| Due to the advantages of green environmental benefits and high construction efficiency,3D printing concrete is one of the hotspots in the field of construction.However,low strength,low toughness and low buildability are one of the main problems hindering its application and further use.Ultra-high performance concrete（UHPC）has superior mechanical properties and high buildability.Therefore,3D printable UHPC has high application value.However,traditional UHPC has poor fluidity with the presence of steel fibers which can easily cause pipeline blockage and extrusion interruption during printing.Cellulose nanocrystal（CNC）is a kind of plant nanofiber with green environmental benefits and excellent physical properties,which can effectively solve the aforementioned problems.Therefore,it is of great significance to prepare CNC/UHPC materials for 3D printing.In this thesis,CNC/UHPC materials were prepared by optimizing the selection of raw materials,preparation process and mixture proportion.The anionic CNC(hereinafter referred to as CNC（-）)and cationic CNC(hereinafter referred to as CNC（+）)with different Zeta potentials were studied.By evaluating the effects of CNC with different Zeta potentials and dosages on the mechanical properties（compressive strength,flexural strength,flexural-compressive ratio,toughness）and 3D printability（fluidity,rheological properties,buildability,setting and hardening properties,and early stiffness）of UHPC,the most suitable CNC type and dosage for 3D printing were determined and verified by 3D printing.The effect of CNC on the hydration heat of UHPC was analyzed by isothermal calorimetry.Through XRD,FTIR,EDS,TEM,BSE-SEM,SEM,MIP and other microscopic test methods,the composition,element distribution,interface bonding,microstructure and pore structure of hydration products of CNC/UHPC materials were systematically analyzed,and the influence mechanism of CNC on UHPC performance was clarified.The main conclusions are as follows:（1）The 28d compressive strength of the prepared UHPC specimen reaches 120MPa,and the flexural strength reaches 22MPa.The mechanical properties meet the requirements of 3D printing high performance concrete.The incorporation of CNC can improve the compressive strength,flexural strength,deformability and internal energy of UHPC.The overall improvement of CNC（+）is greater than that of CNC（-）.（2）Low content of CNC can improve the fluidity of UHPC,and increasing the content can reduce its fluidity,the higher the content is,the greater the decrease is,and the rheological performance evaluation results are consistent with it;CNC can improve the buildability of UHPC,and the higher the content,the higher the buildability;high content of CNC will cause certain retarding effect on UHPC,which may affect 3D printing;within 30 min of standing time,the early stiffness of 0.15wt%CNC/UHPC material is greater than that of the control group,which can withstand enough gravity of the overlying printing layer and meet the requirements of 3D printing for early stiffness;the 0.15wt%CNC/UHPC material is most suitable for 3D printing,and it is verified by 3D printing that the composite material meets the performance requirements of 3D printing.（3）Both room and elevated temperature hydration tests showed that CNC can improve the early hydration rate and 7d hydration degree of UHPC.Heating can activate the activity of supplementary cementitious materials,improve the overall hydration degree of UHPC through pozzolanic reaction,and accelerate the hydration rate.CNC can delay the hydration process of UHPC and likely reduce the development of microcracks caused by excessive hydration heat release rate.（4）XRD and FTIR analysis resulted showed that CNC does not change the composition of UHPC hydration products,but only changes the content of each hydration product.The results of TEM analysis showed that CNC（+）can effectively separate the C-S-H gel formed by each phase and form more hydration channels to better promote the hydration of the slurry.The results of BSE-SEM and EDS analysis showed that CNC can adsorb on the surface of hydrated particles and promote their hydration,and the formation of internal hydration products continues to transition to the formation of external hydration products.Finally,the overall Ca/Si ratio of the hydrated specimen decreases,and more and denser hydration products are formed（high density C-S-H）.The results of SEM analysis showed that CNC can play a‘bridging’role and enhance the interfacial transition zone of UHPC.MIP analysis resulted showed that CNC can effectively reduce the nano-scale pores of UHPC and narrow the pore size range,and the overall porosity of the system was significantly reduced. |
