Study On Resistance Of Cement Based Materials To Sulfate Attack At Low Temperature

Sulfate attack is one of the most important factors that influence the durability of concrete structure. The conventional form of sulfate attack such as ettringite,gypsum and so on has gained extensive research, however, about sulfate attack on conventional cement based materials at low temperature—the thaumasite form of sulfate attack has been studied in short time, as well as many issues and phenomenons have still not gained uniform recognization and explaination, which remain to be resolved further more.The thaumasite form of sulfate attack means that cementitious concrete material exposed to sulfate,carbonate and low temperature enviroment leads to formation of thaumasite(CaCO₃·CaSiO₃·CaSO₄·15H₂O) with no binding ability, which could directly make calcium silicate hydrate(C-S-H) gel distintegrate and make itself lose strength gradually, in final, it can destroy the concrete structure completely by the form of chemical attack, and it is a special form of sulfate attack whose distruction and concealment are more serious than the conventional form of sulfate attack. This paper reports an study on the influence of cement to water ratio,replacement levels of limestone powder,mineral admixture,environmental conditions etc, on the thaumasite formation. The deterioration process and mechanism of the thaumasite form of sulfate attack have been discussed, by which the author has established an mode about sulfate attack at early stage. The research results can be concluded as follows:â‘ The cement pastes with limestone powder addition exposed to 5%MgSO₄ solution can form thaumasite besides gypsum and ettringite at 5¹0℃for 240 days. Such a formation of corrosive can directly make calcium silicate hydrate gel distintegrate and translate itself into a mush which performances more serious than the conventional form of sulfate attack.â‘¡Exposed to 5% magnesium sulfate solution at 6℃, the sulfate attack on the cement paste is less serious as W/C decreasing, which will not happen about cement paste sample immersed in the corrosive media for 240 days when the W/C is below to 0.25; at the same enviroment, the thaumasite form of sulfate attack on the cement paste is more serious as the content of limestone powder increasing.â‘¢It has a complicated influence on the thaumasite form of sulfate attack about mineral admixture at low temperature. In this paper, the addition of fly ash and slag, to a certain extent, can prevent the cement paste from the sulfate attack, and especially incorporating 20% fly ash above, it can effectively inhibit the thaumasite form of sulfate attack.â‘£It shows that the influence of the kind of sulfate solution and its concentration on the sulfate attack is different. At same temperature and humidity enviroment, the corrosive destruction on the cement paste is more serious exposed to magnesium sulfate solution than sodium sulfate, and it will be more serious with the increasing concentration of the same magnesium sulfate solution.⑤It is obvious that temperature has an important iMPact on the thaumasite form of sulfate attack. At 6℃,6¹5℃alternate temperature enviroments, the cement pastes added with limestone powder could be both translated into thaumasite crystal after 240 days exposed to the 5%MgSO₄ solution, which shows that the concrete structure can be destroyed more easily at this extent of temperature by the sulfate attack of thaumasite form.â‘¥The influence of initial curing time on the thaumasite form of sulfate attack progression is obvious. The longer the initial curing time is, the less serious cement paste suffered from the thaumasite form of sulfate attack due to its coMPact structure. According to that, the extension of the initial curing time could delay the paste's resistance to thaumasite form of sulfate attack to a certain extent.⑦The sulfate attack of cement based materials at low temperature gradually develops from the outer into the inner, which exits lots of migration and exchange of ions at the early age.