NMR Studies Of Water And Pore Evolution In Mine Backfill Slurry

With the development of mining technology and further emphasis on the ecological environment of mine in China, the application of backfill method is continuous growing in the actual production of mines. Due to large differences in conditions of mining deposits, the properties of surrounding rock, mineral processing methods, operating costs, technological level, production scale at various mines in China, the specific programs of backfill mining methods vary from one to another.The ratio of filling slurry will directly affect the performance of the filling slurry transportation, the mechanical properties of hardened filling slurry and the production costs and so on. During the hardening process of filling slurry, the moisture, pore size distribution and strength changes with the curing age. The hardened filling slurry is named backfill body. The macroscopic mechanical properties of the filling body, such as the compressive strength is closely linked to the distribution of pore structure and the moisture of filling body. In addition, the moisture and porosity evolution in filling body are closely related to the drying shrinkage of filling body. Therefore, the study on the changes of the moisture and pore size distribution during the hydration and hardening process in backfill slurry has important significance for the analysis of filling slurry hardening mechanism, macroscopic mechanical properties of the filling body.This paper conducted indoor orthogonal matching tests and NMR (Nuclear Magnetic Resonance) tests on filling slurry. To obtain the optimal ratio solution of filling slurry, orthogonal test results were analyzed by using the multi-index comprehensive evaluation method. The water and pore structure evolution in the filling slurry was monitored dynamically by using low-field NMR techniques. The relationship between microscopic pore size distribution and macro-mechanical properties of filling slurry was investigated. In this paper, the studies include the following aspects:(1) On the basis of Analyzing the previous works on the optimization of the filling ratio, the orthogonal ratio test of filling slurry was carried out. The orthogonal test results were analyzed by the multi-index evaluation, thus the optimal ratio of filling slurry was gained. This method which meets the actual production need of mines overcomes the conflicts between single indicators in the analysis of orthogonal experiments.(2) Consecutive NMR CPMG tests on the fresh filling slurry were conducted to study the distribution and evolution of water in the early hydration process of backfill slurry. The results showed that the physically bound water presences three different states in the early hydration process of backfill slurry, namely the adsorbed water on the surface of solid particles, pore water between solid particles and free water in the surface of the fresh filling slurry. TheT2spectrum distribution of the filling slurry shows that the content of adsorbed water is among the largest proportion, which is more than99%. The changes of T2peak areas reflect changes in contents of various states of water and transformation relation between different states of water. This provides evidence for studying water bleeding, hydration process, and assessing the workability of the slurry.(3) To get the evolution role of between T2spectrum distribution and curing time in the filling slurry, continuous observation (0-7d) filling slurry was conducted by using the low-field magnetic resonance CPMG test during the hardening process of filling slurry. The results showed that when the temperature is26℃, the physically bound water and pore size in the filling slurry conformed exponential decay mode, and stabilized after50h. Low-field NMR technique is able to detect the dynamic changes of physically bound water and pore size during the hardening process of filling slurry, which reflect the hardening speed of slurry filling.(4) the low-field NMR technology was used to research pore the differences in pore distribution in filling slurry among different solid concentration, gray sand ratio, superplasticizer, fly ash dosage, sand dosage and water glass dosage. The results showed that adding superplasticizer would delay the hardening process of filling slurry and did not affect the weighted average pore size in the filling slurry. But the pore size is more concentrated and small voids were relatively fewer in the filling slurry with superplasticizer. When the mass concentration and cement tailing ratio in the filling slurry are higher, the weighted average pore size is smaller in hardened filling slurry. After adding fly ash and sand, the weighted average pore size in the hardened filling slurry increased.21d later, compared with filling slurry without fly ash and sand, the weighted average pore diameter increased32%and32%respectively in filling slurry added50%and100%fly ash. For filling slurry with50%sand, its weighted average pore diameter increased52%. After adding water glass, the hardening speed was accelerated in the filling slurry, and the weighted average pore size of its pores was reduced. However, the water glass had no great influence on the weighted average pore size of filling slurry after7days.(5) At different curing time, the uniaxial compressive strength and porosity of backfill and cement paste were obtained by using a universal testing machine and a NMR spectrometer. Mathematical fitting was used to get the time dependence of uniaxial compressive strength and porosity of backfill slurry and cement paste, and the relationship between the uniaxial compressive strength and porosity of cement paste.