Molecular insights into migration of heavy metal ion in calcium silicate hydrate (CSH) surface and intra-CSH (Ca/Si = 1.3)

In order to enhance the efficiency of heavy metal solidification/stabilization (s/s), unexplored atomic-scale interaction mechanisms between cement hydration products and metal ion are important. The atomic underlying mechanism of migration of heavy metals in the main cement hydration products, hydrated calcium silicate (CSH), is investigated under multifactor via molecular dynamics simulation. The multifactor combines: The adsorption at CSH surface determining the ratio of free ions to adsorbed ions; Heavy metals at CSH interlayer causes CSH particles swelling/shrinkage. The pore consisting of CSH will be wider/narrower, in turn acting on the migration of free heavy metal ion. Simulation shows that CSH bonding heavy metal (Cu/Zn/Pb) capacity depends on the ability of Ca exchanging with them as determined by hydration shell stability of heavy metals. Within interphase zone, part heavy metals form a stable chemisorption with silicon chains via metal-OCSH-Si connection and others form fragile physical adsorption near the outside interphase. Moreover, Pb ion swells the particles but Cu and Zn shrink them, making the multilevel pore structures denser/sparser. Different amounts of free ions due to different interaction with CSH migrates in sparser/denser pores, ultimately determining the solidification/stabilization efficiency.