Study On The 3D Molding Of Calcium Carbonate/Geopolymer Composite Material

Geopolymer is a kind of Portland cement-like green building materials,which can be produced by the reaction of alkali-activator and industrial wastes(such as fly ash and slag).Compared with ordinary portland cement(OPC),it has faster condensation time,higher mechanical properties,better durability and better environmental protection performance.After years of research,geopolymer concrete or geopolymer composite materials have been widely used in construction,metallurgy,coating,membrane materials and nuclear waste treatment,etc.However,in its development process,few people have successfully applied geopolymer as a key binder in the field of 3D printing.Limestone powder is a by-product of industrial production and its main component is calcium carbonate.China is rich in limestone resources.A large amount of limestone powder is produced in the process of processing,which pollutes the environment and wastes natural resources.In order to fully use of limestone powder as a cheap inert aggregate,limestone powder has been widely used in the field of cement concrete,but there are few reports on the application of limestone powder with large content in geopolymers.In this paper,alkali activated slag-based geopolymer is used as binder,large amount of calcium carbonate is used as skeleton,and carboxymethyl starch sodium(CMS)is added as modifier to modify it.Carbonate/geopolymer composite both with printing and mechanical properties is designed.Rotary rheometer,veca tester,electronic universal pressure testing machine,FTIR,XRD,SEM and other characterization and testing methods were adopted to explore the changes of fresh properties(setting time,fluidity,rheology,etc.),hardening properties(compression resistance,folding resistance,shrinkage,porosity,etc.),microstructure and 3D printing molding effect of geopolymer materials,calcium carbonate/geopolymer composites and CMS modified composites.The main conclusions of this experiment are as follows:(1)Determination of basic proportion of geopolymer,when the modulus of sodium silicate is 1.7M,the dosage of initiator is 18%,and the water-solid ratio is 28%,the initial setting time of the new slurry of geopolymer is 46 min,and the final setting time is 57 min at 25?,the initial setting time was 22 min and the final setting time was 24 min at 60?.SEM and compressive test results show that the samples prepared under these conditions have dense microstructure and excellent mechanical properties.(2)For calcium carbonate/geopolymer composites,when the calcium carbonate substitution increased from 40% to 90%,the yield stress of the composite slurry decreased from 1.59 to 0.11 Pa and the plastic viscosity decreased from 10.16 to 3.14 Pa·s,the compression and flexural strength and shrinkage of the cured composites were decreased;When the calcium carbonate substitution increased from 40% to 90%,BET test results show that the cumulative pore volume of micropores decreases and the porosity increased from 3.38% to 24.79%;XRD and FTIR results showed that calcium carbonate did not participate in the reaction process of geopolymer;SEM results show that the overall structure of the composite becomes loose and uneven with the increase of calcium carbonate substitute.Based on the above analysis,the test results of the mechanical properties and shrinkage properties of the composite materials,50% calcium carbonate substitute is the best substitute.(3)In the modification experiment of calcium carbonate/geopolymer composite material,when CMS content increased from 0% to 8%,the initial setting time of slurry at 25? increased from 58 min to 252 min,the final setting time from 66 min to 287 min,the yield stress increased from 0.81 to 72.25 Pa,the plastic viscosity increased from 5.35 to 35.09 Pa·s,the compressive strength and the drying shrinkage decreased;The porosity test results showed that when CMS content increased from 0% to 8%,the porosity increased from 3.98% to 15.38%.SEM results showed that the compactness and uniformity of the structure changed significantly with the increase of CMS content.(4)The 3D printing extrusion test on composite materials with different CMS addition amounts show that when the CMS content is 4% and 6%,fresh paste can be continuously extruded at a speed of 30mm/s without any fracture or discontinuity,and the printing layer does not collapse to meet the requirements of extrusion molding.The compressive strength of the molded samples after curing for 28 days is 63 MPa and 35 MPa,the flexural strength is 5.5MPa and 5.5MPa,respectively.