# Experimental Study on Diffusion Process of Sulfate Ion in Cemented Gangue Backfill Material

Authors: Yuxia Guo; Peng Wang; Guorui Feng; Tingye Qi; Xianjie Du; Zehua Wang; Qingdong Li
Journal: Advances in Civil Engineering
 (2020)

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## Abstract

Cemented gangue backfill material (CGBM) is made of coal gangue, fly ash, cement, and water. The backfill body will be corroded by mine water that contains different kinds of corrosive ions. The paper mainly aimed to study the diffusion process of sulfate ion (SO42−) in CGBM. Soluble salt solutions with different concentrations are prepared to simulate various mine water. The SO42− concentration was measured by layered sampling method and chemical analyzing method. The diffusion coefficient was calculated according to the second Fick’s law. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to analyze the degradation mechanisms. The experimental results showed that the SO42− concentration increases in CGBM when the external sulfate solutions concentration increases. With the increasing of magnesium ion (Mg2+) concentration and bicarbonate ion (HCO3−) concentration in the external corrosion solutions, the diffusion coefficient of SO42− was increased, and HCO3− has more significant effects than Mg2+. SO42− reacting with CGBM produced ettringite and gypsum. The ettringite and gypsum changed the pore structure of hardened CGBM and affected the compressive strength of hardened CGBM. Studying the diffusion of SO42− can contribute to predicting the service life of CGBM.


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## Body

## 1. Introduction

Backfill mining is taken to solve the problem of surface subsidence caused by underground coal mining [1]. It has many benefits, such as reducing the accumulation of solid waste, and the occupation of cultivated land, developing and utilizing underground space [2], and protecting groundwater and environment [3]. Cemented gangue backfill material (CGBM), one of the common backfill materials, is made of coal gangue, fly ash, cement, and water [4]. Comparing to other backfill materials, the main raw materials of CGBM, such as coal gangue and fly ash, are plentiful and cheap, and its performance meets the workability and strength requirements of mining.Due to the different levels of mine water, which contain different kinds of corrosive ions in the mine, the backfill body will be corroded by mine water during the service period. Zhang et al. found that the composition of mine water under the mine was complex and diverse, and most of them haveSO42−, HCO3−, Cl−, Mg2+, Na+, and Ga2+ [5]. The mine water composition of 37 measuring points in 12 mining areas in Shanxi and Inner Mongolia was investigated. Most of the mine water contains corrosive ions such as Mg2+, HCO3−, and SO42−, and the ion concentration in the mine water varies greatly. The concentrations of Mg2+, HCO3−, and SO42− are 3.88∼225.00 mg/L, 221.000∼3446.045 mg/L, and 73.89∼3314.00 mg/L, respectively. These ions can be detrimental for hydrated cement in an aqueous environment and for integrity of backfill material consequently [6]. Therefore, it is essential to understand the diffusion process of corrosive ion and the deterioration mechanism of the compressive strength of backfill body.Scholars have conducted a lot of research on the influence of corrosive media on the strength of cement-based materials. Fall et al. found that the strength of the backfill materials under sulfate attack is related to the concentration of sulfate solution, the amount of cement, and the chemical composition [7]. Sun et al. found that the change of backfill body strength under Cl−, SO42−, and Mg2+ corrosion is divided into slow growth segment and accelerated reduction segment, and the greater the concentration, the greater the reduction in strength [8]. Zhao et al. found that the strength of concrete increases first and then decreases in Na2SO4 and Na2SO4 and NaHCO3 complex salt solution and the concrete strength loss rate in the composite solution is greater than the single solution [9].The effective section size of the backfill body will be reduced under corrosive mine water corrosion, and then the bearing capacity will be reduced, which lead the instability to occur. Exploring the diffusion process of corrosive ions inside materials is the basis for determining the redundancy of the section size as the backfill design. Benoit et al. found that sulfate attack was a diffusion reaction process of concrete when exposed to environment with sulfate [10]. Wang et al. immersed cement mortar in sulfate solutions used layered sampling method combined with chemical analysis, and calculated diffusion coefficients of SO42− used XRD and SEM to analyze the formation of reaction products. The experimental results showed that in different kinds of sulfate solutions, cement mortars were immersed in (NH4)2SO4 solutions firstly, MgSO4 solutions took the second place, and Na2SO4 solutions are the last [11]. Zuo and Sun found the SO42− concentration in the concrete had a gradual increase with the increase of diffusion time; finally, the diffusion and reaction reach a dynamic equilibrium [12]. Idiart et al. accounted for the effect of discrete cracks on the transport of ions, and experimental observation indicated ettringite penetration to make an impact on crack propagation [13]. Tai et al. considered that the distribution of pores had influence for the diffusion of SO42− in concrete specimens. Ettringite could fill pores, which would change the diffusion path of the ions [14].Some scholars have establishing models about diffusion coefficient. Nicola et al. used external sulfate to attack concrete structures under partially or fully saturated conditions and established a diffusion model to account for chemical and mechanical damage [15, 16]. In order to calculate the chloride diffusion coefficient (D) in concrete, 3 factors (chloride concentration, curing, and environment temperature) were considered and used to predict concrete’s life of service [17, 18]. Wang et al. used Fick’s second law to calculate diffusion coefficient of chloride ion [19–21].Corrosion of backfill materials by soluble salts is a complex physical and chemical process. Revealing the performance degradation mechanism of backfill materials under corrosive action plays an important role in understanding the deterioration of material properties. Fall et al. found that C-S-H gel in tailings cemented filler material absorbs sulfate and causes its structural quality to decline. The expanded mineral ettringite in the pores of the paste material dissolves at high temperature to roughen the pores of the paste [22]. A few investigators found sulfate attack produced ettringite, and this process had two stages. The ettringite was strengthened due to its crystallization in the first stage; the ettringite was weakened because of overexpansion and modeling to verify its rationality and validity in the second stage [23, 24]. Some investigators also used high resolution synchrotron X-ray diffraction (SyXRD) to analyze microstructural profile of concrete deterioration which was attacked by sulfate [25–27].At present, a lot of researches have been carried out on the strength and corrosive ion diffusion laws of cement-based composite materials such as concrete under salt corrosion environment, and especially the research results of concrete materials are fruitful. But at the same time, it can be found that due to various materials, different corrosive media and test methods, various factors, and research results varying widely, only qualitative results or laws can be referenced and borrowed, and special research should be carried out for different materials in combination with actual working conditions.The main objective of this paper is to study the diffusion process ofSO42− and investigate the relationship between compressive strength and diffusion and analyze the products using XRD and SEM. The paper also considers the time factor to establish a prediction model which is about CGBM’s length of service, which can contribute to predicting the service life of CGBM.


## 2. Materials and Methods

### 2.1. Raw Materials

For ordinary Portland cement (CEM I 42.5 N) (binary system), the initial and final setting time were 165 min and 231 min, respectively, and the compressive strength was 21.9 MPa (3 d curing time) and 54.1 MPa (28 d curing. time). The fly ash sample was obtained from the power plant of Xinyang Colliery, a mine in Shanxi Province of China. The coal gangue samples were obtained from Xinyang Colliery, through breaking, and divided into fine aggregate (<5 mm) and coarse aggregate (5∼15 mm) through breaking; their chemical compositions and physical properties are listed in Table1. The mixture proportion of CGBM is shown in Table 2.Table 1
Chemical and physical properties of the main raw materials of CGBM [4].
Major element
Cement (%)
Fly ash (%)
Gangue (%)
SiO2
22.36
52.42
35.46
Al2O3
5.53
32.48
16.11
Fe2O3
3.46
3.62
3.86
CaO
65.08
0.99
7.15
MgO
2.1
1.01
3.50
TiO2
—
1.26
0.80
Specific gravity
3.1
2.2
2.0
Specific surface (m2/kg)
372
415
499
Fineness (>45μm) (%)
5
42.54
53.78Table 2
Proportion of CGBM mixture.
Cement (kg·m−3)
Fly ash (kg·m−3)
Coal gangue (kg·m−3)
Mass concentration (%)
Fine aggregate
Coarse aggregate
190
380
285
665
80


### 2.2. Salt Solution Corrosion Experiments

#### 2.2.1. Specimen’s Preparation

The specimen’s preparation was according to Chinese standard GB/T50081-2002. The cube specimens with size of 100 mm × 100 mm × 100 mm were casted. The specimens were cured under the temperature of 23 ± 2°C and RH was not less than 90% in molds for 24 hours. Then the specimens were demolded and cured in water under the temperature of 23 ± 1°C for 28 days. Namely, curing time of specimens was 28 days before the immersion test. Specimens were sealed with epoxy resin on the five opposite sides, and one side was exposed to subsequent sulfate attack experiments [11].


#### 2.2.2. Salt Solution Corrosion Conditions

Specimens were soaked in different salt solutions; salt mediums were sodium sulfate (Na2SO4) solutions, magnesium sulfate (MgSO4) solution, and sodium bicarbonate (NaHCO3) solution; and the proportions of these solutions are listed in Table 3. S3, S5, and S10 represent 3%, 5%, and 10% Na2SO4 solutions; M3 and M5 represent 3% and 5% MgSO4 solutions; and H3 and H5 represent 3% and 5% NaHCO3 solutions. To keep the concentration constant, the solutions were renewed for three days.Table 3
Ratio of salt solutions.
Ion name
Na2SO4
MgSO4
NaHCO3
N
0
0
0
S3
3%
0
0
S5
5%
0
0
S10
10%
0
0
S5 + M3
2%
3%
0
S5 + M5
0
5%
0
S5 + H3
5%
0
3%
S5 + H5
5%
0
5%
S5 + M5+H5
0
5%
5%


#### 2.2.3. Layered Sampling Method

Specimens immersed in different salt solutions at the schedule age (30 d/60 d/120 d/180 d) were taken out and dried under the temperature of 60°C for about 24 hours to make the surface dry and easily sampled. Then every 3 mm layered sampling was taken from the exposed surface to center by precision lathe and the powder samples were collected, respectively. After vacuum drying for 24 hours under the temperature of 105 ± 5°C, the samples were sealed for the next experiment. Cutting depth of each specimen was usually six layers which is 18–20 mm. The test process is shown in Figure1.Figure 1
Test process.


#### 2.2.4. Determination ofSO42− of Specimens

Chemical analysis method was carried out to determine the sulfate ion (SO42−) concentration of powder samples in different depths of specimens. Analysis method was according to Chinese standard GB/T 176-2008 “Methods for Chemical Analysis of Cement”. The powder samples were collected by the method of layered sampling, weighed accurately, dissolved by hot hydrochloric acid and filtered. Then the filtrate and Barium chloride solution (BaCl2) was mixed to produce precipitate which was filtered again, and weighed after drying under the 900°C [11].SO
4
2
− concentration was calculated according to the following:(1)wSO42−=m2−m1×0.4116m×100%,where m is the mass of powder sample, g; m1 is the mass of crucible, g; and m2 is the total mass of precipitate and crucible, g.


#### 2.2.5. Testing Compressive Strength

Compressive strength test was conducted with a YAW-200B-type compression-testing machine. The load was implemented at a relatively slow rate (0.3 KN/s). Each test was repeated at least three times and the average value was calculated as the compressive strength of specimens.


#### 2.2.6. Diffusion Coefficient Calculation ofSO42−

Fick’s second law of diffusion was widely used to describe kinds of ions diffusion into concrete [18]. In general, the diffusion of SO42− in CGBM obeyed Fick’s second law. In this paper, the CGBM was assumed as a semi-infinite homogeneous medium and SO42− was not bound to material when they diffused, so the coefficient of SO42− was constant [28], and the following one-dimensional equation was used:(2)∂C∂t=Dc∂2C∂x2,where C is the SO42− concentration, cm2/s; t is the period of corrosion, s; x is the corrosion depth, cm; and D is the diffusion coefficient, cm2/s.When the initial condition wast = 0, x > 0, C = 0; boundary condition was x = 0, t > 0, C=Cs. The solutions were evolved into the following equation:(3)Cx,t=Cs1−erfx2Dct,where Cs is the concentration of SO42− on the surface, cm2/s, and erf is the error function,erf=2/π∫0ue−t2dt.Then diffusion coefficient was evolved into the following equations:(4)Di=xi22tΦ−1mi2,(5)mi=1221−Cix,tCs+0.5,where Φ−1mi can be traceable standard normal distribution table.TheSO42− concentration on the surface and coefficient of variation was evolved into the following equations:(6)ED=∑i=1nDin,(7)VarD=∑i=1nDi2−nE2Dn−1,where n is the number of measured points of the specimen and Var is the coefficient of variation.


## 2.1. Raw Materials

For ordinary Portland cement (CEM I 42.5 N) (binary system), the initial and final setting time were 165 min and 231 min, respectively, and the compressive strength was 21.9 MPa (3 d curing time) and 54.1 MPa (28 d curing. time). The fly ash sample was obtained from the power plant of Xinyang Colliery, a mine in Shanxi Province of China. The coal gangue samples were obtained from Xinyang Colliery, through breaking, and divided into fine aggregate (<5 mm) and coarse aggregate (5∼15 mm) through breaking; their chemical compositions and physical properties are listed in Table1. The mixture proportion of CGBM is shown in Table 2.Table 1
Chemical and physical properties of the main raw materials of CGBM [4].
Major element
Cement (%)
Fly ash (%)
Gangue (%)
SiO2
22.36
52.42
35.46
Al2O3
5.53
32.48
16.11
Fe2O3
3.46
3.62
3.86
CaO
65.08
0.99
7.15
MgO
2.1
1.01
3.50
TiO2
—
1.26
0.80
Specific gravity
3.1
2.2
2.0
Specific surface (m2/kg)
372
415
499
Fineness (>45μm) (%)
5
42.54
53.78Table 2
Proportion of CGBM mixture.
Cement (kg·m−3)
Fly ash (kg·m−3)
Coal gangue (kg·m−3)
Mass concentration (%)
Fine aggregate
Coarse aggregate
190
380
285
665
80


## 2.2. Salt Solution Corrosion Experiments

### 2.2.1. Specimen’s Preparation

The specimen’s preparation was according to Chinese standard GB/T50081-2002. The cube specimens with size of 100 mm × 100 mm × 100 mm were casted. The specimens were cured under the temperature of 23 ± 2°C and RH was not less than 90% in molds for 24 hours. Then the specimens were demolded and cured in water under the temperature of 23 ± 1°C for 28 days. Namely, curing time of specimens was 28 days before the immersion test. Specimens were sealed with epoxy resin on the five opposite sides, and one side was exposed to subsequent sulfate attack experiments [11].


### 2.2.2. Salt Solution Corrosion Conditions

Specimens were soaked in different salt solutions; salt mediums were sodium sulfate (Na2SO4) solutions, magnesium sulfate (MgSO4) solution, and sodium bicarbonate (NaHCO3) solution; and the proportions of these solutions are listed in Table 3. S3, S5, and S10 represent 3%, 5%, and 10% Na2SO4 solutions; M3 and M5 represent 3% and 5% MgSO4 solutions; and H3 and H5 represent 3% and 5% NaHCO3 solutions. To keep the concentration constant, the solutions were renewed for three days.Table 3
Ratio of salt solutions.
Ion name
Na2SO4
MgSO4
NaHCO3
N
0
0
0
S3
3%
0
0
S5
5%
0
0
S10
10%
0
0
S5 + M3
2%
3%
0
S5 + M5
0
5%
0
S5 + H3
5%
0
3%
S5 + H5
5%
0
5%
S5 + M5+H5
0
5%
5%


### 2.2.3. Layered Sampling Method

Specimens immersed in different salt solutions at the schedule age (30 d/60 d/120 d/180 d) were taken out and dried under the temperature of 60°C for about 24 hours to make the surface dry and easily sampled. Then every 3 mm layered sampling was taken from the exposed surface to center by precision lathe and the powder samples were collected, respectively. After vacuum drying for 24 hours under the temperature of 105 ± 5°C, the samples were sealed for the next experiment. Cutting depth of each specimen was usually six layers which is 18–20 mm. The test process is shown in Figure1.Figure 1
Test process.


### 2.2.4. Determination ofSO42− of Specimens

Chemical analysis method was carried out to determine the sulfate ion (SO42−) concentration of powder samples in different depths of specimens. Analysis method was according to Chinese standard GB/T 176-2008 “Methods for Chemical Analysis of Cement”. The powder samples were collected by the method of layered sampling, weighed accurately, dissolved by hot hydrochloric acid and filtered. Then the filtrate and Barium chloride solution (BaCl2) was mixed to produce precipitate which was filtered again, and weighed after drying under the 900°C [11].SO
4
2
− concentration was calculated according to the following:(1)wSO42−=m2−m1×0.4116m×100%,where m is the mass of powder sample, g; m1 is the mass of crucible, g; and m2 is the total mass of precipitate and crucible, g.


### 2.2.5. Testing Compressive Strength

Compressive strength test was conducted with a YAW-200B-type compression-testing machine. The load was implemented at a relatively slow rate (0.3 KN/s). Each test was repeated at least three times and the average value was calculated as the compressive strength of specimens.


### 2.2.6. Diffusion Coefficient Calculation ofSO42−

Fick’s second law of diffusion was widely used to describe kinds of ions diffusion into concrete [18]. In general, the diffusion of SO42− in CGBM obeyed Fick’s second law. In this paper, the CGBM was assumed as a semi-infinite homogeneous medium and SO42− was not bound to material when they diffused, so the coefficient of SO42− was constant [28], and the following one-dimensional equation was used:(2)∂C∂t=Dc∂2C∂x2,where C is the SO42− concentration, cm2/s; t is the period of corrosion, s; x is the corrosion depth, cm; and D is the diffusion coefficient, cm2/s.When the initial condition wast = 0, x > 0, C = 0; boundary condition was x = 0, t > 0, C=Cs. The solutions were evolved into the following equation:(3)Cx,t=Cs1−erfx2Dct,where Cs is the concentration of SO42− on the surface, cm2/s, and erf is the error function,erf=2/π∫0ue−t2dt.Then diffusion coefficient was evolved into the following equations:(4)Di=xi22tΦ−1mi2,(5)mi=1221−Cix,tCs+0.5,where Φ−1mi can be traceable standard normal distribution table.TheSO42− concentration on the surface and coefficient of variation was evolved into the following equations:(6)ED=∑i=1nDin,(7)VarD=∑i=1nDi2−nE2Dn−1,where n is the number of measured points of the specimen and Var is the coefficient of variation.


## 2.2.1. Specimen’s Preparation

The specimen’s preparation was according to Chinese standard GB/T50081-2002. The cube specimens with size of 100 mm × 100 mm × 100 mm were casted. The specimens were cured under the temperature of 23 ± 2°C and RH was not less than 90% in molds for 24 hours. Then the specimens were demolded and cured in water under the temperature of 23 ± 1°C for 28 days. Namely, curing time of specimens was 28 days before the immersion test. Specimens were sealed with epoxy resin on the five opposite sides, and one side was exposed to subsequent sulfate attack experiments [11].


## 2.2.2. Salt Solution Corrosion Conditions

Specimens were soaked in different salt solutions; salt mediums were sodium sulfate (Na2SO4) solutions, magnesium sulfate (MgSO4) solution, and sodium bicarbonate (NaHCO3) solution; and the proportions of these solutions are listed in Table 3. S3, S5, and S10 represent 3%, 5%, and 10% Na2SO4 solutions; M3 and M5 represent 3% and 5% MgSO4 solutions; and H3 and H5 represent 3% and 5% NaHCO3 solutions. To keep the concentration constant, the solutions were renewed for three days.Table 3
Ratio of salt solutions.
Ion name
Na2SO4
MgSO4
NaHCO3
N
0
0
0
S3
3%
0
0
S5
5%
0
0
S10
10%
0
0
S5 + M3
2%
3%
0
S5 + M5
0
5%
0
S5 + H3
5%
0
3%
S5 + H5
5%
0
5%
S5 + M5+H5
0
5%
5%


## 2.2.3. Layered Sampling Method

Specimens immersed in different salt solutions at the schedule age (30 d/60 d/120 d/180 d) were taken out and dried under the temperature of 60°C for about 24 hours to make the surface dry and easily sampled. Then every 3 mm layered sampling was taken from the exposed surface to center by precision lathe and the powder samples were collected, respectively. After vacuum drying for 24 hours under the temperature of 105 ± 5°C, the samples were sealed for the next experiment. Cutting depth of each specimen was usually six layers which is 18–20 mm. The test process is shown in Figure1.Figure 1
Test process.


## 2.2.4. Determination ofSO42− of Specimens

Chemical analysis method was carried out to determine the sulfate ion (SO42−) concentration of powder samples in different depths of specimens. Analysis method was according to Chinese standard GB/T 176-2008 “Methods for Chemical Analysis of Cement”. The powder samples were collected by the method of layered sampling, weighed accurately, dissolved by hot hydrochloric acid and filtered. Then the filtrate and Barium chloride solution (BaCl2) was mixed to produce precipitate which was filtered again, and weighed after drying under the 900°C [11].SO
4
2
− concentration was calculated according to the following:(1)wSO42−=m2−m1×0.4116m×100%,where m is the mass of powder sample, g; m1 is the mass of crucible, g; and m2 is the total mass of precipitate and crucible, g.


## 2.2.5. Testing Compressive Strength

Compressive strength test was conducted with a YAW-200B-type compression-testing machine. The load was implemented at a relatively slow rate (0.3 KN/s). Each test was repeated at least three times and the average value was calculated as the compressive strength of specimens.


## 2.2.6. Diffusion Coefficient Calculation ofSO42−

Fick’s second law of diffusion was widely used to describe kinds of ions diffusion into concrete [18]. In general, the diffusion of SO42− in CGBM obeyed Fick’s second law. In this paper, the CGBM was assumed as a semi-infinite homogeneous medium and SO42− was not bound to material when they diffused, so the coefficient of SO42− was constant [28], and the following one-dimensional equation was used:(2)∂C∂t=Dc∂2C∂x2,where C is the SO42− concentration, cm2/s; t is the period of corrosion, s; x is the corrosion depth, cm; and D is the diffusion coefficient, cm2/s.When the initial condition wast = 0, x > 0, C = 0; boundary condition was x = 0, t > 0, C=Cs. The solutions were evolved into the following equation:(3)Cx,t=Cs1−erfx2Dct,where Cs is the concentration of SO42− on the surface, cm2/s, and erf is the error function,erf=2/π∫0ue−t2dt.Then diffusion coefficient was evolved into the following equations:(4)Di=xi22tΦ−1mi2,(5)mi=1221−Cix,tCs+0.5,where Φ−1mi can be traceable standard normal distribution table.TheSO42− concentration on the surface and coefficient of variation was evolved into the following equations:(6)ED=∑i=1nDin,(7)VarD=∑i=1nDi2−nE2Dn−1,where n is the number of measured points of the specimen and Var is the coefficient of variation.


## 3. Results and Discussion

### 3.1. Influence of Concentration in Diffusion

Specimens were immersed in 3%, 5%, and 10% Na2SO4 solutions; SO42− concentration at different depths in the specimen was obtained according to the above method. The results are shown in Figure 2.Figure 2
The relationship betweenSO42− concentration and immersion time at different diffusion depths. (a) 3% Na2SO4−, (b) 5% Na2SO4, and (c) 10% Na2SO4.
(a)
(b)
(c)It can be seen that the closer to the surface of the specimens, the higher the concentration ofSO42−. The SO42− concentration is decreased with increasing depth of specimens. And the deceleration rate is faster before 6 mm. The SO42− concentration in the specimens decreases slowly and linearly after 6 mm. Because SO42− reacts with CGBM to form gypsum and ettringite, the product will block the surface pores of the materials and change the ion diffusion path, leading to rapid decrease in ion diffusion rate and then slowing down [6, 14]. At 180 days, when the concentration of the external solution Na2SO4 solution is 3%, 5%, and 10%, the SO42− concentration is 0.4634%, 0.5215%, and 0.6712% from the surface of the specimens at 3 mm. SO42− of high concentration in external solution can accelerate the ion diffusion, which indicates that the reaction product is increased, so that the internal pores are expanded in CGBM [29].


### 3.2. Influence of Different Kinds of Salt Solutions in Diffusion

Specimens were immersed in Na2SO4 + MgSO4, Na2SO4 + NaHCO3, and Na2SO4 + MgSO4 + NaHCO3 composite solution; SO42− concentration at different depths in the specimen was obtained according to the above method. The results are shown in Figure 3.Figure 3
The relationship betweenSO42− concentration and immersion time at different diffusion depths. (a) 5% Na2SO4 + 3% MgSO4, (b) 5% Na2SO4 + 5% MgSO4, (c) 5% Na2SO4 + 3% NaHCO3, (d) 5% Na2SO4 + 5% NaHCO3, and (e) 5% Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)
(d)
(e)It can be seen that the change trend ofSO42− concentration is basically consistent with single sulfate corrosion; all of them presented the trend that SO42− concentration is high in outside section and low in inside section. At the same corrosion depth, the concentrations of Mg2+ and HCO3− are higher in the external composite solutions, and concentration of SO42− is higher in the specimens. As is shown in Figures 3(a)–3(e), at 180 days, the SO42− concentrations are 0.495%, 0.5373%, 0.5291%, 0.6011%, and 6471% from the surface of the specimens at 3 mm. It can be found that when the concentration of MgSO4 solution is 3% in external solution, the diffusion of SO42− is inhibited, and when the concentration is 5%, it will accelerate the diffusion of SO42−. When the external solution contains HCO3−, it also accelerates the diffusion of SO42−. And the effect is greater than that of Mg2+. When both ions are present at the same time, the effect is the greatest.Comparison ofSO42− concentrations at depth 15 mm distance from CGBM surface in Na2SO4, Na2SO4 + MgSO4, Na2SO4 + NaHCO3, and Na2SO4 + MgSO4 + NaHCO3 composite solution are shown in Figure 4.Figure 4
SO
4
2
− concentration test results at 15 mm. (a) Na2SO4 + MgSO4, (b) Na2SO4 + NaHCO3, and (c) Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)Effects of different ions on diffusion coefficient ofSO42− in specimens are shown in Figure 5. The changed trend of its diffusion coefficient is divided into two stages: a fast-linear falling section at 30–60 d and a slow falling section after 60 d. The precorrosion period is due to the reaction product blocking the surface pores of the specimens [11, 14], so that the diffusion coefficient is rapidly reduced. As the corrosion time increases, the concentration difference between the inside and the outside gradually becomes balanced, so the diffusion rate slowly decreases.Figure 5
Effect of different ions on diffusion coefficient of SO42− in CGBM. (a) Na2SO4 + MgSO4, (b) Na2SO4 + NaHCO3, and (c) Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 3% MgSO4 composite solutions, as can be seen from Figure 4(a), the SO42− concentration is decreased by 23%, 35%, 17%, and 8% in 30-, 60-, 120-, and 180-day specimens, respectively. As can be seen from Figure 5(a), the diffusion coefficient of SO42− is decreased by 19%, 13%, and 4% and increased by 15% in 30-, 60-, 120-, and 180-day specimens, respectively. At the early age, whether MgSO4 solution is less than 3% can be found, which can inhibit the diffusion of SO42− in CGBM. That is because a small amount of Mg2+ enters into CGBM in the early age and reacts with the hydration product to form brucite (Mg(OH)2); M-H-S and SO42− enter into CGBM to form gypsum and ettringite [13, 25]. These are all water-insoluble precipitate, which can fill the pores of CGBM to improve its compressive strength and prevent SO42− to penetrate [29, 30]. As time goes on, SO42− penetrates the pores gradually and reactants increase, which leads to internal crack expansion in CGBM [31]. Lastly, it will accelerate the diffusion of ions, and the compressive strength decreases.By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 5% MgSO4 composite solutions, from Figure 4(a), the SO42− concentration is increased by 5%, 15%, 21%, and 25% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(a), the diffusion coefficient of SO42− is increased by 0.1%, 11%, 23%, and 49% in 30-, 60-, 120-, and 180-day specimens, respectively. Compared with the previous groups, when MgSO4 solution is more than 5% can accelerate the diffusion of SO42− in CGBM. According to the data, with the corrosion time prolonging, the SO42− concentration is much more in CGBM, and the diffusion coefficient of SO42− is also increasing correspondingly because the reaction product increases, expands the crack of CGBM, and increases the channel of ions diffusion [14, 25, 26].By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 3% NaHCO3 composite solutions, from Figure 4(b), the SO42− concentration is increased by 1%, 9%, 16%, and 12% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(b), the diffusion coefficient of SO42− is increased by 0.1%, 12%, 19%, and 40% in 30-, 60-, 120-, and 180-day specimens, respectively. Because HCO3− enters into CGBM and reacts with it to produce a new corrosion product, which is thaumasite and C-S-H, internal structure is damaged severely more than single sulfate attacks [13, 22]. At the same time, Portland cement contains much C2S which provides enough reactants for thaumasite formation [11].By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 5% NaHCO3 composite solutions, from Figure 4(b), the SO42− concentration is increased by 13%, 30%, 40%, and 47% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(b), the diffusion coefficient of SO42− is increased by 0.2%, 23%, 36%, and 59% in 30-, 60-, 120-, and 180-day specimens, respectively. By comparing the data, single-doped HCO3− has more influence than single-doped Mg2+ for the diffusion of SO42−.By changing the external solutions from 5% Na2SO4 + 5% MgSO4 composite solutions to 5% Na2SO4 + 5% MgSO4 + 5% NaHCO3 composite solutions, from Figure 4(c), the SO42− concentration is increased by 22%, 38%, 35%, and 36% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(c), the diffusion coefficient of SO42− is increased by 7%, 23%, 19%, and 16% in 30-, 60-, 120-, and 180-day specimens, respectively. By comparing the data, composite of the HCO3− and Mg2+ has more influence than single-doped HCO3− or single-doped Mg2+ for the diffusion of SO42−.The compressive strength results are shown in Figure6. The compressive strength increases first and the then decreases. The precorrosion period is due to blocking the pores by the reaction product, which makes the material more solid [7, 8]. As the corrosion time increases, the reaction product increases, which will expand the crack and lead to a decrease in strength [9].Figure 6
Effect of different ions on compressive strength of SO42− in CGBM. (a) Na2SO4 + MgSO4, (b) Na2SO4 + NaHCO3, and (c) Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)


### 3.3. Time Influences for Diffusion Coefficient ofSO42−

According to what the diffusion coefficient ofSO42− shows, it can be concluded that it is not a constant, but a function is considered with time. The basic rule is that the diffusion coefficient decreases with time. Because CGBM is a hydraulic material, its internal structure changes over time. Therefore, equation (3) needs to consider the factor of time [32]. The diffusion coefficient of SO42− changing with time is evolved into the following equation:(8)Dt=D1t−m,where D(t) is the diffusion coefficient of SO42− at time t, cm2/year; D1 is the diffusion coefficient of sulfate ion at 1 year, cm2/year; and m is the empirical coefficient.Establishing a diffusion model to use diffusion coefficientsD1, m and surface concentration Cs, the diffusion model evolves into the following equation:(9)Cx,t=Cs1−erfx2D1t1−m/1−m.The comparison between considering time and not considering time to observe the diffusion coefficients ofSO42− in CGBM is shown in Table 4. The model of diffusion under the composite solutions is shown in Table 5.Table 4
Comparing diffusion coefficients ofSO42− with or without consideration of time in CGBM.
Specimens number
Considering time
Regardless of time
m
D
1 (×10−7cm2/s)
Var.
D
c (×10−7cm2/s)
Var.
S3
0.3689
1.8954 × 10−5
1.1431
2.02502
2.6103
S5
0.5984
8.191 × 10−4
1.6518
2.5845
2.795
S10
0.8354
3.8489 × 10−3
1.4498
3.0405
2.8848
S5 + M3
0.5577
6.0225 × 10−4
0.8071
2.2837
1.5108
S5 + M5
0.6423
2.3004 × 10−3
1.4091
2.8972
2.2574
S5 + H3
0.6324
1.1047 × 10−3
1.7244
2.8475
2.3421
S5 + H5
0.6488
2.607 × 10−3
1.684
3.0508
2.1764
S5 + M5 + H5
0.7724
4.315 × 10−3
1.2453
3.2822
2.1909Table 5
The model of diffusion under the composite solutions.
Specimens number
Diffusion model
S3
C
x
,
t
=
0.4802
1
−
e
r
f
x
/
615
t
0.3155
S5
C
x
,
t
=
0.5539
1
−
erf
x
/
5072
t
0.2008
S10
C
x
,
t
=
0.6833
1
−
erf
x
/
17175
t
0.0823
S5 + M3
C
x
,
t
=
0.5183
1
−
erf
x
/
4144
t
0.2211
S5 + M5
C
x
,
t
=
0.5421
1
−
erf
x
/
9007
t
0.1788
S5 + H3
C
x
,
t
=
0.5346
1
−
erf
x
/
6157
t
0.1838
S5 + H5
C
x
,
t
=
0.5941
1
−
erf
x
/
9677
t
0.1756
S5 + M5 + H5
C
x
,
t
=
0.6221
1
−
erf
x
/
15464
t
0.1138


## 3.1. Influence of Concentration in Diffusion

Specimens were immersed in 3%, 5%, and 10% Na2SO4 solutions; SO42− concentration at different depths in the specimen was obtained according to the above method. The results are shown in Figure 2.Figure 2
The relationship betweenSO42− concentration and immersion time at different diffusion depths. (a) 3% Na2SO4−, (b) 5% Na2SO4, and (c) 10% Na2SO4.
(a)
(b)
(c)It can be seen that the closer to the surface of the specimens, the higher the concentration ofSO42−. The SO42− concentration is decreased with increasing depth of specimens. And the deceleration rate is faster before 6 mm. The SO42− concentration in the specimens decreases slowly and linearly after 6 mm. Because SO42− reacts with CGBM to form gypsum and ettringite, the product will block the surface pores of the materials and change the ion diffusion path, leading to rapid decrease in ion diffusion rate and then slowing down [6, 14]. At 180 days, when the concentration of the external solution Na2SO4 solution is 3%, 5%, and 10%, the SO42− concentration is 0.4634%, 0.5215%, and 0.6712% from the surface of the specimens at 3 mm. SO42− of high concentration in external solution can accelerate the ion diffusion, which indicates that the reaction product is increased, so that the internal pores are expanded in CGBM [29].


## 3.2. Influence of Different Kinds of Salt Solutions in Diffusion

Specimens were immersed in Na2SO4 + MgSO4, Na2SO4 + NaHCO3, and Na2SO4 + MgSO4 + NaHCO3 composite solution; SO42− concentration at different depths in the specimen was obtained according to the above method. The results are shown in Figure 3.Figure 3
The relationship betweenSO42− concentration and immersion time at different diffusion depths. (a) 5% Na2SO4 + 3% MgSO4, (b) 5% Na2SO4 + 5% MgSO4, (c) 5% Na2SO4 + 3% NaHCO3, (d) 5% Na2SO4 + 5% NaHCO3, and (e) 5% Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)
(d)
(e)It can be seen that the change trend ofSO42− concentration is basically consistent with single sulfate corrosion; all of them presented the trend that SO42− concentration is high in outside section and low in inside section. At the same corrosion depth, the concentrations of Mg2+ and HCO3− are higher in the external composite solutions, and concentration of SO42− is higher in the specimens. As is shown in Figures 3(a)–3(e), at 180 days, the SO42− concentrations are 0.495%, 0.5373%, 0.5291%, 0.6011%, and 6471% from the surface of the specimens at 3 mm. It can be found that when the concentration of MgSO4 solution is 3% in external solution, the diffusion of SO42− is inhibited, and when the concentration is 5%, it will accelerate the diffusion of SO42−. When the external solution contains HCO3−, it also accelerates the diffusion of SO42−. And the effect is greater than that of Mg2+. When both ions are present at the same time, the effect is the greatest.Comparison ofSO42− concentrations at depth 15 mm distance from CGBM surface in Na2SO4, Na2SO4 + MgSO4, Na2SO4 + NaHCO3, and Na2SO4 + MgSO4 + NaHCO3 composite solution are shown in Figure 4.Figure 4
SO
4
2
− concentration test results at 15 mm. (a) Na2SO4 + MgSO4, (b) Na2SO4 + NaHCO3, and (c) Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)Effects of different ions on diffusion coefficient ofSO42− in specimens are shown in Figure 5. The changed trend of its diffusion coefficient is divided into two stages: a fast-linear falling section at 30–60 d and a slow falling section after 60 d. The precorrosion period is due to the reaction product blocking the surface pores of the specimens [11, 14], so that the diffusion coefficient is rapidly reduced. As the corrosion time increases, the concentration difference between the inside and the outside gradually becomes balanced, so the diffusion rate slowly decreases.Figure 5
Effect of different ions on diffusion coefficient of SO42− in CGBM. (a) Na2SO4 + MgSO4, (b) Na2SO4 + NaHCO3, and (c) Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 3% MgSO4 composite solutions, as can be seen from Figure 4(a), the SO42− concentration is decreased by 23%, 35%, 17%, and 8% in 30-, 60-, 120-, and 180-day specimens, respectively. As can be seen from Figure 5(a), the diffusion coefficient of SO42− is decreased by 19%, 13%, and 4% and increased by 15% in 30-, 60-, 120-, and 180-day specimens, respectively. At the early age, whether MgSO4 solution is less than 3% can be found, which can inhibit the diffusion of SO42− in CGBM. That is because a small amount of Mg2+ enters into CGBM in the early age and reacts with the hydration product to form brucite (Mg(OH)2); M-H-S and SO42− enter into CGBM to form gypsum and ettringite [13, 25]. These are all water-insoluble precipitate, which can fill the pores of CGBM to improve its compressive strength and prevent SO42− to penetrate [29, 30]. As time goes on, SO42− penetrates the pores gradually and reactants increase, which leads to internal crack expansion in CGBM [31]. Lastly, it will accelerate the diffusion of ions, and the compressive strength decreases.By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 5% MgSO4 composite solutions, from Figure 4(a), the SO42− concentration is increased by 5%, 15%, 21%, and 25% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(a), the diffusion coefficient of SO42− is increased by 0.1%, 11%, 23%, and 49% in 30-, 60-, 120-, and 180-day specimens, respectively. Compared with the previous groups, when MgSO4 solution is more than 5% can accelerate the diffusion of SO42− in CGBM. According to the data, with the corrosion time prolonging, the SO42− concentration is much more in CGBM, and the diffusion coefficient of SO42− is also increasing correspondingly because the reaction product increases, expands the crack of CGBM, and increases the channel of ions diffusion [14, 25, 26].By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 3% NaHCO3 composite solutions, from Figure 4(b), the SO42− concentration is increased by 1%, 9%, 16%, and 12% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(b), the diffusion coefficient of SO42− is increased by 0.1%, 12%, 19%, and 40% in 30-, 60-, 120-, and 180-day specimens, respectively. Because HCO3− enters into CGBM and reacts with it to produce a new corrosion product, which is thaumasite and C-S-H, internal structure is damaged severely more than single sulfate attacks [13, 22]. At the same time, Portland cement contains much C2S which provides enough reactants for thaumasite formation [11].By changing the external solutions from 5% Na2SO4 solutions to 5% Na2SO4 + 5% NaHCO3 composite solutions, from Figure 4(b), the SO42− concentration is increased by 13%, 30%, 40%, and 47% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(b), the diffusion coefficient of SO42− is increased by 0.2%, 23%, 36%, and 59% in 30-, 60-, 120-, and 180-day specimens, respectively. By comparing the data, single-doped HCO3− has more influence than single-doped Mg2+ for the diffusion of SO42−.By changing the external solutions from 5% Na2SO4 + 5% MgSO4 composite solutions to 5% Na2SO4 + 5% MgSO4 + 5% NaHCO3 composite solutions, from Figure 4(c), the SO42− concentration is increased by 22%, 38%, 35%, and 36% in 30-, 60-, 120-, and 180-day specimens, respectively. From Figure 5(c), the diffusion coefficient of SO42− is increased by 7%, 23%, 19%, and 16% in 30-, 60-, 120-, and 180-day specimens, respectively. By comparing the data, composite of the HCO3− and Mg2+ has more influence than single-doped HCO3− or single-doped Mg2+ for the diffusion of SO42−.The compressive strength results are shown in Figure6. The compressive strength increases first and the then decreases. The precorrosion period is due to blocking the pores by the reaction product, which makes the material more solid [7, 8]. As the corrosion time increases, the reaction product increases, which will expand the crack and lead to a decrease in strength [9].Figure 6
Effect of different ions on compressive strength of SO42− in CGBM. (a) Na2SO4 + MgSO4, (b) Na2SO4 + NaHCO3, and (c) Na2SO4 + 5% MgSO4 + 5% NaHCO3.
(a)
(b)
(c)


## 3.3. Time Influences for Diffusion Coefficient ofSO42−

According to what the diffusion coefficient ofSO42− shows, it can be concluded that it is not a constant, but a function is considered with time. The basic rule is that the diffusion coefficient decreases with time. Because CGBM is a hydraulic material, its internal structure changes over time. Therefore, equation (3) needs to consider the factor of time [32]. The diffusion coefficient of SO42− changing with time is evolved into the following equation:(8)Dt=D1t−m,where D(t) is the diffusion coefficient of SO42− at time t, cm2/year; D1 is the diffusion coefficient of sulfate ion at 1 year, cm2/year; and m is the empirical coefficient.Establishing a diffusion model to use diffusion coefficientsD1, m and surface concentration Cs, the diffusion model evolves into the following equation:(9)Cx,t=Cs1−erfx2D1t1−m/1−m.The comparison between considering time and not considering time to observe the diffusion coefficients ofSO42− in CGBM is shown in Table 4. The model of diffusion under the composite solutions is shown in Table 5.Table 4
Comparing diffusion coefficients ofSO42− with or without consideration of time in CGBM.
Specimens number
Considering time
Regardless of time
m
D
1 (×10−7cm2/s)
Var.
D
c (×10−7cm2/s)
Var.
S3
0.3689
1.8954 × 10−5
1.1431
2.02502
2.6103
S5
0.5984
8.191 × 10−4
1.6518
2.5845
2.795
S10
0.8354
3.8489 × 10−3
1.4498
3.0405
2.8848
S5 + M3
0.5577
6.0225 × 10−4
0.8071
2.2837
1.5108
S5 + M5
0.6423
2.3004 × 10−3
1.4091
2.8972
2.2574
S5 + H3
0.6324
1.1047 × 10−3
1.7244
2.8475
2.3421
S5 + H5
0.6488
2.607 × 10−3
1.684
3.0508
2.1764
S5 + M5 + H5
0.7724
4.315 × 10−3
1.2453
3.2822
2.1909Table 5
The model of diffusion under the composite solutions.
Specimens number
Diffusion model
S3
C
x
,
t
=
0.4802
1
−
e
r
f
x
/
615
t
0.3155
S5
C
x
,
t
=
0.5539
1
−
erf
x
/
5072
t
0.2008
S10
C
x
,
t
=
0.6833
1
−
erf
x
/
17175
t
0.0823
S5 + M3
C
x
,
t
=
0.5183
1
−
erf
x
/
4144
t
0.2211
S5 + M5
C
x
,
t
=
0.5421
1
−
erf
x
/
9007
t
0.1788
S5 + H3
C
x
,
t
=
0.5346
1
−
erf
x
/
6157
t
0.1838
S5 + H5
C
x
,
t
=
0.5941
1
−
erf
x
/
9677
t
0.1756
S5 + M5 + H5
C
x
,
t
=
0.6221
1
−
erf
x
/
15464
t
0.1138


## 4. Using XRD and SEM to Analyze Reaction Products in CGBM

Reaction products of CGBM were immersed in Na2SO4, Na2SO4 + MgSO4, Na2SO4 + NaHCO3 and Na2SO4 + MgSO4 + NaHCO3 composite solution separately, then SEM and XRD were used to observe the microstructure of reaction products. Test results are shown in Figure 7.Figure 7
SEM images (180 d) and XRD patterns of CGBM immersed in different solutions. (a) Uneroded, (b) 5% Na2SO4 solution, (c) 5% Na2SO4 + 5% MgSO4 composite solution, (d) 5% Na2SO4 + 5% NaHCO3 composite solution, and (e) 5% Na2SO4 + 5% MgSO4 + 5% NaHCO3 composite solution.
(a)
(b)
(c)
(d)
(e)It can be observed from Figure7(a) that the internal structure is complete, which contains a large amount of calcium hydroxide (Ca(OH)2). Observe Figure 7(b) and find that, when immersed in a solution containing SO42−, the Ca(OH)2 concentration is reduced and the calcium carbonate (CaCO3) is increased, and there are ettringite and gypsum formation. Its chemical reaction is as follows:(10)CaOH2+SO42−+2H2O⟶CaSO4⋅2H2O+2OH−3CaO⋅2SiO2⋅3H2O+3SO42−+8H2O⟶3CaSO4⋅2H2O+6OH−+2SiO2⋅H2OIt can be observed from Figure7(c) that there is much columnar crystal in CGBM after corrosion with SO42− and Mg2+ for 180 d [28], which is arranged orderly. At the same time, a large amount of amorphous matter is wrapped around the columnar crystal [29], its main products are Mg(OH)2, gypsum, ettringite, and M-S-H. The diffraction peaks intensity of M-H-S and Mg(OH)2 is low in the low concentration solutions. It is verified that the formation of Mg(OH)2 can improve sulfate resistance of CGBM in the early composite solutions, so the compressive strength is increased. The number of diffraction peaks of M-H-S and Mg(OH)2 is increased when CGBM is immersed in composite solutions for 180 d, and the high concentration solutions have higher diffraction peaks intensity. In addition, there are also CaCO3 and gypsum with large diffraction peaks intensity. It is indicated that as the concentration of Mg2+ increases and the corrosion time prolongs, SO42− and Mg2+ continuously invade CGBM to form CaSO4. When the protective film of Mg(OH)2 is eroded, SO42− and Mg2+ continuously infiltrate into the interior of CGBM. The C-H-S is decomposed to form M-H-S without bond properties, making the structure of CGBM worse. Its chemical reaction is as follows:(11)Na2SO4+CaOH2+2H2O=2NaOH+CaSO4⋅2H2OMgSO4+CaOH2+2H2O=MgOH2+CaSO4⋅2H2OMgSO4+CaO⋅SiO2⋅H2O⟶MgO⋅SiO2⋅H2O+CaSO4It can be observed from Figure7(d) that a large amount of flocculent material forms in CGBM after corrosion with SO42− and HCO3− for 180 d. In addition, there is much columnar crystal arranged disorderly, which is dispersed in the middle of flocculent material. The flocculent material is gypsum, ettringite, C-S-H, and thaumasite. After immersing for 30 days, the diffraction peaks intensity of Ca(OH)2 is large; it is the hydration product of CGBM. CGBM is immersed in SO42− and HCO3− composite solutions, and the reaction product is CaCO3 and thaumasite. After 180 d, the number of diffraction peaks of ettringite and gypsum increases significantly, and the diffraction peaks intensity of CaCO3 and thaumasite is also increased significantly. Under long-term effects, Ca(OH)2 is increasing because Portland cement contains more C2S and its hydration product is enough, which provides enough reactants to form thaumasite, and CGBM is gradually corroded. Its internal structure is attacked seriously compared to a single sulfate.Most studies on the formation of thaumasite show that it is easy to form at temperatures below 15°C, especially at 4°C to 5°C. However, in recent years, many studies have shown that thaumasite can still be formed at temperatures above 15°C. According to Brown’s research, thaumasite can also be formed at room temperature [33]. Juanhong Liu et al. found that water-rich filling materials in the sodium carbonate solution at a temperature of about 20°C generated a large amount of carbon thaumasite [34]. Cemented gangue backfill material (CGBM) also formed thaumasite at room temperature.Its chemical reaction is as follows:(12)NaHCO3+CaOH2⟶CaCO3+H2O+NaOHC3S2H3+2CaSO4·2H2O+2CaCO3+24H2O⟶2Ca3SiSO4CO3OH6·12H2O+CaOH2C3S2H3+3CaO⋅Al2O3⋅3CaSO4·32H2O+2CaCO3+4H2O⟶2Ca3SiSO4CO3OH6⋅12H2O+CaSO4⋅2H2O+2AlOH3+4CaOH2It can be observed from Figure7(e) that there is much columnar crystal neatly arranged in CGBM when CGBM is etched by SO42−, Mg2+, and HCO3− for 180 d [35–37]. The crystal is sharp and angular, and the surface is straight and smooth. In addition, there are M-S-H, C-S-H, gypsum, ettringite, and thaumasite in CGBM. And the number of diffraction peaks of ettringite and gypsum increases at 180 d. At the same time, the diffraction peaks intensity of M-H-S and thaumasite is increased significantly. When SO42−, Mg2+, and HCO3− exist simultaneously, the hydration product of the cement is OH− in CGBM, which combines with the soluble metal cation to produce more harmful corrosion products [38]. It can reduce the alkalinity of cement and decompose hydrated gel gradually. The corrosion products decompose the structure of CGBM, resulting in loss of its strength.


## 5. Conclusion

Based on the results presented in this paper, the following conclusions are offered:(1)
TheSO42− concentration increases in CGBM when the external sulfate solutions concentration increases. At the early corrosion age, adding less than 3% Mg2+ in external solutions can inhibit the diffusion of SO42− in CGBM. As time goes on, adding Mg2+ and HCO− can accelerate the diffusion of SO42−.(2)
Mg2+ and HCO3− have different effects on SO42− diffusion rate; Mg2+ and HCO3− composite solutions have the greatest impact, HCO3− composite solutions have the second, and Mg2+ composite solutions have minimal.(3)
Gypsum and ettringite are formed by the reaction ofSO42− into the CGBM. In the early 60 days, the reaction product will block the pores on the surface of the material, delay the diffusion reaction speed of the ions, and lead to an increase in strength. After 60 days, due to the increase of reaction products, the pores inside the material expand, which accelerates the diffusion reaction of ions, resulting in a decrease in strength.(4)
Diffusion coefficient ofSO42− is function related with time. As time goes on, the diffusion coefficient of SO42− decreases gradually with an exponential function. Effect of time should be considered when predicting depth of SO42− diffusion in CGBM.



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*Source: 5846397-2020-03-10.xml*