Effect Of Particle Size Distribution Of Silica Fume And Fly Ash On Porosity Of Cryogenic Concrete

Admixtures in modern concrete processes have long become the sixth most indispensable component in the preparation of high performance concrete.Its use for concrete mechanics,durability,to some extent,played a better role in the optimization of concrete.In recent years,many scholars have made remarkable research on the relationship between admixture and concrete structure and properties.,but all based on the qualitative research of admixtures under the traditional conditions.However,the research on the relationship between the granularity and concrete structure based on low temperature is rare.In this paper,the preparation of different particle size of fly ash,silica fume complex admixture,the first of its cementitious material hydration exothermic analysis of different temperatures at different particle sizes admixture of concrete compressive strength,Frost resistance,pore structure parameters,combined with scanning electron microscopy analysis,to find out the rules of particle size and porosity,and to fit the mathematical relationship between particle size and porosity in the corresponding particle size range.The relationship between the particle size variation and the grinding time of different admixtures was studied for the hydration heat of the cementitious material and the compressive strength of the concrete.The results showed that the fixed specific surface area of the silica fume of the two groups A and E was 16.3 m2/g,respectively.The hydration conditions at 20.4 m2/g are relatively close,and the three groups B,C,and D have a silica fume specific surface area of 17.5 m2/g,18.2 m2/g,and 19.1 m2/g,respectively,and the particle size of the fly ash.When it is smaller,the peak of hydration heat release rate will be higher,and it will be advanced,and the heat release will reach relatively high value in a short time,among which the D group,that is,the specific surface area of silica fume is the most obvious at 19.1 m2/g.With the increase of grinding time,the changes of heat release rate and exothermic heat caused by the change of silica ash particle size are more obvious than the change of particle size of fly ash.The silica gel particles with less grinding time showed the largest group in the early hydration rate and the hydration heat release,but the longer the milling time,the smaller the silica gel particles.The effect of the grinding time on hydration is not proportional,and over-grinding will delay the hydration.Under standard curing conditions,the compressive strength of 3d and 7d ages increases with the increase of fineness of fly ash.For the later period,with the extension of curing age,the compressive strength decreases with the particle size of fly ash.The change of the magnitude of change is generally expressed as the 28d compressive strength with the smaller particle size,the first increase followed by the decline;The influence of silica fume particle size on the compressive strength shows that the 3d intensity does not change significantly with the particle size,but for the 7d and 28d compressive strengths,with the increase of fineness of silica fume under the same particle size of fly ash,The compressive strength generally showed a first increase,reaching a peak and then decreased.In the H3 group,the specific surface area of the fly ash was 342.4 m2/kg,and the specific surface area of the silica fume was 19.1 m2/g,and the compressive strength reached a maximum of 54.7 MPa in 28 days.Over-grinding will reduce the strength.At-10℃ curing conditions,the strength values generally decrease.The influence of the particle size of fly ash on the compressive strength shows that at the 3d and 7d ages,as the particle size of the fly ash becomes finer,the compressive strength value generally shows an increasing trend,and at the age of 28d,For group D,that is,the specific surface area of silica fume is 19.1m2/g,when the specific surface area of fly ash reaches 448.5m2/kg,if the fineness decreases,the strength value will decrease.The effect of the silica fume particle size on the compressive strength is that the compressive strength value of the fly ash particle size is increased compared to the same condition,and the specific surface area of the fly ash is within 448.5 m2/kg,and the silica fume becomes finer.The intensity value has generally risen.Whereas the specific surface area of fly ash is 493.9m2/kg,the finer the silica fume,the lower the strength;the 28d age compressive strength increases first and then decreases with the increase of the grinding time.The influence of the variation of fly ash-silica ash particle size on the frost resistance of concrete was studied.The results showed that the particle size of fly ash changed before 50 cycles of freeze-thaw cycles.The mass loss rate of the concrete specimens did not change much.The more significant change in the same group due to the finer particle size was that the D group,that is,the fixed silica fume specific surface area was 19.1 m2/g,in 175 freeze-thaw cycles.The mass loss decreases with the decrease of fly ash particle size,and then it shows an upward trend.When the specific surface area of fixed silica fume was 16.3m2/g and 20.4m2/g,the mass loss rate of the finest sample group exceeded 5%.For each set of test pieces,the relative dynamic elastic modulus of the first test sample and the fourth test sample was smaller than that of the test sample at the intermediate particle size.The same set of test pieces,with the finer granularity of fly ash,will increase first and then decrease.When the other conditions are the same,when the admixture is too thick or too fine,the relative dynamic elastic modulus will be adversely affected.The effect of change in silica ash particle size on frost resistance.With the increase in the number of freeze-thaw cycles,the rate of mass loss continues to increase.Before 50 cycles of freeze-thaw cycles,the mass loss of each set of specimens varied little with particle size.After 75 cycles of freeze-thaw cycles,the specific surface area of fixed fly ash was 288.9 m2/kg and 342.4 m2/kg,respectively.In the 408.4 m2/kg group,the mass loss rate showed a trend of decreasing first and then increasing with the decrease of the silica ash particle size.For the fixed fly ash specific surface area 342.4m2/kg,408.4m2/kg,448.5m2/kg three groups,the mass loss rate decreases with the silica ash particle size,showing the trend of decreasing first and then increasing.In the K group,that is,the specific surface area of the fixed fly ash was 493.9 m2/kg,K 1 and K 5 after 175 freeze-thaw cycles,the mass loss exceeded 5%,and the frost resistance was poor.The relative dynamic elastic modulus of concrete decreases with the particle size of silica fume in multiple freezing and thawing cycles where the specific surface areas of fly ash are 288.9 m2/kg,342.4 m2/kg,and 408.4 m2/kg.After increasing and then decreasing,the fixed fly ash specific surface area of 342.4 m2/kg group reached its maximum after the 175 cycles of freeze-thaw cycles.The specific surface area of silica fume was 18.2 m2/g.Admixtures are either too fine or too coarse to counteract freezing.When the specific surface area of the fly ash fixed is 493.9m2/kg,K group,among the five particle size changes of the silica fume,the frost resistance of the coarsest and finest groups is the worst,and the relative dynamic elastic modulus is reduced to below 60%.The correlation degree between the granularity interval and the concrete porosity was studied by using the grey correlation analysis method.When the specific surface area of silica fume is fixed at 19.1m2/g,the correlation of fly ash particle size in the range of(30～40μm)and(40～50μm)is the strongest for the total porosity.For the pretreatment of silica fume,the variation in the content of each particle size interval is limited,and when the activity difference is not large,especially under low temperature conditions,the degree of hydration is low,and the gradation between coarser particles and fly ash is good,and the pores The better the fill effect.The correlation between the particle size of the silica fume and the porosity of the compounded silica in the two regions(>0.4 μm)and(0.25-0.4 μm)is the largest.The data was analyzed and regression analysis was performed by MATLAB software to establish the mathematical relationship between fly ash particle size interval,silica ash particle size interval and total porosity in concrete under different curing conditions.