Research On Preparation And Performance Of Cement-Based Reactive Powder Concrete For 3D Printing

The lower strength of 3D-printed cement-based material is one of the main problem which restricts its wide application in construction industry.Through orthogonal experiments and theoretical calculations,this paper analyzes the optimization for particle grain gradation of raw materials.Adding a multi-index evaluated,gradual-optimized,and comprehensive strength-evaluated test method,a kind of high-strength cement-base RPC suitable for 3D printing is produced.The working performance,strength and deformation of 3D printed concrete are discussed and researched,the 3D printing process to cement-based RPC is optimized.Through the numerical simulation,the effect of layer forming process based extrusion curing process on strength of 3D-printed building components is researched and analyzed.The best proportion of RPC is obtained,the proportion of the powder material is 0.75:0.06:0.10:0.09 for SAC: FA: SF: SS,the proportion of 0.3mm~0.6mm: 0.6mm ~1.18mm: 1.18-2.36 mm is0.202:0.248:0.55 for quartz sand,1.7-sand ratio,0.27 water-binder,0.8% water-reducer,0.01% retarder,it will provide a certain reference for the popularization and application of 3D printed RPC into construction industry.The main research result are as follows:(1)Based on the Dinger–Funk equation of the closest-packing model,the power optimum proportion of RPC is calculated according to normal distributions of the particle sizes of the raw materials,the theoretical calculation results are verified by the minimum basic water demand test,standard test method for water and orthogonal experiment.The experimental results show that there are some variations between the addition of silica fume and the theoretical results,while the values of the other two mineral admixtures are quite close to the theoretical results.Mineral admixtures have important effects on the work ability,strength and micro-structure of sulphoaluminate cement paste.The addition of mineral admixturesimproves the particle size distribution of cement composite system,adjusts the microstructure of hydration products of sulphoaluminate cement,reduces the thickness of the interface transition region,and increases the density and structural stability of cement stone.The activity of the volcano ash and micro aggregate effect of silica fume may play a vital positive in improving the early strength of the system.However,it affects the fluidity and later strength of the composite system because silica fume requires a larger amount of water for the elevated heat release during the early hydration process.For the reason,silica fume has an important impact on the work performance and strength of cement paste which means that the role of silica fume in the paste needs to be considered comprehensively.The correlation between the minimum basic water demand and the standard consistency method,thus the standard consistency can replace the minimum basic water requirement in determining the optimum proportion of reactive powder concrete.(2)The interfering factors of working performance indexes of 3D-printed RPC are analyzed through orthogonal experiment,the correlation among them is studied,the proportions determined by the orthogonal experiment are optimized step by step through the apparent effect,flow rate and stacking layers of paste,four groups of proportion of RPC meeting the working property of 3D printing are preliminarily obtained.Fluidity,stacking height and extrusion rate have good correlation among them,the correlation between fluidity and thixotropic sensitivity is weak,and two indexes are combined to comprehensively evaluate the working performance of 3D-printed concrete.The influence of water-binder ratio and water-reducing agent on 3D-printed concrete is obviously greater than of mineral admixtures.(3)The effects of different parameters on the mechanical properties of 3D-printed concrete are studied,by means of comprehensive strength evaluation test,the mix proportion which is suitable for printing and has excellent mechanical properties is obtained,the 2h and28 d strength of optimized RPC respectively reach 6.6MPa and 91.4MPa,the strength has astable development without a great sacrifice in later strength.The determination of water binder ratio of 3D-printed concrete should take both working performance and strength into account,the water-binder ratio can be further reduced to improve the strength of concrete on the basis of satisfying the working performance,the polypropylene fiber increases cohesion and mediates the proper fluidity of 3D printed concrete.During the process of layering,the difference of hydration degree of the upper and lower layer of concrete makes the stress more concentrated at the inter-layer interface,which leads to the decrease of strength,the strength loss is nearly 36% when the interval time is close to the initial setting time of concrete.(4)Building components with different shapes and layers are printed through 3D printing technology.The 3D printing process suitable for RPC is acquired,that is,the printing speed is50mm/s,the leveling layer of the bottom is 1-3 layers,the thickness is kept within the range of 5±3mm,and the normal printing thickness is 10 mm.The printed line-width has a greater impact on the size of the printed components.A leveling stage in the early stage of concrete printing is necessary for concrete in plastic state,the printing thickness will tend to a certain value after the leveling stage.The deformation process of printed concrete can be divided into three stages: linear stage,elastic-plastic stage and plastic failure stage.The elastic-plastic stage is the main stage,the transverse extrusion deformation occurs in the stage.The horizontal strain is suitable for evaluating the plasticity of 3D-printed concrete compared with the vertical strain.The inhomogeneity of materials makes the stress-strain curve of the printed component strength fluctuate greatly.In the same printing height,the number of printing layers is more,the strength loss is more,the lowest strength is 60.4MPa,the amount of strength loss is26.3%.The numerical simulation result of the effect of printing process on the strength are slightly higher than the experimental result.(5)The hydration,hardening mechanism and micro-structure is studied through XRD and SEM.,the composition feature of hydration products and microstructure characteristic are discovered.The fundamental reason why RPC still has higher mechanical properties under 3Dprinting process is explored from the static dense packing and secondary hydration filling effect of the compound system,which includes mineral admixtures making up for the strength loss caused inter-layer defects to some extent.