Study On Early Hydration And Ice Content Of Negative Temperature Concrete Based On Nuclear Magnetic Resonance Technology

Negative temperature concrete technology is prevailing in the study of concrete preparation technology and service performance in extreme environments as cold areas.Although the relevant codes and theories are constantly updated and improved,there are still many technical problems that need to be solved urgently.At present,the rapid development of nuclear magnetic resonance technology in material detection provides a fast,convenient,and in-situ,non-destructive detection method for the study of hydration degree and internal pore structure of negative temperature concrete.In this paper,the nuclear magnetic resonance technology was used to test the variation of the liquid water content,the liquid water distribution and the most probable aperture filled with liquid water of the cement paste specimens along with the curing age in negative temperature.The effect of antifreeze on early hydration of negative temperature concrete was analyzed.At the same time,the effects of the types of antifreeze,pre-curing time,water-binder ratio,etc.on the growth law of compressive strength of negative temperature concrete were studied.Then the correlation between compressive strength and microscopic properties such as liquid water content was analyzed.The results show that the nuclear magnetic resonance test of the Portland cement specimens through the CPMG pulse sequence causes more significant interference than the white cement specimens.While contact with iron-testing-instruments,coating with the solidified epoxy resin and a small amount of free water don’t cause obvious interference to the test results.By controlling the testing time and adopting certain insulation methods,it is possible to study the microscopic properties such as liquid water content and ice content in negative temperature concrete.After curing for 4 hours at-15℃,the freezable water inside the cement paste specimens without antifreeze has been completely frozen into ice,the average liquid water content in the specimens is 7.94%,and the most probable aperture filled with liquid water significantly gets lower than normal temperature.After the cement paste specimens with inorganic antifreeze,that the water-cement ratio is 0.46 and 0.29 respectively,was frozen at-15℃ for 4h,the liquid water content of the specimens are 94.9% and 78.5%,and during the 7d of negative temperature curing the liquid water content is continuously decreasing,and the most probable aperture filled with liquid water is continuously refining.The ice content at-7d is significantly lower than the initial frozen state,and the ice content decreases with the extension of the pre-curing time.When the dosage of inorganic antifreeze is 7%,the liquid water in the pores of 10～50nm is continuously decreasing during the negative temperature curing period,that is,the cement is continuously hydrated,and its critical ice content is 10%.With the increasing of the dosage of organic antifreeze,the liquid water content and the most probable aperture filled with liquid water in the cement paste specimens increases.However the effect of organic antifreeze to reduce the freezing point of the pore solution is lower than that of inorganic antifreeze.The liquid water content of the cement paste specimens with organic antifreeze,that has pre-cured for 4h,is decreasing and the most probable aperture filled with liquid water remains basically unchanged during negative temperature curing at-15 ℃.With the increase of pre-curing time,the ice content of the specimens is reducing during the negative temperature curing.With the increasing of pre-curing time,the compressive strength of negative temperature concrete increases.The compressive strength of negative temperature concrete with inorganic antifreeze increases faster in the early age,while the later compressive strength increases slowly,and the compressive strength of negative temperature concrete with organic antifreeze is contrary to that with inorganic antifreeze.The effect of early age frost on the strength development of low water-binder ratio negative temperature concrete is higher than that of higher water-binder ratio concrete.There is a strong correlation between the variation law of liquid water of the cement paste specimen under negative temperature that measured by nuclear magnetic resonance technology and the increase law of compressive strength of negative temperature concrete.