Experimental Study on Mechanical Properties of Paste Backfill with Flue-Gas Desulphurisation Gypsum under Combined Action of Dry–Wet Cycles and Chloride Erosion
Abstract
:1. Introduction
2. Test Materials and Methodology
2.1. Specimen Preparation
2.2. Test Methodology
3. Mechanical Performance Tests
3.1. Effects of Dry–Wet Cycles Coupled with Chloride on Masses of Filled Specimens
3.2. Effects of Dry–Wet Cycles Coupled with Chloride Salt on Uniaxial Compressive Strengths of Backfill Specimens
3.3. AE Characteristics
4. Mesoscopic Tests
4.1. Analysis of Surface Functional Groups via FTIR Spectroscopy
4.2. Crystal Calibration Analysis with XRD Diffraction
4.3. FE-SEM Mesostructure Analysis
5. Discussion of Mechanisms
- Chemical reactions of specimens after mixing
- 2.
- In the fifth and 10th dry–wet cycles, the masses, elastic moduli, and uniaxial strengths of the specimens were increased. Denudation did not obviously occur on the specimen surface. There were more AE signals. At this time, the specimen fractures were mainly vertical. This is because the hydration reaction inside the specimens was not yet complete, and the solution immersion provided additional time for this reaction. Simultaneously, Cl− fully reacted with the fly ash inside the specimen through diffusion and chemically activated the SiO2 and Al2O3 in the fly ash, generating new gel products (C–S–H, C–S–H, AFt, etc.) to fill the internal fractures and pores in the specimens. The reaction process is given by Equations (9) and (10). The FTIR and XRD spectra revealed that AFt was not converted to Friedel’s salt in large quantities, and AFt can provide higher strength to the specimens than Friedel’s salt. This led to increases in the masses and uniaxial strengths of the specimens at the beginning of the dry–wet cycles. As indicated by the FE-SEM images, the mesostructures of the specimens became more compact, which enhanced both the mass and the uniaxial strength. Additionally, AE signals were generated by the refracture of the closed original fractures within the specimens under load.Ca2+ + Al2O3 + 2OH− → CaO·Al2O3·H2OCa2+ + Al2O3 + Cl− + OH− → 3CaO·Al2O3·3CaCl2·10H2O
- 3.
- At the 15th and 20th dry–wet cycles, the specimens’ masses, elastic moduli, and uniaxial strengths were reduced. The denudation of the specimen surfaces was more severe, and the interlaced development of macroscopic vertical and horizontal fractures was observed. This is because, as the internal hydration reaction approached completion, the erosion failure of the specimens due to the chloride salt solution and the deterioration of the specimens due to the dry–wet cycles began to appear. First, the chloride salt entered the specimen’s interior through diffusion and reacted chemically with the previously formed gel products to produce Friedel’s salt. These products had certain swelling properties. The expansion and contraction of the gel products during the dry–wet cycles resulted in new fractures inside the specimens. The reaction process is given by Equation (11). Furthermore, the dry–wet cycles physically denudated the surfaces of the specimens through deterioration, and the internal structures of the specimens were also damaged. Under the coupled effects of these two factors and other related factors, the masses and uniaxial strengths of the specimens were reduced. The IR and XRD spectra revealed that Friedel’s salt started to appear in the late stage of the dry–wet cycles. Additionally, the FE-SEM images indicated that with the increase in the number of dry–wet cycles, the surfaces of the specimens started to deteriorate, and numerous pores and fractures appeared inside the specimen. As a result, the overall structure became loose.3CaO·Al2O3·CaSO4·12H2O + 2Cl− → 3CaO·Al2O3·CaCl2·10H2O + SO42− + 2H2O
- 4.
- A higher-concentration solution can enter the specimen interior and enhance the uniaxial strength of the specimen faster owing to the higher concentration gradient. The strength deterioration rates of the specimens in the higher-concentration solution were lower than those for the low-concentration solution, but the surface denudation was significant. When the concentration of chloride salt in the solution is higher, the erosion on the surface of the specimen is more severe, and the chloride salt is more likely to invade the specimen interior and chemically react with the interior components, which has a positive effect on the specimen’s strength. In a large amount, a solution with a high concentration of chloride salt can react with the internal components of the specimens or precipitate during the dry–wet cycles. This partially fills the fractures and pores inside the specimens and has a buffering effect on the strength deterioration of the specimens under the coupled effects of dry–wet cycles and chloride erosion. Moreover, a high concentration of Cl− can maintain Friedel’s salt; conversely, a lower concentration of Cl− leads to a looser internal structure because it cannot maintain Friedel’s salt, as shown in Equations (12) and (13). These factors led to the considerable strength deterioration of the specimens subjected to dry–wet cycles in the low-concentration solution. The IR and XRD spectra indicated that Friedel’s salt started to appear late in the dry–wet cycles, and the FE-SEM images indicated that the specimens in the high-concentration solution had fewer pores and fractures than those in the low-concentration solution at the same number of dry–wet cycles. The specimens in the high-concentration solution had better integrity than those in the low-concentration solution.CaCl2 + 3Ca(OH)2 + 12H2O → CaCl2·3Ca(OH)2·12H2O3CaO·Al2O3·CaCl2·10H2O (s) → 4Ca2+ + 2Al(OH)4− +2Cl− + 4OH− + 4H2O
6. Conclusions
- At the early stage of dry–wet cycling in a chloride solution, the strengths of paste specimens were improved owing to the continued hydration reaction and chloride catalysis.
- With increasing drying and wetting cycles of paste specimens in a chlorine salt solution, the erosion effect of chlorine salts and the deterioration effect of the dry–wet cycles began to appear, and the strengths of the paste specimens began to decline.
- Compared with the low-concentration chlorine salt solution, the high-concentration solution can improve the strength of the paste faster and have a buffering effect on the strength deterioration of the paste.
- The changes in the mechanical properties of desulphurised gypsum backfill samples under the coupled effects of chloride salt erosion and dry–wet cycles were investigated, and it was determined that the desulphurised gypsum backfill could maintain a certain strength in this special environment. The results provide a laboratory reference and theoretical basis for the field application of desulphurised gypsum backfill.
- In this study, backfill samples were taken as the research objects. There are size differences between the samples and field backfill. Therefore, it is necessary to study the mechanical properties of backfill under the coupled effects of chloride erosion and dry–wet cycles at the field scale.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Abbreviations
FTIR | Fourier-Transform Infrared Spectroscopy |
XRD | X-Ray Diffraction Analysis |
FE-SEM | Field-Emission Scanning Electron Microscopy |
XRF | X-Ray Fluorescence |
Aft | Ettringite |
C–A–H | xCaO·Al2O3·yH2O |
AE | Acoustic Emission |
RA | The Ratio of Rise Time to Amplitude |
PDS | Pressure Density Stage |
LES | Linear Elastic Deformation Stage |
PPYS | Pre-Peak Yielding Stage |
PPDS | Post-Peak Damage Stage |
C–S–H | xCaO·SiO2·yH2O |
C3SH | 3CaO·SiO2·yH2O |
C4AH | 4CaO·Al2O3·H2O |
C3A | 3CaO·Al2O3 |
C4AF | 3CaO·Al2O3·Fe2O3 |
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Raw Materials | Chemical Composition and Content (%) | |||||
---|---|---|---|---|---|---|
SiO2 | Al2O3 | CaO | MgO | SO3 | Fe2O3 | |
Cement | 22.30 | 4.82 | 62.47 | 2.34 | 1.82 | 3.83 |
Fly ash | 56.30 | 18.05 | 11.84 | 3.01 | 1.19 | 4.93 |
Coal gangue | 59.83 | 20.10 | 4.89 | 1.22 | - | 6.52 |
Blast furnace slag | 35.83 | 12.73 | 38.09 | 7.86 | - | 2.79 |
Desulphurised gypsum | 24.40 | 2.62 | 32.25 | 1.33 | 37.58 | 0.73 |
Solution Concentration | Uniaxial Compressive Strength (MPa) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Number of Dry–Wet Cycles | 3% | Average | 5% | Average | 10% | Average | |||||||
0 | 10.77 | 10.74 | 10.14 | 10.49 | - | - | - | - | - | - | - | - | |
5 | 9.41 | 10.97 | 12.02 | 10.80 | 10.47 | 10.61 | 10.86 | 10.64 | 10.20 | 12.19 | 12.90 | 11.76 | |
10 | 12.72 | 11.81 | 10.65 | 11.73 | 10.81 | 11.73 | 9.97 | 10.84 | 10.96 | 13.15 | 10.26 | 11.46 | |
15 | 11.20 | 11.04 | 9.59 | 10.61 | 9.75 | 10.37 | 8.65 | 9.59 | 10.13 | 11.31 | 12.61 | 11.35 | |
20 | 8.95 | 7.94 | 9.94 | 8.94 | 7.90 | 9.59 | 10.25 | 9.25 | 9.14 | 11.98 | 12.08 | 11.07 |
Solution Concentration | Elastic Modulus (MPa) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Number of Dry–Wet Cycles | 3% | Average | 5% | Average | 10% | Average | |||||||
0 | 835.21 | 764.58 | 833.17 | 810.99 | - | - | - | - | - | - | - | - | |
5 | 725.60 | 1019.87 | 902.74 | 882.74 | 945.18 | 819.49 | 912.33 | 892.33 | 756.21 | 1025.90 | 921.05 | 901.05 | |
10 | 1044.36 | 931.05 | 1017.71 | 997.71 | 755.93 | 916.95 | 866.44 | 846.44 | 819.87 | 1030.47 | 955.17 | 935.17 | |
15 | 942.89 | 886.97 | 944.93 | 924.93 | 884.44 | 888.26 | 916.35 | 896.35 | 627.94 | 1004.25 | 846.10 | 826.10 | |
20 | 603.23 | 520.11 | 591.67 | 571.67 | 647.76 | 733.23 | 720.49 | 700.49 | 646.37 | 1096.05 | 901.21 | 881.21 |
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Wang, S.; Wang, F.; Yin, D.; Jiang, T.; Zhang, Z. Experimental Study on Mechanical Properties of Paste Backfill with Flue-Gas Desulphurisation Gypsum under Combined Action of Dry–Wet Cycles and Chloride Erosion. Minerals 2021, 11, 882. https://doi.org/10.3390/min11080882
Wang S, Wang F, Yin D, Jiang T, Zhang Z. Experimental Study on Mechanical Properties of Paste Backfill with Flue-Gas Desulphurisation Gypsum under Combined Action of Dry–Wet Cycles and Chloride Erosion. Minerals. 2021; 11(8):882. https://doi.org/10.3390/min11080882
Chicago/Turabian StyleWang, Sheng, Feng Wang, Dawei Yin, Tianqi Jiang, and Zhen Zhang. 2021. "Experimental Study on Mechanical Properties of Paste Backfill with Flue-Gas Desulphurisation Gypsum under Combined Action of Dry–Wet Cycles and Chloride Erosion" Minerals 11, no. 8: 882. https://doi.org/10.3390/min11080882