Research on improving the durability performance of damaged concrete with mineral admixtures

Concrete can be damaged under long‐term chloride ion action. This article uses mineral admixtures to repair the damaged concrete, and then conducts durability performance tests. Multi‐dimensional chloride ion diffusion tests were conducted under different salt solution immersion environments. The resistance of concrete to chloride ion corrosion was evaluated through RCM chloride ion penetration test, and the internal chloride ion concentration of concrete with different diffusion dimensions was detected through chemical titration analysis. Taking the SW test group as an example, after soaking for 3 months, the three‐dimensional diffusion chloride ion concentration of the SW test group is 2.55 times that of the one‐dimensional diffusion. Under the same three‐dimensional diffusion conditions, at a depth of 10 mm of concrete erosion, the chloride ion concentration of the SW test group after soaking for 3 months is 1.76 times that of the chloride ion concentration after soaking for 1 month. Experiments have shown that multi‐dimensional diffusion can significantly increase the concentration of chloride ions inside concrete, and this effect becomes more pronounced with the increase of erosion time and dimension.

a significant impact on two-dimensional diffusion.When the pore size of concrete is large, the diffusion rate of chloride ions is faster.In concrete, the two-dimensional diffusion of chloride ions is also affected by the moisture state.Generally speaking, when the moisture content of concrete is high, the two-dimensional diffusion rate of chloride ions will also increase accordingly. 18Three-dimensional transport refers to the three-dimensional diffusion of chloride ions in concrete, mainly influenced by factors such as concrete pore structure, moisture state, chloride ion concentration, and environmental temperature and humidity. 19The pore structure in concrete plays a crucial role in three-dimensional diffusion, and different pore sizes and shapes also have different effects on the diffusion rate of chloride ions. 20n concrete, the transport of chloride ions not only exists in one-dimensional, two-dimensional, and three-dimensional transport processes, but may also exist in interactions between different dimensions.For example, in concrete, two-dimensional transmission may coexist with one-dimensional and three-dimensional transmissions, thus having a greater impact on the durability of concrete. 21In order to improve the durability of concrete structures, it is necessary to study the chloride ion transmission in different dimensions.Some researchers have proposed a model based on Fick's diffusion law, and calculated the concentration distribution and migration speed of chloride ions in concrete.For example, in the one-dimensional Diffusion model, researchers usually use Fick Diffusion equation to calculate the diffusion flux of chloride ions and correlate it with the gradient of chloride concentration in concrete.In 2D and 3D Diffusion models, [22][23][24][25] researchers usually use computational fluid dynamics (CFD) methods to simulate the flow and mass transfer process in concrete.In addition to theoretical models, experimental studies have also investigated the mechanisms of chloride ion transport in different dimensions.Some researchers have studied the chloride ion transmission law in different dimensions and the distribution of chloride ion in concrete through the electrochemical test and analysis of concrete specimens, 26,27 while others have used linear sweep voltammetry (LSV) and alternating impedance spectroscopy (EIS) to study the chloride ion transmission law in concrete under one-dimensional diffusion conditions. 28][31][32][33][34][35] Therefore, exploring the multidimensional transport mechanism of chloride ions under the coupling of multiple factors and mastering the migration law of chloride salts inside concrete is of great significance for improving the durability of concrete.

Cement
The cement used in this experiment is Longping brand P.O.52.5 ordinary Portland cement produced by Fujian Longping Group Co., Ltd.The product inspection basis complies with the relevant provisions in GB 175-2007 (General Portland Cement) and GB/T 176-2017 (Methods for Chemical Analysis of Cement).Table 1 shows the chemical composition of the cement, and Table 2 shows the mechanical properties.3 shows the basic performance parameters of the coarse aggregate, and Table 4 shows the basic performance parameters of the fine aggregate.

Mineral admixtures
The Grade I fly ash used in this experiment was produced by Hejin Longjiang Fly Ash Development and Utilization Co., Ltd., and the S95 slag powder was produced by Laoting County Changxu Building Materials Co., Ltd.The test basis complies with the relevant provisions of GB/T 1596-2017 and GBT 18046-2017.Table 5 shows the technical performance of fly ash, and Table 6 shows the technical performance of slag powder.

Admixture
For this experiment, the Point-SS polycarboxylic acid retarder high-efficiency water reducing agent produced by Kezhijie New Materials Group Fujian Co., Ltd. was selected, and the testing basis met the relevant provisions of GB 8076-2008.Table 7 shows the technical performance of the additive.

Design of experiments
This experiment mainly studies the corrosion effect of dry wet cycling, composite salt solution corrosion, and diffusion dimension on concrete.A total of 81,100 specimens were 100 × 100 × 100 mm standard cube concrete test blocks, 3 pieces of 100 × 100 × 400 mm Rectangular cuboid concrete test block and Φ100 × 50 mm cylindrical concrete test block, take 6 cube concrete test blocks and divide them into 2 groups to test the compressive strength of 7 and 28 days, respectively.Rectangular cuboid concrete test blocks to test the flexural strength of 28 days, the cylindrical concrete test block tests the chloride ion diffusion coefficient through the RCM method, 3 standard cube concrete test blocks are soaked in water as

Sample preparation and pretreatment
To ensure the reliability of the test results, all raw materials used in this experiment were from the same batch, and the specimens were poured within the same day.The procedure involved weighing each raw material according to the mix ratio and pre-moistening the mixer.The coarse and fine aggregates, cement, and mineral admixtures were then sequentially added to the concrete mixer for 1 min of dry mixing.The admixtures and tap water were mixed evenly in advance and added to the mixer for 2 min until the slurry was uniformly thick.The mixture was poured into a test mold coated with engine oil and compacted, followed by placing the mold on a vibration table and vibrating it for 1 min.Surface treatment was applied to the overflowing slurry, and the mold was placed in a cool laboratory environment.The temperature and humidity inside the room were controlled to be constant.After 24 h, the mold was demoulded using an air pump and transferred to a curing room for 28 days.The temperature in the curing room was (20 ± 2) • C, and the relative humidity was greater than 95%.The formation and curing process of the concrete specimen are illustrated in Figure 1.
The main focus of this article is to investigate the impact of chloride ion erosion on concrete under the coupling of multiple factors in different dimensions.Therefore, it was necessary to pre-treat cubic concrete specimens with different diffusion dimensions.Epoxy resin was applied to all five surfaces of the one-dimensional diffusion cube concrete test block, leaving only one exposed surface in contact with the salt solution.The two-dimensional diffusion cube concrete test block retained two adjacent surfaces as the contact surface, while the three-dimensional diffusion cube concrete test block retained three adjacent surfaces as the contact surface.The other surfaces were wrapped in epoxy resin, and after the pretreatment was completed, the immersion solution erosion began.The concrete surface before and after applying epoxy resin is shown in Figure 2.

Erosion test
After the pretreatment of concrete test blocks, they are divided into four groups according to the corrosive environment and immersed in different concentrations of salt solution.Each group has 18 cube concrete test blocks, which are subject to one-dimensional, two-dimensional, and three-dimensional salt solution erosion.The erosion time is 30, 60, and 90 days, respectively.The Design of experiments of each group of concrete is shown in Table 8.

Erosion environment
To ensure that the immersed salt solution closely resembles the real marine environment, three seawater samples were collected from Tong'an District of Xiamen City for this experiment.The concentrations of Cl − and SO 4 2− in seawater were measured, and the average values (2.8%NaCl+0.29%Na 2 SO 4 ) were used.High-purity industrial chemical reagents were used for configuration.A dry-wet cycle system with a cycle of 12 h natural drying and 12 h of solution immersion (dry-wet ratio 1:1) was selected.To consider the impact of composite salt solution erosion on concrete durability, four groups with different erosion environments were designed, as shown in Table 9.

One-dimensional diffusion of chloride ions
Figure 3 shows the variation of chloride ion concentration inside the concrete with the depth of erosion for different test groups of concrete specimens under 1-month, 2-month, and 3-month immersion time based on one-dimensional chloride ion diffusion.By analyzing Figure 3, it can be observed that during the same period of immersion for 1 month, the chloride ion concentration inside the concrete gradually decreases with the increase of erosion depth.The entire erosion depth is divided into two parts, with 10 mm as the boundary point.It can be seen that there are significant differences in surface chloride ion concentration among different experimental groups in the first half of the erosion, and as the depth of erosion increases, the concentration of chloride ions decreases rapidly.When the depth of erosion reaches 10 mm, the difference in chloride ion concentration between each experimental group gradually decreases, and the decrease becomes slower in the latter half of the erosion.When the depth of erosion reaches 20 mm, the chloride ion concentration of each experimental group is almost the same and tends to zero.The extremely low chloride ion content of the concrete itself indicates that chloride ions are transported through the concrete, and the concentration difference is a crucial condition.
Figure 4 shows the variation of chloride ion concentration with erosion time at the same erosion depth inside concrete specimens with different immersion periods.By analyzing the diffusion of chloride ions over time at the same depth as shown in Figure 4, it can be observed that as the immersion time of the concrete in the solution increases, the chloride ion concentration of the four groups of concrete test blocks at a depth of 10 mm from the diffusion surface increases.Compared to the SW test group and Cl5 test group, the chloride ion concentration inside the concrete in the SWC test group and Cl5S10 test group is higher under short-term immersion, indicating that both dry and wet cycles and the action of composite salt solution can improve the transfer rate of chloride ions on the surface of concrete.This effect is more pronounced when the service time of the concrete specimen is short, and as the depth and time of erosion increase, this effect gradually weakens.
F I G U R E 3 Variation of chloride ion concentration with erosion depth under one-dimensional diffusion. (A) Soak for 1 month, (B) soak for 2 months, and (C) soak for 3 months.

F I G U R E 4
Change of chloride ion concentration with erosion time.(A) At a depth of 1 mm inside the concrete.(B) At a depth of 10 mm inside the concrete.

Two-dimensional diffusion of chloride ions
Figure 5 shows the variation of chloride ion concentration inside the concrete with the depth of erosion for different test groups of concrete specimens based on the two-dimensional diffusion of chloride ions during soaking times of 1 month, 2 months, and 3 months.In the two-dimensional diffusion test of chloride ions, the chloride ion concentration on the surface of concrete increased, with the Cl5S10 test group having the highest chloride ion concentration on the surface, followed by the Cl5 test group.However, the chloride ion concentration inside the concrete of both test groups decreased rapidly.
Although the chloride ion concentration on the surface of the SWC test group was not high, the chloride ion concentration inside it decreased slowly.At an erosion depth of 10 mm, the chloride ion concentration in the SWC test group is almost the same as that in the Cl5 and Cl5S10 test groups.However, in the erosion solution, the chloride ion concentration in the Cl5S10 and Cl5 test groups is 1.78 times that of the SWC test group.When the erosion depth reaches 20 mm, the chloride ion concentration in the SWC test group has far exceeded that in the Cl5 and Cl5S10 test groups.concentration of chloride ions inside the concrete is higher, and this performance becomes more obvious as the erosion time increases.Taking the SWC test group as an example, at the same immersion time of 3 months and the erosion depth of 10 mm, the chloride ion concentration of three-dimensional diffusion is 2.2 times that of one-dimensional diffusion.

Three-dimensional diffusion of chloride ions
At the erosion depth of 20 m, the chloride ion concentration of three-dimensional diffusion has reached 4.08 times that of one-dimensional diffusion.By comparing Figures 4-6, it can be observed that regardless of the soaking time, the chloride ion concentration inside the four soaking numbers of concrete specimens decreases with the increase of erosion depth, indicating that in natural environments, chloride ions are always transported from high concentration areas to low concentration areas.Comparing the chloride ion concentrations in the SW test group and the SWC test group at the same time, it can be found that the chloride ion concentration inside the concrete in the SW test group is lower, which simulates the actual working condition of completely submerged concrete components in the underwater area.In contrast, the SWC test group undergoes dry and wet cycles, and the chloride ion concentration on both the surface and inside of the concrete is higher than that in the SW test group.This represents the tidal zone in actual engineering concrete components in the splash area.This is because under the action of dry-wet cycles, the concrete structure undergoes changes in both saturated and unsaturated states.When the concrete changes from a dry state to a saturated wet state, the salt solution outside the concrete will drive more chloride ions to transfer to the internal pores of the concrete, thereby increasing the content of chloride ions in the concrete.This also proves that in actual working conditions, concrete members in tidal and splash areas tend to have higher chloride ion content than those in underwater areas, and the reinforcement inside the concrete is more prone to redox, which will cause corrosion expansion and damage the concrete structure.

CONCLUSION
This article tests the concentration of chloride ions at different depths inside concrete, and analyzes the effects of different erosion environments, erosion times, and diffusion dimensions on the distribution of chloride ions inside concrete through graphical analysis.The summary is as follows: 1.The concentration of chloride ions in concrete is positively correlated with the concentration of chloride ions in the erosion solution, chloride ion diffusion dimension, and erosion time, while negatively correlated with the depth of concrete erosion.2. Under the same conditions, the chloride ion concentration inside the concrete affected by multi-dimensional diffusion is significantly higher than that of one-dimensional diffusion, and this difference becomes more apparent with the increase of erosion time and dimension.3. Taking the SW experimental group as an example, after soaking for 3 months, the chloride ion concentration in the three-dimensional diffusion of the SW experimental group is 2.55 times that of one-dimensional diffusion.4. In the future research, we will carry out the application of new materials in damaged concrete.And conduct proportional experiments as much as possible to obtain more accurate chloride ion transport patterns.

1
Concrete specimen forming and curing.(A) Forming of concrete specimens.(B) Standard legislative body test block maintenance.(C) Curing of RCM test blocks.(A) (B) F I G U R E 2 Concrete specimen pretreatment.(A) Before pretreatment of test blocks.(B) After pretreatment of the test block.

Figure 6
Figure6shows the variation of chloride ion concentration inside the concrete with the depth of erosion for different experimental groups of concrete specimens based on the three-dimensional diffusion of chloride ions.The specimens are soaked for 1 month, 2, and 3 months.It can be that under three-dimensional diffusion conditions, the

F I G U R E 6
Variation of chloride ion concentration with erosion depth under three-dimensional diffusion.(A) Soak for 1 month, (B) soak for 2 months., and (C) soak for 3 months.

E 2 Mechanical Properties of Cement. Setting time (min) Flexural strength (MPa) Compressive strength (MPa) Stability Specific surface area (m 2 /kg) Initial setting Final setting 3d 28d 3d 28d
natural gravel produced by Taining County Wanxing Building Materials Co., Ltd. was used as the coarse aggregate in this test, and the fine aggregate is the natural river sand produced by Xiamen Shunlei Building Materials Co., Ltd.The detection basis of the aggregate meets the relevant provisions in JGJ52-2006 and JTS 202-2011.Table The Basic performance parameters of fine aggregate.Technical performance of slag powder.Technical performance of admixtures., and the remaining 72 standard cube concrete test blocks are divided into 4 groups on average, Each group is immersed in different service environments, and 18 standard cubic concrete specimens from the same service environment are coated with epoxy resin to reserve different numbers of diffusion surfaces.By controlling the number of diffusion surfaces in contact with the solution, one-dimensional, two-dimensional, and three-dimensional diffusion of chloride ions are achieved.
TA B L E 3Basic performance parameters of coarse aggregate.TA B L E 5Technical performance of fly ash.
Test plan design.Test number and erosion environment.
TA B L E 8