【作者】 刘赞群；

【导师】 邓德华； Geert De Schutter；

【作者基本信息】 中南大学 ， 土木工程材料， 2010， 博士

【摘要】 作为混凝土耐久性的一个主要方面,硫酸盐侵蚀的相关研究已有100多年的历史,但由于试验模拟方法和实际工程暴露条件间的差异,因而仍认为混凝土硫酸盐侵蚀研究现状是一个“混乱的世界”。本文从实际出发,系统深入分析实际混凝土结构和外界环境之间的相互关系和工程实例,针对实际工程中两种暴露方式-半埋和全埋,设计并进行了一系列较切合实际工程的室内试验研究,提出了相应的混凝土硫酸盐侵蚀机理。半埋混凝土-构件一部分埋入地下水土中或与水土紧密接触,而另一部分暴露在相对干燥的空气中。工程实例表明,半埋混凝土暴露于空气中的部分发生了类似冻融剥落的严重劣化,通常认为盐结晶是其主要破坏机理。然而,工程实例、长期野外试验和相关试验室研究结果均没有充分证明这个观点。本文通过混凝土孔溶液压滤试验结果表明,由于“灯芯效应”的“传输-浓缩”过程,在较短的时间内会在暴露于空气中的混凝土中形成一高硫酸盐浓度(接近饱和)高pH值(13.0左右)孔溶液区。微观分析结果表明,暴露在温湿度稳定或剧烈变化的环境中,未能在水泥净浆与混凝土试件内观测到硫酸盐结晶体,但发现了钙矾石、石膏和氢氧化镁等化学侵蚀产物,只在混凝土碳化区观测到硫酸盐结晶体。混凝土宏观力学性能试验显示：混凝土碳化、潮湿的外界环境、矿物掺合料都会加剧硫酸盐对半埋混凝土的侵蚀破坏作用,混凝土表面涂层能有效提高半埋混凝土的抗硫酸盐侵蚀性能。1.根据试验结果和理论分析,首先阐述了混凝土内部不能产生盐结晶破坏的原因：(1)化学反应消耗了孔溶液中的硫酸盐,其消耗速度随孔溶液浓度而加快,使得孔溶液难以满足盐结晶所需条件-过饱和,致使半埋混凝土内部孔溶液中很难发生硫酸盐结晶。(2)发生在单个孔隙中的盐结晶不会导致材料破坏,只有晶体在多孔材料内部不同大小,不同形状的孔隙内结晶生长产生的压力才会导致材料的破坏。但在混凝土中,由于硫酸盐和水泥水化产物之间的化学结合作用,晶体和孔壁之间的溶液薄膜消失,晶体停止生长,也不会产生结晶压力破坏。(3)对于半埋混凝土试件,通过“灯芯效应”在混凝土内形成了孔溶液区,很难在内部形成过饱和浓度的干湿界面区引起内部结晶破坏。2.在此基础上,提出了由“灯芯效应”(wick action)的传输-浓缩过程形成的高浓度、高pH值硫酸盐孔溶液区内发生的严重化学侵蚀是导致半埋混凝土中暴露于空气部分较快严重破坏的主要机理。本机理能较好地解释以下一些事实：(1)掺加矿物掺合料不能改善半埋混凝土的抗硫酸盐性能,甚至加剧劣化。其原因是：一方面,掺加矿物掺和料有利于因“灯芯效应”在混凝土内部形成更广的高浓度、高pH值硫酸盐孔溶液区,另一方面,在高浓度、高pH值的硫酸盐孔溶液作用下,矿物掺合料中残留的活性组分会被激发而生成膨胀性产物。(2)潮湿环境中半埋混凝土发生严重侵蚀破坏的区域广,其原因是潮湿环境有利于因“灯芯效应”在混凝土内部形成更广的高浓度、高pH值硫酸盐孔溶液区。(3)混凝土碳化将加剧半埋混凝土硫酸盐侵蚀破坏作用。因一方面混凝土碳化能使盐结晶发生；另一方面能提高混凝土内的毛细管吸附作用,形成更广的高浓度、高pH值硫酸盐孔溶液区。(4)混凝土表面涂层处理能有效提高半埋混凝土抗硫酸盐侵蚀能力,因涂层可以阻碍在混凝土内部形成高浓度、高pH值硫酸盐孔溶液区。3.最后,提出了半埋混凝土两阶段硫酸盐侵蚀劣化模式：首先在较短时间内形成高浓度、高pH值硫酸盐孔溶液区；然后发生化学侵蚀破坏过程,其特点是在骨料界面处生成石膏,其他部分内主要生成钙矾石。劣化模式描述了半埋混凝土在硫酸盐侵蚀过程中的内部微观变化,解释了混凝土沿骨料界面处发生涨裂性破坏是造成半埋混凝土发生类似冻融剥落破坏的主要原因。全埋混凝土-全部位于地表下,或者深埋在地表面下建筑物的一部分,实际破坏工程实例显示,碳硫硅钙石是引起全埋混凝土破坏的主要产物。通过对在混合硫酸盐溶液以及高浓度硫酸镁溶液中各种水泥基材料的生成产物的研究,提出了碳硫硅钙石侵蚀是发生在一种随着时间累积,侵蚀环境浓度不断升高(大于现场地下水测量值),同时也是高pH值(大于12.5)的硫酸盐环境中的破坏机理。综合以上两种情况,认为,在含有硫酸盐的服役环境中,不管是全埋混凝土还是半埋混凝土,发生的严重破坏都是由经时累积的高硫酸盐含量下的化学侵蚀引起的。更多还原

【Abstract】 Sulfate attack on concrete is a major issue within concrete durability. This detrimental process has been reported to be a cause of damage to concrete structures for over a century. However, due to the discrepancy between laboratory tests and field cases, sulfate attack on concrete is still a confused world.In order to avoid the above discrepancy, first, according the extensive and in-depth literature review on the analysis of field cases and the relationship between concrete structures and their environment, two types of sulfate attack on concrete were defined as follows:sulfate attack on fully buried concrete and sulfate attack on partially exposed concrete. Then, a series of tests based on the appearance of damaged field cases are performed. At last, the basic mechanisms of sulfate attack on these two types of concrete are proposed.Concerning the partially exposed concrete (a part of concrete in contact with sulfate environment and another part exposed to relative dry air), the part of concrete exposed to air is always severely damaged in a manner similar to that of freezing-and-thawing scaling, however the part of concrete in contact with sulfate environment is still sound. This appearance is normally taken for granted to be caused by physical sulfate attack on or salt crystallization in concrete. However, the field cases, the experimental results of long term field experiences, and the indoor laboratory studies do not show strong evidence to prove this view.In this thesis, pore solution expression disclosed that an aquiferous zone containing a large amount of high SO42- concentration (almost saturated) and high pH value (around 13.0)pore solution was generated in the upper parts of cement paste in contact with air due to wick action. On the other hand, results obtained by means of micro-analysis did not show any evidence to prove that salt crystallization can occur in the hydrated cement phase in spite of cement paste or concrete and in spite of constant exposure condition or sharply fluctuated exposure condition. However, the chemical reaction products, ettringite, gypsum and brucite, were the main causes that induce the failure of cementitious materials. Salt crystallization can occur in the calcite crystals, the carbonated products of hydrated cement phases. The mechanical performances of partially exposed concrete cylinders showed that mineral additions, concrete carbonation and high relative humidity exposure condition could accelerate the concrete damage by sulfate attack, and the surface coating could effectively improve the sulfate resisting performance of concrete.First, the reasons why salt crystallization distress cannot result in concrete damage are as follows:(1) Supersaturation is the key factor to generate salt crystallization, however the chemical reactions between sulfates and cement hydrated phases would consume a lot of sulfate ions and the higher concentration and the faster reaction rate. So, it is difficult to reach supersaturation in the concrete.(2) The stress existing in a single pore cannot cause failure, for fracture to occur the crystals must propagate through a region of the network with a lot of pores with different sizes and shapes. However, during the growth of crystals, the chemical adsorption between sulfates and hydrated cement phases makes the pore wall attractive to the salt, rather than repelling. This results in the situation that the thin liquid film between crystals and pore walls cannot be formed. Crystals will grow into contact with the pore walls, and will stop growing, so that the crystallization stress disappears.(3) During the process of wick action, it is difficult to form the dry-wet interface in the bulk of concrete to form sub-efflorescence due to the rich pore solution in the concrete.Secondly, based on the above results, the real damage mechanism of sulfate attack was proposed that the severely chemical attack occurring in the aquiferous zone containing almost saturated sulfate and high pH value pore solution caused the concrete damage. The following appearances can be logically explained by the above mechanism:(1) The blend concretes are always deteriorated more severely than the normal concretes when they are partially exposed to sulfate envrionment. There are two factors responsible for this appearance, due to wick action a more extensive aquiferous zone can be formed in the blend materials. On the other hand, reactive aluminum present in blended cementitious materials is activated by high concentration sodium sulfate solution with high pH value, as generated in the aquiferous zone. This leads to more ettringite formation, and a more severe deterioration than in case of ordinary cementitious materials.(2) When the partially exposed concrete is stored in a higher relative humidity environment, a more extensive aquiferous zone can be generated than in a lower relative humidity environment, this results in more severe damage.(3) The carbonation of concrete plays a negative role in the sulfate resisting performance of concrete. It enables salt crystallization in concrete. Furthermore, carbonation can improve the capillary suction of concrete and promote the formation of aquiferous zone in the upper part of concrete in contact with air.(4) The surface coating is the most effective way to improve the sulfate resisting performance of concrete because it can prevent the formation of aquiferous zone due to wick action in the concrete.At last, a two-stage model involving the formation of aquiferous zone and the progress of sulfate attack was proposed. First, the aquiferious zone can reach equilibrium in a relative short period. Then, during the process of sulfate attack, gypsum would accumulate in the interface zone and ettringite would be formed in the binding medium. Small piece of concrete spalled along the interface zone due to the accumulation gypsum is the main reason why the partially exposed concrete was damage in a manner similar to that of freezing-and-thawing scaling.Concerning sulfate attack on fully buried concrete (concrete fully burid under the earth surface or deeply buried in the soil), according to the survey of field cases, the thaumasite sulfate attack (TSA) was attributed to the failure of concrete. In this thesis, the effects of mixed sulfates solution and high concentration magnesium sulfate solution on the thaumasite formation were studied. Combining previous research and field experiences, it can be concluded that thaumasite sulfate attack took place in the perched sulfates solution with high pH value (>12.5) and high concentrations sulfate solution (higher than the monitored value of local groundwater).In summary, one major conclusion can be drawn. In the field case, if there is concrete damage by sulfate attack, whether it is in fully buried concrete or in partially exposed concrete, the main cause is the chemical sulfate attack occurred in a high concentration sulfate solution formed by a long time accumulation.更多还原