Study On The Properties Of Green Concrete At Room Temperature And After Fire Considering The Theory Of Wet Packing Density

The 14th Five-Year Plan clearly proposes to accelerate the promotion of green and low-carbon development,continuously improve environmental quality,enhance ecosystem quality and stability,and comprehensively improve resource utilization efficiency.In May2021,China made it clear that it would achieve emission peak by 2030 and carbon neutrality by 2060.It is well known that concrete is the most widely used building material.But cement,as the most important raw material for concrete,emits a lot of carbon dioxide in the production process.According to statistics,the carbon dioxide released by the production of cement accounts for 8% of global carbon emissions.Therefore,scientifically reducing the use of cement in the field of engineering is conducive to the realization of carbon peaking and carbon neutrality goals and the sustainable development of social economy.Silica fume(SF),granulated blast furnace slag(GGBFS)and fly ash(FA)is the by-products in industrial production.Using SF,GGBFS or FA to replace part of cement can reduce the amount of cement while improving the performance of concrete,but also to solve the industrial by-product stacking and treatment problems,to ensure high-quality social,economic development and the achievement of carbon peaking and carbon neutrality goals.Previously,researchers have studied the effect of cementitious materials on the mechanical properties and durability of concrete,but no scholar has yet analyzed the changes in concrete properties from the perspective of the impact of green materials on the microstructure of concrete.After the cement is replaced partly with SF,GGBFS and FA,the internal structure of the concrete changes,which in turn affects its macroscopic performance.Therefore,the priority is to discuss the influence mechanism of green materials on the internal structure and macroscopic mechanical properties of green concrete.In order to solve the above problems,based on the wet packing density theory,this paper will carry out the research on the effects of SF,GGBFS and FA on the internal structure,compressive strength,chloride ion permeability and post-fire apparent changes,compressive strength loss and quality loss of concrete.In this paper,30 groups of concrete were designed with water-binder ratio of 1.24 and 1.86 by volume(water-binder ratio by weight of about 0.4and 0.6),the amount of Superplasticizer unit powder surface area of 1.5×10-10m3/m2,and green replacement materials SF,GGBFS and FA.First,SF(replacement rates 5%,10% and15%),GGBFS(replacement rates 15%,25% and 35%)or FA(replacement rates 15%,25%and 35%)were used to replace cement to concreting,and its wet packing density(WPD),7,28 and 90 days compressive strength,and chloride ion permeability resistance were determined.The results show that in all the mix ratios,the WPD of SF concrete and FA concrete increases with the increase of material replacement rate.When the water-binder ratio by volume is 1.24,the WPD of GGBFS concrete reaches a maximum at a replacement rate of25%,followed by a decrease.When the water-binder ratio by volume is 1.86,the WPD of GGBFS increases with the increase in replacement rate.The compressive strength of SF concrete increases with the rate of replacement and age.The early compressive strength of GGBFS concrete and FA concrete is lower than that of reference concrete,and the higher the replacement rate,the lower the compressive strength,but the later compressive strength of these two concretes develops faster than that of reference concrete.Whether it is a water-binder ratio of 1.24 or 1.86 by volume,the compressive strength of GGBFS concrete with a 15% replacement rate of 90 d is higher than that of reference concrete and other GGBFS concrete.At a water-binder ratio of 1.24,the 90 d compressive strength of FA concrete with a replacement rate of 15% and 25% was higher than that of the reference group,of which 25% FA concrete had the highest compressive strength.When the type of cement replacement material is certain,with the increase of WPD,the resistance to chloride ion permeation increases.In addition,the performance of concrete after high temperature will change dramatically,especially when the temperature is above 600 ℃.Its compressive strength and quality will be greatly lost.So understanding the residual properties of concrete after fire will be helpful to avoid major casualties and property losses caused by building collapse after fire.Therefore,this paper also studies the post-fire residual properties of three kinds of green concrete.The results show that the post-fire quality loss rate of the three green concretes is higher than that of the concrete of the reference group,and the performance of the post-fire compressive strength of SF concrete at all temperatures is not as good as that of the reference group.At400°C,GGBFS concrete and FA concrete have an increase in compressive strength relative to that of room temperature.The residual compressive strength ratio of the two at 400 °C and600 °C is higher than that of the reference group and the residual compressive strength ratio after heating at 800°C and 1000°C is lower than that of the reference group.