Stabilization/Solidification Of Heavy Metal Containing Sediments Using Concrete Additive

The pollution of heavy metals presents a serious threat to the environment ofXiawan Stream with the rapid development of industry since the1960s. Highconcentrations of heavy metals in sediments have potentially hazardous effects onwater quality of Xiangjiang River and thus need to be solidified.An improved method was adopted herein for the solidification/stabilization (S/S)of sediments with cement-based additives. Sediments containing high concentrationsof Cu, Cd, and Pb were solidified/stabilized by binders of cement and fly ash.Additives, such as calcium nitrate, sodium nitrite, sodium aluminate, sodium sulfate,sodium lignosulfonate, sodium lauryl sulfate, triethanolamine, and bentonites, wereused. Binder content (200g/kg cement,200g/kg fly ash,30g/kg bentonite, and25g/kg additive) was determined by orthogonal experiments. Results demonstrate thatthe addition of concrete additives has a significant positive effect on the S/S ofsediments.A sequential extraction method was used to determine the fraction of heavymetals present with an increase in curing time. In nearly all fractions, theconcentrations of heavy metals changed significantly after solidification. Percentagesof Cu, Cd, and Pb in the residual fraction increased with longer curing times. pHincrease in samples after solidification could possibly result in the dissolution of otherforms and improve the ability to combine carbonate, which reduces the organicfraction of Cu and carbonate fractions of Cu, Cd, and Pb. After60days of curing, thepercentage of the five fractions showed no significant changes.SEM microstructural analyses clearly identified typical hydration products, suchas ettringite, calcium silicate hydrate, and calcium hydroxide. In the first period,ettringite was the main form of hydration products observed, which later turned intoCSH. Two of these hydrated products played an important role in enhancing thesolidification of the sediments studied. The results provide further insights into thetransformation of heavy metals during S/S.