Recycling contaminated sediment into eco-friendly paving blocks by a combination of binary cement and carbon dioxide curing

Abstract

Dredging is a regular maintenance for navigation in harbours and water channels; however, disposal of dredged sediment poses environmental liability and leads to shortfall of disposal facilities. This study demonstrated an innovative approach to transform contaminated sediment into eco-friendly sediment blocks by applying binary cement and CO2 curing. The binary use of magnesium oxide cement (MOC) and ordinary Portland cement (OPC) with air curing improved the compatibility of heavy metals and cement by provision of sufficient magnesium hydrates for metal sequestration. However, an introduction of MOC weakened compressive strength and increased water absorption of sediment blocks. Therefore, 1-d CO2 curing was employed to transform soluble magnesium hydrate into stable carbonates, and densify the microstructure and reduce the porosity from 18.5% to 16.9%, which enabled a substantial enhancement in strength (2.8 times) as well as carbon sequestration (3.98 wt%) in 40% MOC sample. A subsequent 7-d air curing with moisturizing facilitated continuous carbonation and rehydration of binary cement, resulting in a deeper carbonation and higher strength by the provision of additional nucleation sites of reactive magnesium oxide. Although accelerated carbonation had an adverse influence on metal immobilization in OPC sample due to pH decrease, it provided synergistic interaction with binary cement samples due to sufficient hydrates and pH buffering capacity. Therefore, binary cement with CO2 curing presents a novel and green stabilization/solidification technology for recycling contaminated sediment as valuable and eco-friendly construction materials.