Experimental Study On Dynamic Change Of Pore Structure Of Natural Pumice Concrete At Low Temperatures

In the cold area of northwest China,winter lasts for about 5 months.Most concrete buildings in this area are affected by low temperature environment.The properties of concrete(physical properties,mechanical properties,durability)will change in varying degrees.Among them,mechanical properties(such as compressive strength,elastic modulus,etc.)are important performance indexes of Engineering materials.Therefore,testing and studying the effect of low temperatures on the mechanical properties of concrete could more accurately predict the service performance and service life of concrete materials.In this paper,the mechanical properties of natural pumice,natural pumice concrete and ordinary concrete are tested by self-made cryogenic pressure device,and the dynamic changes of pore structure of natural pumice,natural pumice concrete and ordinary concrete at low temperatures(0,5,10,15,20)are tested by nuclear magnetic resonance technology.The following aspects are discussed in this paper:(1)The uniaxial compressive strength and modulus of elasticity of natural pumice at low temperatures are tested,and the macro-mechanical properties and pore structure of natural pumice at different temperatures are correlated with the porosity,pore size distribution and nuclear magnetic resonance imaging analysis of natural pumice.(2)The macro-mechanical properties of natural pumice concrete and ordinary concrete at low temperatures are tested,and the trend of mechanical properties of concrete with temperature is analyzed.Through the analysis of porosity and pore size distribution of two kinds of concrete at low temperatures,the freezing situation and the change of pore structure of concrete at different low temperatures are discussed.(3)Analysing the relationship between the pore structure and the mechanical properties of concrete at low temperatures.The theoretical model between pore structure and strength of concrete is used to verify the experimental data.The model analysis and verification are carried out from single factor and multi-factor aspects,and the theoretical model suitable for the pore structure and strength of concrete at low temperatures is found.Through experiments and analysis,the following conclusions are drawn:(1)The uniaxial compressive strength and elasticity modulus of saturated natural pumice are increased with the decrease of temperature at low temperatures.With the decrease of temperature,the porosity of natural pumice decreases,too.the volume of non capillary pores and capillary pores gradually decreases,and the volume of gel pores increases gradually.(2)The compressive strength and elasticity modulus of natural pumice concrete increase with the decrease of temperature at low temperatures.The growth process of natural pumice concrete is divided into slow growth stage,linear development stage and stable stage.The growth rate of compressive strength and elasticity modulus of natural pumice concrete is larger than ordinary concrete.The porosity of natural pumice concrete is larger than ordinary concrete,and the reduction rate is lower than ordinary concrete.With the temperature decreasing,the volume of pores in concrete decreases and the proportion of gel pores increases.(3)The relationship between porosity,temperature and compressive strength of natural pumice concrete at low temperatures is more suitable for logarithmic and power function;the relationship between porosity,temperature and compressive strength of ordinary concrete is more suitable for logarithmic and exponential;the relationship between temperature,porosity,average pore size distribution and cement content and compressive strength of two kinds of concrete at low temperature tends to exponential distribution.