Study On Self-healing Potential Of Cement-based Materials Based On BSE Images

The healing of cracks in cement-based materials due to continuous hydration and carbonization is called spontaneous self-healing.In recent years,the ultra-high toughness cement-based composite materials represented by ECC have developed rapidly.The parallel fine and multi-crack and low water-to-binder ratio are very beneficial to self-healing.This has been confirmed by many scholars.The distribution of unhydrated cement?UHC?particles in the matrix is one of the main factors affecting the self-healing effect.Common test methods such as hydration heat method,chemical combined water method,calcium hydroxide method,etc.can only measure the degree of hydration and cannot give the distribution status of UHC.Analysis of cement hydration degree in composite systems containing mineral admixtures is more difficult.Based on the above reasons,firstly,this paper explores and improves the backscattered electron?BSE?image analysis method.Next,the BSE method was used to analyze the hydration degree,UHC volume fraction,particle size distribution and surface state of cement-based materials with different ratios and ages.Finally,the theoretical limit crack width model is established,and the self-healing potential of cement-based materials with different water-cement ratio and age is quantitatively evaluated.The main research results are as follows:?1?The existing gray threshold selection method has poor applicability and low efficiency,so the OTSU fast gray threshold selection method is introduced.For the mis-selected fly ash,the arc recognition algorithm is designed to improve the accuracy of the hydration analysis in the composite system.In addition,the influence of median filtering and mean filtering on UHC volume fraction is analyzed.The results show that the UHC volume fraction obtained by each filter is ranked as follows:V_{Vmean filtering}<V_V median filtering<VV without filtering.?2?With the increase of age,the influence of water-to-binder ratio on the degree of cement hydration is gradually becoming significant.For the 28d-old cement-fly ash composite system,the degree of cement hydration is linear with the equivalent water-to-binder ratio,and fly ash hardly participates in the reaction;after 56d age,when the equivalent water-to-binder ratio is less than 0.3 and greater than 0.47,the degree of cement hydration is linear with the equivalent water-to-binder ratio,and the slope of the former is greater than the latter.When the equivalent water-to-binder ratio is between 0.3 and 0.47,the degree of cement hydration is not sensitive to changes in the equivalent water-to-binder ratio.?3?According to the thermogravimetric analysis results,for the pure cement hardened body,the non-evaporated water and Ca?OH?₂ content increase monotonically with the increase of the water-to-binder ratio.For the cement-fly ash composite system with 35%or 55%fly ash content,the non-evaporated water volume increases monotonously with the increase of water-to-binder ratio,while the Ca?OH?₂ content is not sensitive to the change of water-to-binder ratio.?4?For the pure cement hardened body made of the benchmark cement,when the water-cement ratio is smaller than the theoretical full hydration water demand rate?,the water-cement ratio has a stronger influence on the degree of hydration.The higher the age,the more significant this phenomenon.In addition,the smaller the age,the larger the proportion of small particle size UHC in the overall UHC.The UHC particle size distribution between samples with different water-cement ratios is not much different,UHC with 10³0 micron particle size accounts for the highest proportion of UHC.And overall,the UHC particle size generally does not exceed 120 microns.The water-cement ratio has a negative correlation with the weighted roundness of the UHC surface.That is,the smaller the water-cement ratio,the simpler the UHC edge shape and the closer to the circle.?5?A simplified healing potential evaluation model was established based on the width value that can be achieved when the theoretical hydration product formed by the full hydration of UHC in cement-based hardening is densely filled with cracks.Substituting the test data,it is found that the smaller the age,the smaller the water-cement ratio,the greater the limit crack width,that is,the healing potential is better,but the limit crack width is usually only 4⁸?m.In addition,although the healing potential characterization value,d_c,is affected by both V_V and f?r?,overall,the effect of volume fraction is much stronger than the particle size distribution.