Immobilization of Cu (Ⅱ), Ni (Ⅱ) and Zn (Ⅱ) in silica fume blended Portland cement: Role of silica fume

Silica fume (SF) is extensively used as supplementary cementitious materials to improve the mechanical and durable properties of Portland cement (PC) and thus has great potential to be applied in PC immobilization technique for the treatment of heavy metal contaminants. This research aims to elucidate the influencing mechanism of SF on the immobilization of heavy metal cations (Cu²⁺, Ni²⁺ and Zn²⁺) in SF-PC blended systems. SF accelerates the early hydration kinetics of PC immobilization systems due to the filler effect. This offsets the retardation impacts of Cu²⁺ and Zn²⁺ on the early hydration of PC. The reaction of SF with calcium hydroxide also promotes the later hydration. This decreases the capillary pore volume and increases the compressive strength of PC immobilization systems. The addition of SF induces the formation of C-S-H with high Si/Ca ratio and thus introduces additional [tbnd]SiO⁻ sites to bind Cu²⁺ and Ni²⁺. This enhances the capacity of C-S-H for the immobilization of Cu²⁺ and Ni²⁺. Zn²⁺ is transformed to Zn(OH)₄²⁻ in the high alkaline pore solution of PC. High Si/Ca ratio of C-S-H due to the addition of SF reduces the [tbnd]SiOCa⁺ and ([tbnd]SiOOH)₂AlCa⁺ sites to bind Zn(OH)₄²⁻. This declines the immobilization efficiency of Zn in PC.