复杂盐环境下硫酸盐诱导的裂纹损伤对混凝土介质传输特性的影响

Cracks are one of the key factors to the mass transfer property of concrete. Previous studies mainly concentrate on those cracks caused by the shrinkage and loading. Whereas now a large number of concretes is in a complex surroundings, such as the oceans containing Magnesium sulfate chloride salt and also the saline alkali soil, however less researches were operated on influences on the mass transfer property of concrete because of the extensions of the cracks caused by the continuous sulfate damages. This project aims to investigate the influences of the erosion products and its microstructural evolution on the formation and extension of cracks suffering the continuous sulfate damages, in order to illuminate the mechanism and the evolution of the crack formations. Simultaneously, the dynamic relationship between the crack extensions and the transmission characteristic of the water and ions(Cl-、SO42-、Mg2+) in the concrete is also explored under the cooperation of sulfate, chlorine and magnesium, as a result of establishing the unsteady model of the chlorine transmission in the concrete with the crack damages induced by the sulfate. Thereafter,time dependent laws of the interface, pore structure and both the micro and macro properties of concrete will be discussed being the surroundings filled with magnesium, sulfate and chloride, and then the degradation mechanism will be revealed. Meanwhile, concerned admixtures and microbial capsules will be used to inhabit the formation of ettringite and repair the micro cracks, which can improve the anti-permeability of the concrete. All above results can be useful to the establishment of the life production models of the concrete served in complex surroundings and also can be the guidance of designing and repairing the debility of the concrete structure.

裂纹是影响混凝土传质性能的关键因素，相关研究大多关注收缩、载荷作用产生的裂纹。现在越来越多混凝土处于硫酸盐-氯盐-镁盐复合的海洋或盐碱土壤环境，而对该环境中硫酸盐持续损伤引起的裂纹扩展对混凝土传质性能影响的研究较少。本项目拟研究硫酸盐持续损伤下侵蚀产物相、微结构演变对裂纹形成和扩展的影响，阐明硫酸盐损伤下混凝土中裂纹的形成机制和演变过程；研究硫酸盐-氯盐-镁盐作用下混凝土中水分、离子（Cl-、SO42-、Mg2+）传输特性与裂纹扩展间的动态关系，建立硫酸盐诱导裂纹损伤下混凝土中氯离子的非稳态传输模型；研究硫酸盐-氯盐-镁盐环境下混凝土界面、孔结构和细微观性能的时变规律，揭示该环境下混凝土的劣化机制，并探究抑制钙矾石形成和修复微裂纹的方法，以提高硫酸盐-氯盐-镁盐环境下混凝土的抗渗性。成果将有利于多因素环境下服役寿命预测模型的建立、指导多因素环境下混凝土结构耐久性设计和修复。

在硫酸盐侵蚀混凝土的实际环境中常伴随着其它有害离子的存在，如Mg2+、Cl-等，而硫酸盐侵蚀混凝土过程中硫酸盐与水泥基材料中的铝相会形成钙矾石造成损伤裂缝，裂缝的存在又会加快这些有害离子向混凝土内部的渗透并侵蚀混凝土，本项目探究了硫酸盐、氯盐、镁盐环境下硫酸盐持续损伤对混凝土裂纹形成与演变的影响，并对复杂盐环境下混凝土中水及离子传输与硫酸盐诱导裂纹扩展的动态关系进行了分析，最后阐明了硫酸盐-氯盐-镁盐多离子耦合作用下混凝土的损伤机制。本项目分别研究了C3A单矿和砂浆在硫酸根离子、硫酸根离子+镁离子、硫酸根离子+氯离子+镁离子三种环境下试件的裂纹的形成和扩展情况。研究结果表明在硫酸盐环境中，水泥石会与侵入的硫酸根离子反应生成钙矾石，引起膨胀开裂损伤破坏。铝酸三钙含量从6.7%增加15.1%，砂浆内部钙矾石的长度从1-2μm变为3-4μm，分布范围更广，形成的微观裂纹更多，裂缝更宽。在侵蚀后期高含量铝酸三钙砂浆中的钙矾石变为长度为5-6μm棒状钙矾石，密集的堆积在砂浆内部，砂浆具有明显的开裂特征。氯离子存在时，氯离子会先于硫酸根离子与钙离子和铝酸根离子反应生成水化氯铝酸钙，从降低了硫酸盐侵蚀砂浆过程中钙矾石含量，减缓了膨胀开裂损伤破坏。镁离子的存在，会生成氢氧化镁沉淀附着在单矿水泥石表面，一方面减缓了硫酸根离子的向内渗透，减缓了单矿水泥石在硫酸盐溶液中的破坏，另一方面降低孔溶液中pH值，会有利于石膏的生长。在Cl-与SO42-复合环境下，氯离子的存在减缓了硫酸钠溶液中砂浆的劣化损伤速度，氯离子会先于硫酸根离子渗入砂浆内部与其中的钙离子和铝酸根离子反应生成水化氯铝酸钙，从降低了硫酸盐侵蚀砂浆过程中钙矾石和石膏的含量，减缓了砂浆的裂纹损伤破坏。在Cl-、SO42-与Mg2+复合环境下，氯离子的存在加快了硫酸镁溶液中砂浆的劣化损伤速度，当砂浆浸泡在硫酸镁与氯化钠复合溶液中时，一方面镁离子的存在会降低砂浆孔溶液的pH，虽然氯离子的存在减少了钙矾石的生成量，但是孔溶液中游离的硫酸根离子浓度的增加会有利于石膏的生成，造成明显的强度损失，引起性能的快速劣化。