Shrinkage Compensation Of Metakaolin Base Geopolymer By Different Internal Curing Methods

Geopolymer has become a good substitute for traditional silicate cement due to its excellent properties such as being green and low carbon,superior mechanical performance,corrosion resistance and high temperature resistance.However,the drying shrinkage caused by the loss of internal moisture is the biggest obstacle to its engineering application.The shrinkage behaviors promotes the formation and development of cracks,reduces the durability and volume stability of building materials,induces the overall failure of building structures,and poses a threat to production and human life.Therefore,reducing the drying shrinkage behavior of building materials through internal curing has become a research hotspot at home and abroad.In this paper,we consider metal-based polymers.Due to their problems such as large dry shrinkage and easy cracking,SAP was used as an internal curing agent and various active Mg O was used as a shrinkage compensating agent to improve the shrinkage cracking of metakaolin-based geopolymers.The following conclusions were reached:（1）The SAP-doped geopolymer sample can significantly mitigate dry shrinkage and cracking.No macroscopic cracks were observed on the surface of the sample when the SAP content was greater than 0.4%,and optimum shrinkage compensation was obtained when the SAP content was 0.5%and the dry shrinkage was mitigated by 70.7%.At the same time,the addition of SAP introduces a large pore content into the sample matrix and its porosity gradually increases.The SAP was found to have a negative effect on the mechanical properties of the geopolymer sample.When the SAP content is 0.1%,the dry shrinkage can be reduced by 49.5%and the compressive strength is improved compared to the blank sample.Furthermore,by analyzing the hydration progress of the geopolymer,it was found that the addition of SAP geopolymer would retard the geopolymerization and make the internal moisture change of the sample relatively stable,but SAP itself did not participate in the geopolymerization.（2）To mitigate the change in slurry solid-liquid ratio due to dry blending of SAP into the geopolymer,specimens with 10 times additional alkali exciter（AA）or water were introduced at constant SAP blending ratios（0.1,0.2 and 0.3wt%）with reduced h macroscopic cracks,denser structure and optimized pore structure.Characterization of the pore structure of the specimens revealed that the gel pores,capillaries and macropores of the biased metakaolin-base geopolymer specimens were optimized to varying degrees,with reduced porosity and enhanced mechanical properties of the specimens compared to the control group.The number of cracks produced on the surface of the specimens was less when additional water supplementation was applied to SAP,compared to the additional introduction of AA where fine mesh cracks were produced on the surface and the porosity,compressive strength and degree of hydration were weaker than that of the AA supplemented specimens.This demonstrates that the additional 10times AA supplementation compensates for the change in the solid-liquid ratio of the entire cementitious system due to the solution absorbed by the dry doped SAP within the specimen.（3）In order to reduce the large pores left by SAP after water release in geopolymer samples,active Mg O under different calcination conditions was added to the geopolymer samples cured in SAP,and the hydration was used to generate expanded Mg（OH）₂,which physically filled the large holes and cracks in the samples,thus achieving the purpose of shrinkage compensation for geopolymer cured in SAP.Among the geopolymers,the active Mg O has the best comprehensive performance under the condition of 700℃calcination.The porosity of sample M700-4 decreased by 17.4%and the drying shrinkage decreased by 8.2%.The compressive strength of the sample at 3 d,7 d and 28 d were 45.5 MPa,61.2 MPa and 66.4 MPa respectively.At the same time,with the increase of MS calcination temperature（600℃,700℃,800℃）,the compressive strength and porosity of metakaolin-based geopolymer cured in SAP gradually decreased.XRD analysis of the sample showed that after the addition of active Mg O,hydrotalcite and Mg（OH）₂ were formed in the hydration reaction process.Hydrotalcite was formed by active Mg O and metakaolin in the geopolymerization reaction,and Mg（OH）₂ was formed by the reaction of active Mg O and water.Hydrotalcite and Mg（OH）₂ with expansion effects can compensate for partial shrinkage in geopolymer samples and fill internal pores and cracks.In this work,we investigate the shrinkage compensation of metakaolin-based geopolymers by different internal curing methods.The drying shrinkage,macroscopic cracking and other physical and chemical properties（pore structure,compressive strength,hydration and microstructure characteristics）of metakaolin base polymer were studied with the dosage of internal curing material（SAP）and expansion agent（Mg O under different calcination conditions）as control variables.The results show that SAP as internal curing material and active Mg O as expansion agent play an important role in improving the shrinkage compensation of geopolymer.