Study On The Micro-particles Effect On Ultra-high Performance Hydraulic Concrete

Research and application of ultra-high performance concrete(UHPC)is one of the most promising developing directions of cement-based materials developments and it has been started from 1990s in China.Yet its application on the hydraulic structure was restricted due to lack of fundamental researches and high cost.Supported by the National Key Basic Research and Development Program of China(973)“Research on long life-span of concrete materials and structures under severe environment(2015CB655101)w and National Natural Science Foundation Program of China“Structural parameter diversity of nano-particles of hydraulic concrete and performance control(51379163)",this thesis had launched investigations on workability,mechanical property,dry shrinkage as well as hydration process,hydration degree,micro-mechanical property and pore structures of ultra high performance hydraulic concrete(UHPHC)produced by silica fume(SF),limestone powder(LP),ground blast furnace slag(GBFS)and fly ash micro-beads(FAM)with employment of simplex-lattice design method and some other advanced observation technologies,including micro-calorimeter,scanning electron microscope(SEM),comprehensive thermal analysis,as well as mercury intrusion method and micro hardness,and taking silicon as the common components,compared the differences of physical,mechanical and durability properties of UHPHC prepared by compounding with limestone powder and those produced by compounding with reactive mineral admixtures such as slag and FAM.It had also proposed the key factors influencing macro performances of UHPHC with object of providing theoretical support for UHPHC design and application,especially for the application of LP in UHPHC in the future.The primary conclusions were forwarded as follows:(1)Based on the Compressible Packing Model(CPM),the calculation procedure of bulk density of multi-component composite of cementitious material was compiled,the optimization efficiency of complex components was improved.(2)The influence mechanism of close packing effect of multi-component cementitious powder on its macroscopic properties was analyzed,and explored the relationship between content of mineral admixture and particles grading effect.The fact that the bulk density of the cement-silica fume-limestone system was higher than that of the cement-silica fume-slag powder system was explained through the effect of particle accumulation,and the reason why UHPHC can still maintain higher physical and mechanical properties after cement replaced by limestone powder in large amount was further revealed.(3)By means of the simplex-lattice design method and the contour map,it was found that the interaction of silica fume with LP,slag and FAM was able to elevate the compressive strength at 28d of UHPHC.Even if cement replacing by LP by mass of 50%,the compressive strength of LP-cement system at 28d reached 85%of pure cement system,and approximate to FAM-cement system,which indicated the advantage of UHPHC can retain high mechanical properties after cement replaced by limestone powder in large amount.(4)It was revealed that the hydration degree,crystalline degree,morphology and amount of hydration products of cementitious system were different from conventional system on the condition of extremely low water cement ratio by a variety of modern micro test methods.The cement-silica fume-slag system had the highest hydration degree,followed by cement-silica fume-FAM system,and cement-silica fume-limestone system had the lowest hydration degree,contacting with the test results of mechanical properties,it shows that the strength of limestone binder system didn't depend on it's hydration degree,but the close packing effect.In addition,limestone powder plays the role of nucleation when mixing in the cementitious system yet very low hydration degree of LP was observed,the micro-crystalline nuclei and the micro-aggregate effects played an important role in the development of early strength.Porosity of ternary system with incorporation of SF and LP,slag and FAM under extreme low water/binder ratio was relatively low and the effect of pore refinement was obvious.(5)Based on the UHPHC characteristics of "low water-cement ratio,powder dense filling,high strength and high durability as well as the optimization of mineral admixture varieties,and considering the design requirements of hydraulic concrete,the UHPHC with the compressive strength acheve to 100MPa,good anti-abrasion performance and good anti-freezing performance could be prepared under the condition of the low water-cement ratio,low-active limestone powder used as filler material and adding 10mm iron tailings aggregate and steel fiber.The multi-gel system of mineral admixtures improved the micro-structure of the slurry-aggregate interface area and the pore structure of system 5 the concrete bubble spacing coefficient was less than 200?m,and the most probable porosity was in the(10nm?20nm)range.The incorporation of LP,slag powder and FAM was favorable to reducing drying shrinkage of UHPHC.