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.

Graphical abstractThe mineralogy of hydrothermal synthesis products of alkali activated slag–fly ash–metakaolin mixtures was dependent on the Si/Al molar ratio of the starting materials. The average compressive strength of the samples was more than 60 MPa after hydrothermal process.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Zeolitic phases cementitious materials were prepared by hydrothermal method. ► The main mineral phases of the products are analcime and zeolite NaP1. ► Si/Al molar ratio has a strong effect on mineralogy of products. ► The products exhibit a lower porosity and higher compressive strength.