Effect Of Material Composition On The Mechanical Properties Of UHPC Paste Prepared At Room Temperature

Ultra-high performance concrete is a cementitious material with excellent properties,such as high strength,high toughness,favorable volume stability and good durability.However,UHPC is still limited in terms of engineering applications.Excluding the high price of silica fume and steel fiber,one of the important reasons why UHPC is difficult to apply in practical projects is that UHPC requires pressure molding and steam curing during the preparation process,these methods are difficult to apply in practical projects.Therefore,to increase the usability of UHPC and the strength of UHPC under the condition of curing at normal temperature is a problem in the research of ultra-high performance concrete.In order to explore the influence of material composition on the mechanical properties of UHPC paste prepared at room temperature,this article mainly carried out the following work:Firstly,UHPC paste under normal temperature curing was prepared by using different types of cement,silica fume,water reducing agent,different water-binder ratio and silica fume content,and the mechanical properties were tested.The results showed that:Water-binder ratio is the biggest factor affecting the flowability and strength of UHPC paste,and the compressive strength of UHPC is highest when the paste fluidity reaches 180 mm.The type of cement has little effect on the highest compressive strength UHPC can achieve;it has a greater impact on fluidity,and therefore influences the compressive strength at different ratios through flowability.The fluidity of 52.5 ordinary Portland cement is smaller than that of 42.5 moderate heat cement.UHPC paste has the best fluidity with 15% content of original silica fume and semi-densified silica fume;the fluidity increases with the content increases of zirconium-silice fume and white silica fume;the flexural strength of the paste was the highest when the silica fume content was 15%.The UHPC paste’s compressive strength of 52.5 ordinary portland cement is the highest when the silica fume content is 15%,while the compressive strength of the 42.5 moderate heat cement is the highest when the silica fume content is 20%.Silica fume is to increase the fluidity of UHPC paste: zirconium silica fume,white silica fume,original silica fume,semi-densified silica fume.The enhancement effect of white silica fume on the strength of UHPC paste is the greatest,and the zirconium silica fume is the smallest.For UHPC with original silica fume and semi-densified silica fume,the type of water-reducer has little effect,while UHPC with white silica fume should use polycarboxylic acid powder water-reducing agent.Then,the mechanical properties of the UHPC paste at room temperature curing and 90°C thermal curing,and the contribution of the silica fume ash effect to the strength of the paste were compared,the results showed that: The flexural strength and compressive strength of UHPC paste after standard water curing for 7 days and 28 days are consistent with the change trend of the silica fume content.The strength of UHPC paste after 7 days standard water curing is 70% to 80% of the 28 days.However,the flexural strength and compressive strength of UHPC paste increase with the increase of silica fume content under the condition of 90°C heat curing.Finally,thermogravimetric tests,mercury injection tests and SEM analysis on UHPC paste specimens cured for 7 days and 28 days at room temperature and 2 days heat curing were carried out.The results showed that: The calcium hydroxide content of UHPC paste under normal temperature curing and heat curing conditions decreased with the increase of silica fume content.The UHPC paste cured at room temperature is densest at 15% silica fume content,while the UHPC paste under the heat curing becomes more compact with the addition of silica fume.Under normal temperature curing conditions,the porosity and harmful pores of UHPC paste with 15% silica fume content were significantly lower than those with 30% addition.Under the condition of heat curing,the porosity of UHPC paste decreases with the increase of silica fume content,and the number of harmful pores also decreases.