The influence of crack control materials such as shrinkage and expanding admixture on compressive strength and shrinkage properties of concrete were studied.The hydration characteristics of cement and the evolution law of pore structure were evaluated by low field nuclear magnetic resonance and scanning electron microscope tests.The results show that the drying shrinkage of the sample decreased significantly.Concerning drying shrinkage performance,the effect of expanding admixture is better than that of shrinkage admixture.Compared with the control sample,the drying shrinkage of the shrinkage and expanding admixture samples at 90 days decreased by from 4.1%to 10.3%and from 14%to 21.7%,respectively.Meanwhile,the strain development rate coefficient of the shrinkage admixture sample decreased from 80.48 to 54.59,and that of the expanding admixture sample decreased to 41.69.The microscopic test results show that the expanding admixture can improve the early hydration of cement.The hydration degree of the later stage is lower than that of the control sample.The addition of shrinkage and expanding admixture weakened the concrete matrix's microstructure,increased the content of harmful pores in the matrix,and reduced the content of pores volume with pore size<50 nm.
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