Effects of Superplasticizer on the Hydration, Consistency, and Strength Development of Cemented Paste Backfill

Jian Zhang 1,2 ID , Hongwei Deng 1, Abbas Taheri 2, Junren Deng 1,* and Bo Ke 3 1 School of Resources and Safety Engineering, Central South University, Changsha 410083, China; zhangj@csu.edu.cn (J.Z.); denghw208@126.com (H.D.) 2 School of Civil, Environmental and Mining Engineering, The University of Adelaide, Adelaide 5005, Australia; abbas.taheri@adelaide.edu.au 3 School of Resource and Environment Engineering, Wuhan University of Technology, Wuhan 430070, China; boke@whut.edu.cn * Correspondence: 0201140113@csu.edu.cn


Introduction
Large amounts of mining tailings are deposited after useful minerals have been extracted by the ore processing plant.In China, 1.2 billion tons of mining tailings are deposited every year.These solid wastes do great harm to the environment, therefore, the mining tailings should be managed properly to protect the environment [1].Some scholars suggested that one of the most valuable ways to re-use tailings is to generate cemented paste backfill (CPB) materials in underground mines [2].In practical engineering, CPB is the main method to fill the underground void spaces and support the open stopes.CPB has been widely employed in many mines around the world [3,4].
CPB material is often made by an engineered mixture of binder, tailings, and water.Ordinary Portland Cement (OPC) is often used as the main binder.However, in some cases, fly ash or blast furnace slag substitute part of the OPC to reduce the costs and improve the performance [5][6][7].It has been demonstrated that the addition of fly ash increases the amount of calcium silica hydrates (C-H-S), which may contribute to the strength development of the CPB mixture [8].

Materials and Methods
In this study, full mining tailings from the Shuiyindong gold mine located in the southeast of Guizhou Province of China were selected as the tailings material.OPC is used as the main binder.The average price of OPC is CNY350/t.Fly ash, which was generated by the power plant of the mine, is also selected to substitute part of the cement to decrease the backfilling costs.Mine water and polynaphtalene sulfonate were also added to the tailings material.

Tailings
Many researchers observed the important effects of physical and chemical properties of tailings on the strength and fluidity.Particle size and chemical composition were the most influential parameters affecting the properties of CPB.In this study, the particle size is measured by the sieve analysis method (SAM).The results of the particle size distribution are shown in Table 1 and Figure 1.It may be seen that particles with a size less than 0.002 mm comprise 21.4% and particles with a size less than 0.05 mm comprise 84.0%.The parameter d 50 , which means the median particle size, defined by the particle size when the accumulative percentage reaches 50%, can be calculated from Figure 1.By Figure 1, d 50 is 0.012.Chemical Composition Analysis Centre at Central South University also tested the chemical composition, and the results are shown in Table 2. Table 2 demonstrates that the content of SiO2 and Al2O3 is 35.42% and 14.65%, respectively.The low content of cementitious oxides is not favourable for CPB strength gain.Therefore, in this study, fly ash with a high content of SiO2 and Al2O3 is added to the CPB mixture.

Binders
The most commonly used binder in CPB is OPC.The cost of the binder is nearly 50-75% of the total costs of a typical CPB plant [37,38].A substitution binder can reduce the cost of backfilling significantly.As the main solid waste of power plants, fly ash has potential cementitious properties due to the high content of SiO2, Al2O3, and other kinds of oxides.On the other hand, the replacement of part of the cement by fly ash in the CPB mixtures is often seen as an environmental value-added benefit.Moreover, the use of fly ash in CPB can significantly reduce the total costs of a paste backfill plant due to its lower cost [13].As shown in Table 1, the fine tailings (<20 m) comprise 21.4%.Fall and Benzaazoua [11] suggested that a high content of fine tailings (<20 m) is not favourable for CPB strength gain.Therefore, in this paper, a relatively high binder content is adopted.In order to improve the particle size distribution of CPB mixture, as well as to reduce the backfill costs, a mix of 66.7% of fly ash and 33.3% OPC is used as the binder.The chemical compositions of OPC and fly ash are given in Table 3.

Superplasticizer and Water
In the cement and concrete industry, the mechanical strength of cemented pastes can be improved by reducing the moisture content of the paste, while maintaining the fluidity of the paste by adding a superplasticizer [39].In this study, a polynaphtalene sulfonate is used as a superplasticizer.The dosages of additive superplasticizer are 0.2%, 0.4%, and 0.8% of the binder by weight based on the backfill practice of this mine.The water discharged from the ore concentrator is used to make the CPB mixtures.Chemical Composition Analysis Centre at Central South University also tested the chemical composition, and the results are shown in Table 2.

Binders
The most commonly used binder in CPB is OPC.The cost of the binder is nearly 50-75% of the total costs of a typical CPB plant [37,38].A substitution binder can reduce the cost of backfilling significantly.As the main solid waste of power plants, fly ash has potential cementitious properties due to the high content of SiO 2 , Al 2 O 3 , and other kinds of oxides.On the other hand, the replacement of part of the cement by fly ash in the CPB mixtures is often seen as an environmental value-added benefit.Moreover, the use of fly ash in CPB can significantly reduce the total costs of a paste backfill plant due to its lower cost [13].As shown in Table 1, the fine tailings (<20 µm) comprise 21.4%.Fall and Benzaazoua [11] suggested that a high content of fine tailings (<20 µm) is not favourable for CPB strength gain.Therefore, in this paper, a relatively high binder content is adopted.In order to improve the particle size distribution of CPB mixture, as well as to reduce the backfill costs, a mix of 66.7% of fly ash and 33.3% OPC is used as the binder.The chemical compositions of OPC and fly ash are given in Table 3.

Superplasticizer and Water
In the cement and concrete industry, the mechanical strength of cemented pastes can be improved by reducing the moisture content of the paste, while maintaining the fluidity of the paste by adding a superplasticizer [39].In this study, a polynaphtalene sulfonate is used as a superplasticizer.The dosages of additive superplasticizer are 0.2%, 0.4%, and 0.8% of the binder by weight based on the backfill practice of this mine.The water discharged from the ore concentrator is used to make the CPB mixtures.

Specimen Preparation
In this study, the tailing-binder ratio is 3:1, and the solid concentration of CPB is 70%.All the specimens were prepared by a standard cubical mould (i.e., 7.07 × 7.07 × 7.07 cm 3 ) suggested by the Ministry of Construction of the PRC [40].The specimens were then cured in the curing box, being wrapped after a 6 h knockout time.The curing temperature was kept at 20 • C, and the humidity at 95%.Curing times for strength test specimens were 1 day (d), 7 d, 14 d, and 28 d, respectively.For each curing time, three specimens were prepared.

Nuclear Magnetic Resonance Tests
Previous studies demonstrated that NMR technology is suitable for evaluating the hydration of cement [41][42][43].In this study, a low-field NMR system (see Figure 2a) was used to evaluate the hydration process of the CPB specimens.

Specimen Preparation
In this study, the tailing-binder ratio is 3:1, and the solid concentration of CPB is 70%.All the specimens were prepared by a standard cubical mould (i.e., 7.07  7.07  7.07 cm 3 ) suggested by the Ministry of Construction of the PRC [40].The specimens were then cured in the curing box, being wrapped after a 6 h knockout time.The curing temperature was kept at 20 °C , and the humidity at 95%.Curing times for strength test specimens were 1 day (d), 7 d, 14 d, and 28 d, respectively.For each curing time, three specimens were prepared.

Nuclear Magnetic Resonance Tests
Previous studies demonstrated that NMR technology is suitable for evaluating the hydration of cement [41][42][43].In this study, a low-field NMR system (see Figure 2a) was used to evaluate the hydration process of the CPB specimens.For the short-duration tests, the CPB mixture was placed into a plastic cylinder tube and put into the NMR system to measure the T2 relaxation time at 10 min, 1 h, 2 h, and 8 h after mixing.For the long-duration tests, after 1 d, 7 d, 14 d, and 28 d of curing the CPB specimens were tested by the NMR system.
Before the test, the T2 relaxation time of pure water was measured and presented in Figure 3.The figure shows that the T2 relaxation time of pure water is about 800-1600 ms.

Unconfined Compressive Tests
After the NMR tests, all the specimens cured for the strength test were placed in a servohydraulic material test system (see Figure 2b  For the short-duration tests, the CPB mixture was placed into a plastic cylinder tube and put into the NMR system to measure the T 2 relaxation time at 10 min, 1 h, 2 h, and 8 h after mixing.For the long-duration tests, after 1 d, 7 d, 14 d, and 28 d of curing the CPB specimens were tested by the NMR system.
Before the test, the T 2 relaxation time of pure water was measured and presented in Figure 3.The figure shows that the T 2 relaxation time of pure water is about 800-1600 ms.

Specimen Preparation
In this study, the tailing-binder ratio is 3:1, and the solid concentration of CPB is 70%.All the specimens were prepared by a standard cubical mould (i.e., 7.07  7.07  7.07 cm 3 ) suggested by the Ministry of Construction of the PRC [40].The specimens were then cured in the curing box, being wrapped after a 6 h knockout time.The curing temperature was kept at 20 °C , and the humidity at 95%.Curing times for strength test specimens were 1 day (d), 7 d, 14 d, and 28 d, respectively.For each curing time, three specimens were prepared.

Nuclear Magnetic Resonance Tests
Previous studies demonstrated that NMR technology is suitable for evaluating the hydration of cement [41][42][43].In this study, a low-field NMR system (see Figure 2a) was used to evaluate the hydration process of the CPB specimens.For the short-duration tests, the CPB mixture was placed into a plastic cylinder tube and put into the NMR system to measure the T2 relaxation time at 10 min, 1 h, 2 h, and 8 h after mixing.For the long-duration tests, after 1 d, 7 d, 14 d, and 28 d of curing the CPB specimens were tested by the NMR system.
Before the test, the T2 relaxation time of pure water was measured and presented in Figure 3.The figure shows that the T2 relaxation time of pure water is about 800-1600 ms.

Unconfined Compressive Tests
After the NMR tests, all the specimens cured for the strength test were placed in a servohydraulic material test system (see Figure 2b) after 1 d, 7 d, 14 d, and 28 d.The force-displacement curve during loading was recorded automatically by the computer.The

Unconfined Compressive Tests
After the NMR tests, all the specimens cured for the strength test were placed in a servohydraulic material test system (see Figure 2b) after 1 d, 7 d, 14 d, and 28 d.The force-displacement curve during loading was recorded automatically by the computer.The specimens were loaded by the displacement control method with a loading rate of 0.01 mm/s.Then the uniaxial compressive strength can be calculated.The capacity of the testing machine is 200 KN and the accuracy is 0.5%.

Effects of Polynaphtalene Sulfonate on the Fluidity
Slump tests were undertaken on CPB mixtures to investigate the effects of polynaphtalene sulfonate on the consistency.Figure 4 shows the sample with different dosage of polynaphtalene sulfonate in a slump test.The average slump of CPB mixtures with different polynaphtalene sulfonate dosages is given in Table 4.As shown in Figure 4 and Table 4, the addition of polynaphtalene sulfonate improves the fluidity of CPB significantly.Similar to previous studies, it can be concluded that the superplasticizer has a positive effect on the fluidity of CPB [27,29].The fluidity increased with an increase in the polynaphtalene sulfonate dosage.
Minerals 2018, 7, x 5 of 14 specimens were loaded by the displacement control method with a loading rate of 0.01 mm/s.Then the uniaxial compressive strength can be calculated.The capacity of the testing machine is 200 KN and the accuracy is 0.5%.

Effects of Polynaphtalene Sulfonate on the Fluidity
Slump tests were undertaken on CPB mixtures to investigate the effects of polynaphtalene sulfonate on the consistency.Figure 4 shows the sample with different dosage of polynaphtalene sulfonate in a slump test.The average slump of CPB mixtures with different polynaphtalene sulfonate dosages is given in Table 4.As shown in Figure 4 and Table 4, the addition of polynaphtalene sulfonate improves the fluidity of CPB significantly.Similar to previous studies, it can be concluded that the superplasticizer has a positive effect on the fluidity of CPB [27,29].The fluidity increased with an increase in the polynaphtalene sulfonate dosage.

Effects of Polynaphtalene Sulfonate on the Strength
The correlation between the strength of CPB and dosage of polynaphtalene sulfonate is shown in Figure 5.It shows that the strength of CPB increases with the increase of curing time.Additionally, with the increased dosage of polynaphtalene sulfonate, the 1 d strength remained nearly constant.However, the 7 d, 14 d, and 28 d strengths increase rapidly with an increase in polynaphtalene sulfonate dosage.This means that the polynaphtalene sulfonate has a negligible effect on the 1 d strength of CPB, while it has a positive effect on the 7 d, 14 d, and 28 d strength gain.

Effects of Polynaphtalene Sulfonate on the Strength
The correlation between the strength of CPB and dosage of polynaphtalene sulfonate is shown in Figure 5.It shows that the strength of CPB increases with the increase of curing time.Additionally, with the increased dosage of polynaphtalene sulfonate, the 1 d strength remained nearly constant.However, the 7 d, 14 d, and 28 d strengths increase rapidly with an increase in polynaphtalene sulfonate dosage.This means that the polynaphtalene sulfonate has a negligible effect on the 1 d strength of CPB, while it has a positive effect on the 7 d, 14 d, and 28 d strength gain.
Minerals 2018, 7, x 5 of 14 specimens were loaded by the displacement control method with a loading rate of 0.01 mm/s.Then the uniaxial compressive strength can be calculated.The capacity of the testing machine is 200 KN and the accuracy is 0.5%.

Effects of Polynaphtalene Sulfonate on the Fluidity
Slump tests were undertaken on CPB mixtures to investigate the effects of polynaphtalene sulfonate on the consistency.Figure 4 shows the sample with different dosage of polynaphtalene sulfonate in a slump test.The average slump of CPB mixtures with different polynaphtalene sulfonate dosages is given in Table 4.As shown in Figure 4 and Table 4, the addition of polynaphtalene sulfonate improves the fluidity of CPB significantly.Similar to previous studies, it can be concluded that the superplasticizer has a positive effect on the fluidity of CPB [27,29].The fluidity increased with an increase in the polynaphtalene sulfonate dosage.

Effects of Polynaphtalene Sulfonate on the Strength
The correlation between the strength of CPB and dosage of polynaphtalene sulfonate is shown in Figure 5.It shows that the strength of CPB increases with the increase of curing time.Additionally, with the increased dosage of polynaphtalene sulfonate, the 1 d strength remained nearly constant.However, the 7 d, 14 d, and 28 d strengths increase rapidly with an increase in polynaphtalene sulfonate dosage.This means that the polynaphtalene sulfonate has a negligible effect on the 1 d strength of CPB, while it has a positive effect on the 7 d, 14 d, and 28 d strength gain.

Effects of Polynaphtalene Sulfonate on the Hydration
In CPB mixtures there are four different water phase states, i.e., chemically-bound water, which corresponds to the water that is combined with chemical compounds with hydrogen bonds, bonded water in the gel phase, which corresponds to the water that is enveloped by the C-H-S structure, pore water, which corresponds to the water physically bound between particles, and free water.According to the theory of NMR, different water phase states have different T 2 relaxation times.For example, the T 2 relaxation time of chemically-bound water in the cement mixture is about 12 µs [43] which is difficult to obtain by NMR [44,45].Therefore, in this study, only three water phase states (i.e., bonded water in the gel phase, pore water, and free water) were detected by the NMR system.
The T 2 relaxation time of short-duration (10 min-8 h) hydration of CPB mixture is shown in Figures 6 and 7.It can be seen that the T 2 relaxation time curve has three peaks, indicating three main water phase states.The three peaks, which are shown in Figure 6a, are named here from left to right as the 1st peak, the 2nd peak, and the 3rd peak.It demonstrated that the centre of the 1st peak is 1 ms-10 ms, while the centre of the 2nd and the 3rd peaks are about 100 ms and 1000 ms, respectively.Thus, it can be judged that the 1st peak is the signal of bonded water in the gel phase, while the 2nd and the 3rd peaks are the signals of pore water and free water, respectively.Figure 6 demonstrated that the peak value of the 1st peak decreased with an increase in hydration time, which means that the amount of the bonded water in the gel phase decreased.The decrease in bonded water in the gel phase is mainly due to the reaction with the binder, leading to the decreasing of a detectable water molecule.Additionally, it can be seen that the 2nd and the 3rd peaks do not change much, identifying that the amount of pore water and free water changes little during the short-duration hydration.In addition, it is implied that during the hydration time of 10 min to 1 h, the peak value of the 1st peak decreases more than during 1 h-8 h.This is mainly because during the hydration process of CPB mixture the reaction between water and binder is stronger in the first 30 min [22].
As shown in Figure 7, an increase in the dosage of polynaphtalene sulfonate leads to the decrease in the peak value of the 1st peak, indicating that the bonded water in the gel phase decreased.However, the 2nd and the 3rd peaks showed almost no changes, which means that the amount of pore water and free water has little change with an increased dosage of polynaphtalene sulfonate.It was also observed that, during hydration times of 10 min to 1 h, when the dosage of polynaphtalene sulfonate increased from 0% to 0.2%, the peak value of the 1st peak slightly decreased.However, when the dosage of polynaphtalene sulfonate increased from 0.2% to 0.4%, the peak value of the 1st peak decreased significantly.On the other hand, during the hydration time of 2 h to 8 h, the peak value of the 1st peak decreased in a nearly uniform manner.This result demonstrated that the addition of 0.4% polynaphtalene sulfonate promoted the hydration of this CPB material during the 10 min to 1 h hydration time, which may contribute to the strength increase.Furthermore, the addition of polynaphtalene sulfonate has a stronger influence on the hydration of this CPB material during the hydration time of 10 min to 1 h than from 2 h to 8 h.

Effects of Polynaphtalene Sulfonate on the Hydration
In CPB mixtures there are four different water phase states, i.e., chemically-bound water, which corresponds to the water that is combined with chemical compounds with hydrogen bonds, bonded water in the gel phase, which corresponds to the water that is enveloped by the C-H-S structure, pore water, which corresponds to the water physically bound between particles, and free water.According to the theory of NMR, different water phase states have different T2 relaxation times.For example, the T2 relaxation time of chemically-bound water in the cement mixture is about 12 s [43] which is difficult to obtain by NMR [44,45].Therefore, in this study, only three water phase states (i.e., bonded water in the gel phase, pore water, and free water) were detected by the NMR system.
The T2 relaxation time of short-duration (10 min-8 h) hydration of CPB mixture is shown in Figure 6 and Figure 7.It can be seen that the T2 relaxation time curve has three peaks, indicating three main water phase states.The three peaks, which are shown in Figure 6a, are named here from left to right as the 1st peak, the 2nd peak, and the 3rd peak.It demonstrated that the centre of the 1st peak is 1 ms-10 ms, while the centre of the 2nd and the 3rd peaks are about 100 ms and 1000 ms, respectively.Thus, it can be judged that the 1st peak is the signal of bonded water in the gel phase, while the 2nd and the 3rd peaks are the signals of pore water and free water, respectively.Figure 6 demonstrated that the peak value of the 1st peak decreased with an increase in hydration time, which means that the amount of the bonded water in the gel phase decreased.The decrease in bonded water in the gel phase is mainly due to the reaction with the binder, leading to the decreasing of a detectable water molecule.Additionally, it can be seen that the 2nd and the 3rd peaks do not change much, identifying that the amount of pore water and free water changes little during the short-duration hydration.In addition, it is implied that during the hydration time of 10 min to 1 h, the peak value of the 1st peak decreases more than during 1 h-8 h.This is mainly because during the hydration process of CPB mixture the reaction between water and binder is stronger in the first 30 min [22].
As shown in Figure 7, an increase in the dosage of polynaphtalene sulfonate leads to the decrease in the peak value of the 1st peak, indicating that the bonded water in the gel phase decreased.However, the 2nd and the 3rd peaks showed almost no changes, which means that the amount of pore water and free water has little change with an increased dosage of polynaphtalene sulfonate.It was also observed that, during hydration times of 10 min to 1 h, when the dosage of polynaphtalene sulfonate increased from 0% to 0.2%, the peak value of the 1st peak slightly decreased.However, when the dosage of polynaphtalene sulfonate increased from 0.2% to 0.4%, the peak value of the 1st peak decreased significantly.On the other hand, during the hydration time of 2 h to 8 h, the peak value of the 1st peak decreased in a nearly uniform manner.This result demonstrated that the addition of 0.4% polynaphtalene sulfonate promoted the hydration of this CPB material during the 10 min to 1 h hydration time, which may contribute to the strength increase.Furthermore, the addition of polynaphtalene sulfonate has a stronger influence on the hydration of this CPB material during the hydration time of 10 min to 1 h than from 2 h to 8 h.The T2 relaxation time of long-duration (1 d-28 d) hydration is shown in Figures 8 and 9. Figure 8 presents that, similar to Figure 6, the peak value of the 1st peak decreased with an increase in curing time, which demonstrated that the amount of bonded water in the gel phase decreased.In addition, there is only one peak of the long-duration T2 relaxation time curve, identifying that during the hydration time 1 d-28 d, there is almost no pore water and free water in the CPB specimens.On the other hand, as presented in Figure 9, during long-duration hydration, the peak values of the 1st peak showed no conclusive trend with the increased dosage of polynaphtalene sulfonate.This result further demonstrated that the polynaphtalene sulfonate has a stronger influence on the short-duration hydration than the long-duration hydration.This has been explained in the next part.The T2 relaxation time of long-duration (1 d-28 d) hydration is shown in Figures 8 and 9. Figure 8 presents that, similar to Figure 6, the peak value of the 1st peak decreased with an increase in curing time, which demonstrated that the amount of bonded water in the gel phase decreased.In addition, there is only one peak of the long-duration T2 relaxation time curve, identifying that during the hydration time 1 d-28 d, there is almost no pore water and free water in the CPB specimens.On the other hand, as presented in Figure 9, during long-duration hydration, the peak values of the 1st peak showed no conclusive trend with the increased dosage of polynaphtalene sulfonate.This result further demonstrated that the polynaphtalene sulfonate has a stronger influence on the short-duration hydration than the long-duration hydration.This has been explained in the next part.The T 2 relaxation time of long-duration (1 d-28 d) hydration is shown in Figures 8 and 9. Figure 8 presents that, similar to Figure 6, the peak value of the 1st peak decreased with an increase in curing time, which demonstrated that the amount of bonded water in the gel phase decreased.In addition, there is only one peak of the long-duration T 2 relaxation time curve, identifying that during the hydration time 1 d-28 d, there is almost no pore water and free water in the CPB specimens.On the other hand, as presented in Figure 9, during long-duration hydration, the peak values of the 1st peak showed no conclusive trend with the increased dosage of polynaphtalene sulfonate.This result further demonstrated that the polynaphtalene sulfonate has a stronger influence on the short-duration hydration than the long-duration hydration.This has been explained in the next part.

Physico-Chemical Mechanism of Superplasticizer in the Mixture
When the CPB materials were mixed with water, the cement particles envelop the water molecules to form a flocculent structure, leading to the free water content decreasing.Thus, the fluidity of the CPB mixture decreased.However, when polynaphtalene sulfonate was added to the mixture, the fluidity increased.The chemical mechanism is demonstrated in Figure 10.As shown in Figure 10a, without using any additive, free water is surrounded by cement particles.By adding polynaphtalene sulfonate to the mixture, the radical group −SO 3 Na (see Figure 10b) in polynaphtalene sulfonate hydrolysed into −SO 3 − , which is an ion with a negative charge.−SO 3 − ions then were attracted by the cement particles due to the attractive force between negative and positive charges.Thus, the cement particles were surrounded with −SO 3 − , leading to the separation of cement particles due to the electrostatic repulsion (see Figure 10c).In this regard, the flocculent structure will be destroyed, resulting in the increase of fluidity.On the other hand, the free water enveloped by the flocculent structure will be released as a result of the destruction of the flocculent structure (see Figure 10d).The amount of water that contributes to the mixture fluidity increased.Therefore, with the increase of dosage of polynaphtalene sulfonate the fluidity of CPB increases.In addition, the water molecules attached by the cement particles formed hydrated films (see Figure 11a), which increased the lubrication between cement particles.This is another reason for the fluidity increase when polynaphtalene sulfonate was added to the mixture.Figure 11 demonstrates the strength gain under the effect of polynaphtalene sulfonate.It can be seen that, when polynaphtalene sulfonate was added to the mixture, more water molecules were attracted by the hydrophilic radical −SO 3 − attached on the surface of cement particles (see Figure 11a), leading to the increase of the hydration extent of CPB.In this regard, the strength of CPB increased.On the other hand, as presented in Figure 11b, the addition of polynaphtalene sulfonate destroyed the flocculent structure, making the cement particles more dispersed.Thus, the cement particles can enter the void spaces between tailing particles more easily.Therefore, the Figure 11 demonstrates the strength gain under the effect of polynaphtalene sulfonate.It can be seen that, when polynaphtalene sulfonate was added to the mixture, more water molecules were attracted by the hydrophilic radical −SO − 3 attached on the surface of cement particles (see Figure 11a), leading to the increase of the hydration extent of CPB.In this regard, the strength of CPB increased.On the other hand, as presented in Figure 11b, the addition of polynaphtalene sulfonate destroyed the flocculent structure, making the cement particles more dispersed.Thus, the cement particles can enter the void spaces between tailing particles more easily.Therefore, the polynaphtalene sulfonate reduces the pore diameter of the backfill [46] and increases the long-term strength.

Conclusions
In order to investigate effects of superplasticizer on the hydration, consistency, and strength of cement paste backfill, a polynaphtalene sulfonate is used as a superplasticizer and mixed with ordinary Portland cement, fly ash, full tailings, and discharged water from the ore concentrator to generate CPB materials.An NMR system is adopted to investigate the hydration of CPB.The fluidity and strength evolution, as well as the physico-chemical mechanism of the effect of polynaphtalene sulfonate on the fluidity and strength, are also presented.The following conclusions can be drawn: (c) During the short-duration hydration of CPB, the amount of bonded water in the gel phase decreases with the increase of the hydration time, while the amount of pore water and free water do not change significantly.However, during long-duration hydration, no pore water and free water were detected via the NMR test.It was also found that polynaphtalene sulfonate has a stronger effect on the short-duration hydration than the long-duration hydration.Due to the fact that the CPB properties are strongly affected by the hydration of the CPB mixture, more attention should be paid to the hydration of the CPB mixture with different admixtures.In addition, new methods should be developed to investigate the microscopic mechanism of CPB hydration.

Table 1 .Figure 1 .
Figure 1.Particle size distribution obtained by the sieve analysis method.

Figure 1 .
Figure 1.Particle size distribution obtained by the sieve analysis method.

Figure 2 .
Figure 2. Test apparatus (a) low-field NMR system and (b) a uniaxial compressive strength testing machine.
) after 1 d, 7 d, 14 d, and 28 d.The force-displacement curve during loading was recorded automatically by the computer.The

Figure 2 .
Figure 2. Test apparatus (a) low-field NMR system and (b) a uniaxial compressive strength testing machine.

Figure 2 .
Figure 2. Test apparatus (a) low-field NMR system and (b) a uniaxial compressive strength testing machine.

Figure 3 .
Figure 3. T 2 relaxation time of pure water.

Figure 5 .
Figure 5. Strength evolution of CPB specimens with different polynaphtalene sulfonate dosages.

Figure 7 .
Figure 7.The short-duration T 2 relaxation time of CPB mixture with different polynaphtalene sulfonate dosages when the hydration time is: (a) 10 min, (b) 1 h, (c) 2 h, and (d) 8 h.

Figure 9 .
Figure 9.The long-duration T 2 relaxation time of CPB mixture with different polynaphtalene sulfonate dosages when the hydration time is: (a) 1 d, (b) 7 d, (c) 14 d, and (d) 28 d.

Figure 9 .
Figure 9.The long-duration T2 relaxation time of CPB mixture with different polynaphtalene sulfonate dosages when the hydration time is: (a) 1 d, (b) 7 d, (c) 14 d, and (d) 28 d.

Figure 10 .
Figure 10.Physico-chemical mechanism of the effect of polynaphtalene sulfonate on the consistency of CPB: (a) flocculent structure, (b) chemical structure of polynaphtalene sulfonate, (c) electrostatic repulsion effect and (d) destruction of the flocculent structure.

Figure 10 .
Figure 10.Physico-chemical mechanism of the effect of polynaphtalene sulfonate on the consistency of CPB: (a) flocculent structure, (b) chemical structure of polynaphtalene sulfonate, (c) electrostatic repulsion effect and (d) destruction of the flocculent structure.

Figure 11 .
Figure 11.Physico-chemical mechanism of the effect of polynaphtalene sulfonate on the strength of CPB: (a) adsorption effect and (b) filling effect.
(a) The polynaphtalene sulfonate has a positive effect on the fluidity of CPB mixture.With an increase in the dosage of polynaphtalene sulfonate, the slump of CPB mixture increased.(b) The superplasticizer has a negligible effect on the 1 d strength of CPB, while it increases the 7 d, 14 d, and 28 d strength of CPB significantly.With the increased dosage of superplasticizer, the strength also increased.

Table 2 .
The chemical composition of full tailings.

Table 3 .
Chemical compositions of OPC and fly ash.

Table 2
demonstrates that the content of SiO 2 and Al 2 O 3 is 35.42% and 14.65%, respectively.The low content of cementitious oxides is not favourable for CPB strength gain.Therefore, in this study, fly ash with a high content of SiO 2 and Al 2 O 3 is added to the CPB mixture.

Table 2 .
The chemical composition of full tailings.

Table 3 .
Chemical compositions of OPC and fly ash.

Table 4 .
The average slump of CPB with different dosages of polynaphtalene sulfonate.

Table 4 .
The average slump of CPB with different dosages of polynaphtalene sulfonate.

Table 4 .
The average slump of CPB with different dosages of polynaphtalene sulfonate.