Study On The Frost Resistance Of Cement-based Materials In High Salt Concentration

Over the years,freeze-thaw damage of road concrete in cold regions by the coupling effect of de-icing salts and freeze-thaw cycles is very common,mainly manifested by surface scaling,structural cracking,flaking and disintegration of cement-based materials.Clarifying the corrosion mechanism of high concentration de-icing salt solutions under freeze-thaw cycles and the controlling methods to freeze-thaw resistance of cement-based materials is an essential part to ensure the long-term durability of concretes in cold region.In previous study,the understanding towards corrosion mechanism of de-icing salts under freeze-thaw cycles remains insufficient,such as the effect of erosion product content on macroscopic properties,the universality and persistence of erosion product,and the influence of freeze-thaw cycle environment on the macroscopic deterioration behavior of cement-based materials.Based on aforementioned facts,this paper relying on the Inner Mongolia Autonomous Region Major Science and Technology Project,systematically investigates the influence of freeze-thaw cycles and ion corrosion on the macroscopic properties of cement-based materials under high concentration salt environment and focuses on the correspondence between the corrosion product content and the macroscopic property damage of the materials under this environment.The main research contents and conclusions are as follows:(1)By characterizing the evolution of the hydration products of cement-based materials after freeze-thaw cycles,the microscopic morphology of the erosion products and the physical properties,the mechanisms of the deterioration of hardened cement pastes under high concentrations of salt ion attack are explored.The results show that the calcium hydroxide from hydration product of cement can reacts with the calcium and chloride ions in the salt solution to form calcium oxychloride,as a swelling destructive complex salt,it fills the pores of the paste and grows along the cracks of the matrix.Then the swelling stress caused by the crystal transformation causes the material to be flaking and disintegration with the increase of the corrosion product yield,and the cracking and calcium hydroxide leaching caused by the corrosion products is the main reason for the material to disintegration in the late stage of freeze-thaw cycle.(2)The effect of coal-metakaolin on the ion corrosion process is discussed by characterizing the degree of macroscopic properties,the evolution of macroscopic mechanical properties,and the pore structure change process of cement-based materials during the salt-freezing cycle.The results show that,benefiting from the large specific surface area and high volcanic ash activity of coal-metakaolin,the material system can be depleted of calcium hydroxide while optimizing the pore structure and thus hindering the transport of harmful ions,thus reducing the corrosion product generation.(3)A threshold analysis of cement compounds based on the strength reduction was derived by linearly fitting the material macroscopic property loss to the content of hydration and corrosion products,and the results showed that the risk of freeze-thaw damage of cement-based materials in high salt concentration environment can be effectively reduced when the content of calcium hydroxide and calcium oxychloride in the material system was reduced to 9.2g/100 g paste and7.1g/100 g paste.(4)The effect of salt concentration on the freeze-thaw resistance of cement mortar was discussed by characterizing the degree of apparent damage and the process of macroscopic mechanical property changes in cement mortar under different salt freeze-thaw environments.The results show that under the low concentration salt environment,the main cause of damage of cement-based materials is surface scaling caused by physical damage of salt freeze-thaw;under the high concentration salt environment,the main cause of performance deterioration of cement-based materials is swelling and cracking caused by corrosion damage of ions;under the cumulative concentration salt environment,cement-based materials show the characteristics of obvious deterioration at the early stage and reduced damage at the later stage;In the dilute concentration salt environment,cement-based materials show the characteristics of less damage in the early stage and a sharp increase of damage in the later stage.Meanwhile,coal-metakaolin can effectively reduce the macroscopic deterioration of cement-based materials caused by different deterioration mechanisms.