Study On The Effects Of Nano-CACO₃on The Properties Of Ultra High Performance Concrete

As a new kind of cement-based composite material, Ultra High PerformanceConcrete （UHPC） has the characteristic of ultra high strength, high toughness andhigh durability which plays more and more important role in the engineeringapplication. The paper introduced nano-CaCO₃in ultra high performance concrete inorder to design and prepare ultra high performance concrete with goodperforcemence. Besides the influences of nano-CaCO₃on ultra high performanceconcrete which can provide a train of thoughts how to improve its performance in thepractical engineering are also studied.The hydration process such as bound water content, exothermic rate andautogenous shrinkage, characteristics of hydration products, spread value andmechanic performance of ultra high performance concrete which containing Nano-CaCO₃particles were investigated using calorimetry, thermogravitymetric analysis,non-contact concrete shrinkage deformation tester, SEM, fluidity, mechanic tests.Research results show that:（1）Studies on the influence of nano-CaCO₃on the rheological property offreshly mixed cement paste of UHPC show that with the increase quantity of nano-CaCO₃the fluidity of freshly mixed cement paste decreased gradually. These mainbecause of its too fine particle size and high specific surface area, the water demandof nano-CaCO₃is higher. While the effect will be weaken througth the method ofchanging water-binder ratio, using super reactive slag powder partly replace glasspowder and optimizing superplasticizer.（2）With the micro-aggregates packing effect, crystal nucleus effect and thepinning effect, nano-CaCO₃will have a big influence on the mechanical propertiesof UHPC. After the incorporation of steel fiber, basalt fiber and stainless steel microfiber, nano-CaCO₃will also optimize its mechanical properties.（3） Through analysis the microstructure of UHPC containing nano-CaCO₃byscanning electronic microscope technology, it is clear that adding nano-CaCO₃intothe system will optimize its grading and improve the density of matrix. Besides theamounts of attachments on cement particles increase and the degree of hydrationprocess will be higher. Meanwhile the SEM photos of ultra high performanceconcrete with basalt fibers are also presented to study the bonding of matrix and fiber interface.（4）The incorporation of nano-CaCO₃into UHPC system will shorten the initialsetting time as well as final setting time and increase the exothermic rate ofhydration. Moreover it will also make the exothermic peak on hydration exothermiccurve appear in advance. And the amounts of hydration heat and the amounts ofhydration products are also increased.（5）At the early time of hydration UHPC will expand because of hydration heat.While it will appear shrinkage phenomenon with hydration going on and the ratio ofautogeneous shrinkage will be relatively steady after3days. As for the influence ofnano-CaCO₃on UHPC, it will increase the autogenous shrinkage with the contentincrease.