Influence of seawater on the mechanical and microstructural properties of lime-incineration sewage sludge ash pastes

Traditional disposal methods of dredged marine sediment are expensive and environmentally harmful. Reusing marine sediment as construction materials by stabilisation/solidification (S/S) has been an appealing option. This paper reports on a study on evaluating the potential of using incineration sewage sludge ash (SSA) as the principal binder together with anhydrous lime and hydrated lime for S/S in a seawater environment. This study evaluated the effects of seawater on the mechanical performance and microstructural properties of the binder as compared to the use of freshwater. Generally, seawater accelerated and promoted the strength development of the lime-SSA binder. Friedel's salts (3CaO·Al₂O₃·CaCl₂·10H₂O) were detected in the seawater pastes by X-ray diffraction (XRD) and thermogravimetric (TG) analysis. It was found that the role of Friedel's salt contributed differently to the strength development of hydrated lime (CH)-SSA and anhydrous lime (lime)-SSA pastes mixed with seawater particularly at the later curing age. The 28-day strength of the lime-SSA paste mixed with seawater was more than double that of mixed with freshwater, whereas the CH-SSA paste mixed with seawater only showed a comparable 28-day strength with the freshwater mix. This was primarily due to the initial porous structure of the fresh lime-SSA paste allowing the growth of the Friedel's salts. Therefore, the use of anhydrous lime may mitigate the strength reduction caused by Friedel's salts at the later ages.