Research on the Working Performance and the Corresponding Mechanical Strength of Polyaluminum Sulfate Early Strength Alkali-Free Liquid Accelerator Matrix Cement

Liquid accelerating agents have the advantages of simple operation and fast construction, and have become indispensable admixtures in shotcrete. However, most liquid accelerating agents in the market at present contain alkali or fluorine, which adversely affect concrete and seriously threaten the physical and mental health of workers. Therefore, in view of the above deficiencies, it is necessary to develop a new type of alkali-free fluorine-free liquid accelerating agent. In this paper, the polyaluminum sulfate early strength alkali-free liquid accelerator is prepared using polymeric aluminum sulfate, diethanolamine, magnesium sulfate heptahydrate and nano-silica. The influence of this agent on the setting time of fresh cement paste and compressive strength of the corresponding cement mortar is determined. Thermogravimetric analysis curves, X-ray diffraction and scanning electron microscopy images are obtained to investigate the mechanism. Findings show that the initial setting time and the final setting time of cement paste are 2 min 30 s and 7 min 25 s. The compressive strengths of cement mortar cured for 1 d, 28 d and 90 d are 2.4 MPa, 52.2 MPa and 54.3 MPa respectively. Additionally, the corresponding flexural strengths are 3.4 MPa, 9.8 MPa, 11.8 MPa. When the mass rate of accelerator is 7%, the mechanical strengths of cement mortar are the highest. The additions of fly ash and blast furnace slag can affect the mechanical of cement mortar mixed with accelerator. When the mass ratio of the fly ash and blast furnace slag is 15%, the mechanical strengths of cement mortar reach the highest. Moreover, the hydration heat release rate of cement is increased by the accelerator and the corresponding time of hydration heat peak is decreased by the accelerator. The accelerator can decrease the amount of needle-like hydration products and improve the compactness. The mechanical strengths are improved by consuming a large amount of Ca(OH)2 and forming more compact hydration products. It is recommended that the optimum dosage range of the polyaluminum sulfate early strength alkali-free liquid accelerator is 7%.


Introduction
A quick-setting agent is an admixture that can quickly condense and harden concrete, which is widely used in tunnels, mines and other projects [1][2][3][4]. In recent years, with the development of the spray concrete wet spraying process [5,6], liquid quick-setting agent has become an indispensable material in the construction of sprayed concrete, which has the characteristics of significantly reducing the amount of rebound and dust concentration in the spraying construction process. Moreover, the thickness of single injection is increased and the speed of concrete condensation and hardening accelerated, gradually replacing the past powder quick-setting agent [7][8][9] From the perspective of green environmental protection, energy conservation and emission reduction, it is imperative to use large-scale wet spraying for the construction of sprayed concrete, and the demand for alkali-free liquid fast-setting agent has shown explosive growth [10][11][12][13]. Alkali-free liquid quicksetting agent has the advantages of green, environmental protection and high efficiency, The water-reducing agent is polycarboxylate superplasticizer (water reduction of 25%) produced by Jiangsu Subote New Materials Co., Ltd., Nanjing, China and naphthalene superplasticizer (water reduction of 20%) produced by Hebei Guangming Chemical Technology Co., Ltd., Shijiazhuang, China. The experimental water is deionized water produced by Shanghai Chuangsai Technology Co., Ltd., Shanghai, China. The density of deionized water was 1.000 g/mL.

Sample Preparation
The preparing method of the new alkali-free and fluorine-free liquid accelerator is shown in Figure 1. Each component is weighed according to the mass percentage, and the proportions. The mass ratio of polyaluminum sulfate: diethanolamine: ethylene glycol: magnesium sulfate heptahydrate: nano-silica: QM stabilizer: dilute sulfuric acid: water is 57:6:1:2:2:3:1:28. The new alkali-free and fluorine-free liquid accelerator is made by the following steps.
Secondary fly ash with the specific surface area of 386 m 2 /kg produced by Beijing Jingyeda New Building Materials Co., Ltd., Beijing, China and S95 blast furnace slag powder (BFS) (specific surface area of 452 m 2 /kg) produced by Jintaicheng Technology Group Co., Ltd., Shahe, China are used as mineral admixture. The chemical compositions of different type of cement are shown in Table 1. The water-reducing agent is polycarboxylate superplasticizer (water reduction of 25%) produced by Jiangsu Subote New Materials Co., Ltd., Nanjing, China and naphthalene superplasticizer (water reduction of 20%) produced by Hebei Guangming Chemical Technology Co., Ltd., Shijiazhuang, China. The experimental water is deionized water produced by Shanghai Chuangsai Technology Co., Ltd., Shanghai, China. The density of deionized water was 1.000 g/mL.

Sample Preparation
The preparing method of the new alkali-free and fluorine-free liquid accelerator is shown in Figure 1. Each component is weighed according to the mass percentage, and the proportions. The mass ratio of polyaluminum sulfate: diethanolamine: ethylene glycol: magnesium sulfate heptahydrate: nano-silica: QM stabilizer: dilute sulfuric acid: water is 57:6:1:2:2:3:1:28. The new alkali-free and fluorine-free liquid accelerator is made by the following steps. The fresh cement paste can be produced by the following steps. NJ-160 cement paste mixer produced by Wuxi Jianyi Instrument Machinery Co., Ltd., Wuxi, China is used to stir the cement paste. The stirring process can be divided into the following steps. Firstly, the weighed gelling material and water are added into the stirring pot then 30 s stirring with the speed of 62 r/min. When the stirring is finished, the liquid accelerator is injected with a 50 mL syringe to the cement paste mixer and stirred for 1 min. After the mixing is completed, the fresh cement paste is used for the measurement of the setting time by following the process in Chinese standard GB/T 35159-2017 [32]. JJ-5 cement mortar mixer produced by Wuxi Jianyi Instrument Machinery Co., Ltd., Wuxi, China is used for stirring the cement mortar. Firstly, the weighed cementitious material is put into the mixer and mixed at a speed of 140 r/min for 1 min, then the uniform mixed water and accelerator is The fresh cement paste can be produced by the following steps. NJ-160 cement paste mixer produced by Wuxi Jianyi Instrument Machinery Co., Ltd., Wuxi, China is used to stir the cement paste. The stirring process can be divided into the following steps. Firstly, the weighed gelling material and water are added into the stirring pot then 30 s stirring with the speed of 62 r/min. When the stirring is finished, the liquid accelerator is injected with a 50 mL syringe to the cement paste mixer and stirred for 1 min. After the mixing is completed, the fresh cement paste is used for the measurement of the setting time by following the process in Chinese standard GB/T 35159-2017 [32]. JJ-5 cement mortar mixer produced by Wuxi Jianyi Instrument Machinery Co., Ltd., Wuxi, China is used for stirring the cement mortar. Firstly, the weighed cementitious material is put into the mixer and mixed at a speed of 140 r/min for 1 min, then the uniform mixed water and accelerator is added and stirred at a speed of 140 r/min for another 1 min. Finally, the standard sand is added and 2 min stirring with the speed of 285 r/min is carried out. When the stirring process is finished, the fresh mixture is poured into molds sized 40 × 40 × 160 mm 3 . The mixing proportion of specimens is shown in Table 2. The compressive strength of cement mortar is tested by using the YAW-300 microcomputer controlled electrohydraulic servo pressure testing machine produced by Shanghai Sansi Hengheng Machinery Manufacturing Co., Ltd., Shanghai, China. Three samples are required for the flexural experiment and six samples are required for the compressive test. The loading rates of the flexural and compressive strengths are 0.01 kN/s and 2.4 kN/s respectively. The measuring process is carried out referring to the Chinese standard GB/T 17671-1999 [33].

Measurement of Hydration Heat
In order to test the hydration heat, the liquid accelerator with different dosage is mixed with water to prepare a mixed solution, then weighed cement is put into the sample bottle of isothermal calorimeter to mix evenly and is used for testing. The measuring process is carried out following the Chinese standard GB/T 12959-2008 [34].

Measurement of SEM and XRD
The hardened cement paste with smooth surface at different curing ages is immersed in the absolute ethanol to prevent hydration. The samples are dried in a 60 • C drying oven for 3 days until the mass is constant. When these steps are finished, the samples of soybean size are sprayed with gold and moved for measurement with SEM. SEM model geminisem300 (Zeiss), producing area for Oberkochen, Germany. Some dried samples are ground and sieved through a 0.075 mm square-hole sieve. The measuring process is implemented according to the Chinese standard SY/T 5162-1997 [35].

Basic Properties of Cement Paste with Accelerator
The initial setting time and the final setting time of fresh cement paste are shown in Figure 2. As shown in Figure 2, the initial setting time and the final setting time decrease with the increasing dosages of the accelerator. As depicted in Figure 2, when the dosage of accelerating agent is 0%, the initial setting time is 198 min and the final setting time is 259 min. When the dosage is 3%, the setting time has an obvious downward trend, showing the initial and final setting time of 36.4 min and 123 min, with decreasing rates of 81.6% and 52.5%, respectively. The dosage of 4% is the threshold value. This is ascribed to the fact that when the dosage of accelerator is 3%, the polyaluminum sulfate is consumed in the hydration of cement, and the Ca 2+ in cement minerals is not completely consumed. At the same time, the trisulfide hydrated calcium sulfoaluminate and hydrated calcium silicate generated cannot make the slurry set rapidly. Zhang et al. obtained a similar conclusion that the low dosage of accelerating agent resulted in less hydration production and the coagulation-promoting effect was not obvious [36,37].

Measurement of SEM and XRD
The hardened cement paste with smooth surface at different curing ages is imme in the absolute ethanol to prevent hydration. The samples are dried in a 60 °C drying for 3 days until the mass is constant. When these steps are finished, the samples of soy size are sprayed with gold and moved for measurement with SEM. SEM model g nisem300 (Zeiss), producing area for Oberkochen, Germany. Some dried samples ground and sieved through a 0.075 mm square-hole sieve. The measuring process is plemented according to the Chinese standard SY/T 5162-1997 [35].

Basic Properties of Cement Paste with Accelerator
The initial setting time and the final setting time of fresh cement paste are show Figure 2. As shown in Figure 2, the initial setting time and the final setting time decr with the increasing dosages of the accelerator. As depicted in Figure 2, when the do of accelerating agent is 0%, the initial setting time is 198 min and the final setting tim 259 min. When the dosage is 3%, the setting time has an obvious downward trend, sh ing the initial and final setting time of 36.4 min and 123 min, with decreasing rates of 8 and 52.5%, respectively. The dosage of 4% is the threshold value. This is ascribed to fact that when the dosage of accelerator is 3%, the polyaluminum sulfate is consume the hydration of cement, and the Ca 2+ in cement minerals is not completely consume the same time, the trisulfide hydrated calcium sulfoaluminate and hydrated calcium cate generated cannot make the slurry set rapidly. Zhang et al. obtained a similar con sion that the low dosage of accelerating agent resulted in less hydration production the coagulation-promoting effect was not obvious [36,37].     Figure 3, the compressive and flexural strengths of cement mortar increase with the increasing curing age and firstly increases and then decreases with the increasing addition of accelerator. When the dosage of accelerator is 7%, the mechanical strengths of the cement mortar is the highest. This is attributed to the fact that when the dosage of accelerator increases from 0% to 7%, a large amount of Al 3+ and SO 4 2− is increased by polyaluminum sulfate hydrolysis in the accelerator, which reacts quickly with Ca 2+ produced by hydration of cement clinker and generates a large amount of needle-like hydrated 3CaO·Al 2 O 3 , forming a skeleton structure. Therefore, the mechanical strengths are increased. However, when the increasing dosages of accelerator is higher than 7%, the excessive SO 4 2− induced by accelerator can react with Ca 2+ , forming a large amount of dihydrate gypsum. The newly generated dihydrate gypsum can continue to react to generate a large amount of expansive hydrated calcium sulfoaluminate; therefore, the mechanical strengths are decreased by the addition of accelerator [38][39][40]. The error values are small and the test results are accurate. yaluminum sulfate hydrolysis in the accelerator, which reacts quickly with Ca produced by hydration of cement clinker and generates a large amount of needle-like hydrated 3CaO·Al2O3, forming a skeleton structure. Therefore, the mechanical strengths are increased. However, when the increasing dosages of accelerator is higher than 7%, the excessive SO4 2-induced by accelerator can react with Ca 2+ , forming a large amount of dihydrate gypsum. The newly generated dihydrate gypsum can continue to react to generate a large amount of expansive hydrated calcium sulfoaluminate; therefore, the mechanical strengths are decreased by the addition of accelerator [38][39][40]. The error values are small and the test results are accurate.

Influence of Fly Ash on Basic and Compressive Performances
The setting time of fresh cement paste with different dosage of fly ash and 7% accelerator is shown in Figure 4. As depicted in Figure 4, the initial and final setting time increase with the equation of the mass ratio of fly ash. This is attributed to the fact that the contents of 3CaO·SiO2, 2CaO·SiO2,3CaO·Al2O3 and 4CaO·Al2O3·Fe2O3 decrease with the addition of fly ash [41]. Moreover, as reported in prior research, fly ash contains active SiO2 and active Al2O3, and hydration reaction can occur only under the excitation of alkaline conditions, which makes the early activity of fly ash difficult to excite [42]. Therefore, the hydration rate of cement is decreased with the increasing dosages of fly ash, leading eventually to increasing the setting time, as observed in Figure 4, and relationships between the setting time and the mass rate of fly ash can be deduced as cubic functions.

Influence of Fly Ash on Basic and Compressive Performances
The setting time of fresh cement paste with different dosage of fly ash and 7% accelerator is shown in Figure 4. As depicted in Figure 4, the initial and final setting time increase with the equation of the mass ratio of fly ash. This is attributed to the fact that the contents of 3CaO·SiO 2 , 2CaO·SiO 2 ,3CaO·Al 2 O 3 and 4CaO·Al 2 O 3 ·Fe 2 O 3 decrease with the addition of fly ash [41]. Moreover, as reported in prior research, fly ash contains active SiO 2 and active Al 2 O 3 , and hydration reaction can occur only under the excitation of alkaline conditions, which makes the early activity of fly ash difficult to excite [42]. Therefore, the hydration rate of cement is decreased with the increasing dosages of fly ash, leading eventually to increasing the setting time, as observed in Figure 4, and relationships between the setting time and the mass rate of fly ash can be deduced as cubic functions.
by hydration of cement clinker and generates a large amount of needle-like hydrated 3CaO·Al2O3, forming a skeleton structure. Therefore, the mechanical strengths are in creased. However, when the increasing dosages of accelerator is higher than 7%, the ex cessive SO4 2-induced by accelerator can react with Ca 2+ , forming a large amount of dihy drate gypsum. The newly generated dihydrate gypsum can continue to react to generate a large amount of expansive hydrated calcium sulfoaluminate; therefore, the mechanica strengths are decreased by the addition of accelerator [38][39][40]. The error values are smal and the test results are accurate.

Influence of Fly Ash on Basic and Compressive Performances
The setting time of fresh cement paste with different dosage of fly ash and 7% accel erator is shown in Figure 4. As depicted in Figure 4, the initial and final setting time increase with the equation of the mass ratio of fly ash. This is attributed to the fact that the contents of 3CaO·SiO2, 2CaO·SiO2,3CaO·Al2O3 and 4CaO·Al2O3·Fe2O3 decrease with the addition of fly ash [41]. Moreover, as reported in prior research, fly ash contains active SiO2 and active Al2O3, and hydration reaction can occur only under the excitation of alka line conditions, which makes the early activity of fly ash difficult to excite [42]. Therefore the hydration rate of cement is decreased with the increasing dosages of fly ash, leading eventually to increasing the setting time, as observed in Figure 4, and relationships be tween the setting time and the mass rate of fly ash can be deduced as cubic functions.    [43]. The cement mortar with 15% fly ash shows the maximum compressive and flexural strengths. As illustrated in Figure 5, the values of error bars are lower than 0.09, indicating that the test results are accurate. Figure 5 shows the compressive and flexural strengths of cement mortar with different dosages of fly ash. As illustrated in Figure 5, the compressive and flexural strengths of cement mortar cured for 1 day decrease with the increasing mass of fly ash. Meanwhile, when the curing age is 28 days, the compressive and flexural strengths of cement mortar firstly increase and then decrease with the increasing dosage of fly ash. Moreover, the pozzolanic activity of fly ash can lead to secondary hydration reaction, enhancing the late strength and making up for the loss of compressive strength of mortar [43]. The cement mortar with 15% fly ash shows the maximum compressive and flexural strengths. As illustrated in Figure 5, the values of error bars are lower than 0.09, indicating that the test results are accurate.

Influence of Blast Furnace Slag on Basic and Compressive Performances
The initial setting time and final setting time of fresh cement paste with different dosages of blast furnace slag are shown in Figure 6. In this part, the accelerator is kept at 7% by mass of binder materials. It can be observed from Figure 6, the initial setting time and final setting time increase in the form of cubic function with the mass ratio of blast furnace slag, due to the decreased hydration rate by the addition of BFS, which is similar to the reason for fly ash.

Influence of Blast Furnace Slag on Basic and Compressive Performances
The initial setting time and final setting time of fresh cement paste with different dosages of blast furnace slag are shown in Figure 6. In this part, the accelerator is kept at 7% by mass of binder materials. It can be observed from Figure 6, the initial setting time and final setting time increase in the form of cubic function with the mass ratio of blast furnace slag, due to the decreased hydration rate by the addition of BFS, which is similar to the reason for fly ash. ent dosages of fly ash. As illustrated in Figure 5, the compressive and flexural strength of cement mortar cured for 1 day decrease with the increasing mass of fly ash. Meanwhile when the curing age is 28 days, the compressive and flexural strengths of cement morta firstly increase and then decrease with the increasing dosage of fly ash. Moreover, th pozzolanic activity of fly ash can lead to secondary hydration reaction, enhancing the lat strength and making up for the loss of compressive strength of mortar [43]. The cemen mortar with 15% fly ash shows the maximum compressive and flexural strengths. As il lustrated in Figure 5, the values of error bars are lower than 0.09, indicating that the tes results are accurate.

Influence of Blast Furnace Slag on Basic and Compressive Performances
The initial setting time and final setting time of fresh cement paste with differen dosages of blast furnace slag are shown in Figure 6. In this part, the accelerator is kept a 7% by mass of binder materials. It can be observed from Figure 6, the initial setting tim and final setting time increase in the form of cubic function with the mass ratio of blas furnace slag, due to the decreased hydration rate by the addition of BFS, which is simila to the reason for fly ash.    Figure 7 shows the compressive and flexural strengths of cement mortar with 7% accelerator and different dosages of blast furnace slag. It can be observed from Figure 7 that the compressive and flexural strengths of cement mortar cured at 1 day decrease with the increasing dosage of BFS. However, when the curing age is 28 d, the compressive and flexural strengths firstly increase as the dosage of BFS increases from 0% to 15%. Meanwhile, when the dosage of BFS increases from 15% to 25%, the compressive and flexural strengths of cement mortar cured for 28 days decrease. Prior research points out that with the decreasing addition of slag, the amount of hydration products of early cement lead to reducing the early strength of cement. Moreover, with the increase of curing age, the active substances in the slag undergo secondary hydration in the alkaline environment, resulting in hydration products, which improves the compressive strength of the mortar in the later stage [44,45]. The error bars of 1 d compressive strength and flexural strength are lower than 0.8, which ensures the accuracy of the research results. the increasing dosage of BFS. However, when the curing age is 28 d, the compressive and flexural strengths firstly increase as the dosage of BFS increases from 0% to 15%. Meanwhile, when the dosage of BFS increases from 15% to 25%, the compressive and flexural strengths of cement mortar cured for 28 days decrease. Prior research points out that with the decreasing addition of slag, the amount of hydration products of early cement lead to reducing the early strength of cement. Moreover, with the increase of curing age, the active substances in the slag undergo secondary hydration in the alkaline environment, resulting in hydration products, which improves the compressive strength of the mortar in the later stage [44,45].

Heat of Hydration Analysis
According to the above results, the cement mortar with 7% accelerator shows the maximum mechanical strengths; therefore, in this part, cement mortar with 7% accelerator is selected for the measurement of heat of hydration. The hydration heat release rate curves are illustrated in Figure 8. It can be observed from Figure 8, the shapes of the hydration exothermic curves with 0% and 7% accelerator are similar. The first exothermic peak of the hydration exothermic curves with 0% and 7% accelerator appears in 3.66 min and 3.54 min, while the corresponding second exothermic peak appears in 9.03 h and 8.2 h. As shown in Figure 8, the first peak values of the two hydration exothermic curves are 120.73 m·Wg −1 and 80.07 m·Wg −1 , respectively. Meanwhile, the second peak values of the two hydration exothermic curves are 3.96 m·Wg −1 and 4.08 m·Wg −1 , respectively. The main reason is that Al 3+ and SO4 2-in the accelerator consume Ca 2+ in the liquid phase, leading to generating ettringite and releasing more heat and promoting the hydration of cement [44].

Heat of Hydration Analysis
According to the above results, the cement mortar with 7% accelerator shows the maximum mechanical strengths; therefore, in this part, cement mortar with 7% accelerator is selected for the measurement of heat of hydration. The hydration heat release rate curves are illustrated in Figure 8. It can be observed from Figure 8, the shapes of the hydration exothermic curves with 0% and 7% accelerator are similar. The first exothermic peak of the hydration exothermic curves with 0% and 7% accelerator appears in 3.66 min and 3.54 min, while the corresponding second exothermic peak appears in 9.03 h and 8.2 h. As shown in Figure 8, the first peak values of the two hydration exothermic curves are 120.73 m·Wg −1 and 80.07 m·Wg −1 , respectively. Meanwhile, the second peak values of the two hydration exothermic curves are 3.96 m·Wg −1 and 4.08 m·Wg −1 , respectively. The main reason is that Al 3+ and SO 4 2− in the accelerator consume Ca 2+ in the liquid phase, leading to generating ettringite and releasing more heat and promoting the hydration of cement [44]. The total hydration heat release of cement is illustrated in Figure 9. It can be observed from Figure 9, the hydration heat release firstly increases sharply and then increases steadily with the time. This is attributed to the fact that the hydration rate of cement is relatively fast when at the early curing age. However, with the increasing curing age, the hydration rate can descend. The samples with 7% agent show higher hydration heat release than the The total hydration heat release of cement is illustrated in Figure 9. It can be observed from Figure 9, the hydration heat release firstly increases sharply and then increases steadily with the time. This is attributed to the fact that the hydration rate of cement is relatively fast when at the early curing age. However, with the increasing curing age, the hydration rate can descend. The samples with 7% agent show higher hydration heat release than the blank samples. This is ascribed to the fact that the addition of accelerator leads to increasing the hydration reaction of 3CaO·Al 2 O 3 in cement. Moreover, due to the pozzolanic activity of ultrafine nano-silica in the accelerator, the accelerator can react with Ca(OH) 2 generating hydrated calcium silicate gel and improving the hydration rate [45]. Therefore, the total amount of hydration heat release of the experimental group is higher than that of the blank group. The total hydration heat release of cement is illustrated in Figure 9. It can be observe from Figure 9, the hydration heat release firstly increases sharply and then increases stead ily with the time. This is attributed to the fact that the hydration rate of cement is relativel fast when at the early curing age. However, with the increasing curing age, the hydratio rate can descend. The samples with 7% agent show higher hydration heat release than th blank samples. This is ascribed to the fact that the addition of accelerator leads to increas ing the hydration reaction of 3CaO·Al2O3 in cement. Moreover, due to the pozzolanic ac tivity of ultrafine nano-silica in the accelerator, the accelerator can react with Ca(OH) generating hydrated calcium silicate gel and improving the hydration rate [45]. Therefore the total amount of hydration heat release of the experimental group is higher than tha of the blank group.  Figure 10 shows the X-ray diffraction spectrum of blank cement paste and cemen paste with 7% accelerator. It can be depicted in Figure 10, the 3CaO·SiO2, 2CaO·SiO 3CaO·Al2O3 and 4CaO·Al2O3·Fe2O3 can be found [46][47][48]. With the extension of curing age the characteristic diffraction peaks of Ca(OH)2 became more obvious. Obviou 3CaO·Al2O3 diffraction peaks can be found in the samples with accelerator at all curin ages, indicating that a certain amount of 3CaO·Al2O3 is formed at 6 min of hydration However, in the blank group, the aft diffraction peak could not be found until the curin age is 6 h.  Figure 10 shows the X-ray diffraction spectrum of blank cement paste and cement paste with 7% accelerator. It can be depicted in Figure 10, the 3CaO·SiO 2 , 2CaO·SiO 2 , 3CaO·Al 2 O 3 and 4CaO·Al 2 O 3 ·Fe 2 O 3 can be found [46][47][48]. With the extension of curing age, the characteristic diffraction peaks of Ca(OH) 2 became more obvious. Obvious 3CaO·Al 2 O 3 diffraction peaks can be found in the samples with accelerator at all curing ages, indicating that a certain amount of 3CaO·Al 2 O 3 is formed at 6 min of hydration. However, in the blank group, the aft diffraction peak could not be found until the curing age is 6 h. Figure 11 shows the scanning electron microscopy (SEM) photos of the blank specimens and the specimens with 7% accelerator cured for 6 min, 6 h, 1 d, 3 d and 28 d, respectively. It can be observed in Figure 11 that with increasing curing age, the amount of needle-like products decreases and the hexagonal flake and flocculent products increases, due to improved hydration degree. As depicted in Figure 11, the addition of accelerator leads to decreasing the needle-like products and improving the compactness of the hydration products at early curing age (the curing age is lower than 3 d). However, when the curing age is 28 d, little difference can be found in the SEM photos of the blank specimens and the specimens with 7% accelerator. This is because the accelerator can accelerate the early hydration process of the cement.  Figure 10. X-ray diffraction patterns of specimens. (a) X-ray diffraction p X-ray diffraction pattern of the sample with 7% accelerator. Figure 11 shows the scanning electron microscopy (SEM) ph mens and the specimens with 7% accelerator cured for 6 min, 6 spectively. It can be observed in Figure 11 that with increasing cu needle-like products decreases and the hexagonal flake and floccu due to improved hydration degree. As depicted in Figure 11, the leads to decreasing the needle-like products and improving the dration products at early curing age (the curing age is lower tha the curing age is 28 d, little difference can be found in the SEM ph

Conclusions
The purpose of this paper was to study the basic properties and micromechanisms of polyaluminum sulfate early strength alkali-free and fluorine-free liquid accelerator. The following conclusions are drawn.
The The setting time and mechanical strengths of cement past and cement mortar are affected obviously by the additions of accelerant, fly ash and blast furnace slag, where the dosages of accelerant, fly ash and blast furnace slag are 7%, 15% and 15%, respectively, and the mechanical strengths are the highest.
From the results of microscopic analysis, the addition of accelerating agent increases the hydration heat release rate of cement hydration and the overall hydration heat release in the hydration process. The accelerator can decrease the amount of needle like hydration products and improve the compactness. The mechanical strengths are improved by consuming a large amount of Ca(OH)2 and forming more compact hydration products.
This study has found a new type of early strength alkali-free liquid accelerator and the optimum dosages in the cement matrix. This technique will be applied in actual projects in the future. These will provide technical support and theoretical basis for the preparation and application of alkali-free liquid accelerators in the future.

Conclusions
The purpose of this paper was to study the basic properties and micromechanisms of polyaluminum sulfate early strength alkali-free and fluorine-free liquid accelerator. The following conclusions are drawn.
The The setting time and mechanical strengths of cement past and cement mortar are affected obviously by the additions of accelerant, fly ash and blast furnace slag, where the dosages of accelerant, fly ash and blast furnace slag are 7%, 15% and 15%, respectively, and the mechanical strengths are the highest.
From the results of microscopic analysis, the addition of accelerating agent increases the hydration heat release rate of cement hydration and the overall hydration heat release in the hydration process. The accelerator can decrease the amount of needle like hydration products and improve the compactness. The mechanical strengths are improved by consuming a large amount of Ca(OH) 2 and forming more compact hydration products.
This study has found a new type of early strength alkali-free liquid accelerator and the optimum dosages in the cement matrix. This technique will be applied in actual projects in the future. These will provide technical support and theoretical basis for the preparation and application of alkali-free liquid accelerators in the future.