Hydrothermal synthesis and characterization of alkali-activated slag-fly ash-metakaolin cementitious materials

In the present work, the alkali-activated slag-fly ash-metakaolin cementitious materials were prepared by hydrothermal method. The as-synthesized products were characterized by X-ray diffraction (XRD), fourier transform-infrared spectrum (FT-IR), scanning electron microscopy (SEM), mercury porosimetry and universal testing machine. The results showed that the main mineral phases of the products were zeolite NaP1, analcime and aluminosilicate gel (zeolite precursor) when Si/Al/Ca molar ratio of the starting materials was 1.7:1.0:0.25. While Si/Al/Ca molar ratio of the system changed to 2.3:1.0:0.45, zeolite NaP1 transformed to analcime, and a tobermorite phase appeared. The quantities and types of zeolites synthesized depended on the Si, Al and Ca molar ratio of starting materials. The average compressive strength of the products was more than 60 MPa after hydrothermal process. And the products exhibited a porosity of less than 36% and a median pore diameter (area) of approximately 12 nm. The reaction degree results indicated that about 75% of starting materials transformed to zeolitic structure and aluminosilicate gel after alkaline activation and hydrothermal process.