Study On The Deterioration Mechanism Of Cement- Based Rich-water Filling Material In Corrosive Environment

Gob-filling mining is an important part of green mining technology, which has the advantages of energy-saving and emission-reduction, safety and environmental protectioa It has been widely used in the underground mined-out area caused by the metal mine and coal mine located under villages, railroads and surface buildings. As the underground mine water environment is very complex, the filling body is in the environment containing a large number of corrosive medium. The rich-water filling material will be effected by detrimental ions in the process of hydration and hardening, which causes the reduction of stability and durability, decomposition of hardening crystallization, leading to destruction and instability of the main structure of filling body. The existing researches focus on the basic mechanical properties, hydration mechanism and weathering properties of rich-water filling material The corrosion resistance of the filled hardening body in the complex mine water environment is almost not involved. And there is lack of the analysis on deterioration mechanism of rich-water filling material in corrosive environment.The rich-water filling material which meets the requirments of fast-hardenging was prepared. On this basis, the basic components were optimized and the new-type rich-water filling material with compound mineral admixture was developed. The deterioration mechanism of rich-water filling material was analyzed by the means of macroscopic property test and SEM, XRD and infrared spectroscopy. The main contents and conclusions are as follow.?1?The rich-water filling material is consists of A components ?sulphoaluminate cement?, B components ?dihydrate gypsum and lime?. After studing on the type and dosage of admixture, the compound retarder and early strength agent, suspending and accelerator for sulphoaluminate cement should be added in rich-water filling material. When the ratio of A and B components is 10:?5+2?, the rich-water filling material with a gelling time of 10 min and 28 days compressive strength of 2.61MPa are prepared. On this basis, slag and fly ash are added into A components to optimize the mixture ratio, and the new-type rich-water filling material is prepared when the ratio of A and B components is ?90+6+4?:?40+15?. The compressive strength is up to 1.22MPa when water-solid ratio is 3.5. The materials with different water-solid ratio are curing for 360 days, and the compressive strength shows a tendency to increase first, and decrease and then increase. The first intensity peak of new-type rich-water filling material appeared later. The main hydration product of rich-water filling material is ettringite, which can form a dense network structure to provide strength for material. The compressive strength of rich-water filling material under water curing gradually decreased. The internal microstructure of material became loose and the porosity increased.?2?After immersing in acid solution with tha pH value of 1, the surface of rich-water filling material is greatly affected, occurring the phenomenon of softening off angle. With the pH value increases, the impact on the appearance of material is gradually weakened. The compressive strength decreases with the extension of immersing time and reduction of pH value, and it tost completely after immersed in hydrochloric acid and sulfuric acid solution with pH value of 1 for 180 and 360 days respectively. The compressive strength of filling material in sulfuric acid solution is lower than that of hydrochloric acid solution under the conditions of same solution concentration, water-solid ratio and curing ages. With the increase of immersing time and concentration of the solution, features and kinds of internal hydration product change. The main hydrated product, ettringite, decreased, and the hardening structure gradually became loose. The production of hydrocalumite ?Friedel's salt? in hydrochloric acid solution shows that the material has a solidification effect on chloride ions, but the effect is weak. The material mainly gets dissolved corrosion caused by the neutralization of H+. A large number of dehydrate gypsum is generated in the hardening body of filling material immersed in the sulfuric acid solution The material in sulfuric acid solution mainly gets dissolved corrosion of hardening body and expansion corrosion caused by dihydrate gypsum.?3?The compressive strength of rich-water filling material in the chloride solution with the extension of immersing time, increase of solution concentration and water-solid ratio. The compressive strength of material in magnesium chloride solution are lower than that of sodium chloride solution under various immersing conditions. When the compressive strength is tested, the material in magnesium chloride solution is found that the phenomenon of 0.8cm layer of peeling off The filling material in sodium chloride solution generate hydrocalumite with the extension of immersing time, increase of water-solid ratio and solution concentration, and found that a different number of sodium chloride crystals. The hardening structure gradually loose. The diversification of microstructure of the material in magnesium chloride solution is similar to that of sodium chloride solution, and the difference is that it is gradually forms hydrotalcite and gypsum in the immering process. In addition to the corrosive effect of Cl- ton's permeation, the material in magnesium chloride solution gets cross-corrosion caused by Mg²⁺, which results in more serious corrosion on the filling material.?4?The rich-water filling material appears phenomenon of mud from outside to the inside after immersed in sodium carbonate solution, and the compressive strength is afffected serious impact, reducing significantly. The compressive strength of material is lost after immersed in the 15% concentration of solution for 90 days. The compressive strength of the material is lost after immersed for 180 days under the immersing conditions except that of in 5% concentration of solution. After immerse, the number of ettringite gradually reduces with the increase of immersing time, and calcium monocaruminonsilicate and thaumasite are generated. Since the material is in the environment with a large number of CO₃^2- ions, the ettringite transforms to produce tots of thaumasite, which will cause serious damage.?5?After immersed in the acidic solution with pH value of 3 and the soluble salt solution at the concentration of 10%?chloride solution, carbonate solution?, the compressive strength toss rate of rich-water filling material with water-solid ratio of 1.7 is the lowest. It is recommented that the water-solid ratio of 1.7:1 ?water volume fraction of 83%? is used in acid, chloride and carbonate environment.?6?After immerse in corrosive solution, the change of new-type filling material is different from that of the original filling material. The toss rate of compressive strength is not all increases, and it shows a trend of first increasing and then descending. The first intensity peak appears almost completely at the immersing time for 180 days. And the compressive strength increases with the immersing time in acidic environment. The material immersed in sodium carbonate solution appears the phenomenon of mud, and the appearance are more complete in other solutions. The main hydrated product of material in acidic environment is ettringite, and the hydrocaluminate is formed in chloride solution, in which the sodium chloride is crystallized in sodium chloride solution, and the gypsum is formed in magnesium chloride solution. The thaumasite is formed in sodium carbonate solution. In constract to the original rich-water filling material, the ability of new-type to resist the corrosion of aggressive solution has been greatly improved.