Effect Of Curing Humidity On Volume Deformation Of MgO Shrinkage-compensating Concrete And Its Mechanism Analysis

MgO expansive agent was originally mainly used to compensate temperature shrinkage in hydraulic mass concrete,and has been gradually applied to industrial and civil buildings in recent years.Nowadays,in the study of volume deformation of MgO shrinkage-compensating concrete,the internal relative humidity and its humidity deformation are uniformly described as the unified internal cause of autogenous shrinkage and drying shrinkage of concrete,but there is no quantitative analysis of autogenous shrinkage,drying shrinkage and shrinkage compensation.In this paper,C40 shrinkage-compensating concrete is taken as the research object,and the influence and mechanism of different humidity conditions（standard curing,sealing curing,dry curing,urea curing）on the volume deformation of MgO shrinkage-compensating concrete are systematically studied.The hydration process,hydration degree and pore structure of hardened paste of MgO shrinkage-compensating composite cementitious materials are systematically studied by means of hydration heat,mercury intrusion and thermogravimetric test methods,so as to carry out the influence of curing humidity on the volume deformation of MgO shrinkage-compensating concrete and explore its mechanism.The following conclusions are drawn:（1）Under different humidity conditions,when the content of MgO expansive agent is the same,the free expansion of concrete under sealed curing condition is the largest,followed by urea curing and dry curing.Under standard curing conditions,the limited expansion rate of concrete is the largest,followed by sealing curing,and dry curing is the smallest.The free volume deformation of concrete sealed curing with MgO expansive agent is expansion,showing a trend of rapid expansion first and then slow expansion.The test group of urea curing and dry curing showed expansion first,and then began to shrink around 7 days.Under the same strength grade（C40）,the larger the water-binder ratio,the more the expansion source Mg（OH）₂generated in the system,the greater the free expansion amount and the limited expansion rate;with the extension of curing age,the expansion source Mg（OH）₂ generated in the system shows an increasing trend.MgO expansive agent can produce expansion to compensate the drying shrinkage and autogenous shrinkage of concrete.The free deformation of coated urea curing 28d increases by about 25×10^-6-40×10^-6,and the lower the water-binder ratio,the more obvious the increase of expansion value;the sealing expansion is the largest,followed by dry expansion,and the free deformation of shrinkage compensation will increase by more than 100×10^-6 in 28days.Under standard curing,dry curing and sealed curing,the expansion stability window of MgO expansive agent is 0d-28d,and the expansion is more stable after 28d.The change of curing environment only changes the final expansion amount,but has little effect on the time when the expansion reaches stability.（2）With the extension of age,the internal relative humidity of concrete decreases.Under the condition of sealed curing,the internal relative humidity decreases to about 92%when the water-binder ratio is 0.35 and decreases to about 95%when the water-binder ratio is 0.45.Under the condition of dry and coated urea curing,the internal relative humidity of 0.35 water-binder ratio decreased to about 75%and 0.45 water-binder ratio decreased to about 87%at 28d.At the same curing age,the internal relative humidity and hydration degree of MgO shrinkage-compensating concrete cured by sealed curing and urea curing are higher than those cured by dry curing.The internal relative humidity of shrinkage compensating concrete with MgO expansive agent is less than that of concrete without MgO expansive agent.The greater the water-binder ratio,the slower the internal relative humidity decreases,and the greater the hydration degree of MgO;the internal humidity and deformation show good dependence and synchronization.The end point of expansion deformation is not completely consistent with the time when the internal relative humidity decreases rapidly,and the latter is later than the former.（3）The compressive strength of MgO shrinkage-compensating concrete is increased by about 2%-8%by sealing curing and 1.5%-3%by smearing urea curing.With the increase of MgO content from 9%to 12%,the peak value of the second exothermic peak decreases,the heat release decreases,and the compressive strength of concrete decreases as a whole.At28 d,it decreases by about 19.4%,which reduces two strength grades.Under the condition of30kg（9%）MgO expansive agent,the addition of MgO expansive agent will increase the heat release in the system,but will delay the peak time.The compressive strength of concrete with MgO expansive agent is higher than that of concrete without MgO expansive agent.Under the same strength grade（C40）,the larger the water-binder ratio,the more the heat release,the higher the peak of the second exothermic peak,and the greater the hydration degree of MgO.（4）For MgO shrinkage-compensating concrete,the smaller the water-binder ratio,the denser the structure.The smaller the pores in the concrete,the greater the capillary pressure and the greater the deformation;the capillary pressure increases with the decrease of the internal humidity of the slurry,showing a trend of first fast and then slow.The total pore volume and the most probable pore size of sealed curing and coated urea curing are less than those of dry curing.With the increase of age,the pore volume of harmless pores and less harmful pores gradually increased.Urea curing increases the proportion of harmful pores and less harmful pores,and sealing curing increases the pore volume less than 50 nm.The incorporation of MgO expansion agent increases the pore volume greater than 50 nm.With the increase of water-binder ratio,the porosity of the slurry structure is larger,the structure is looser,and the pores in each pore size range increase,especially the pores larger than 200 nm.