Influence On Hydration Reaction And Sulfate Attack Of Cement Concrete By Action Of Electrical Pulse

Sulfate attack is an important factor for the durability of cement concrete. Compared with other disasters, sulfate attack harms more seriously at cement concrete and its mechanism of deterioration is rather complex. In order to guarantee the durability of concrete and accurately predict the resistance of sulfate attack of concrete, various methods have been proposed to quickly evaluate the resistance of sulfate attack of cement concrete by accelerated sulfate attack, such as increasing the area in contact between concrete and sulfate solution, raising the concentration and temperature of the solution, enhancing the crystallization pressure of sulfates, etc. However, these methods all have shortcomings so that it is necessary to put forward a more reasonable rapid method. Other studies have shown that accelerated test method of sulfate attack resistance using electrical pulse is feasible. However, the effect of electrical pulse on the hydration of cement and the mechanism of sulfate attack is needed to be further studied.In this paper, electrical pulse was employed to accelerate the sulfate attack on cement concretes. The effect of electrical pulse on strength development, hydration reaction and sulfate products was investigated. The results indicate that:① Under the condition without aggressive medium, electrical pulse promoted hydration of the cement. The compressive strength increased at early stage, and then the coefficient of strength was gradually decreased with the development of age and finally was lower than 1. The p H of cement stone was gradually decreased due to the directional migration of OH- under electrical field.② Electrical pulse accelerated the hydration of unhydrated cement, yet it would not influence the species of cement hydration products. The decalcification of hydrated cement was exhibited because of the directional migration of Ca2+ ions under the action of electrical pulse. At the end of test, a portion of Ca(OH)2 decomposed to form a great number of holes, resulting in a uncompacted structure within concrete.③ Under the condition of sulfate solution, electrical pulse has beneficial effect on accelerating sulfate attack on concrete. Mg SO4 was more corrosive and resulted in a larger strength loss compared to Na2SO4. The greater concentration of SO42- ions led to more deterioration of concrete. Under electrical pulse, the closer to the cathode side, the decrease of the p H of cement stone was greater with the development of time.④ Under the condition of Na2SO4 attack, electrical pulse did not change the varieties of sulfate products, but would change the number of the sulfate products. The massive formation of ettringite and gypsum resulted in the rough and porous concrete structure, along with a large amount of cracks. Under the condition of Mg SO4 attack, Mg2+ ions did not migrate into specimen form the cathode side because of electrical field, for which the sulfate products were other than those in immersion test. As with Na2SO4 attack, a large number of cracks were also noted within the specimen under Mg SO4 attack.⑤ Electrical pulse could be a new acceleration test for evaluating the sulfate resistance of cement-based materials. However, in the following research the aggressive medium should be reasonably selected, and it needs to choose a more effective way to control the p H of solutions in both cathode and anode sides.