concrete, which is prepared by any of the molding processes described herein.

[0020] Thirdly, the present invention provides the application of any of the molding processes described in improving the interfacial transition zone of high-volume fly ash concrete.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The molding process of this invention can target and improve the interface transition zone, reducing the porosity of the interface transition zone of high-volume fly ash concrete to 23.5%, with high precision and a simple method for targeting and improving the interface transition zone. Attached Figure Description

[0023] Figure 1 It is a backscattered image of the interface transition zone of concrete with high fly ash content.

[0024] Figure 2 It is a grayscale image of the pore segmentation in the transition zone of concrete with high fly ash content.

[0025] Figure 3 This is a surface diagram showing the influence of cement paste coating coarse aggregate preparation parameters on the porosity of the interfacial transition zone of high-volume fly ash concrete. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0027] Example 1:

[0028] 1. Raw materials

[0029] This invention uses high-volume fly ash concrete as an example. The cementitious materials in the cement paste used to coat the coarse aggregate include P·II 42.5 type cement and slag. The coarse aggregate is continuously graded crushed stone in a saturated surface-dry state. After the slag-modified cement paste is used to coat the surface of the coarse aggregate, high-volume fly ash concrete is prepared using the coated coarse aggregate. The cementitious materials in the preparation process of high-volume fly ash concrete include P·II 42.5 type cement and Grade II fly ash. The fine aggregate is silica sand with a fineness modulus of 2.7.

[0030] 2. Experimental Design for Coarse Aggregate Wrapped in Cement Paste

[0031] A five-factor, five-level central composite design was used to select parameters for the preparation process of cement paste encapsulating coarse aggregate. The five factors were: X1, water-cement ratio of cement paste (W / CM); X2, slag content in cement paste (S / CM); X3, mass ratio of coarse aggregate to modified cement paste (CA / CP); and X4, mixing time of coarse aggregate and modified cement paste (T). m X5, drying time (T) of coarse aggregate and modified cement paste after mixing. d The value ranges for each factor are as follows: X1, 0.3-0.4; X2, 0-10%; X3, 1.5:1-4.5:1; X4, 2-10 min; X5, 0-8 h. A total of 50 groups of coarse aggregates coated with slag-modified cement paste were obtained, and the specific parameters are shown in Table 1 below. The preparation of the cement paste-coated coarse aggregates was carried out using an experimental concrete mixer. Cement, slag, and water were first mixed for 5 min to obtain a homogeneous slag-modified cement paste. Then, coarse aggregates were added and the mixture was stirred again. m (min), the coarse aggregate covered with slag-modified cement paste is dried at room temperature. d (h) prepares for the subsequent preparation of large-volume fly ash concrete.

[0032] Table 1 Selection of parameters for preparing coarse aggregate coated with cement paste

[0033]

[0034]

[0035]

[0036] 3. Mix proportion of high-volume fly ash concrete

[0037] The mix proportions of high-volume fly ash concrete prepared using the aforementioned coarse aggregates are shown in Table 2 below. High-volume fly ash concrete prepared with different coarse aggregates have the same fly ash content (60% of the cementitious material) and the same water-cement ratio (0.3). The high-volume fly ash concrete was prepared using a laboratory concrete mixer. Cement, fly ash, fine aggregates, and the previously prepared coarse aggregates were first dry-mixed for 5 minutes, then water was added and mixed for another 5 minutes to achieve better workability. After mixing, the fresh concrete was poured into cubic molds with sides of 100 mm and vibrated for approximately 5 seconds to fill the molds. All sample surfaces were covered with a plastic film and then placed in a room temperature environment for 24 hours. Afterward, the molds were removed and the samples were placed in a curing room to cure to the required testing age.

[0038] Table 2. Mix proportions of high-volume fly ash concrete (kg / m³) 3 )

[0039]

[0040] 4. Test methods

[0041] To investigate the effects of cement paste coating parameters on the porosity of the interfacial transition zone in high-volume fly ash concrete, a TESCAN scanning electron microscope was used to observe the interfacial transition zone and obtain backscattered images. At 90 days of curing, 1cm × 1cm thin samples were cut from the center of the high-volume fly ash concrete. After drying, the samples were immersed in low-viscosity epoxy resin. After the resin hardened, they were polished with oil-based diamond powder, progressing from coarse to fine, until the sample surface was sufficiently smooth. After polishing, the samples were ultrasonically cleaned in anhydrous ethanol to ensure surface cleanliness. Finally, the sample surface was sputter-coated with gold. The operating voltage of the scanning electron microscope was set to 30kV. Figure 1 This image shows the backscattered image of the interface transition zone of high-volume fly ash concrete using Group 48 coarse aggregate. In the backscattered image, the brightest areas are considered to be unhydrated cement particles, the black areas are pores and cracks, and the gray to dark gray areas generally correspond to CH, CSH, unhydrated fly ash particles, and other hydration products. Figure 1 The presence of spherical fly ash particles and pores is clearly visible.

[0042] To quantify the porosity of the interfacial transition zone in high-fly ash concrete, ImageJ software was used to process backscattered images and separate the pores. During the separation process, the inflection points of the grayscale histogram of the backscattered image were determined as the grayscale values ​​that distinguish the pores from the original image and display them as black. The resulting binarized image is shown below. Figure 2 As shown, after separating the pores from the image, the porosity of the interface transition zone can be calculated based on the area fraction of the black pore region relative to the interface transition zone. In the calculation process, 50 different interface transition zones were selected for porosity calculation to reduce the error from a single image and improve statistical accuracy.

[0043] 5. Test Results

[0044] 5.1 Porosity of high-volume fly ash concrete

[0045] Table 3 lists the porosity of the interfacial transition zone in 50 groups of high-volume fly ash concrete.

[0046] Table 3. Porosity (%) of the interfacial transition zone in high-fly ash concrete.

[0047]

[0048] Using the porosity of the transition zone at the interface of high-fly ash concrete as the dependent variable, a fitting formula between significant factors and the porosity can be established:

[0049]

[0050] Based on the fitting formula between the porosity of the transition zone at the interface of high-volume fly ash concrete and significant influencing factors, a 3D visualization of the response surface of different influencing factors to the porosity of the transition zone at the interface of high-volume fly ash concrete can be obtained, indicating possible directions for subsequent optimization of the porosity of the transition zone at the interface of high-volume fly ash concrete. Figure 3 This shows the effect of the water-cement ratio and slag content of cement paste on the porosity of the concrete interface transition zone in high-volume concrete. Figure 3 The water-cement ratio and slag content of the cement paste were increased from 0.3 to 0.4 and from 0 to 10%, respectively. The mass ratio of coarse aggregate to cement paste, the mixing time, and the drying time after mixing were fixed at 3, 6 min, and 4 h, respectively. Figure 3 It can be seen that although there is no significant interaction between the water-cement ratio and slag content of cement paste, their effects on the porosity of the interfacial transition zone in high-volume fly ash concrete are not linear. When the water-cement ratio and slag content reach their respective suitable values, the pozzolanic reaction of slag in cement paste is more complete. The microstructure of the interfacial transition zone formed by slag-modified cement paste coating coarse aggregate in high-volume fly ash concrete is more compact, resulting in a decrease in the porosity of the interfacial transition zone.

[0051] 5.2 Optimization of porosity in the interfacial transition zone of high-volume fly ash concrete

[0052] Based on the influence of different factors on the porosity of the interfacial transition zone of high-volume fly ash concrete, five influencing factors were optimized with the goal of minimizing the porosity of the interfacial transition zone of high-volume fly ash concrete. During the optimization process, the water-cement ratio of cement paste, slag content, the mass ratio of coarse aggregate to cement paste, and the mixing time and drying time after mixing were given equal weights. The optimization results are shown in Table 4 below.

[0053] As shown in Table 4, the optimal preparation parameters for coating coarse aggregate with cement paste to achieve the minimum porosity in the interfacial transition zone of high-fly ash concrete are: a water-cement ratio of 0.35, a slag content of 3.80%, a mass ratio of coarse aggregate to cement paste of 3.2, a mixing time of 10 min, and a drying time of 1 h. Under these optimal conditions, the predicted porosity of the interfacial transition zone in high-fly ash concrete is 22.88%.

[0054] Table 4. Optimization of the interfacial transition zone in high-volume fly ash concrete.

[0055]

[0056] 5.3 Verification of the optimal solution for porosity in high-volume fly ash concrete

[0057] High-volume fly ash concrete was prepared based on the optimal preparation parameters for coating coarse aggregate with cement paste. After 90 days of standard curing, the porosity of the interfacial transition zone was tested, and the measured value was 23.5%. The error between the predicted and measured values ​​of the interfacial transition zone in high-volume fly ash concrete was within 3%, indicating the accuracy of targeted improvement of the interfacial transition zone in high-volume fly ash concrete by coating coarse aggregate with modified cement paste.

Claims

1. A molding process for targeted improvement of the interfacial transition zone in high-volume fly ash concrete, characterized in that, Includes the following steps: (1) Weigh out slag-modified cement paste and mix and coat the coarse aggregate. After drying, obtain pretreated coarse aggregate covered with a layer of slag-modified cement paste. (2) The pretreated coarse aggregate obtained in step (1) is mixed with cement, fly ash, fine aggregate and water to obtain high-volume fly ash concrete. The water-cement ratio of the slag-modified cement paste mentioned in step (1) is 0.

3. 0.4; the slag content in the slag-modified cement paste is 1. 10%; the mass ratio of the coarse aggregate to the slag-modified cement paste is 1.5:

1. 4.5:1; The stirring time in step (1) is 2 seconds. 10 min; the drying time is 1 8h; The fly ash mentioned in step (2) accounts for 50% of the total mass of cement and fly ash. 70%, the water-cement ratio of high-volume fly ash concrete is 0.

3. 0.

4.

2. The molding process for targeted improvement of the interfacial transition zone of high-volume fly ash concrete as described in claim 1, characterized in that, In step (1), the water-cement ratio of the slag-modified cement paste is 0.35, the slag content in the slag-modified cement paste is 3.80%, and the mass ratio of the coarse aggregate to the slag-modified cement paste is 3.2:

1.

3. The molding process for targeted improvement of the interfacial transition zone of high-volume fly ash concrete as described in claim 1, characterized in that, The stirring time in step (1) is 10 min, and the drying time is 1 h.

4. The molding process for targeted improvement of the interfacial transition zone of high-volume fly ash concrete as described in claim 1, characterized in that, The coarse aggregate is continuously graded crushed stone in a saturated surface-dry state.

5. The molding process for targeted improvement of the interfacial transition zone of high-volume fly ash concrete as described in claim 1, characterized in that, The fine aggregate mentioned in step (2) is sand; the fineness modulus of the sand is 2.

7.

6. A high-volume fly ash concrete, characterized in that, By claim 1 It is prepared by any one of the molding processes described in item 5.

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