Research On The Change Law Of Pore Structure Of Natural Pumice Concrete Based On Nuclear Magnetic Resonance Technology

In the cold area of North China,the concrete in water conservancy structure is used in the bad environment.The environment has high requirements for the mechanical properties and frost resistance of concrete.Natural pumice concrete,a new building material,has the advantages of saving construction cost and improving construction service life.The micro pore structure(porosity,pore volume,pore size distribution)of natural pumice concrete is closely related to its macro performance.In the environment close to the actual use of natural pumice concrete,the compressive strength,frost resistance and NMR pore structure of natural pumice concrete were tested.The relationship between the evolution of pore structure of natural pumice concrete and the changes of compressive strength and frost resistance is analyzed.This paper discusses how to optimize the macroscopic properties of materials by improving the microporous structure.The paper includes:(1)When the curing age is 3d,7d,14 d,28d,60 d and 90 d,the change rule of compressive strength and pore structure of natural pumice concrete is analyzed.The results show that the change of porosity has experienced two stages: rapid growth and slow growth.The porosity of natural pumice concrete is negatively related to its compressive strength.The higher the porosity,the lower the compressive strength.During the curing period of 3-28 days,the proportion of multiple harmful holes with pore diameter greater than 0.2 ?m decreased significantly,and the increase of compressive strength was larger;during the curing period of 28-90 days,the proportion of multiple harmful holes changed little,and the increase of compressive strength tended to be stable.(2)In the freeze-thaw environment,the frost resistance(mass loss rate and relative dynamic modulus of elasticity)and microporous structure of natural pumice concrete are analyzed.The results show that the porosity of natural pumice concrete increases with the increase of freeze-thaw cycles,which can be divided into slow growth period and rapid growth period.With the increase of freeze-thaw cycles,the proportion of multi damaged holes with pore diameter larger than 0.2 ?m increased significantly,and the frost resistance of natural pumice concrete decreased.(3)Introduce the concept of fractal dimension and analyze the distribution of the pore size of natural pumice concrete.The pore size of pore structure evolves to small pores as the curing period increases,and the fractal dimension of pore volume shows an increasing trend.Increase the number of freeze-thaw cycles,the connectivity between pores increases,small pores evolve into large pores,and the fractal dimension of pore volume shows a decreasing trend;the optimal relationship model between fractal dimension and compressive strength and frost resistance is a quadratic function,the best relationship model of fractal dimension and porosity is linear function and quadratic function.The study found that the pore structure that affects the macro performance of natural pumice concrete is mainly related to the size of the multi-harmful pores in the total porosity.Reducing the proportion of multi-harmful pores in the pore structure can effectively improve the macro performance of natural pumice concrete.