| Compared with the traditional construction industry materials,the preparation process of geopolymer of the green cementing materials is simple and the emission of carbon dioxide in the production process is low.It has a wide range of sources and low cost and most of them were taken from industrial waste materials,so they exhibit a widely development prospects in the field of civil engineering.In this paper,fine sand and the metakaolin of geopolymer were selected to prepare geopolymer composites in order to study the influence of the particle sizes and the amount of fine sand on the composite specimens,so as to improve the properties of the metakaolin-based geopolymer.The composition of raw materials,mechanical properties and microstructure were explored by the unconfined compressive strength test,three-point flexural strength test,dry shrinkage test,particle flow software PFC3 D simulation,X-ray diffraction measurement(XRD),Fourier infrared transform spectroscopy(FTIR),scanning electron microscope(SEM-EDS),etc.(1)Adding standard sand to the geopolymer slurry can increase the strength of the geopolymer specimens to a certain extent,but when the mixing amount reached 20% of the maximum mass fraction,the strength of the specimens decreased owing to the inconsistency of the standard sand particle size distribution.Therefore,the fine sand with smaller particle size and more uniform distribution was chosen to mix into the geopolymer slurry,and it was found that fine sand can significantly improve the strength of the geopolymer specimens,and the smaller the fine sand particle size,the higher the strength of the composite.When mixed with 15% B5 fine sand,the compressive strength of the specimens reached 41.51 MPa after 28 days of curing,and the flexural strength reached 4.02 MPa.As the content of fine sand increased,the strength of the specimens decreased,but its strength was still higher than that of the undisturbed metakaolin specimens under the same curing conditions.(2)When the particle size of the fine sand in the composite was large,the internal micro-cracks of the specimen mainly appear in the form of tensile cracks.The total number of cracks in group B5 was small and the cracks were concentrated and evenly distributed on the upper end,and particles destruction appear only at the upper end.After compression,the porosity reached the maximum change amplitude,which indicated that the decrease of fine sand particle size increased the degree of porosity change of the specimens before and after compression,and the internal structure was more compacted,while the fine sand content increased,the degree of change of the porosity before and after compression was reduced.(3)The standard sand or fine sand mixed in the geopolymer would not chemically react with it,and none of new mineral were formed.The strength of the geopolymer composite specimen was mainly derived from the geopolymer coagulation glue.The incorporation of a small amount of standard sand would hinder the evaporation of water molecules and reduce the free water in the specimens,while the size of the fine sand could not cause significant effect.The standard sand in the specimens was accumulated and could not be fully mixed with the geopolymer slurry,and the internal flatness of the specimen was low.Incorporating fine sand could significantly improve the internal structure of the specimens,and the smaller the particle size of the fine sand,the more uniform and dense the internal structure.Therefore,the B5 fine sand geopolymer composite specimens possese the most excellent mechanical properties after 28 days of curing. |
