Effects Of LDHs On Chloride Migration And Their Interaction In Alkali-activated Slag

Alkali-activated slag(AAS)cement has excellent performance.AAS can not only meet the performance requirements,but also reduce carbon emissions when AAS is used to replace Portland cement.Therefore,AAS has a considerable application prospect.At present,AAS cement is still facing the problem of durability.This study is committed to using layered double hydroxides(LDHs)to improve the chloride penetration behavior of AAS cement.Firstly,the hydration products of calcined Layered Double Hydroxides(CLDH)in the pore solution of alkali-activated slag were studied separately.The effects of reaction temperature and silicate modulus of activators on the hydration mechanisms of CLDH were analyzed by XRD and SEM/TEM.The hydration exothermic process of CLDH was recorded by isothermal microcalorimeter.The results show that CLDH can rapidly hydrate in deionized water and sodium hydroxide solution to form hydrotalcitelike phase.At 20℃,CLDH can fully hydrate and form hydrotalcite-like products in low modulus(Ms = 0.5 and 1.0)activators,while the hydration of CLDH in the activator with modulus of 1.5 is inhibited by the silicate.At 80℃,the hydration of CLDH tend to form mainly M(-A)-S-H and Na-A zeolite,particularly at Ms=1.0 and1.5,and a small amount of hydrotalcite-like phase with poor crystallization.Chloride binding is of great significance to delay the corrosion of steel bars.In this paper,the effect of CLDH on the chloride binding capacity of AAS paste was studied by equilibrium adsorption method,and the phase change of AAS paste before and after soaking in external chloride solution was analyzed by means of micro characterization.The results show that the addition of CLDH to AAS can enhance the chloride binding capacity,and the effect of CLDH is more obvious in sodium hydroxide system.There is a threshold value of CLDH in AAS,and further increase of CLDH content will not significantly change the chloride binding capacity.The chloride binding capacity of AAS not only comes from the physical adsorption of the gel phase,but also includes the chemical binding of the hydrotalcite-like phase and the Fridel’s salt.The addition of CLDH increased the production of hydrotalcite-like phase and Fridel’s salt in the hydration products of AAS.Based on NT Build 492,this paper focuses on the influence of CLDH on the migration coefficient of AAS mortar.At the same time,combined with the compressive strength and water-permeability porosity of AAS mortar,the correlation between pore structure and compressive strength and migration coefficient is analyzed and discussed.The results show that the macro properties of AAS mortar develop in a favorable direction with the increase of activator modulus,but when the modulus increases to1.5,the trend of macro properties reverses.The addition of CLDH can improve the macro properties of AAS mortar to a certain extent.On the one hand,the hydration of CLDH reduces the actual water/binder ratio and increases the alkalinity of pore solution,resulting in the acceleration of hydration rate and the enhancement of hydration degree;On the other hand,CLDH plays a filler effect and nucleation effect in sodium silicate activated slag,which promotes the formation of more hydration products.CLDH can reduce the migration coefficient by reducing the connectivity between the pore structures of AAS mortar.In addition,CLDH can also increase the chloride binding amount of hardened paste in the process of chloride transport.