Study On The Evolution Of Mechanical,Water Transport Properties And Microscopic Mechanism Of Air-entrained Mortar Before And After Freeze-thaw Cycles

In cold regions,water infiltration can accelerate the freeze-thaw deterioration of cement-based materials.At present,adding the air entraining agent into the concrete has become the main measure to improve the frost resistance and impermeability of concrete.However,the introduction of excessive pores will have a negative impact on the mechanical properties of cement-based materials.Therefore,it is essential to systematically study the influence of air entrainment on the macro mechanical properties,water transport and microstructure of cement-based materials during freeze-thaw cycles.Among them,pore structure is the most critical factor affecting the durability of cement mortar.However,the research at this stage mainly focuses on qualitative analysis or preliminary quantitative analysis.Based on the above considerations,this paper took cement mortar specimens with different air entraining agent contents（0%,0.005%,0.010%,0.015%,0.020%）as the research objects,and used traditional testing methods and advanced testing technology to study the mechanical properties,microstructural characteristics and water transport laws of air entrained mortar before and after freeze-thaw cycles.The study creatively combined mercury intrusion porosimetry（MIP）and low-field nuclear magnetic resonance（LF-NMR）in the experiment.The volume fractions of different types of voids were calculated by using the ratio of the volume of different types of voids to the total volume of the specimen corresponding to the pore size distribution curves and the T₂ spectrum distribution curves,so as to comprehensively consider the pore structure characteristics of air entrained mortar.At the same time,the role of pores in the air entrained mortar in the process of water absorption was observed by weighing methods and one-dimensional NMR water absorption tests.This study can provide some insights to investigate the microstructural characteristics and water transport of air entrained mortar,and provide theoretical support for the design of frost resistance of buildings in cold regions.The main research contents and achievements of this paper are as follows:（1）The changes of compressive strength and flexural strength of air entrained mortar before and after freeze-thaw cycles were studied,and the changes of dynamic elastic modulus of air entrained mortar before and after freeze-thaw cycles were studied by ultrasonic testing method.The results show that the mechanical property indexes such as flexural strength,compressive strength and dynamic elastic modulus of air entrained mortar change significantly with the amount of air entraining agent incorporated and the number of freeze-thaw cycles.（2）The microstructure of air entrained mortar in saturated state was tested by nuclear magnetic resonance technology,and the T₂ spectrum of water saturated sample was obtained.The results show that the addition of air entraining agent increases the number of mesopores and macropores in cement mortar and changes the pore size distribution of cement mortar.The volume fractions of different types of voids were calculated by using the ratio of the volume of different types of voids to the total volume of the specimen corresponding to the pore size distribution curves and the T₂ spectrum distribution curves,and it was found that the volume fractions of pores determined by the NMR method and the MIP method were relatively close to each other.The ink bottle effect underestimates the pore size of large-size holes in air entrained mortar during mercury injection test.Therefore,the combination of the two test methods can better reflect the pore size distribution of pores.（3）The microstructure of air entrained mortar after freeze-thaw damage was studied by the combination of nuclear magnetic resonance technology and scanning electron microscope（SEM）.The results show that the proportion of crack volume in mortar with 0%and 0.010%air entraining agent increases gradually with the increase of freeze-thaw cycles.However,under the same number of freeze-thaw cycles,the proportion of crack volume of mortar without air entraining agent is larger than that of mortar with air entraining agent content of 0.010%.Finally,the micro mechanism of macro performance evolution of mortar specimens is explored with the help of microstructure characterization parameters such as porosity.（4）The capillary water absorption processes of mortars with different air entraining amounts before and after freeze-thaw cycles were quantitatively characterized by the traditional weighing method of water absorption tests.The results show that within a certain range of air entrainment,the capillary water absorption coefficient of mortar decreases with the increase of air entrainment.The introduction of pores cuts off the connectivity,resulting in more twists and turns of water transmission channel.The one-dimensional water absorption test by nuclear magnetic resonance shows that the air voids can not effectively transfer water.Among the five air entraining agent dosages,0.015%air entraining agent is more effective in delaying water transmission.The water absorption curve of air entrained mortar after freeze-thaw cycles shows bilinear characteristics,and with the increase of the number of freeze-thaw cycles,the capillary water absorption coefficient of mortar without air entraining agent gradually increases,and the capillary water absorption coefficient of mortar with 0.010%of air entraining agent shows a trend of basic stability first and then gradually increasing.Based on the results of nuclear magnetic resonance tests during one-dimensional water absorption of air entrained mortar,the correlation between the initial capillary water absorption coefficient and its microstructure parameters of air entrained mortar was explored.