喷射混凝土的硝酸侵蚀:孔溶液H^+与NO_3^-的扩散规律及侵蚀机理

Nitric Acid Immersion Corrosion of Shotcrete Lining: Diffusion Law of Hydrogen Ions (H^+) and Nitrite Ions (NO_3^-), and the Corrosion Mechanism

一般大气环境下长大公路隧道中,喷射混凝土单层衬砌往往会遭受硝酸侵蚀,导致耐久性恶化及使用寿命降低。本研究以硝酸溶液为侵蚀介质,针对制备的普通喷射混凝土、钢纤维喷射混凝土和模筑混凝土开展浸泡试验。通过测试混凝土不同深度处孔溶液pH值和NO_3^-含量,分析喷射混凝土硝酸侵蚀过程。对硝酸侵蚀后试件的矿物组成和微观结构进行表征,以研究孔溶液中H^+和NO_3^-的扩散机理。实验结果表明,喷射混凝土孔溶液H^+和NO_3^-含量均小于模筑混凝土,即喷射混凝土抗硝酸侵蚀耐久性更优。侵蚀溶液pH值不大于2时,钢纤维喷射混凝土孔溶液中H^+和NO_3^-含量与普通喷射混凝土差异较小,说明钢纤维对较高浓度硝酸侵蚀环境下喷射混凝土耐久性能的提升作用不明显。针对喷射混凝土硝酸侵蚀机理的分析表明,硝酸侵蚀喷射混凝土的过程可分为水化产物反应、水化硅酸钙凝胶分解和钙矾石及骨料侵蚀等三个阶段。因此,孔溶液H^+在侵蚀早期增长速度快于侵蚀后期;NO_3^-含量则随侵蚀过程的推移而快速增大。

The commonly used single-layer shotcrete linings in long highway tunnels under general atmospheric environment usually suffer nitric acid corrosion,and as consequences,the durability and service lifetime are prone to deterioration.This paper presents the experimental study and the mechanism analysis over the diffusion of hydrogen ions(H^+)and nitrate ions(NO3^-)in the pore solution of shotcrete lining experiencing immersion corrosion in nitric acid.The nitric acid(with pH values of 1,2 and 3)immersion tests upon a series of specimens of ordinary shotcrete,steel-fiber-reinforced shotcrete and cast concrete were conducted.The changes in physical and mechanical properties of the specimens during the entire 180 d immersion process were measured,and the H^+and NO3^-concentration gradients along vertical(depth)direction for the pore solution within the three kinds of concrete specimens were investigated systematically by varying the nitric acid pH value and immersion duration.Moreover,the mineral composition determination and microstructure observation for the corrosion products were also carried out,and the diffusion laws of H^+and NO3^-in the pore solution were analyzed.As was expected,both of the H^+and NO3^-concentrations of pore solution within shotcrete specimens were found to be lower than those for cast concrete specimens,confirming a higher resistance against nitric acid corrosion for shotcrete.The steel fiber addition has inconspicuous effect for the durability improvement of shotcrete under relatively high-concentration nitrate corrosive circumstances,as nearly no difference could be identified for H^+/NO3^-concentrations between ordinary shotcrete specimens and steel-fiber-reinforced shotcrete specimens,both of which were immersed in pH≤2 nitric acid.It was concluded according to the corrosion mechanism analysis that the corrosion process of shotcrete in nitric acid includes three steps:reaction of hydration products(with the acid),decomposition of calcium silicate hydrate gel,and corrosion of ettringite and aggregate.Therefore,the pore solution within shotcrete displayed an H^+content change tendency of rapid increase(early stage)and subsequent slower increase(late stage),while on the other hand,a NO3^-content change tendency of always fast increase,as the corrosion getting processed.