Hydration mechanism and phase assemblage of blended cement with iron-rich sewage sludge ash

The hydration of Portland cement blended with iron-rich sewage sludge ash (IRSSA) was investigated. The addition of IRSSA results in a retarding effect on cement clinker hydration during early ages, especially C3S, while the reaction degree of cement clinker increases at later ages. The second peak observed in the deceleration period of heat flow curves in blended systems is due to the reaction between aluminum/iron phases in IRSSA and portlandite, generating additional AFt and AFm. Iron can also be incorporated and absorbed in C-S-H, leading to the increase in the Fe/Ca atomic ratios. Special areas with disorder distribution of bright spots can be observed in BSE images, which is due to the formation of C-S-H induced by IRSSA. The low mobility of Fe3+ in the pore solution leads to a higher Fe/Ca atomic ratios in the C-S-H induced by IRSSA than that in the C-S-H induced by cement clinker.