| Dual-carbon strategy has become an important strategic task of national development and the effectiveness of carbon emission reduction in cement industry is directly related to the success or failure of national"double-carbon"strategy.Carbonate-activated cementitious materials（CACM）are a kind of alkali-activated cementitious materials which use carbonate as activator to stimulate the activity of silicate-alumina vitreous.With the characteristics of low environmental impact,easy operation and good material properties,it is a research hotspot in the field of low-carbon cementitious materials.Slow early reaction rate,long setting and hardening time and low early strength are the main problems restricting the engineering application of CACM.In view of the main difficulties in the application of CACM,the early hydration reaction mechanism of CACM was deeply studied in this paper.It is revealed that the early hydration products of CACM are in a supersaturated state for a long time under the action of salt effect,which leads to the phenomenon of long setting time and low early strength.Furthermore,a material performance optimization method was proposed to accelerate the formation of hydration products by layered double hydroxide and C-S-H seed.The carbonation resistance and chloride ion penetration resistance of modified CACM were deeply explored and the structure-activity relationship between seed crystal and the durability of CACM was established.A kind of early strength CACM with short setting time,high early strength and good durability was successfully designed and prepared.The main research contents of this paper are as follows:（1）The structure-activity relationship among the amount of activator,reaction rate,product composition,mechanical properties and microstructure was illustrated and the early hydration reaction mechanism of CACM was studied.The design and performance improvement direction of CACM based on seed crystal were clarified.The results show that the calcium concentration in the early hydration of CACM is low and under the action of salt effect,the main products of C-S-H gel and hydrotalcite have been over saturated for a long time,resulting in slow hydration of CACM.Due to the induction period of hydration reaction is controlled by the nucleation rate,the introduction of seed crystal to promote the nucleation and growth of C-S-H gel and hydrotalcite can effectively improve the early properties of CACM.（2）A method for optimizing the properties of CACM was formed by using C-S-H seed and layered double hydroxide seeds to promote the early hydration reaction,accelerate the formation of hydration products and improve the early mechanical properties.The two kinds of crystal seeds improve the early mechanical properties of sodium carbonate activated slag,shorten the initial setting and final setting time of the sample and the early hydration reaction rate of the samples was promoted.The addition of crystal seeds introduces nucleation sites and reduces the heterogeneous nucleation barrier.Layered double hydroxides（LDHs and CLDHs）seeds can induce the formation of hydrotalcite phase in situ,which consumes carbonate ions to increase the alkalinity of the solution and introduces heterogeneous nucleation sites,thus promoting the precipitation of C-S-H gel.The addition of C-S-H seeds can induce the formation of C-S-H gel phase.With the nucleation of Si,Ca and other elements on the seed surface,the continuous dissolution of Mg2+,Al3+and other elements in the slag and the seed as heterogeneous nucleation sites,the hydrotalcite products increase.（3）The relationship between the type and content of crystal seed and carbonation depth,porosity,mechanical properties,phase composition and microstructure was studied and the mechanism of crystal seed improving the durability of CACM by hindering the migration of chloride ion was revealed.The technology to improve the durability of CACM based on seed induction was obtained.CLDHs can induce hydrotalcite formation to promote C-S-H gel precipitation,which absorbs carbonate ions and some water molecules into the interlayer during the process of structural reconstruction,further improves alkalinity,reduces porosity and enhances the carbonation resistance of samples.Layered double hydroxide crystal seed and its induced hydrotalcite phase can hinder chloride ion migration by surface physical adsorption,which is helpful to improve the chloride ion permeability of CACM. |
