Optimization of kaolin into Metakaolin: Calcination Conditions, mix design and curing temperature to develop alkali activated binder

The natural kaolin deposits are crystalline inert with low CaO contents and surface area. Therefore, kaolin is not a suitable material used to produce Alkali Activated Binders (AAB) due to the low strength resulting. This work aims to optimize kaolin into metakaolin and use them later as AAB. Initially, kaolin was characterized to determine its chemical compositions and reactivity before and after calcination through XRF, XRD and Chapelle test. Kaolin is thermally treated at different firing conditions (500, 550, 600, 650 and 700 °C for 15, 30 and 60 min, i.e., 15 firing trials). The optimum calcination parameter revealed according to the Chapelle method is 600 °C for 60 min. Paste specimens were fabricated from selected metakaolin according to different pozzolanicity using 5, 10 and 14 wt% NaOH as activation media. The paste specimens were cured at 40, 60 and 80 °C for 24 hrs, then cured at room temperature with 98 ± 2 % relative humidity for 28 days. It is found that metakaolin paste with high optimum pozzolanicity produces AAB with higher mechanical compressive strength values (20.9 MPa) after curing at 60 °C. The latter mechanical compressive strength is maximized (27.09 MPa) when metakaolin pastes are activated by 14 wt% NaOH and 10 wt% Na₂SiO₃ mixture.