Abstract
Management strategies for the safe disposal of contaminated dredged marine sediment constitute a global-scale environmental issue. The stabilization/solidification method was investigated as a sustainable approach to the recycling of the sediment as a construction material. A systematic study of the factors affecting the mechanical performance and contaminant release was performed. The physico-chemical variables selected to assess the potential re-use of the sediment treated with Ordinary Portland cement (OPC) and Ground Granulated Blast Furnace Slag (GGBS) in an aquatic environment were: curing duration (7, 28, 56 and 98 days), curing temperature (5, 20 and 40 °C) and ambient (leachate) pH (1, 4, 7 and 10). Unconfined compressive strength (UCS) tests were conducted and extended-duration tank leaching tests were used to characterize the long-term leaching of Al, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba, Pb. The results showed that S/S methods provide excellent immobilization of metals in marine sediment at a pH range of 4 to 10. Immobilization efficiencies of >99.9% for Mn, Fe, Zn, As, Ba, Pb and >97.8% for Al, Cu and Zn are reported over 100 days. GGBS replacement is an effective way to further improve sediment properties by enhancing strength, mitigating sediment alkalization and offering a better immobilization capacity for Fe, Ni and Zn. The release of metals (Al, Mn, Cu, As, Ba and Pb) was strongly associated with a coupling effect of the physico-chemical factors, with metal-specific responses to curing temperature, curing duration and pH. Mn mobility showed a dramatic sensitivity to ambient pH while Ba was less pH-dependent. Al release is related to strength and leached out by dissolution in all situations considered. Considering that dredged marine sediments may contain multiple metal contaminants which exhibit individual responses to remediation, treatment with GGBS may be considered a potentially suitable management option.
Highlights
Dredging is carried out periodically in channels and ports worldwide in order to maintain sufficient depth for navigation, resulting in the transfer of several hundred million cubic metres of sediments from the aquatic to the terrestrial environment annually (Lirer et al, 2017; Snellings et al, 2016)
In order to investigate the effect of curing duration on the strength of the Ordinary Portland cement (OPC)-only sediment and OPC/Ground Granulated Blast Furnace Slag (GGBS) sediments, the Unconfined compressive strength (UCS) values for the 12C and 6C6G cases are presented in Figure 4 as a function of curing time
The UCS values for the 6C6G mixtures were much greater than those for the 12C mixtures for all stages of curing, including early stages. This reflects an acceleration of cement hydration by the mutual activation of OPC and GGBS, allowing earlier development of pozzolanic reactions
Summary
Dredging is carried out periodically in channels and ports worldwide in order to maintain sufficient depth for navigation, resulting in the transfer of several hundred million cubic metres of sediments from the aquatic to the terrestrial environment annually (Lirer et al, 2017; Snellings et al, 2016). Exposure to elevated ambient temperatures likely causes the ‘crossover’ effect in cement-based GGBS mixtures, whereby the early stage formation of hydrates occurs too quickly, leading to the non-uniform dispersion of the hydration products This results in the formation of a hard paste in the pores and the hydrated shells and a low permeability at the surface of the particle (Aziz et al, 2012; Castellano et al, 2016; Tanyildizi, 2009). The immobilization efficiency of marine sediment stabilized/solidified with cement and GGBS requires further investigation, given the increased number of commercial projects considering this option To this end, a comprehensive suite of mechanical and leaching tests was performed on a contaminated dredged sediment stabilized with cement and cement/GGBS binders, and subjected to ranges of curing time, curing temperature and ambient leachate pH. The specific aims included: 1) a comparative evaluation of the effect of curing duration, curing temperature, leachate pH and water immersion on the unconfined compressive strength of cement-only and cement/GGBS mixtures; 2) an assessment of the effect of curing duration, curing temperature and pH on the leachability of selected metals, as investigated through tank leaching tests, in addition to a consideration of the prevailing leaching mechanisms for various contaminants
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