SiO₂ Aerogel Effect Mechanism On The Early Shrinkage And Cracking Properties Of Geopolymer

Geopolymer is an inorganic non-metallic material made of alkali-activated silica-aluminate.The raw materials for preparation mainly come from industrial waste.Geopolymer is a new type of construction material with low energy consumption and low carbon emission and promotes the protection of the ecological environment while applied to production and construction.In this paper,the aerogel-modified geopolymer material was prepared by using slag and fly ash as raw materials,sodium hydroxide and water glass as alkali exciter,and adding hydrophilic SiO₂ aerogel on the basis.And then,the effects of slag admixture,SiO₂ aerogel admixture,exciter alkali concentration,and curing age on the workability,early shrinkage,mechanical properties,and shrinkage cracking properties of the modified geopolymers were systematically investigated.Work performance studies have shown that the elevated calcium content in the geopolymer system has the potential to improve the flowability of the slurry.However,with the increasing SiO₂ aerogel admixture,the flowability of the geopolymer slurry under the three slag admixture levels showed a decreasing trend.And the problem of difficulty in entering the mold appeared when the doping level reached 12wt%.In addition,as the increase of slag admixture in raw materials,the setting time of the geopolymer became shorter.Early shrinkage studies showed that the chemical shrinkage of the geopolymer slurry was significantly increased by the elevated calcium content in the system.When the slag dosing reached 30wt%,the difference in chemical shrinkage rate of geopolymer is less than2%with different dosages of SiO₂ aerogel.When slag content was 10wt%,aerogels content had the greatest effect on geopolymer self-shrinkage.The 56d self-shrinkage rate of geopolymer in the 8wt%aerogel group was increased by 0.4%compared with the reference group,and that in the 12wt%aerogel,group was increased by 0.9%compared with the reference group.When slag content is 30wt%,the effect of aerogel content change on self-shrinkage is the weakest,and the difference in the shrinkage rate of each group is less than0.1%.In addition,the self-shrinkage of the geopolymer material increased significantly with the increase of Na OH concentration in the alkali exciter.There was intensive cracking or even fracture at 40d for the specimens with higher alkali concentrations.For the geopolymer drying shrinkage specimens,the shrinkage was weakest at 30wt%slag dosage,and the drying shrinkage was reduced by 0.25%relative to the reference group when the aerogel doping reached 8wt%.The strength of geopolymer after the self-shrinking shows an increasing trend with the increasing of slag admixture.As the slag admixture reached 50wt%,the strength of the specimen increased significantly.And the compressive strength reached 36.45MPa when the aerogel dosing reached 8wt%.In addition,with the increasing alkali admixture in the alkali exciter,the mechanical properties after the self-shrinkage showed a trend of first increasing and then decreasing.The compressive strength reached 26.92MPa at 3.4mol/L Na OH concentration.The mechanical properties of the 1d age geopolymer specimens under a dry environment increased with the increase of slag admixture.When the curing age reached28d,the mechanical properties of the material increased and then decreased with the increased of slag admixture.Among them,the 28d compressive strength of geopolymer containing 30wt%slag and 8wt%aerogel reached 28.86MPa.The analysis of shrinkage cracking performance showed that the improvement effect of SiO₂ aerogel on the material shrinkage cracking was not obvious under the shorter maintenance age.As the age increases,the internal maintenance function of the aerogel compensates for the moisture loss inside the specimen in a dry environment and delays the drying shrinkage cracking for the geopolymer material.