Investigation On The Temperature Effect Of Pore Structure For Cement-based Materials Through Low-field Nuclear Magnetic Resonance Technique

Since the microstructure of cement-based materials is closely related to its mass transport property,it is of great significance to study the behaviors of pore structure under environmental actions to reveal the degradation mechanism of durability for concrete structures.In order to characterize the pore structure of cement-based materials accurately and non-destructively,advanced low-field magnetic resonance technique(LF-NMR)is utilized to test the pore structures of white cement mortars at saturated conditions,which have been conditioned under different environmental conditions.The measured pore structure is compared with that of mortars at the dry state,which are determined through the mercury intrusion method(MIP).This research focuses on the effects of high temperature of 80 degrees and freeze-thaw cycles on the microstructure of cement-based materials,and strengthens the analysis of high-temperature effects on cement-based materials from the permeability of the materials at macroscale as well as the calcium silicate hydrated(C-S-H)gel at nanoscale.The main research results of the paper are as follows:(1)By the aid of the LF-NMR test,it is found that the high temperature of 80�C leads to a significant change in the pore structure of the mortar,which is due to the polymerization of the C-S-H gel.The lower the water-cement ratio,the more obvious the polymerization of the C-S-H gel,and the larger the increase in volume porosity.The results of the LF-NMR test agree well with those of the mass method.In addition,the results of LF-NMR relaxation analysis show that the pore structures of the mortars are significantly coarsened after high temperature curing,and the critical pore size and percolation pore size increase by 0.7 to 1.5 times,resulting in an increase in macroscopic water permeability of 1.6 to 3.9 times.The temperature of 80�C causes the polymerization and rearrangement of the C-S-H gel,which reduces the volume of interlayer pores slightly but increases the volume of gel pores significantly.Meanwhile,the water content within the C-S-H gel,as well as the pore space out of C-S-H gel both increases.The 29 Si MAS NMR test shows that the intensity of the Q1 peaks of the silicon-oxygen tetrahedron of the white cement mortars decrease while the Q2 peaks increase correspondingly,and the Q3 and Q4 peaks are basically unchanged.This result validates the polymerization of C-S-H gels at the end of the silicone chain.The condensation reaction between the silicon hydroxyl groups at the ends converts the-OH group into water,which is the fundamental reason for the increase of the total porosity and the interlayer pore size,as well as the decrease of the interlayer pore volume.(2)The pore size distributions of saturated mortars measured through LF-NMR technique are used to calculate the water permeability of mortars.It is found that both the Katz-Thompson equation and the Kozeny-Carman equation can give an accurate prediction of the water permeability for cement-based materials.The relative error between the predicted value and the experimental measured value ranges from-18.82% to 63.7%.The accuracy of this prediction is considerably high compared with the predicted results of related models in the literature,which are generally 2 orders of magnitude larger.These results highlight that when predicting the permeability of specific fluids in cement-based materials based on the pore structure,great attention must be paid to the significant differences in the pore structure when saturated with various fluids,which is actually the significant correlation between the pore structures and pore fluids.Through analyzing the quantitative relationship between the macroscopic permeability and the pore size distribution at the saturated state,it can be concluded that the optimal value of the proportionality factor of the Katz-Thompson equation is 1/195,and the optimal value of the proportionality factor of the Kozeny-Carman equation is 1.09.From the theoretical analysis and tests on the permeability of mortar specimens conditioned at normal temperature and high temperature treatments,it is quantitatively validated that the high temperature conditioning will significantly affect the pore structure.Meanwhile,the good applicability of classic permeability model to cement based materials is also verified.(3)The pore structures of mortar specimens undergoing certain freeze-thaw cycles are monitored through LF-NMR technique,it is found that during the freeze-thaw damage,the pore size distribution changes from a unimodal curve to a bimodal one.Pores within gels represented by the left peak basically do not change with the increase of freeze-thaw cycles,while the area of the right peak,whose average logarithmic pore radius is from 187 nm to 782 nm,gradually increases with freeze-thaw cycles.The water curing at 80�C reduces the frost resistance of mortars,while the surface sealing treatment enhances the frost resistance.However,the deterioration of the pore structure of the mortar during freeze-thaw degradation is roughly the same.The volume for pores of several hundred nanometers within the mortar increases continually and eventually lead to the damage of the material.