The graphite tailing causes serious environmental pollution, and the pollution problem becomes worse and worse with the increase in graphite demands. This paper focuses on the graphite tailing concrete, which can alleviate the environment problem through utilizing graphite tailings. With the orthogonal experimental design, 16 groups of specimens were designed to investigate the compressive strength of the graphite tailing concrete, and each group had 6 specimens. The significance sequence of the influencing factors for the compressive strength was studied, including the ratio of water to cement, sand ratio, graphite tailings content, and carbon fiber content. The optimal contents of graphite tailings and carbon fiber were obtained from the further experimental study on the electrical characteristics of the graphite tailing concrete, and a regression analysis was conducted to develop the predictive mixture design relationships for the electrical resistivity of the conductive graphite tailing concrete. The experimental results show that the conductive concrete mixture containing graphite tailings and carbon fiber has satisfactory mechanical strength along with well electrical conductivity. With the increase in graphite tailings content, the compressive strength decreases slowly, but the electrical resistivity decreases much more obviously. Predictions with the proposed relationship are in reasonable agreement with experimental results. This study provides references for the graphite tailing utilization alleviating the environment problems.
Graphite tailings are derived from froth flotation process of graphite ores [
Conductive concrete is a kind of heterogeneous material that is composited by binding and conductive materials, dielectric aggregates, and water [
Steel fiber and carbon fiber are other two conductive materials used in concrete usually [
Graphite tailing concrete can be applied as conductive concrete if the electrical resistivity is good enough [
In order to alleviate the environment problem through utilizing graphite tailings, the following studies are performed. Graphite tailings are added to the conventional concrete to make the conductive graphite tailing concrete, and carbon fibers are added to further improve the electrical conductivity. The effects of the ratio of water to cement, sand ratio, graphite tailings content, and carbon fiber content on mechanical characteristics are studied experimentally based on the orthogonal experimental design, and then, the electrical characteristics are studied with the four-electrode method. The optimal contents of graphite tailings and carbon fiber are studied, and the mixture design relationships for the electrical resistivity are predicted by regression analysis.
The graphite tailing sample on experiment is obtained from a graphite mine in Jixi City, China. The chemical composition is analyzed with X-ray fluorescence, and the results are shown in Table
Chemical composition of graphite tailings.
Composition | Fe2O3 | MgO | Al2O3 | SiO2 | CaO | K2O | C | V2O5 | Loss |
---|---|---|---|---|---|---|---|---|---|
Content (%) | 5.073 | 2.326 | 10.205 | 62.498 | 15.547 | 2.26 | 1.13 | 0.308 | 0.653 |
The mineral composition of tailings was analyzed with X-ray diffraction mineral identification. The main mineral compositions of the tailings are quartz, grossular, lead bismuth vanadium oxide, graphite, and so on. Table
The cement used was Portland cement PI 42.5 provided by Harbin Swan cement factory, China. Standard river sand was used as fine aggregates. Crushed stone was used as coarse aggregates, and the particle size was no more than 20 mm. Chopped carbon fibers of 7
There are four factors that affect the concrete mechanical and electrical properties to be investigated in this study: (A) water/cement material (
Orthogonal experimental design is based on the probability theory, the mathematical statistics, and the standardized orthogonal table. Orthogonal experimental design can select the right amount of representative samples from a large number of experimental designs to arrange experiments reasonably. To design an optimal test, it should have reasonable indicator and reference for selecting factor and the corresponding level. In this test, all the factors and corresponding levels are shown in Table
Factors and levels.
Level |
|
Sand ratio (%) | Tailings content (%) | Carbon fiber content (%) |
---|---|---|---|---|
Level 1 | 0.49 | 30 | 0 | 0 |
Level 2 | 0.54 | 33 | 5 | 0.15 |
Level 3 | 0.60 | 36 | 10 | 0.30 |
Level 4 | 0.68 | 39 | 15 | 0.45 |
The foundation of the orthogonal experimental design is the orthogonal table, and it forms as
According to the orthogonal experimental design table of
The composition of the tested mixture per cubic metre.
Specimen | Water (kg/m3) | Cement (kg/m3) | Sand (kg/m3) | Crushed stone (kg/m3) | Graphite tailings (kg/m3) | Carbon fiber (kg/m3) |
---|---|---|---|---|---|---|
1 | 195 | 397.96 | 542.11 | 1264.93 | 0 | 0 |
2 | 195 | 397.96 | 556.72 | 1130.32 | 120 | 2.64 |
3 | 195 | 397.96 | 564.13 | 1002.91 | 240 | 5.28 |
4 | 195 | 397.96 | 564.35 | 882.69 | 360 | 7.92 |
5 | 195 | 361.11 | 517.17 | 1206.72 | 120 | 5.28 |
6 | 195 | 361.11 | 608.48 | 1235.41 | 0 | 7.92 |
7 | 195 | 361.11 | 534.2 | 949.69 | 360 | 0 |
8 | 195 | 361.11 | 625.52 | 978.37 | 240 | 2.64 |
9 | 195 | 325 | 492 | 1148 | 240 | 7.92 |
10 | 195 | 325 | 501.6 | 1018.4 | 360 | 5.28 |
11 | 195 | 325 | 676.8 | 1203.2 | 0 | 2.64 |
12 | 195 | 325 | 686.4 | 1073.6 | 120 | 0 |
13 | 195 | 286.76 | 467.47 | 1090.76 | 360 | 2.64 |
14 | 195 | 286.76 | 553.82 | 1124.42 | 240 | 0 |
15 | 195 | 286.76 | 647.36 | 1150.87 | 120 | 7.92 |
16 | 195 | 286.76 | 748.11 | 1170.12 | 0 | 5.28 |
Hydroxyethyl was first added into water and left for 20 minutes letting it dissolve completely, and then carbon fibers and defoamer were added into water and stirred gently. The rest of the mixing water was poured into the mixer followed by the superplasticizer. Then, the cement was added and stirred by a rotary mixer for 3 minutes. The mixer was stopped, and the carbon fibers were poured into the mixer. When the mixer was run for 1 minute, the sand and the tailings were added and stirred for 3 minutes. Finally, the crushed stones were added and stirred for 3 minutes. After the mixture was poured into an oiled mold, the electrode (if applicable) was laid in fresh concrete. Then an external vibrator was used to facilitate compaction and decrease the amount of air bubbles. The samples were demolded after 24 hours and then cured under the standard condition at a temperature of 20°C and a relative humidity of 100% for 28 days [
The size of the concrete specimens for compressive strength tests was 150 mm × 150 mm × 150 mm, and each group with same mix proportion had 6 specimens for compressive strength tests as shown in Figure
Compressive strength test. (a) Test specimen and (b) servo-controlled hydraulic testing machine.
The compressive strengths are listed in Table
Compressive strength.
Specimen |
|
Sand ratio (%) | Tailings content (%) | Carbon fiber content (%) | Compressive strength (N/mm2) | Standard deviation of compressive strength |
---|---|---|---|---|---|---|
1 | 0.49 | 30 | 0 | 0 | 43.7 | 1.315 |
2 | 0.49 | 33 | 5 | 0.15 | 41.2 | 0.834 |
3 | 0.49 | 36 | 10 | 0.30 | 39.6 | 0.942 |
4 | 0.49 | 39 | 15 | 0.45 | 31.2 | 0.994 |
5 | 0.54 | 30 | 5 | 0.30 | 38.5 | 0.529 |
6 | 0.54 | 33 | 0 | 0.45 | 42.6 | 1.227 |
7 | 0.54 | 36 | 15 | 0 | 24.9 | 1.058 |
8 | 0.54 | 39 | 10 | 0.15 | 34.3 | 0.962 |
9 | 0.60 | 30 | 10 | 0.45 | 32.9 | 1.183 |
10 | 0.60 | 33 | 15 | 0.30 | 24.3 | 0.752 |
11 | 0.60 | 36 | 0 | 0.15 | 35 | 0.617 |
12 | 0.60 | 39 | 5 | 0 | 31.6 | 0.897 |
13 | 0.68 | 30 | 15 | 0.15 | 20.4 | 0.825 |
14 | 0.68 | 33 | 10 | 0 | 22.8 | 0.529 |
15 | 0.68 | 36 | 5 | 0.45 | 30.9 | 0.776 |
16 | 0.68 | 39 | 0 | 0.30 | 31.3 | 1.269 |
Range analysis on compressive strength (N/mm2).
Factors |
|
Sand ratio | Tailings content | Carbon fiber content |
---|---|---|---|---|
Kam 1 | 38.925 | 33.875 | 38.15 | 30.75 |
Kam 2 | 35.075 | 32.725 | 35.55 | 32.725 |
Kam 3 | 30.95 | 32.6 | 32.4 | 33.425 |
Kam 4 | 26.35 | 32.1 | 25.2 | 34.4 |
|
12.575 | 1.775 | 12.95 | 3.65 |
Figure
Relationship between compressive strength and affecting factors. (a)
Figure
Adopted sand ratio in the tests derives from the rational range of Standard for Test Method of Mechanical Properties on Ordinary Concrete, and the variations among levels are not very large as shown in Figure
The arithmetic mean of compressive strength decreases with the increase of graphite tailings content as shown in Figure
Compressive strength and target strength.
Specimen |
|
Tailings content (%) | Compressive strength | Target strength | Compressive strength (%) |
---|---|---|---|---|---|
3 | 0.49 | 10 | 39.6 | 40 | −1.0 |
8 | 0.54 | 10 | 34.3 | 35 | −2.0 |
9 | 0.60 | 10 | 32.9 | 30 | 9.7 |
14 | 0.68 | 10 | 22.8 | 25 | −8.8 |
The chopped carbon fibers have a little reinforcing effect on compressive strength as shown in Figure
The size of the specimens for electrical properties was 150 mm × 150 mm × 300 mm. Each group with the same mix proportion had 3 specimens for electrical property tests. The effect of
The selection and arrangement of electrodes significantly affect the experiment on conductive properties. There are several test methods for measuring the electrical resistivity of concrete such as two-electrode method [
The electrical schematic of the four-electrode method is shown in Figure
The measurement circuit of the four-electrode method. (a) Electrical schematic with electrical connections and (b) electrical resistivity measurement with the four-electrode method.
As shown in Figure
Electrical resistivity with different tailings content and carbon fiber content.
From an economic view, the optimum dosage of conductive materials should approach the percolation threshold. So, an appropriate percentage of carbon fiber is 0.3% if the requirement for electrical resistivity is not very high. To get better conductivity, graphite tailings can be added more to the concrete if the concrete can satisfy the demand for strength. This experimental study shows that the appropriate percentage for tailings is 10% for good enough strength.
The electrical resistivity can be lower than 1 kΩ⋅cm as shown in Table
Measured resistivity and estimated resistivity.
Specimen | Tailings (%) | Carbon fiber (%) | Measured resistivity (kΩ·cm) | Estimated resistivity (kΩ·cm) | Relative error (%) |
---|---|---|---|---|---|
1 | 0 | 0 | 63.68 | 63.805 | −0.20 |
2 | 5 | 0 | 47.8 | 47.408 | 0.82 |
3 | 10 | 0 | 37.24 | 37.692 | −1.21 |
4 | 15 | 0 | 32.28 | 32.156 | 0.38 |
5 | 0 | 0.15 | 48.04 | 49.163 | −2.34 |
6 | 5 | 0.15 | 34.56 | 33.096 | 4.23 |
7 | 10 | 0.15 | 23.48 | 24.050 | −2.43 |
8 | 15 | 0.15 | 19.6 | 19.429 | 0.87 |
9 | 0 | 0.3 | 27.2 | 26.672 | 1.94 |
10 | 5 | 0.3 | 11.32 | 11.276 | 0.39 |
11 | 10 | 0.3 | 3.53 | 3.591 | −1.74 |
12 | 15 | 0.3 | 0.837 | 0.826 | 1.32 |
13 | 0 | 0.45 | 21.36 | 21.000 | 1.69 |
14 | 5 | 0.45 | 6.36 | 6.517 | −2.47 |
15 | 10 | 0.45 | 0.643 | 0.689 | −7.16 |
16 | 15 | 0.45 | 0.435 | 0.453 | −4.06 |
An inverse trigonometric function is used to simulate threshold effect of electrical resistivity. The comparison between measured resistivity and estimated resistivity is shown in Table
Electrical resistivity results with proposed regression equation.
Predictions with the proposed relationship are in reasonable agreement with experimental results measured by the four-electrode method. Fitting curved surface is rational and acceptable at not only experimental points but also transitional regions.
This paper proposed the method of the graphite tailing utilization for conductive concrete, and the compressive strength and electrical conductivity of the conductive concrete are investigated by test. The conductive concrete mixture containing graphite tailings and carbon fiber shows a good electrical conductivity and an enough mechanical strength. Some of the conclusions from the study are as follows: For graphite tailing concrete, it is more reasonable that the sand ratio is near 30% so that compressive strength is high enough with good workability. The compressive strength decreases with the increasing graphite tailings content. When tailings content is 10%, the compressive strength does not reduce too much with respect to the target strength because compressive strength can be slightly improved by carbon fiber. The increase of graphite tailings can obviously decrease the electrical resistivity of specimens. Graphite tailing concrete can be used in deicing or snow melting. From an economic view, an appropriate percentage for carbon fiber is 0.3% if the requirement for electrical resistivity is not very high. Predictions with the proposed relationship are in reasonable agreement with experimental results measured by the four-electrode method. Fitting curved surface is rational and acceptable at not only experimental points but also transitional regions.
The data used to support the findings of this study are available from the corresponding author upon request.
The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.
The authors declare no conflicts of interest.
This work was supported by the National Natural Science Foundation of China (Grant nos. 51008094 and 51678221) and Heilongjiang Natural Science Foundation (Grant no. LC2017025).