盐湖侵蚀环境喷射混凝土耐久性能劣化规律及机理研究

Durability deterioration and corrosion mechanism of shotcrete in salt lake environment

我国西部地区盐湖分布广泛,土壤及地下水中含有高浓度硫酸盐、镁盐及氯盐,与衬砌喷射混凝土发生一系列物理化学反应,造成其结构耐久性能下降。为系统研究盐湖侵蚀环境喷射混凝土耐久性能劣化规律及机理,以5%Na2SO4+5%MgSO4+3.5%NaCl混合溶液为侵蚀介质,采用干湿交替法,分别模拟盐湖环境地下水及隧道衬砌侵蚀方式,开展喷射混凝土耐久性试验,对其物理力学性能、侵蚀产物矿物组成及含量、微观形貌、孔结构和离子含量进行测试分析。喷射混凝土SO42-含量随侵蚀时间快速增大,Cl-和Na+含量缓慢增大,而Ca2+和混凝土pH值降低。喷射混凝土侵蚀过程包含水镁石、石膏及钙矾石形成阶段,C-S-H分解和碳硫硅钙石形成阶段,M-S-H形成等三个阶段。最终,在碳硫硅钙石、水镁石、石膏、钙矾石以及结晶盐所形成的膨胀应力和结晶压力共同作用下,喷射混凝土内部孔数量及空气含量增大,形成网状裂纹,性能快速劣化。模筑混凝土微裂缝在盐结晶形成的结晶压作用下快速开裂,与气孔形成宏观裂缝导致断裂。钢纤维可显著消减喷射混凝土内部应力,提高其抗侵蚀能力。

Salt lake is widely distributed in western China.The high concentrations of chlorine salt,sulfate,and magnesium salt in the soil and groundwater are the major causes leading to the deterioration of lining shotcrete and the corrosion of reinforcement.In order to systematically study the deterioration rules and mechanism of the durability of shotcrete lining in salt lake corrosion environment,the dry-wet alternate method was employed to simulate the corrosion of tunnel lining,and the durability experiment of shotcrete was carried out with5%Na2 SO4+5%MgSO4+3.5%NaCl mixed solution.Relative dynamic elastic modulus,mass change and relative compressive strength were measured.After experiencing corrosion,mineral composition and microstructure of corrosion products,pore structure and ion content were tested by using XRD,TG-DSC,SEM&EDX,liner traverse method and electrochemical method.Acid-soluble sulfate content may obviously increase with the increase of the dry-wet cycles,and water-soluble chloride and sodium content slowly increased.However,the pH value of water-soluble calcium and concrete may decrease.The corrosion process of shotcrete is composed of three stages,including the formation stage of brucite,gypsum and ettringite,the stage of C-S-H decomposition and thaumasite formation,and the formation stage of M-S-H.Eventually,under the coactions of swelling stress and crystallization pressure from thaumasite,brucite,gypsum,ettringite and crystallized salt,the amount of pores and air content in shotcrete were increased,so that the formation of netty cracks could result in the quick deterioration of shotcrete in physical and mechanical properties.The microcracks of mould concrete quickly propagated under the crystallization pressure of salt and truned to macrocracks after being connected with pores.Steel fibers can remarkably alleviate the internal stress of shotcrete,and hence can improve the corrosion resistance of shotcrete.