Investigation Of Preparation And Properties Of Layered Double Hydroxide De-icing Additives For Asphalt Mixture

In winter,snow and ice on asphalt pavement not only greatly reduce the road transport efficiency,but also easily lead to accidents.How to timely melt or remove snow and ice on the pavement to reduce traffic accidents and ensure safe and smooth traffic has become the urgent problem to be solved by each country in the world.Traditional way of removing ice and snow is to spread chloride salt,but chloride salt seriously harms the surrounding ecological environment and leads to failures in the asphalt pavement.In order to mitigate the damaging effects of chloride salt,a few of environmental friendly deicing salts?such as acetate salts?began to be applied.However,these deicing salts not only cost expensively,but also have great harmfulness to the asphalt pavement.Incorporating the chloride deicing agents into the asphalt mixture to prepare asphalt pavement with the functions of self-melting ice has attracted much attention in recent years.But chloride salt still gradually release under the rain and vehicle load.These released salts not only damage environment and road construction,but also reduce the long term of melting ice.Therefore,it is essential to develop a new deicing additive which does not harm the environment and pavement construction as well as melts ice or snow sustainably.Layered double hydroxides?LDHs?are composed of metal cation layers with positive charges and interlayer anions with negative charges.Our previous research found that the interlayer anions can lead to the effect of decreasing the freeze point of water.The elements of metal cation layers and types of interlayer anions directly influence the function of decreasing the freeze point.This thesis takes Mg/Al CO₃^2-LDHs with different element ratios of magnesium and aluminum as raw materials.LDHs containing different interlayer anions?chloride anions,formate anions and acetate anions?were prepared by calcination recovery method.The effects of the elements of metal cation layers and types of interlayer anions on decreasing freeze point of water were systematically investigated.The new deicing additives and asphalt mixtures incorporated LDHs with various interlayer anions were prepared.The effects of new deicing additives on the deicing properties and performance of asphalt mixture were studied.The main research conclusions are as follows:?1?MgAl LDHs with various interlayer anions were prepared by calcination recovery method.Elemental spectroscopy and Fourier transform infrared spectroscopy imply that chloride anions,formate anions and acetate anions have been successfully entered into the interlayers of LDHs.X-ray diffraction analysis showed that the introduction of chloride anions,formate anions and acetate anions increases the interlayer distance of original Mg/Al CO₃^2-LDHs.Scanning electron microscopy indicated that LDHs intercalated with chloride anions,formate anions and acetate anions still have lamellar structure.Transmission electron microscope images further confirmed that the interlayer distances between LDHs lattices increases more than that of pristine LDHs.?2?Element ratios of metal cation layers have a significant impact on melting ice capability of LDHs.Differential scanning calorimetry tests showed:Compared to that the element ratios of magnesium and aluminum are 3 and 4,when the element ratio of magnesium and aluminum is 2,the freeze points of slurries of LDHs intercalated with chloride anions,formate anions and acetate anions are lower.The freeze points of 5wt%content of slurries are-8.15°C?-9.32°C and-10.68°C,respectively.This because when the element ratio of magnesium and aluminum is comparatively low,the relative content of trivalent aluminum anions and the total number of positive charges in metal cation laminates increase,subsequently,more anions can be intercalated into lamellas.?3?Different anions have a significant impact on the melting ice capability of LDHs.When the element ratio of magnesium and aluminum is 2,5%of LDHs intercalated with chlorine anions?Mg₂/Al Cl^-LDH?,LDHs intercalated with formate anions?Mg₂/Al Fo^-LDH?,LDHs intercalated with acetate anions?Mg₂/Al Ac^-LDH?and LDHs intercalated with carbonate anions(Mg₂/Al CO₃^2-LDH)were mixed with water to obtain slurries,the corresponding freeze points are-8.1°C,-8.8°C,-9.3°C and-1.2°C.This indicates that the intercalations of chloride anions,formate anions and acetate anions can endow LDHs with the superb capability of melting ice.LDHs intercalated with acetate anions have the superior effect of decreasing the freeze point.This is ascribed to that calcined LDHs has the highest amount of acetate anions.The unit content of interlayer anions test results show that unit contents of interlayer anions of LDHs intercalated with chloride anions,formate anions and acetate anions are 0.0035 mol/g,0.0041 mol/g and0.0054 mol/g.?4?The preparation technique of calcination recovery and the mass ratios of calcined LDHs and intercalation agents also apparently influence the capability of melting ice of LDHs.When the intercalation reaction temperatures are 70°C?75°C and 80°C,the capabilities of melting ice for LDHs intercalated with chloride anions,LDHs intercalated with formate anions and LDHs intercalated with acetate anions are superior.This because the activities of the reaction system rose with the increase of temperature at preliminary stage,subsequently,the intercalation of LDHs by external anions became more and more easy.When the temperature further increased to exceed the optimal reaction temperature,small amount of hydroxyl?-OH?began to release from LDHs to compete with external anions.As a result,the amount of intercalated anions decreased.The mass ratios between calcined LDHs?CLDH?and intercalation agent also affect the freeze point of LDHs intercalated with various anions.When the mass ratios of CLDH/sodium chloride,CLDH/sodium formate and CLDH/potassium acetate are 3,2.5 and 2,the intercalated LDHs have superior effect of decreasing freeze point of water.This is because the mass ratios of CLDH/sodium chloride,CLDH/sodium formate and CLDH/potassium acetate decrease to a saturated value,there is no extra intercalation sites to accept more external anions.?5?LDHs have the confinement effect on the intercalated anions.Solution of silver nitrate titration showed that there are no chloride anions in the filtrated liquid of Mg₂/Al Cl^-LDH.The conductivity test indicates that there are no free formate anions and acetate anions in the filtrated liquid of Mg₂/Al Fo^-LDH and Mg₂/Al Ac^-LDH.X-ray photoelectron spectroscopy test results reveal that the bonding energies between interlayer anions and layer metal anions increased after the intercalation,indicating that metal cation layers have better confinement effect on the three types of anions.?6?LDHs intercalated with the three types of anions can endow the asphalt mixtures with excellent long-term ability.The freeze point tests on the surface of asphalt mixture showed:the freeze points on the surface of asphalt mixtures containing 5wt%of Mg₂/Al Cl^-LDH?Mg₂/Al Fo^-LDH and Mg/Al Ac^-LDH are-5.0°C?-6.5°C and-7.2°C.The ice adhesion tests on the surface of asphalt mixture indicate that LDHs deicing additives can decrease the adhesion force between ice and asphalt mixture.After the repeated immersion and mechanical abrasions,the freeze point and adhesion force do not change.?7?LDHs deicing additives can improve the moisture stability and high-temperature property of asphalt mixture.Moisture stabilities of asphalt mixtures containing 5wt%of Mg₂/Al Cl^-LDH,Mg₂/Al Fo^-LDH and Mg/Al Ac^-LDH are 85.9%?86.2%and 87.5%,respectively,but moisture stability of asphalt mixture containing mineral powder is 82.5%;Dynamic stabilities of asphalt mixtures containing 5wt%of Mg₂/Al Cl^-LDH,Mg₂/Al Fo^-LDH and Mg/Al Ac^-LDH are 3920cycles/mm?4120 cycles/mm and 4370 cycles/mm,respectively,but dynamic stability of asphalt mixture containing mineral powder is 3520 cycles/mm.