Quantitative Analysis Of Carbonate In Carbonation Of Cement Pastes With Fly Ash

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.