Alkali-Activated Metakaolin and Fly Ash as Unfired Ceramic Bonding Systems

Metakaolin (MK) prepared by the calcination of kaolin at 550 degrees C and fly ash (FA) from the combustion of black coal in a granulating boiler were used to prepare unfired ceramic bonding systems via the alkali activation process. A long-term stability of the mechanical properties of the prepared samples similar to the unfired ceramic systems was observed. The optimal metakaolin and fly ash ratio, the type of the activator (NaOH or water glass) and its concentration were evaluated after the hydration in: a) laboratory conditions; b) hydration box; and c) under the hydrothermal activation. Raw materials and the samples prepared by alkali activation process were characterized by XRD, XRF, TG/DTA, and FTIR methods. The mechanical properties of the prepared samples were tested using a compressive strength test after 2, 28 and 56 days of hydration. The compressive strengths of 16 and 24 MPa after 28 days of hydration were reached for FA samples activated with water glass. The alkali activation of MK was successful in the NaOH solution of the molar concentration above 5 M. The compressive strength values of metakaolin, activated hydrothermally and hydrated at laboratory conditions, reached 11.2 and 5.5 MPa, respectively, for 5 M activator of NaOH.

Automated summary

Unfired ceramic bonding systems were prepared via the alkali activation process using metakaolin and fly ash. Long-term stability of mechanical properties was observed, with FA samples activated with water glass reaching compressive strengths of 16 and 24 MPa after 28 days. Metakaolin activated in NaOH solution above 5 M achieved a compressive strength of 11.2 MPa after hydration at laboratory conditions.