This paper presents the results of an experimental study on the priming effect of sodium silicate gel (SS) on cemented tailing backfill (CTB) that contains lead-zinc smelting slag. CTB and cemented paste (CP) containing lead-zinc smelting slag samples with SS of 0 and 0.4% of the mass of the slag were prepared and cured at 20°C for 1, 3, 7, and 28 days. Mechanical test and pore structure analyses were performed on the studied CTB samples, microstructural analyses (X-ray diffraction analysis and thermal gravity analysis) were performed on the studied CP samples, whereas the electrical conductivity of CTB was monitored. The results reveal that SS has a significant positive effect on cementitious activity of binder mixed by cement and lead-zinc smelting slag. This activation leads to the acceleration of binder hydration process, the formation of more cement hydration products in the CTBs, and the refinement of their pore structure, which is favorable for the strength development of CTB.
It is known that cemented tailing backfill (CTB) technology is a common method for ground support and mine waste disposal during mining operations [
Sodium silicate gel (SS), Na2SiO3·9H2O, is a kind of water-soluble silicate which appears as crystallization of orthogonal double cone or white-to-gray lump or powder. SS is often used as a binder and an activator in cement-based materials to improve the strength and stability [
Early age (≤28 days) mechanical strength, which controls the mechanical stability and supporting ability of CTB structure, is considered as one of the most important performances of CTB. To ensure safe underground working conditions, different mechanical strengths are required depending on the function and application of the backfill. For example, up to 1 MPa is needed for free-standing wall, and strength values higher than 4 MPa are required for roof support [
Considering the current situation mentioned above, the main purpose of this paper is to experimentally research and present the priming effect and the effect of sodium silicate gel (SS) on CTBs that contain lead-zinc smelting slag at early ages.
Portland cement type I (PCI) blended with grounded lead-zinc smelting slag in the weight ratio of 50/50 was used as a binder in all samples. Grain size distribution analyses were conducted by laser particle size analyzer, a Mastersizer 2000. Analytical analyses were conducted on the nonferrous slag by using X-ray fluorescence spectroscopy. The apparatus is Philips Pw2400. Determination of the specific gravity of the tailing particles is in accordance with ASTM D854. Figure
Grain size distribution curve of the tailing material used.
Main chemical compositions of the studied lead-zinc smelting slag.
Element unit | SiO2 (wt.%) | Fe2O3 (wt.%) | Al2O3 (wt.%) | TiO2 (wt.%) | CaO (wt.%) | MgO (wt.%) | Na2O (wt.%) | K2O (wt.%) | MnO (wt.%) | P2O5 (wt.%) | LOI (wt.%) |
---|---|---|---|---|---|---|---|---|---|---|---|
15.5 | 30.41 | 6.20 | 0.284 | 2.92 | 0.55 | 0.35 | 0.50 | 0.47 | 0.15 | 40.24 |
Main chemical compositions of the cement used.
Element unit | SO3 (wt.%) | Fe2O3 (wt.%) | Al2O3 (wt.%) | SiO2 (wt.%) | CaO (wt.%) | MgO (wt.%) | Relative density |
---|---|---|---|---|---|---|---|
PCI | 3.82 | 2.70 | 4.53 | 18.03 | 62.82 | 2.65 | 3.1 |
Figure
Main mineral compositions of the tailings used.
Mineral (wt.%) | Quartz | Calcite | Magnetite | Muscovite | Albite |
---|---|---|---|---|---|
Content (%) | 0.52 | 0.16 | 0.05 | 0.35 | 99.00 |
Powdered sodium silicate gel was used in order to be completely mixed with tailing and binder particles.
Distilled water (DW) was used in this study.
All CTB samples were prepared with a water-to-binder ratio of 7 and a binder content of 4.5% in terms of mass. Weighted tailings, binder, SS of 0 and 0.4% of the mass of the lead-zinc smelting slag, and mixing water were mixed and homogenized until obtaining a homogeneous paste. The produced mixes were poured into concrete curing cylinders, 5 cm in diameter and 10 cm in height. The specimens were then sealed (this avoids the evaporation of water) and cured at 20°C for periods of 1, 3, 7, and 28 days. Cylinders sized 10 cm in diameter and 20 cm in height were used for monitoring tests. Samples of lead-zinc smelting slag cement paste (SCP), with a water-cement (w/c) ratio of 2, were prepared in the same procedure for microstructural analysis.
Following ASTMC 39, UCS tests were performed on the CTB specimens. The loading capacity and load rate are 50 kN and 1 mm/min, respectively. Each test was repeated at least twice, and the average was chosen as the strength of the tested sample.
To assess the SS effect on binder hydration products in the CTBs, a series of microstructural analyses that included examination of the pore structure and evolution of binder hydration products were carried out to evaluate the microstructural properties of the studied CTB. The microstructure of the studied CTB samples was investigated by mercury intrusion porosimetry (MIP) and thermal (TG/DTG) and XRD analyses. MIP tests were performed by PMI mercury/nonmercury intrusion porosimeter to evaluate the pore size distribution and the total porosity. The CTB samples prepared for the MIP tests were cured up to 28 days. Prior to tests, all samples were first oven-dried at 50°C until mass stabilization. Drying at this temperature did not appear to cause cracking.
Electrical conductivity (EC) monitoring, which can assess the progress of cement hydration that occurs within the hydrating cementitious materials, was performed on the CTB samples. An EC sensor (5TE) was placed in the middle of the CTB samples to measure the EC evolution. All the sensors are connected to a data logger (Em 50) so that the data could be recorded.
Active effect of sodium silicate gel on cemented tailing backfill that contain lead-zinc smelting slag can be seen in the strength evolution of CTBs at early ages. Figure
Strength evolution of CTB for different SS contents at early ages.
Sodium silicate gel caused a refinement influence on the pore structure which significantly affects the strength of CTB. Generally, finer pore structure is associated with higher strength of the porous medium [
Differential pore size distribution curves of CTB that contain lead-zinc slag with SS and without SS.
More amounts of hydration products were formed due to the addition of SS in CTBs. The results of the thermal analyses are presented in Figure
TG (a)/DTG (b) diagrams for FA-cemented paste of FA-CTB cured for 28 days.
The formation of more hydration products related to SS can be demonstrated by the XRD results of SCP samples that contain lead-zinc smelting slag activated by SS and without SS and cured at 28 days, which are presented in Figures
XRD results of (a) CTB without SS and (b) CTB that contains SS.
Finally, SS shows obvious acceleration of binder hydration in CTBs that contain lead-zinc smelting slag. Figure
Results of electrical conductivity monitoring of CTB that contains NFS with SS and without SS.
This paper presents the experimental results of a study which focuses on the effect of SS on CTBs that contain lead-zinc smelting slag. It is found that SS can cause an activation effect and increase the strength value of CTB made of Portland cement and lead-zinc smelting slag. This activation includes the refinement of the pore structure of CTB, the formation of more cement hydration products in the CTBs, and the acceleration of the binder hydration process, which has been confirmed by the results of microstructural analyses (MIP, TG/DTG, and XRD) and electrical conductivity (EC) monitoring, respectively. It can be concluded that the SS can cause an active effect on CTBs that contain lead-zinc smelting slag.
The authors declare that they have no conflicts of interest.
This research was supported by the International Science & Technology Cooperation Program of China (no. 2014DFA70760), the National Key Research and Development Program of China (2016YFC0600709 and YS2017YFGH000511), and the Youth Science and Technology Innovation Fund Project of BGRIMM (Q-28).