【作者】 张廷雷；

【导师】 万朝均；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 硕士

【摘要】 混凝土内部孔溶液的pH值通常为12.5~13.5,在这种碱性环境中钢筋表面形成厚约2~6nm的钝化膜,起着保护钢筋的作用。混凝土的碳化,逐渐使其内部pH值降低,完全碳化混凝土的pH值约为8.5~9.0甚至更低,使混凝土中的钢筋脱钝化,易于产生锈蚀。铁锈的体积一般要增长2~4倍,造成钢筋截面膨胀,混凝土保护层开裂,构件承载能力降低,结构延性降低,进而导致混凝土结构破坏,因此碳化被认为是影响混凝土耐久性的主要因素之一。粉煤灰在水泥混凝土中大量应用,改善了混凝土的性能,节约水泥,有利于环保、节能,是混凝土可持续发展的必然趋势。但是,粉煤灰在发挥火山灰效应的同时,会消耗混凝土中的氢氧化钙,具有降低混凝土抗碳化能力和钢筋抗锈蚀能力的可能。因此,进一步研究进而提高粉煤灰混凝土的抗碳化能力,克服其在混凝土中应用的负面效果具有理论与实践意义。本论文通过净浆试件的加速碳化试验,分层测定碳化试件中碳酸盐的含量,绘制“碳酸盐含量—深度”曲线,较系统、深入的对粉煤灰水泥石的抗碳化性能及各影响因素进行了研究,主要得出以下结论:①碳化产物CaCO3主要以方解石为主。XRD分析和DSC分析均表明碳化产物几乎不含有文石和球霰石或含量较少;完全碳化区(即酚酞不显色区域)仍有少量Ca(OH)2晶体未被碳化,主要是因为这些晶体被C-S-H凝胶包裹,很难参与碳化反应;②水泥石的抗碳化性能随粉煤灰掺量的增加而变差。大掺量(50%和70%)粉煤灰水泥石的抗碳化性能很差,限制了其在实际工程中的应用;③大掺量粉煤灰水泥石的碳化程度随水胶比的增大而加重;④湿度越小,碳化程度越严重,尤其当湿度小于40%时,其完全碳化区长度和部分碳化区长度均有明显的增大。试验中并未产生相关文献[8,19,102]所述的:“湿度过低时(≤20%),碳化反应因缺水而受到限制的现象;湿度较大时(≥80%),虽然碳化反应很快,但由于气体向水泥石内部的扩散十分缓慢,使得部分碳化区长度较小”;⑤碳化前,水泥石在60℃下烘干24小时,使孔隙中水分蒸发,填充水甚少,加速碳化后,试件的碳化十分严重(碳酸盐含量较高),且出现了碳化不均匀现象,酚酞显色区域性不明显,即无明显的完全碳化区、碳化反应区(部分碳化区)和未碳化区之分,CaCO3含量曲线也没有明显的下降段;⑥水泥石的抗碳化性能与养护龄期之间无明显的规律性,似乎养护龄期增长对大掺量粉煤灰水泥石的抗碳化性能并无改善;⑦龄期为28天的试件,密封与饱和Ca(OH)2溶液两种养护制度下的碳化情况无明显差异,这可能是由于28天的龄期内,密封养护并未出现水化缺水现象,两种养护制度下水泥的水化程度基本相同,水泥石内部孔隙率及孔结构也未出现显著差异,因此CO2的扩散速度及碳化速度差异不大;⑧随着碳化时间的延续,碳化深度(即完全碳化区)不断增大,部分碳化区的长度也不断增大,且其可能与完全碳化区的长度存在一定的比例关系或某种特定关系,并可以利用模型公式加以计算,但其仍有待进一步深入研究;⑨高温养护和粉煤灰的磨细对大掺量粉煤灰水泥石的抗碳化性能有较好的改善效果;补足碱对大掺量粉煤灰水泥石抗碳化性能的改善作用不大,单掺NaOH时甚至出现抗碳化性能变差的现象。本论文中所采用的试验方法及装置为笔者及课题组成员共同设计发明,其有如下创新点:①采用净浆试件对水泥石碳化进行试验研究,消除了骨料对碳化及碳酸盐含量测定的影响;②自制了碳酸盐含量测定装置,由于自制装置控制措施较好,其精密性和精确性较相关文献[82-83]均佳;③对碳化试件由表及里逐层做碳酸盐含量测定,并绘制“CaCO3含量-深度”曲线,以此来研究水泥石碳化情况,该方法能清晰的反映出水泥石碳化情况,区分出完全碳化区和碳化反应区并能测量其长度;④用密封袋做CO2气体的容器,用过饱和盐溶液控制袋内湿度,用温湿度计监控湿度、温度变化,自制水泥石加速碳化装置;该装置的特点是:1) CO2浓度高;2)装置操作简单,便于控制。更多还原

【Abstract】 The pH of pore solutions in concrete is commonly in the range of 12.5 to 13, in this alkaline environment, passive film with depth of about 2~6nm was formed on the surface of steel bars and protected steel bars from corrosion. Carbonation of concrete gradually decreases the pH, when the carbonation is completed, the pH is down to about 8.5~9.0 even lower, thus the steel bars in concrete were depassivated and prone to ccorrosion. The volume of steel rust was generally increased two to four times, as a result, the cross-section of rebars expanded thus cracking appeared in concrete cover, the load-bearing capacity of the concrete member and the ductility of the structure were decreased, even the concrete structure was failed. Therefore, carbonation is considered as one of the major factors which affect the durability of concrete.Fly ash is widely used in concrete, which inproved properties of concrete, reduced amount of cement, benefited environment protection and energy saving, thus is the inevitable trend to the sustainable development of the concrete. However, while playing the role of pozzolanic effect, fly ash can consume Ca(OH)2 in concrete ,gradually reduced the ability of carbonaion resistance and corrosion resistance. So theoretically and practically, it is worthy further investigating carbonation property of fly ash concrete , finding out the measurements to improve the carbonation resistance and overcoming the disadvantages when used in concrete.In this paper, Samples of cement paste were chosen for the accelerated carbonation tests, and calcium carbonate content in the sample was measured and plotted as a function of depth on specimens, effectuating a systematically and comprehensively investigation on the carbonation resistance of fly ash blended cement paste and how those relative characteristics affect its performance. The following conclusions can be drawn:①A combination of XRD and DSC suggested that calcite constituted most part of the carbonation product while little aragonite and vaterite were ensued from carbonation. It was observed that a small amount of calcium hydroxide crystals still remain in the fully carbonated zone, which was mainly because these crystals have been encapsulated in the C-S-H gel, it is difficult to taking part in the combination reaction.②Carbonation resistance of cement paste decreased with the increase in fly ash content. Cement pastes contain high-contents of fly ash (50% and 70% in mass) exhibit severe carbonation which limited the use of fly ash in practical.③At high fly ash content, apparently, the carbonation extent of cement paste increased linearly with water-binder ratio.④Carbonation coefficient increased with the decrease in relative humidity, moreover, especially when relative humidity is lower than 40%, both the fully carbonated zone and the partial carbonated zone extended significantly. This observation was inconsistent with the conclusion as described in some published works[8,19,102]: when in low relative humidity(lower than 20%), the carbonation process would be limited from lacking of water; when in high relative humidity(higher than 80%), the presence of a liquid phase facilitates the carbonation reaction significantly, however, the carbonation process would be blocked from lacking of carbon dioxide and the extent of the partial carbonated zone is pretty small, since the carbon dioxide gas diffusion coefficient is rather low on this condition.⑤As regards specimens dried in a furnace at 60℃for 24 hours before the carbonation test, water was vaporized out of the pore solution gradually and little remained, carbonation turned out to be severe(suggested by a relatively high carbonate content) and non-uniform. According to the phenolphthalein test, a fairly rough border line of the carbonated zone was revealed, that is, boundaries between the fully carbonated zone and the partial carbonated zone were vague. Quantitative analysis also indicated that calcium carbonate content did not drop significantly with the increase in depth.⑥The specimens in this study did not show strong correlations between carbonation performance and age of cement pastes; it seemed that prolonged curing does not improve the carbonation resistance of cement paste blended with a high content of fly ash.⑦At the age of 28 days, the amount of carbonates and the depth of carbonation did not differ much for specimens under sealed or saturated solution of Ca(OH)2 curing, probably indicating that there were enough water for hydration even for sealed curing specimens at the age of 28 days and testing specimens curing under these two regimes were alike, that is, they were of similar degree of hydration and the amount of porosity and pore structure did not differ much, therefore, the carbon dioxide gas diffusion coefficient and carbonation rate were alike.⑧As the carbonation process went on, the depth of the fully carbonation region extended gradually, as well as the partially carbonation region. It is likely that, there is a certain relationship between the depth of the fully carbonation region and of the partially carbonation region, moreover, this kind of relationship could be calculated in the light of an analytical model. Further investigation on this issue is to be conducted.⑨The use of fine fly ash and high curing temperature significantly improved the carbonation resistance of blended cement paste with a high content of fly ash, while additive alkali showed little effect, what’s more, the solely addition of NaOH even aggravated the circumstance.The testing method and its relating apparatus used in this experiment are designed and devised by the author and his colleagues, the innovations are as follows:①Aggregate is known to affect the carbonation process and the measured calcium carbonate content; therefore, for better investigation on the mechanism of carbonation, tests were carried on cement paste so as to eliminate the effects brought by aggregate.②The apparatus applied for the quantitative analysis of carbonates is especially developed by the author and his colleagues. The developed apparatus has a better control over testing environment, and practically, it advanced in precision and accuracy than the other apparatus applied in references[82-83].③The developed quantitative analysis apparatus has allowed us to measure the amount of the carbonates in the testing specimens at different depths. Samples of cement paste were chosen, and calcium carbonate content was measured and plotted as a function of depth on specimens, clearly illustrating the carbonation performance of cement paste. This method enables us to differentiate and quantify the fully carbonation region from the partially carbonation region undetectable by the phenolphthalein spray test. Therefore, apparently, this method is superior to the classical phenolphthalein spray.④The accelerated carbonation also took place in an especially developed chamber, using a sealed bag as the carbonate dioxide gas container. The humidity in the chamber was controlled by saturated salt solution and a hygrometer was used to monitor the relative humidity and the temperature.更多还原