The strength and consistency of cemented paste backfill (CPB) are of key concerns in the stope stability and cost control for underground mines. It is common practice to use additives, such as superplasticizer, to improve the performance of CPB. This study mainly focuses on the effects of superplasticizer on the hydration, consistency, and strength of CPB. In this study, a polynaphtalene sulfonate was used as the superplasticizer. The binder is a mix of 33.3% ordinary Portland cement and 66.7% fly ash. The CPB specimens with a tailings-binder ratio of 3:1 and a solid concentration of 70% were then tested by a low field nuclear magnetic resonance system after different hydration times. Effects of polynaphtalene sulfonate on the hydration, fluidity, and strength were investigated. Results showed that the polynaphtalene sulfonate has a strong influence on short-duration hydration, which may contribute to the strength increase of CPB. It has been demonstrated that the polynaphtalene sulfonate improved the fluidity of the CPB mixture. With the increased dosage of polynaphtalene sulfonate, the slump increased. It was also found that the polynaphtalene sulfonate dosage has a negligible effect on the 1 day (d) strength while it has a strengthening effect on the 7 d, 14 d, and 28 d strength of CPB specimens.
Liquid carbon dioxide blasting technology as a new environmentally safe rock cracking technology has been widely used in mining, enhancing the permeability of coal seam, road construction, and other fields. In this study, a liquid carbon dioxide blasting experiment was conducted on a concrete specimen of size 500mmÂ500mmÂ400 mm firstly, and the crack distribution and blast hole morphology were observed. Then, the peridynamics model of rock fracturing under liquid carbon dioxide blasting was established, and the crack propagation in liquid carbon dioxide blasting and the influence of explosion pressure and numbers and radii of releasing holes on the liquid carbon dioxide blasting effects were analyzed. The results showed that the broken concrete specimen had no obvious smash district and the direction of crack propagation was perpendicular to that of carbon dioxide release after liquid carbon dioxide blasting. The failure of the specimen was only caused by the blasting stress wave, and the quasi-static effect of highpressure carbon dioxide did not have sufficient time to play its role. With an increase in the stress wave strength and the numbers and radii of releasing holes, the fracturing velocity of liquid carbon blasting will increase. The cracks produced by liquid carbon dioxide blasting were mainly distributed in the middle of the adjacent releasing holes. The research results had a good guiding significance for understanding the mechanism of liquid carbon dioxide rock breaking and optimizing and improving the liquid carbon dioxide fracturing device.
The pore structure of rock has a great influence on its physical and mechanical properties. Factors such as chemical corrosion and temperature changes affect the pore structure evolution. In this paper, the pore structure of sandstone was investigated under rapid freeze-thaw (F-T) cycles and chemical corrosion. A nuclear magnetic resonance (NMR) testing system is used to study the pore structure of tight sandstone samples immersed in different chemical solutions after 10, 20, and 30 F-T cycles. Permeability is determined by using empirical method. Results found that permeability is strongly affected by the erosion of NaOH and NaCl solutions. The pores in the rock were divided into three categories based on the pore size, i.e., minipores, mesopores, and macropores. The results showed that the amount of mini-pores and mesopores both decreased with an increase in the number of F-T cycles while the amount of macropores increased for groups of NaOH, NaCl, and pure water. No conclusive trend can be found in the H 2 SO 4 group. Fractal analysis of the pore structure revealed that no conclusive trend was observed for fractal dimension of mini-pores D 1 . Fractal dimension of mesopores D 2 ranged from 2.79 to 2.93, indicating a medium complexity pore structure of the mesopores. Fractal dimension of macropores D 3 was over 2.9, implying that the pore structure of the macropores is the most complex. The fractal dimension of the T 2 spectrum D NMR ranged from 2.55 to 2.77. Correlations between the fractal dimensions and porosity are also presented. Results showed that D 2 and D 3 can be good indicators for the pore size volume of sandstone samples immersed in H 2 SO 4 , NaOH and NaCl solutions, while D NMR is a good indicator for the pore size volume of sandstone samples immersed in NaOH solution and pure water.INDEX TERMS Pore structure, freeze-thaw, chemical corrosion, nuclear magnetic resonance, fractal analysis.
In order to figure out the damage characteristics and mechanisms of sandstone under coupled effect of acid erosion and recurrent freezing-thawing cycles, sulfuric acid is chosen as acid solution, and sandstone, representative stone in Sichuan province, is chosen as a sample to conduct the freezing-thawing cycling test. In the meantime, chemical component of the solution is also tested and recorded in the progress of freezing-thawing cycling. Then, the nuclear magnetic resonance (NMR) test and magnetic resonance images (MRI) test on samples are conducted with the help of the AniMR-150 NMR imaging system. After a series of tests, the sample’s appearance, dry mass, porosity, T2 spectrum, solution’s pH, solution’s metallic ion concentration, and magnetic resonance images are obtained and analyzed. The results show that dry mass loss and porosity grow with F-T cycles increase and pH value decrease. Montmorillonite, illite, and clay in sandstone react with sulfuric acid solution; as a consequence, K+, Mg2+, Fe3+, and Al3+ separate out, and the solution’s pH and concentration of K+, Mg2+, Fe3+, and Al3+ increase with F-T cycles. Acid erosion and F-T cycling lead to the generation of new micropores and expansion of micropores at the beginning; when the acid solution is exhausted, new micropores generate under the effect of freezing-thawing cycling, and micropores in samples keep developing and expanding with the increase of freezing-thawing cycles. Coupled effect of acid corrosion and recurrent freezing-thawing cycling causes much more serious deterioration to sandstone samples than acid corrosion.
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