Study On Calcium Corrosion Process Of Cement - Based Materials Under Water Delivery And Its Effect On Chloride Ion Transport

The cement mortar lining of ductile iron pipe which is in hydrodynamic environment for a long time will cause calcium leaching damage due to calcium hydroxide dissolution and CSH gel decalcification, cast iron corrosion due to chloride penetration for the reason that there is little amount of calcium ions and a certain amount of chloride ions in the low pH water, further, it will reduce the durability and service life of the ductile iron pipe. Calcium leaching and chloride penetration are important questions in the area of the durability of cement-based materials, and it has important theoretical value to the basic scientific research of cement based materials. Therefore, to analysis the corrosion mechanism of cement based materials in the hydrodynamic environment, establish the control equation of calcium ion in cement based materials under calcium leaching, and build the chloride ion transmission model under calcium leaching, is very important to the design of the durability of structures.This paper investigated the calcium leaching and chloride penetration of cement mortar lining of ductile iron pipe in hydrodynamic environment. By theoretical analysis, theoretical modeling, experimental verification and numerical simulation, we analysis the corrosion law of cement based materials and the change of chloride penetration process induced by calcium leaching. The main contents and conclusions are as follows:(1) The calcium leaching model of cement mortar lining of ductile iron pipe in hydrodynamic environment is established. The seepage velocity of calcium ions produced by the water pressure in the process of calcium leaching is analyzed, and the basic leaching parameters of cement based materials after harden is formed.(2) By means of solving the discrete equation based on the Crank-Nicolson difference scheme of the finite element method, the analysis program was developed using MATLAB language. According to the hierarchical feature of lining mortar, the hierarchical problem caused by centrifugal process is considered in editing the program and inputting the parameters.(3) The numerical simulation of calcium leaching process of cement mortar in deionized water is carried out using the MATLAB software, and the calcium ion concentration in the box is compared with the simulation value, the result shows that they fit well. The effect of time step, position step, water cement ratio, temperature and seepage to the leaching process of the cement mortar lining is analyzed according to the Matlab program, and the leaching depths are in good agreement with the literature value.(4) The difference between constant volume and infinite volume corrosion solution in the leaching process of cement based materials was analyzed. The leaching process in the ammonium chloride solution is simulated, and the cement paste containing slag is also analyzed. The results show that the constant volume and infinite volume corrosion solution has little influence on the leaching process in a reasonable period, the cement based materials with moderate slag can improve the anti-corrosion ability.(5) The chloride ion transmission equation under calcium leaching is formed, and the chloride ion transmission process under calcium leaching in paste lining is compared with the situation of traditional Fick’s model. The results show that the penetration process of chloride ion in the cement based materials is accelerated by calcium leaching, and the chloride ion binding capacity is gradually reduced as the corrosion degree increases in the leaching areas.