Study On The Mechanism Of Hydration And Structural Evolution Of Slag Cement Modified With Keggin-Al₁₃

Utilization as a component of supplementary cementitious materials(SCMs)is a key technology for consuming and recyling the vitreous industrial wastes.The glassy aluminosilicate structure in industrial wastes react with alkaline pore solution in the paste,acting as latent hydraulic materials and forming the binding phase.Promoting its hydration degree not only improves the consuming capacities of the low carbon and sustainable SCMs to wastes,but also enhances the binding performance,durability and service life of SCMs.A dual-beam focused ions beam plus scanning electron system(FIB plus SEM)is used to prepare the sample of TEM,facilitating to accurately find the region of unhydrated slag grains and inner products.This study presents a near in-situ observation of nanostructural evolution of unhydrated slag grains-inner product and outer product.A descriptive model of the hydration mechanism of slag grains and novel theory in which Keggin-Al13 adjusts the hydration process and structural evolution of inner product are proposed.The main innovative research results are as follows:(1)A orientationally distributed LDHs and C-A-S-H gel in the region of inner product is observed.Nano-crystals of LDHs are enriched in the region near the initial boundary of slag grains,forming a loose and porous structure.Amorphous C-A-S-H gel and poorly crystalline LDHs are enriched in the region near unhydrated slag,forming a dense structure.The low porosity of amorphous C-A-S-H gel inhibits the ions diffusion through hydration layer from slag to outer region,causing a low hydration degree of slag.Adjusting the structure of dense C-A-S-H gel to increase the porosity is the key method to promote the hydration degree of slag at late ages.(2)This study reveals that the orientationally distribution of LDHs and C-A-S-H gel is caused by the difference of nucleation and growth mechanism.At early ages,the dissolved Ca and Si ions diffused to the region of outer product,nucleating and precipitating as C-A-S-H gel,while the Mg ions nucleates at the etches on the dissolved surface of slag and grows as nano-crystals of LDHs from initial boundary to inner region.Thus,the region inner product at early ages enriches nano-crystals of LDHs.At late ages,the growth of LDHs crystals is inibited,causing the nucleation and precipitation of amorphous C-A-S-H gel and poorly crystalline LDHs in the region of inner product near the unhydrated slag.The above process of dissolution-nucleation and precipitaion-ions diffusion are the novel descriptive model of hydration mechanism of slag proposed in this study.(3)This study proposes a novel theory that Keggin-Al13 adjusts the structural evolution of inner product to promote the hydration degree of slag.Keggin-Al13promtes the growth of nano-crystals of LDHs,preventing the presence of dense product layer of amorphous C-A-S-H.The formation of a loose and porous inner product with Keggin-Al13 weakens the inhibiting effect on the ions diffusion induced by dense inner product,promoting the ions diffusion between slag grains and hydration product.(4)This study describes an innovative technical scheme in which Keggin-Al13improves the mechanical properties of SCMs and the immobilization ability of trace amount of hazardous metal cations in waste with the intrinsic structure of hydration product.Outer C-A-S-H gel reduce the porosity of paste,enhancing the binding performance.Trace amount of Cu²⁺and Cr³⁺ions are immobilized into the main plate of LDHs,reducing the leaching of hazardous metal cations.This study presents a new insight into the hydration mechanism of slag,which provide a better understanding on the hydration efficiency of slag.A new theroy in which Keggin-Al13 adjusts the hydration process of slag are proposed.The knowledge and theroy in this study are expected pave a way to a utilization with higher efficiency of indstrust wastes and a low carbon and sustainble development of SCMs.