Studies On The Relationship Between Composition And Microstructure Of UHPC Matrix

Ultra-high performance concrete(UHPC)as a new high-strength,high toughness and high durability of the new cement-based composite materials,compared to ordinary concrete,it is through the use of lower water-cement ratio,high dosage of cementing materials and Increase the activity of mineral admixtures,in the performance has been significantly improved.The active admixtures such as silica fume,ultrafine ore powder,fly ash and so on through the micro-aggregate effect and volcanic ash effect,can increase the density of UHPC matrix,thereby greatly improving its compressive strength and durability.In this paper,the gradation of mineral admixtures and cement particles in UHPC matrix was studied.Under the guidance of the compressible stacking(CPM)model,the most dense gradation ranges of single mineral admixture and cement particles were studied,and the mixing ratio design was carried out under the most dense state.The Nyquist graphs of UHPC substrates with different compositions,different water-binder ratios and curing conditions were also studied by using an equivalent circuit by AC impedance method.Finally,the water absorption of different UHPC substrates was studied.Ca(OH)2 Analysis of content and pore structure.Through the study and analysis of the above experiments,the following results are achieved:(1)The theoretical method of compressible stacking(CPM)model is introduced in detail,and several kinds of cases of solid particle system are deduced in detail.At the same time,the following two key problems of powder particle gradation are studied:1)It is proved that the method of solving the bulk density is solved by using Matlab,that is,the minimum solution of the nonlinear equation is obtained.2)The most dense gradation range of the mineral admixture and the cement particles is double-Piling up the mix design.(2)Under the conditions of room temperature maintenance 3d,the quasi-Randles shape of the Nyquist diagram of each group was not obvious,and no obvious electrochemical reaction occurred.The filling effect of the mineral admixture particles was mainly in the early stage,And the hydration reaction has been started under the condition of room temperature curing.In addition to the pure cement group,the quasi-Randles morphology of the other Nyquist graphs has improved greatly compared with the room temperature maintenance 3d,Showing a more typical Randles graphics.(3)The particle size and activity of the blend,the ratio of water to cement,and the particle size of the composites have a significant effect on the volume resistivity of the UHPC matrix.In the case of mixed with silica fume,the volume resistivity of the pure cement group can be increased by an order of magnitude compared with the volume resistivity of the base of the pulverized coal mixed with the fly ash.After adding the silica fume,at the appropriate water-The volume resistivity of UHPC matrix can be increased by two orders of magnitude compared with pure cement group.When the water-cement ratio is kept at 0.18～0.20 and the silica fume content is more than 20%,the volume resistance value is improved significantly.Therefore,the incorporation of appropriate amount of silica fume,to maintain a low water-cement ratio,can improve the resistance of ultra-high performance concrete matrix.The volume resistivity of UHPC matrix decreases with the increase of water-cement ratio.Low water ratio,high content of silica fume can optimize the matrix pore structure,increase the non-connected pores,improve the substrate resistance.(4)Pore water absorption test results show that the water absorption of pure cement and fly ash group increases with time,the powder group remained basically unchanged after 9h,indicating that the pore water absorption capacity of the slag group reached saturation,And the porosity of the matrix was lower than that of 0.22,and the water absorption of the matrix increased with the increase of the water absorption time.When the water-cement ratio was more than 0.22,the water absorption of the matrix increased with time The increase in the water absorption,the connectivity of the pore structure increases.Therefore,in the preparation of UHPC matrix,control the appropriate water-cement ratio can reduce the base of the water absorption pore volume.(5)Compared with the pure cement group,the content of Ca(OH)2 in the fly ash and mineral powder group was reduced by 50%,while the incorporation of silica ash group significantly reduced the Ca(OH)2 content,UHPC When the content of ash is more than 20%and the water cement ratio is 0.18,the content of Ca(OH)2 is relatively small,and the matrix has strong corrosion resistance.(6)Particle size and activity of the admixture,the particle size of the cementitious material,the ratio of water to cement is the key factor to determine the microstructure of the matrix.When the content of silica fume is more than 20%,the ratio of water to cement is controlled at 0.18～0.20 range,you can get high durability of ultra-high performance concrete matrix.