Investigating the Effects of Cooling Methods and Aggregate Sizes on the Microstructure of Cement Mortars Based on MRI
To study the effects of cooling methods and aggregate sizes(AS)on the microscopic pore distribution char-acteristics and damage of cement mortar samples,the cement mortar samples with AS of 0.63-1.25 mm,1.25-2.5 mm,2.5-5.0 mm,and 5.0-10.0 mm were heated to 400℃and then placed in air and water for cooling,respec-tively.Low-field nuclear magnetic resonance(NMR)and digital acoustic wave instruments were used to test the T2 spectrum,nuclear magnetic resonance imaging(MRI),and P-wave velocity,respectively.The effects of cooling meth-ods and AS were studied on the pore size distribution,geometric mean pore radius,pore volume,P-wave,and damage degree of cement mortar samples.The relationship between the eigenvalues of MRI pixel values(mean ω,standard de-viation δ,and entropy ∈)and the degree of damage was deeply explored.The results show that the pore geometric mean radius,pore volume,ω,δ,and ∈ all increased after high temperature,but the amplitude of change was different due to the effect of the cooling method and AS:On the one hand,the damage degree of water cooling was greater than that of air cooling.After high-temperature water cooling,the increased amplitude of T2 peak area of pores,pore geometric mean radius,pore volume,and ω were all greater than that of air cooling.On the other hand,the T2 peak area of the gel pore decreased under water cooling,while the geometric mean radius and initial δ of the pore under air cooling were al-most unaffected by the aggregate size.At the same time,the aggregate size was negatively correlated with the T2 peak area of capillary pores,the geometric mean radius of pores under water cooling,and the increase of pore volume,and positively correlated with ω and ∈ under air cooling.After high temperature,the bright spots of MRI expanded on the existing spots and connected to form brighter and larger spots,and the MRI pixel values were normally distributed.There is an exponential functional relationship between the damage degree and ω and ∈(R2 were 0.91 and 0.76,re-spectively),and ω can be used to characterize the damage degree of porous media.
magnetic resonance imaging(MRI)aggregate sizescooling methodpore size distributiondamage degreecement mortar
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