Study On Geopolymer For 3D Printing Prepared With Recycled Micro Powder From Construction And Demolition Waste

Recycled micro powder is a particle with a particle size less than 75μm produced during the preparation of recycled aggregate.The recycled micro powder used in this study is collected from the bag filter on the crushing production line.Previous studies have shown that the thixotropy of geopolymer paste can be improved by adding micro powder of construction waste within a certain range,which is suitable for 3D printing process.Therefore,if the recycled micro powder geopolymer that can be used for 3D printing can be prepared,it can not only increase the utilization rate of recycled micro powder,reduce the use cost,reduce the environmental pressure,but also solve the problem of large amount of glue material and no low carbon in the 3D printing process.At the same time,it can also make up for the lack of material strength caused by the low activity of the recycled micro powder through the characteristics of high strength and fast hardening of the geopolymer,Finally,it provides a reference for realizing the resource utilization of recycled micro powder.In this paper,the recycled micro powder geopolymer mortar that can be used for 3D printing is prepared by using the recycled micro powder,and on this basis,the influence of the rheological properties,printability,mechanical properties and drying shrinkage of the geopolymer mortar is studied,and the following main research conclusions are obtained:（1）A kind of fine geopolymer of recycled micro powder with good printability has been developed.Its mixing ratio range is:PP fiber content is 0.7%（volume content）,PVA content is 0.2%,water retaining thickener content is 0.7%,Zn Cl₂ content is 1.5%,recycled micro powder content is less than 20%,water binder ratio is 0.4-0.5,cement sand ratio is 0.6-0.8,and nano-Si O₂ content is 1%-2%,providing a scheme for the utilization of recycled micro powder.（2）When the content of recycled micro powder is less than 20%,the printability can be improved;The constructability of mortar can be improved by properly reducing the water binder ratio and cement sand ratio;The printability of mortar can be improved by adding nano silica;Increasing the content of recycled micro powder and PP fiber and reducing the water binder ratio and the binder sand ratio can effectively inhibit the drying shrinkage of recycled micro powder geopolymer.（3）Based on the existing standards,the quantifiable evaluation indexes-extrusion uniformity and cumulative deformation rate are established to evaluate the extrudability and constructability of printable properties respectively.The results show that the extrudability of the geopolymer increases first and then decreases with the increase of fluidity,and increases with the decrease of dynamic yield stress;The constructability decreases with the increase of settlement degree and with the decrease of plastic viscosity and static yield stress.（4）The mechanical properties of standard mortar test block and 3D printing mortar test block are compared and analyzed by using the reduction rate of compressive strength.The results show that when the content of recycled micro powder is less than 20%,the impact on the flexural and compressive strength of geopolymer mortar is small;The mechanical properties of geopolymer bonded sand can be improved by decreasing the water binder ratio and increasing the binder sand ratio;Adding nano silica can improve the mechanical properties of geopolymer mortar,but the content should not exceed 1%;The law of influencing factors of mortar specimen strength is also applicable to 3D printed specimens,and is not affected by the process;The reduction rate of compressive strength of 3D printed specimens can be reduced by increasing the curing age,the mechanical properties of the material itself and the interlaminar properties;Enhancing the interlaminar bond strength can improve the compressive strength anisotropy of 3D printed specimens.