Experimental Study On Frost Resistance And Pore Structure Of Nano-TiO₂ Concrete Under Freeze–thaw Environment

Nano-titanium dioxide（Nano-TiO₂）is the most widely used photocatalytic material at present.As a high-quality carrier for loading Nano-TiO₂,concrete is mostly studied for its photocatalytic performance.Most of concrete buildings serve in cold areas in winter.Freeze–thaw damage is the main reason that seriously affects its service life.Nano-TiO₂has the performance of photocatalysis,and due to its smaller particle size and larger specific surface area,the internal micropore structure can be changed when it is added into concrete,thus affecting its macroscopic frost resistance and mechanical properties.In this paper,Nano-TiO₂concrete with photocatalytic performance was prepared by adding Nano-TiO₂equal quality replacement cement（0%,2%,4%,6%）into concrete.The freeze–thaw cycle test,photocatalytic test,compressive strength test,pore structure test and water ice phase change test were carried out on the concrete specimens with different contents of Nano-TiO₂to explore the frost resistance,photocatalytic performance,mechanical properties and microscopic pore structure of Nano-TiO₂concrete.Meanwhile,the freezing failure mechanism of Nano-TiO₂concrete was analyzed based on pore structure data and concrete freezing stress model.The main conclusions are as follows:（1）With the increase of freeze–thaw cycles,the cumulative mass loss rate of Nano-TiO₂concrete specimens increased,while the relative dynamic elastic modulus and compressive strength decreased.In the process of freeze–thaw cycles,the frost resistance of 2%Nano-TiO₂concrete specimens is the best.（2）Before the freeze–thaw cycle,the pore distribution of each specimen is mainly dominated by small pores;during the freeze–thaw cycle,the proportion of small pores in each specimen gradually decreased,while the proportion of mesopores and macropores gradually increased.With the increase of freeze–thaw cycles,the porosity and free water saturation of all specimens increased,while the irreducible water saturation decreased.Compared with other concrete specimens with Nano-TiO₂content,the porosity and free water saturation of 2%Nano-TiO₂concrete specimens remained the minimum values during the freeze–thaw cycle,so the frost resistance is the best.（3）With the increase of the number of freeze–thaw cycles,the photocatalytic degradation efficiency of rhodamine B decreased.With the increase of Nano-TiO₂content,the photocatalytic degradation efficiency of rhodamine B increased.The effect of freeze–thaw cycles on photocatalytic degradation efficiency is related to the content of Nano-TiO₂.The more the content of Nano-TiO₂,the less affected by the freeze–thaw cycles.（4）The freezing stress of concrete is the result of the combined action of hydrostatic pressure,crystallization pressure and pore wall pressure caused by volume expansion during the freezing process of pore water.Its value is closely related to the distribution of pore structure in concrete.The larger the pore size,the smaller the liquid phase saturation,the greater the ice phase saturation,the greater the final freezing stress,and the more serious the macroscopic damage degree of concrete.The concrete specimen containing2%Nano-TiO₂has the minimum freezing stress and the minimum damage during freeze–thaw cycle.（5）In the process of water ice phase change of Nano-TiO₂concrete specimens,the pore water freezing always lags behind the melting process.The actual icing content of2%Nano-TiO₂concrete specimens is lower than that of undoped specimens,and the actual icing content of 4%and 6%Nano-TiO₂concrete specimens is higher than that of undoped specimens.The water ice phase transition rate of 4%and 6%Nano-TiO₂concrete specimens is higher than that of undoped and 2%Nano-TiO₂concrete specimens.