The purpose of this paper was using gold mine tailings and cemented materials with low alkalinity to fabricate baking-free bricks. The obtained baking-free brick samples were evaluated by unconfined compressive strength (UCS), water absorption percentage, freezing-thawing cycle, and drying-wetting cycle. The microstructures of the baking-free brick samples were analyzed using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. The baking-free brick specimens cured for 28 days with the addition of 10% mixing water consumption and 1:6 cement/tailing ratio tended to obtain favorable comprehensive properties such as a high compressive strength of 15.15 MPa, a low water absorption percentage of 11.8%, excellent freezing-thawing resistance with a 8.9% UCS loss rate after 15 freezing-thawing cycles and good drying-wetting resistance with a 11% UCS loss rate after 10 drying-wetting cycles. The XRD and SEM test results verified that different kinds of hydrate products including C-S-H and C-S-A-H gels, and ettringite were produced during hydration process, which were responsible for good physical, mechanical properties, and durability of the obtained baking-free bricks. Therefore, the experimental results showed that it was practical and reasonable to utilize the homemade cementitious materials in our laboratory to stabilize the gold tailings for production of baking-free bricks, which still met the requirements of major regional construction standards in some countries.
Gold has long been considered a symbol of wealth due to its scarcity and performance stability. According to incomplete statistics, the mined gold from gold ores has been up to 190,000 tons since 1950 [
In recent years, a variety of researches have been performed on developing effective techniques to utilize these tailings widely including the recovery of associated valuable elements, the production of construction materials, glass fibers, and ceramics [
In the present work, the factors influencing the performances of the baking-free bricks that were fabricated by the raw materials of gold mine tailings together with low alkalinity binders were investigated in detail. The effects of mixing water consumption, cement/tailing ratio, freezing-thawing cycle, and drying-wetting cycle on the performances of the baking-free bricks were studied using single-factor experiments. The current results will give a potential route for solving the safety and environmental pollution problems caused by the massive accumulation of gold mine tailings.
The gold mine tailing used in this work was provided by Zhaoyuan Mining Co., Ltd., located in Shandong Province of China. The chemical compositions of the main raw materials used in the current work were identified by an X-ray Fluorescence Spectrometer (XRF, ARL ADVANT’X Intellipower™ 3600, Thermo Fisher Scientific, USA). The results are displayed in
Raw materials | SiO2 | Al2O3 | K2O | Na2O | CaO | Fe2O3 | MgO | TiO2 | SO3 |
---|---|---|---|---|---|---|---|---|---|
Gold mine tailings | 70.14 | 17.30 | 4.95 | 3.78 | 1.87 | 0.72 | 0.54 | – | – |
Slag powder | 32.90 | 17.10 | – | 0.85 | 37.60 | 0.56 | 7.90 | 1.40 | 0.75 |
Fly ash | 56.17 | 29.01 | 2.26 | 0.32 | 5.42 | 3.72 | 1.29 | 0.77 | 0.80 |
42.5 ordinary Portland cement | 22.00 | 9.60 | – | – | 59.80 | 3.00 | 1.60 | 0.45 | 2.10 |
The particle size distribution of the gold mine tailing was measured by a particle size analyzer (Rise-2006, Jinan Runzhi Technology Co., Ltd., China) and the result is shown in
The cemented materials were synthesized by our lab using slag powder, fly ash, 42.5 ordinary Portland cement, Na2SO4, CaO, and CaSO4 according to a designed proportion. The pH value of the leaching solution of the as-prepared cemented materials was about 9.8, which was measured using a digital display pH–meter (PHS-25, Shanghai INESA Scientific Instrument Co., Ltd., China).
Grade-S95 slag powder with a density of 2.90 g/cm3 and a specific surface area of 410 m2/kg was taken from Anhui Magang Jiahua New Building Materials Co., Ltd. of China. Its quality coefficient
The calculated
Grade-I fly ash with a density of 2.29 g/cm3 and a specific surface area of 242 m2/kg was purchased from the Maanshan No. 2 power plant of China. Its activity index
where
In the experiment, 42.5 ordinary Portland cement with a specific surface area of 360 m2/kg was provided by Anhui Conch Cement Co., Ltd. of China, its chemical composition is shown in
Series | Mixing water consumption (%) | Cement/tailings ratio | Curing age (days) |
---|---|---|---|
BW | 6 | 1:6 | 28 |
8 | |||
10 | |||
12 | |||
BC | 10 | 1:4 | 3, 7, 28 |
1:6 | 3, 7, 28 | ||
1:8 | 3, 7, 28 | ||
1:10 | 3, 7, 28 |
Initially, a certain amount of slag powder, fly ash, 42.5 ordinary Portland cement, Na2SO4, CaO, and CaSO4 were weighed using an electronic balance accurately, respectively. Then these powders were mechanical mixed and ball-milled in a horizontal ball mill for about 60 min using steel balls with different diameters as milling media to obtain the cementitious binders with low alkalinity. It is necessary for the gold mine tailings to perform a pretreatment since the fresh tailings generally contain 20–40 wt% moisture. In this study, the tailings were stored for about 48 h, performing a centrifugal solid-liquid separation. Then, they were dried in a drying oven and were ground by a 100-mesh sieve. Subsequently, the sieved tailings and the cementitious binders were weighed according to various cement/tailings ratios and were mixed homogeneously in a manual mixer. The desired water was added slowly to the mixture for obtaining a uniform paste. The paste was poured into a cylindrical mould with a diameter of 30 mm and a height of 30 mm, and was compacted under a pressure of 20 MPa for 5 min. After being removed from the die, the compacted samples were sealed with plastic wrap and were cured at 25 ± 3°C and relative humidity of 90 ± 5% in a curing box. After reaching a desired curing age, a series of performance tests on the baking-free bricks were conducted.
Based on the Chinese standard GB/T 4111-2013, the unconfined compressive strength (UCS) of the baking-free brick was performed using an electronic universal testing machine (WDW-30, Jinan Metus Testing Technology Co., Ltd., China) with the maximum capacity of 30 kN. The peak failure load
where
The water absorption capacity can be characterized by the 24 h water absorption rate of baking-free bricks (see
Following the 28 days curing age, the baking-free brick samples suffered the drying-wetting cycle. In this work, the drying-wetting cycles were 0, 3, 5, 7, and 10, respectively. One drying-wetting cycle was defined as that a baking-free brick sample was wetted for 48 h via immersing it in tap water at room temperature (20°C) and then it was dried to its initial dry weight, then wetted again to swell. Finally, the UCS of these samples was measured using an electronic universal testing machine. The typical baking-free sample after 10 drying-wetting cycles test is displayed in
After curing for 28 days, the baking-free brick samples were subjected to the freezing-thawing cycle test. In this work, the freezing-thawing cycles were 0, 3, 7, 15, and 20, respectively. A freezing-thawing cycle was composed of thawing the samples in water at 20°C for 4 h followed by freezing them at −15°C for 4 h in a low-temperature freezer. Subsequently, the UCS of the samples was measured and then the UCS loss was obtained by comparing with the value before the freezing-thawing cycle. In the case of dry freezing, wrapped a fresh-keeping film on the surface of the baking-free brick sample to prevent the loss of water, and then placed it in the low-temperature test chamber. In the case of wet freezing, soaked the baking-free brick sample in water for 1 day, took it out, wiped the surface water with a dry cloth, put on a fresh-keeping film, and placed it in a low-temperature freezer for the freezing-thawing cycle test. The typical baking-free sample after 20 freezing-thawing cycles test is presented in
To explore the potential environmental effect of the baking-free bricks, the concentration of leached heavy metals in the leachate of the baking-free bricks was determined by an ICP emission spectrometer ICPS-7510 PLUS of Japan according to the Chinese standards GB 5085.3-2007. By mixing 100 water with 10 g powder of crushed brick specimens at 28 days curing age, a turbid liquid was prepared. Subsequently, the turbid liquid was magnetically stirred at a speed of 200 r/min for 2 h. Finally, the leachate was obtained by filtering the turbid liquid.
Phase structure of the sample after 28 days of moist curing was performed using a diffractometer system XRD D8 Advance of Germany with Cu Kα radiation. The sample was ground to a fine powder (passing through 100 mesh square hole sieve) and then the XRD analyses were conducted with a 2θ range of 20–80°C with a scanning speed of 4 °C/min. Before XRD identification, the samples that have been subjected to UCS test were stored into ethanol for 3 days to terminate the hydration process and were dried for about 8 h to obtain the constant weight. Microstructure observation was carried out by a SEM machine JSM6490 of Japan with a working voltage of 20 kV. Before observation, the fragments of the crushed samples were coated with a layer of gold.
Mixing water consumption is one of the significant factors to influence the performances of baking-free bricks. Therefore, we investigated its effect on the UCS of the baking-free bricks using single-factor experiment, as shown in
Since the raw materials and technical parameters in the manufacture of baking-free bricks differ from each other, the existing researches cannot give any specific advices on appropriate mixing water consumption. Zhang et al. [
Cement/tailing ratio | 3 days strength development rate/% | 7 days strength development rate/% |
---|---|---|
1:4 | 47.1 | 77.8 |
1:6 | 52.7 | 80.3 |
1:8 | 53.7 | 76.2 |
1:10 | 54.0 | 74.5 |
The 28 days UCS value (15.15 MPa) of the brick with the 1:6 cement/tailing ratio and 10% mixing water consumption is higher than those of the bricks using ordinary Portland cement, lime (natural hydraulic lime and calcareous lime), fly ash(or coal ash waste), and nano-SiO2 as cementitious binders with similar fabrication parameters [
In the entire cement/tailing ratio, the water absorption percentage increases continuously when the cement/tailings ratio decreases, as observed from
Freezing-thawing resistance is another important performance of baking-free bricks. Freezing-thawing usually causes the damage of construction materials in many areas, especially in cold regions. Generally, the quality loss or the strength loss after the freezing-thawing cycle is used to characterize the freezing-thawing resistance of bricks.
It can be seen from
Higher UCS loss rate under the wet-freezing condition can be seen by comparing
In addition, steady increases in the UCS loss of around 8.6%–10.0% and 13.9%–16.0% when the brick samples undergo respectively 20 freezing-thawing cycles under the dry-freezing condition and the wet-freezing condition, which agree well with the tendency of water absorption percentage. Clearly, the higher the water absorption percentage of the brick samples, the higher the loss rate of freezing-thawing strength. This consistency can be explained based on the microstructure level. A higher water absorption percentage indicates that the brick samples have a looser microstructure and more interstices and hence the water will migrate to these interstices. When the water located in these interstices is frozen, local stress will be produced, leading to damage into the structure of the bricks.
Drying-wetting resistance is also an important property of baking-free bricks. In the shallow ground, cyclic drying-wetting is a quite common phenomenon because of an alternation of dry and wet seasons. In this work, the drying-wetting cycles that the brick samples were subjected to were 0, 3, 5, 7, and 10 cycles. The variation in the UCS of the brick specimens with a 1:6 cement/tailing ratio after different drying-wetting cycles is shown in
To better understanding of the hydration reaction products of the baking-free bricks, X-ray diffraction (XRD) patterns analyses were conducted. The analyzed results of the phase composition for the BC series samples with the curing age of 28 days is presented in
The fracture surface morphologies of the BC series samples after 3 days and 28 days of curing age were observed using SEM technology and the results are shown in
The baking-free brick sample with a cement/tailings ratio of 1:6 and a mixing water consumption of 10% cured for 28 days, which is the mix design that yielded the favorable comprehensive properties, was evaluated for environmental effect. The concentrations of leached heavy metals in the leachate of the baking-free brick sample are presented in
Baking-free bricks | GB 5085.3-2007 maximum allowed concentration in leachate (mg/L) | |
---|---|---|
As | 0.71 | 5 |
Cr | 0.07 | 5 |
Cu | 0.02 | 100 |
Zn | 0.02 | 100 |
Pb | ND | 5 |
Ba | 0.04 | 100 |
Cd | ND | 5 |
Mn | 0.18 | 5 |
This investigation successfully fabricated the baking-free bricks made from gold mine tailings and slag powder-cement-fly ash binders along with a small amount of chemical activators. A series of experimental tests were carried out to study the influences of mixing water consumption, and the cement/tailing ratio on the water absorption percentage, unconfined compressive strength, freezing-thawing resistance, and drying-wetting resistance of the baking-free bricks. Furthermore, the phase composition and microstructure of the typical baking-free brick samples were also evaluated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The main conclusions in this investigation were summarized as follows: A binder with slag powder and fly ash wastes blended with cement and chemical additives showed an ability to stabilize gold mine tailings. Therefore, our study provided a technical strategy for the efficiently comprehensive utilization of industrial waste and mine waste. Mixing water consumption is one of the significant factors to influence the performances of the baking-free bricks. The 28 days unconfined compressive strength of the baking-free bricks increased from 13.17 MPa to 15.15 MPa as the mixing water consumption increased from 6% to 10%. However, the unconfined compressive strength sharply decreased from 15.15 MPa to 12.22 MPa as the mixing water consumption was 12%. The unconfined compressive strength decreased with the decrease of the cement/tailing ratio. However, the effect of the cement/tailings ratio on the strength development of the brick sample with various curing ages was different. Compared with the cement/tailing ratio, the curing age had a more significant effect on the early strength of the baking-free bricks. The unconfined compressive strength of the baking-free brick specimens decreased with the increase of times of freezing-thawing cycle or drying-wetting cycle. The damage of 20 freezing-thawing cycles or 10 drying-wetting cycles for the baking-free bricks was in an acceptable range (≤15% strength loss rate), exhibiting good freezing-thawing and drying-wetting resistances.