Experimental Study On Mechanical Properties And Microstructure Of Nano-TiO₂ Concrete After Carbonization And Freeze-thaw

Nano-TiO₂ concrete has become a research hotspot due to its photocatalytic and self-cleaning properties and has been widely used in practical engineering.The reduction of concrete alkalinity caused by carbonization is an important prerequisite for steel bar corrosion.In addition to CO₂in the air,nano-TiO₂concrete can oxidize and reduce CO in the air to CO₂,which will lead to further carbonization of nano-TiO₂concrete.In addition,in the cold areas of northern China,concrete will also be subject to freeze-thaw damage.Therefore,studying the mechanical properties and durability of nano-TiO₂ concrete after carbonization,freeze-thaw and post-carbonization freeze-thaw is of great significance for predicting the service life of nano-TiO₂concrete.In this paper,nano-TiO₂ concrete carbonization test,freeze-thaw test and post-carbonization freeze-thaw test are carried out.The evolution of carbonization depth,compressive strength,morphology,mass loss and relative dynamic modulus of elasticity were studied.The change of pore structure of nano-TiO₂concrete under the action of different factors were analyzed by nuclear magnetic resonance instrument,and the effects of different factors on the microscopic morphology characteristics of nano-TiO₂concrete were observed by scanning electron microscope.After carbonization,nano-TiO₂ concrete has smaller carbonization depth and higher compressive strength.Carbonization optimizes the pore structure of concrete,increasing the number of large harmless pores and reducing the number of harmful pores.The pore size variation of nano-TiO₂concrete is smaller than that of ordinary concrete,and the pore size distribution is relatively stable,indicating that nano-TiO₂concrete is less affected by carbonization.Through scanning electron microscopy,it is found that there are continuous and complete hydration products in nano-TiO₂ concrete,and the number of pores and micro-cracks is reduced.The critical content of nano-TiO₂to improve the anti-carbonization performance of concrete is 3%.Nano-TiO₂ reduces the mass loss rate and increases the relative dynamic modulus value after freeze-thaw cycles;the concrete mixed with 1%nano-TiO₂has the largest compressive strength.Nano-TiO₂has a good filling effect,so during the freeze-thaw process,the pore size changes less,indicating that the effect of nano-TiO₂on the improvement of the concrete pore structure is greater than the effect of freeze-thaw damage.Carbonation reduces the freeze-thaw damage of concrete to a certain extent.The porosity growth rate of concrete after post-carbonization freeze-thaw is lower than that of single freeze-thaw,and carbonization reduces the pore connectivity to a certain extent.Nano-TiO₂inhibits the production of ettringite and makes concrete matrix denser,thereby improving the freeze-thaw resistance of concrete.The damage model of concrete after carbonization and freeze-thaw cycles is established,with a high correlation coefficient,which has practical value in saving resources and guiding the design and construction of such projects.