Research On Performance Enhancement And Shrinkage Characteristics Of Fa-ggbfs Geopolymer

Ordinary Portland cement（OPC）has always been a common component in concrete preparation,but its clinker production stage produces a large amount of carbon dioxide and industrial waste,which is contrary to China’s strategic goal of achieving"carbon neutrality"and"carbon peak".It is urgent to find an acceptable green cementitious material substitute.The concept of geopolymer was proposed by the Frenchman Davidovits in 1978.It is an inorganic polymer with a three-dimensional network structure composed of Al O₄ and Si O₄ tetrahedral structural units,prepared from industrial waste with high silicon aluminum content after alkali excitation.It has many advantages such as simple preparation,good mechanical properties,low carbon emissions,and provides a new way for the treatment of industrial waste.During the fifty years since the discovery of geopolymers,scholars have conducted numerous studies on the precursors of geopolymers.The research on fly ash slag geopolymer is the most extensive among them.This is because fly ash,as an industrial waste generated by the combustion of coal-fired power plants,has advantages such as wide sources and low prices.The application of fly ash geopolymers provides new ways for the reuse of waste materials.However,during the preparation stage of fly ash geopolymers,thermal curing is often required,and the strength is too low under room temperature curing conditions.So it is often achieved by adding slag to improve the drawbacks of high temperature curing and low strength of fly ash geopolymers.However,with the increase of slag content,the shrinkage of fly ash slag geopolymer will also correspondingly increase.Excessive shrinkage may lead to the generation of cracks in practical applications,bringing certain engineering drawbacks.Based on this,in order to improve the current situation of high slag content with high shrinkage and low strength,this article first studies the mechanical properties and shrinkage characteristics of geopolymers under different fly ash slag ratios,and further optimizes the performance of fly ash slag geopolymers from the perspective of external admixtures.The main work and achievements are as follows:（1）Firstly,the ratios of fly ash to slag were set as 9:1,8:2,7:3 and 6:4 to investigate the setting time,mechanical properties,shrinkage characteristics,microscopic mechanisms,and the effect of increasing calcium components in the precursor on the performance of fly ash slag geopolymer under different slag dosages.The results show that the setting time decreased with the increased of slag content.The compressive strength and flexural strength increased with the increased of slag content.However,as the replacement rate of slag increased,the shrinkage value increased.The increase in slag content also brought certain drawbacks.In order to avoid excessive shrinkage caused by an increase in slag content,this paper selects slag 20%as a substitute for fly ash,and further explores the technology of enhancing the performance of fly ash slag on the basis of ensuring mechanical properties.（2）On the basis of the above mix ratio,explored the effect of zeolite powder as an external admixture on fly ash slag geopolymer.Based on the response surface method,established the relationship between the three factors of zeolite powder dosage,alkali activator dosage,and alkali activator modulus and the two response values of 3-day compressive strength and 28-day compressive strength;Using the Box Behnken model to fit the quadratic polynomial equations of compressive strength at 3 and 28 days;Subsequently,microscopic analysis was conducted based on SEM,FTIR,XRD,and NMR experiments.Through experiments,it was found that the maximum compressive strength at 3 and 28 days after the addition of zeolite powder was 40.2MPa and 70.9MPa,respectively.Compared to the absence of zeolite powder,the compressive strength increased by 58.9%and 20.0%,respectively.By optimizing the mix ratio with the highest compressive strength as the target value,the optimal mix ratios for zeolite powder dosage,alkali activator dosage,and alkali activator modulus were 13.3%,40.2%,and 1.2,respectively.The microscopic analysis results indicated that zeolite powder reduced the total pore volume,optimized the pore structure,made the microstructure of the geopolymer denser,and improved the strength of the geopolymer.However,excessive addition of zeolite powder led to high water absorption during the reaction process,which was not conducive to the hydration reaction.（3）The addition of zeolite powder effectively improved the compressive strength of the geopolymer,but the flexural strength did not significantly improve.Find another additive,polyvinyl alcohol（PVA）fiber,and study the effect of fiber on the compressive strength,flexural strength,and shrinkage characteristics of fly ash slag geopolymer.At the same time,XRD,SEM,and EDS were used to study the microstructure and mechanism of composite materials,in order to improve the high brittleness and poor crack resistance of fly ash-slag geopolymer.The results showed that at room temperature,the compressive strength of fly ash slag geopolymer at 28 days increased by 18.51%,and the bending strength increased by 78.72%.Fiber had a significant effect on improving the bending resistance of geopolymer.Due to the bridging effect of PVA fibers,the toughness of the polymer matrix was increased.However,excessive fiber length may lead to fiber aggregation,increased the porosity of the geopolymer,and instead lead to a downward trend in compressive strength and flexural strength.The 6mm length PVA fiber reduced the drying shrinkage of fly ash-slag geopolymer mortar by 49.7%at a 1.0%volume fraction,minimizing the drying shrinkage and brittleness of the geopolymer.The research conducted in this article provides reference for the utilization of industrial waste such as fly ash and slag,and also provides new ideas for improving the performance of fly ash slag geopolymers.