硝酸侵蚀/冻融循环共同作用下喷射混凝土耐久性能(II)——pH值及NO -3扩散#br#

为进一步研究硝酸侵蚀/冻融循环共同作用喷射混凝土耐久性能，试验采用固液萃取法和电化学法，测试喷射混凝土pH值和含量，讨论冻融损伤和喷射混凝土配合比对其孔溶液离子扩散的影响。结果表明，冻融损伤喷射混凝土中微裂缝数量增多并扩展，继而相互连通形成裂缝，硝酸扩散速度加快。喷射混凝土pH值随冻融循环次数增大逐渐降低，含量增大。高水胶比及粉煤灰掺量喷射混凝土pH值低，含量大。与普通喷射混凝土相比，喷射钢纤维混凝土离子含量变化幅度较小，抗冻性能较高。共同作用下喷射混凝土同时遭受H+、及冻融损伤三重作用。在硝酸化学侵蚀作用下，混凝土pH值快速下降，含量增加，水化产物被侵蚀或发生分解，使其矿物组成及微结构发生改变，促进微裂缝萌生。同时，在与冻融循环形成的盐冻作用下，混凝土表面水泥砂浆及骨料快速剥蚀，最终导致混凝土结构酥松，物理力学性能退化。

Study on the durability performance of shotcrete under coupling effect of nitric acid attack and freeze-thaw cycles — Part II: pH value andNO -3diffusion

In order to further study the durability degradation mechanism of shotcrete under coupling effect of nitric acid attack and freeze-thaw cycles, by using solid liquid extraction method and electrochemical method, pH value and nitrate ion content testing are carried out considering the influence of frost damage and shotcrete mix proportions. The results show that micropores in shotcrete under coupling effect continue to be formed and transformed into macropores which interconnect into cracks. With the increase of freeze-thaw cycles, pH value of shotcrete gradually decreases and nitrate ion content increases. For shotcrete with high water-binder ratio and/or high content of fly ash, pH value is low and nitrate ion content is high. Compared with shotcrete without steel fiber, the variation range of ions content of steel fiber reinforced shotcrete is relatively small. Therefore, the frost resistance of steel fiber reinforced shotcrete is high. Coupling effects make shotcrete suffer from hydrogen ion, nitrate ion and freeze-thaw cycle. Under the chemical attack of nitric acid, pH value of concrete quickly decreases and nitrate ion content increases, hydration products are eroded and/or decomposed which leads to the hange of mineral composition. Meanwhile, under the influence of salt frost damage produced by nitrate ion and freeze-thaw cycles, mortar and aggregate on the shotcrete surface peel off, leading to loose and air-void structure as well as deterioration of the physical and mechanical properties.