# Effect of Salt Solution on the Mechanical Behaviours of Geopolymer Concrete under Dry-Wet Cycles

Authors: Zhiyu Tang; Nianchun Deng
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Seawater splashing and frequent rainfall in the coastal area will often cause the concrete members to be affected by dry-wet cycles. In order to investigate the effect of salt solution on the mechanical behaviours of geopolymer concrete under dry-wet cycles, a series of tests on erosion of geopolymer concrete by three different salt solutions were carried out by using the mass loss rate, compressive strength, erosion resistance coefficient, and elastic modulus. The results show that, with the increase of dry-wet cycles, the mass of the specimen increases slightly at 20 dry-wet cycles and then decreases gradually. Then, the stress-strain curve of geopolymer concrete can be divided into three stages: linear growth stage, deceleration growth stage, and slow decline stage, which correspond to the elastic deformation stage, elastic-plastic deformation stage, and crushing stage, respectively. The overall trend of the stress-strain curve is similar under different dry-wet cycles, and the peak stress decreases with the increase of dry-cycle. The relationship between stress and strain of geopolymer concrete samples with different salt solutions is similar, while their strength characteristics have changed obviously. Furthermore, with the increase of dry-wet cycles, the compressive strength and the relative elastic modulus of geopolymer concrete gradually decrease, which reflects that sulfate erosion will lead to the strength loss of geopolymer concrete, causing its internal damage, thus having adverse effects on it. Different salt solutions have the great influence on the mass loss rate, compressive strength, erosion resistance coefficient, and elastic modulus of geopolymer concrete. In detail, the mixed salt solution erosion results in the greatest damage to geopolymer concrete and accelerates the damage. Chloride salt solution erosion causes middle damage to geopolymer concrete. Compared with the other two salt solutions, sulfate solution erosion leads to the least damage to geopolymer concrete and then sulfate has a certain inhibitory effect on the damage of geopolymer concrete. In addition, based on the test results, the constitutive model considering dry-wet cycles damage of salt solution is proposed, and the correctness is verified. This study has a good guiding value for geopolymer concrete engineering in coastal areas.


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## Body

## 1. Introduction

Guangxi province is located in the coastal area of China. Seawater splashing and frequent rainfall in the coastal area will often cause the concrete members to be affected by dry-wet cycles. The erosion of salt solution to concrete members is common [1]. Dry-wet cycles also intensify the erosion damage of salt solution to concrete members. However, the influence of salt solution on geopolymer concrete members is not clear at present. Geopolymer was first proposed by Davidovits in the 1970s, a series of chemical reactions occur between its main raw materials to form polymers with stable physical and chemical properties. Therefore, geopolymer is often used in many industrial fields for new cementitious materials [2, 3]. The production of geopolymer uses the waste of thermal power plants. Meanwhile, the use of geopolymer can also reduce the production of cement, which is also of great significance in environmental protection and waste reuse [4–7].The influence of dry-wet cycles on civil engineering materials has been focused by many scholars, such as asphalt [8], concrete [9], soft rock [10–13], and soil [14]. Laboratory test is a common method to study the mechanical properties of geopolymer concrete. Olivia and Nikraz [15] optimized the mix proportion of geopolymer concrete and found that geopolymer concrete has well durability, tensile, and compressive properties. Palom et al. [16] pointed out that using Na2O∙nSiO2 and NaOH as alkali activator can enhance the strength of geopolymer concrete. Assi et al. [17] studied the effects of different sources of fly ash on the properties of geopolymer concrete. Nath and Sarker [18] found that adding cement to geopolymer concrete can improve its strength development rate, but also reduce its workability. Ghosh et al. [19] evaluated the mechanical properties and durability parameters such as water sorptivity, efflorescence, and acid resistance of geopolymer concrete synthesized from fly ash of three different origins. Long et al. [20] studied the deterioration and microstructural evolution of the fly ash geopolymer concrete against MgSO4 solution. Lee et al. [21] focused on the durability of geopolymer concrete after nine months of an indoor and outdoor curing period and evaluated the influence of sodium aluminate, wollastonite additions, and NaOH concentration on the microstructure, physical, and mechanical properties. Luo et al. [22] studied the sulfate corrosion resistance of limestone powder concrete and the improvement mechanism of fly ash/slag were studied geopolymer concrete. Liu et al. [23] researched the damage of geopolymer concrete under the action of saltwater and freeze-thaw. In the research of salt solution eroding concrete, Chin et al. [24] studied the characterizes sorption and transport of distilled water, salt solution, and a simulated concrete pore solution in free films of vinyl ester, isophthalic polyester (isopolyester), and epoxy resins. Mass loss was observed for the isopolyester in salt water and concrete pore solution at 60°C, suggesting hydrolysis that was accelerated by the high temperature exposure. Xu et al. [25] investigated the corrosion mechanism and damage characteristic of steel fiber concrete under the effect of stray current and salt solution. Wang et al. [26] researched the effect of salt solution wet-dry cycling on the bond behavior of FRP-concrete interface. However, the effect of salt solution on geopolymer concrete under dry-wet cycle was still few reported.Through the analysis of the existing references, it can be seen from the above analysis, there are some studies on the mix proportion design of geopolymer concrete, the content of raw materials, and freeze-thaw damage, but there is still insufficient research on the mechanical properties of geopolymer concrete eroded by different salt solutions under dry-wet cycles. Therefore, based on the existing results, preparing geopolymer concrete specimens to study the mechanical properties (such as the mass loss rate, compressive strength, erosion resistance coefficient, and elastic modulus) after standard curing and to propose the constitutive model considering dry-wet cycles damage of salt solution.


## 2. Test Materials and Methods

### 2.1. Test Materials

The main test materials used in this test were fly ash, cement,NaOH solution, Na2SiO3 solution, NaCl solution, fine aggregate, and coarse aggregate. The fly ash used in the test belongs to the category F and level II produced by Datang Huayin You County thermal power plant, and the chemical composition is obtained by XRD (X-ray diffraction) test as shown in Figure 1. It can be seen from Figure 1 that the chemical composition of the fly ash is mainly Si2O3 and Al2O3. Meanwhile, the fly ash also contains some CaO, Fe2O3, MgO, and SO3. The early strength of the geopolymer concrete is slow to develop because only fly ash is added, and adding a certain amount of cement can effectively improve the early strength [18]. The cement used in this experiment was P∙O42.5 ordinary portland cement. NaOH solution was prepared by dissolving NaOH flake alkali (with a purity of 99% and produced in Wuxi city Yatai United Chemical Co. Ltd. of Wuxi city) into water, and the solution concentration was 12 Mol/L. The alkali activator was composed of NaOH solution with 12 Mol/L and sodium silicate (i.e., Na2SiO3, produced in Yatai United Chemical Co. Ltd. of Wuxi city), and the mass ratio of NaOH solution to sodium silicate solution was 0.4. The fine aggregate was medium sand with fineness modulus of 2.25 and apparent density of 2658 kg/m3. The coarse aggregate was granite after mechanical crushing.Figure 1
Main chemical composition of fly ash.


### 2.2. Choice of Mix Proportions

The optimum mix proportion of geopolymer concrete was determined by using the principle of orthogonal experimental design. The more cement content occurs with the higher cost, but the lower cement content will affect its strength. According to the existing research [22], the mix proportion of geopolymer concrete samples are as follows, 200, 400, and 600 kg/m3 for fly ash, 35.3, 45.7, and 48.6 kg/m3 for NaOH solution, and 72.5, 90.8, and 114.3 kg/m3 for Na2SiO3 solution, respectively. In addition, cement, fine aggregate and coarse aggregate are taken as 20 kg/m3, 650 kg/m3, and 1210 kg/m3, which has the same content in each specimen. The orthogonal experimental design is shown in Table 1.Table 1
The orthogonal experimental design.
NumbersFly ashNaOH solutionNa2SiO3 solutionCementOT1200kg/m335.3kg/m372.5kg/m320kg/m3OT2200kg/m345.7kg/m390.8kg/m320kg/m3OT3200kg/m348.6kg/m3114.3kg/m320kg/m3OT4400kg/m335.3kg/m390.8kg/m320kg/m3OT5400kg/m345.7kg/m3114.3kg/m320kg/m3OT6400kg/m348.6kg/m372.5kg/m320kg/m3OT7600kg/m335.3kg/m3114.3kg/m320kg/m3OT8600kg/m345.7kg/m372.5kg/m320kg/m3OT9600kg/m348.6kg/m390.8kg/m320kg/m3Firstly, fly ash, cement, fine aggregate, and coarse aggregate following the mix proportion shown in Table1 were mixed well. Then, the alkali activator cooled to room temperature was added and all test materials were fully stirred. After that, cube specimens with the size of 150 mm × 150 mm × 150 mm for geopolymer concrete were prepared, and all of the specimens were cured under standard curing conditions. The selection of cube samples mainly refers to [27]. Finally, the specimens were taken out after curing for 28 days under standard curing conditions and the residual water on the surface of samples was lightly wiped and dried for using.The compressive strength of each specimen was obtained using the RYL-600 servo shear rheometer. According to the treatment method of orthogonal test results, the compressive strength of sample number 5 is the largest. Therefore, the mix proportion of geopolymer concrete for sample number 5 is used to study the effect of salt solution on the mechanical behaviours of geopolymer concrete under dry-wet cycles.


### 2.3. Test Methods

Following the above analysis, the mix proportion of geopolymer concrete sample was shown in Table2. The test methods refer to the GB/T 50082-2009 Standard for test methods of long-term performance and durability of ordinary concrete [28] to carry out the effect of salt solution on the strength behaviours of geopolymer concrete under dry-wet cycles.Table 2
The mix proportion of geopolymer concrete sample.
MaterialsFly ashCementNaOH solutionNa2SiO3 solutionFine aggregateCoarse aggregateUnit weight400kg/m320kg/m345.7kg/m3114.3kg/m3650kg/m31210kg/m3Na2SO4 solution with 5% by mass, NaCl solution with 5% by mass, and the mixed solution of Na2SO4 solution with 5% by mass and NaCl solution with 5% by mass were used as the erosion solution under dry-wet cycles in this test. The water surface of the prepared solution should be at least 10 mm higher than the specimen. Experiment of salt solution eroding geopolymer concrete under dry-wet cycles was carried out in the automatic concrete sulfate testing machine of the college of engineering and architecture at Guangxi University. The value of the drying temperature was taken as 50 ± 5°C according to the possible temperature of concrete members in summer of Guangxi province. In this experiment, the salt solution eroding process was simulated in the laboratory under dry-wet cycles. In order to speed up the simulation time, the salt solution eroding time was 12 h and the drying time was 12 h. Repeating once salt solution eroding and once drying treatment meant a complete dry-wet cycle and a whole cycle spent 24 h. The salt solution was replaced regularly every month. The dry-wet cycles of salt solution eroding geopolymer concrete selected 0, 20, 40, 80, 120, and 160, respectively.The specimens were taken out from the machine after achieving the specified cycle numbers. The experiment of strength properties of geopolymer concrete was carried out according to the GB/T 50081-2019 standard for test methods of concrete physical and mechanical properties [29]. RYL-600 servo shear rheometer was used as the test loading equipment, achieving 600 kN axial load, automatic collecting, and storing test data during the experiment can be provided by this instrument. In addition, the data accuracy is less than ±0.3%, and the loading rate is 0.01 mm/s. The test was completed when the specimen was obviously broken. The test flow is shown in Figure 2.Figure 2
Test flow chart.In this study,NaCl solution, Na2SO4 solution, and the mixed solution of Na2SO4 and NaCl are numbered as SE1, SE2, and SE3, respectively. In order to ensure the reliability of the test results, each experiment was repeated for 4 times.


## 2.1. Test Materials

The main test materials used in this test were fly ash, cement,NaOH solution, Na2SiO3 solution, NaCl solution, fine aggregate, and coarse aggregate. The fly ash used in the test belongs to the category F and level II produced by Datang Huayin You County thermal power plant, and the chemical composition is obtained by XRD (X-ray diffraction) test as shown in Figure 1. It can be seen from Figure 1 that the chemical composition of the fly ash is mainly Si2O3 and Al2O3. Meanwhile, the fly ash also contains some CaO, Fe2O3, MgO, and SO3. The early strength of the geopolymer concrete is slow to develop because only fly ash is added, and adding a certain amount of cement can effectively improve the early strength [18]. The cement used in this experiment was P∙O42.5 ordinary portland cement. NaOH solution was prepared by dissolving NaOH flake alkali (with a purity of 99% and produced in Wuxi city Yatai United Chemical Co. Ltd. of Wuxi city) into water, and the solution concentration was 12 Mol/L. The alkali activator was composed of NaOH solution with 12 Mol/L and sodium silicate (i.e., Na2SiO3, produced in Yatai United Chemical Co. Ltd. of Wuxi city), and the mass ratio of NaOH solution to sodium silicate solution was 0.4. The fine aggregate was medium sand with fineness modulus of 2.25 and apparent density of 2658 kg/m3. The coarse aggregate was granite after mechanical crushing.Figure 1
Main chemical composition of fly ash.


## 2.2. Choice of Mix Proportions

The optimum mix proportion of geopolymer concrete was determined by using the principle of orthogonal experimental design. The more cement content occurs with the higher cost, but the lower cement content will affect its strength. According to the existing research [22], the mix proportion of geopolymer concrete samples are as follows, 200, 400, and 600 kg/m3 for fly ash, 35.3, 45.7, and 48.6 kg/m3 for NaOH solution, and 72.5, 90.8, and 114.3 kg/m3 for Na2SiO3 solution, respectively. In addition, cement, fine aggregate and coarse aggregate are taken as 20 kg/m3, 650 kg/m3, and 1210 kg/m3, which has the same content in each specimen. The orthogonal experimental design is shown in Table 1.Table 1
The orthogonal experimental design.
NumbersFly ashNaOH solutionNa2SiO3 solutionCementOT1200kg/m335.3kg/m372.5kg/m320kg/m3OT2200kg/m345.7kg/m390.8kg/m320kg/m3OT3200kg/m348.6kg/m3114.3kg/m320kg/m3OT4400kg/m335.3kg/m390.8kg/m320kg/m3OT5400kg/m345.7kg/m3114.3kg/m320kg/m3OT6400kg/m348.6kg/m372.5kg/m320kg/m3OT7600kg/m335.3kg/m3114.3kg/m320kg/m3OT8600kg/m345.7kg/m372.5kg/m320kg/m3OT9600kg/m348.6kg/m390.8kg/m320kg/m3Firstly, fly ash, cement, fine aggregate, and coarse aggregate following the mix proportion shown in Table1 were mixed well. Then, the alkali activator cooled to room temperature was added and all test materials were fully stirred. After that, cube specimens with the size of 150 mm × 150 mm × 150 mm for geopolymer concrete were prepared, and all of the specimens were cured under standard curing conditions. The selection of cube samples mainly refers to [27]. Finally, the specimens were taken out after curing for 28 days under standard curing conditions and the residual water on the surface of samples was lightly wiped and dried for using.The compressive strength of each specimen was obtained using the RYL-600 servo shear rheometer. According to the treatment method of orthogonal test results, the compressive strength of sample number 5 is the largest. Therefore, the mix proportion of geopolymer concrete for sample number 5 is used to study the effect of salt solution on the mechanical behaviours of geopolymer concrete under dry-wet cycles.


## 2.3. Test Methods

Following the above analysis, the mix proportion of geopolymer concrete sample was shown in Table2. The test methods refer to the GB/T 50082-2009 Standard for test methods of long-term performance and durability of ordinary concrete [28] to carry out the effect of salt solution on the strength behaviours of geopolymer concrete under dry-wet cycles.Table 2
The mix proportion of geopolymer concrete sample.
MaterialsFly ashCementNaOH solutionNa2SiO3 solutionFine aggregateCoarse aggregateUnit weight400kg/m320kg/m345.7kg/m3114.3kg/m3650kg/m31210kg/m3Na2SO4 solution with 5% by mass, NaCl solution with 5% by mass, and the mixed solution of Na2SO4 solution with 5% by mass and NaCl solution with 5% by mass were used as the erosion solution under dry-wet cycles in this test. The water surface of the prepared solution should be at least 10 mm higher than the specimen. Experiment of salt solution eroding geopolymer concrete under dry-wet cycles was carried out in the automatic concrete sulfate testing machine of the college of engineering and architecture at Guangxi University. The value of the drying temperature was taken as 50 ± 5°C according to the possible temperature of concrete members in summer of Guangxi province. In this experiment, the salt solution eroding process was simulated in the laboratory under dry-wet cycles. In order to speed up the simulation time, the salt solution eroding time was 12 h and the drying time was 12 h. Repeating once salt solution eroding and once drying treatment meant a complete dry-wet cycle and a whole cycle spent 24 h. The salt solution was replaced regularly every month. The dry-wet cycles of salt solution eroding geopolymer concrete selected 0, 20, 40, 80, 120, and 160, respectively.The specimens were taken out from the machine after achieving the specified cycle numbers. The experiment of strength properties of geopolymer concrete was carried out according to the GB/T 50081-2019 standard for test methods of concrete physical and mechanical properties [29]. RYL-600 servo shear rheometer was used as the test loading equipment, achieving 600 kN axial load, automatic collecting, and storing test data during the experiment can be provided by this instrument. In addition, the data accuracy is less than ±0.3%, and the loading rate is 0.01 mm/s. The test was completed when the specimen was obviously broken. The test flow is shown in Figure 2.Figure 2
Test flow chart.In this study,NaCl solution, Na2SO4 solution, and the mixed solution of Na2SO4 and NaCl are numbered as SE1, SE2, and SE3, respectively. In order to ensure the reliability of the test results, each experiment was repeated for 4 times.


## 3. Test Results and Analysis

### 3.1. Mass Loss Analysis

The mass of geopolymer concrete specimen will be lost after several dry-wet cycles, which can reflect the damage of geopolymer concrete specimen. In this paper, the mass loss rate is used for quantitative analysis, which defined as(1)∆m=m0−mnm0×100%,where ∆m is the mass loss rate of geopolymer concrete specimen (%), m0 is the initial mass of the specimen (kg), and mn is the mass of the specimen after N cycles (kg).Figure3 shows the relationship between the mass loss rate of geopolymer concrete and dry-wet cycles. It can be seen from Figure 3 that, with the increase of the number of dry-wet cycles, the mass of the specimen increases slightly at 20 dry-wet cycles and then decreases gradually. The main reason of such appearance is that, after soaking the solution in the early stage of the test, the pores and cracks in the specimen are filled with salt solution. Then, the salt solution may react with geopolymer concrete and remain in the specimen, resulting in the increase of the mass of the specimen in the early stage of test. However, with the further increase of the number of dry-wet cycles, the damage effect caused by dry-wet cycles of salt solution is greater than the internal hydration reaction, resulting in the decrease of specimen mass and obvious damage accumulation effect. This also shows that there is a critical point between 20–40 dry-wet cycles, which can make the mass of the specimen neither lose nor increase. At 160 dry-wet cycles, the mass loss rate of the specimen in the three solutions is more than 3% and reached 3.9% at the maximum. It indicates that the spalling of the geopolymer concrete surface is serious, and absorbing the salt solution causes the gradual collapse of the geopolymer concrete and the erosion resistance of concrete is degraded.Figure 3
The relationship between the mass loss rate and the dry-wet cycles.In the early stage of the test (when the number of dry-wet cycles is less than 80), the effects of the three salt solutions on the mass loss rate of geopolymer concrete are relatively close. With the increase of dry-wet cycles, various salt solutions have different influences on the mass loss rate of geopolymer concrete. It mainly shows that the mass loss rate of the specimen in the mixed solution ofNa2SO4 and NaCl is the largest, followed by NaCl solution, and Na2SO4 solution is the weakest.


### 3.2. Relationship between Stress-Strain

The relationship between stress and strain of geopolymer concrete specimens eroded by different salt solutions under dry-wet cycles is presented in Figures4–6. As can be seen from Figures 4–6 that the stress-strain curve of geopolymer concrete can be divided into three stages. The first stage is the linear growth stage, and the curves show an approximate linear growth. The second stage is the deceleration growth stage, and the curves growth rate slow down gradually. The third stage is the slow decline stage, and the curves decrease gradually. The three stages of the stress-strain curve correspond to the elastic deformation stage, elastic-plastic deformation stage, and crushing stage, respectively.Figure 4
The relationship between stress and strain (NaCl solution, i.e., SE1 specimen).Figure 5
The relationship between stress and strain (Na2SO4 solution, i.e., SE2 specimen).Figure 6
The relationship between stress and strain (the mixed solution ofNa2SO4 and NaCl, i.e., SE3 specimen).The overall trend of the stress-strain curve is similar under different dry-wet cycles, and the peak stress and residual stress decrease with the increase of dry-wet cycles. The relationship between stress and strain of geopolymer concrete samples with different salt solutions is similar. However, their strength characteristics have changed obviously, which will be analyzed in the following paper.


### 3.3. Analysis of Strength and Elastic Modulus

Further, the influence of dry-wet cycles on the compressive strength of geopolymer concrete is analyzed, and the compressive strength is the maximum value captured from the relationship between stress and strain in Figures4–6. Figure 7 displays the relationship between compressive strength and dry-wet cycles. As shown in Figure 7, with the increase of dry-wet cycles, the compressive strength of geopolymer concrete decrease gradually. This reflects that the erosion of salt solution will lead to the strength loss of geopolymer concrete, causing its internal damage, thus having adverse effects on it.Figure 7
The relationship between compressive strength and dry-wet cycles.Different salt solutions have the great influence on the compressive strength of geopolymer concrete. In detail, the compressive strength of geopolymer concrete specimen is the smallest in the mixed solution ofNa2SO4 and NaCl is the middle in the NaCl solution and is the biggest in the Na2SO4 solution.The erosion resistance of geopolymer concrete is analyzed, and the erosion resistance coefficient is defined as(2)K=fc160fc0×100%,where K is the erosion resistance coefficient of geopolymer concrete (%), fc160 is the strength of the specimen after 160 dry-wet cycles (MPa), and fc0 is the strength of the specimen with different erosion degree (MPa).The erosion resistance coefficient of geopolymer concrete can be calculated according to equation (2). Figure 8 shows the relationship between erosion resistance coefficient and dry-wet cycles. As can be seen from Figure 8, with the increasing dry-wet cycles, the erosion resistance coefficient of geopolymer concrete increases gradually, which also shows that the erosion of salt solution to geopolymer concrete gradually accumulates with the increasing dry-wet cycles. Among the three different salt solutions, the erosion resistance coefficient in the mixed solution of Na2SO4 and NaCl is the smallest, which reflects that the erosion degree of the mixed salt solution to geopolymer concrete is the most intense. In addition, the erosion resistance coefficient in the Na2SO4 solution is the largest, followed by NaCl solution.Figure 8
The relationship between erosion resistance coefficient and dry-wet cycles.In order to facilitate comparison, the calculated elastic modulus is normalized, which is called relative elastic modulus. Figure9 presents the relative elastic modulus versus dry-wet cycles. It can be seen from Figure 9, with the increase of dry-wet cycles, the relative elastic modulus of geopolymer concrete gradually decrease. The decrease of elastic modulus indicates that there is a certain internal damage after dry-wet cycles, that is, the salt solution erosion under dry-wet cycles has a great impact on its performance. The relative elastic modulus with mixed solutions of Na2SO4 and NaCl in three different salt solutions is the smallest, indicating that it is more prone to deformation.Figure 9
The relationship between relative elastic modulus and dry-wet cycles.


### 3.4. Discussion

For the SE1 samples (i.e., the salt solution isNaCl solution) during 160 dry-wet cycles, the maximum mass loss of the sample is 3.70%, the compressive strength reduces by 32.15%, and the relative elastic modulus decreases by 11.21%. Chloride salt solution erosion causes obvious damage to geopolymer concrete under dry-wet cycles according to above parameters. The above parameters are the middle values compared with the other two salt solutions. These show that chloride salt has an adverse effect on the durability of geopolymer concrete. Lee and van Deventer [30] indicated that chloridion will gradually induce the precipitation and crystallization of aluminosilicate gel (geopolymer binding phase), thereby reducing the durability of geopolymers.With regard to specimens in theNa2SO4 solution (in other words SE2 specimen) during 160 dry-wet cycles, the maximum mass loss of the sample is 3.44%, the compressive strength reduces by 25.87%, and the relative elastic modulus decreases by 8.42%. Compared with the other two salt solutions, sulfate solution erosion leads to the least damage to geopolymer concrete under dry-wet cycles. It should be noted that, Rashad et al. [31] found that using Na2SO4 as an activator of geopolymer can lead to a long-lasting polymerization, causing more dense geopolymer structure. These show that compared with the other two salt solutions, sulfate has a certain inhibitory effect on the damage of geopolymer concrete.In terms of the specimens in the mixed solution ofNa2SO4 and NaCl (SE3 specimen), the maximum mass loss of the sample is 3.91%, the compressive strength reduces by 42.25%, and the relative elastic modulus decreases by 15.01%. Compared with the other two salt solutions, mixed salt solution erosion results in the greatest damage to geopolymer concrete under dry-wet cycles. These show that the mixed salt solution accelerates the damage of geopolymer concrete.


## 3.1. Mass Loss Analysis

The mass of geopolymer concrete specimen will be lost after several dry-wet cycles, which can reflect the damage of geopolymer concrete specimen. In this paper, the mass loss rate is used for quantitative analysis, which defined as(1)∆m=m0−mnm0×100%,where ∆m is the mass loss rate of geopolymer concrete specimen (%), m0 is the initial mass of the specimen (kg), and mn is the mass of the specimen after N cycles (kg).Figure3 shows the relationship between the mass loss rate of geopolymer concrete and dry-wet cycles. It can be seen from Figure 3 that, with the increase of the number of dry-wet cycles, the mass of the specimen increases slightly at 20 dry-wet cycles and then decreases gradually. The main reason of such appearance is that, after soaking the solution in the early stage of the test, the pores and cracks in the specimen are filled with salt solution. Then, the salt solution may react with geopolymer concrete and remain in the specimen, resulting in the increase of the mass of the specimen in the early stage of test. However, with the further increase of the number of dry-wet cycles, the damage effect caused by dry-wet cycles of salt solution is greater than the internal hydration reaction, resulting in the decrease of specimen mass and obvious damage accumulation effect. This also shows that there is a critical point between 20–40 dry-wet cycles, which can make the mass of the specimen neither lose nor increase. At 160 dry-wet cycles, the mass loss rate of the specimen in the three solutions is more than 3% and reached 3.9% at the maximum. It indicates that the spalling of the geopolymer concrete surface is serious, and absorbing the salt solution causes the gradual collapse of the geopolymer concrete and the erosion resistance of concrete is degraded.Figure 3
The relationship between the mass loss rate and the dry-wet cycles.In the early stage of the test (when the number of dry-wet cycles is less than 80), the effects of the three salt solutions on the mass loss rate of geopolymer concrete are relatively close. With the increase of dry-wet cycles, various salt solutions have different influences on the mass loss rate of geopolymer concrete. It mainly shows that the mass loss rate of the specimen in the mixed solution ofNa2SO4 and NaCl is the largest, followed by NaCl solution, and Na2SO4 solution is the weakest.


## 3.2. Relationship between Stress-Strain

The relationship between stress and strain of geopolymer concrete specimens eroded by different salt solutions under dry-wet cycles is presented in Figures4–6. As can be seen from Figures 4–6 that the stress-strain curve of geopolymer concrete can be divided into three stages. The first stage is the linear growth stage, and the curves show an approximate linear growth. The second stage is the deceleration growth stage, and the curves growth rate slow down gradually. The third stage is the slow decline stage, and the curves decrease gradually. The three stages of the stress-strain curve correspond to the elastic deformation stage, elastic-plastic deformation stage, and crushing stage, respectively.Figure 4
The relationship between stress and strain (NaCl solution, i.e., SE1 specimen).Figure 5
The relationship between stress and strain (Na2SO4 solution, i.e., SE2 specimen).Figure 6
The relationship between stress and strain (the mixed solution ofNa2SO4 and NaCl, i.e., SE3 specimen).The overall trend of the stress-strain curve is similar under different dry-wet cycles, and the peak stress and residual stress decrease with the increase of dry-wet cycles. The relationship between stress and strain of geopolymer concrete samples with different salt solutions is similar. However, their strength characteristics have changed obviously, which will be analyzed in the following paper.


## 3.3. Analysis of Strength and Elastic Modulus

Further, the influence of dry-wet cycles on the compressive strength of geopolymer concrete is analyzed, and the compressive strength is the maximum value captured from the relationship between stress and strain in Figures4–6. Figure 7 displays the relationship between compressive strength and dry-wet cycles. As shown in Figure 7, with the increase of dry-wet cycles, the compressive strength of geopolymer concrete decrease gradually. This reflects that the erosion of salt solution will lead to the strength loss of geopolymer concrete, causing its internal damage, thus having adverse effects on it.Figure 7
The relationship between compressive strength and dry-wet cycles.Different salt solutions have the great influence on the compressive strength of geopolymer concrete. In detail, the compressive strength of geopolymer concrete specimen is the smallest in the mixed solution ofNa2SO4 and NaCl is the middle in the NaCl solution and is the biggest in the Na2SO4 solution.The erosion resistance of geopolymer concrete is analyzed, and the erosion resistance coefficient is defined as(2)K=fc160fc0×100%,where K is the erosion resistance coefficient of geopolymer concrete (%), fc160 is the strength of the specimen after 160 dry-wet cycles (MPa), and fc0 is the strength of the specimen with different erosion degree (MPa).The erosion resistance coefficient of geopolymer concrete can be calculated according to equation (2). Figure 8 shows the relationship between erosion resistance coefficient and dry-wet cycles. As can be seen from Figure 8, with the increasing dry-wet cycles, the erosion resistance coefficient of geopolymer concrete increases gradually, which also shows that the erosion of salt solution to geopolymer concrete gradually accumulates with the increasing dry-wet cycles. Among the three different salt solutions, the erosion resistance coefficient in the mixed solution of Na2SO4 and NaCl is the smallest, which reflects that the erosion degree of the mixed salt solution to geopolymer concrete is the most intense. In addition, the erosion resistance coefficient in the Na2SO4 solution is the largest, followed by NaCl solution.Figure 8
The relationship between erosion resistance coefficient and dry-wet cycles.In order to facilitate comparison, the calculated elastic modulus is normalized, which is called relative elastic modulus. Figure9 presents the relative elastic modulus versus dry-wet cycles. It can be seen from Figure 9, with the increase of dry-wet cycles, the relative elastic modulus of geopolymer concrete gradually decrease. The decrease of elastic modulus indicates that there is a certain internal damage after dry-wet cycles, that is, the salt solution erosion under dry-wet cycles has a great impact on its performance. The relative elastic modulus with mixed solutions of Na2SO4 and NaCl in three different salt solutions is the smallest, indicating that it is more prone to deformation.Figure 9
The relationship between relative elastic modulus and dry-wet cycles.


## 3.4. Discussion

For the SE1 samples (i.e., the salt solution isNaCl solution) during 160 dry-wet cycles, the maximum mass loss of the sample is 3.70%, the compressive strength reduces by 32.15%, and the relative elastic modulus decreases by 11.21%. Chloride salt solution erosion causes obvious damage to geopolymer concrete under dry-wet cycles according to above parameters. The above parameters are the middle values compared with the other two salt solutions. These show that chloride salt has an adverse effect on the durability of geopolymer concrete. Lee and van Deventer [30] indicated that chloridion will gradually induce the precipitation and crystallization of aluminosilicate gel (geopolymer binding phase), thereby reducing the durability of geopolymers.With regard to specimens in theNa2SO4 solution (in other words SE2 specimen) during 160 dry-wet cycles, the maximum mass loss of the sample is 3.44%, the compressive strength reduces by 25.87%, and the relative elastic modulus decreases by 8.42%. Compared with the other two salt solutions, sulfate solution erosion leads to the least damage to geopolymer concrete under dry-wet cycles. It should be noted that, Rashad et al. [31] found that using Na2SO4 as an activator of geopolymer can lead to a long-lasting polymerization, causing more dense geopolymer structure. These show that compared with the other two salt solutions, sulfate has a certain inhibitory effect on the damage of geopolymer concrete.In terms of the specimens in the mixed solution ofNa2SO4 and NaCl (SE3 specimen), the maximum mass loss of the sample is 3.91%, the compressive strength reduces by 42.25%, and the relative elastic modulus decreases by 15.01%. Compared with the other two salt solutions, mixed salt solution erosion results in the greatest damage to geopolymer concrete under dry-wet cycles. These show that the mixed salt solution accelerates the damage of geopolymer concrete.


## 4. Damage Constitutive Model

### 4.1. Establishment of Damage Constitutive Model

According to the variation law of the relationship between stress and strain for geopolymer concrete in the above Section3.2, the following piecewise constitutive model could be used to describe [32].(3)σσu=aεεu+3−2aεεu2+a−2εεu3,ε/εubε/εu−12+ε/εu,where σ and ε are the stress and strain, respectively, σu is the maximum stress, and εu is the strain with the maximum stress.The constitutive model shown in equation (3) is used to analyze the relationship between stress and strain. The model parameters obtained by regression analysis are shown in Figures 10–12, and all of the correlation coefficients are bigger than 0.9. It can be seen from the correlation coefficients that the constitutive model of equation (3) can well describe the relationship between stress and strain of geopolymer concrete.Figure 10
Model parameters (SE1 specimen).Figure 11
Model parameters (SE2 specimen).Figure 12
Model parameters (SE3 specimen).According to the damage mechanics, the damage variables are defined as follows(4)D=1−E∗E0,where E0 and E∗ are the elastic modulus without damage and after damage.The damage variable of geopolymer concrete erosion of salt solution under dry-wet cycles can be obtained through equation (3). By regression analysis between the obtained damage variable and the above constitutive model parameters, it can be captured from Figures 10–12 that, the relationship between them meets the exponential function.(5)a=cedD,(6)b=eefD,where c, d, e, and f are the coefficient of exponential function, and the values are shown in Table 3.Table 3
The coefficient of exponential function.
Specimen numbercdR2efR2SE13.3643−0.0020.91340.67850.00330.9716SE22.8725−0.0030.95390.47910.00290.9848SE34.4431−0.0010.90860.43670.00820.9771All of the correlation coefficients in Table3 are larger than 0.9, thus equations (5) and (6) can well reflect the relationship between the damage variable and the constitutive model parameters. Next, substituting equations (5) and (6) into equation (3), the constitutive model considering dry-wet cycles damage of salt solution is as follows:(7)σσu=cedDεεu+3−2cedD×εεu2+cedD−2εεu3,ε/εueefDε/εu−12+ε/εu.


### 4.2. Model Validation

The constitutive model considering dry-wet cycles damage of salt solution (i.e., equation (7)) is compared with the test results shown in Figures 4–6, and then the results show that the theoretical calculation is in good agreement with the test results. Here, only one group of data (0 dry-wet cycles of SE2 specimen) is randomly selected to display, because the graph of each group of data is the same. It can be seen from comparison results and Figure 13 that the theoretical calculation is very close to the test results, which verifies the correctness of the model.Figure 13
Model validation.


## 4.1. Establishment of Damage Constitutive Model

According to the variation law of the relationship between stress and strain for geopolymer concrete in the above Section3.2, the following piecewise constitutive model could be used to describe [32].(3)σσu=aεεu+3−2aεεu2+a−2εεu3,ε/εubε/εu−12+ε/εu,where σ and ε are the stress and strain, respectively, σu is the maximum stress, and εu is the strain with the maximum stress.The constitutive model shown in equation (3) is used to analyze the relationship between stress and strain. The model parameters obtained by regression analysis are shown in Figures 10–12, and all of the correlation coefficients are bigger than 0.9. It can be seen from the correlation coefficients that the constitutive model of equation (3) can well describe the relationship between stress and strain of geopolymer concrete.Figure 10
Model parameters (SE1 specimen).Figure 11
Model parameters (SE2 specimen).Figure 12
Model parameters (SE3 specimen).According to the damage mechanics, the damage variables are defined as follows(4)D=1−E∗E0,where E0 and E∗ are the elastic modulus without damage and after damage.The damage variable of geopolymer concrete erosion of salt solution under dry-wet cycles can be obtained through equation (3). By regression analysis between the obtained damage variable and the above constitutive model parameters, it can be captured from Figures 10–12 that, the relationship between them meets the exponential function.(5)a=cedD,(6)b=eefD,where c, d, e, and f are the coefficient of exponential function, and the values are shown in Table 3.Table 3
The coefficient of exponential function.
Specimen numbercdR2efR2SE13.3643−0.0020.91340.67850.00330.9716SE22.8725−0.0030.95390.47910.00290.9848SE34.4431−0.0010.90860.43670.00820.9771All of the correlation coefficients in Table3 are larger than 0.9, thus equations (5) and (6) can well reflect the relationship between the damage variable and the constitutive model parameters. Next, substituting equations (5) and (6) into equation (3), the constitutive model considering dry-wet cycles damage of salt solution is as follows:(7)σσu=cedDεεu+3−2cedD×εεu2+cedD−2εεu3,ε/εueefDε/εu−12+ε/εu.


## 4.2. Model Validation

The constitutive model considering dry-wet cycles damage of salt solution (i.e., equation (7)) is compared with the test results shown in Figures 4–6, and then the results show that the theoretical calculation is in good agreement with the test results. Here, only one group of data (0 dry-wet cycles of SE2 specimen) is randomly selected to display, because the graph of each group of data is the same. It can be seen from comparison results and Figure 13 that the theoretical calculation is very close to the test results, which verifies the correctness of the model.Figure 13
Model validation.


## 5. Conclusions

(1)
With the increase of the number of dry-wet cycles, the mass of the specimen increases slightly at 20 dry-wet cycles and then decreases gradually. However, with the further increase of the number of cycles, the damage effect caused by dry-wet cycles of salt solution is greater than the internal hydration reaction, resulting in the decrease of specimen mass and obvious damage accumulation effect. In the early stage of the test, the effects of the three salt solutions on the mass loss rate of geopolymer concrete are relatively close. However, when the number of dry-wet cycles is greater than 80, the mass loss rate of the specimen shows that in the mixed solution ofNa2SO4 and NaCl is the largest, followed by NaCl solution, and Na2SO4 solution is the weakest.(2)
The stress-strain curve of geopolymer concrete can be divided into three stages: linear growth stage, deceleration growth stage, and slow decline stage, which correspond to the elastic deformation stage, elastic-plastic deformation stage, and crushing stage, respectively. The overall trend of the stress-strain curve is similar under different dry-wet cycles, and the peak stress and residual stress decrease with the increase of dry-cycle. The relationship between stress and strain of geopolymer concrete samples with different salt solutions is similar, while their strength characteristics have changed obviously.(3)
With the increase of dry-wet cycles, the compressive strength and the relative elastic modulus of geopolymer concrete gradually decrease, which reflects that sulfate erosion will lead to the strength loss of geopolymer concrete, causing its internal damage, thus having adverse effects on it. Different salt solutions have the great influence on the compressive strength and the relative elastic modulus of geopolymer concrete. In detail, the compressive strength and the relative elastic modulus of geopolymer concrete specimen is the smallest in the mixed solution ofNa2SO4 and NaCl is the middle in the NaCl solution, and is the biggest in the Na2SO4 solution.(4)
Based on the test results, the constitutive model considering dry-wet cycles damage of salt solution is proposed and the correctness is verified.



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*Source: 9120821-2022-07-07.xml*