Effect of SiO₂-modified calcined layered double hydroxides on the properties of cement-based material: Crucial role of the phase-transformation induced by alkaline pore solution

Calcined layered double hydroxides (CLDH) and nano-SiO₂ are commonly added to cement-based materials to enhance their resistance against chloride ions (Cl⁻) and mechanical performance, respectively. In this work, the CLDH and nano-SiO₂ were combined to synthesize SiO₂@CLDH, aiming for a synergistic effect between the two materials. The deposition of nano-SiO₂ on the CLDH substrate enhanced the utilization of nano-SiO₂ by promoting their dispersion in the cement pastes. Interestingly, the alkaline pore solution could induce a phase-transformation of SiO₂@CLDH, leading to the formation of a hydrated calcium silicate (C-S-H) layer on the rehydrated LDH surface (i.e., C-S-H@LDH). The C-S-H layer could enhance the surface adsorption for Cl⁻; effectively hinder the displacement of interlayer Cl⁻ by SO₄²⁻ and also improve the carbonization resistance of C-S-H@LDH. However, the amount of nano-SiO₂ loading on the CLDH surface exhibits a ‘trade-off’ effect. Excessive nano-SiO₂ loading leads to an overabundance of nascent C-S-H gel, which envelops the CLDH and hinders the rehydration of CLDH. Moreover, the nascent C-S-H gel layer could act as the cross-linker between rehydrated LDH and cement pastes, thereby increasing the compressive strength of cement. In general, the incorporation of SiO₂@CLDHs into cement could significantly increase the hydration degree, the compressive strength, adsorption of Cl⁻, and optimize the pore size of cement pastes. This study highlighted that when investigating the effects of doping materials on cement performance, it is crucial to recognize that cement may concurrently influence the physicochemical properties of the doping materials.