Chemical effect of sewage sludge ash on early-age hydration of cement used as supplementary cementitious material

Sewage sludge ash has potential to be used as supplementary cementitious material in the production of building materials. The chemical effect of sewage sludge ash on early-age hydration of cement was investigated in this study. Two types of sludge ash calcined at 600 degrees C (5600) and 800 degrees C (5800) were blended with cement for analysis of hydration heat evolution, solid phase assemblage and aqueous composition. The aluminate dissolution of 5600 ash promoted the formation of ettringite and consumption of gypsum, resulting in a high initial hydration heat. However, high concentrations of Al and Si caused by continuous dissolution of aluminate and silicate in 5600 ash inhibited significantly the C3S dissolution. Interestingly, 5800 ash had slight effect on early cement hydration since higher calcination temperature decreased the activity of aluminate. As compared with the reference, cumulative hydration heat of blended paste with 30% 5800 ash at 7 days was increased by 18.72% indicating the occurrence of reaction between sludge ash and cement. Further study is recommended to focus on the long-term performance of cement-based materials blended with sludge ash.

Automated summary

This study investigated the chemical effect of sewage sludge ash on early-age hydration of cement. Two types of sludge ash calcined at 600 degrees C and 800 degrees C were blended with cement for analysis. The dissolution of aluminate and silicate in the ash significantly inhibited the C3S dissolution and caused high initial hydration heat. However, higher calcination temperature decreased the activity of aluminate in the 5800 ash, resulting in slight effect on early cement hydration.