# Effect and Mechanism of Superplasticizers on Performance of Ultrafine Sulfoaluminate Cement-Based Grouting Materials

Authors: Jianwu Zhang; Xiao Wang; Biao Jin; Xiaoting Zhang; Zhixin Li; Xuemao Guan
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

A novel ultrafine sulfoaluminate cement-based grouting material (USCBGM) has been prepared in the previous research, which had better engineering application performance, such as fast setting and hardening, fast strength development, stone body microexpansion, and other properties compared with the traditional ultrafine cement. However, as an ultrafine particle material, it is also necessary to select an appropriate superplasticizer to improve the fluidity of USCGBM paste. Based on this, the effects of polycarboxylic acid (PCE) and naphthalene (FDN) superplasticizers on the performance of USCBGM were studied in this study. The results showed that both PCE and FDN can effectively improve the initial fluidity of USCBGM paste, but the modification effect of PCE was better. The incorporation of PCE had little effect on the setting time of USCBGM paste. Unlike PCE, the addition of FDN can significantly prolong the setting time of paste. XRD, DSC-TG, and SEM results showed the addition of PCE significantly increased the amount of ettringite hydration products in the hardened paste at the early hydration stage. Correspondingly, the hourly strength of hardened paste increased with the addition of PCE. Similar to PCE, the addition of FDN can also promote the formation of more ettringite hydration products at the early hydration stage. However, the difference is that the addition of FDN can lead to the cluster distribution of ettringite crystals, which significantly affected the crystal skeleton effect of ettringite and markedly reduced the early strength of hardened paste.


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## Body

## 1. Introduction

Ultrafine cement is a new kind of high-performance cement-based grouting material, which is prepared by grinding cement-based material with common particle size by relevant grinding means [1–4]. Due to the smaller particle size, ultrafine cement has good permeability and grouting ability similar to organic chemical grouting liquid [5]. In addition, the stone body of ultrafine cement also has higher strength, durability, and good environmental protection compared to chemical grouting liquid [6–8]. Based on its good performance, ultrafine cement has been widely concerned and applied in underground space engineering fields such as mining and tunnel reinforcement [9, 10]. At present, most ultrafine cements are prepared by fine grinding and modification on the basis of Portland cement. Moreover, due to the small particle size of ultrafine cement particles, large water consumption is often required in order to obtain good grouting paste performance. Therefore, the ultrafine cement generally has some defects, such as slow setting and hardening, low early strength, and easy shrinkage of stone body, which seriously affect its grouting reinforcement effect [11–13].Based on the problems existing in the field of ultrafine cement, the authors developed a novel ultrafine sulfoaluminate cement-based grouting material (abbreviated as USCBGM) in the course of the preliminary study [14, 15]. The results show that the USCBGM has the excellent characteristics of fast setting and hardening, rapid strength development, and microexpansion of the stone body compared with the traditional ultrafine Portland cement. Therefore, it can better meet the needs of various underground space projects. However, although USCBGM has good physical and mechanical properties, it should also be endowed with good fluidity and other working properties in order to ensure its application effect in the actual grouting reinforcement process. Due to the agglomerating effect between the ultrafine particles, part of the free water is wrapped up when USCBGM is mixed with water, which leads to poor fluidity of the paste and significantly affects its injection and permeability.Superplasticizers are usually added to improve the flow performance of paste in the field of cement-based materials [16–19]. Superplasticizers can effectively change the surface properties of particles, break the agglomeration between particles, release the wrapped free water, and significantly improve the fluidity of cement-based material paste [20, 21]. At present, naphthalene superplasticizer (FDN) and polycarboxylic acid superplasticizer (PCE) are two kinds of superplasticizers commonly used in the field of cement-based materials. FDN is mainly adsorbed on the particle surface of cement-based materials by electrostatic adsorption to change the charge properties on the particle surface and achieve the purpose of particle dispersion [22]. Different from FDN, PCE mainly relies on the steric hindrance effect to disperse particles due to its complex molecular structure [23]. Many research results have shown that FND and PCE can effectively improve the fluidity of Portland cement-based materials and have good adaptability [24].In fact, both FDN and PCE can improve the fluidity of sulfoaluminate cement-based materials [25–27]. The author's previous research results have also proved this point [14]. However, many scholars found that superplasticizers not only can improve the working performance of sulfoaluminate cement-based materials but also cause changes in setting time, strength, and other properties. Moreover, there are great differences or even contradictions between the research conclusions obtained by different scholars [28, 29]. It can be seen that it is necessary to fully consider the adaptability between superplasticizer and sulfoaluminate cement-based materials in the process of paste modification.Based on the above analysis, this study carried out the effects of FDN and PCE on the fluidity, setting time, and mechanical performance of USCBGM. At the same time, the effect of FDN and PCE on the phase composition and hydration product morphology of hardened paste was studied by X-ray diffraction, DSC-TG synchronous thermal analysis, and SEM. In addition, this study deeply expounded the influence mechanism of PCE and FDN on the mechanical performance of USCBGM according to the action principle of superplasticizers.


## 2. Materials and Test Methods

### 2.1. Materials

The sulfoaluminate cement clinker and anhydrite were purchased from the Huayan cement plant. Quicklime was bought from the Taihang lime plant. Three kinds of ultrafine powders for the experiment were obtained by ultrafine grinding of raw materials with a fluidized bed air mill as shown in Figure1. Tables 1 and 2 show the chemical composition of the ultrafine sulfoaluminate cement clinker and ultrafine anhydrite, respectively. The calcium oxide content in ultrafine quicklime was 70.3 wt. %. The particle size distribution of three ultrafine powders is shown in Figure 2. In addition, PCE was bought from Shandong Bock Chemical Co., Ltd., with a solid content of 40%. FDN was purchased from Jiangxi Weitai Co., Ltd. for building materials.Figure 1
Fluidized bed-type jet mill used in the experiment.Table 1
Chemical composition of the ultrafine sulfoaluminate clinker (wt. %).
C4A3S¯β-C2SC4AFf-SO3f-CaOCaO·TiO274.5418.253.860.812.023.01Table 2
Chemical composition of the ultrafine anhydrite (wt. %).
LossSiO2Fe2O3MgOAl2O3CaOSO3Alkali6.141.040.182.640.2338.6350.110.12Figure 2
Particle size distribution of three ultrafine powders.


### 2.2. Test Methods

#### 2.2.1. Preparation of the UCBGM Paste

As a double liquid grouting material, USCBGM was mainly composed of paste A and paste B. Paste A was made of ultrafine sulfoaluminate cement clinker mixed with water, and paste B was prepared by adding water to ultrafine anhydrite and ultrafine quicklime in a certain proportion. The mass ratio between ultrafine sulfoaluminate cement clinker, ultrafine anhydrite, and ultrafine quicklime was determined to be 100: 80: 20 according to the previous research results [14]. During the experiment, paste A and paste B were first separately mixed with water according to the water powder ratio of 1 : 1. Then, the two kinds of paste were blended according to the same volume and evenly stirred to obtain the USCBGM paste. It should be noted that the specified amount of superplasticizer should be dissolved in the mixing water that is used to prepare paste A and paste B in advance when the USCBGM paste doped with superplasticizer was prepared.


#### 2.2.2. Fluidity Test

Figure3 shows the schematic diagram of the USCBGM paste fluidity test device. First, the prepared USCBGM paste was quickly injected into the truncated cone circular die at the center of the horizontal glass plate and scraped flat with a scraper. Then, the truncated cone circular mold was lifted in the vertical direction and timed until the paste freely flowed on the glass plate for 30 seconds. Then, the maximum diameter of the two directions perpendicular to each other of the flowing part was measured with a ruler and the average value was taken as the fluidity of the grouting paste.Figure 3
Schematic diagram of grouting paste fluidity test.


#### 2.2.3. Setting Time and Mechanical Performance Test

The setting time of the USCBGM paste was performed by utilizing a Vicat apparatus and following the Chinese National Standard (GB/T 1346-2011). In order to study the effect of superplasticizer on the mechanical performance of USCBGM, a series of 40 mm × 40 mm × 40 mm samples were formed and prepared. After forming, the samples were placed in an environment of 20°C and 90% relative humidity to curing to the specified age. A pressure testing machine that modeled with TYE-10C was used to test the compressive strength of samples. The specific test process is described as follows. First, the samples were taken out of the curing chamber and the surfaces were dried with a suede fabric. Second, the sample was placed on the sample table of the pressure testing machine for compressive strength testing at a loading rate of 0.5 kN/s. Finally, the strength data were recorded when the failure load limit was reached. The compressive strength of the sample was calculated according to formula .(1)Rc=FcA,where RC represented the compressive strength of the sample, expressed in MPa. FC was the maximum load when the sample was damaged, and N. A represented the area of stress, mm2. Each group of samples had three test results. The average value of the three measured values was used as the strength representative value of the group of samples when the difference between the three measured values did not exceed 15%. If one of the maximum and minimum values of the three measured values exceeded 15% of the median value, the measured value should be removed and the average value of the remaining two should be taken as the strength representative value of this group of samples. If the maximum and minimum values of the three measured values were more than 15% of the median value, then the group of samples should be invalid, and it was necessary to reprepare and test samples.


#### 2.2.4. X-Ray Diffraction and DSC-TG Test

The specimens collected from a crushed piece of the compressive strength test sample were stopped by using ethanol. After 24 h, the specimens were dried in an environment with a temperature of 35°C and a vacuum of 0.08 MPa. The dried specimens were crushed by hand with a diameter of less than 0.063 mm for XRD and DSC-TG tests. The X-ray diffraction patterns of the hardening pastes were tested on a Bruker D8 ADVANCE X-ray diffractometer with Cu Ka radiation (k = 0.154 nm), in which the scanning range was from 5 to 50 with a step size of 0.02. The DSC-TG curves were collected at a rate of 10°C/min in a nitrogen atmosphere through a synchronous thermal analyzer with model STA449F3.


#### 2.2.5. SEM Test

Unground dried specimens were measured by using Quanta 450 scanning electron microscope at 25 kV and 5,000 magnification in order to observe the microstructure of hardened paste. First, the dried specimens were sprayed with gold. Then, the treated samples were placed in the scanning electron microscope and vacuumed. Finally, a secondary electronic image of the sample’s surface can be observed and obtained. The influence of the superplasticizer on the microstructure of hardened paste would be revealed by comparing the secondary electronic images of each sample.


## 2.1. Materials

The sulfoaluminate cement clinker and anhydrite were purchased from the Huayan cement plant. Quicklime was bought from the Taihang lime plant. Three kinds of ultrafine powders for the experiment were obtained by ultrafine grinding of raw materials with a fluidized bed air mill as shown in Figure1. Tables 1 and 2 show the chemical composition of the ultrafine sulfoaluminate cement clinker and ultrafine anhydrite, respectively. The calcium oxide content in ultrafine quicklime was 70.3 wt. %. The particle size distribution of three ultrafine powders is shown in Figure 2. In addition, PCE was bought from Shandong Bock Chemical Co., Ltd., with a solid content of 40%. FDN was purchased from Jiangxi Weitai Co., Ltd. for building materials.Figure 1
Fluidized bed-type jet mill used in the experiment.Table 1
Chemical composition of the ultrafine sulfoaluminate clinker (wt. %).
C4A3S¯β-C2SC4AFf-SO3f-CaOCaO·TiO274.5418.253.860.812.023.01Table 2
Chemical composition of the ultrafine anhydrite (wt. %).
LossSiO2Fe2O3MgOAl2O3CaOSO3Alkali6.141.040.182.640.2338.6350.110.12Figure 2
Particle size distribution of three ultrafine powders.


## 2.2. Test Methods

### 2.2.1. Preparation of the UCBGM Paste

As a double liquid grouting material, USCBGM was mainly composed of paste A and paste B. Paste A was made of ultrafine sulfoaluminate cement clinker mixed with water, and paste B was prepared by adding water to ultrafine anhydrite and ultrafine quicklime in a certain proportion. The mass ratio between ultrafine sulfoaluminate cement clinker, ultrafine anhydrite, and ultrafine quicklime was determined to be 100: 80: 20 according to the previous research results [14]. During the experiment, paste A and paste B were first separately mixed with water according to the water powder ratio of 1 : 1. Then, the two kinds of paste were blended according to the same volume and evenly stirred to obtain the USCBGM paste. It should be noted that the specified amount of superplasticizer should be dissolved in the mixing water that is used to prepare paste A and paste B in advance when the USCBGM paste doped with superplasticizer was prepared.


### 2.2.2. Fluidity Test

Figure3 shows the schematic diagram of the USCBGM paste fluidity test device. First, the prepared USCBGM paste was quickly injected into the truncated cone circular die at the center of the horizontal glass plate and scraped flat with a scraper. Then, the truncated cone circular mold was lifted in the vertical direction and timed until the paste freely flowed on the glass plate for 30 seconds. Then, the maximum diameter of the two directions perpendicular to each other of the flowing part was measured with a ruler and the average value was taken as the fluidity of the grouting paste.Figure 3
Schematic diagram of grouting paste fluidity test.


### 2.2.3. Setting Time and Mechanical Performance Test

The setting time of the USCBGM paste was performed by utilizing a Vicat apparatus and following the Chinese National Standard (GB/T 1346-2011). In order to study the effect of superplasticizer on the mechanical performance of USCBGM, a series of 40 mm × 40 mm × 40 mm samples were formed and prepared. After forming, the samples were placed in an environment of 20°C and 90% relative humidity to curing to the specified age. A pressure testing machine that modeled with TYE-10C was used to test the compressive strength of samples. The specific test process is described as follows. First, the samples were taken out of the curing chamber and the surfaces were dried with a suede fabric. Second, the sample was placed on the sample table of the pressure testing machine for compressive strength testing at a loading rate of 0.5 kN/s. Finally, the strength data were recorded when the failure load limit was reached. The compressive strength of the sample was calculated according to formula .(1)Rc=FcA,where RC represented the compressive strength of the sample, expressed in MPa. FC was the maximum load when the sample was damaged, and N. A represented the area of stress, mm2. Each group of samples had three test results. The average value of the three measured values was used as the strength representative value of the group of samples when the difference between the three measured values did not exceed 15%. If one of the maximum and minimum values of the three measured values exceeded 15% of the median value, the measured value should be removed and the average value of the remaining two should be taken as the strength representative value of this group of samples. If the maximum and minimum values of the three measured values were more than 15% of the median value, then the group of samples should be invalid, and it was necessary to reprepare and test samples.


### 2.2.4. X-Ray Diffraction and DSC-TG Test

The specimens collected from a crushed piece of the compressive strength test sample were stopped by using ethanol. After 24 h, the specimens were dried in an environment with a temperature of 35°C and a vacuum of 0.08 MPa. The dried specimens were crushed by hand with a diameter of less than 0.063 mm for XRD and DSC-TG tests. The X-ray diffraction patterns of the hardening pastes were tested on a Bruker D8 ADVANCE X-ray diffractometer with Cu Ka radiation (k = 0.154 nm), in which the scanning range was from 5 to 50 with a step size of 0.02. The DSC-TG curves were collected at a rate of 10°C/min in a nitrogen atmosphere through a synchronous thermal analyzer with model STA449F3.


### 2.2.5. SEM Test

Unground dried specimens were measured by using Quanta 450 scanning electron microscope at 25 kV and 5,000 magnification in order to observe the microstructure of hardened paste. First, the dried specimens were sprayed with gold. Then, the treated samples were placed in the scanning electron microscope and vacuumed. Finally, a secondary electronic image of the sample’s surface can be observed and obtained. The influence of the superplasticizer on the microstructure of hardened paste would be revealed by comparing the secondary electronic images of each sample.


## 2.2.1. Preparation of the UCBGM Paste

As a double liquid grouting material, USCBGM was mainly composed of paste A and paste B. Paste A was made of ultrafine sulfoaluminate cement clinker mixed with water, and paste B was prepared by adding water to ultrafine anhydrite and ultrafine quicklime in a certain proportion. The mass ratio between ultrafine sulfoaluminate cement clinker, ultrafine anhydrite, and ultrafine quicklime was determined to be 100: 80: 20 according to the previous research results [14]. During the experiment, paste A and paste B were first separately mixed with water according to the water powder ratio of 1 : 1. Then, the two kinds of paste were blended according to the same volume and evenly stirred to obtain the USCBGM paste. It should be noted that the specified amount of superplasticizer should be dissolved in the mixing water that is used to prepare paste A and paste B in advance when the USCBGM paste doped with superplasticizer was prepared.


## 2.2.2. Fluidity Test

Figure3 shows the schematic diagram of the USCBGM paste fluidity test device. First, the prepared USCBGM paste was quickly injected into the truncated cone circular die at the center of the horizontal glass plate and scraped flat with a scraper. Then, the truncated cone circular mold was lifted in the vertical direction and timed until the paste freely flowed on the glass plate for 30 seconds. Then, the maximum diameter of the two directions perpendicular to each other of the flowing part was measured with a ruler and the average value was taken as the fluidity of the grouting paste.Figure 3
Schematic diagram of grouting paste fluidity test.


## 2.2.3. Setting Time and Mechanical Performance Test

The setting time of the USCBGM paste was performed by utilizing a Vicat apparatus and following the Chinese National Standard (GB/T 1346-2011). In order to study the effect of superplasticizer on the mechanical performance of USCBGM, a series of 40 mm × 40 mm × 40 mm samples were formed and prepared. After forming, the samples were placed in an environment of 20°C and 90% relative humidity to curing to the specified age. A pressure testing machine that modeled with TYE-10C was used to test the compressive strength of samples. The specific test process is described as follows. First, the samples were taken out of the curing chamber and the surfaces were dried with a suede fabric. Second, the sample was placed on the sample table of the pressure testing machine for compressive strength testing at a loading rate of 0.5 kN/s. Finally, the strength data were recorded when the failure load limit was reached. The compressive strength of the sample was calculated according to formula .(1)Rc=FcA,where RC represented the compressive strength of the sample, expressed in MPa. FC was the maximum load when the sample was damaged, and N. A represented the area of stress, mm2. Each group of samples had three test results. The average value of the three measured values was used as the strength representative value of the group of samples when the difference between the three measured values did not exceed 15%. If one of the maximum and minimum values of the three measured values exceeded 15% of the median value, the measured value should be removed and the average value of the remaining two should be taken as the strength representative value of this group of samples. If the maximum and minimum values of the three measured values were more than 15% of the median value, then the group of samples should be invalid, and it was necessary to reprepare and test samples.


## 2.2.4. X-Ray Diffraction and DSC-TG Test

The specimens collected from a crushed piece of the compressive strength test sample were stopped by using ethanol. After 24 h, the specimens were dried in an environment with a temperature of 35°C and a vacuum of 0.08 MPa. The dried specimens were crushed by hand with a diameter of less than 0.063 mm for XRD and DSC-TG tests. The X-ray diffraction patterns of the hardening pastes were tested on a Bruker D8 ADVANCE X-ray diffractometer with Cu Ka radiation (k = 0.154 nm), in which the scanning range was from 5 to 50 with a step size of 0.02. The DSC-TG curves were collected at a rate of 10°C/min in a nitrogen atmosphere through a synchronous thermal analyzer with model STA449F3.


## 2.2.5. SEM Test

Unground dried specimens were measured by using Quanta 450 scanning electron microscope at 25 kV and 5,000 magnification in order to observe the microstructure of hardened paste. First, the dried specimens were sprayed with gold. Then, the treated samples were placed in the scanning electron microscope and vacuumed. Finally, a secondary electronic image of the sample’s surface can be observed and obtained. The influence of the superplasticizer on the microstructure of hardened paste would be revealed by comparing the secondary electronic images of each sample.


## 3. Results

### 3.1. Fluidity of Pastes

The effect of PCE and FDN on the initial fluidity of USCBGM paste is shown in Figure4. Figure 4(a) shows the initial fluidity of paste without superplasticizer that was 155 mm. The addition of PCE can significantly improve the initial fluidity of paste. In addition, the more the content of PCE, the initial fluidity of the paste was greater. The initial fluidity of the paste reached a maximum of 320 mm when the content of PCE was 0.2%. Subsequently, the initial fluidity of the paste did not significantly change with the continuous increase in PCE content. Figure 4(b) shows the addition of FDN can also significantly improve the initial fluidity of the paste. Similar to PCE, FDN also had saturated content in USCGBM. The paste can have a maximum fluidity of 308 mm when FDN reached the saturated content of 1.5%. It can be seen the modification effect of PCE on the fluidity of USCGBM paste was significantly better than that of FDN under the condition of saturated content.Figure 4
Effect of superplasticizer on initial fluidity of USCBGM paste: (a) PCE and (b) FDN.
(a)(b)The fluidity test results showed that both PCE and FDN can effectively improve the fluidity of the USCBGM slurry. As a surfactant, PCE can be effectively adsorbed on the surface of active particles after being mixed with USCBGM. Due to the steric hindrance effect caused by a more branched-chain structure in PCE molecule, the aggregate particles in USCBGM paste can be dispersed. As a result, the free water was released, which significantly improved the fluidity of the paste. FDN can also be adsorbed on the surface of active particles in USCBGM. Different from PCE, FDN mainly relies on electrostatic repulsion to achieve the purpose of particle dispersion and the fluidity improvement of paste due to the rigid straight-chain structure of FDN molecules.


### 3.2. Setting Time of Pastes

Figure5 shows the effect of PCE and FDN superplasticizers on the setting time of USCBGM paste. The paste without superplasticizers reached initial and final coagulation after only 6 min and 10 min, respectively, from Figure 5(a). The incorporation of PCE can slightly prolong the setting time of paste but not significantly. For example, the initial setting time and final setting time of paste with 0.2% PCE were only extended by 15 s and 12 s, respectively, compared with the paste without a superplasticizer. The effect of FDN on the setting time of USCBGM paste is shown in Figure 5(b). It can be seen that FDN can significantly prolong the initial and final setting times of the paste to a certain extent. Moreover, the more the content of FDN, the more significant the effect. The initial setting time and final setting time of the paste were significantly prolonged by 270 s and 156 s compared to the paste without superplasticizer when FDN reached the saturated content. Subsequently, the setting time of the paste did not significantly change with the increase in the content of FDN.Figure 5
Effect of superplasticizers on setting time of USCBGM grouting paste: (a) PCE and (b) FDN.
(a)(b)The main reasons why PCE and FDN affected the setting time of USCGBM paste were as follows. PCE can preferentially adsorb on the surface of sulfoaluminate cement clinker particles with higher activity when it was incorporated into USCBGM paste. At this time, the carboxyl, sulfonic acid, and other groups contained in PCE molecules can combine with the Ca2+ ions released by the dissolution of clinker particles through complexation and form a calcium-rich protective layer around the clinker particles, which hindered the contact between clinker particles and water, delayed the hydration reaction, and slightly prolonged the setting time of USCBGM paste [30]. Similar to PCE, FDN can also be adsorbed on the surface of positively charged cement clinker particles. Moreover, the sulfonic acid groups contained in FDN molecules can also react with the Ca2+ ions in the liquid phase around the clinker particles and form a calcium-rich protective layer on the surface of the clinker particles, which hindered and delayed the setting and hardening of USCBGM paste [31]. It may be that the rigid branch chain structure of FDN molecules enabled it to adsorb on the surface of clinker particles more than PCE, resulting in a thicker calcium-rich layer and significantly delaying the condensation of USCBGM paste.


### 3.3. Compressive Strength

In order to analyze the effect of superplasticizer on the early mechanical properties of USCBGM, the compressive strength of blank sample (namely, hardened paste without superplasticizer), sample doped with 0.2% PCE and 1.5% FDN, respectively, was tested as shown in Figure6. For the blank sample, the compressive strength reached 8.7 MPa after 4 h hydration age. After hydration to 1 d age, the compressive strength significantly increased to 16.7 MPa. At the age of 1 d–7 d, the compressive strength of the blank sample no longer significantly increased. The addition of PCE can improve the early compressive strength of USCBGM to a certain extent. For example, the compressive strength of the sample doped with PCE at 4 h and 1 d age reached 10.5 MPa and 17.3 MPa, respectively, which was increased by 20.7% and 3.6% compared with the blank sample. Similar to the blank sample, the compressive strength of the sample doped with PCE did not significantly change at the age of 1 d–7 d. For the sample doped with FDN, the early strength was obviously low. The compressive strength of the sample with FDN was only 4.5 MPa and 11.2 MPa at the age of 4 h and 1 d hydration age, respectively, which decreased by 48.3% and 32.9% compared with the blank sample. Similarly, the compressive strength of the sample doped with FDN did not significantly increase in the age of 1 d–7 d.Figure 6
Effect of superplasticizers on compressive strength of USCBGM.


### 3.4. X-Ray Diffraction Analysis

Figure7 shows the XRD spectrum of each sample at 4 h and 7 d age. It can be seen that only after 4 h hydration age, a large number of ettringite hydration products were detected in the three groups of samples. At the same time, the characteristic peaks of unreacted ye’elimite and anhydrite were also detected. Generally, the greater the intensity of the crystal diffraction peak, the more the crystals. Comparing the intensity of (100) crystal plane diffraction peak of ettringite crystal in the three groups of samples, it can be seen that the amount of ettringite in the two groups of samples doped with PCE and FDN was higher than that in the blank sample, indicating that PCE and FDN can accelerate the early hydration reaction rate of USCBGM to a certain extent. Moreover, FDN played a stronger role in promoting hydration compared with PCE. Figure 7(b) shows the amount of ettringite in the three groups of samples increased further after 7 d of age. The ettringite content in the three groups was similar by comparison to peak strength. Accordingly, anhydrite and ye’elimite were largely consumed due to the in-depth hydration reaction. After 7 d of age, only weak anhydrite characteristic peaks were detected in each sample. The characteristic peak of ye’elimite was not detected, which indicated that it has been exhausted.Figure 7
XRD spectra of each sample: (a) 4 h and (b) 7 d.
(a)(b)


### 3.5. Thermogravimetric Analysis

The thermal analysis test results of each sample at the age of 4 h and 7 d are shown in Figure8. The endothermic peak of ettringite was detected near 120°C in all samples after hydration of 4 h. In addition, a few endothermic peaks of AFm and AH3 hydration products were detected near 150°C and 250°C, respectively. According to TG curve data, the amount of ettringite in the three groups of samples can be calculated by using the method described in literature [32]. The calculated results showed that the yield of ettringite was 51.4%, 55.5%, and 56.2% for blank sample, sample doped with PCE, and sample doped with FDN after 4 h hydration, respectively. It can be seen the amount of ettringite in the samples mixed with PCE and FDN was significantly higher than that of the blank sample. Moreover, the amount of ettringite in the sample doped with FDN was slightly higher than that of the sample doped with PCE. This further showed that the addition of PCE and FDN can accelerate the early hydration reaction of USCBGM. The amount of ettringite in each sample further increased after 7 d. Moreover, the amount of ettringite in each sample was the same as the weight loss curve. This was consistent with the test results of XRD. In addition, after 7 d of age, AH3 was still detected in each sample, but the endothermic peak of AFm disappeared, which may be caused by its transformation into ettringite crystals.Figure 8
Thermal analysis test results of each sample: (a) 4 h and (b) 7 d.
(a)(b)


### 3.6. Scanning Electron Microscopy

Figure9 shows the micrographs of each sample at the age of 4 h and 7 d. It can be seen from Figures 9(a)–9(c) that a large number of fine-needle ettringite crystals were generated in the three groups of samples after hydration for 4 h. For the blank sample and the sample doped with PCE, the ettringite production obviously had a high degree of overlapping and interleaving, which was very beneficial to fully develop the strength skeleton effect of ettringite crystals and improve the early strength of hardened paste. However, different from the blank sample and the sample doped with PCE, the ettringite produced in the sample doped with FDN tended to adhere to the clinker particles and grow in clusters. This distribution characteristic of ettringite crystals was obviously not conducive to the exertion of its skeleton effect. Accordingly, the early strength of the sample doped with FDN was significantly lower. Figures 9(d)–9(f) show the micrographs of three groups of samples at the hydration age of 7 d. It can be seen that the amount of ettringite in each sample increases further after hydration to 7 d. The blank sample and the sample doped with PCE obviously became denser. Ettringite crystals formed in the sample doped with FDN were still distributed in clusters after 7 d of hydration, which reflected poor overlap between ettringite crystals and relatively lower 7 d strength of hardened paste.Figure 9
SEM photograph for each sample: (a) blank sample for 4 h; (b) sample doped with PCE for 4 h; (c) sample doped with FDN for 4 h; (d) blank sample for 7 d; (e) sample doped with PCE for 7 d; (f) sample doped with FDN for 7 d.
(a)(b)(c)(d)(e)(f)


## 3.1. Fluidity of Pastes

The effect of PCE and FDN on the initial fluidity of USCBGM paste is shown in Figure4. Figure 4(a) shows the initial fluidity of paste without superplasticizer that was 155 mm. The addition of PCE can significantly improve the initial fluidity of paste. In addition, the more the content of PCE, the initial fluidity of the paste was greater. The initial fluidity of the paste reached a maximum of 320 mm when the content of PCE was 0.2%. Subsequently, the initial fluidity of the paste did not significantly change with the continuous increase in PCE content. Figure 4(b) shows the addition of FDN can also significantly improve the initial fluidity of the paste. Similar to PCE, FDN also had saturated content in USCGBM. The paste can have a maximum fluidity of 308 mm when FDN reached the saturated content of 1.5%. It can be seen the modification effect of PCE on the fluidity of USCGBM paste was significantly better than that of FDN under the condition of saturated content.Figure 4
Effect of superplasticizer on initial fluidity of USCBGM paste: (a) PCE and (b) FDN.
(a)(b)The fluidity test results showed that both PCE and FDN can effectively improve the fluidity of the USCBGM slurry. As a surfactant, PCE can be effectively adsorbed on the surface of active particles after being mixed with USCBGM. Due to the steric hindrance effect caused by a more branched-chain structure in PCE molecule, the aggregate particles in USCBGM paste can be dispersed. As a result, the free water was released, which significantly improved the fluidity of the paste. FDN can also be adsorbed on the surface of active particles in USCBGM. Different from PCE, FDN mainly relies on electrostatic repulsion to achieve the purpose of particle dispersion and the fluidity improvement of paste due to the rigid straight-chain structure of FDN molecules.


## 3.2. Setting Time of Pastes

Figure5 shows the effect of PCE and FDN superplasticizers on the setting time of USCBGM paste. The paste without superplasticizers reached initial and final coagulation after only 6 min and 10 min, respectively, from Figure 5(a). The incorporation of PCE can slightly prolong the setting time of paste but not significantly. For example, the initial setting time and final setting time of paste with 0.2% PCE were only extended by 15 s and 12 s, respectively, compared with the paste without a superplasticizer. The effect of FDN on the setting time of USCBGM paste is shown in Figure 5(b). It can be seen that FDN can significantly prolong the initial and final setting times of the paste to a certain extent. Moreover, the more the content of FDN, the more significant the effect. The initial setting time and final setting time of the paste were significantly prolonged by 270 s and 156 s compared to the paste without superplasticizer when FDN reached the saturated content. Subsequently, the setting time of the paste did not significantly change with the increase in the content of FDN.Figure 5
Effect of superplasticizers on setting time of USCBGM grouting paste: (a) PCE and (b) FDN.
(a)(b)The main reasons why PCE and FDN affected the setting time of USCGBM paste were as follows. PCE can preferentially adsorb on the surface of sulfoaluminate cement clinker particles with higher activity when it was incorporated into USCBGM paste. At this time, the carboxyl, sulfonic acid, and other groups contained in PCE molecules can combine with the Ca2+ ions released by the dissolution of clinker particles through complexation and form a calcium-rich protective layer around the clinker particles, which hindered the contact between clinker particles and water, delayed the hydration reaction, and slightly prolonged the setting time of USCBGM paste [30]. Similar to PCE, FDN can also be adsorbed on the surface of positively charged cement clinker particles. Moreover, the sulfonic acid groups contained in FDN molecules can also react with the Ca2+ ions in the liquid phase around the clinker particles and form a calcium-rich protective layer on the surface of the clinker particles, which hindered and delayed the setting and hardening of USCBGM paste [31]. It may be that the rigid branch chain structure of FDN molecules enabled it to adsorb on the surface of clinker particles more than PCE, resulting in a thicker calcium-rich layer and significantly delaying the condensation of USCBGM paste.


## 3.3. Compressive Strength

In order to analyze the effect of superplasticizer on the early mechanical properties of USCBGM, the compressive strength of blank sample (namely, hardened paste without superplasticizer), sample doped with 0.2% PCE and 1.5% FDN, respectively, was tested as shown in Figure6. For the blank sample, the compressive strength reached 8.7 MPa after 4 h hydration age. After hydration to 1 d age, the compressive strength significantly increased to 16.7 MPa. At the age of 1 d–7 d, the compressive strength of the blank sample no longer significantly increased. The addition of PCE can improve the early compressive strength of USCBGM to a certain extent. For example, the compressive strength of the sample doped with PCE at 4 h and 1 d age reached 10.5 MPa and 17.3 MPa, respectively, which was increased by 20.7% and 3.6% compared with the blank sample. Similar to the blank sample, the compressive strength of the sample doped with PCE did not significantly change at the age of 1 d–7 d. For the sample doped with FDN, the early strength was obviously low. The compressive strength of the sample with FDN was only 4.5 MPa and 11.2 MPa at the age of 4 h and 1 d hydration age, respectively, which decreased by 48.3% and 32.9% compared with the blank sample. Similarly, the compressive strength of the sample doped with FDN did not significantly increase in the age of 1 d–7 d.Figure 6
Effect of superplasticizers on compressive strength of USCBGM.


## 3.4. X-Ray Diffraction Analysis

Figure7 shows the XRD spectrum of each sample at 4 h and 7 d age. It can be seen that only after 4 h hydration age, a large number of ettringite hydration products were detected in the three groups of samples. At the same time, the characteristic peaks of unreacted ye’elimite and anhydrite were also detected. Generally, the greater the intensity of the crystal diffraction peak, the more the crystals. Comparing the intensity of (100) crystal plane diffraction peak of ettringite crystal in the three groups of samples, it can be seen that the amount of ettringite in the two groups of samples doped with PCE and FDN was higher than that in the blank sample, indicating that PCE and FDN can accelerate the early hydration reaction rate of USCBGM to a certain extent. Moreover, FDN played a stronger role in promoting hydration compared with PCE. Figure 7(b) shows the amount of ettringite in the three groups of samples increased further after 7 d of age. The ettringite content in the three groups was similar by comparison to peak strength. Accordingly, anhydrite and ye’elimite were largely consumed due to the in-depth hydration reaction. After 7 d of age, only weak anhydrite characteristic peaks were detected in each sample. The characteristic peak of ye’elimite was not detected, which indicated that it has been exhausted.Figure 7
XRD spectra of each sample: (a) 4 h and (b) 7 d.
(a)(b)


## 3.5. Thermogravimetric Analysis

The thermal analysis test results of each sample at the age of 4 h and 7 d are shown in Figure8. The endothermic peak of ettringite was detected near 120°C in all samples after hydration of 4 h. In addition, a few endothermic peaks of AFm and AH3 hydration products were detected near 150°C and 250°C, respectively. According to TG curve data, the amount of ettringite in the three groups of samples can be calculated by using the method described in literature [32]. The calculated results showed that the yield of ettringite was 51.4%, 55.5%, and 56.2% for blank sample, sample doped with PCE, and sample doped with FDN after 4 h hydration, respectively. It can be seen the amount of ettringite in the samples mixed with PCE and FDN was significantly higher than that of the blank sample. Moreover, the amount of ettringite in the sample doped with FDN was slightly higher than that of the sample doped with PCE. This further showed that the addition of PCE and FDN can accelerate the early hydration reaction of USCBGM. The amount of ettringite in each sample further increased after 7 d. Moreover, the amount of ettringite in each sample was the same as the weight loss curve. This was consistent with the test results of XRD. In addition, after 7 d of age, AH3 was still detected in each sample, but the endothermic peak of AFm disappeared, which may be caused by its transformation into ettringite crystals.Figure 8
Thermal analysis test results of each sample: (a) 4 h and (b) 7 d.
(a)(b)


## 3.6. Scanning Electron Microscopy

Figure9 shows the micrographs of each sample at the age of 4 h and 7 d. It can be seen from Figures 9(a)–9(c) that a large number of fine-needle ettringite crystals were generated in the three groups of samples after hydration for 4 h. For the blank sample and the sample doped with PCE, the ettringite production obviously had a high degree of overlapping and interleaving, which was very beneficial to fully develop the strength skeleton effect of ettringite crystals and improve the early strength of hardened paste. However, different from the blank sample and the sample doped with PCE, the ettringite produced in the sample doped with FDN tended to adhere to the clinker particles and grow in clusters. This distribution characteristic of ettringite crystals was obviously not conducive to the exertion of its skeleton effect. Accordingly, the early strength of the sample doped with FDN was significantly lower. Figures 9(d)–9(f) show the micrographs of three groups of samples at the hydration age of 7 d. It can be seen that the amount of ettringite in each sample increases further after hydration to 7 d. The blank sample and the sample doped with PCE obviously became denser. Ettringite crystals formed in the sample doped with FDN were still distributed in clusters after 7 d of hydration, which reflected poor overlap between ettringite crystals and relatively lower 7 d strength of hardened paste.Figure 9
SEM photograph for each sample: (a) blank sample for 4 h; (b) sample doped with PCE for 4 h; (c) sample doped with FDN for 4 h; (d) blank sample for 7 d; (e) sample doped with PCE for 7 d; (f) sample doped with FDN for 7 d.
(a)(b)(c)(d)(e)(f)


## 4. Discussion

As a kind of sulfoaluminate cement-based material, the mechanical properties of USCBGM were closely related to the formation law of ettringite hydration products. Generally, the greater the ettringite concentration, the higher the strength of the hardened paste. However, Figures6–8 show that it was found that the amount of ettringite in the sample doped with FDN was higher than that of the blank sample after hydration of 4 h, but its compressive strength was significantly lower. It can be inferred that the amount of ettringite was not the only factor determining the physical properties of the hardened paste. In fact, the mechanical properties of USCBGM hardened paste were also related to the distribution of ettringite. According to SEM test results, ettringite generated in USCBGM was mainly distributed in clusters after the addition of FDN, which significantly reduced the overall lap degree between the hydration products of each crystal, resulted in the difficulty of crystal skeleton effect of ettringite, and markedly reduced the mechanical properties of the hardened paste. Similar to FDN, PCE can also increase the amount of ettringite in hardened paste in the early stage. However, the addition of PCE did not affect the crystal skeleton effect of ettringite in the hardened paste. It was due to these effects of PCE that the early strength of USCBGM hardened paste was improved to some extent.The reason why superplasticizer can significantly affect the formation and distribution of ettringite in USCBGM was mainly related to its action mechanism. Now it was explained through three schematic diagrams. Figure10 shows the formation mechanism of ettringite in the blank sample. It was well known that the formation of ettringite mainly followed the liquid-phase dissolution precipitation theory in sulfoaluminate cement-based material system. Therefore, the formation process of ettringite crystal can be described as follows in the blank sample. First, AlO2− ions released from sulfoaluminate cement clinker particles are combined with two OH− ions and two H2O molecules to form [Al(OH)6]3-. Then, the {Ca6[Al(OH)6]224H2O}6+ is formed by the combination of [Al(OH)6]3- with Ca2+ and H2O molecules in the liquid phase. Finally, the {Ca6[Al(OH)6]224H2O}6+ is further combined with the SO42− ions released by the dissolution of anhydrite to form a complete ettringite crystal. Due to the slow dissolution rate of anhydrite, the concentration of SO42− ions in the liquid phase was low and the diffusion potential energy was relatively small, which was difficult to diffuse around the clinker particles in a short time. At this time, the {Ca6[Al(OH)6]224H2O}6+ formed near the clinker particles can have sufficient time to diffuse to the far liquid-phase region and combine with SO42− ions to form an ettringite crystal nucleus [15]. The ettringite crystals formed in the liquid phase showed a high degree of interlacing growth and precipitation due to the lack of crystalline-attached particles. In addition, due to the agglomeration effect between the ultrafine particles in the blank sample, the reaction particles cannot fully contact and dissolve with water, which affects the formation rate of ettringite in the blank sample at the early hydration stage to a certain extent.Figure 10
Schematic diagram of ettringite formation in blank sample.The schematic diagram of ettringite formation in the sample doped with PCE is shown in Figure11. PCE can be preferentially adsorbed on the surface of clinker particles with strong activity when it was added to USCBGM. Due to the complex comb-like structure of PCE molecules, it cannot significantly affect the dissolution of ye’elimite included in clinker particles and the formation of {Ca6[Al(OH)6]224H2O}6+ ions in the liquid phase. In addition, due to the hydrophobic effect of the PCE molecular branch, it was difficult for SO42− ions in the liquid phase to diffuse to the liquid phase on the surface of clinker particles. At the same time, the SO42− ions released by the dissolution of anhydrite in the liquid phase will be difficult to quickly diffuse into the liquid phase around the clinker particles due to the low diffusion potential energy and the hydrophobic effect of the PCE molecular branch chain. Similarly, the {Ca6[Al(OH)6]224H2O}6+ ions formed in the liquid phase around the clinker particles can be able to diffuse to a long distance, combine with SO42− ions to form ettringite crystals, and exhibit intricate staggered distribution.Figure 11
Schematic diagram of ettringite formation in sample doped with PCE.The schematic diagram of ettringite formation in a sample doped with FDN is shown in Figure12. Similar to PCE, the FDN can also preferentially adsorb on the surface of highly active clinker particles when it was added to the USCBGM. Unlike PCE, FDN molecules had a straight-chain structure and small molecular weight. At the initial stage of hydration, FDN molecules can be adsorbed on the surface of clinker particles in horizontal adsorption form, which seriously hindered the dissolution of ye’elimite minerals and the formation of {Ca6[Al(OH)6]224H2O}6+. At this time, the SO42− ions released from anhydrite dissolution into the liquid phase can have enough time to diffuse around the clinker particles and combine with the {Ca6[Al(OH)6]224H2O}6+ formed in the liquid phase on the surface of the clinker particles to form ettringite crystal nucleus. Due to the close proximity to the clinker particles, ettringite nuclei will adhere to the surface particles of the clinker particles and develop and grow in obvious clusters.Figure 12
Schematic diagram of ettringite formation in sample doped with FDN.


## 5. Conclusions

In this study, the effect and mechanism of PCE and FDN superplasticizers on the performance of USCBGM were studied. Based on the results of the investigation, the following conclusions can be drawn:(1)
Both PCE and FDN can effectively improve the initial fluidity of USCBGM paste. The modification effect of PCE was significantly better than that of FDN under the condition of saturated content.(2)
The initial setting time and final setting time of USCGBM paste were only 6 min and 10 min, respectively. The incorporation of PCE had no significant effect on the setting time of the USCBGM paste. However, the addition of FDN can significantly prolong the setting time of the paste. The higher the content of FDN, the longer the setting time was. The initial setting time and final setting time of paste were prolonged by 270 s and 156 s, respectively, when the amount of FDN reached the saturated content.(3)
The addition of PCE can accelerate the hydration reaction rate of USCBGM paste and improve the production of ettringite in the early hydration stage. Accordingly, the early strength of hardened paste increased with the addition of PCE. Similar to PCE, FDN can also increase the formation of ettringite in the hardened paste at early hydration age. Unlike PCE, the incorporation of FDN can lead to the cluster distribution of ettringite, which affected the exertion of the ettringite crystal skeleton effect, resulting in a significant reduction in the early strength of hardened paste.



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*Source: 3836517-2022-04-16.xml*