Research On Optimal Composition Design And Properties Of 3D Printed White Portland Cement-based Materials

As a new and green intelligent manufacturing technology,3D printing technology is based on the digital model to prepare three-dimensional solid structure by stacking materials layer by layer.3D printing technology is widely used for its advantages of saving templates,construction costs and time,and increases the safety of construction sites and the flexibility of architectural design,which can realize the automation of construction machinery and greatly reduce labor intensity.At present,the application of cement-based 3D printing technology is expanding from printing buildings to printing special-shaped color decorative components and landscape components.In addition,white Portland cement is widely used as an important architectural decoration material,due to its controllable colorability and setting time,which is more suitable for combining with 3D printing technology to construct artistic,landscape and colorful decorative components.Therefore,this thesis aimed to break through the limitations of existing3D printed white Portland cement-based materials（3DP WPCMs）,such as high production cost,low early strength,severe efflorescence,poor decoration and durability.The mix proportion and properties of 3DP WPCMs were optimized by means of mineral admixtures and admixture regulation,fiber reinforcement and pigment modification,so as to meet the needs of intelligent construction and aesthetic effects of 3D printed special-shaped decorative components.The main research results are as follows:（1）The basic proportion system of 3DP WPCMs was studied and determined.Silica fume（SF）,metakaolin（MK）and lithium slag（LS）were used to optimize the existing basic system,and their effects on the printability and mechanical properties of 3DP WPCMs were studied.Meanwhile,the rheological properties were systematically studied to clarify the effect of mineral admixtures on the yield stress,viscosity and creep.The results showed that the dynamic yield stress and plastic viscosity of the paste can be improved by SF,MK and LS.When the content of mineral admixtures increased to 12%,the linear viscoelastic zone was significantly higher than that of the control.In addition,based on the correlation calculation,the thixotropy measured by creep recovery method can better reflect the printability of the paste.When the SF content is 12%,the structure deformation of paste decreased to 3.52%,the highest3d compressive strength can reach 48.70 MPa,the 28d efflorescence rate decreased to 0.48g/m^2·h.（2）The regulation mechanism of setting time and rheological properties of 3DP WPCMs with early strength accelerator was studied.The relationship among hydration,rheology and printability was revealed by investigating the effects of aluminum sulfate（AS）and triethanolamine（TEA）on hydration,rheology and mechanical properties of 3D WPCMs.The results showed that AS can realize flexible control of paste within 50～144 min and improve its printability.The structure deformation of 3D printed component containing 2%AS can reduce to 5.04%.However,TEA reduced the yield stress,which guaranteed the continuous extrusion of the paste while was not conducive to the structure stability.In addition,AS can also significantly improve the mechanical properties of samples.Compared with the control,when the AS content reaches to 2%,the 3-day and 7-day compressive strength of the samples can increase by 17.93%and 8.22%,respectively.When the content of AS increased to 4%,the 7d efflorescence rate decreased to 1.25 g/m²·h.（3）Mineral,one and two dimensional fiber reinforced 3DP WPCMs system were constructed.The effects of wollastonite and short glass fiber（GF）on rheological,printability and mechanical properties of 3DP WPCMs were studied.In addition,the feasibility of external glass fiber textile（GFT）was explored,and the mechanism of GFT was revealed based on load deflection curve.The results showed that the mechanical properties of the 3DP WPCMs can be greatly improved by the introduction of three types of fibers,and the yield stress and elastic modulus of the paste can be significantly improved by the appropriate amount of wollastonite and GF,so as to improve the buildability and deformation resistance of the paste.Compared with the control,the 3-day and 7-day flexural strength of the specimens with 8%wollastonite increased by 30.61%and 14.99%,respectively.When the GF content was 1%,the flexural strength perpendicular to the printing direction increased to 10.14 MPa,and the compressive strength parallel to the printing direction increased to 53.56 MPa.The GFT have a more obvious effect on the flexural strength of 3D printed components.When the GFT layer number was 7,the first cracking flexural strength and flexural strength increased by 273%and 608%,respectively.（4）The effects of three pigments on the durability and components of 3DP WPCMs were studied.By controlling the properties of the material itself and its influence on the rheological properties of the paste,the entire printing process was guided to reduce the number of interlayer pores and achieve the long-term durability of 3D printed cement-based materials.The results showed that the rheological parameters and printability of the paste increased with the increase of the content of the three pigments.The introduction of three pigments with 3%content can improve the buildability of the paste significantly,and the printed component had no obvious deformation.Besides,three 3D printed colored cement-based materials showed excellent color durability.Compared with the control,the samples containing organic red and yellow pigment had lower chloride ion permeability coefficient,while the samples containing compound blue pigment had much higher chloride ion permeability coefficient.Based on Image analysis,the inhibition of organic red and yellow pigment to efflorescence was stronger than that of compound blue pigment.