Deterioration of concrete under the coupling action of freeze – thaw cycles and salt solution erosion

: In this article, the deterioration law of concrete under the coupling action of freeze – thaw cycles and salt solution erosion is studied through the comprehensive thermal analysis, the ﬁ eld emission electron microscope, and the nuclear magnetic resonance, and the in ﬂ uence of salt solution type and salt solution concentration is con -sidered. The results show that the freeze – thaw damage of concrete in the salt solution is the combined e ﬀ ect of the expansion pressure of the freeze – thaw erosion product, the crystallization pressure of the salt solution, and the frost heave pressure of the fresh water; the damage degree increases with the increase of freezing and thawing cycles; the damage degree of concrete in the chloride solu tion is greater than that in the sulfate solution and fresh water before 150 freezing and thawing cycles; the damage degree of concrete in the sulfate solution is greater than that in the chloride solution after 150 freezing and thawing cycles; the pores size of concrete in the salt solution is larger than that in fresh water, the main peak of the dif ference of pore size proportion shifts to the harmful pore area, and the secondary peak and third peak appear in the seriously harmful pore area; the pore diameters of the main peak, the secondary peak and the third peak of concrete in di ﬀ erent salt solution, and the limiting pore diameters are 0.0662, 1.145, and 10.116 μ m, respectively; the safe service life of concrete in salt solution environ ment after freezing and thawing cycles is predicted by the Weibull distributed life evaluation model. The max imum life after the freeze – thaw cycle is 33 years, which is at least 42% lower than that in the fresh water envi ronment after freezing and thawing cycles.


Introduction
The Eurasian continent, North America continent, and southeastern South America at 40-60°north-south latitude belong to the typical temperate continental climate. Due to the long distance from the ocean or the obstruction of topographical features, it is difficult for the humid ocean airflow to reach the area, so the climate is dry and the rainfall is low, which leads to the extremely high salt content in rocks, lakes, and groundwater, most of which are salt solutions composed of corrosive ions such as chloride ions, sulfate ions, sodium ions, and magnesium ions [1][2][3][4]. The erosion of sulfate ions and chloride ions is one of the main reasons that cause concrete damage [5]. Sulfate ions and chloride ions chemically reacted with the internal ions of the concrete to generate expansive and extensible reaction products, which cause the internal microstructure of the concrete to expand, deteriorate, and collapse, and ultimately lead to the destruction of the concrete. In addition, the mid-to-high latitudes above 40°n orth-south latitudes lasted for a long time in winter, with extremely low temperatures, large temperature differences between day and night, and the concrete structure often suffered from the freeze-thaw damage [6], and hence, concrete structures in mid-to-high latitudes were damaged by the coupling of salt solution erosion and freezing and thawing cycles.
Currently, some scholars have carried out some research on the durability of concrete under the condition of salt solution. An et al. [7] investigated the freeze-thaw damage of concrete in 5% NaCl solution and found that internal damage of concrete could be repaired by filling pores less than 100 nm with reaction products. Yan et al. [8] found that the water absorption and porosity of highstrength concrete gradually increased with the increase of the number of freeze-thaw cycles. Yuan et al. [9] reported that the sulfate solution had both inhibitory and promoting effects on the freeze-thaw damage of concrete, and the promoting effect increased with the increase of sulfate concentration. Liu et al. [10] investigated the sodium sulfate salt crystallization distress on carbonized concrete and found that the sodium sulfate crystallization was the main cause of concrete damage. Tan et al. [11] found that the magnesium sulfate and the elastic load had a significant impact on the mechanical properties of concrete. Alnahhal et al. [12] studied the crack evolution and performance degradation of concrete under the magnesium sulfate environment. Zhou et al. [13] and Wang et al. [14] investigated the performance evolution and mechanism of asphalt under the action of chloride salt erosion. Tian et al. [15,16] investigated the bond strength between rebar and recycled coarse aggregate concrete and found that the bond strength decreased with the increase of salt-frost cycles. However, the influence of sulfate on the frost resistance of concrete was not considered in these studies. Wen [17], Hu and Wu [18], and Tian et al. [19,20] conducted research on the salt frost resistance of concrete and found that the concrete damage in the salt solution was greater than that in fresh water under the same freeze-thaw cycles. However, they did not consider the effect of different salt solutions on concrete performance.
In summary, the current research is limited to a single concentration and single type of salt solution, and there are few studies on the freeze-thaw damage and degradation laws of concrete in different concentrations and different types of salt solutions. The objective of this article is to investigate the deterioration law of concrete under the coupling action of freeze-thaw cycle and salt solution erosion by considering different types of salt solutions and different salt solution concentrations, and to evaluate the freeze-thaw durability life of concrete based on the freeze-thaw damage theory.

Experimental program 2.1 Materials
In this article, the coarse aggregate (CA) was gravel with a particle size of 4.75-31.5 mm, and fine aggregate (FA) was natural river sand. The typical physical characteristics of CA and FA are listed in Tables 1 and 2, respectively. The sieving results of CA and FA are shown in Figures 1 and 2, respectively. The cement was P. O. 42.5 ordinary Portland cement produced by Hohhot Jidong Cement Co., Ltd., and the cement properties are listed in Table 3. The fly ash obtained was Grade II fly ash produced by Hohhot Jinqiao Thermal Power Co., Ltd, and the fly ash properties are listed in Table 4. The water-reducing agent adopts naphthalenebased powder water-reducing agent, and the waterreducing efficiency was 16% ( Table 5). Table 5 shows the mixture proportions of concrete. The specimens were cured for 36 h after being fabricated, then demolded and cured for 28 days. During the curing process, the concrete cubic compressive strength of

Specimens
The working condition design of concrete specimens is presented in Table 7.

Test method
The concrete specimens cured for 24 days were put into the solution under different working conditions for 4 days to make the specimens saturated with water. Then, the rapid freeze-thaw test was carried out according to the Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete (GBT50082-2009) [21]. Sigma 500/VP field emission scanning electron microscope (SEM) and TG/DTA7300-integrated thermal analyzer were used to observe the internal microstructure of concrete in different salt solution types and analyze the mineral composition and element content of the erosion products.
Before the nuclear magnetic resonance test, the concrete was cut into the concrete specimen of Φ = 50 mm, and the specimens absorbed water for 24 h in a vacuum environment of −0.1 MPa to reach the saturated water absorption state, and then, specimens are placed into the low magnetic field of nuclear magnetic resonance testing machine for testing. The T 2 spectrum can be obtained after inversion of the measured spectrum signal, and the pore characteristic quantity of concrete can be measured at the same time [22].

Microstructure analysis
The microscopic morphology of concrete after freezethaw cycles is shown in Figure 3. show that the interior of the concrete was relatively dense before the freeze-thaw cycle, but there was still incompletely hydrated gel material inside, which was mainly concentrated in the interface transition zone at the small-     [23]. At the same time, the layered Ca (OH) 2 and the clustered C-S-H gel were formed inside. Due to the large amount of Ca(OH) 2 , it formed needleshaped ettringite crystals (AFt) in some small cavities with water and sulfate ions, which compacted the internal pores of the concrete. Yuebo et al. [24] found that the hydration products of concrete in the chloride environment would combine with Cl-to generate a large amount of Friedel salt, and OH-would be released during the reaction, which would combine with Ca ions to form Ca (OH) 2 . Although there were no large-scale developed cracks in the concrete after freezing and thawing cycles, a large amount of hexagonal plate-shaped Friedel salt appeared in the ettringite crystal (AFt) cluster in the transition zone between the cement paste and the aggregate interface. At the same time, the amount of Ca(OH) 2 increased. The production of Friedel salt and Ca(OH) 2 reduced the interfacial cohesive force, leading to the freeze-thaw damage of concrete, which was similar to that reported by Zongxi et al. [25]. Figure 3(c) shows that after freeze-thaw cycles in the sulfate solution, the amount of Ca(OH) 2 and cluster-like C-S-H gel decreased, gradually separated and decomposed, and obvious developmental cracks appeared in the concrete internal structure, and a lot of AFt growing in all directions were generated in the gaps and a large number of AFt also caused the cracks to develop rapidly, eventually leading to the expansion failure of concrete.
The comprehensive thermal analysis diagram of concrete is shown in Figure 4. It can be seen that the hydration products in concrete gradually decreased as the number of freeze-thaw cycles increased, while the types and content of erosion products gradually increased. By comparing with the literature [25][26][27], it can be seen that the endothermic peak at 70°C was the evaporation of nonchemically bound water; the endothermic peak at 80-140°C was the primary dehydration of monosulfide hydrated calcium sulfoaluminate (AFm) and C-S-H; the endothermic peak at 260-280°C was the secondary 160 298 1,214 654 74 1.12   dehydration of AFm and C-S-H; the endothermic peak at 420°C was caused by the decomposition of Ca(OH) 2 , and the endothermic peak at 670°C was caused by the thermal decomposition of calcite in the aggregate. As shown in Figure 4(b), the endothermic peak at 80-140°C and 420°C of the concrete in the salt solution after freezing and thawing cycles was significantly reduced compared with that without freezing and thawing cycles, indicating that C-S-H and Ca(OH) 2 were consumed in the erosion process and converted into other substances. Yuebo et al. [24] and Ying [29] had confirmed that the conversion products were Friedel salt and Ettringite. The endothermic peak of concrete in the salt solution after freezing and thawing cycles increased at 80-140°C, 150-200°C, and 850-900°C, which is due to the increase of ettringite and mirabilite content in the erosion products. At 150-200°C, the corrosion product mirabilite was dehydrated into sodium sulfate, and gypsum was dehydrated into hemihydrate gypsum. At 850°C, the sodium sulfate melted by absorbing heat. The concrete in the chlorine solution after freezing and thawing cycles had an obvious endothermic peak at 360°C, which is due to the increase in the content of Friedel salt as the number of freezing and thawing cycles increases.
In summary, the freeze-thaw damage of concrete in the salt solution was the combined effect of the expansion pressure of the freeze-thaw erosion product, the crystallization pressure of the salt solution, and the frost heave pressure of the fresh water. Under the influence of this, the surface cracks of the concrete appeared like net, and the erosion occurred, resulting in a decrease in physical properties of concrete. As the freeze-thaw cycle progresses, the damage gradually deepened, eventually the aggregates were exposed and the concrete broke.

Deterioration of concrete durability 3.2.1 Deterioration law
The life of concrete was reduced in the salt solution after the freezing and thawing cycles, which is essentially due to the damage of concrete with the freezing and thawing cycles and the decrease of internal compactness.
The mass loss of concrete after freezing and thawing cycles is shown in Figure 5. It can be seen that the mass loss of concrete in the chlorine solution and sulfate solution increased with the increase of freezing and thawing cycles, which means that the performance of concrete gradually deteriorated with the increase of freezing and thawing cycles, and its external damage was gradually increased. In addition, the mass loss of concrete in the chloride solution was greater than that in the sulfate solution, and the mass loss of concrete increased with the increase of chloride concentration. After 150 freezing and thawing cycles, the mass loss of concrete in the sulfate solution showed a greater degree of external erosion, and the mass loss exceeded that in the chloride solution. In this experiment, the relative dynamic elastic modulus was used as the concrete performance index, and the damage degree D n expressed by the relative dynamic elastic modulus was used to express the damage degree of the concrete [28], as shown in equation (1): where E 0 is the initial relative dynamic elastic modulus of concrete and E n is the relative dynamic elastic modulus of concrete after n freezing and thawing. The damage degree D n after freezing and thawing cycles is presented in Table 8 and Figure 6. It can be seen that the damage degree D n increased with the increase of freezing and thawing cycles, which means that the internal damage of concrete increased with the increase of freezing and thawing cycles. The damage degree of concrete in different solutions was different. The damage degree D n in the chloride salt solution increased linearly, and the damage rate was basically unchanged. The damage degree D n in the sulfate solution first increased linearly and then accelerated.
Before 150 freezing and thawing cycles, the damage degree of concrete in the chloride solution was greater than that in the sulfate solution and fresh water. The loss of elastic modulus of concrete in the chloride solution was 1.2-1.6 times that of in the sulfate solution. After 150 freezing and thawing cycles, the damage degree of concrete in the sulfate solution increased rapidly, which was greater than that in the chloride solution.
To further explain the destruction mechanism of concrete in the salt solution environment, the ionic reaction in concrete was analyzed from the basic chemistry. The negative electric charge particles (Cl − and − SO 4 2 ) in the erosion solution reacted with the positively charged particles (such as Ca 2+ and Si 4+ ) and hydration products of the concrete to generate corrosion destructive products. It was known from SEM and comprehensive thermal analysis that the destructive products were Friedel salt, gypsum, Aft, and mirabilite [29][30][31][32]. The different salt solution concentrations resulted in the concentration gradient, which provided a driving force for the inward directional diffusion of Cl − and − SO 4 2 . As the ions invaded the concrete were consumed by the reaction, the concentration gradient continued to exist, and Cl − and − SO 4 2 continued to diffuse inwardly. According to the Goldschmidt radius proposed by Goldschmidt [33], the contact radius of Cl − ions was smaller than that of   and gypsum, and mirabilite crystals were produced due to the enrichment of − SO 4 2 ions. When the expansion stress of the erosion products was greater than the tensile strength of the concrete pores, the micro-damage of the concrete was aggravated, leading to the formation of internal microcracks, − SO 4 2 ions quickly penetrated the interior of the concrete, a large number of expansive products were generated, and the performance of the concrete was rapidly degraded. In the linear destruction stage of − SO 4 2 ions, the concrete was mainly physically damaged, that is, the frost heave pressure damage. According to the research of Gaifei et al. [34], the frost heave pressure damage was the initial damage after the destructive ion surface penetrates at the initial stage of freezing and thawing cycles.

Nuclear magnetic resonance spectrum analysis
The nuclear magnetic resonance signal was processed according to the principle of nuclear magnetic resonance [35,36], and the nuclear magnetic resonance T 2 spectrum of concrete after freezing and thawing cycles was obtained, as shown in Figure 7. It can be seen that the T 2 relaxation time value of the concrete without freezing and thawing cycles was smaller than that after freezing and thawing cycles (the T 2 relaxation time represents the energy level transition time of all hydrogen atoms in the pore, and the larger the pore size, the more the hydrogen atoms and the longer the relaxation time). In addition, the concrete without freezing and thawing cycles had a significant single peak, and the single peak value was lower than that after freezing and thawing cycles. The single peak value of concrete in fresh water after 200 freezing and thawing cycles was significantly larger than that without freezing and thawing cycles, and the single peak value shifted to the right at the relaxation time,  which means that the internal pores of the concrete were developing in the direction of large pores, but the moving value was smaller than that of concrete in the salt solution. Therefore, it can be seen that the freeze-thaw damage of the concrete in the salt solution was greater than that in fresh water.
The pores inside the concrete were extremely small and could be approximated as a spherical shape. In addition, the T 2 relaxation time of concrete was directly proportional to the pore diameter, the relationship between the relaxation time and the pore diameter can be expressed by equation (2): where ρ proe is the transverse relaxation strength, μm·s −1 , and the empirical value is 5 μm·s −1 ; S is the internal pore surface area of the concrete, μm 2 ; and V is the internal pore volume of the concrete, μm 3 . According to equation (2), the T 2 spectrum of concrete could be transformed into the pore size distribution of concrete. According to Wu Zhongwei's concrete pore type classification theory [37], it can be known that pore radius less than 0.02 μm was harmless pore, pore radius between 0.02 and 0.05 μm was less harmful pore, pore radius between 0.05-0.20 μm was harmful pore, and pore radius greater than 0.20 μm was seriously harmful pore. The pore size proportion of concrete after freezing and thawing cycles can be obtained, as shown in Figure 8, and the difference in pore size proportion of concrete after freezing and thawing cycles is shown in Figure 9. Figure 9 shows that the pore size was mainly concentrated between 0.001 and 23.27 μm after 200 freezing and thawing cycles. The pores of concrete in fresh water after freezing and thawing were mainly harmless pores and less harmful pores, the main peak of the difference in pore size proportion was in the harmless pore area, and these were no obvious secondary peaks. The pore size of concrete in the salt solution was larger than that in fresh water, the main peak of the difference of pore size proportion shifted to the harmful pore area, and the secondary peak and third peak appeared in the seriously harmful pore area. The transfer range of the main peak of the difference of pore size proportion increased with the increase of the salt solution concentration, and the porosity of concrete in the salt solution after freezing and thawing cycles was greater than that without freezing and thawing cycles and greater than that in fresh water after freezing and thawing cycles. This showed that concrete damage in the salt solution was larger than that in fresh water, and it increased with the increase of the salt solution concentration.
The main peak position of the difference of pore size proportion in the chloride solution was slightly to the right than that in the sulfate solution, and the secondary peak and the third peak position were to the right and the peak values were lower. This situation can be explained from the ion reaction theory and the Goldschmidt radius: the contact radius of Cl − is smaller than − SO 4 2 , and Cl − is more likely to enter the interior of the concrete and undergo degradation reactions, so the less harmful pore changes of concrete in the chloride environment are more obvious. The reaction between − SO 4 2 and cement paste is an expansive product, which changes the original pores from small pores to large pores, so the concrete pores in the sulfate solution environment will change more obviously in the seriously harmful pore area.
The porosity decreased with the increase of the salt solution concentration. It is because that with the increase of the salt solution concentration, the Friedel salt formed by the reaction of the higher concentration of Cl − ions with the cement paste further separated the cement paste from the aggregate.
In addition, Figure 9 shows that when the pore diameter of concrete exceeded the peak, the pore diameter changed rapidly to the next peak. Therefore, the pore diameter at the peak position can be regarded as the limiting pore diameter (the limiting pore diameter is the minimum pore diameter when the concrete was frozen in the salt solution). The limiting pore diameter deteriorated as the concrete froze, and the concrete will be eroded and damaged when the pore diameter of the concrete exceeded the final limiting pore diameter. The main peak position, the second peak position, and the third peak position of the difference of pore size proportion in different salt solution environment were similar. Therefore, the concrete in different salt solution environment had the same limiting pore diameters. Fitting the pore diameters of the main peak, the secondary peak and the third peak of concrete in different salt solution, the limiting pore diameters were 0.0662, 1.145, and 10.116 μm, respectively.

Life evaluation of concrete in salt solution after freeze-thaw cycles
Concrete is a multidirectional nonuniform material, and so the freeze-thaw damage of concrete is also multidirectional random. Therefore, the life evaluation of concrete in the salt solution after freeze-thaw cycles should be carried out from the perspective of probability theory [38].
Weibull distribution can use probability values to infer distribution parameters [39], and it is widely used in data processing of various life tests and the design of strength failure distribution functions. In this article, Weibull distribution was used as the theoretical basis of concrete life evaluation, and the life evaluation model was established through the least-square method.
Assuming that the freeze-thaw cycle life of concrete (N) conforms to the Weibull distribution, the three major influencing parameters (scale, shape, and position parameters) of the Weibull distribution can be used to obtain the concrete performance degradation distribution, which can ultimately evaluate the life of concrete in the salt solution after freezing and thawing cycles. The Weibull distribution of freeze-thaw cycle life of concrete (N) was established according to the three-parameter model, as shown in equation (3): where α is the position parameter, which represents the minimum life threshold of concrete; b is the Weibull shape parameter, which determines the shape distribution of the distribution function F(N) and the density function ( ) f n ; ε is the scale parameter (the values of α, b, and ε are all greater than or equal to 0); and n is the test value of concrete freeze-thaw cycle life N.
The concrete has a relative maximum life before freeze-thaw cycle, and its values is greater than or equal to 0, so that = α 0, and take α into equation  After the freeze-thaw cycles, the relative maximum life showed the decreasing trend, until the relative maximum life was reduced to 0, and the damage degree D n = 1.
Carry out the Weibull transform of equation (4): and equation (5) can be transformed into a linear regression equation of one variable: According to Table 8 and equations (4)-(6), the Weibull calculations were performed to obtain the Weibull parameter b and c of concrete life in the salt solution after freezing and thawing cycles, as shown in Figure 10 and Table 9. The regression relationship coefficient R 2 was greater than 0.9. The parameters in the corresponding test were brought into the Weibull regression equation of concrete life in the salt solution after freezing and thawing cycles, and the evaluation life of concrete in salt solution after freeze-thaw cycles were done.
In addition, the average number of freeze-thaw cycles in the northwestern region was 8.8 [40], and the safe service life of concrete in the salt solution was obtained after freezing and thawing cycles. Table 10 presents the evaluation life and the safe service life of concrete in the salt solution after freezing and thawing cycles. It can be seen that Cl − and − SO 4 2 made the performance of concrete decline rapidly, and the safe service life of concrete was greatly shortened. Compared with the fresh water environment, the safe service life of concrete in the salt solution environment was shortened by at least 42%.

Conclusion
1. The freeze-thaw damage of concrete in the salt solution is the combined effect of the expansion pressure of the freeze-thaw erosion product, the crystallization pressure of the salt solution, and the frost heave pressure of the fresh water. 2. The damage degree increases with the increase of freezing and thawing cycles. Before 150 freezing and thawing cycles, the damage degree of concrete in the chloride solution is greater than that in the sulfate solution and fresh water. After 150 freezing and thawing cycles, the damage degree of concrete in the sulfate solution is greater than that in the chloride solution. 3. The pore size of concrete in the salt solution is larger than that in fresh water, the main peak of the difference in pore size proportion shifts to the harmful pore area, and the secondary peak and third peak appear in the seriously harmful pore area. The pore diameters of the main peak, the secondary peak and the third peak   of concrete in different salt solution, and the limiting pore diameters are 0.0662, 1.145, and 10.116 μm, respectively. 4. The safe service life of concrete in the salt solution environment after freezing and thawing cycles is at least 42% lower than that in a fresh water environment after freezing and thawing cycles.