Wewnętrzna korozja siarczanowa betonu (The internal sulphate corrosion of concrete)

Owsiak, Zdzisława (2008) Wewnętrzna korozja siarczanowa betonu (The internal sulphate corrosion of concrete). Monografie, Studia, Rozprawy (M6). Wydawnictwo Politechniki Świętokrzyskiej, Kielce.

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The causes of the occurrence of internal sulphate corrosion in concrete resulting from the delayed creation of ettringit and the concomitant damaging of concrete have been the topic of systematic research, which was carried out by numerous scientific centres for a dozen or so years. The expansive product created during this reaction may lead to a decrease in the durability of the concrete, or even to the total destruction of the concrete elements. For some time now, data has been reported in the literature concerning the damage of concrete elements that is caused by the simultaneously reaction of alkalies with silica and the delayed creation of ettringit. The research that has been carried out by a great many authors tends to concentrate on issues connected with the factors that cause the expansion of those concrete elements that are subjected to infusion. Laboratory studies have been preceded by an analysis of the available literature data on this topic, which cover primarily the following issues: the role of ettringit in fresh and hardened concrete, the hypotheses explaining the mechanism of the expansion of concrete with the delayed creation of ettringit, the technological factors influencing the delayed creation of ettringit in concrete, the simultaneous reaction of alkalies with silica, and the delayed creation of ettringit. In light of the discussion carried out in the literature it is evident that the issues of the simultaneous reaction of alkalies with silica and the delayed creation of ettringit, or occurring shortly after the first reaction, are neither experimentally documented, nor fully explained. For this reason, we performed a number of tests that were aimed at gathering experimental materials that could be used to further clarify these problems. The programmed experiments made it possible for us to determine the role of both processes in the destruction of the microstructure of concrete. The mechanism governing the influence of mineral additives on both of these processes is also unclear. Model tests were performed on mortars that were prepared with cements with varying sulphate contents and an increased content of alkalies and reactive aggregate with alkalies. The results of the tests cover both the mortars cured at increased temperatures (infused) and the mortars cured under natural conditions. The study also includes the results of the research into mortars concerning the influence of mineral additives and the type of aggregate on the course of internal concrete corrosion. The tests included linear changes in the mortar closures, the designation of the phase composition using the Roentgen analysis method, analysis of the microstructure and composition of the products that were created by the reaction of alkalies with aggregate and during the process of the delayed creation of ettringit using scanning microscopy connected with Roentgen analysis in microareas. Microsection surfaces were observed utilising reversely dissipated electrons. The research was carried out by the author over a period of many years into the reactions of aggregate with alkalies, the processes of the delayed creation of ettringit, and the simultaneous occurrence of both reactions has made it possible to formulate conclusions concerning the impact of the content of sulphates, sodium and potassium in the cement on the delayed creation of ettringit, the role of both processes in the destruction of the microstructure of concrete, and the role of aggregate and mineral additives. The increased content of sodium and potassium hydroxide in cement results in an increase in the expansion of the mortar samples containing an addition of opal and that are subjected to thermal processing during the first phase of expansion connected with the reaction of alkalies with opal. However, not all the results can be so simply explained. For example, the non-expansion of thermally processed mortars containing opal from cement with a high content of alkalies, but devoid of C3A, or with a smaller quantity of SO3, is unclear. The clarification of this issue requires further research. The delayed creation of ettringit caused by the thermal processing of mortars at a temperature of 90oC increases the expansion of the samples, whereas this greater expansion is to a certain extent dependent on its magnitude in relation to the reaction of opal with alkalies; the greater the first expansion, the greater the second. The experiments that were conducted show that the accelerated method of researching mortars appropriately reflects the assessment of the hazard that is posed to concrete by the process of the delayed creation of ettringit, for the expansion curves that were obtained using this method, which aptly correlate with the course of expansion attained applying other methods. The experiments that were conducted indicated that in the case of mortars that are a few years old and that have not been subjected to infusion, the delayed creation of ettringit also increases expansion, which correlates with the content of SO3 in cement. The results obtained indicate that the reaction of alkalies with the aggregate is conducive to the creation of secondary ettringit, in which in all probability this is not only due to the earlier development of cracks, but also as a result of the changes in the pH of the solution in the pores resulting from a decrease in the concentration of sodium and potassium hydroxide connected by silicate ions in gel.

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