Effect And Mechanism Analysis Of Modified Admixtures On Properties Of Mortar Under Partial Sulfate Immersion

Sulphate ions mainly reach the concrete interior through the transport of water,and then cause the expansion,cracking and peeling of concrete.This erosion will affect the bearing capacity and service function of the concrete structure.The ability of concrete to resist sulfate attack is usually improved by increasing hydrophobicity.At present,domestic and foreign scholars have done a lot of research on mortar resistance to sulphate partial immersion erosion,including improving pore structure and surface protection.There is still a lack of systematic research on improving the overall hydrophobicity of mortar to resist sulfate attack by superhydrophobic materials.In this paper,by coupling mica powder/metakaolin with silane coupling agent(KH550)and polydimethylsiloxane(PDMS),a new type of waterproof admixtures modified mica powder and modified metakaolin were prepared.Then,the effects of modified admixtures on cement hydration properties,mechanical properties,water absorption and loss capacity,pore structure,hydrostatic contact Angle,microscopic morphology and properties of mortar soaked in sulfate were studied.The main research contents are as follows:(1)The effects of the content of modified mica powder and modified metakaolin on the water consumption,setting time,water absorption,water loss and compressive strength of mortar with standard consistency were studied.The results show that the two modified hydrophobic materials can increase the water consumption and setting time of cement at standard consistency,and reduce the water absorption and water loss rate of cement mortar to a great extent.With the increase of the content of modified hydrophobic material,the flexural strength and compressive strength of mortar decrease slightly.(2)The effects of the content of modified mica powder,modified metakaolin and PVA fiber on the appearance,size,strength,quality and relative dynamic modulus of mortar partially soaked in sulfate were studied.The results show that under partial sulfate immersion,the modified admixture can overinhibit the crystallization and crack development of mortar,and also inhibit the size increase of mortar specimen in the length direction.At the same time,it can reduce the influence of sulfate immersion on the strength,quality and relative dynamic elastic modulus of mortar,and improve the durability of mortar.PVA fibers accelerate the erosion of sulfate erosion under partial immersion,which is bad for the durability of mortar.(3)The effects of the modified admixtures on pore structure and surface hydrostatic contact Angle of the samples,as well as the hydration properties and microstructure of the cement paste before and after partial sulfate immersion and erosion were studied,and the transport movement of sulfate ions under onedimensional sulfate erosion was analyzed.The results show that the modified admixtures with hydrophobic properties can refine the pore size,reduce the connectivity of the pores and improve the pore structure.In addition,the addition of modified admixtures increases the hydrophobicity of the mortar surface and the hydrophobicity of the mortar.The lamellar modified admixtures provide a waterrepellant barrier through the pore channels,and the evaporated water in the pore channels is inhibited and retained in the mortar,restricting the penetration and transmission of sulfate ions in the mortar.Under one-dimensional sulfate erosion,the sulfate concentration at the same depth showed a decreasing trend with the increase of the content of modified mica powder and modified metakaolin.