Study On Rheological And Setting Properties Of Gypsum-based Materials Used For Three-dimensional Robocasting

Gypsum components with complex shapes and structures have been widely used in the field of architectural structure and decoration.However,3D powder printing,which is widely used for printing gypsum components,has some defects such as complicated printing process,low printing efficiency and poor performance of printing specimens.3D robocasting(3DR)is a new type of 3D printing technology,which can effectively solve the above problems and shows great advantages especially in the printing of large special-shaped gypsum components.In order to realize the 3DR of gypsum-based materials(GM),the difficulty of controllable setting and rheological behavior of GM must be solved.GM pastes are essentially multi-scale active suspensions formed by the dispersion of gypsum particles and admixtures with different dimensions in the medium water.The setting and rheological behaviors of the suspension are closely related to the evolution of microstructure.Therefore,the evolution of microstructure of multi-scale active suspensions is the core scientific problem in the study of setting and rheological behavior of GM pastes.The solution of this problem has important theoretical value and practical significance for controlling the setting and rheological behavior of GM pastes and realizing their 3DR as well.In this paper,the influence of admixtures with different dimensions,including chemical admixtures(10?100 nm),polymers(100?1000 nm),mineral admixtures(1 to 100 microns)and fibers(0.1?20 mm)on setting and rheological properties of GM pastes were studied.In addition,their effecting mechanisms on GM pastes were also revealed from the perspective of the microstructural evolution of multi-scale active suspensions.On this basis,the initial-state rheological model of multi-scale suspensions was established by introducing the microstructural parameter of flocculation rate,and the time-dependent rheological model of active suspensions was established by introducing the time-varying parameter of relative hydration degree at the same time.Finally,this paper presents the basic idea of composition design of GM pastes used for 3DR,the comprehensive evaluation method and criteria of setting and rheological properties of GM pastes,and systematically studies the physical and mechanical properties and microstructure of GM samples prepared by 3DR.Results indicate that chemical admixtures and polymers are mainly adsorbed onto the surface of gypsum particles or dispersed in interstitial solutions,thus modifying rheological properties of GM pastes through the influence of flocculation structure formation/destruction or the change of interstitial solutions viscosity.At the same time,chemical admixtures and polymers would also change the hydration process and setting properties of GM pastes by further affecting the dissolution of hemihydrate gypsum particles and the nucleation,growth and interlocking of dihydrate crystals.Fibers and mineral admixtures are mainly dispersed among gypsum particles as the second dispersed phase,increasing the solid volume fraction and promoting the formation of flocculation network structure as well,and therefore changing the rheological properties of GM pastes.At the meanwhile,fibers and mineral admixtures can further change the hydration process and setting properties of pastes by water absorption or acting as crystal nucleating agents.Besides,admixtures can be divided into two categories,that is thickener and plasticizer according to their different effects on rheological properties of GM pastes.Meanwhile,their different effects on rheological parameters can be buried in the microstructural parameter of flocculation rate.Thus,the initial-state rheological model of GM pastes can be obtained as follows:(?)Furthermore,the change of rheological properties of GM pastes with time is related to both initial-state rheological properties and hydration degree,and the time-dependent rheological model of GM pastes can be expressed as follows:(?)In addition,setting time and rheological properties are two evaluation index for 3DR-GM pastes.It has been proved that 3DR-GM pastes should behave as significant shear-thinning and have good shape retention ability.Besides,it is essential that 3DR-GM pastes should have much longer setting time than the printing time.As a result,adjusting the water-to-gypsum ratio and selecting the appropriate thickener and retarders type and dosage to achieve the above properties are the basic ideas of the composition design for 3DR-GM pastes.In general,3DR can improve the internal pore structures and physical and mechanical properties of the printed gypsum samples,thus having great potential in the field of preparing large-scale gypsum structures.