| The cement industry is limited by raw materials and production processes and has become a major emitter of carbon dioxide.Under the background of ’double carbon’,carbon emission reduction in the cement industry has become an inevitable trend.Geopolymer is a new type of green building material,which is expected to replace ordinary Portland cement(OPC)as a green binder for concrete.Geopolymer materials have a wide range of raw materials.They can be made from industrial solid wastes such as red mud,blast furnace slag,fly ash,rice husk ash,metakaolin,ceramic tiles,roof tiles,recycled clay bricks,hollow bricks,and glass.Moreover,geopolymer has a simple preparation process,low energy consumption,extremely low carbon dioxide emissions,excellent mechanical properties,thermal stability,durability,corrosion resistance,and extremely low porosity,which is a hot topic in the field of building materials.However,the key challenge to prevent geopolymers from being widely used in engineering is based on the composition of aluminosilicate source materials and the variability of alkaline activators,as well as the lack of understanding of how the chemical composition,reaction mechanism,and liquidsolid ratio of alkaline activators affect the microstructure,machinability,and strength of geopolymer cementitious materials.Therefore,the research goal of this paper is to link the chemical composition of the alkaline activator,the liquid-solid ratio(the ratio of the mass of the alkaline activator to the mass of the aluminosilicate source material),and the performance of the geopolymer cementitious material through experiments,providing a theoretical basis for its practical engineering application,and as a necessary step in the design of sustainable,low-carbon dioxide building materials.Based on the current research progress,this paper uses ultrafine metakaolin as the main aluminosilicate source material,and the mixture of sodium silicate solution and sodium hydroxide solution as an alkaline activator to prepare geopolymer.The main research contents and research results of this paper are as follows:(1)The geopolymer was prepared by using ultrafine metakaolin as aluminosilicate source material and the mixture of sodium silicate solution and sodium hydroxide solution as an alkaline activator under the condition of room temperature curing.The effect of sodium hydroxide molar concentration in composite alkaline activator on the fluidity and mechanical properties of geopolymer paste was studied by single factor test.The results show that the fluidity of geopolymer paste increases with the increase of sodium hydroxide molar concentration.Increasing the molar concentration of sodium hydroxide,the compressive strength of the geopolymer also increases.The 3-day compressive strength of 6M geopolymer is 6.41 MPa,and the 3-day compressive strength of 14M geopolymer is 48.77 MPa.The improvement in compressive strength is very significant.The results of the stress-strain analysis show that with the increase of sodium hydroxide molar concentration,the failure mode of geopolymer changes from plastic failure to brittle failure.(2)The effect of sodium hydroxide molar concentration on the microstructure,pore structure,and pore distribution of geopolymer was further analyzed.The phase of geopolymer was characterized by X-ray diffraction(XRD).The relationship between NASH gel and sodium hydroxide molar concentration was quantified by simultaneous thermal analysis(TG-DTG)and XRD.The microstructure,pore distribution,and pore structure of geopolymer were characterized by scanning electron microscopy(SEM)and nuclear magnetic resonance hydrogen spectrum(1HNMR).The results show that the XRD analysis results show that the ultrafine metakaolin only forms the NASH phase in the geological polymerization reaction,and the change of the molar concentration of sodium hydroxide will not change the phase of the geopolymer.The results of TG-DTG and XRD analysis showed that with the increase of sodium hydroxide molar concentration,the NASH gel formed became more,and the crystallinity of the sample increased.The results of SEM and 1H-NMR showed that the geopolymer made of the high molar concentration of sodium hydroxide had sufficient hydration and a dense structure.By comparing and analyzing the characteristics of pore distribution and pore structure of geopolymers,it is found that the porosity of geopolymers decreases with the increase of sodium hydroxide molar concentration,and the proportion of small pores and micro-cracks does not change much,while large pores gradually evolve into mesopores.During the whole curing process,the curing time had no significant effect on the microstructure and strength development of the geopolymer.(3)Based on UMK-12M,the effect of the liquid-solid ratio on the fluidity and mechanical properties of geopolymer paste was studied.The results show that with the increase of the liquidsolid ratio,the fluidity of geopolymer paste increases.The fluidity of the liquid-solid ratio 1.0 is 114.17 mm,and the fluidity of the liquid-solid ratio 1.3 is 158.46 mm.With the increase of the liquid-solid ratio,the compressive strength of the geopolymer decreases.The 3-day compressive strength of liquid-solid ratio 1.0 is 36.00 MPa,and the 3-day compressive strength of liquid-solid ratio 1.3 is 29.69 MPa.The results of the stress-strain analysis showed that the liquid-solid ratio had no significant effect on the failure mode of geopolymer,and all samples showed brittle failure.(4)The effects of the liquid-solid ratio on the microstructure,pore structure,and pore distribution of geopolymer paste were analyzed.The phase of geopolymer was characterized by XRD.The relationship between NASH gel and the liquid-solid ratio was quantified by TG-DTG and XRD.The microstructure,pore distribution,and pore structure of geopolymer were characterized by SEM and 1H-NMR.The results show that the XRD analysis results show that with the change of liquid-solid ratio,the phase of geopolymer changes locally,showing more quartz phase,and the structure and polymerization degree of NASH gel inside geopolymer is also different.TG-DTG analysis showed that with the increase of the liquid-solid ratio,the bound water of NASH gel decreased.The results of SEM and 1H-NMR showed that the geopolymer generated at a high liquid-solid ratio would generate more water,resulting in more pores and loose structure in the geopolymer.By comparing and analyzing the characteristics of pore distribution and pore structure of geopolymers,it is found that the porosity of geopolymers increases with the increase of liquid-solid ratio,in which mesopores gradually evolve into macropores,and macropores gradually evolve into microcracks.In this paper,experimental methods and theoretical analysis are used to study the mechanical properties and microstructure of geopolymer under normal temperature curing conditions.The research results are of great significance for geopolymers to replace cement in hydraulic structures. |
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