Microstructure and chloride transport of aeolian sand concrete under long-term natural immersion

: River sand was consumed in large quantities, and alternatives to river sand were urgently needed. There are a large number of natural resources of aeolian sand in western China. Aeolian sand was prepared into aeolian sand concrete (ASC). It can greatly reduce the consumption of river sand and inhibit the process of deserti-ﬁ cation to protect the environment. ASC is a new type of concrete material prepared by using aeolian sand as ﬁ ne aggregate. To clarify the chloride ion transport behaviour in the ASC under long-term natural immersion, the aeolian sand was 100% substituted for the river sand to prepare the full ASC with three water – binder ratios. The ASC was naturally immersed in 3 and 6% NaCl solutions for a long time, and nuclear magnetic resonance and microscopic scanning electron microscopy techniques were used. The change rule of chloride ion content at di ﬀ erent depths of the ASC was studied, and its microstructure characteristics under di ﬀ erent erosion times were analysed. The results showed that the free chloride ion concentration at different depths of the ASC increased with increasing water – binder ratio, immersion time, and chloride concentration. After soaking in the salt solution, the hydration products in the ASC reacted with chloride ions to form Friedel salt, which ﬁ lled the internal pores and microcracks of the ASC, improved its interface transition zone structure, and increased the compactness of the test piece. The porosity of the three groups of ASC with di ﬀ erent water – binder ratios decreased by 0.95, 1.03, and 1.15% after soaking in 6% salt solution for 12 m. To study the di ﬀ usion law of chloride ions in ASC, combined with in ﬂ uencing factors such as temperature, humidity, D value, deterioration e ﬀ ect and chloride ion combination, Fick ’ s second law was modi ﬁ ed, and a chloride ion di ﬀ usion model of ASC with high accuracy was established, with a ﬁ tting correlation number above 0.93, which provided a reference for the research and application of ASC in saline areas.


Introduction
China is one of the countries with the most serious desertification, with desertification land accounting for 20% of its land resources in 2020 [1].The research and application of aeolian sand concrete (ASC) can absorb a large amount of desert sand resources and alleviate the shortage of river sand resources [2].With the implementation of China's "Belt and Road" strategy, a large number of concrete structures need to serve in complex environments, such as western salt lakes and coastal environments, where a large number of chloride ions accelerate the deterioration of concrete [3].Therefore, it is of great significance to study the chloride ion transport of ASC under long-term natural immersion.
To clarify the transport behaviour of chloride ions in concrete, Dong et al. [4][5][6] studied the transport of chloride ions in ASC and found that chloride salt erosion caused a variety of crystals inside the concrete, which filled the internal cementation hole and blocked the diffusion of chloride ions.Chloride ion diffusion is affected by many factors, such as aggregates and pores [7,8].Cao et al. [9][10][11] improved the performance of the interfacial region by adding carbon fibre.The results showed that the addition of carbon fibre can significantly improve the mechanical properties of the material, which leads to better durability and working performance.Gao et al. [12,13] studied the chloride ion transport behaviour in the interface area of recycled concrete and found that modified recycled coarse aggregate could make the interface transition zone (ITZ) structure more compact, which was beneficial to resist chloride ion erosion.However, when the ITZ structure of old aggregate and old mortar was dense, chloride ions could penetrate into the ITZ structure of recycled aggregate and new mortar and then enter the mortar matrix through pores and cracks, weakening the corrosion resistance of concrete.Tian et al. [14][15][16][17] proved that ITZ increased the diffusion rate of chloride ions by establishing a microscopic model of chloride ion diffusion in saturated concrete.Qiao et al. [18,19] simulated the transport of chloride ions in saturated and unsaturated concrete through numerical simulation.The research showed that the diffusion of chloride ions was dominant when the concrete was saturated, and convection was dominant in the unsaturated state.At present, most chloride ion diffusion models are established through Fick's law, and there are various chloride ion transport models and simulation methods [20][21][22][23].However, there are many factors affecting chloride ion diffusion, and the existing models have difficulty accurately reflecting the chloride ion transport law.Due to the complexity of concrete structures, simulation analysis is only an auxiliary research tool and cannot completely replace tests.
In summary, there are few studies on the chloride ion transport behaviour of ASC under long-term natural immersion, and most of the studies are numerical simulations.Based on this, this study uses chemical titration, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) to analyse the transport law of chloride ions in ASC under long-term natural immersion in different concentrations of NaCl solution and establish a more accurate chloride ion diffusion model, which provides a reference for the research and engineering application of chloride ion transport law in ASC.
2 Experimental design and method

Materials
The cement was P•O 42.5 Portland cement (Inner Mongolia, China) was used.The Grade-II fly ash was used.The coarse aggregate has a size of between 5 and 25 mm.The aeolian sand was collected from the surface of the Kubuqi Desert in north central China, with a particle size range of 0.075-0.25 mm, and the physical indicators are shown in Table 1.The tap water and admixture polycarboxylate superplasticiser were used.

Concrete mix proportion and preparation
In accordance with Chinese code (Specification for mix proportion design of ordinary concrete JGJ 55-2016), the aeolian sand was 100% replaced by river sand to prepare 100 mm × 100 mm × 100 mm cube specimens.The design water-binder ratios were 0.4, 0.45, and 0.55.Table 2 shows the mix ratio of ASC.

Test method
The ASC test block is sealed with epoxy resin on five sides, leaving one side as the erosion surface.The oven temperature was adjusted to 105°C and dried to constant weight.The test piece was placed in different concentrations of salt solution (3 and 6% NaCl) so that the liquid level was 1-2 cm higher than the test piece.After soaking 0, 1, 3, 5, 8, and 12 m, the powder was obtained by layered drilling (powder depth 0-5, 5-10, 10-15, 15-20, 20-25, 25-30, and 30-35 mm).
The soaking solution was replaced once a month.The content of free chloride ions in the powder was titrated by chemical titration according to Chinese code (technical specification for concrete test and inspection of waterway engineering JTS/T 236-2019), and the relevant data were recorded.
The microstructure of the samples soaked in 3% NaCl solution for 12 m was observed by SEM.To improve the conductivity of the sample and make the observation clear, the sample was glued to the tray with conductive tape before the observation and then gold-plated in the vacuum diffraction gold spray instrument.The micromorphology of the sample was observed at an accuracy of 20 μm.
The pore structure of the samples was analysed by an NMR analyser.The specimens without soaking treatment and 3 and 6% NaCl solution soaking for 12 m were drilled and sampled, and the sampling site was the centre of the specimen.The sample was made into a Ф48 mm × H50 mm cylinder and placed in a 0.1 MPa vacuum saturation device for vacuum saturation.An NMR test was performed after the sample reached saturation.

Quality loss rate
Figure 1 shows the relationship between the mass loss rate and time of ASC after long-term immersion.It can be seen from Figure 1 that when the concentration of the erosion solution is 3%, the mass loss rate increases rapidly and tends to be stable in the eighth month.When the concentration of the erosion solution is 6%, it tends to be stable after five months of long-term immersion, and the growth rate of the mass loss rate is extremely slow in 5-12 months.In the 12th month, the final mass loss rates of ASC-1, ASC-2, and ASC-3 in the 3% NaCl erosion solution are −0.55,−0.58, and −0.62%, respectively, and the final mass loss rates in the 6% NaCl erosion solution are −0.67,−0.7, and −0.75%, respectively.When the concentration of NaCl solution increases from 3 to 6%, the mass loss rates of ASC-1, ASC-2, and ASC-3 decrease further, and the reduction rates are −0.12,−0.12, and −0.13%, respectively.
The phenomenon of negative growth in the mass loss rate caused by concrete immersion in corrosive solution is that in the early stage of erosion, the hydration of concrete has just been completed, the internal pores are dry, and the capillary absorption capacity is strong.When concrete is placed in the corrosive solution, the solution enters the concrete through capillary absorption, which increases the quality of the concrete.The greater the water-binder ratio of the concrete is, the more internal capillary pores there are, and the stronger the capillary absorption capacity.When the water-binder ratio is the same and the  Ion transport of aeolian sand concrete  3 external solution concentration is different, the greater the concentration difference between the inside and outside of the concrete, the faster the NaCl in the external erosion solution enters the concrete through capillary absorption in the state of Na + and Cl − .In the 3 and 6% NaCl corrosion solutions, the mass loss rate of ASC produces inflection points at 8 and 5 m, respectively, and then, the loss rate tends to be balanced because Na + and Cl − had physicalchemical reactions with Ca(OH) 2 in the concrete, which is adsorbed at the capillary pore diameter or generates tiny particles to block the pores [24,25], preventing the external solution from entering the concrete.

Compressive strength
Figure 2 shows the compressive strength variation of ASC under long-term immersion.The compressive strength of ASC with a high water-binder ratio decreases.This is due to the poor gradation of aeolian sand, which cannot improve the gradual gap, increase the specific surface area of the aggregate, and reduce the encapsulation of the aggregate by the mortar matrix.The larger the water-binder ratio, the dosage of cement gradually decreases, and the cementitious material cannot fully encapsulate the sand aggregate, so that the bonding force between the mortar and the aggregate decreases, and a large number of tiny pores are formed inside the concrete.The structure of the concrete ITZ becomes weaker, resulting in a decrease in the compressive strength of the concrete.
Taking the ASC-1 group as an example, the compressive strength of 12 m immersed in 6% NaCl solution is 66.05 MPa, which is 3.75% higher than that of 3% NaCl solution 63.66 MPa.The compressive strength of ASC specimens immersed in 6% NaCl solution is better than that of ASC specimens immersed in 3% NaCl solution.This is because when concrete is immersed in a high-concentration solution, chloride ions enter the concrete specimen.When the ion concentration reaches a certain value, the ions will adsorb in the channel, react with the hydration products, fill the capillary channel, and make the ITZ structure of the concrete more compact.Macroscopically, the compressive strength of concrete increases [26].

Distribution of free chloride ion content
Figure 3 shows the distribution of free chloride ion content in ASC-1, ASC-2, and ASC-3 after soaking in 3 and 6% NaCl solutions at different times.Figure 3 shows that with increasing water-binder ratio, salt solution concentration, and soaking time, the amount and depth of chloride ion intrusion increased.The larger the water-binder ratio is, the worse the compactness of the concrete is, the better the connectivity between the pores is, and the chloride ion is easier to transport into the concrete.Therefore, the free chloride ion content in concrete with a large water-binder ratio is higher.The higher the concentration of salt solution is, the greater the concentration difference during immersion, which promotes the erosion of chloride ions into the interior and increases the free chloride ion content in the specimen.
To accurately analyse the transmission behaviour of free chloride ions in ASC with different water-binder ratios after soaking for 1, 3, 5, 8, and 12 m, the integral area of free chloride ion content is used to quantify the free chloride ion content at different depths in ASC, as shown in Figure 4.
Combined with Figures 3 and 4, it can be seen that ( 1) with increasing soaking time, the integral area of the free chloride ion content in the ASC increases.Taking ASC-3 immersed in 3% NaCl solution as an example, the free chloride ion content of the specimens with immersion times of 12, 8, 5, and 3 m at 2.5 mm increased by 70.15, 40.71, 29.44, and 18.37%, respectively, compared with that at 1 m, and the integral area of the free chloride ion content increased by 90.73, 45.42, 29.51, and 15.83%, respectively.(2) When the salt solution concentration and soaking time are the same, the larger the water-binder ratio of the ASC is, the larger the free chloride ion content and the integral area, and the easier the chloride ion migrates into the ASC.For example, after ASC-3 is immersed in a 3% NaCl solution for 12 m, the free chloride ion content at 2.5 mm increases by 9.54 and 4.49% compared with ASC-1 and ASC-2, and the integral area of the free chloride ion content increases by 16.69 and 12.31%.(3) When the water-binder ratio and soaking time were the same, with increasing salt solution concentration, the free chloride ion content at different depths of ASC in each group showed an increasing trend.For example, after the ASC-1 group is immersed in a salt solution for 12 m, the free chloride ion concentration at 2.5 mm in the 6% NaCl group increases by 0.125% compared with that in the 3% NaCl group.

SEM analysis
To analyse the microstructure change characteristics of ASC after long-term natural immersion in NaCl solution, the internal hydration products and ITZ structure of ASC-1 and ASC-3 specimens after immersion in 3% NaCl solution for 12 m were observed by SEM.Figures 5 and 6   Ion transport of aeolian sand concrete  5 microstructure of the ASC after soaking.Figures 5 and 6 show the microstructure of the ASC after soaking.It can be seen from Figure 5 that after long-term natural immersion in chloride salt, the structure of the ITZ is improved, and the compactness is good.It can be seen from Figure 6 that after long-term natural immersion in chloride salt, the structure of the ITZ is improved, and the compactness is good.There are no cracks at the junction of the aggregate and mortar matrix, but there are microcracks in the mortar matrix, with a small number and width, and no obvious holes are observed.This shows that the longterm natural immersion of NaCl solution has no obvious damage to the internal microstructure of ASC but has an enhancement effect on the ITZ structure.The reason is that the ITZ structure is the weak part of the ASC.The combination of aggregate and mortar matrix is prone to cracks, and the porosity of the nearby mortar matrix is high, which is the main channel for chloride ion transport.When the salt solution enters the interior of the ASC, it reacts with the hydration products to form Friedel salt [27,28], which fills the ITZ and microcracks, making the interior denser, reducing the porosity, and improving the structure.
From Figure 6, it can be seen that after soaking for 12 m, most of the hydration products are lamellar, and  some products are "nested" with each other, showing good integrity.At the same time, Friedel salts can be observed on the surface of the hydration products, some of which are distributed in a "needle-like" or "flocculent" shape, and some form layered structures.In addition, a small amount of lamellar Ca(OH) 2 and hexahedral salt crystals can be observed in the hydration products.

NMR analysis
Figures 7 and 8 show the NMR T 2 spectrum and T 2 spectrum area of ASC after soaking in 3 and 6% NaCl solutions for 12 m.According to Figure 7, the T 2 spectrum of ASC shows 3-4 peaks, and the first peak accounts for the largest proportion.With increasing NaCl solution concentration, the T 2 spectrum moves to the left, and the total area and the first peak signal amplitude decay.In Figure 8, the total T 2 spectrum area of the ASC-1, ASC-2, and ASC-3 groups decreases from 5286.691, 7491.198, and 8321.922 to 1826.208, 2541.198, and 3618.209,respectively.The total T 2 spectrum area decreases by 9.54% when ASC-3 is immersed in a 3% salt solution for 12 m and decreases by 51.98% when the solution concentration increases from 3 to 6%.
The reason is that after chloride ions enter the ASC, physical adsorption and chemical bonding occur with the cement hydration products, filling the internal pores and reducing the pore size and the number of pores, resulting in a left shift of the T 2 spectrum and a decrease in the first peak area.The increasing amount of chloride ion intrusion causes salt crystallisation in the ASC, and the reaction of chloride ions with hydration products reduces the porosity, resulting in a decrease in the total area of the T 2 spectrum.With increasing NaCl solution concentration, the concentration difference between the inside and outside of the specimen becomes larger, the transmission of  Ion transport of aeolian sand concrete  7 chloride ions to the inside of the specimen increases, and the porosity decreases more.The decrease in porosity increases the tortuosity of pores, resulting in an increase in the viscous resistance of Cl-and water during transport, a decrease in transport rate and depth, and a slower increase in chloride ion content in the later period.
According to Bout's classification method, the pore structure is divided into four categories: gel pores (r < 10 nm), transition pores (10 nm < r < 100 nm), capillary pores (100 nm < r < 1,000 nm), and macropores (r > 1,000 nm). Figure 9 shows the proportion of pore size classification of ASC after soaking in 3% NaCl and 6% NaCl solutions for 12 m.The comparative analysis shows that with the increase in the concentration of chloride solution, the number of gel pores of ASC increased significantly after soaking.ASC-1 increased from 68 to 90%, an increase of 22%, and ASC-3 increased from 33 to 86%, an increase of 53%.The number of transition holes decreased significantly, ASC-1 decreased from 24 to 7%, decreasing by 17%, and ASC-3 decreased from 51 to 10%, decreasing by 41%.The number of pores and macropores decreased slightly.This is because with increasing capillary absorption time, the amount of chloride ion intrusion increases, the internal hydration reaction continues, and the physical and chemical combination of chloride ions and hydration products produces Friedel salt, which increases the interior density and the number of holes.
The changes in porosity and saturation of ASC after soaking in NaCl solution are shown in Figure 10. Figure 10 shows that with increasing soaking time and salt solution concentration, the free fluid saturation and porosity of concrete decrease, and the bound fluid saturation increases.After soaking in 6% NaCl solution for 12 m, the free fluid saturation of the ASC-1, ASC-2, and ASC-3 groups decreases by 15, 17, and 19%, respectively, and the porosity decreases by 0.95, 1.03, and 1.15%, respectively.
With increasing soaking time, the amount of water and chloride ion intrusion increases, and the internal hydration products are further hydrated.Coupled with the reaction of chloride ions and hydration products, the reaction products fill the interior of concrete, improve the internal compactness, reduce the porosity, and deteriorate the connectivity between pore structures.With increasing NaCl solution concentration, the concentration difference between the inside and outside of the pores increases the amount of chloride ions and water intrusion, the connectivity of the  internal pore structure worsens, the number of closed pores increases, the number of open pores decreases, and the porosity and free fluid saturation decrease.

Chloride ion diffusion model of aeolian sand concrete
The diffusion of chloride ions in concrete generally satisfies Fick's second law.The use of Fick's second law requires three points: (1) concrete is regarded as a semi-infinite homogeneous medium; (2) Cl − does not react with porous media in the diffusion process; and (3) the diffusion coefficient D is a fixed value.In the actual test, Cl − will react with the gel material inside the concrete, and the reaction product will hinder the diffusion of chloride ions and reduce the diffusion rate.In addition, due to the influence of multiple factors, such as temperature, humidity, soaking period, and physical damage, the chloride ion diffusion coefficient D is not a fixed value.In summary, to establish a more accurate chloride ion diffusion model of ASC, the chloride ion diffusion model is modified based on Fick's second law.Chloride ion diffusion is sensitive to temperature.The higher the temperature is, the faster the chloride ion diffusion rate [29].The effect of temperature on the chloride diffusion coefficient is shown in the following equation: where T ref and D ref are the absolute temperature of curing 28 days and chloride diffusion coefficient at T ref ; T, D(T), and f 1 (t) are absolute temperature when calculating, chloride diffusion coefficient at temperature T, and temperature influence coefficient, respectively; and U is activation energy of diffusion process, When the ordinary Portland cement water-binder ratio is 0.4, 41,800 (J/mol) is taken, and 44,600 (J/mol) is taken when the water-binder ratio is 0.5; R is gas constant (8.314J/mol K).The water content in concrete is mainly affected by saturation, which is usually characterised by relative humidity.The effect of relative humidity on chloride ion diffusion in concrete is shown in the following equation [30]: where H, f 2 (H), and D(H) are the relative humidity, humidity influence coefficient, and chloride diffusion coefficient at relative humidity, respectively; H c and D(H c ) are the critical relative humidity, generally 75%, and the chloride diffusion coefficient at critical relative humidity H c .The functional relationship between the chloride ion diffusion coefficient D and age is shown in the following equation [31]: where f 3 (t) and t are the age influence coefficient and time, respectively; m is the attenuation coefficient; t ref , D ref , and D(t) are the reference time, chloride diffusion coefficient when the reference time is t ref , and chloride diffusion coefficient at time t, respectively.The effect of damage on the chloride diffusion coefficient is shown in the following equation [32]: where K and D ref are the degradation index and D of defect-free concrete, respectively.Ion transport of aeolian sand concrete  9 When the salt concentration gradient changes, the bound chloride ion concentration remains unchanged, so Fick's second law can be expressed as: where C b , C f , f 4 , and D ref are the combined chloride ion concentration, free chloride concentration, chloride binding coefficient and initial chloride diffusion coefficient, respectively.
Taking into account the impact of the five factors mentioned above on the diffusion coefficient of chloride ions, it can be expressed as By Fick's second law, we consider the initial condition: C(x,0) = C0; when the boundary conditions are C(0,t) = C s , C(∞,t) = C 0 , the following equation can be obtained by solving: where C s , C 0 , and erf(z) are the surface chloride ion concentration, initial chloride ion concentration, and Gauss error function, respectively.D is obtained by fitting, which means the value of concrete exposed to chloride solution [33].It is mainly calculated by the age of exposure to the chloride ion erosion environment that is the value of the reference time tref in equation ( 3): Bring equation (3) into equation ( 8): where t is time and Substituting equation ( 9) into equation ( 7), a chloride ion diffusion model considering time dependence can be obtained, that is: (10) D ref in equation ( 10) is replaced by D in equation ( 6), and influencing factors such as temperature, humidity, age, deterioration effect, and chloride binding effect are considered.The chloride ion diffusion model of ASC can be obtained: In the engineering application of the chloride ion diffusion model, it is necessary to give the values of the main parameters in the model, including m, C s , K, and k of the chloride ion diffusion coefficient.At present, the deterioration coefficient is used to evaluate the influence of concrete deterioration on chloride ion intrusion in practical engineering.The test environment is long-term natural immersion of chloride salt, which is close to the actual environment, so the deterioration coefficient K = 1; according to the Japanese Harbour Research Institute [34], k = 0.2302.
The value of m mainly depends on the composition of the concrete.Through Fick's second law fitting, the chloride ion diffusion coefficient of ASC with different water-binder ratios under different chloride salt concentrations and longterm natural immersion is obtained, as shown in Table 3.
According to formula (9), the corresponding m value can be obtained, as shown in Table 4.
In the long-term natural immersion process of chloride salt, the surface chloride ion concentration of the concrete structure is not constant but a process from low to high and gradually saturates.C s and t show simple linear, complex  According to the experimental data fitting, the C s = at b power function fitting effect is better and more in line with the actual situation, and the fitting results are shown in Table 5.
The free chloride ion concentration calculated by the modified chloride ion diffusion model of ASC is compared with the free chloride ion concentration test data of ASC after long-term natural immersion in 3 and 6% NaCl solutions for 12 m.The results are shown in Figure 11.It can be seen from Figure 11 that the difference between the theoretical value and the measured value of the free chloride ion concentration of ASC after soaking for 12 m is small, which proves that the established chloride ion diffusion model of ASC has high reliability (Table 6).

Main parameters and discussion of long-term soaking aeolian sand concrete
In practice, the Cl − diffusion will undergo a physical or chemical reaction with the gel material inside the concrete, which reduces the concentration of the chloride ions and the diffusion rate of the chloride ions.The chloride ion diffusion coefficient is not a fixed value and is affected by temperature, humidity, age, physical damage, and other factors.The main parameters of ASC after long-term immersion are shown in Table 7.It is shown that the diffusion coefficient of full ASC is similar to that of ordinary concrete.Full ASC with a low water-to-binder ratio has a strong resistance to chloride ion penetration, which is sufficient for practical engineering applications of ASC.Due to its similar impermeability to ordinary concrete, it is sufficient to ensure that the steel rods inside the concrete remain intact.In the future, ASC can be considered preliminary application in small buildings, such as warehouses and poultry houses.In this article, the ion diffusion behaviour of ASC is analysed in the form of long-term free diffusion of chloride ions.Compared with Zhou and Dong [35,36].In the result of the test, it was found that in the freeze-thaw cycle test of the whole ASC, due to the large pores of the whole ASC, the pore evolution first changed from large pores to small pores, and then the pores expanded and cracked.The ability to resist freeze-thaw cycles is stronger than that of ordinary concrete.A comparison with Li et al. [37] found that the impermeability of full ASC was better, but the compressive strength of partially mixed specimens was higher than that of fully mixed specimens.On the whole, if the ASC is applied to practical engineering, the performance of the concrete is the best when the content of aeolian sand is 20%.At this time, the ASC has higher compressive strength and the diffusion coefficient is similar to that of ordinary concrete.

Conclusion and foresight
It is important to note that the total ASC has good impermeability, and its compressive strength is lower than that of ordinary concrete.Its good anti-permeability performance is due to the large pores in the pore structure of the whole ASC.After the solution is inhaled, a stable or semi-stable meniscus shape is formed at the interface between the solution and the air, which hinders further Free chloride concentration/% Penetration depth/mm  entry of the solution.However, its large pore size will reduce the compressive strength.Therefore, more research is needed to apply ASC to important buildings such as bridges and dams.Through experimental analysis, this study draws the following conclusions: 1.The mass loss rates of ASC-1, ASC-2, and ASC-3 were −0.55, −0.58, and −0.62%, respectively, when they were immersed in 3% NaCl solution for 12 m and −0.67, −0.7, and −0.75%, respectively, when they were immersed in 6% NaCl solution.The greater the water-binder ratio of concrete, the greater the capillary pore diameter, the faster the chloride ion penetration, and the increase in corrosion solution concentration accelerated the chloride ion penetration rate, resulting in an increase in the mass loss rate.2. The compressive strength of ASC-1, ASC-2, and ASC-3 soaked in 6% NaCl solution at 12 is 3.75, 3.06, and 3.38% higher than that of 3% NaCl solution.The concentration of chloride ions in the outside world increased, the rate of entering the concrete was accelerated, and the hydration products reacted.The compressive strength of the ASC block soaked in 6% NaCl solution is better than that soaked in 3% NaCl solution.3. The larger the water-binder ratio, the easier the chloride ion was transmitted to the inside of the ASC, and the free chloride ion concentration and its integral area inside the specimen were relatively large.After the ASC-3 group was immersed in a 3% NaCl solution for 12 m, the free chloride ion content at 2.5 mm was 9.54 and 4.49% higher than that of the ASC-1 and ASC-2 groups, and the integral area of the free chloride ion content was increased by 16.69 and 12.31%.Under the same water-binder ratio and soaking time, the free chloride ion concentration at different depths of the ASC increased with increasing salt solution concentration.4.After soaking in NaCl solution for 12 m, the reaction products of chloride ions filled the internal pores and microcracks of the ASC and improved the ITZ structure of the concrete.With increasing salt solution concentration, the gel pores in ASC increased, and the transition pores and capillary pores decreased.Compared with the 3% salt solution, the gel pores increased by 53%, and the transition pores decreased by 41% in the ASC-3 group after soaking in the 6% salt solution for 12 m.The filling effect of reaction products worsens the connectivity between the pore structures of ASC, resulting in a decrease in free fluid saturation and an increase in bound fluid saturation.After soaking in 6% salt solution for 12 m, the free fluid saturation of the ASC-1, ASC-2, and ASC-3 groups decreased by 15, 17, and 19%, respectively.5. Considering the influence of temperature, humidity, age, deterioration effect, and chloride ion binding on the chloride ion diffusion coefficient of ASC, Fick's second law was modified, and the modified chloride ion diffusion model of ASC was established.
acquisition, AS: performed data collation, writingreview and editing, and MZ: methodology, writingreview and editing.

Conflict of interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 11 :
Figure 11: Fitting curve of the chloride diffusion model.

Table 1 :
Main physical indexes of fine aggregate Note: Percentage in the table is the mass fraction.

Table 2 :
Mix proportion of aeolian sand concrete

Table 3 :
Chloride ion diffusion coefficient of aeolian sand concrete

Table 5 :
Power function fitting between surface chloride concentration and time

Table 6 :
Fitting correlation coefficient of the chloride ion diffusion model for aeolian sand concrete