| Ultra High Performance Concrete（UHPC）is a new type of cement-based composite material with ultra-high strength,ultra-strong toughness,high volume stability and good durability.Compared with ordinary concrete,UHPC has higher production costs and greater shrinkage.Desulfurization slag is a kind of bottom slag discharged from the circulating fluidized bed of a power plant.It has pozzolanic activity,self-hardening and micro-expandable solid waste.Fly ash is a kind of fine ash collected from the flue gas of coal-fired power plants.Its particles are porous and have good activity.In this paper,on the basis of modifying the desulfurization slag and fly ash physical grinding,the two are creatively used as composite admixtures for the preparation of ultra-high performance concrete,and the response surface method is used to design the mix ratio.The content of composite admixtures,water-to-binder ratio and volume of steel fiber are used as influencing factors to explore its influence on the working performance,mechanical properties and shrinkage properties of UHPC.It is optimized on the basis of the test results,and the best test mix ratio and test results are predicted.The main research contents and results are as follows:（1）Using response surface method to design the mix ratio,systematically study the influence of the compound admixture content,water-binder ratio and steel fiber volume content on the working performance of UHPC,and predict the best mix ratio and test results.The results show that:with the increase of the compound admixture content,the fluidity of UHPC gradually decreases.The water-binder ratio has a more obvious effect on the fluidity of UHPC,which is linear.The larger the water-binder ratio,the greater the concrete fluidity;when the volume of steel fiber is not more than 2%,the fluidity of UHPC is due to the volume of steel fiber.Increase,its fluidity gradually decreases;when the volume of steel fiber content is greater than 2%,its fluidity declines in a steep slope,agglomeration phenomenon occurs obviously.Response surface method analysis shows that the interaction between steel fiber and water-binder ratio has a significant impact on the performance of UHPC.（2）The experiment researched the influence of compound admixture content,water-binder ratio and steel fiber volume content on the compressive strength of UHPC at different ages,analyzed the microscopic mechanism,and predicted the best mix ratio and test results.The study found that when the content of the composite admixture does not exceed 20%,the compressive strength of UHPC at 7d age will gradually increase with the increase of the content;when the content exceeds 20%,the strength will decrease accordingly;The increase in the glue ratio will gradually reduce the compressive strength of UHPC at 7d age;the compressive strength of UHPC at 7d age has an obvious linear relationship with the increase of the volume of steel fiber.When the volume of steel fiber is higher than 2%,And the reunion phenomenon is more serious.The response surface method analysis shows that the interaction between the content of composite admixture,water-binder ratio,and steel fiber content has a significant impact on the compressive strength of UHPC at 7d age.The compressive strength test results at 28d age show that the compressive strength of UHPC gradually decreases with the increase of the compound admixture content;the compressive strength of UHPC decreases with the increase of water-to-binder ratio;steel fiber The more volume content,the more significant the improvement of UHPC compressive strength.In the 90-d age compressive strength test,the compressive strength is slightly higher than that of 28d age,and its influence relationship is the same as that of 28d age.（3）The experiment researched the influence of compound admixture content,water-binder ratio and steel fiber volume content on UHPC shrinkage,and predicted the best mix ratio and test results.The results show that:the impact of composite admixtures on the shrinkage performance of UHPC can be divided into three stages:a rapid growth period,a rapid development period,and a stable period.When the composite admixture content is30%,the water-binder ratio is 0.16,and the steel fiber volume content is 3%,the shrinkage rate reaches the maximum,and the shrinkage rate is-138.8×10-6;the composite admixture content is 20%,the water-binder ratio is 0.21,and the volume of steel fiber is 2%when the shrinkage rate is the smallest,and the shrinkage rate is-204.3×10-6.Response surface analysis shows that the interaction of water-binder ratio and composite admixture has a significant impact on the shrinkage performance of UHPC.With the increase of the amount of composite admixture,the expansion of UHPC（the shrinkage rate of UHPC in the test is negative is expansion）gradually increases;the larger the water-binder ratio,the smaller the expansion of UHPC;the volume of steel fiber The contractility of UHPC has no obvious effect.Therefore,it can be concluded that the expansion of the composite admixture in UHPC can significantly offset its shrinkage and expand.（4）After optimization by response surface method,it is predicted that when the content of the composite admixture is 20%,the water-binder ratio is 0.18,and the steel fiber content is 2%,its comprehensive performance（working performance,compressive strength and shrinkage）The optimal value can be obtained,the fluidity is 284mm,the compressive strength of 7d,28d and 90d age reaches 69.14MPa,86.57MPa and 88.92MPa respectively,and the shrinkage rate is-183.35×10-6. |
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