# Optimization and Hydration Mechanism of Composite Cementing Material for Paste Filling in Coal Mines

Authors: Jinxiao Liu; Wenxin Li; Feng Zhang; Xinguo Zhang; Lianjun Chen; Yongle Liu
Journal: Advances in Materials Science and Engineering
 (2019)

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## Abstract

The low early strength of materials for paste filling in mines caused by low early strength of composite cementing material has been a severe issue. In this study, the effects of sulphoaluminate cement and gypsum on strengths of composite cementing material were investigated experimentally by employing the constrained formulation uniform design. With the content of the sulphoaluminate cement below 14% and the content of the gypsum below 16%, the compressive strengths of composite cementing materials increased, especially early strength. However, the initial and final setting time does not meet the engineering requirements in this case. Optimization tests of composite additives demonstrated that H2BO3(0.3%) + Na2SO4(0.1%) and H2BO3(0.3%) + NaNO2(0.1%) were ideal setting retarding and early strengthening composite additives as they can both reduce the initial and final setting time and enhance compressive strengths of composite cementing material. Investigations by XRD and SEM revealed that the hydration products of composite cementing material were dominated by AFt (ettringite) at the early stage and by C-S-H (hydrated calcium silicate) gel + CH (calcium hydroxide) gel at the middle and late stages. The hydration products of ratio-optimized composite cementing material do not restrain each other due to the generation sequence. Instead, they grew interactively and were coupled, thus facilitating the growth of the hardened body. This study can provide references for optimization of composite cementing material for paste filling in coal mines.


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## Body

## 1. Introduction

In cementing filling, cementing agents (e.g., cement, red mud, and gypsum) were added into the filling materials, which were pumped to the underground to generate fillers with moderate strengths. The cement filler does not collapse even after partial or full relievation of its limiting conditions, thus supporting surrounding rocks and pillars. Hence, cementing filling technologies in mining can effectively extract limited resources and relieve its environment damages [1–3].Cementing filling technologies have been widely employed since 1960s. For instance, pillars were extracted in the Mount Isa Mine in Australia using tailing cementing filling techniques, and the cement content was 12%. In 1980s and 1990s, new processes such as high-concentration filling, paste filling, waste stone cementing filling, and full tailing cementing filling have been developed and applied in various mines, including the Kidd Creek Mine, the Golden Giant Mine and the Louvicourt Mine in Canada, the Köln Mine in Germany, and the Cannington Mine in Australia [4–6].In China, the development of cementing filling technologies can be divided into three stages. At the first stage (1960s), graded tailings were used as aggregates in cementing filling technologies. In 1968, cementing filling (in the Fankou lead-zinc mine) by graded tailings and cement was reported for the first time. At the second stage (1980s), full tailing and high-concentration cementing filling [7–12], high water rapid setting and full tailing cementing filling [13–15], and block stone cementing filling [16–21] dominated. At the third stage (1990s), paste and paste-like pumping cementing filling technology was developed [22–28].Previous studies demonstrated that the properties of cementing material are key factors affecting its parameters such as filler strength and setting time [29–67]. As the requirement on early strength of fillers by paste filling in metal mining is not high, the cost-effective and readily accessible Portland cement can be used as the paste of cementing material. However, unlike paste filling in metal mining, the mining and filling processes of longwall workface in coal mines are strictly alternative (each for 8–10 h). Hence, paste filling in coal mining requires relatively high early strength. Owing to its long setting time and slow growth of early strength, the early performance of Portland cement must be enhanced before being used as dominant cementing fillers in paste filling.Based on the practical requirements on performances (e.g., compressive strength and setting time) of filling paste cementing material for coal mining, Portland cement was used as the dominant material, with sulphoaluminate cement and gypsum as additives to enhance its early strength. The initial and final setting time of composite cementing material was controlled in 2.5∼4 h and 3∼4 h, respectively. First, ratios of Portland cement, sulphoaluminate cement, and gypsum in composite cementing material were optimized experimentally; then, additives were used to modify the composites. Finally, the hydration process was identified by investigating the hydration mechanism of composite cementing material to obtain that it meets the requirements of paste filling in coal mines.


## 2. Materials, Equipment, and Methods

### 2.1. Composition and Optimization Scheme of Composite Cementing Material

In this study, Portland cement was used as the main ingredient for paste cementing material, with sulphoaluminate and gypsum as supplemental ingredients to enhance the early strength of composite cementing material. The formulation of composite cementing material was determined by constrained formulation uniform design, and then the mechanical performances of the proposed composites were tested. Specifically, compressive strengths and the setting time of composite cementing materials with different ratios (D01∼D15) at different ages were measured.


#### 2.1.1. Raw Materials

The No. 42.5 Portland cement (chemical and mineral compositions are shown in Table1) was purchased from Shandong Sunnsy Cement Group, the No. 42.5 rapid hardening sulphoaluminate cement (mineral composition is shown in Table 2) was from Shandong Jinyu Cement Co., Ltd., and the hardened gypsum (chemical composition is shown in Table 3) from Shandong Taihe Dongxin Group was calcinated.Table 1
Chemical and mineral compositions of portland cement.
Material
Chemical constituents (%)
Mineral compositions (%)
SiO2
Al2O3
Fe2O3
CaO
SO3
C3S
C2S
C3A
C4AF
Portland cement
21.38
4.23
3.58
66.49
0.1
59.95
12.02
5.94
13.53Table 2
Chemical and mineral compositions of sulphoaluminate cement.
Material
Chemical constituents (%)
Mineral compositions (%)
SiO2
Al2O3
Fe2O3
CaO
SO3
C4A3S¯
β-C2S
C2F
f-SO3
Sulphoaluminate cement
11.85
29.64
2.68
43.22
6.94
57.37
25.55
6.56
1.92Table 3
Chemical composition of hardened gypsum.
Material
Chemical constituents (%)
CaO
Al2O3
Fe2O3
SO3
SiO2
MgO
Na2O
Ignition loss
Gypsum
38.15
3.18
0.32
44.86
1.73
2.57
0.08
8.38


#### 2.1.2. Instruments

Table4 summarizes the testing instruments.Table 4
Testing instruments for composite cementing material.
Instrument
Supplier
NJ-160A cement paste mixer
Cangzhou Luda Building Instrument Factory
Cement Vicat apparatus
Cangzhou Jilu Testing Instrument Co., Ltd.
SC-145 cement consistometer
Cangzhou Jilu Testing Instrument Co., Ltd.
GZ-85 cement mortar shaking table
Wuxi Jianyi Laboratory Equipment Co., Ltd.
JJ-5 cement mortar mixer
Wuxi Jianyi Laboratory Equipment Co., Ltd.
70.7 × 70.7 × 70.7 mm3 triplex testing mold
Cangzhou Jilu Testing Instrument Co., Ltd.
YH-40B thermostatic and constant humidity box
Hebei Kexi Instruments and Equipment Co., Ltd.
YAW-400 pressure tester
Jinan Haiweier Instrument Co., Ltd.
CP2202S electronic balance
Shenzhen Mingke Chemicals Co., Ltd.
Beakers and measuring cylinders
Shenzhen Ruixinda Scientific and Educational Instruments Co., Ltd.


#### 2.1.3. Testing of Performances

In this study, the composite cementing materials consist of Portland cement, sulphoaluminate cement, and gypsum in different ratios. With guaranteed early strength of composite cementing material, the content of sulphoaluminate cement shall be minimized to reduce the cost. With reference of the results from other researchers, the optimized contents of sulphoaluminate cement, gypsum, and Portland cement were determined to be 0∼20%, 0∼20%, and 60%∼100%, respectively. The constraints of corresponding formulation tests are as follows:(1)X1+X2+X3=1,0.6≤X1≤1,0≤X2≤0.2,0≤X3≤0.2.Then, the corresponding testing scheme (15 sample groups, see Table5) was developed using the constrained formulation design in the Uniform Design Version 3.0. The uniform design table used was U31∗3110 with D = 0.0908 (see Appendix).Table 5
Formulations of composite cementing material.
Test number
Proportioning (%)
Portland cement
Sulphoaluminate cement
Gypsum
D01
94.9
3.7
1.4
D02
91.2
3.8
5.0
D03
88.6
1.7
9.7
D04
86.6
11.9
1.5
D05
84.8
9.0
6.2
D06
83.2
5.2
11.6
D07
81.7
0.4
17.9
D08
80.3
14.9
4.8
D09
79.1
9.8
11.1
D10
77.9
4.0
18.1
D11
75.6
15.3
9.1
D12
74.6
8.6
16.8
D13
71.7
14.1
14.2
D14
68.3
11.8
19.9
D15
65.9
18.1
16.0The raw materials were mixed and stirred to generate 70.7 × 70.7 × 70.7 mm3 stock samples (Figure 1), which were cured at a relative humidity of 90% and a temperature of 20 ± 2°C to designated ages to obtain testing samples. According to the GB177-85 cement mortar strength testing method, the compressive strengths of samples at different ages were measured using the MTS-815 mechanical tester. In addition, the GB1346-89 cement standard consistency water consumption, setting time, and stability testing method were adopted to measure the setting time. The initial setting time is defined as the period from the start to the moment when slurry loses flow ability, during which the plasticity of cementing material paste remained constant. After initial setting, the cementing material paste was further hardened until its complete loss of plasticity, which corresponds to the final setting.Figure 1
Some of the samples.


### 2.2. Modification Scheme of Optimized Composite Cementing Material

According to the results of mechanical performance tests, optimized formulations of composite cementing material with adequate early compressive strength were identified. The setting time was selected as the performance indicator of composite cementing material. The hydration of sulphoaluminate cement was initiated after 10 min and considerable quantity of gel was generated, resulting in too short setting time of composite cementing material. Therefore, the setting time shall be adjusted using composite additives such as setting retarding agents (e.g., H2BO3) and setting retarding and early strengthening agents (e.g., Li2CO3, Na2SO4, and NaNO2).In accordance with Section 2.1, the paste samples were 70.7 × 70.7 × 70.7 mm3 cubes and were cured under standard conditions. The setting time and compressive strengths of the samples were measured according to GB1346-89 and GB177-85, respectively.


### 2.3. Hydration Mechanism of Modified Composite Cementing Material

After being mixed with water, the materials are exposed to a series of physical and chemical reactions and then a hardened body with certain strengths was obtained as cementing pastes which gradually lose fluidity. This process is defined as hydration hardening of cementing material [55–64]. The mechanical performances of the hardened body are closely related to hydration products and microstructures of the samples, which vary during the hydration process. Therefore, studies of hydration hardening of cementing material can be conducted in terms of hydration products and microstructures. Herein, the D09 composite cementing material was selected and made into paste samples according to standard consistency. The samples were cured in a standard curing box for 8 h, 16 h, 1 d, 3 d, and 7 d, respectively. Subsequently, the hydration products and microstructures of prepared paste samples at designated ages were analyzed.


#### 2.3.1. Characterizations by X-Ray Diffraction (XRD)

The X′ Pert Pro MPD diffractometer (by Malvern Panalytical, see Figure2) was employed for quantitative and qualitative analysis of hydration products of the modified composite cementing material. The conditions are as follows: Cu target, Kα diffraction, and scanning angle of 5°∼75°. For investigations by XRD, the surface carbonized layers of samples at designated ages were removed and the samples were rinsed twice by acetone and alcohol in CO2 free environment to achieve complete dehydration. Then, the samples were grinded by using an agate mortar, dried, and sieved by using a 4900 mesh/cm2 sieve. After each cycle, the mortar was rinsed with HCl.Figure 2
X′ Pert Pro MPD diffractometer.


#### 2.3.2. Characterization by SEM

The microstructures of hydration products were investigated using Apreo high-resolution SEM (see Figure3). Sample blocks at different ages were dried at 70°C for 2 h, sprayed with gold in vacuum, and then characterized to investigate hydration products and micromorphologies.Figure 3
Apreo high-resolution SEM.


## 2.1. Composition and Optimization Scheme of Composite Cementing Material

In this study, Portland cement was used as the main ingredient for paste cementing material, with sulphoaluminate and gypsum as supplemental ingredients to enhance the early strength of composite cementing material. The formulation of composite cementing material was determined by constrained formulation uniform design, and then the mechanical performances of the proposed composites were tested. Specifically, compressive strengths and the setting time of composite cementing materials with different ratios (D01∼D15) at different ages were measured.


### 2.1.1. Raw Materials

The No. 42.5 Portland cement (chemical and mineral compositions are shown in Table1) was purchased from Shandong Sunnsy Cement Group, the No. 42.5 rapid hardening sulphoaluminate cement (mineral composition is shown in Table 2) was from Shandong Jinyu Cement Co., Ltd., and the hardened gypsum (chemical composition is shown in Table 3) from Shandong Taihe Dongxin Group was calcinated.Table 1
Chemical and mineral compositions of portland cement.
Material
Chemical constituents (%)
Mineral compositions (%)
SiO2
Al2O3
Fe2O3
CaO
SO3
C3S
C2S
C3A
C4AF
Portland cement
21.38
4.23
3.58
66.49
0.1
59.95
12.02
5.94
13.53Table 2
Chemical and mineral compositions of sulphoaluminate cement.
Material
Chemical constituents (%)
Mineral compositions (%)
SiO2
Al2O3
Fe2O3
CaO
SO3
C4A3S¯
β-C2S
C2F
f-SO3
Sulphoaluminate cement
11.85
29.64
2.68
43.22
6.94
57.37
25.55
6.56
1.92Table 3
Chemical composition of hardened gypsum.
Material
Chemical constituents (%)
CaO
Al2O3
Fe2O3
SO3
SiO2
MgO
Na2O
Ignition loss
Gypsum
38.15
3.18
0.32
44.86
1.73
2.57
0.08
8.38


### 2.1.2. Instruments

Table4 summarizes the testing instruments.Table 4
Testing instruments for composite cementing material.
Instrument
Supplier
NJ-160A cement paste mixer
Cangzhou Luda Building Instrument Factory
Cement Vicat apparatus
Cangzhou Jilu Testing Instrument Co., Ltd.
SC-145 cement consistometer
Cangzhou Jilu Testing Instrument Co., Ltd.
GZ-85 cement mortar shaking table
Wuxi Jianyi Laboratory Equipment Co., Ltd.
JJ-5 cement mortar mixer
Wuxi Jianyi Laboratory Equipment Co., Ltd.
70.7 × 70.7 × 70.7 mm3 triplex testing mold
Cangzhou Jilu Testing Instrument Co., Ltd.
YH-40B thermostatic and constant humidity box
Hebei Kexi Instruments and Equipment Co., Ltd.
YAW-400 pressure tester
Jinan Haiweier Instrument Co., Ltd.
CP2202S electronic balance
Shenzhen Mingke Chemicals Co., Ltd.
Beakers and measuring cylinders
Shenzhen Ruixinda Scientific and Educational Instruments Co., Ltd.


### 2.1.3. Testing of Performances

In this study, the composite cementing materials consist of Portland cement, sulphoaluminate cement, and gypsum in different ratios. With guaranteed early strength of composite cementing material, the content of sulphoaluminate cement shall be minimized to reduce the cost. With reference of the results from other researchers, the optimized contents of sulphoaluminate cement, gypsum, and Portland cement were determined to be 0∼20%, 0∼20%, and 60%∼100%, respectively. The constraints of corresponding formulation tests are as follows:(1)X1+X2+X3=1,0.6≤X1≤1,0≤X2≤0.2,0≤X3≤0.2.Then, the corresponding testing scheme (15 sample groups, see Table5) was developed using the constrained formulation design in the Uniform Design Version 3.0. The uniform design table used was U31∗3110 with D = 0.0908 (see Appendix).Table 5
Formulations of composite cementing material.
Test number
Proportioning (%)
Portland cement
Sulphoaluminate cement
Gypsum
D01
94.9
3.7
1.4
D02
91.2
3.8
5.0
D03
88.6
1.7
9.7
D04
86.6
11.9
1.5
D05
84.8
9.0
6.2
D06
83.2
5.2
11.6
D07
81.7
0.4
17.9
D08
80.3
14.9
4.8
D09
79.1
9.8
11.1
D10
77.9
4.0
18.1
D11
75.6
15.3
9.1
D12
74.6
8.6
16.8
D13
71.7
14.1
14.2
D14
68.3
11.8
19.9
D15
65.9
18.1
16.0The raw materials were mixed and stirred to generate 70.7 × 70.7 × 70.7 mm3 stock samples (Figure 1), which were cured at a relative humidity of 90% and a temperature of 20 ± 2°C to designated ages to obtain testing samples. According to the GB177-85 cement mortar strength testing method, the compressive strengths of samples at different ages were measured using the MTS-815 mechanical tester. In addition, the GB1346-89 cement standard consistency water consumption, setting time, and stability testing method were adopted to measure the setting time. The initial setting time is defined as the period from the start to the moment when slurry loses flow ability, during which the plasticity of cementing material paste remained constant. After initial setting, the cementing material paste was further hardened until its complete loss of plasticity, which corresponds to the final setting.Figure 1
Some of the samples.


## 2.1.1. Raw Materials

The No. 42.5 Portland cement (chemical and mineral compositions are shown in Table1) was purchased from Shandong Sunnsy Cement Group, the No. 42.5 rapid hardening sulphoaluminate cement (mineral composition is shown in Table 2) was from Shandong Jinyu Cement Co., Ltd., and the hardened gypsum (chemical composition is shown in Table 3) from Shandong Taihe Dongxin Group was calcinated.Table 1
Chemical and mineral compositions of portland cement.
Material
Chemical constituents (%)
Mineral compositions (%)
SiO2
Al2O3
Fe2O3
CaO
SO3
C3S
C2S
C3A
C4AF
Portland cement
21.38
4.23
3.58
66.49
0.1
59.95
12.02
5.94
13.53Table 2
Chemical and mineral compositions of sulphoaluminate cement.
Material
Chemical constituents (%)
Mineral compositions (%)
SiO2
Al2O3
Fe2O3
CaO
SO3
C4A3S¯
β-C2S
C2F
f-SO3
Sulphoaluminate cement
11.85
29.64
2.68
43.22
6.94
57.37
25.55
6.56
1.92Table 3
Chemical composition of hardened gypsum.
Material
Chemical constituents (%)
CaO
Al2O3
Fe2O3
SO3
SiO2
MgO
Na2O
Ignition loss
Gypsum
38.15
3.18
0.32
44.86
1.73
2.57
0.08
8.38


## 2.1.2. Instruments

Table4 summarizes the testing instruments.Table 4
Testing instruments for composite cementing material.
Instrument
Supplier
NJ-160A cement paste mixer
Cangzhou Luda Building Instrument Factory
Cement Vicat apparatus
Cangzhou Jilu Testing Instrument Co., Ltd.
SC-145 cement consistometer
Cangzhou Jilu Testing Instrument Co., Ltd.
GZ-85 cement mortar shaking table
Wuxi Jianyi Laboratory Equipment Co., Ltd.
JJ-5 cement mortar mixer
Wuxi Jianyi Laboratory Equipment Co., Ltd.
70.7 × 70.7 × 70.7 mm3 triplex testing mold
Cangzhou Jilu Testing Instrument Co., Ltd.
YH-40B thermostatic and constant humidity box
Hebei Kexi Instruments and Equipment Co., Ltd.
YAW-400 pressure tester
Jinan Haiweier Instrument Co., Ltd.
CP2202S electronic balance
Shenzhen Mingke Chemicals Co., Ltd.
Beakers and measuring cylinders
Shenzhen Ruixinda Scientific and Educational Instruments Co., Ltd.


## 2.1.3. Testing of Performances

In this study, the composite cementing materials consist of Portland cement, sulphoaluminate cement, and gypsum in different ratios. With guaranteed early strength of composite cementing material, the content of sulphoaluminate cement shall be minimized to reduce the cost. With reference of the results from other researchers, the optimized contents of sulphoaluminate cement, gypsum, and Portland cement were determined to be 0∼20%, 0∼20%, and 60%∼100%, respectively. The constraints of corresponding formulation tests are as follows:(1)X1+X2+X3=1,0.6≤X1≤1,0≤X2≤0.2,0≤X3≤0.2.Then, the corresponding testing scheme (15 sample groups, see Table5) was developed using the constrained formulation design in the Uniform Design Version 3.0. The uniform design table used was U31∗3110 with D = 0.0908 (see Appendix).Table 5
Formulations of composite cementing material.
Test number
Proportioning (%)
Portland cement
Sulphoaluminate cement
Gypsum
D01
94.9
3.7
1.4
D02
91.2
3.8
5.0
D03
88.6
1.7
9.7
D04
86.6
11.9
1.5
D05
84.8
9.0
6.2
D06
83.2
5.2
11.6
D07
81.7
0.4
17.9
D08
80.3
14.9
4.8
D09
79.1
9.8
11.1
D10
77.9
4.0
18.1
D11
75.6
15.3
9.1
D12
74.6
8.6
16.8
D13
71.7
14.1
14.2
D14
68.3
11.8
19.9
D15
65.9
18.1
16.0The raw materials were mixed and stirred to generate 70.7 × 70.7 × 70.7 mm3 stock samples (Figure 1), which were cured at a relative humidity of 90% and a temperature of 20 ± 2°C to designated ages to obtain testing samples. According to the GB177-85 cement mortar strength testing method, the compressive strengths of samples at different ages were measured using the MTS-815 mechanical tester. In addition, the GB1346-89 cement standard consistency water consumption, setting time, and stability testing method were adopted to measure the setting time. The initial setting time is defined as the period from the start to the moment when slurry loses flow ability, during which the plasticity of cementing material paste remained constant. After initial setting, the cementing material paste was further hardened until its complete loss of plasticity, which corresponds to the final setting.Figure 1
Some of the samples.


## 2.2. Modification Scheme of Optimized Composite Cementing Material

According to the results of mechanical performance tests, optimized formulations of composite cementing material with adequate early compressive strength were identified. The setting time was selected as the performance indicator of composite cementing material. The hydration of sulphoaluminate cement was initiated after 10 min and considerable quantity of gel was generated, resulting in too short setting time of composite cementing material. Therefore, the setting time shall be adjusted using composite additives such as setting retarding agents (e.g., H2BO3) and setting retarding and early strengthening agents (e.g., Li2CO3, Na2SO4, and NaNO2).In accordance with Section 2.1, the paste samples were 70.7 × 70.7 × 70.7 mm3 cubes and were cured under standard conditions. The setting time and compressive strengths of the samples were measured according to GB1346-89 and GB177-85, respectively.


## 2.3. Hydration Mechanism of Modified Composite Cementing Material

After being mixed with water, the materials are exposed to a series of physical and chemical reactions and then a hardened body with certain strengths was obtained as cementing pastes which gradually lose fluidity. This process is defined as hydration hardening of cementing material [55–64]. The mechanical performances of the hardened body are closely related to hydration products and microstructures of the samples, which vary during the hydration process. Therefore, studies of hydration hardening of cementing material can be conducted in terms of hydration products and microstructures. Herein, the D09 composite cementing material was selected and made into paste samples according to standard consistency. The samples were cured in a standard curing box for 8 h, 16 h, 1 d, 3 d, and 7 d, respectively. Subsequently, the hydration products and microstructures of prepared paste samples at designated ages were analyzed.


### 2.3.1. Characterizations by X-Ray Diffraction (XRD)

The X′ Pert Pro MPD diffractometer (by Malvern Panalytical, see Figure2) was employed for quantitative and qualitative analysis of hydration products of the modified composite cementing material. The conditions are as follows: Cu target, Kα diffraction, and scanning angle of 5°∼75°. For investigations by XRD, the surface carbonized layers of samples at designated ages were removed and the samples were rinsed twice by acetone and alcohol in CO2 free environment to achieve complete dehydration. Then, the samples were grinded by using an agate mortar, dried, and sieved by using a 4900 mesh/cm2 sieve. After each cycle, the mortar was rinsed with HCl.Figure 2
X′ Pert Pro MPD diffractometer.


### 2.3.2. Characterization by SEM

The microstructures of hydration products were investigated using Apreo high-resolution SEM (see Figure3). Sample blocks at different ages were dried at 70°C for 2 h, sprayed with gold in vacuum, and then characterized to investigate hydration products and micromorphologies.Figure 3
Apreo high-resolution SEM.


## 2.3.1. Characterizations by X-Ray Diffraction (XRD)

The X′ Pert Pro MPD diffractometer (by Malvern Panalytical, see Figure2) was employed for quantitative and qualitative analysis of hydration products of the modified composite cementing material. The conditions are as follows: Cu target, Kα diffraction, and scanning angle of 5°∼75°. For investigations by XRD, the surface carbonized layers of samples at designated ages were removed and the samples were rinsed twice by acetone and alcohol in CO2 free environment to achieve complete dehydration. Then, the samples were grinded by using an agate mortar, dried, and sieved by using a 4900 mesh/cm2 sieve. After each cycle, the mortar was rinsed with HCl.Figure 2
X′ Pert Pro MPD diffractometer.


## 2.3.2. Characterization by SEM

The microstructures of hydration products were investigated using Apreo high-resolution SEM (see Figure3). Sample blocks at different ages were dried at 70°C for 2 h, sprayed with gold in vacuum, and then characterized to investigate hydration products and micromorphologies.Figure 3
Apreo high-resolution SEM.


## 3. Results and Discussion

### 3.1. Compressive Strength and Setting Time of Composite Cementing Materials

#### 3.1.1. Compressive Strength

Table6 summarizes the setting time and compressive strengths of Portland cement and composite cementing samples measured experimentally at different ages.Table 6
Setting time and compressive strengths of samples at different ages.
Test number
Setting time (min)
Compressive strength (MPa)
Initial
Final
8 h
16 h
1 d
3 d
7 d
28 d
Portland cement
145
265
2.4
12.8
21.5
29.7
35.8
42.5
D01
22
29
2.9
16.4
25.4
32.7
37.4
44.8
D02
18
25
2.8
18.0
24.6
34.5
39.6
46.3
D03
87
135
2.6
14.8
23.8
30.2
37.6
43.6
D04
8
13
3.2
15.4
28.3
36.4
40.8
47.3
D05
12
20
3.8
17.8
26.4
40.1
43.7
48.9
D06
70
124
3.6
17.4
28.4
38.9
43.8
50.1
D07
187
304
0.6
3.2
5.4
23.2
20.7
18.2
D08
4
8
3.6
13.4
19.3
24.7
18.3
17.2
D09
42
64
4.2
18.2
29.3
39.7
44.5
52.7
D10
85
102
1.4
7.0
12.4
24.1
23.1
21.3
D11
10
13
5.4
10.3
13.2
24.3
20.2
18.9
D12
57
68
2.1
7.9
10.5
14.2
12.1
11.7
D13
12
15
2.7
9.2
14.1
21.3
20.7
19.2
D14
38
52
3.7
6.8
8.7
19.3
18.7
17.1
D15
17
22
3.1
7.2
7.9
18.4
17.2
16.9As the hydration rate and duration of composites are directly related to chemical compositions of the raw materials, the couplings of the hardened body produced by hydrations inevitably affect the compressive strength of the composite samples. According to the results, the compressive strengths of samples are dependent on the contents of sulphoaluminate cement and gypsum. Figure4 shows the trends of compressive strengths as a function of composition of composite samples.Figure 4
Trends of compressive strengths as a function of sample composition when the gypsum content is (a) 0–5%, (b) 5–10%, (c) 10–15%, and (d) 15–20%. CSA-sulphoaluminate cement; P-gypsum.
(a)
(b)
(c)
(d)As shown in Table6 and Figure 4, both the early and late compressive strengths of composite cementing material decreased significantly once the content of sulphoaluminate cement exceeded 14% or the content of gypsum exceeded 16%. Indeed, the compressive strengths of composite cementing material may shrink after 3 d in this case. With the content of sulphoaluminate cement below 14% and content of gypsum below 16%, the compressive strengths of composites increased, especially the early strength. The descending sequence of compressive strength is D09 > D06 > D05 > D04 > D02 > D01 > D03. Specifically, compared with Portland cement, compressive strengths of sample D09 at 8 h, 16 h, 1 d, 3 d, 7 d, and 28 d increased by 75%, 42%, 36%, 34%, 24%, and 23%, respectively; meanwhile, compressive strengths of sample D06 at 8 h, 16 h, 1 d, 3 d, 7 d, and 28 d increased by 50%, 36%, 32%, 31%, 22%, and 17%, respectively.In summary, the early strength of composite cementing samples can be significantly enhanced by ratio optimization of Portland cement, sulphoaluminate cement, and gypsum, as well as the late strength. In terms of structures, hardened bodies by early and late hydration were coupled with each other so that composites exhibited performances superior to the Portland cement single clinker. With compressive strength as the indicator, sample D09 is regarded as the optimized sample in this study.


#### 3.1.2. Setting Time

Figures5 and 6, respectively, show the initial and final setting time of samples measured experimentally. As observed, the trends of initial and final setting time were consistent: increased as the content of gypsum increased and decreased as the content of sulphoaluminate cement increased. This can be attributed to sulphoaluminate, which accelerates early hydration of the composite samples. As a result, the hydration rate of samples at the early stage increased and the large quantity of hydration products led to reduced setting time. On the other hand, the too high content of gypsum hinders hydration of the Portland cement, resulting in extended setting time of samples.Figure 5
Relationship between initial setting time and components of composite cementing material.Figure 6
Relationship between final setting time and components of composite cementing material.In summary, despite its significantly enhanced compressive strength, sample D09 exhibited severely reduced initial and final setting time. Indeed, its initial and final setting time was 42 min and 64 min, respectively. The paste filling materials used for mining shall not solidify in channels for 3-4 h. Therefore, the setting time of composite sample D09 optimized by uniform design still does not meet the requirement; thus, modifications by additives are required.


### 3.2. Modification Results of Optimized Composite Cementing Material

Table7 summarizes the experimentally obtained effects of the setting retarding agent (H2BO3) and early strengthening agents (e.g., Li2CO3, Na2SO4, and NaNO2) on performances of composite cementing material D09.Table 7
Effects of additives on performances of composite cementing material.
Test number
Contents of additives (%)
Setting time (min)
Compressive strength (MPa)
Initial
Final
8 h
16 h
1 d
3 d
7 d
28 d
J1
D09
42
64
4.2
18.2
29.3
39.7
44.5
52.7
J2
Li2CO3 (0.15)
21
32
6.1
23.5
26.7
30.2
34.4
40.5
J3
Li2CO3 (0.30)
18
27
8.4
25.4
24.1
27.6
32.7
38.4
J4
H2BO3 (0.15)
125
184
2.0
12.7
20.7
34.2
39.5
51.2
J5
H2BO3 (0.3)
324
348
1.2
10.4
18.6
32.1
38.8
50.7
J6
H2BO3 (0.3) + Li2CO3 (0.1)
152
182
5.8
24.6
28.7
32.2
36.4
39.3
J7
H2BO3 (0.3) + Na2SO4 (0.1)
182
185
5.8
22.8
34.2
46.5
49.7
58.4
J8
H2BO3 (0.3) + NaNO2 (0.1)
214
232
5.2
19.5
32.9
43.2
47.3
54.1As observed, Li2CO3 and H2BO3 had significant effects on the setting time of composite cementing material. As the content of Li2CO3 increased, the initial and final setting time decreased and the early strength was enhanced, while the late strength decreased. As the content of H2BO3 increased, the initial and final setting time increased, while the early strength decreased. The results indicated that at the expense of reduced late strengths, additive J6 had desired effects on the setting time and early strength, while additives J7 and J8 can enhance both setting time and compressive strengths. Therefore, H2BO3 (0.3%) + Na2SO4 (0.1%) and H2BO3 (0.3%) + NaNO2 (0.1%) are ideal setting retarding and early strengthening additives for composite cementing material.In summary, appropriate additives can enhance the performances (e.g., setting time and compressive strengths) of composite cementing material. In this study, samples J7 and J8 are ideal additives for cementing material in coal mining filling.


### 3.3. Hydration Mechanism of Modified Composite Cementing Material

#### 3.3.1. Hydration Process

The composite cementing material consists of 2CaO·SiO2, 3CaO·SiO2, 3CaO·Al2O3, 4CaO·Al2O3·Fe2O3 (Portland cement), 3CaO·3Al2O3·CaSO4, 2CaO·SiO2 (sulphoaluminate cement), and CaSO4·2H2O (gypsum). Owing to the presence of gypsum, the reaction rate of 3CaO·Al2O3 and 3CaO·SiO2 was higher than that of 2CaO·SiO2 and 4CaO·Al2O3·Fe2O3. Meanwhile, 3CaO·3Al2O3·CaSO4 in sulphoaluminate is an early hydration product. Therefore, 3CaO·3Al2O3·CaSO4, 3CaO·Al2O3, and 3CaO·SiO2 react with each other upon addition of water:3CaO·3Al2O3·CaSO4 + 38H2O = 3CaO·Al2O3·CaSO4·32H2O + 2(Al2O3·H2O)3CaO·Al2O3 + 3CaSO4·2H2O + 26H2O = 3CaO·Al2O3·3CaSO4·32H2O2(3CaO·SiO2) + 11H2O = 3CaO·2SiO2·8H2O + 3Ca(OH)2As the hydration proceeds, the concentration of Ca(OH)2 increases and hydration products react with gypsum:Al2O3·H2O + 3Ca(OH)2 + 3CaSO4·2H2O + 22H2O = 3CaO·Al2O3·3CaSO4·32H2OIn the middle and late hydration stages, 2CaO·SiO2 and 4CaO·Al2O3·Fe2O3 are activated:2(2CaO·SiO2) + 9H2O = 3CaO·2SiO2·8H2O + Ca(OH)2C4AF + 2CH + 6CaSO4·2H2O + 50H = 2C3(A, F)·3CaSO4·32H2OIn summary, main hydration products of the samples include ettringite (AFt), calcium hydroxide (CH) gel, and hydrated calcium silicate (C-S-H) gel. The generation of AFt at an early stage is the main cause of high early strength of samples.


#### 3.3.2. Hydration Products

Figure7 shows the XRD spectra of samples at different hydration ages. Several conclusions can be drawn:(1)
The main hydration products include AFt, CH gel, and C-S-H gel, while their relative contents vary with the hydration age, according to their diffraction peak intensities(2)
After 8 h hydration, a considerable quantity of AFt was generated, resulting in high early strength of samples(3)
CaCO3 was observed in hydration products. This can be attributed to the exposure of AFt to CO2(4)
The diffraction peaks of clinker minerals (e.g., SiO2 and 3CaO·3Al2O3·CaSO4) in samples decreased, indicating that the contents of clinker minerals decreased as the hydration age increased(5)
After 8 h of hydration, no diffraction peak corresponding to CaSO4 was observed, demonstrating complete hydration of CaSO4, whose product is AFtFigure 7
XRD spectra of samples at different hydration ages. ★-AFt; ●-C-S-H; ◆-CH; ■-CaCO3; □-SiO2; △-3CaO·3Al2O3·CaSO4. (a) 8 h. (b) 16 h. (c) 1 d. (d) 3 d. (e) 7 d.
(a)
(b)
(c)
(d)
(e)


#### 3.3.3. Microstructures

Figure8 shows the microstructures of samples at different hydration ages. Several conclusions can be drawn:(1)
After 8 h of hydration, fine, scattered, needle-shaped AFt was observed. The AFt grew interactively and radially, with high density.(2)
After 16 h, the density of AFt increased, and its growth followed the intersecting pattern.(3)
After 1 d of hydration, sizes of AFt increased drastically and were distributed in clusters, while C-S-H gel flakes appeared.(4)
After 3 d, the density of C-S-H gel increased and the flakes were turned into prisms, demonstrating that the contents of C-S-H gel and CH gel increased and the hydration of Portland cement was accelerated.(5)
After 7 d, the matrix AFt followed a cloud distribution while the density of needle-shaped AFt decreased. In some areas, large C-S-H gel prisms were observed.(6)
According to the microstructures of hydration products, the early and late strength of sample was dominated by AFt and C-S-H gel, respectively, as AFt grew rapidly while C-S-H gel grew slowly. The hydration products grew interactively and were coupled, thus facilitating the growth of the hardened bodies of cementing material, instead of restraining each other due to the generation sequence.Figure 8
SEM images of samples at different ages. (a) 8 h. (b) 16 h. (c) 1 d. (d) 3 d. (e) 7 d.
(a)
(b)
(c)
(d)
(e)


## 3.1. Compressive Strength and Setting Time of Composite Cementing Materials

### 3.1.1. Compressive Strength

Table6 summarizes the setting time and compressive strengths of Portland cement and composite cementing samples measured experimentally at different ages.Table 6
Setting time and compressive strengths of samples at different ages.
Test number
Setting time (min)
Compressive strength (MPa)
Initial
Final
8 h
16 h
1 d
3 d
7 d
28 d
Portland cement
145
265
2.4
12.8
21.5
29.7
35.8
42.5
D01
22
29
2.9
16.4
25.4
32.7
37.4
44.8
D02
18
25
2.8
18.0
24.6
34.5
39.6
46.3
D03
87
135
2.6
14.8
23.8
30.2
37.6
43.6
D04
8
13
3.2
15.4
28.3
36.4
40.8
47.3
D05
12
20
3.8
17.8
26.4
40.1
43.7
48.9
D06
70
124
3.6
17.4
28.4
38.9
43.8
50.1
D07
187
304
0.6
3.2
5.4
23.2
20.7
18.2
D08
4
8
3.6
13.4
19.3
24.7
18.3
17.2
D09
42
64
4.2
18.2
29.3
39.7
44.5
52.7
D10
85
102
1.4
7.0
12.4
24.1
23.1
21.3
D11
10
13
5.4
10.3
13.2
24.3
20.2
18.9
D12
57
68
2.1
7.9
10.5
14.2
12.1
11.7
D13
12
15
2.7
9.2
14.1
21.3
20.7
19.2
D14
38
52
3.7
6.8
8.7
19.3
18.7
17.1
D15
17
22
3.1
7.2
7.9
18.4
17.2
16.9As the hydration rate and duration of composites are directly related to chemical compositions of the raw materials, the couplings of the hardened body produced by hydrations inevitably affect the compressive strength of the composite samples. According to the results, the compressive strengths of samples are dependent on the contents of sulphoaluminate cement and gypsum. Figure4 shows the trends of compressive strengths as a function of composition of composite samples.Figure 4
Trends of compressive strengths as a function of sample composition when the gypsum content is (a) 0–5%, (b) 5–10%, (c) 10–15%, and (d) 15–20%. CSA-sulphoaluminate cement; P-gypsum.
(a)
(b)
(c)
(d)As shown in Table6 and Figure 4, both the early and late compressive strengths of composite cementing material decreased significantly once the content of sulphoaluminate cement exceeded 14% or the content of gypsum exceeded 16%. Indeed, the compressive strengths of composite cementing material may shrink after 3 d in this case. With the content of sulphoaluminate cement below 14% and content of gypsum below 16%, the compressive strengths of composites increased, especially the early strength. The descending sequence of compressive strength is D09 > D06 > D05 > D04 > D02 > D01 > D03. Specifically, compared with Portland cement, compressive strengths of sample D09 at 8 h, 16 h, 1 d, 3 d, 7 d, and 28 d increased by 75%, 42%, 36%, 34%, 24%, and 23%, respectively; meanwhile, compressive strengths of sample D06 at 8 h, 16 h, 1 d, 3 d, 7 d, and 28 d increased by 50%, 36%, 32%, 31%, 22%, and 17%, respectively.In summary, the early strength of composite cementing samples can be significantly enhanced by ratio optimization of Portland cement, sulphoaluminate cement, and gypsum, as well as the late strength. In terms of structures, hardened bodies by early and late hydration were coupled with each other so that composites exhibited performances superior to the Portland cement single clinker. With compressive strength as the indicator, sample D09 is regarded as the optimized sample in this study.


### 3.1.2. Setting Time

Figures5 and 6, respectively, show the initial and final setting time of samples measured experimentally. As observed, the trends of initial and final setting time were consistent: increased as the content of gypsum increased and decreased as the content of sulphoaluminate cement increased. This can be attributed to sulphoaluminate, which accelerates early hydration of the composite samples. As a result, the hydration rate of samples at the early stage increased and the large quantity of hydration products led to reduced setting time. On the other hand, the too high content of gypsum hinders hydration of the Portland cement, resulting in extended setting time of samples.Figure 5
Relationship between initial setting time and components of composite cementing material.Figure 6
Relationship between final setting time and components of composite cementing material.In summary, despite its significantly enhanced compressive strength, sample D09 exhibited severely reduced initial and final setting time. Indeed, its initial and final setting time was 42 min and 64 min, respectively. The paste filling materials used for mining shall not solidify in channels for 3-4 h. Therefore, the setting time of composite sample D09 optimized by uniform design still does not meet the requirement; thus, modifications by additives are required.


## 3.1.1. Compressive Strength

Table6 summarizes the setting time and compressive strengths of Portland cement and composite cementing samples measured experimentally at different ages.Table 6
Setting time and compressive strengths of samples at different ages.
Test number
Setting time (min)
Compressive strength (MPa)
Initial
Final
8 h
16 h
1 d
3 d
7 d
28 d
Portland cement
145
265
2.4
12.8
21.5
29.7
35.8
42.5
D01
22
29
2.9
16.4
25.4
32.7
37.4
44.8
D02
18
25
2.8
18.0
24.6
34.5
39.6
46.3
D03
87
135
2.6
14.8
23.8
30.2
37.6
43.6
D04
8
13
3.2
15.4
28.3
36.4
40.8
47.3
D05
12
20
3.8
17.8
26.4
40.1
43.7
48.9
D06
70
124
3.6
17.4
28.4
38.9
43.8
50.1
D07
187
304
0.6
3.2
5.4
23.2
20.7
18.2
D08
4
8
3.6
13.4
19.3
24.7
18.3
17.2
D09
42
64
4.2
18.2
29.3
39.7
44.5
52.7
D10
85
102
1.4
7.0
12.4
24.1
23.1
21.3
D11
10
13
5.4
10.3
13.2
24.3
20.2
18.9
D12
57
68
2.1
7.9
10.5
14.2
12.1
11.7
D13
12
15
2.7
9.2
14.1
21.3
20.7
19.2
D14
38
52
3.7
6.8
8.7
19.3
18.7
17.1
D15
17
22
3.1
7.2
7.9
18.4
17.2
16.9As the hydration rate and duration of composites are directly related to chemical compositions of the raw materials, the couplings of the hardened body produced by hydrations inevitably affect the compressive strength of the composite samples. According to the results, the compressive strengths of samples are dependent on the contents of sulphoaluminate cement and gypsum. Figure4 shows the trends of compressive strengths as a function of composition of composite samples.Figure 4
Trends of compressive strengths as a function of sample composition when the gypsum content is (a) 0–5%, (b) 5–10%, (c) 10–15%, and (d) 15–20%. CSA-sulphoaluminate cement; P-gypsum.
(a)
(b)
(c)
(d)As shown in Table6 and Figure 4, both the early and late compressive strengths of composite cementing material decreased significantly once the content of sulphoaluminate cement exceeded 14% or the content of gypsum exceeded 16%. Indeed, the compressive strengths of composite cementing material may shrink after 3 d in this case. With the content of sulphoaluminate cement below 14% and content of gypsum below 16%, the compressive strengths of composites increased, especially the early strength. The descending sequence of compressive strength is D09 > D06 > D05 > D04 > D02 > D01 > D03. Specifically, compared with Portland cement, compressive strengths of sample D09 at 8 h, 16 h, 1 d, 3 d, 7 d, and 28 d increased by 75%, 42%, 36%, 34%, 24%, and 23%, respectively; meanwhile, compressive strengths of sample D06 at 8 h, 16 h, 1 d, 3 d, 7 d, and 28 d increased by 50%, 36%, 32%, 31%, 22%, and 17%, respectively.In summary, the early strength of composite cementing samples can be significantly enhanced by ratio optimization of Portland cement, sulphoaluminate cement, and gypsum, as well as the late strength. In terms of structures, hardened bodies by early and late hydration were coupled with each other so that composites exhibited performances superior to the Portland cement single clinker. With compressive strength as the indicator, sample D09 is regarded as the optimized sample in this study.


## 3.1.2. Setting Time

Figures5 and 6, respectively, show the initial and final setting time of samples measured experimentally. As observed, the trends of initial and final setting time were consistent: increased as the content of gypsum increased and decreased as the content of sulphoaluminate cement increased. This can be attributed to sulphoaluminate, which accelerates early hydration of the composite samples. As a result, the hydration rate of samples at the early stage increased and the large quantity of hydration products led to reduced setting time. On the other hand, the too high content of gypsum hinders hydration of the Portland cement, resulting in extended setting time of samples.Figure 5
Relationship between initial setting time and components of composite cementing material.Figure 6
Relationship between final setting time and components of composite cementing material.In summary, despite its significantly enhanced compressive strength, sample D09 exhibited severely reduced initial and final setting time. Indeed, its initial and final setting time was 42 min and 64 min, respectively. The paste filling materials used for mining shall not solidify in channels for 3-4 h. Therefore, the setting time of composite sample D09 optimized by uniform design still does not meet the requirement; thus, modifications by additives are required.


## 3.2. Modification Results of Optimized Composite Cementing Material

Table7 summarizes the experimentally obtained effects of the setting retarding agent (H2BO3) and early strengthening agents (e.g., Li2CO3, Na2SO4, and NaNO2) on performances of composite cementing material D09.Table 7
Effects of additives on performances of composite cementing material.
Test number
Contents of additives (%)
Setting time (min)
Compressive strength (MPa)
Initial
Final
8 h
16 h
1 d
3 d
7 d
28 d
J1
D09
42
64
4.2
18.2
29.3
39.7
44.5
52.7
J2
Li2CO3 (0.15)
21
32
6.1
23.5
26.7
30.2
34.4
40.5
J3
Li2CO3 (0.30)
18
27
8.4
25.4
24.1
27.6
32.7
38.4
J4
H2BO3 (0.15)
125
184
2.0
12.7
20.7
34.2
39.5
51.2
J5
H2BO3 (0.3)
324
348
1.2
10.4
18.6
32.1
38.8
50.7
J6
H2BO3 (0.3) + Li2CO3 (0.1)
152
182
5.8
24.6
28.7
32.2
36.4
39.3
J7
H2BO3 (0.3) + Na2SO4 (0.1)
182
185
5.8
22.8
34.2
46.5
49.7
58.4
J8
H2BO3 (0.3) + NaNO2 (0.1)
214
232
5.2
19.5
32.9
43.2
47.3
54.1As observed, Li2CO3 and H2BO3 had significant effects on the setting time of composite cementing material. As the content of Li2CO3 increased, the initial and final setting time decreased and the early strength was enhanced, while the late strength decreased. As the content of H2BO3 increased, the initial and final setting time increased, while the early strength decreased. The results indicated that at the expense of reduced late strengths, additive J6 had desired effects on the setting time and early strength, while additives J7 and J8 can enhance both setting time and compressive strengths. Therefore, H2BO3 (0.3%) + Na2SO4 (0.1%) and H2BO3 (0.3%) + NaNO2 (0.1%) are ideal setting retarding and early strengthening additives for composite cementing material.In summary, appropriate additives can enhance the performances (e.g., setting time and compressive strengths) of composite cementing material. In this study, samples J7 and J8 are ideal additives for cementing material in coal mining filling.


## 3.3. Hydration Mechanism of Modified Composite Cementing Material

### 3.3.1. Hydration Process

The composite cementing material consists of 2CaO·SiO2, 3CaO·SiO2, 3CaO·Al2O3, 4CaO·Al2O3·Fe2O3 (Portland cement), 3CaO·3Al2O3·CaSO4, 2CaO·SiO2 (sulphoaluminate cement), and CaSO4·2H2O (gypsum). Owing to the presence of gypsum, the reaction rate of 3CaO·Al2O3 and 3CaO·SiO2 was higher than that of 2CaO·SiO2 and 4CaO·Al2O3·Fe2O3. Meanwhile, 3CaO·3Al2O3·CaSO4 in sulphoaluminate is an early hydration product. Therefore, 3CaO·3Al2O3·CaSO4, 3CaO·Al2O3, and 3CaO·SiO2 react with each other upon addition of water:3CaO·3Al2O3·CaSO4 + 38H2O = 3CaO·Al2O3·CaSO4·32H2O + 2(Al2O3·H2O)3CaO·Al2O3 + 3CaSO4·2H2O + 26H2O = 3CaO·Al2O3·3CaSO4·32H2O2(3CaO·SiO2) + 11H2O = 3CaO·2SiO2·8H2O + 3Ca(OH)2As the hydration proceeds, the concentration of Ca(OH)2 increases and hydration products react with gypsum:Al2O3·H2O + 3Ca(OH)2 + 3CaSO4·2H2O + 22H2O = 3CaO·Al2O3·3CaSO4·32H2OIn the middle and late hydration stages, 2CaO·SiO2 and 4CaO·Al2O3·Fe2O3 are activated:2(2CaO·SiO2) + 9H2O = 3CaO·2SiO2·8H2O + Ca(OH)2C4AF + 2CH + 6CaSO4·2H2O + 50H = 2C3(A, F)·3CaSO4·32H2OIn summary, main hydration products of the samples include ettringite (AFt), calcium hydroxide (CH) gel, and hydrated calcium silicate (C-S-H) gel. The generation of AFt at an early stage is the main cause of high early strength of samples.


### 3.3.2. Hydration Products

Figure7 shows the XRD spectra of samples at different hydration ages. Several conclusions can be drawn:(1)
The main hydration products include AFt, CH gel, and C-S-H gel, while their relative contents vary with the hydration age, according to their diffraction peak intensities(2)
After 8 h hydration, a considerable quantity of AFt was generated, resulting in high early strength of samples(3)
CaCO3 was observed in hydration products. This can be attributed to the exposure of AFt to CO2(4)
The diffraction peaks of clinker minerals (e.g., SiO2 and 3CaO·3Al2O3·CaSO4) in samples decreased, indicating that the contents of clinker minerals decreased as the hydration age increased(5)
After 8 h of hydration, no diffraction peak corresponding to CaSO4 was observed, demonstrating complete hydration of CaSO4, whose product is AFtFigure 7
XRD spectra of samples at different hydration ages. ★-AFt; ●-C-S-H; ◆-CH; ■-CaCO3; □-SiO2; △-3CaO·3Al2O3·CaSO4. (a) 8 h. (b) 16 h. (c) 1 d. (d) 3 d. (e) 7 d.
(a)
(b)
(c)
(d)
(e)


### 3.3.3. Microstructures

Figure8 shows the microstructures of samples at different hydration ages. Several conclusions can be drawn:(1)
After 8 h of hydration, fine, scattered, needle-shaped AFt was observed. The AFt grew interactively and radially, with high density.(2)
After 16 h, the density of AFt increased, and its growth followed the intersecting pattern.(3)
After 1 d of hydration, sizes of AFt increased drastically and were distributed in clusters, while C-S-H gel flakes appeared.(4)
After 3 d, the density of C-S-H gel increased and the flakes were turned into prisms, demonstrating that the contents of C-S-H gel and CH gel increased and the hydration of Portland cement was accelerated.(5)
After 7 d, the matrix AFt followed a cloud distribution while the density of needle-shaped AFt decreased. In some areas, large C-S-H gel prisms were observed.(6)
According to the microstructures of hydration products, the early and late strength of sample was dominated by AFt and C-S-H gel, respectively, as AFt grew rapidly while C-S-H gel grew slowly. The hydration products grew interactively and were coupled, thus facilitating the growth of the hardened bodies of cementing material, instead of restraining each other due to the generation sequence.Figure 8
SEM images of samples at different ages. (a) 8 h. (b) 16 h. (c) 1 d. (d) 3 d. (e) 7 d.
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## 3.3.1. Hydration Process

The composite cementing material consists of 2CaO·SiO2, 3CaO·SiO2, 3CaO·Al2O3, 4CaO·Al2O3·Fe2O3 (Portland cement), 3CaO·3Al2O3·CaSO4, 2CaO·SiO2 (sulphoaluminate cement), and CaSO4·2H2O (gypsum). Owing to the presence of gypsum, the reaction rate of 3CaO·Al2O3 and 3CaO·SiO2 was higher than that of 2CaO·SiO2 and 4CaO·Al2O3·Fe2O3. Meanwhile, 3CaO·3Al2O3·CaSO4 in sulphoaluminate is an early hydration product. Therefore, 3CaO·3Al2O3·CaSO4, 3CaO·Al2O3, and 3CaO·SiO2 react with each other upon addition of water:3CaO·3Al2O3·CaSO4 + 38H2O = 3CaO·Al2O3·CaSO4·32H2O + 2(Al2O3·H2O)3CaO·Al2O3 + 3CaSO4·2H2O + 26H2O = 3CaO·Al2O3·3CaSO4·32H2O2(3CaO·SiO2) + 11H2O = 3CaO·2SiO2·8H2O + 3Ca(OH)2As the hydration proceeds, the concentration of Ca(OH)2 increases and hydration products react with gypsum:Al2O3·H2O + 3Ca(OH)2 + 3CaSO4·2H2O + 22H2O = 3CaO·Al2O3·3CaSO4·32H2OIn the middle and late hydration stages, 2CaO·SiO2 and 4CaO·Al2O3·Fe2O3 are activated:2(2CaO·SiO2) + 9H2O = 3CaO·2SiO2·8H2O + Ca(OH)2C4AF + 2CH + 6CaSO4·2H2O + 50H = 2C3(A, F)·3CaSO4·32H2OIn summary, main hydration products of the samples include ettringite (AFt), calcium hydroxide (CH) gel, and hydrated calcium silicate (C-S-H) gel. The generation of AFt at an early stage is the main cause of high early strength of samples.


## 3.3.2. Hydration Products

Figure7 shows the XRD spectra of samples at different hydration ages. Several conclusions can be drawn:(1)
The main hydration products include AFt, CH gel, and C-S-H gel, while their relative contents vary with the hydration age, according to their diffraction peak intensities(2)
After 8 h hydration, a considerable quantity of AFt was generated, resulting in high early strength of samples(3)
CaCO3 was observed in hydration products. This can be attributed to the exposure of AFt to CO2(4)
The diffraction peaks of clinker minerals (e.g., SiO2 and 3CaO·3Al2O3·CaSO4) in samples decreased, indicating that the contents of clinker minerals decreased as the hydration age increased(5)
After 8 h of hydration, no diffraction peak corresponding to CaSO4 was observed, demonstrating complete hydration of CaSO4, whose product is AFtFigure 7
XRD spectra of samples at different hydration ages. ★-AFt; ●-C-S-H; ◆-CH; ■-CaCO3; □-SiO2; △-3CaO·3Al2O3·CaSO4. (a) 8 h. (b) 16 h. (c) 1 d. (d) 3 d. (e) 7 d.
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## 3.3.3. Microstructures

Figure8 shows the microstructures of samples at different hydration ages. Several conclusions can be drawn:(1)
After 8 h of hydration, fine, scattered, needle-shaped AFt was observed. The AFt grew interactively and radially, with high density.(2)
After 16 h, the density of AFt increased, and its growth followed the intersecting pattern.(3)
After 1 d of hydration, sizes of AFt increased drastically and were distributed in clusters, while C-S-H gel flakes appeared.(4)
After 3 d, the density of C-S-H gel increased and the flakes were turned into prisms, demonstrating that the contents of C-S-H gel and CH gel increased and the hydration of Portland cement was accelerated.(5)
After 7 d, the matrix AFt followed a cloud distribution while the density of needle-shaped AFt decreased. In some areas, large C-S-H gel prisms were observed.(6)
According to the microstructures of hydration products, the early and late strength of sample was dominated by AFt and C-S-H gel, respectively, as AFt grew rapidly while C-S-H gel grew slowly. The hydration products grew interactively and were coupled, thus facilitating the growth of the hardened bodies of cementing material, instead of restraining each other due to the generation sequence.Figure 8
SEM images of samples at different ages. (a) 8 h. (b) 16 h. (c) 1 d. (d) 3 d. (e) 7 d.
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## 4. Conclusions

Formulation tests of composite cementing materials were conducted based on constrained formulation design. The effects of compositions (e.g., Portland cement, sulphoaluminate cement, and gypsum) of composite cementing materials on their performances were investigated using mechanical performance tests of samples, and the optimized samples were tested and modified. The following conclusions can be drawn:(1)
The setting time of samples was negatively related to the content of sulphoaluminate cement and positively related to the content of gypsum, and the trends of initial and final setting time were consistent.(2)
Both the early and late compressive strengths of composite cementing materials decreased significantly if the content of sulphoaluminate cement exceeded 14% or the content of gypsum exceeded 16%. Indeed, the compressive strengths of composite cementing material may shrink after 3 d in this case. With the content of sulphoaluminate cement below 14% and the content of gypsum below 16%, the compressive strengths of composites increased. The composition corresponding to optimized compressive strength was Portland cement (79.1%) + sulphoaluminate cement (9.8%) + hardened gypsum (11.1%).(3)
Additives H2BO3(0.3%) + Na2SO4(0.1%) and H2BO3(0.3%) + NaNO2(0.1%) are regarded as setting retarding and early strengthening composite additives because they can enhance both the setting time and the strength of composite cementing material.(4)
Investigations by XRD and SEM revealed that the hydration products were dominated by AFt at the early stage and C-S-H gel and CH gel at the middle and late stages. Meanwhile, hydration products of composite cementing material with optimized ratios grew interactively and were coupled, thus facilitating the growth of the hardened body of cementing material, instead of restraining each other due to the generation sequence.In this study, the optimized ratios of Portland cement, sulphoaluminate cement, and gypsum in composite cementing material were determined, the hydration mechanism of composite cementing material was identified, and the composite cementing material was modified using appropriate additives to meet the requirements by paste filling in coal mines.



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*Source: 3732160-2019-12-07.xml*