Study on the Relationship between Chloride Ion Penetration and Resistivity of NAC-Cement Concrete

Abstract

To explore the effect of nano-attapulgite clay (NAC) on the durability of concrete, two kinds of NAC (calcined raw ore and calcined high viscosity ore: calcination at 650 °C for 2 h) were used to study their effects on the durability of concrete, mainly discussing the changes of chloride ion permeability and the resistivity of concrete with NAC. The effect of NAC on the strength of concrete was analyzed by testing the compressive strength of concrete. The two-electrode method, four-electrode method, and concrete resistivity tester were used to analyze the relationship between the testing method and concrete resistivity, and the effect of NAC on concrete resistivity was analyzed. The influence of NAC on the chloride corrosion resistance of concrete was analyzed by measuring the chloride diffusion coefficient, and the relationship between the chloride diffusion coefficient and resistivity was established. The diffusion process of chloride ions in concrete was analyzed by theoretical derivation and numerical simulation. The results show that: calcining raw ore NAC can improve the compressive strength of concrete, while calcining high-viscosity ore reduces the compressive strength of concrete. At the age of 28 days, the strength of concrete mixed with calcined raw ore is about 7.10% higher than that of ordinary concrete, while the compressive strength of concrete mixed with calcined high-viscosity ore is about 4.32% lower than that of common concrete. The resistivity of concrete mixed with calcined raw ore increases the fastest, and the 56 days age is about 15.8% and 29.6% higher than that of ordinary concrete and calcined high-viscosity ore. There is a good negative correlation between concrete resistivity and chloride diffusion coefficient. At 28 days, the incorporation of calcined raw ore concrete decreased by about 19.9% and 49.4% compared with ordinary concrete and calcined high-viscosity ore, respectively. After 10 years of decline, the chloride ion content is 11.1% and 23.2% lower than that of ordinary concrete and concrete mixed with calcined high viscosity ore.

1. Introduction

Cement mortar and concrete with cement are widely used in various building structures. However, due to the adverse natural environment, seawater erosion, road deicing salt, and the industrial environment resulting in premature deterioration of materials, this leads to most building structures far from reaching their design life. In the past 40 years of reform and opening up, China has carried out large-scale infrastructure construction, but there is poor durability to varying degrees. Especially in the northern offshore or marine environment, concrete is easy to crack and surface spall. Steel corrosion under the combined action of seawater erosion, freeze-thaw cycle, and load factors, which leads to the destruction of reinforced concrete structures, which have to be repaired frequently, and even need to be rebuilt, resulting in a significant waste of funds and resources. For example, the Qingdao trestle in Shandong was built in 1892, and eight significant repairs and reconstructions were carried out. Especially in 2013, more than 30 m of collapse occurred on the bridge’s east side, which lasted for three months and cost CNY 12 million [1,2,3]. Therefore, it is an inevitable trend to develop high-performance concrete to improve structural durability.

The durability failure of a structure originates from the durability failure of components, and the durability failure of components begins with the materials used. Therefore, improving the quality of materials used is crucial in improving durability. At present, incorporating mineral admixtures is one of the leading technical ways to improve the durability of cementitious cement materials. In the standards formulated by the Chinese Society of Civil Engineering, it is emphasized in selecting concrete and other materials that mineral admixtures should be added to the chemical corrosion environment. Many mineral admixtures should be added when the environmental grade is C or above [4]. Reinforced concrete in a chloride environment should adopt a low water-cement ratio concrete with many mineral admixtures. Mineral admixtures are usually excellent particles and even can reach nanometer levels. Theoretically, its specific surface area, surface energy, and potential activity are far greater than ordinary materials. Nano-silica, nano-calcium carbonate, nano-kaolin, and carbon nanotubes are mainly incorporated into cement-based materials [5,6,7]. The addition of cementitious cement materials can improve the internal microporous structure, thereby changing the density of the material and improving its resistance to chloride ion diffusion. At the same time, nanomaterials have a large surface area, high activity, and apparent pozzolanic effect. Incorporating nanomaterials can improve mechanical properties, thereby improving the overall durability of building structures. However, despite the incomparable advantages of nano-additives, the preparation of nano-silica and carbon nanotubes is complex, with different standards and high prices, which is difficult to be applied in practical engineering. Nano calcium carbonate and kaolin are difficult to disperse in the matrix. The test results are discrete, and the application in practical engineering is slow [8].

Nano-attapulgite clay rod crystal is only tens of nanometers, has many internal pores, a large specific surface area, and has particular filling and chemical activity. The application of cement-based materials can make up for the cracks and pores produced in the hydration process of cement and improve its strength and impermeability [9,10,11,12,13,14,15,16,17]. Alani et al. [9] studied the effect of modified NAC on cement mortar’s operational performance and stability. It was found that NAC reduced the layered degree of mortar, improved water retention, and improved operational performance. The flexural strength and compressive strength of the mortar decreased slightly, mainly because the modified NAC increased the viscosity of the mortar. When maintaining the same fluidity of mortar, more water was needed to reduce the strength of the mortar slightly. Niu et al. [10] studied the effect of NAC on the power and shrinkage of alkali slag cementitious materials. It is found that the 28 day compressive strength of the sample with 5% raw ore NAC is 16.7% lower than that of the ordinary model, and the flexural and compressive strength of the selection with 5% calcined nano-attapulgite clay is increased by 38% and 11%, respectively. The shrinkage value of the specimen with 10% NAC addition increased by 46.8% compared with that of the ordinary model. The shrinkage value did not change significantly when the content was 5%. Adel et al. [11,12] studied the blending of calcined NAC in dry powder mortar. It was found that the consistency, water retention, compressive strength, bond strength, shrinkage, and the impermeability of mortar were improved after adding 20% cement to calcined NAC. The 28 day compressive strength of mortar with NAC calcined at 850 °C was 102.1% higher than that of ordinary samples, the bond strength was 39.7% higher, and the impermeability was 25% higher. It was found by X-ray diffraction that a large number of active silica and active alumina were produced in the calcined NAC, which high volcanic ash activity and had a high reinforcement effect on mortar. The NAC calcined at 850 °C for 2 h dramatically reduces the concentration of Ca(OH)₂ in the cement hydration products, which can well improve the pore size structure of mortar and significantly improve the strength and impermeability of mortar.

In this paper, the effects of calcined NAC on the resistivity, compressive strength, frost resistance, and chloride ion diffusion coefficient of concrete are analyzed by mixing calcined NAC in concrete. The effect of NAC on the chloride ion diffusion of concrete was analyzed from the perspective of resistivity. Based on the heat transfer equation, the chloride ion diffusion model is established. The chloride ion diffusion process of the cylindrical pier is simulated by Matlab, which provides a scientific reference for non-destructive testing of chloride ion diffusion resistance and anti-corrosion treatment of structures in practical engineering.

2. Materials and Methods

2.1. Raw Materials

In the experiment, PO·42.5R ordinary Portland cement was produced by Xiaoyetian, Dalian. Nano-attapulgite clay was calcined with raw ore (C) and calcined high-viscosity ore (D) after calcination at 650 °C for 2 h, the chemical composition of the NAC is presented in

Table 1. Composition of NAC.

Component	CaO	SiO2	Al2O3	Fe2O3	MgO	K2O	Na2O	Lol
Content/%	1.8–2.5	50.4–61.3	9.4–9.5	4.0–5.0	9.3–10.5	0.2–0.6	0.5–1	11.94–13.46

, the appearance/TEM/XRD of NAC are shown in Figure 1. River sand is selected as a fine aggregate with a fineness modulus of 2.5. The particle size distribution of coarse aggregate stone is 5~25 mm.

The content of nano-attapulgite clay is 5% of the total mass of cementitious materials [18,19,20], and the exact mix proportion is shown in

Table 2. Mixing proportion of concrete kg·m⁻³.

Specimen Number	Cement	Stone	Sand	Water	Nano-Attapulgite Clay
CMA_0	570	1104	660	228	0
CMA_C	541.5	1104	660	228	28.5
CMA_D	541.5	1104	660	228	28.5

Note: CMA_C represents the concrete with mineral admixture doped with calcined raw ore (C), CMA_D represents the concrete with mineral admixture doped with calcined high-viscosity ore (D), and 0 represents the concrete without mineral admixture.

. Mixing using artificial mixing, first nano-attapulgite clay into water ultrasonic dispersion for 15 min, then the dry mixing uniform, and finally the concrete mixing. The concrete mixing method is referred to as ‘Highway Engineering Cement and Cement Concrete Test Specification’ (JTG E30-2005).

2.2. Preparation of Specimen

The experiment made nine groups of specimens according to different test requirements shown in

Table 3. Experimental grouping.

Specimen Number	Test Index	Test Age/Days
CMA_0	compressive strength, Chloride ion diffusion coefficient, resistivity	3, 7, 28
CMA_C
CMA_D

, three groups of 100 mm × 100 mm × 100 mm cube compressive strength specimens; three groups of Φ100 mm × 50 mm cylinder anti-chloride ion diffusion specimens; three groups of 100 mm × 100 mm × 400 mm prisms resistivity test specimens. After pouring, the specimen was moved into the standard maintenance room at a temperature of 20 ± 3 °C and a relative humidity of 95% for maintenance, and the mold was removed after 1 day of curing. Then the specimen was put into water at a temperature of 20 ± 3 °C to continue maintenance until the specified test age.

2.3. Experimental Method and Process

2.3.1. Chloride Diffusion Coefficient

According to the Standard Test Method for Long-term Performance and Durability of Ordinary Concrete’ (GB/T50082-2009), the chloride diffusion coefficients of different specimens were tested by rapid chloride ion migration (RCM) proposed by Tang and Nilsson [21]. The principle is that the external solution concentration gradient is used to drive the chloride ion transmission in cement-based materials. The shallow electric field potential gradient is used to accelerate the chloride ion movement speed. Figure 2 shows the schematic diagram of the chloride ion diffusion coefficient tester.

The temperature of the test room shall be controlled at 20 ± 1 °C the test piece shall be subjected to ultrasonic saturation for 15 min, and the test piece shall be loaded into a silica gel cylinder. Then, put the silica gel cylinder installed with the test piece into the electrolytic cell. After the anode is installed, inject 300mLKOH solution with a concentration of 0.2 mol/L into the silica gel cylinder, so that the anode plate and the surface of the test piece are immersed in the solution. Inject 5% NaCl and 0.2 mol/L KOH solution into the electrolytic cell until it is flush with the KOH solution level in the rubber cylinder. Correctly connect the positive and negative electrodes, put the temperature sensor into the silica gel cylinder, and turn on the host power of the RCM-DAL chloride diffusion coefficient tester for electromigration.

2.3.2. Resistivity

The two-electrode method measured four resistivities of concrete specimens and the four-electrode way. The Resist-400 resistivity tester measured the resistivity of concrete, as shown in Figure 3. When measuring the resistivity of concrete, the foam contact electrode is thoroughly infiltrated and checked before each measurement. The measurement position is at the median line of the side of the concrete specimen. From left to right, the average value is measured at multiple points.

Pre apply 30 V DC regulated power supply to the electrodes at both ends of the test piece, measure the current I in the circuit, and then measure the potential difference U between the middle two electrodes with a voltmeter (accurate to 0.1 V), and then calculate the resistance value R.

2.3.3. Compressive Strength

The of the concrete samples was measured following JTG E30-2005 using the 2000 kN electro-hydraulic servo compressive testing system (YAW-YAW2000A). The loading rate on the concrete samples was 0.5 MPa/s. For each mixture at each age, three samples per batch were tested; the average value was taken to be the representative strength.

3. Analysis and Discussion of Test Results

3.1. Compressive Strength

The specimen is cured to the specified age, and the compressive strength test is carried out. The results are shown in Figure 4. It can be seen that the early age compressive strength (3 d) of three kinds of concrete is the same. With the increase of age, the compressive strength of concrete mixed with calcined raw ore grows the fastest, while that of concrete mixed with calcined high-viscosity ore grows the slowest. After curing for7 days, the strength of concrete mixed with calcined raw ore is about 19.97% higher than that of concrete without NAC. The compressive strength of concrete mixed with calcined high viscosity ore is approximately 14.01% lower than that of concrete without NAC. That is mainly due to the high water absorption of calcined high-viscosity ore, which reduces the water-binder ratio around the cement particles, thereby reducing the degree of hydration. In addition, after the addition of calcined high-viscosity ore, the slump of concrete is reduced, the operating performance is reduced, and it is difficult to vibrate and compact. A large number of internal defects are introduced, and many harmful pores are contained in the concrete, which reduces the compactness of the concrete and its strength. At the age of 28 days, the power of concrete mixed with calcined raw ore is about 7.10% higher than that of ordinary concrete, while the compressive strength of concrete mixed with calcined high viscosity ore is about 4.32% lower than that of ordinary concrete. It can be seen that the compressive strength of concrete can be improved by adding calcined raw ore, because the calcined NAC has high pozzolanic activity, and the fibrous rod crystal structure of nano-attapulgite clay is also conducive to improving the internal microstructure of concrete and improving the compactness of microstructure. In addition, the influence of NAC on the slump of concrete is reduced by calcination, and the workability of concrete is improved [5,22,23].

The displacement and load are collected during the loading test, and the load-displacement curve is shown in Figure 5. It can be seen that the elastic modulus (the slope of the linear fitting curve in the load displacement curve during the initial compression process.) difference between the three kinds of concrete in the early stage is slight. Still, with the increase of age, the elastic modulus of concrete mixed with calcined raw ore increases rapidly. The elastic modulus at 7 days and 28 days is significantly higher than that of the other two kinds of concrete. The elastic modulus of calcined high-viscosity ore and ordinary concrete is the same.

3.2. Chloride Diffusion Coefficient

The chloride diffusion coefficient of the specimen was measured at the specified age, and the results are shown in Figure 6. It can be seen that after various curing ages, the chloride ion diffusion coefficient of concrete mixed with calcined high viscosity ore is the largest. The resistance to chloride ion erosion is the worst. The chloride diffusion coefficient of concrete mixed with calcined ore is slightly smaller. At 28 days, the concrete mixed with calcined raw ore is about 19.9% and 49.4% lower than that of ordinary concrete and concrete mixed with calcined high-viscosity ore, respectively. It shows that calcined raw ore can improve concrete’s chloride ion penetration resistance, mainly due to its pozzolanic effect and filling effect. The calcined NAC has high pozzolanic activity, which can not only promote the formation of C-S-H gel but also improve the internal microstructure of concrete and strengthen the bonding force of aggregate interface and improve the impermeability. Moreover, due to the tiny rod crystal diameter of NAC, it can fill the pores between cement particles, improve the pore structure of concrete, reduce harmful porosity, and improve the compactness of the matrix, which can effectively improve the resistance to chloride ion penetration of concrete [19]. However, the concrete mixed with calcined high viscosity ore has a low slump under the same water cement ratio. It is challenging to vibrate and compact under the same vibrating condition. There are many internal initial defects and more harmful pores, which increase the chloride ion permeability channel and reduce the overall resistance to chloride ion permeability [22].

3.3. Chloride Diffusion Prediction Model

3.3.1. Two-Dimensional Diffusion Model

In this paper, through theoretical calculation and referring to the heat transfer equation, the analytical solution of the chloride ion diffusion process in a two-dimensional cylindrical pier under a chloride environment is established. The relationship between the distribution of chloride ion and diffusion coefficient, diffusion time, and protective layer thickness, is analyzed. The pier radius is R0, located in the immersion zone. Ignoring the influence of water pressure, the material is isotropic, and the chloride diffusion coefficient is a. When the cross-section size and shape are constant along the height direction, and the chloride ion concentration is constant along the height method, the cylinder can be simplified as a plane problem, similar to the plane strain.

The coaxial cylindrical surface is an equal-content surface, the chloride ion concentration in the environment is ρ_w, the chloride ion concentration in the cylinder is f(r), and the convection-diffusion coefficient α is uniform everywhere on the pier surface. Then, the chloride diffusion differential equation is:

$$\frac{\partial u}{\partial t}=a^2\left(\frac{{\partial }^{2}u}{\partial r^2}+\frac{1}{r}\frac{{\partial }^{}u}{\partial r^{}}\right) \left(t>0,0<r<R\right)$$

(1)

Initial conditions:

$$u(r,0)=f(r)$$

Boundary conditions:

$$\left\{\begin{array}{l}r=0,u(0,t)=0\\ r=R,\lambda \frac{\partial u}{\partial r}=\alpha ({\rho }_w-u)\end{array}\right.$$

Introducing new variables:

$$R(r,t)=u(r,t)-{\rho }_w$$

(2)

Equation (1) becomes:

$$\left\{\begin{array}{l}\frac{\partial R}{\partial t}=a^2\left(\frac{{\partial }^{2}R}{\partial r^2}+\frac{1}{r}\frac{{\partial }^{}R}{\partial r^{}}\right)\\ R(r,0)=f(r)-{\rho }_w\\ r=0,R(0,t)<\infty \\ r=R_0,\lambda \frac{\partial R}{\partial r}=-\alpha R\end{array}\right.$$

(3)

Solving Equation (3):

$$R(r,t)=C{J}_0(\beta r)e^{a{\beta }^{2}t}$$

(4)

where β is the constant introduced by solving the differential equation.

According to boundary conditions:

$$-\lambda \beta C{J}_1(\beta R_0)e_{}^{-a{\beta }^{2}t}=-\alpha C{J}_0(\beta R_0)e_{}^{-a{\beta }^{2}t}$$

(5)

Bessel function is approximately equal to:

$$\left\{\begin{array}{l}{J}_0(x)=\sqrt{\frac{1}{\pi x}}(\cos x+\sin x)\\ J_1(x)=\sqrt{\frac{1}{\pi x}}(\cos x-\sin x)\end{array}\right.$$

(6)

Bringing Equation (6) into Equation (5):

$$-\tan (x+\frac{\pi }{4})=\frac{1}{B}x$$

(7)

It can be seen from Figure 7 that the root of Equation (7) is the abscissa of the intersection points of y₁ and y₂ in the figure: x = x₁, x₂, x₃, x₄…x_n, x_n+1, x_n+2…; obviously, no matter how the slope of y₂ function changes, there must be a finite real number n, making the nth root x_n of the equation close to nπ − 3π/4, which greatly simplifies the calculation. x is denoted as {x_n, n$\in$R}.

Since the differential equation is linear, its infinite linear combination of solutions is still its solution, then the solution of Equation (3):

$$R(r,t)={\displaystyle \sum _{n=1}^{\infty }}{C}_n{J}_0(x_n\frac{r}{R_0})e^{-a{(\frac{x_n}{R_0})}^{2}t}$$

(8)

The two-dimensional diffusion model of chloride ion in concrete is obtained by introducing Formula (8) into (4) transformation:

$$u(r,t)={\rho }_w-{\displaystyle \sum _{n=1}^{\infty }\frac{{\rho }_w\sin (x_n\frac{r}{R_0}+\frac{\pi }{4})}{\sqrt{\pi x_n}{R}_0^2}{\displaystyle {\int }_0^{R_0}r}\sin (x_n\frac{r}{R_0}+\frac{\pi }{4})dr\times e^{-a{(\frac{x_n}{R_0})}^{2}t}}$$

(9)

3.3.2. Numerical Model

According to the relationship between concrete age and chloride diffusion coefficient, the finite element numerical model was established. Considering the influence of age on the diffusion coefficient of chloride ions in concrete (shown in Figure 8), the diffusion process of chloride ions in an underwater pile foundation is simulated. Because the diffusion coefficient decreases with time and the diffusion coefficient are regarded as a constant, the relationship between chloride ion concentration and time at 5 cm of the concrete protective layer is shown in Figure 9. The relationship between chloride ion content and protective layer thickness is shown in Figure 10 when the erosion time is 10 years.

3.3.3. Comparative Analysis of Different Diffusion Prediction Models

Ignoring the influence of circumferential chloride ion content on diffusion, the chloride ion diffusion coefficient is regarded as a constant. According to the theoretical model established by Formula (6) and the numerical model, the change of chloride ion content in concrete at different depths during 10 years of erosion is obtained, shown in Figure 11. It can be seen that the two calculation results have a reasonable correlation. Still, when the protective layer thickness is small, the calculated value is larger than the simulated value due to ignoring the influence of circumferential chloride ion content on diffusion.

3.3.4. Chloride Ion Erosion Prediction of Different Nano-Clay Concrete

Ignoring the influence of convection-diffusion and temperature and humidity on diffusion, ignoring the relationship between seawater concentration and time, the chloride ion content of three kinds of concrete is simulated, as shown in Figure 12, and the relationship between chloride ion content and depth after 10 years of erosion is shown in Figure 13.

It can be seen from Figure 13 that at the protective layer of 5 cm, the chloride ion content is at the early stage, and the difference among the three mix proportions of concrete is negligible. However, with the increase of time, the chloride ion content in the concrete mixed with calcined high-viscosity ore increases rapidly, while the chloride ion content in the concrete mixed with calcined raw ore is the lowest. After 10 years of erosion, the chloride ion content is reduced by 11.1% and 23.2%, respectively, compared with ordinary concrete and the concrete mixed with calcined high-viscosity ore. It can be seen from Figure 13 that the chloride ion content of concrete with three mixing ratios varies with depth. It can be seen that the chloride ion content of concrete mixed with calcined raw ore is the smallest, followed by ordinary concrete, and the largest is mixed with calcined high viscosity ore. It indicates that calcined raw ore can reduce the chloride ion content of concrete under superficial erosion. However, calcination of high-viscosity ore does not lessen the diffusion coefficient of chloride ions but leads to high chloride ion content in concrete.

3.4. Resistivity

3.4.1. Effect of Test Method on Resistivity of Concrete

After the curing period of specimens, concrete resistivity was measured by the two-electrode method, four-electrode method, and concrete resistivity tester. The test results are shown in Figure 14.

It can be seen that the resistivity of concrete measured by the three methods increases rapidly in the early stage and tends to be flat with the increase of age. The resistivity measured by the concrete resistivity tester is significantly higher than that measured by the four-electrode method and two-electrode method. The resistivity measured by the four-electrode method is slightly higher than that measured by the two-electrode method. This is mainly because the embedded electrode is inside the concrete and the concrete resistivity tester measures on the surface of the specimen. Due to the effect of water diffusion, the surface resistivity of the concrete is significantly higher than that inside the concrete. The resistivity measured by the concrete resistivity tester fluctuates greatly, and the variance is much higher than that of the four-electrode and the two-electrode methods. This is mainly because the external conditions greatly influence the test results of the concrete resistivity tester. The moisture of the concrete surface, the moisture of the sponge infiltrated at the end, and the temperature and humidity of the test environment interfere with it. Moreover, because the end is directly attached to the concrete surface, and even the pressure can affect the test results, there is inevitable contact resistance. In addition, due to the small size of the laboratory specimen and the size effect, the measurement position also dramatically influences the measurement results. This paper’s selected measurement point is the concrete side’s median position. The combined effects of these factors lead to large fluctuations in the measured resistivity of concrete and poor repeatability.

It is also found that there is little difference between the resistivity of concrete measured by the two-electrode method and the four-electrode method. The resistivity of concrete measured by the two-electrode method meets the superposition principle, which is inconsistent with the results of Han et al. [24]. The resistivity variance measured by different test methods is small in the early age, and gradually increases with age. This is mainly due to the large amount of free water filled in the early concrete has a great impact on the resistivity, and the resistivity of each measuring section has little difference. With the deepening of the hydration process, free water becomes less and less, and the weight of the influence of the concrete structure on the resistivity increases. There is a difference between each measuring section (the capacitance and polarization effect increase) and the measurement error causes the difference between each measuring section to change [17,25].

The resistivity of ordinary concrete and calcined raw ore concrete measured by the same method is shown in Figure 15, and the resistivity measured by the four-electrode concrete method with three mixing ratios is shown in Figure 16. It can be seen that the resistivity measured by the concrete resistivity meter is much higher than that measured by the four-electrode method and the two-electrode method. It can be seen from Figure 16 that in the early stage, the resistivity difference between the three kinds of concrete is slight. With the increase of age, it can be seen that the resistivity of concrete with calcined raw ore increases fastest, and the rise of 56 days of age is about 15.8% and 29.6% compared with ordinary concrete and calcined high viscosity ore. This is mainly because the calcined ore can promote the hydration of cement, promote the formation of C-S-H gel, improve the internal microstructure of concrete, strengthen the bonding force of the aggregate interface, and reduce the water content of the matrix. In addition, due to the small diameter of the NAC rod crystal, it can fill the pores between cement particles, improve the pore structure of concrete, reduce the harmful porosity, and improve the compactness of the matrix, thereby improving the resistivity of concrete.

3.4.2. Effect of Concrete Resistivity on Chloride Diffusion Coefficient

The experiment is to force the chloride ion to migrate to the anode under the action of electric field force, and the resistivity of the specimen is crucial to the chloride ion diffusion. For porous material concrete, the relationship between chloride diffusion coefficient and resistivity of concrete can be reflected by the Nernst-Einstein equation, as shown in Equation (10):

$$D_i=\frac{RT}{Z^2{F}^2}\times \frac{t_i}{r_i{c}_i\rho }$$

(10)

D_i is chloride diffusion coefficient, m²/s; R is a gas constant, J/mol·K; T is the absolute temperature, K; Z is chloride ion valence; F is Faraday constant, C/mol; t_i is the number of chloride ion migration, m²/s; r_i is chloride activity coefficient, S/m; c_i is the concentration of chloride ion in pore water mol/m³; ρ is resistivity of concrete, kΩ·m.

For specific temperature and humidity conditions, Formula (10) can be simplified to:

$$D_i=k\frac{1}{\rho }$$

(11)

D_i is chloride diffusion coefficient, m²/s; k is a constant, kΩ·m³/s; ρ is resistivity of concrete, kΩ·m.

It can be seen from Formula (11) that the chloride diffusion coefficient of concrete is inversely proportional to the resistivity, and Figure 17 shows the relationship between the chloride diffusion coefficient and the resistivity of concrete with three mixing ratios. It can be seen that the resistivity of concrete decreases with the increase of resistivity, and the two have a good linear negative correlation. Because concrete resistivity measurement is non-destructive, in practical engineering, the relationship curve between concrete resistivity and chloride diffusion coefficient can be determined in advance, and the resistance to chloride diffusion can be indirectly reflected by testing concrete resistivity.

4. Conclusions

The effects of calcined NAC on the compressive strength, electrical resistivity, and the chloride diffusion resistance of concrete are analyzed. The influence of three testing methods on the test results was analyzed by the two-electrode method, the four-electrode method, and resistivity tester. The relationship between the resistivity and chloride diffusion coefficient was established. Based on the partial differential equation, the distribution of chloride ions in concrete was analyzed. The main conclusions are as follows:

(1)

Calcining raw ore NAC can improve the compressive strength of concrete while calcining high-viscosity ore reduces the compressive strength of concrete. At the age of 28 days, the strength of concrete mixed with calcined raw ore is about 7.10% higher than that of ordinary concrete, while the compressive strength of concrete mixed with calcined high-viscosity ore is about 4.32% lower than that of common concrete.

(2)

The resistivity difference between the two-electrode method and the four-electrode method of concrete is slight, and both methods meet the superposition principle; three test methods are early discrete small, later gradually larger. The resistivity of concrete mixed with calcined raw ore increases the fastest, and the 56 days of age is about 15.8% and 29.6% higher than that of ordinary concrete and calcined high-viscosity ore.

(3)

Calcining raw ore nano-attapulgite clay can improve the chloride corrosion resistance of concrete. At 28 days, the incorporation of calcined raw ore concrete decreased by about 19.9% and 49.4% compared with ordinary concrete and calcined high-viscosity ore, respectively. There is a good negative correlation between concrete resistivity and chloride diffusion coefficient.

(4)

The chloride diffusion model of the cylindrical pier is established based on the heat transfer equation and finite element theory. Considering the relationship between chloride ion diffusion coefficient and time, the chloride ion corrosion of concrete is weaker than considering the diffusion coefficient as a constant. The chloride ion content decreased rapidly with the change of protective layer depth, and it changed exponentially with time and diffusion coefficient. Calcining raw ore can reduce the chloride ion content in concrete erosion. After 10 years of decline, the chloride ion content is 11.1% and 23.2% lower than that of ordinary concrete and concrete mixed with calcined high viscosity ore.