Reduction of heavy metals leaching and pore volume in high-volume fly ash cement pastes by adding nano-SiO₂

This study investigated the compressive strengths and leaching of heavy metals in high-volume fly ash cement (HVFAC) pastes with and without nano-SiO₂ (NS). Further, scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry were used to characterize the microstructure of the specimens. The results showed that the addition of NS increased the compressive strength of HVFAC specimens. NS accelerated the hydration of both cement and fly ash to form more hydration products, and it reacted with portlandite to simultaneously generate more C-S-H gel. Thus, the total pore volume, especially the capillary pores, in the NS-mixed HVFAC specimens was reduced, and their microstructure became denser because of the filling of more hydration products in the pores. This resulted in the reduction of the leaching of heavy metals (Cr, Pb, Cd, As, Zn, and Cu) in the NS-mixed HVFAC specimens compared with the HVFAC specimen containing no NS. According to the Chinese national standard (GB 3838-2002), the water quality of leachates from the HVFAC specimens mixed with both 3% and 5% NS reached grade I, thus causing nearly no harm to the water and soil environment.