Study On Degradation Mechanisms Of Cement-based Materials Under Multiple-salts Action In Marine Environment

Building a maritime power is of great and far-reaching significance to promote the sustained and healthy development of economy and safeguard national sovereignty,security and development interests.The cement-based material is one of the most important materials in the marine development and reefs construction.However,the cement-based materials used in the offshore environment subjected to the environment of high temperature,high humidity,high salt and high sunshine for a long time,and combined with the corrosive media such as chloride ion,sulfate ion,magnesium ion in seawater,which make different degrees physical and chemical failures of marine concrete structures.This paper aims to the durability of marine concrete and to simulate the ocean splash zone environment,and systematically investigate the coupling factors of sulfate,magnesium and chloride salt together with drying-wetting cycles on the degradation progress of cement-based materials.The damage variations of cement-based materials under seawater action are studied emphatically.In addition,the chloride ion diffusion in cement-based materials under the seawater action is explored and the diffusion model of chloride ion is established.The research contents and achievements are as follows:(1)The chloride diffusion of cement-based materials exposed to the single and multiple salts actions of chloride,magnesium and sulfate salt indicated that,in contrast to the Na Cl solution,the SO₄^2-+Cl^-increased the chloride binding capacity and the presence of SO₄^2-had limited influence on chloride ion diffusion.While the Mg²⁺+Cl^-and Mg²⁺+SO₄^2-+Cl^-decreased the binding capacity and increased the chloride diffusion.Among them,the presence of single Mg²⁺had the greatest influence on the binding capacity.The chloride binding capacity in the multiple salts solutions decreased with increasing the exposure time.(2)The experimental results under the drying-wettig cycle and multiple salts action showed that the drying-wetting cycles increased the concentration of SO₄^2-in the composite solution of SO₄^2-+Cl^-,which inhibit the chloride diffusion at the early stage,but at the later stage promote the chloride diffusion.The presence of SO₄^2-and SO₄^2-+Mg²⁺in chloride environment increased the chloride diffusion,and Mg²⁺exhibited the largest increase of chloride diffusion under drying-wetting cycle condition.The chloride diffusion coefficient of cement-based materials decreased with the increasing of curing age.In addition,ultraviolet radiation increased the chloride diffusion coefficient of cement-based materials at the early stage under the drying-wetting cycle and multiple salts action,but at the late stage of corrosion,the effects of ultraviolet radiation on chloride diffusion is negligible.(3)Based on the chloride diffusion mechanism of cement-based materials exposed to full immersion zone and variable water level zone in marine environment,the transportation model of saturated state and drying-wetting cycle state in seawater was established.The results were in good agreement with the experimental data by COMSOL software calculating the model.(4)The relative dynamic elastic modulus and mass loss rate are used to characterize the damage of cement-based materials under multiple salts and drying-wetting cycle.The results showed that the deterioration process of cement-based materials includes three stages,namely performance improvement stage,performance stability stage and performance deterioration stage.In the composite environment of SO₄^2-+Cl^-,the presence of SO₄^2-initially improved the internal structure,but generated the expansive products at the later stage of corrosion,causing matrix damage and accelerating deterioration.While the Mg²⁺+SO₄^2-+Cl^-multiple salts further aggravated the deterioration of cement-based materials.(5)Compared with the single chloride salt under drying-wetting cycles,the presence of SO₄^2-and SO₄^2-+Mg²⁺increased the transformation of the mixed phases of AFm and AFt and decreased the content of CH and C-S-H gel of cement-based materials.Meanwhile,the combined actions of SO₄^2-and SO₄^2-+Mg²⁺decreased the MCL of C-S-H gel in the paste,which aggravated the calcification of C-S-H gel and promoted the formation of AFt.The SO₄^2-+Cl^-generated more defective pores,while the SO₄^2-+Mg²⁺+Cl^-caused the greatest damage to pore structure in the matrix,which further promoted the formation of connected pores,and resulted in the ingress of soluble salt.Moreover,the combined actions of SO₄^2-and SO₄^2-+Mg²⁺in chloride environment increase the formation of micro-cracking and porosity of matrix.(6)The mineral admixtures FA BFS and MK enhanced the resistance to degradation of cement-based materials exposed to the SO₄^2-+Mg²⁺+Cl^-multiple salts solution and drying-wetting cycles.Among them,the incorporating of 50%BFS significantly improved the stability and increase the MCL of C-S-H gel,which restrained the decalcification and impeded the formation of AFt effectively.Meanwhile,it reduced the defective pores,decreased the connected pores and larger pore volumes,and refined pore size and optimized the pore structure of cement-based materials.