Low carbon stabilization of Hong Kong marine deposits by sewage sludge ash for land reclamation

Abstract

Fill material is highly demanded in land reclamation. In Hong Kong, around 3000 tonnes of dredged marine deposits are produced per day which can be used as fill material after stabilization. Traditionally, cement is used to stabilize marine deposits, however, cement production results in high energy consumption and carbon emissions. Therefore, this study aims to recycle dredged marine deposits to serve as fill material and replace cement with sewage sludge ash (SSA) to reduce the carbon footprint. The samples have a constant binder content of 20%, in which cement is partially replaced by SSA with a replacement ratio (R) varying from 0 to 60%. Unconfined compression tests were performed to study their mechanical behaviors. Although larger R is preferred to replace more cement, the unconfined compressive strength (qu) decreases exponentially with R and reduces by around 50% when R is 60%. qu also decreases with water to binder ratio (W/B), while increasing with curing time. The increase of strength can be attributed to more hydration products, which could bond the soil particles and fill the pores. This is supported by the X-ray diffraction (XRD) results and scanning electron microscopy (SEM) images. Based on the results, a modified basicity coefficient is proposed and validated for the strength prediction of ash-stabilized soil considering chemical compositions in different binders, binder content, water content, and curing days. This study found that it is feasible to replace more than 50% of cement with SSA, while still achieving the required strength of 200 kPa for fill material at 28 days with an in-situ water content of 100%. The replacement of cement with a low-carbon and cost-free waste of SSA in marine deposit stabilization can significantly enhance the greenness and sustainability of construction.