Hydration mechanisms of sewage sludge ashes used as cement replacement

The influence of sewage sludge ash (SSA) on the hydration of Portland cement (OPC) was investigated in this study. Blending OPC with SSA was found to influence the kinetic of hydration, notably the alite dissolution. This retardation effect has been attributed to the presence of the orthophosphate ions (PO43-). However, SSA releases only a small fraction of PO43- into solution, which is deemed insufficient to precipitate calcium phosphates compounds on noticeable levels at least. On the other hand, an enhancement of the engineering-scale properties (i.e. mechanical and durability) and portlandite consumption are observed when SSA is used. This is explained by the formation of AFm phases, which leads to changes in the total volume of the solid and thus the properties of the systems. These experimental results were confirmed by thermodynamic modelling, showing that the additional alumina released by SSA results in the formation of higher amount of AFt and AFm phases.