Research On The Printability And Early Strength Of 3D Printing Concrete Mixed With Magnesium Phosphate Cement

With the proposal of carbon emission reduction goals and the continuous progress of science and technology,concrete 3D printing technology is being applied and promoted with its advantages such as modeless,automation,and low-carbon.However,this technology also faces development bottlenecks,such as low printing success rate,susceptibility to cracking,and lack of early strength.The traditional methods of improving the early strength of concrete,including adding accelerating agents,early strength agents,or short cut fibers,still face shortcomings such as difficulty in grasping the rapid setting effect and long-term loss of strength and durability.Magnesia Phosphate Cement(MPC),as a fast hardening and early strength cement,can improve early strength without changing the composition of hydration products by adding MPC to ordinary Portland cement.At the same time,it can obtain a wide range of ratio choices,reduce the risks of 3D printed concrete deformation,collapse,cracking,and ensure sufficient long-term strength and durability.This article introduces the use of magnesium phosphate cement with different mass fraction ratios in 3D printed concrete materials to improve the properties of 3D printed concrete,and applies the variation law of static yield stress and the Mohr Coulomb criterion to establish a comprehensive evaluation method for rheological properties.The printability of 3D printed concrete is comprehensively evaluated from the aspects of flowability,setting ability,extrudability,and constructability under different MPC dosages,Clarify the rheological properties of concrete materials with different dosages that meet the requirements for stable printing;At the same time,evaluate the early mechanical properties of 3D printed concrete from aspects such as early strength development,flexural strength,compressive strength,and splitting tensile strength;Analyze the strength data of different loading directions to obtain the variation pattern of specimen anisotropy under different MPC dosages.The specific research results are as follows:(1)In the environment of 10～20 °C,as the MPC content increases from 0% to 10%,the paving diameter after vibration in the 3D printing mortar fluidity test is also reduced from 180 mm to 150mm;The initial coagulation time was reduced from 270 min to 145 min,the final coagulation time was reduced from 480 min to240min,the initial coagulation time was reduced by 86%,and the final coagulation time was reduced by 100%.When the MPC content is 0%,10%,20%,the normal printing work can be carried out,and with the gradual increase of the MPC content,the lateral deformation is gradually decreasing,the maximum vertical deformation under 10% content is 8mm,and when20%,the printing process will be stuttered;The initial setting time and opening time of 30%,40% and 50% content of concrete materials are too short to meet the printing requirements and cannot be effectively printed.(2)In the early strength development,compressive strength,flexural strength,splitting tensile strength test,the increase of MPC content is conducive to the improvement of early strength,when the MPC content is increased from 0% to 10%,the flexural strength in the X,Y,Z direction is increased by 10.5%,13.7%,14.9%,the compressive strength is increased by 29%,45%,15%,when the MPC content is increased from 10% to 20%,the flexural strength in the X,Y,and Z directions is increased by 9% and-9% respectively,5%,compressive strength increased by 12.9%,12.5% and 22% respectively.In terms of the magnitude of the intensity increase,the MPC content increased from 0% to 10%.3D printing components show obvious anisotropy due to their special construction methods,with the increase of MPC content,the anisotropy coefficient of different dosage specimens is gradually decreasing,but the anisotropy between different loading directions of the same specimen is gradually increasing.Based on the above research results,it can be found that the increase in MPC content will have a significant modification effect on the printability of 3D printing mortar,and also enhance the early strength,but the MPC content is too large(more than 20%)will lead to too short concrete initial setting time,poor extricability,cannot be effectively formed,so the best MPC content in this study is 10%,under this dosage,concrete has good printability,can obtain good early strength in the early stage of printing,for the rapid structure of 3D printing,a large number of construction to provide important reference significance.