Hydration behavior and chloride binding of seawater-mixed sintered sludge cement paste: Experimental and thermodynamic study

Inspired by the easy chemical binding of chloride ions with aluminum, dredged sludge (DS) has the potential to be used in cement to reduce the adverse effects of seawater mixing and accelerate its hardening. Herein, a detailed investigation into the fresh properties, hydration behavior and chloride binding of seawater-mixed sintered sludge cement (SSSC) paste is carried out utilizing methods such as Vicat apparatus, jumping table test, isothermal calorimetry, the bellows technique, SEM, MIP, DTG, ion chromatography and thermodynamic simulation. The research reveals that calcination at 800 ℃ promotes the thermodynamic decomposition of Clinochlore, Calcite and Muscovite in DS, with amorphous phase content ranging from 9.3% to 47.2% and mainly composed of activated alumina. Compared to fresh water mixing, the initial and final setting times of SSSC paste shortens by up to 12.1% and 10.9%, respectively. Meanwhile, the addition of sintered sludge ash (SSA) results in an accelerated onset of autogenous shrinkage in the cement paste, with the degree and pace of shrinkage rising of up to 29.7% and 110.9%, respectively. Amorphous alumina released by SSA facilitates the re-dissolution of aluminate clinker and expedites the formation of AFm phases when mixed with seawater, especially Friedel's salt. Comparatively, seawater mixing results in lower Portlandite content, heightened autogenous shrinkage, and increased cumulative heat release, suggesting a pronounced improvement in the pozzolanic activity of SSA. Moreover, the concentration of leached chloride ions shows a gradual decrease with the rising addition of SSA, which is attributable to the chemical binding of Friedel's salt and physical adsorption due to increased pore tortuosity.