Study On Accelerated Corrosion Behavior And Microstructure Of Alkali-activated Slag Grouting Material In Flowing NaHCO₃ Solution

Chemical Corrosion of cement concrete will occur in corrosive water（including industrial waste water,nuclear waste water,groundwater,etc.）,calcium Portland cement silicate（C-S-H）and Ca（OH）₂react with corrosive water,the structure is destroyed to form soluble calcium salts,causes Calcium to run off with environmental water,the strength of concrete decreases,and the p H of pore solution decreases,further accelerating the corrosion of concrete.Karst water is a typical kind of erosive groundwater,the main corrosive ion is HCO₃^-.At present,there is no evaluation method for accelerated dissolution of Karst water,in this paper,5 wt%Na HCO₃flowing karst water is used as corrosion medium,the effects of different alkalinity,water glass modulus and different dosage of Metakaolin on the mass loss rate,expansion rate,corrosion depth,mechanical properties and microstructure of alkali-activated slag grouting material in 5wt%Na HCO₃flowing karst water environment were studied.The results show that:（1）As the modulus and alkalinity of sodium silicate increase,the alkali-activated slag grouting material had lower mass loss rate,lower expansion rate,higher compressive strength and flexural strength,and lower erosion depth.When the modulus of sodium silicate was 2.0 M and the alkalinity was 6%,the mass loss rate was 0.33%,the expansion rate was-0.045%,the compressive strength was increased by 80%,the flexural strength was increased by 54%.（2）From the microstructure and pore structure,it was found that with the increase of the modulus of water glass,the calcium-silicon ratio of C-A-S-H gel decreased,the pores were partially filled with calcium carbonate and silica gel formed by decalcification of surface hydration products,the Pore size of the gel structure decreased and the strength increased,Prevented HCO₃^-attack alkali-activated slag grouting material inside.（3）With the increase of the content of Metakaolin,the mass loss rate,erosion depth and expansion rate decreased at first and then increased of alkali-activated slag grouting material.When the dosage of Metakaolin was 10%,the mass loss rate was 0.05%,and the erosion depth was 1.875 mm,compared with the standard curing condition,the flexural strength was increased by 0.8 MPa,the compressive strength was increased by 11 MPa,and the karst erosion resistance was good.With the addition of Metakaolin,N-A-S-H gel increases,the pore size of gel decreases,the calcium solution decreases,the strength increases,and the ability of resisting hydrogencarbonate erosion increases.Adding too much metakaolin,the unactivated metakaolin can increase the porosity,decreased the strength and reduced the resistance to karst erosion of grouting material.（4）The corrosion of alkali-activated slag grouting material in flowing Na HCO₃solution was a comprehensive process of carbonation and acid corrosion.The flowing Na HCO₃solution contacted the surface of the sample,and HCO₃^-reacted with C-A-S-H gel to form calcium carbonate and Silica gel deposited in the pores.The ionized H⁺of HCO₃^-continued to react with the calcium carbonate in the pores to form soluble hydrogencarbonate which were lost by flowing water.After the surface hydration product was completely decalcified,pore size increased,HCO₃^-contin to spread inside the sample,the reaction between HCO₃^-and C-A-S-H gel in the middle layer resulted in the deposition of calcium carbonate in the pores to form a compact structure,prevented HCO₃^-attack alkali-activated slag grouting material inside.