Study On The Properties Of Underwater 3D Printing Cement-based Materials

As one of the most popular emerging technologies in the 21 st century,3D printing technology,which has the advantages of high efficiency,energy saving and environmental protection,has broad prospects in the construction industry.As the ocean development of China and the rapid construction of Guangdong-Hong Kong-Macao Greater Bay Area,3D printing technology is becoming more and more important in the field of hydraulic engineering and marine engineering.The important basis for the application of 3D printing technology in the field of construction is excellent 3D printing cement-based materials.Scholars at home and abroad have carried out research on this topic,but there are still few studies on underwater 3D printing cement-based materials.This research focused on the research and development of 3D printing building materials for application in marine or underwater construction projects,and designd the main scheme for this study.The materials used in this study were mainly silica fume(MS),nano-silica(NS),glass fiber(GF),hydroxypropyl methylcellulose(HPMC).Applying the type of admixtures as the manin controlled variable 15 groups of freshwater river-sand mortar and 15 groups of seawater coral-sand mortar were designed,and normal printing experiments and underwater printing experiments were carried out to evaluate the fresh mixing porperty,printability and mechanical properties of the printed mortar.The comprehensive effects of several admixtures on the properties of 3D printing mortar were deeply explored.The main work and conclusions are as follows:(1)By referring to relevant research and trial-mixing,the experimental mortar mix ratio was set,in which the water/binder ratio was selected as 0.3,the MS content was 10%,the NS content was 0.5%,the GF content was 0.2%,and the HPMC content was 0.1%,and the dosage of the water reducing agent(WRA)was achieved according to the fluidity of the printed mortar during the trial-mixing.(2)Through the workability test of 3D printing mortar,the following phenomena were found: in order to make the fluidity of the printing mortar meet the requirements,the dosage of WRA required for the seawater coral-sand group is greater than that of the freshwater river-sand group;MS,NS,GF and HPMC would reduce the fluidity of the printing mortar;when printing in air,the suitable flow expansion of the printing mortar is 180?220mm;when printing underwater,the suitable flow expansion of the printing mortar is recommended to be adjusted to 180?210mm in order to properly reduce the dispersion of the mortar in water.(3)Through the printability test of 3D printing mortar,the following conclusions are drawn: the extrudability of the seawater coral-sand mortar group is better than that of the freshwater river-sand mortar group,while the deformability and buildability of the seawater coral-sand mortar group are weaker than those of the freshwater river-sand mortar;MS can slightly reduce the extrudability of the mortar,and greatly improve the deformability and buildability of the mortar;adding NS would improve the extrudability,deformability,and buildability of the printing mortar;adding GF will significantly reduce the extrudability of the mortar,but improve the deformability and buildability;the incorporation of HPMC can improve the extrudability and deformability of the mortar,but reduce the buildability.When the mortar is printed underwater,the anti-dispersibility of seawater coral-sand mortar is better than that of freshwater river-sand mortar,but its deformability and buildability are weaker than those of freshwater river-sand mortar;the incorporation of MS and HPMC can significantly improve the anti-dispersibility of the mortar.For the anti-dispersibility,NS can increase slightly,whearas GF decrease adversly to make the printing failed.(4)Through the mechanical properties test of 3D printing mortar,the following conclusions are obtained: in the normal printing test,the flexural strength of printed specimens is higher compared with that of the standard specimen,but the compressive strength of printed specimen is lower than that of standard specimen;in the underwater printing test,the flexural strength and compressive strength of printed specimens were reduced to a certain extent compared with the standard specimens.Incorporating MS,NS,GF and HPMC can increase the flexural strength of the mortar;in terms of compressive strength,the singly addition of MS,NS,GF,and HPMC will increase,but when HPMC is mixed with the other three materials,the compressive strength will decrease the compressive strength;the incorporation of MS,NS,GF,HPMC will improve the interlayer bond strength of the printing mortar.(5)Through the optimized analysis of mix proportion,the following suggestions are given: considering the printability,it is necessary to mix MS and HPMC into the underwater printing mortar,whereas GF is not suitable for the underwater printing mortar;considering the mechanical properties,The addition of MS and NS can improve the mechanical properties of the underwater printing mortar,whereas HPMC will decrease to a certain extent,but the combined addition of MS,NS,and HPMC will offset the negative effect of HPMC on the strength.Therefore,by comprehensive consideration,for the underwater printing mortar,the mortar with MS,NS and HPMC is suggested to apply in underwatrer printing due to the excellent underwater printability and good mechanical properties.