边界条件对水泥基材干湿循环下氯离子扩散对流峰值和对流深度的影响

The Influence of Boundary Conditions on the Peak Value and Depth of Chloride Ion Diffusion Convection under Dry-Wet Cycles in Cement-Based Materials

摘要: 本文研究了干湿循环条件下氯离子在水泥基材料中的扩散行为，分析了初始饱和度及干湿时间比对氯离子扩散深度和浓度峰值的影响。利用COMSOL Multiphysics软件建立二维耦合传输模型，通过实验验证了模型的准确性，探讨了氯离子在非饱和混凝土中的迁移机制。结果表明，干湿循环显著加速了氯离子的扩散过程，这一效应主要源于孔隙结构和浓度梯度的动态变化。实验数据显示，初始氯离子浓度和干湿时间比对对流深度和浓度峰值具有显著影响。以初始浓度3%~6%下，在50天的干湿循环中，氯离子扩散主要局限于表层区域，扩散深度为4.32~4.74 mm；在100天后，扩散深度在6.88~7.01 mm范围中，浓度峰值则从3.35%~6.68%提升至4.42%~8.83%。干湿时间比的变化同样显著影响氯离子的迁移行为，当干湿时间比为1:2时，对流深度和峰值浓度分别达到最大值9.33 mm和4.75%，相比1:0组提升了58.3%。通过干湿循环模拟还发现，湿润阶段扩散效应与干燥阶段浓度梯度作用叠加，共同推动了氯离子在基材内的迁移与积聚。孔隙结构的动态变化及水化产物的微观反应为氯离子的扩散提供了通道，进一步强化了其时间依赖性。本文研究为沿海及湿热环境下混凝土结构的耐久性设计提供了科学依据，同时为材料性能优化和侵蚀行为预测提供了理论支持。未来研究可深入探讨温度、湿润时间及盐分浓度等多因素对侵蚀行为的协同影响，为复杂环境下工程材料的开发和应用提供指导。

Abstract: This study investigates the diffusion behavior of chloride ions in cement-based materials under dry-wet cycling conditions, analyzing the effects of initial saturation and dry-wet time ratios on diffusion depth and concentration peaks. A two-dimensional coupled transport model was established using COMSOL Multiphysics software, and its accuracy was validated through experiments. The migration mechanism of chloride ions in unsaturated concrete was further explored. Results indicate that dry-wet cycling significantly accelerates chloride ion diffusion, primarily due to dynamic changes in pore structure and concentration gradients. Experimental data show that initial chloride concentration and dry-wet time ratios have a significant impact on convection depth and concentration peaks. For initial concentrations ranging from 3% to 6%, chloride ion diffusion during 50 days of dry-wet cycling was mainly confined to the surface layer, with a diffusion depth of 4.32~4.74 mm. After 100 days, the diffusion depth increased to 6.88~7.01 mm, and the concentration peak rose from 3.35%~6.68% to 4.42%~8.83%. Variations in dry-wet time ratios also significantly influenced chloride migration; when the ratio was 1:2, the convection depth and peak concentration reached maximum values of 9.33 mm and 4.75%, respectively, representing a 58.3% increase compared to the 1:0 group. Simulations further revealed that the combined effects of diffusion during the wetting phase and concentration gradient during the drying phase jointly promote the migration and accumulation of chloride ions within the substrate. The dynamic changes in pore structure and the microstructural responses of hydration products provide pathways for chloride diffusion, enhancing its time dependency. This study offers scientific insights for designing durable concrete structures in coastal and humid environments, while also providing theoretical support for material optimization and corrosion behavior prediction. Future research could explore the synergistic effects of factors such as temperature, wetting time, and salt concentration on corrosion behavior, offering guidance for the development and application of engineering materials in complex environments.