Efflorescence And Inhibition Improvements Of Alkali Activated Materials

Alkali activated material is a type of unconventional cementitious material formed by the reaction of active aluminosilicate raw materials and alkaline activators.Compared with traditional cement,alkali activated material have the characteristics of early high strength,fire resistance,acid resistance,radiation resistance,freeze resistance,durability,and well stabilization and solidification,but severe shrinkage,carbonation and efflorescence.The shrinkage cracks,carbonation,and efflorescence are all important reasons for the degradation of alkali activated material.Various active raw materials,different alkaline activators and alkaline dosages,and variable environmental conditions cause the complexity of alkali activated material.This paper selected active aluminosilicate materials（slag,metakaolin and fly ash）with different calcium contents and performance as raw materials with the Na OH-modified water glass activator,to study the effect of aluminosilicate raw materials on the efflorescence performance of alkali-activated materials.Investigate the effect of modulus and dosages of activators on the efflorescence of alkali-activated slag in a high calcium system and its mechanism.On this basis,the efflorescence resistence and compressive strength under different mixture parameters,and optimized by epoxy resin modification with the factors of raw material composition,modulus,and dosages of activators were evaluated.Also,the paper explores the technical measures to prevent the efflorescence of alkali activated materials,to provide theoretical technical support for the application research of geopolymers.The main conclusions obtained are as follows:（1）Adding metakaolin to slag can effectively mitigate the efflorescence of alkali activated materials.The addition of fly ash will aggravate the efflorescence of alkali activated materials and reduce the 28 d compressive strength.The composite reaction product of metakaolin and slag excited by alkali has a good complementary effect.SG-MK paste 28 d sealed curing compressive strength is 61.4MPa.Metakaolin improves the microstructure of SG paste.C-（N-）A-S-H in the main product of SG-MK paste is more stable than C-A-S-H,and the 28 d efflorescence compressive strength is still as high as 56.4 MPa,which is 21.8%and 9.5%higher than that of SG paste and MK paste after 28 d efflorescence,respectively.（2）Activator modulus and dosage affect the efflorescence performance of high calcium system alkali-activated materials as the main reason.The paste with a low modulus of the activator tends to cause the efflorescence phenomenon due to the high content of free Na⁺.But,too high or too low activator dosages of alkali-activated materials will also cause a more severe efflorescence phenomenon.Early and severe cracking of the alkali activated slag paste resulted in the growth of efflorescence carbonate crystals along the cracks below the surface of the paste.The exterior of alkali activated slag with high efflorescence tends to form"fluffy"products Na₂CO₃·n H₂O.Yet,the paste exterior with a low degree of efflorescence tends to form Na₂Ca（CO₃）₂·n H₂O.The 28 d efflorescence coefficient of alkali activated slag paste with different curing methods climbs and then declines with the increase of activator dosages.The efflorescence rate and 28 d appearance changes of alkali activated slag paste are very similar when the activator-solid ratio is greater than 0.5.Excessive activator dosage leads to an increase in the hydrated carbonate content of the product.The alkali-activated slag paste tends to form subflorescence layers in the cracked or connected pore structures.The degree of efflorescence on the surface of the alkali-activated slag paste decreases when the activator-solid ratio is 0.5.（3）The 28 d efflorescence compressive strength of SG-MK paste is heavily involved with the parameters of the activator and raw material.Especially ranges of efflorescence coefficients of paste are as high as about 25.4 and 10.3,respectively,with the activator modulus and the admixture.The efflorescence degree of paste appearance gradually decreased with the lower SG:MK ratio in raw material parameters.SG-MK paste efflorescence rate slowdown with a higher activator modulus and epoxy resin dosage.The epoxy resin reduces the water absorption rate of the SG-MK paste and hinders the internal transport of water during weathering.The epoxy resin partially filled open pores in the SG-MK paste with stabilization,which makes the pore structure more compact,slows down the migration of Na⁺and Ca²⁺,and inhibits the formation of surface carbonate crystals.In the optimized prediction results,when the SG:MK is 1,the alkali excitation modulus is 1.2,and the activator-solid ratio is 0.9,adding 5%epoxy resin can effectively improve the 28 d efflorescence compressive strength of the paste,And the influence on its appearance is small.