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The relations among the uniaxial compressive strength of cemented paste backfill (CPB) with solid phase mass fraction, cement sand ratio, and curing age were studied. The UCS of CPB samples increased exponentially with the increase of solid phase mass fraction and curing age but increased linearly with the increase of cement sand ratio. The results of X-ray diffraction (XRD) and scanning electron microscope (SEM) showed that the strength was determined by the amount of ettringite and C-S-H gelling. With the increase of ettringite and C-S-H gelling, the strength became larger. The triaxial compression test was conducted by selecting four kinds of CPB samples. The results showed that, with the increase of confining pressure, peak and residual strength became larger, but the elastic modulus decreased. When the ratio of confining pressure and uniaxial strength is about 1 : 2, the CPB samples show significant ductility characteristics and the ratio of residual strength and peak strength increased obviously.

Filling method is one of the three major mining methods in metal mine [

At present, the mixture ratio design of CPB refers to the relevant theory of concrete. Mitchell and Wong established the strength model by a large quantity of tests which show that the uniaxial compressive strength relates to the porosity, water cement ratio, and binder content [

However, there was obvious difference between CPB material and concrete material [

Related experiments have shown that the strength of CPB was closely related to solid mass fraction of tailings slurry, cement sand ratio, and curing age [

The tailings materials used in this study were collected from Daye Iron Mine in Hubei province of China. The chemical analysis method was used to study the chemical constituents of the whole backfill, which are shown in Table

Chemical constituent of whole tailings of Daye Iron Mine.

Constituents | Content (%) |
---|---|

SiO_{2} | 26.30 |

TFe | 20.79 |

SFe | 20.02 |

CaO | 12.45 |

FeO | 10.90 |

Al_{2}O_{3} | 6.07 |

MgO | 5.55 |

S | 1.315 |

Ag | 0.38 |

TiO2 | 0.232 |

Cu | 0.228 |

MnO | 0.164 |

P | 0.15 |

Au | 0.101 |

SrO | 0.046 |

Zn | 0.026 |

V2O5 | 0.023 |

Co | 0.013 |

Ni | 0.10 |

Pb | 0.006 |

As | 0.001 |

It can be seen from the fact that tailings mainly consisted of SiO_{2}, CaO, Al_{2}O_{3}, and so forth. It means that Daye iron ore tailings were mainly composed of clay minerals, quartz, and silicate composition. Secondly, there are CaO, MgO, NaO, SO_{3}, P_{2}O_{5}, Au, Ag, and so on; in addition, they also contain a small amount of Co, Ni, Pb, As, and so forth.

Particle size composition was measured by a laser particle size analyzer. According to the test results, particle size distribution curve of whole tailings was gotten as is shown in Figure

Characteristic value of particle size.

Fineness/mesh | Particle size/ | Percentage/% | Cumulative percentage/% |
---|---|---|---|

<500 | 0~25 | 56.7 | 56.7 |

500~325 | 25~45 | 10.8 | 67.5 |

325~200 | 45~75 | 11.2 | 78.7 |

200~150 | 75~106 | 7.4 | 86.1 |

150~100 | 106~150 | 7.8 | 93.9 |

100~80 | 150~180 | 3.2 | 97.1 |

>80 | >180 | 2.9 | 100 |

Distribution curves of tailings particle diameter.

The physical properties are the inherent properties of tailings. The test results are shown in Table

The list of physical properties of whole tailings.

Specific gravity/ | Bulk density/t·m^{−3} | Porosity | Specific surface area ^{2}·cm^{−3} |
---|---|---|---|

1.47 | 3.2 | 57 | 6400 |

Ordinary Portland cement (OPC, 42.5R) used in this study was obtained from Jidong Cement Plant. The chemical composition and physical characteristics of the OPC were given in Table

The list of physical properties of binders.

Specific gravity/ | Bulk density/t·m^{−3} | Porosity |
---|---|---|

1.3 | 3.10 | 58.06 |

CBP samples were made using Daye Iron Mine whole backfill as aggregate and OPC (42.5R) as binder. The number of samples was 75. Those samples were divided into five groups taking cement sand ratio as standard, which contained 1 : 4, 1 : 5, 1 : 6, 1 : 8, and 1 : 10. And every group was divided into three parts taking solid phase mass fraction as standard, which contained 65%, 68%, 70%, 73%, and 75%. Curing age of three samples in each section was 3 days, 7 days, and 28 days.

CPB samples were made using square mould whose length was 100 mm, and the curing temperature is (

CPB samples.

Following a predetermined period (7, 14, and 28 days) of curing, the CPB samples were tested for unconfined compressive strength (UCS) according to ISO 1920-4-2005 and T0553-2005. The UCS tests were performed using WDW digital display type mechanical testing equipment with a normal load capacity of 50 KN and a displacement speed of 1 mm/min. All of the experiments were carried out in triplicate, and the mean UCS values were presented in the results. All data regarding the test were collected by using a computerised data logging system.

The phase analysis of samples was carried out by X-ray (diffraction XRD, X-ray) diffraction instrument which is D/Max-RC diffraction instrument produced by the Rigak company of Japan. The scanning range is

The fractured samples obtained from UCS tests were used in SEM studies. The EVO18 type tungsten filament scanning electron microscope produced by the German Carl Zeiss Company was adopted.

Uniaxial compressive strength mechanics test was conducted for every sample, and the final test results are shown in Table

Uniaxial compression strength of CPB (unit: MPa).

Cement sand ratio | Curing age/d | Solid phase mass fraction/% | ||||
---|---|---|---|---|---|---|

65 | 68 | 70 | 73 | 75 | ||

1 : 4 | 3 | 0.28 | 0.36 | 0.46 | 0.6 | 0.84 |

7 | 0.81 | 0.96 | 1.39 | 1.61 | 1.96 | |

28 | 2.15 | 2.88 | 3.68 | 4.62 | 5.48 | |

| ||||||

1 : 5 | 3 | 0.27 | 0.31 | 0.39 | 0.48 | 0.66 |

7 | 0.72 | 0.81 | 1.34 | 1.58 | 1.91 | |

28 | 1.80 | 2.22 | 3.02 | 3.91 | 5.00 | |

| ||||||

1 : 6 | 3 | 0.26 | 0.28 | 0.34 | 0.44 | 0.51 |

7 | 0.69 | 0.80 | 1.30 | 1.53 | 1.89 | |

28 | 1.34 | 1.60 | 2.13 | 2.85 | 3.74 | |

| ||||||

1 : 8 | 3 | 0.18 | 0.22 | 0.28 | 0.36 | 0.41 |

7 | 0.31 | 0.39 | 0.56 | 0.66 | 0.70 | |

28 | 1.05 | 1.36 | 1.78 | 2.11 | 2.61 | |

| ||||||

1 : 10 | 3 | 0.15 | 0.20 | 0.26 | 0.34 | 0.38 |

7 | 0.24 | 0.36 | 0.44 | 0.58 | 0.63 | |

28 | 0.61 | 0.79 | 1.14 | 1.43 | 1.81 |

In Daye Iron Mine where open stope mining and subsequent filling method has been used, CPB placed into underground voids provides stability during the mining of the adjacent stopes. According to the rock mechanical properties and the mining requirement, a minimum strength of 2.0 MPa after 28 days is suggested to the UCS criteria. It can be seen from Table

Taking cement sand ratio as invariant, the change in strength of samples of different curing age with solid phase mass fraction was studied. Besides, the tendency equation is introduced, respectively, by exponential fitting method. The strength histogram of samples of different curing age and exponential fitting curve are shown in Figure

Relationships of solid phase mass fraction and the strength. (a) Curing age was 3 days; (b) curing age was 7 days; (c) curing age was 28 days.

The average value of exponential fitting complex correlation coefficient was 0.988, which means the influence law of solid phase mass fraction on the strength is obvious exponential function. It means that the greater the solid phase mass fraction, the more intense the influence on the strength. The greater the cement sand ratio, the bigger the slope of curves, which means that, with the increase of the cement sand ratio, the influence of solid phase mass fraction on the UCS increases.

It could be seen that the exponential curve of UCS development of CPB samples with different solid phase mass fraction at 3 and 28 days of curing age distributed evenly from down to up, but the distribution of exponential curve at 7 days of curing age is discrete. The exponential curve of the 1 : 8 and 1 : 10 of cement sand ratio at 7 days of curing age is far lower than these of the 1 : 4, 1 : 5, and 1 : 6 of cement sand ratio.

Above all, the influence of solid phase mass fraction on strength could be expressed as the following formula:

Taking the solid phase mass fraction as invariant, the change in strength of samples of different curing age with cement sand ratio was studied. Besides, the tendency equation is introduced, respectively, by linear fitting method. The strength histogram of different curing age and linear fitting curve were shown in Figure

Relationships of cement sand ratio and the strength. (a) Curing age was 3 days; (b) curing age was 7 days; (c) curing age was 28 days.

The average value of exponential fitting complex correlation coefficient was 0.932, which means the influence law of cement sand ratio on the strength is obvious linear function. It could be seen from Figure

Yin et al. [

Above all, the influence of cement sand ratio on strength could be expressed as the following formula:

Taking the CPB samples whose solid phase mass fraction is 68% and 73% as the research object, the increasing law of UCS of CPB with different cement sand ratio along with the increase of curing age was studied. The strength histogram of samples and exponential fitting curve were shown in Figure

Relationships of curing age and the strength. (a) Solid phase mass fraction was 68%; (b) solid phase mass fraction was 73%.

Relationships of curing age and the strength. (a) Cement sand ratio was 1 : 8; (b) cement sand ratio was 1 : 5.

It could be seen from Figures

Above all, the influence of curing age on strength could be expressed as the following formula:

The above analysis shows that UCS of CPB samples had ties with cement sand ratio, solid phase mass fraction, and curing age but the influence susceptibility was different. Taking five kinds of CPB samples with different cement sand ratio and solid phase mass fraction as research object, the strength data in Table

Distribution of strength of different CPB samples.

From the above analysis it can be observed that, with the increase of cement sand ratio, solid phase mass fraction, and curing age, the UCS increases in various ways. However, it could be seen from Figure

In addition, from Table

The phases of CPB samples of different curing age were analyzed using X-ray diffraction (XRD). The results are shown in Figure

XRD diffraction of hydration products.

It has been reported that [

It can be seen from Figure

The SEM images of CPB samples of different conditions are shown in Figure

SEM images of CPB samples of different condition.

From Figures

The SEM images of CPB samples with 70% of solid phase mass fraction at 1 : 4 of cement sand ratio at 3 days, 7 days, 28 days, and 56 days of curing age are shown in Figures

Based on the analysis, the microstructure and components are closely related to the curing age, cement sand ratio, and solid phase mass fraction. The factors affecting the strength development mainly include the C-S-H, ettringite, and porosity. Increasing the binder dosage apparently improved the microstructure of the SPB samples with the formation of additional C-S-H gelling [

When the curing age is 56 days the secondary gypsum was clearly detected, which was claimed to be responsible for the reduction of strength of CPB samples after 56 and 90 days, respectively, due to its expansive properties [

In fact, the CPB was under triaxial stress conditions after being filled into the stopes. It was significantly important to know well the mechanical behavior of cemented paste backfill under triaxial stress.

The CPB samples were tested for confined compression strength according to ASTM C 39. The solid phase mass fraction and cement sand ratio of selected samples were as follows: (1) Type 1: cement sand ratio was 1 : 6, and solid phase mass fraction was 65%; (2) Type 2: cement sand ratio was 1 : 6, and solid phase mass fraction was 70%; (3) Type 3: cement sand ratio was 1 : 8, and solid phase mass fraction was 65%; (4) Type 4: cement sand ratio was 1 : 6, and solid phase mass fraction was 70%. The samples were conserved in isothermal curing box for 28 days. These samples are shown in Figure

Samples of triaxial compression test.

The loading mode was axial displacement control and the rate was 1 × 10^{−4} mm/s. The confining pressures include 0.4 MPa, 0.6 MPa, 0.8 MPa, and 1.0 MPa. Mohr’s circles are shown in Figure

Mohr stress circle of different CPB samples.

The cohesion (

List of cohesion and internal friction angles.

1 : 6 | 1 : 8 | |||
---|---|---|---|---|

65% | 70% | 65% | 70% | |

Cohesion/MPa | 1.012 | 1.361 | 0.779 | 1.156 |

Internal friction angle ( | 34° | 31° | 37° | 36° |

The test data are shown in Table

Experimental data list.

Serial number | Confining pressure/MPa | Peak load/MPa | Residual load/MPa | Peak strength/MPa | Residual strength/MPa | The ratio of residual and peak strength/% |
---|---|---|---|---|---|---|

1-1 | 0.4 | 6.11 | 5.12 | 5.25 | 4.40 | 83.80 |

1-2 | 0.6 | 7.34 | 6.64 | 5.94 | 5.11 | 90.46 |

1-3 | 0.8 | 8.48 | 7.75 | 6.82 | 6.23 | 91.39 |

1-4 | 1.0 | 9.19 | 9.01 | 7.39 | 7.39 | 91.95 |

2-1 | 0.4 | 7.80 | 6.39 | 5.99 | 4.91 | 81.92 |

2-2 | 0.6 | 8.18 | 6.67 | 6.61 | 5.55 | 83.99 |

2-3 | 0.8 | 9.19 | 8.16 | 7.12 | 6.27 | 92.83 |

2-4 | 1.0 | 10.53 | 9.98 | 9.07 | 8.60 | 94.81 |

3-1 | 0.4 | 5.32 | 4.51 | 4.76 | 3.88 | 84.77 |

3-2 | 0.6 | 6.33 | 5.50 | 5.52 | 4.93 | 86.89 |

3-3 | 0.8 | 7.44 | 6.38 | 6.39 | 5.51 | 85.75 |

3-4 | 1.0 | 8.65 | 8.51 | 7.55 | 7.39 | 98.38 |

4-1 | 0.4 | 7.06 | 4.84 | 6.06 | 4.34 | 68.56 |

4-2 | 0.6 | 7.26 | 6.21 | 6.28 | 5.42 | 85.54 |

4-3 | 0.8 | 8.33 | 7.52 | 7.19 | 6.46 | 90.28 |

4-4 | 1.0 | 9.64 | 8.81 | 8.37 | 7.69 | 91.39 |

Complete stress-strain curves.

1 : 6, 65%

1 : 6, 70%

1 : 8, 65%

1 : 8, 70%

Elastic stage (OB) of complete stress-strain curve can be expressed as straight line, which is divided into two parts containing initial compaction stage (OA) and the liner elastic stage (AB). There is a very significant initial compaction stage which means that CPB samples largely deformed under little stress. It is because CPB is artificial materials in which there are many void defects. The deformation law is very complex in this stage. Linear elastic stage (AB) is the stage of strength formation of CPB samples. The internal pores have been completely closed resulting in stress-strain curves of this stage being straight lines. The deformation characteristics of CPB samples could be described by elastic modulus (

Nonlinear failure stage (BC) is the stage of generating, development, and accumulating of micro cracks in CPB samples. When the internal cracks link up with each other, yield surface begins to form. With the development of yield surface, CPB samples reach the ultimate bearing capacity. The greater the confining pressure, the longer the stage, and the greater the peak strength. Then the CPB samples enter strain softening stage (CD). With increase of strain, the stress gradually decreases after peak strength and CPB samples show progressive failure and fracture surface gradually forms. After point D, the stress has no significant change. After that, CPB samples enter into plastic flow stage where the stress no longer reduces with increase of strain.

For the same type of samples, with the increase of confining pressure, peak point becomes not noticeable. When the confining pressure was less than or equal to 0.6 MPa, with the increase of strain, the stress became significantly smaller in the strain softening stage (CD). When the confining pressure is more than 0.6 MPa, there are no clear boundaries between nonlinear failure stage and strain softening stage. The average ratios of residual strength and peak strength under different confining pressure are 72.56%, 86.72%, 90.06%, and 94.13%. The solid phase mass fraction has significant effect on the three-dimensional mechanical properties of CPB samples. The greater the solid phase mass fraction is, the more intense the strength decreased after peak strength under the same confining pressure.

It could be seen from above analysis that samples deformation shows significant brittle features and strain softening phenomenon is obvious after peak strength. Stress-strain curve shows the ideal elastic-plastic characteristics. The destruction form is conjugate shear failure. The failure surface is rough and sample appeared as obviously drum-shaped. As shown in Figure

Failure characteristics of CPB samples. (a) Conjugate shear; (b) transitional shear failure; (c) single shear failure.

In this study, the effects of solid phase mass fraction, curing age, and cement sand ratio on the unconfined compressive strength (UCS) of cemented tailings backfill at 3, 7, and 28 days of curing age were investigated. Additionally, the microstructure and failure mechanism under triaxial compression of backfill were also investigated. Based on the obtained results, the following conclusions can be drawn:

Test results show that there is a definite functional relationship between the strength of CPB samples and solid phase mass fraction, cement sand ratio, and curing age.

The effects of solid phase mass fraction, curing age, and cement sand ratio on the development of UCS of cemented tailings backfill were different. When the other two factors were kept constant, the UCS of CPB samples increased exponentially with the increase of solid phase mass fraction, increased linearly with the increase of cement sand ratio, and increased exponentially with the increase of curing age.

The effect of curing age had the greatest impact on the UCS of CPB samples. With the increase of curing age, the degree of influence of solid phase mass fraction gradually weakened and the degree of influence of cement sand ratio gradually became larger.

The state and type of hydrate were quite different in each curing age. The amount of ettringite and C-S-H gel materials gradually increased. Hydrates gradually bonded together which caused the increase of strength.

The deformation of CPB samples was composed of initial compaction deformation, elastic deformation of the matrix, and inelastic deformation caused by crack propagation. Accumulation of elastic deformation and local stress concentration caused further damage of CPB samples. Damage would lead to anisotropic material and the evolution of damage led to fracture failure.

Confining pressure had obvious impact on the mechanical properties. With the increase of confining pressure, peak strength, peak strain, and the residual strength became larger, but the elastic modulus decreased. So CPB closely contacted with the surrounding rock only can achieve the best effect.

The authors declare that they have no competing interests.

Financial supports for this work, provided by the National Natural Science Foundation of China (no. 5137403) and the Fundamental Research Funds for the Central Universities (no. FRF-TP-15-042A1), are gratefully acknowledged. Finally, the first author would like to thank colleagues at USTB for their support.