Study On The Composition,structure And Properties Of 3D Printed Calcium Sulphoaluminate Cement-based Materials

3D printing technology of construction materials can significantly improve the preparation efficiency of building components and products,which can bring a disruptive change in product design and manufacturing mode,and realize the automatic and intelligent,accurate,and controllable manufacturing.Meanwhile,it also can eliminate the pollution of dust,simplify the production process,and decrease the cost,which is particularly suitable for the preparation of fast building,special building components,and the complicated structure and decoration components with special shape.3D printing technology of construction materials becomes a research hotspot in the world.Although 3D printed construction materials have made certain progress in the aspect of composition,structure,and performance optimization,but the scientific research and application developed slowly.The main restrictive factor is lack of the printable cement-based materials with superior and controllable performance.Therefore,it is necessary to explore a new way or to find a new method to break the limitation in the composition of traditional cement-based materials fundamentally.To meet the high quality of the special shape,the decoration components design and landscape beautification,the preparation of 3D printed cement-based materials with superior performance,controllable structure,and good durability is also necessary.In this paper,the calcium sulphoaluminate（CSA）cement with high early strength was used as the main cementitious materials to establish a new composition of 3D printed cement-based materials with high properties.it can not only realize the stable control of the 3D printed structure and suppress the deformation and cracking of the material,but also improve the matching of model design and printing process.This way also can lay a theoretical and technical foundation for preparing 3D printed construction materials with controllable rheological properties,high mechanical properties,and stable printed structure.The main research contents and results are as follows:（1）The basic composition system of 3D printed CSA cement-based materials was determined.Hydroxypropyl methyl cellulose（HPMC）,water reducer agent（WRA）and lithium carbonate（Li₂CO₃）were used to study the effects on the rheological properties and structural deformation,aiming to clarify the variation rules among shear stress,viscosity,and printed structure.The results showed that the printed paste can realize the basic stacking when the dosage of HPMC is more than 0.2%.In addition,based on RSM results,the admixtures can well control the rheological properties of printed materials and the optimal range of HPMC,WRA and Li₂CO₃ is 0.15%-0.35%,0.1%-0.4%and 0-0.015%,respectively,and 1d compressive strength can reach 35.8 MPa.（2）The regulation mechanism of retarder on the building time and rheological properties of 3D printed CSA cement-based materials was determined.Tartaric acid（TA）,boric acid（BA）and sodium gluconate（SG）were used as retarders to control the building time and rheological properties of 3D printed materials and improve the printable properties.The results showed that TA,BA,and SG can significantly prolong the building time in the range of 18～90 min and improve the printability.In addition,the introduction of diatomite can significantly reduce the structural deformation（less than 10%）and improve the mechanical properties of the printed material by regulating the rheological properties without losing the building time,and the 1d compressive strength can reach 41.5 MPa.Furthermore,the correlation between static yield stress and building time indicates that the 3D printed structure is mainly supported by static yield stress when the building time is long enough.（3）The composition system of high thixotropic 3D printed CSA cement-based materials was constructed and its thixotropic regulation mechanism was determined,and it revealed the influence law of the paste flow state and structural deformation from linear viscoelastic zone to nonlinear viscoelastic zone.The results showed that the addition of metakaolin,bentonite and paper sludge can improve the thixotropy and static yield stress of the paste,which is conducive to the reduction of structural deformation,and the 1d compressive strength can reach 46.9 MPa.Meanwhile,the correlation between thixotropy and rheological parameters verified that the yield stress of 3D printed CSA cement-based materials can predict the thixotropy,and significantly affects the construction of printed structure.In addition,based on the results of Lissaju-Bowditch curve,the thixotropic re-construction during the period around can significantly affect the structural deformation of the paste.（4）The effect of polypropylene（PP）and polyvinyl alcohol（PVA）fiber on the rheology,mechanical properties and structural stability of 3D printed CSA cement-based materials was determined,and revealed the influence law of rheological parameters on structural deformation.The results showed that the fibers can increase the elastic modulus and static yield stress of paste and decrease the printed structure deformation.When the content of polypropylene and polyvinyl alcohol fiber reaches 0.75%and 1.00%,respectively,the flexural strength of 3D printed CSA cement-based materials increases by about 111%and191%,respectively,reaching the maximum value of 9.23 MPa and 12.72 MPa.Based on the radar map correlation,the rheological parameters of 3D printed CSA cement-based materials have less or worse effect on the structure deformation when the fiber content is low,and small changes in rheological parameters can greatly affect the deformation of the structure when the fiber content is high.（5）The flow state and flow field distribution of the printed paste were determined by the simulation of the paste flow in the 3D printing extrusion device,so as to reflect the influence of the paste flow state on the extrusion state.The flow state of CSA cement-based materials in air pump and screw extrusion device was simulated by using the fluid module.The results showed that Herschel-Bulkley model can more accurately simulate the flow state of paste in the extrusion system,and as the increase of slag content,the speed at the nozzle decreases gradually.Compared with the air pump extrusion system,the screw extrusion system can stabilize the flow state of the paste,which is beneficial to the improvement of the paste extrusion.（6）The effects of rheological parameters and printability on the durability of 3D printed CSA cement-based materials were determined.Slag was used as an ultrafine admixture to regulate the printed structure and improve the durability of CSA cement-based materials.The results showed that the rheological parameters and printability affected the durability of the printed materials significantly.The durability of 3D printed CSA cement-based materials was improved significantly due to the high static yield stress and thixotropy when the content of slag is less than 10%.However,when the content of slag exceeds 10%,the durability became worse due to the bad printability and the increase of inert materials.