Evolution of metakaolin thermal and chemical activation from natural kaolin

In the present paper, we study the combined effect of thermal activation (600 °C/2 h and 750 °C/2 h) and chemical activation with 1% ZnO on the reactivity of metakaolinite (MK) obtained from natural kaolin. The phases are identified by chemical (ICP/MS), mineralogical (XRD), and morphological (SEM/EDX) characterization of all products, as well as the evolution and stability over time of the hydrated phases generated during the reaction, to determine their use as pozzolan in the manufacture of cements. The stability analysis for the kaolin/lime system activated chemically and thermally at 600 °C/2 h shows that the C-S-H gels are thermodynamically stable after one day of reaction, evolving the system to the stability field of stratlingite for the other analyzed times. At 750 °C/2 h, the thermodynamically stable reaction phases are C-S-H gels. Calcination at 600 °C/2 h and the addition of 1% ZnO are the optimal conditions for thermal and chemical activation, to improve the pozzolanic reaction and promote the replacing part of the cement for developing secondary reaction products.