Study Of Leaching Behavior And Leaching Kinetics Of Alkali-activated Slag Materials In Karst Water Solutions

With continued exposure to flowing karst water,calcium leaching,chemical reaction and flowing scour accelerated the structural damage of concrete due to the large concentration of HCO₃^-in karst water.Alkali-activated slag showed excellent properties including fast hardness,high strength and good durability,which has the potential to partially replace ordinary Portland cement.Leaching behavior and leaching kinetics of alkali-activated slag in karst environment are yet to be studied.In this thesis,1X karst water solution and other HCO₃^-concentration karst water solutions were prepared according to the ion concentration of karst water in the southern region（changing the mass concentration of HCO₃^-,with fixed concentrations of other ions）.The early and the long-term accelerated leaching processes of alkali-activated slag in different karst water were investigated,the leaching kinetic equations were established,and the effect of different metakaolin doping on leaching behavior of alkali-activated slag were studied.The experimental results are as follows:（1）The leaching process of karst water is slow,and the signs of leaching are not obvious.During immersion process of alkali-activated slag,mass change showed a fast growth in the early stage and a flat growth in the late stage,water absorption of calcium silicate aluminate hydrate（C-A-S-H）gel phase and precipitation of calcium carbonate contributed to mass growth,calcium carbonate dissolution and calcium precipitation slowed down mass growth.Leaching leaded to the decrease in p H value,the increase in Ca²⁺concentration and leaching depth,disappearance of hydrotalcite and increase in pore size.With increase of HCO₃^-concentration,dissolution of alkali-activated slag was more severe.With the linear fitting of leaching depth and Ca²⁺concentration in the first layer,taking 1x karst water sample as a reference,leaching kinetic equation was established,and it was speculated that the fitting results of Ca²⁺concentration in the first layer was more suitable.（2）The leaching mechanism is complex and the applicability of early leaching kinetics needs to be verified in long-term leaching experiments.The long-term mass growth rate,p H value,the variation trends of Ca²⁺concentration and leaching depth in alkali-activated slag were similar to those of in early experiments,their numerical variation ranges was smaller than those in earlier experiments.The early accelerated leaching kinetics was used to calculate the long-term Ca²⁺concentration in outer layer（first layer）and long-term leaching depth,the comparative analysis showed that long-term leaching kinetics fitted by the Ca²⁺concentration of outer layer had a certain applicability.（3）The leaching mechanism of alkali-activated slag in karst water is:HCO₃^-in karst water diffused inward through surface pore channels and part of alkali in the pore solution were neutralized.C-A-S-H gel reacted with HCO₃^-and decalcifed to form calcium carbonate and silica gel,and gradually carbonated from the outside to the inside.Calcium carbonate reacted with HCO₃^-to produce soluble calcium salts which were lost with karst water flowing.After leaching,the sample were roughly divided into three layers,those were the surface dissolved gel layer,the intermediate carbonated barrier layer and the internal unresolved layer.（4）Taking cement as a reference,the calcium content in the gel products was reduced by adding different metakaolin in the mixture,zeolite-like（N-A-S-H）gels were generated to enhance the leaching resistance of alkali-activated slag.The trends of mass growth rate and p H value of various materials in long-term experiments were similar to and different from those in alkali-activated slag,and Ca²⁺concentrations in the inner and outer layers were more complex.Long-term leaching kinetics were fitted linearly using partial data of Ca²⁺concentration in outer layer.Compared with cement in 1x karst water,the acceleration factor of 20%MK（alkali-activated slag with 20%metakaolin）is smaller than that of cement,and the leaching resistance of20%MK is stronger than cement.