Effect Of Silica Fume Content And Particle Size Distribution On Compactness Of Negative Temperature Concrete

The microscopic pore structure of the material determines its various macroscopic properties.The pore structure of the concrete not only affects the concrete strength,but also affects the durability of the concrete.Silica fume,an excellent mineral admixture,could optimize the pore size distribution,refine the average pore size,reduce the porosity of the concrete and thus improving its pore structure.In this paper,silica fume was added into concrete as a kind of mineral admixture.The mechanical properties,carbonation resistance and the changes of pore structure and microstructure of concrete were investigated from the perspectives of content of silica fume,the particle size distribution of silica fume and the curing system of concrete.Furthermore,the relationship was established by fractal dimension method with regards to the content of silica fume and the particle size distribution and the pore structure and strength of concrete.Conclusions are as followed:The effects of the amount of silica fume,the particle size distribution of silica fume and the concrete curing regime on the compressive strength of concrete were studied,the results show that,silica fume can improve the compressive strength,and with the increase of curing age,the compressive strength of concrete doped with silica fume was significantly higher than that of concrete without silica fume.When the particle size of silica fume was constant,the compressive strength of concrete with the content of silica fume 10%was the largest.When the content of silica fume and curing period of concrete are certain,the compressive strength of concrete corresponding to silica fume with the particle size of F2(d50=0.17?m,d90=0.3?m)is larger,and the concrete strength growth rate under standard curing obviously higher than the constant negative temperature curing.In the depth of carbonation testing,the influence of silica fume content and particle size distribution on the carbonation depth of concrete with different carbonation age was studied after different curing systems.The results show that,the micro-aggregate filling effect of silica fume is very effective in enhancing the carbonation resistance of concrete.No matter what carbonation age,the depth of carbonization in the test group with 10%silica fume is relatively small,and the 6%is much deeper.The carbonized depth of silica fume with the particle size of F2(d50=0.17?m,d90=0.3?m)was the lowest in the experimental group.After different conditions curing,the concrete with same carbonation age and particle size of silica fume has a great difference in carbonation depth,and the carbonation depth of concrete which after standard curing was significantly lower than the constant negative temperature curing.Through the mercury intrusion test and scanning electron microscopy analysis,it was found that,an appropriate amount of silica fume can significantly improve the compactness of the internal structure of concrete,reduce the porosity of concrete and improve the pore size distribution of concrete.Whatever curing system,when the particle size of silica fume is F2(d50=0.17?m,d90=0.3?m)and 10%content,the concrete is relatively dense,the content of harmless pores(<20nm)and few harmful pores(20-50nm)is relatively high,and the contents of harmful pores(50-200nm)and multiple harmful pores(>200nm)are relatively low.The pore fractal dimension of concrete increases with the content of silica fume increased,and then with the decrease of particle size of silica fume,it increased firstly and then decreased,the pore fractal dimension of concrete corresponding to 10%silica fume content with particle size of F2(d50=0.17?m,d90=0.3?m)is the largest than that of other test groups,the pore fractal dimension under standard curing is obviously higher than that under constant low temperature curing.The compressive strength of concrete under the two kinds of curing system has a positive correlation with the pore fractal dimension.