Study On The Effect Of Mineral Admixtures On The Mechanical Properties And Durability Of Recycled Concrete

Against the backdrop of the rapid development of infrastructure in China,with the demolition of old buildings and the reconstruction and renovation of roads,the amount of construction waste,mainly waste concrete,is increasing,and the problems of natural resources scarcity such as sand and gravel,and ecological environmental pollution are becoming increasingly acute.The use of recycled concrete provides a new way to solve these problems,saving natural aggregate resources while reducing environmental pollution caused by construction waste,and achieving a green transformation of construction materials.However,the poor performance of recycled concrete itself limits its engineering application,while mineral admixtures such as fly ash and silica fume,as industrial wastes,can partially replace cement and be added to concrete to improve its performance.Based on this,this thesis takes the variation of the content of fly ash and silica fume in the case of single or double admixtures based on the replacement ratio of recycled coarse aggregate and 100% replacement ratio as variable factors,and studies the impact of various factors on the mechanical properties and durability of recycled concrete through indoor experiments and theoretical analysis,focusing on the changes in mineral admixtures.The main research contents and conclusions are as follows:(1)The basic physical performance tests were conducted to compare and analyze the basic performance differences between the prepared recycled coarse aggregate and natural coarse aggregate.The results show that the water absorption,crushing index,and needle flake particle content of the prepared recycled coarse aggregate are higher than those of the natural coarse aggregate,while the apparent density,bulk density,and compact density are lower than those of the natural coarse aggregate,meeting the requirements of the national standard for Class II recycled coarse aggregate.(2)Based on the compressive strength test and flexural strength test,the influence of the content change of recycled coarse aggregate on the replacement ratio and 100%replacement ratio of fly ash and silica fume in the case of single or double admixtures was analyzed.The research shows that the compressive strength and flexural strength of recycled concrete decrease with the increase of the replacement ratio of recycled coarse aggregate;The single addition of fly ash has a negative impact on the early mechanical properties of recycled concrete,with a maximum negative impact at 30%,while at 60 days of age,fly ash has a good improvement effect on the mechanical properties.The optimal content of fly ash for compressive strength performance is 20%,and the flexural strength is 10%;The improvement of compressive and flexural strength by adding silica fume alone showed a trend of first increasing and then decreasing with the increase of the content,and the performance was optimal at 6% content;The mechanical performance of double mixed fly ash and silica fume is superior to that of single mixed fly ash.When 20%fly ash and 6% silica fume are mixed together,the compressive and flexural strength can be greatly improved,reaching the level of ordinary concrete.(3)Accelerated carbonization test,rapid freeze-thaw test,and sulfate dry wet cycle test were conducted to study the effect of dosage changes on durability performance under the condition of single or double addition of fly ash and silica fume based on the replacement rate and 100% replacement rate of recycled coarse aggregate.The results show that each specimen group meets the requirements for carbonization depth,F150 frost resistance level,and KS90 sulfate resistance level;Recycled coarse aggregate has adverse effects on the carbonation resistance,frost resistance,and sulfate resistance of recycled concrete,and the negative effects increase with the increase of substitution rate;The single addition of fly ash is unfavorable for the carbonation resistance of recycled concrete,and the effect becomes more obvious with the increase of the dosage.However,it has a good improvement in frost resistance and sulfate resistance,with an optimal dosage of 20%;Adding silica fume alone will greatly deepen the carbonization depth,and significantly improve the frost resistance and sulfate resistance.When the dosage is 6%,the best effect is to slow down the carbonization rate,and the frost resistance performance is the best.The sulfate resistance performance of the 9% dosage group is slightly better than that of the 6% dosage group;The performance of double mixed fly ash and silica fume is superior to that of single mixed fly ash.When 10% fly ash is mixed with 6% silica fume,the carbonization resistance is the best,and when 20% fly ash is mixed with 6%silica fume,the frost resistance and sulfate resistance are the best.The performance of frost resistance and sulfate resistance is better than that of ordinary concrete.(4)Analyze the microstructure and hydration products of the interface transition zone of recycled concrete through SEM and XRD.The results show that there are many weak parts in the interface transition zone of recycled concrete,and the addition of fly ash and silica fume can form a good particle gradation to fill the micropores and generate more secondary hydration products such as C-S-H gel to improve the internal microstructure.