Study On Salt Freezing Resistance Of Calcined Hydrotalcite Cement-based Composites

The deicing salt is often used to ensure the passage of vehicles in winter rain and snow weather,resulting in the erosion and cracking of concrete road surface.At present,salt freeze damage has become the most common form of damage in concrete roads in northern China,so it is necessary to study the resistance of concrete to salt freeze.Calcined layered double hydroxide(CLDH),as a new type of anion adsorbent,has good chloride ion adsorption capacity.In this work,the feasibility of incorporating CLDH into cement to cure chloride ions is firstly verified through experiments,and the curing ability of CLDH cement-based composites to chloride ions and the performance and mechanism of salt freezing resistance are further studied.In this work,CLDH was obtained by calcining magnesium aluminum carbonate layered double hydroxides(LDH)at 500℃ for 5 hours.CLDH can absorb anion to restore the layered structure in aqueous solution and the ability of cure chloride ions in cement was used.Firstly,simulated pore fluid was prepared to test the feasibility of CLDH for curing chloride ions.The dosage of CLDH was selected to be 0%,2%,4% and 6% to replace cement of the same quality to prepare cement-based composite materials,and the water-cement ratio was determined as 0.35.The setting time,mechanical properties and dynamic elastic modulus of CLDH cement-based composite materials were tested.The properties of CLDH cement-based composite materials was comprehensively analyzed by SEM and XRD.Using a 3% Na Cl solution at-20℃-20℃,the CLDH cement-based composite material was subjected to 50 salt freeze cycles(air-freezing and water-melting)to test the salt-freezing resistance performance of the CLDH cement-based composite material;The concentration changes of free chloride and total chloride in simulated pore fluid of cement-based composite material with different freeze-thaw cycles were tested;The penetration depth of chloride ion after soaking in 5% Na Cl solution for different time was tested by silver nitrate color method.The mercury intrusion method was used to test the porosity and pore size distribution of the cement-based composite materials;In addition,the grey correlation method was used to analyze the influence of CLDH dosage on the salt-freezing resistance of CLDH cement-based composites.The main results are as follows:(1)CLDH has the strongest ability to cure chloride ions compared with traditional fly ash,slag powder and kaolin.CLDH has low water solubility,but the proper amount of CLDH has no adverse effect on the setting time and workability of cement.With the increase of CLDH,the mechanical properties and dynamic elastic modulus of CLDH cement-based composites show a trend of increasing first and then decreasing.The mechanical properties are the best when the dosage of CLDH is 2%,but the overall change is not large;SEM and XRD separately was used to analyze the morphology and composition of different dosage of CLDH cement-based composites.The results show that CLDH cement-based composites also produced a great amount of hydration products.(2)After the same salt freezing cycle,as the proportion of CLDH replacing cement increases,the salt freezing damage of CLDH cement-based composites gradually decreases;The mechanical properties and relative dynamic elastic modulus of CLDH cement-based composites both show an increase and then decrease trend.The mechanical properties and relative dynamic modulus of cement-based materials are the best when the content of a is 2%.The concentration of free chloride ions and the penetration depth of chloride ions in the simulated pore fluid of cement-based composites with the same number of salt-freezing cycles are gradually reduced with the increase of CLDH dosage.It indicating that CLDH can significantly improve the ability of cement-based composite to cure chloride ion and resist chloride ion penetration.(3)The results of mercury intrusion method showed that the total pore volume and total pore surface area of cement-based composites was decreased after adding CLDH.In addition,the pore structure and distribution of cement-based composites was improved and the salt freezing resistance was increased.The grey relation theory shows that the influence of CLDH on the properties of cement-based composites decreases gradually with the increase of CLDH dosage.Therefore,the dosage of CLDH in cement-based composites should be reasonably selected according to the actual situation.