Study On Stability Of Inactive Admixture In Low Clinker System

The technological progress and wide use of high-efficiency superplasticizer make it easier for concrete to obtain high strength and create conditions for the application of inactive admixture in concrete.The low clinker cementing system has gradually attracted people's attention for environmental protecting,energy saving and emission reduction.In recent years,due to the excessive fineness of the cement and the high strength of the concrete in the early stage,the early cracking sensitivity of concrete is increasing.Besides that,the cracking problem of the concrete structure is remarkable and the structural durability get worse.The research and application of inactive admixtures in concrete are more and more attractive.But there are relatively few studies on the application of inactive admixture in the cementing system of low clinker.There is little research on stability in low-clinker systems.The research and utilization of inactive admixture in concrete can not only effectively consume solid waste,but also partly solve the problem of short supply of high quality admixture like fly ash.In this paper,limestone powder,iron tailings fine powder and steel slag powder(Its activity is not low,but due to poor water demand,it is studied as an inactive admixture)are selected as the research objects.The effects of its addition on the workability,strength,stability and fine microstructure of the low-clinker system that it is composed of cement-S95 slag powder-fly ash(mass ratio 40:35:25)were studied.The results show that in the same flow state of concrete mixture,the content of 10% limestone powder or iron tailings fine powder can reduce the dosage of water reducer,which can be reduced by 11.4% and 7.1% respectively.Overall,the strength of the hardened body increases with the increase of age and decreases with the increase of dosage.The addition of inactive admixture makes the strength of low-clinker cement mortar and concrete system decrease in the early stage,but the strength develops rapidly in the later stage.In mortar experiment,the strength of the iron tailings powder or steel slag powder mortar mixed with 10% is not reduced compared with the control group,and the strength value of inactive admixture is not much different from the control group when the content is 15%;In the test of the mechanical properties of concrete,for medium and low strength concrete,when the content of the three admixtures is 10%,the late strength of concrete does not decrease compared with the control group,among which the steel slag powder group is the highest,and the compressive strength at 360 d is 3.6MPa higher than the control group.When the content of inactive admixture is less than 20%,the strength of concrete at 28 d meets the requirements of C30;For medium and high strength concrete,the addition of inactive admixture will reduce the compressive strength of concrete to different degrees.The compressive strength of the group with 10% steel slag powder is lower than the control group by 4.8MPa at 360 d.When the content of limestone powder or iron tailings powder is 20% or steel slag powder is 10%,the strength of concrete at 28 d meets the requirements of C50.With the further increase of the content of inactive admixture,the strength of concrete at long age increases obviously,and the strength of the long-term is guaranteed.Stability refers to volume stability and durability.In terms of volume stability,when the content of inactive admixture is 15%,it can effectively delay the early cracking time of low-clinker cement slurry or mortar,and reduce the crack width of the same period.Limestone powder or iron tailings powder can reduce concrete shrinkage at the content of 15%,while steel slag powder can increase concrete shrinkage;In terms of concrete durability,all durability indexes of concrete meet the requirements when the inactive admixture is within 20%,among which the limestone powder group is superior to the iron tailings fine powder group,and the steel slag powder group is relatively inferior.From the water absorption test of concrete surface,when the content of inactive admixture is less than 20%,it has little influence on the concrete surface water absorption rate;If the dosage is too large,it has great influence on concrete.The surface water absorption rate of concrete can be doubled when the content is 40%.SEM test was conducted to observe the microstructure of the hardened body for 28 days.It can be concluded that appropriate amount of inactive admixture can increase the compactness of the hardened body.The hydration heat test shows that the addition of limestone powder can reduce the peak heat release rate and early heat release of the cementing system of low clinker,and the larger the amount of limestone powder,the more obvious.