Applicability of steel slag as a substrate in eelgrass ( Zostera marina L.) beds restoration in coastal Japan

•Adding dredged material to slag mitigated the increase of pH in interstitial water.•The addition of dredged material to slag and sand enhanced the growth of eelgrass.•No difference was found in eelgrass growth between slag- and sand-based sediments.•No difference was found in benthic fauna between slag- and sand-based sediments.