Study On The Effects Of Nano-materials On The Strength Of Ultra High Performance Concrete

Along with the development of concrete,high performance or ultra highperformance have become the main developing trend of concrete materials.As a newkind of cement-based composite materials,Ultra High Performance Concrete （UHPC）has played more and more important role in the engineering application due to itsultra high performance.But,as a brittle material,it still has some defects,so how toimprove the strength and toughness of UHPC has attracted increasing attention ofconcrete researchers.Nano-material has been applied in various fields due to its special properties,including used in ordinary concrete area also.But the application of nano-material inUHPC its still rarely,and there were a lit reports about the effects of nano-material onthe strength of Ultra-high Performance Concrete（UHPC） technology at present.Thus,On the basis of current research achievements about UHPC,round the nano-materialand UHPC of these two respects,the effects of Nano-CaCO₃or Nano-SiO₂on thestrength of UHPC were mainly studied through the experimental research in thispaper,the major research achievements were summarized as follows:（1）Studies on the influence of nano-material on the rheological property offreshly mixed cement paste of UHPC show that with the increase of nano-materialquantity the fluidity of freshly mixed cement paste decreased gradually,these mainbecause of its too fine micro-powder particle size and high specific surface area,thewater demand of nano-material is higher,lead to the fluidity of paste decreased.（2）When added nano-SiO₂（NS） in UHPC singly,the result shows that with thequantities of NS increased appropriately,the compressive strength and bendingstrength of UHPC are both enhanced compared with the groups that without addedNS,especially the flexural strength improved significantly,when added1%NS,themaximize growth rate of flexural strength can up to40.7%,the optimal fraction of NSin UHPC is0.5%¹%in this paper.Through observe the strength of UHPC that afterbeen standard cured of7days,28days and hot water cured of48h,we found that thestrength of hardened cement paste with NS at early age was increased obviously,certainly the late strength also enhanced,beside the hot water curing is the besteffective one to enhance the UHPC strength if under the same condition.In addition,the ratio of flexural to compressive strength of UHPC with NS also increased,it shows the concrete toughness enhanced.（3）When added nano-CaCO₃（NC） singly,it directs that the strength of UHPCwas found to be extremely enhanced higher than added NS,the optimal fraction of NCis3%in this paper.The compressive strength of UHPC with NC was increased up to16.7%compared with that without add NC,the flexural strength was enhanced39%when under hot water curing condition, beside,the ratio of flexural to compressivestrength of UHPC increased,NC displayed its enhancement and toughen effectsgradually.（4）Combination of nano-SiO₂and nano-CaCO₃received an unfavorablestrengthening effect,its just a little better than add NS singly,but a little inferior thanthe singe application of NC.（5）Through analysis the microstructure of UHPC by scanning electronicmicroscope technology,the mechanism of nano-material strengthened and toughenedto UHPC were shown as follows:1)The micro-aggregates packing effect:the gaps ofgelled material system can be filled up by nano-powders when just added them,thosedecreased the cement stone porosity and improved the density of matrix.2)Thepozzolanic effect:nano-SiO₂can react with Ca（OH）2easily to produce hydratedcalcium silicates gel due to its high pozzolanic activity,and inducted the C-S-H gelgrown in the direction of acicular also,then improved the interface structure of matrixand reduced the structural defects of cement paste.3)The crystal nucleus effect:nano-CaCO₃can absorb the calcium ion from the hydration of C3S because of itssurface high activity,those leaded the dense distribution and beamed form of CHcrystal at interface between cement paste and block decreased and the contents ofC-S-H at interface increased, then improved the strength and toughness of hardenedcement paste at last.4)The pinning effect:the nano-powders can produce a pinningeffect at interface of cement stone matrix also,when the tiny cracks just formed incement paste interior that nano-powder reflect and retard the crack growth velocity ofinterface cracks in the same time,and the energy was consumed,the initiation andgrowth of cracks was restricted effectively,then improved the fracture toughness ofcement matrix materials.