Hydration process of the aluminate 12CaO7Al ₂O ₃-assisted Portland cement-based solidification/stabilization of sewage sludge

The high organic content in sewage sludge is the main obstacle to Portland cement-based solidification/stabilization (S/S). A novel aluminate 12CaO7Al ₂O ₃ was prepared as accelerator in order to improve the performance of cement-based S/S making the sludge disposal and recycling possible. The behavior of pastes fabricated with various mass ratios of aluminate/cement has been analyzed in terms of mechanical strength, hydration products, microstructure and leaching characteristics. The incorporation of aluminate significantly improved the cement-based S/S performance. The solid matrix obtained with the aluminate/cement ratio of 4/6 and binder addition of 10 wt.% presented 28 day-strength of 157.2 kPa, in contrast, 25.1 kPa or so obtained for the cement only-sludge mix. X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC) and scanning electron microscopy (SEM) analysis revealed that the presence of aluminate counteracted the interference from organic matters, favored the formation of crystalline phases viz. ettringite C ₆AS ₃H ₃₂, calcite CaCO ₃ and quartz, and therefore the strength development. Environmental assessment of the final products in compliance leaching tests demonstrated that the concentration of heavy metals were below the certain legal limits (GB 5085.3-2007) set in China, though the addition of aluminate slightly fell the resistance of solidified mortars to acidic environments.