Properties And Mechanism Of Vitamin-C Regulation On The Performance Of Alkali-Activated Slag Materials

Under the policy of"carbon neutrality"and"emission peak",alkali-activated slag materials（AAS）have the advantages of low energy consumption and low carbon emission.Moreover,AAS shows excellent mechanical property,low hydration heat,high resistance against sulfate attack and chloride ingress,high-temperature resistance and frost resistance,thereby emerging as the most promising new cementitious material today.However,regardless of these favorable advantages,AAS sets and hardens too quickly,the significant shrinkage limit its application in construction engineering.In this paper,slag was used as raw material,with NaOH or Na₂SiO₃as an activator to prepare AAS.The effect of VC as a natural multifunctional admixture on the properties of AAS were studied under two different activator systems.Specifically,this paper studied the hydration regulation properties of VC on AAS by isothermal calorimetry,XRD,TGA,ICP-MS,SEM and NMR,characterized the reaction products and microstructure,and revealed its related working mechanism to provide theoretical basis for the application of VC as natural multifunctional admixture in AAS.The specific research results are as follows:（1）As a natural multifunctional admixture,VC can effectively regulate the hydration process of AAS,which is manifested by retarding the hydration reaction.Under the two different activator systems of NaOH or Na₂SiO₃,the retarding effectis more obvious with the increase of VC dosage.The retarding mechanisms of VC on AAS can be attributed to both the surface adsorption and complexation with Ca²⁺.（2）The hydration,mechanical and shrinkage characteristics of AAS when NaOH activated slag were investigated using different amounts of VC.The addition of VC was able to reduce the content of C-（A）-S-H gel and increase the content of hydrotalcite.The addition of 0.5%VC showed optimal increments in the compressive strength of mortar samples and the values were 26.88%,16.67%and 19.60%at 3d,7d,and 28d,respectively.Meanwhile,combined with ²⁹SiNMR and Ca/Siratio analysis,it was found that AAS had the highest mean chain length and the lowest Ca/Siratio at VC5.In addition,the incorporation of VC can obviously reduce the autogenous shrinkage of AAS.With the increase of VC dosage,the shrinkage reduction is up to 63.74%.This can be attributed to the decrease of gel pores and internal relative humidity in AAS.3)The hydration and mechanical properties of AAS with Na₂SiO₃activated slag were investigated using different amounts of VC.The addition of VC significantly increased the content of hydration products C-（A）-S-H gel and hydrotalcite.The optimal increments in the compressive strength of AAS mortar samples at only 0.1%VC and the values were 14.43%,9.27%and 21.43%at 3d,7d,and 28d,respectively.VC promotes the formation of hydration products by providing nucleation sites for the hydration reaction,refines the size of hydration products and promotes the formation of finer particles.The hydration products were uniformly distributed and compactly packed,resulting in a denser pore structure and microstructure of the AAS mortar and improved compressive strength.（4）The hydroxyl group on the enol-form ring of VC is chemically reactive and can chelate Ca²⁺,thus increasing the Ca²⁺concentration in the pore solution.These will promote nucleation,improve particle packing and generate more uniform hydration products and microstructure,resulting in AAS mortars with excellent properties.Figure[42]Table[11]References[156]...