Research On Properties And Micromechanism Of UHPC With Zeolite Powder Replacing Part Of Silica Fume

Compared with traditional concrete,ultra-high performance concrete（UHPC）is a new type of composite cementitious material developed in recent years and conforming to the basic requirements of modern engineering construction,with ultra-high mechanical strength,toughness and durability.To ensure excellent performance,UHPC generally adopts low water-binder ratio,and the silica fume with high activity and price is added.The extensive use of silica fume makes the preparation cost of UHPC very high,which also hinders the development of UHPC and the engineering application,especially in some developing countries.Compared with silica fume,zeolite powder is a local mineral admixture with low price and abundant natural resources.It has good pozzolanic activity and characteristics of water absorption and release.In order to achieve good economic benefits,UHPC was prepared by substituting zeolite powder for part of silica fume.First,the effects of different amounts of zeolite powder on mechanical properties and shrinkage of UHPC were studied,so as to determine the optimal zeolite powders content.Secondly,under the condition of this amount,the effects of different water-binder ratios on mechanical properties and shrinkage of UHPC containing zeolite powder were studied,and the optimal water-binder ratio was determined.Then,based on the established zeolite powder content and water-binder ratio,the influence of steel fiber in different volume content on the mechanical properties of UHPC containing zeolite powder was studied.Then,the phases of composition of UHPC containing zeolite powder were analyzed by x-ray diffraction（XRD）,the microstructures were observed by scanning electron microscope（SEM）,and the microscopic mechanical properties were analyzed based on nano-indentation experiment,and the macroscopic properties of UHPC containing zeolite powder were analyzed by the microscopic mechanism.Finally,combined with the macroscopic properties and microscopic results,the suggestions on the mixture ratio of UHPC containing zeolite powder were put forward.The main conclusions are as follows:（1）The 3-day strength of UHPC was reduced by replacing part of silica fume with zeolite powder.With the increase of age,zeolite powder with replacing rate of15%increased the strength,while with replacing rate of 30%had little influence on strength.Zeolite powder contributes to toughness of UHPC in later stage.（2）The mechanical strength of UHPC containing zeolite powder reduced with the increase of water-binder ratio,but the difference in strength between specimens with water-binder ratio of 0.14 and 0.16 was not significant.The strength was improved obviously by adding appropriate content of steel fiber in volume,while the excessive steel fiber reduced the strength.（3）Zeolite powder significantly reduced the shrinkage of UHPC,and with the increase of the replacing rate of zeolite powder,the shrinkage of UHPC containing zeolite powder decreased successively.Increasing water-binder ratio reduced the shrinkage.（4）XRD results showed that UHPC with zeolite powder contained almost no Ca（OH）₂（CH）.Zeolite powder with replacing rate of 15%reduced content of CH and Si O₂,thus promoting production of calcium silicate hydrates（CSH）in UHPC,and zeolite powder had an adverse effect on the formation of hydration products when replacing rate was 60%.（5）SEM results showed that the hydration products of UHPC with zeolite powder did not contain ettringite.After zeolite powder replaced silica fume,the type of hydration products did not change.Zeolite powder with replacing rate of 15%reduced the porosity of matrix and improved the pore structure.The effects of zeolite powder with 30%on the microstructure and pore structure of UHPC were not obvious.As the zeolite powder continued to increase,the overall compactness in matrix reduced,and the crack width of interfacial transition zone（ITZ）increased.The steel fiber with volume content of 2.5%had no obvious influence on the densification of ITZ in UHPC containing zeolite powder.（6）The results of nano-indentation showed that the main hydration products of UHPC with zeolite powder were high density CSH（HD CSH）and ultra high density CSH（UHD CSH）.Zeolite powder with replacing rate of 15%could reduce the thickness of ITZ,and improved the elastic modulus of UHPC.The zeolite powder with 30%had no effect on the thickness and modulus of ITZ.With the increase of zeolite powder,the thickness and porosity of ITZ increased,and the content of UHD CSH decreased.Steel fiber slightly reduced the elastic modulus and increased the thickness of ITZ.（7）When the sand-binder ratio was 1.1 and the dosage of water reducing agent was 22.9kg/m³,it was suggested that the rate of zeolite powder replacing silica fume was 30%,the water-binder ratio was 0.16,and the steel fiber volume content was2.0%-3.0%.When specimens were cured under standard treatment for 28 days,the maximum compressive strength and flexural strength of UHPC containing zeolite powder were 153.4MPa and 21.9MPa,respectively.