Design optimization and characterization of a green product by combined geopolymerization of sewage sludge ash with metakaolin.

This study aimed at developing sewage sludge ash (SSA) as a green binding material through geopolymerization for saving natural resources and reducing problems with the waste disposal. Geopolymer pastes based on a precursor comprising 25 wt% of metakaolin (MK) and 75 wt% of SSA were synthesized using the dry-mixed method. Compressive strength tests of the specimens revealed that geopolymers produced from mixes with a Na 2 O/SiO 2 molar ratio of 0.30 and a SiO 2 /Al 2 O 3 molar ratio in the range of 3.00 to 3.60 achieved the highest compressive strength after 28 days of curing, regardless of temperature. The microstructural analyses showed the formation of N-A-S-H gels in the specimens. The initial curing temperature had a significant influence on the compressive strength of the geopolymers. Specimens initially cured at 40 °C for 7 days and then stored at room temperature showed progressive gain in compressive strength with time. However, raising the initial curing temperature from 40 °C to 60 °C resulted in the formation of cracks in the matrix and new mineral of zeolite P (Na) which caused a notable loss of late-age strength. Leaching toxicity tests showed that the concentrations of heavy metal(loid)s in leachates did not exceed the specified limits. [Display omitted] • Sewage sludge ash/metakaolin geopolymers were synthesized by the dry-mixed method. • Microstructural analyses proved the formation of geopolymer gels. • The optimal Na 2 O/SiO 2 and SiO 2 /Al 2 O 3 molar ratios were identified. • Strength improvement was achieved at 40 °C. • Further rise in the initial curing temperature generated zeolite crystallites and crack features. [ABSTRACT FROM AUTHOR]