Expansion Inhibition Mechanism Of Steel Slag With Hydrophobic Surface And Properties Of Its Asphalt Mixture

The volume expansion of steel slag is the main problem to restrict the application of steel slag to asphalt pavement instead of nature coarse aggregate.In this paper,steel slag with partice size larger than 4.75 mm produced by Nanjing Steel Slag Treatment Company is taken as the research object.The surface modification of steel slag is carried out with the waterborne hydrophobic coating based on silicone rubber emulsion,and a hermetic hydrophobic film is formed on the surface of steel slag,which can prevent the steel slag from contacting with water and inhibit the volume expansion of steel slag.In addition,the properties of the modified steel slag and performances of its asphalt mixture are evaluated.Firstly,a laboratory process is designed to prepare the waterborne hydrophobic coating.The hydrophobic coating is a two-component condensed room temperature vulcanized silicone rubber emulsion,which is mainly composed of film-forming material,crosslinking agent,coupling agent,reinforcing agent,film forming auxiliary,defoaming agent,catalyst,viscosity increaser and disperse medium.The experimental results show that the prepared hydrophobic coating has the good hydrophobicity,fluidity,adhesion and permeability and can be used to modify steel slag.Then,the steel slag is treated by immersing in the hydrophobic coating and cured at room temperature for 2~5h to solidify the silicone rubber emulsion.At the end,the modified steel slag with hydrophobic layer is obtained.The basic properties of modified steel slag,original steel slag and basalt aggregate,including hydrophobicity,water absorption rate,high temperature stability,adhesion with asphalt and immersion expansion rate,are tested and evaluated.In hydrophobic test,the contact angle of droplets on the surface of modified steel slag varies from 84.7° to 101.3°,indicating the steel slag with hydrophobic film has better hydrophobicity to reduce the adhension between droplets and aggregate.In the water absorption rate test,the result shows that the original steel slag has the highest water absorption rate and does not meet the specification requirements,while the water absorption rate of modified steel slag with different particle sizes(4.75~9.5mm,9.5~13.2mm and 13.2~16mm)are reduced by 29.7%,26.2% and 23.6% respectively.In the high temperature stability test,the water absorption rate of modified steel slag with different particle sizes which has been treated by high temperature are further reduced by 23.4%,49.7% and 67.6% respectively,revealing that after high temperature treatment the surface hydrophobic film of the modified steel slag becomes more dense and hydrophobic.In the adhesion with asphalt test,we put forward a new evaluation index called the asphalt spalling rate that can be used to quantitatively evaluate the adhesion between aggregate and asphalt.Compared with original steel slag,the asphalt spalling rate of modified steel slag is reduced by 76.7%.In the immersion expansion test,the smaller the particle size of original steel slag,the higher the immersion expansion rate.Compared with original steel slag,the final expansion rate of modified steel slag with 4.75~9.5mm and 9.5~13.2mm particle size decreases by 46.0% and 21.2% respectively,while the final expansion rate of that with 13.2~16mm particle size is basically the same,which demonstrates that the steel slag has a good inhibition effect on the volume expansion of immersion after modification.Finally,AC-13 asphalt mixture is prepared by using modified steel slag as coarse aggregate instead of nature aggregate,and its moisture resistance is evaluated.The results show that the optimal asphalt-aggregate ratio of the modified steel slag asphalt mixture is reduced by 0.5%.The volume stability,immersion marshal test and freeze-thaw splitting test of the modified steel slag asphalt mixture are greatly improved and meet the requirements of specification.