Pozzolanic reactivity of three kaolinitic clays (R1, R2, and R3) was assessed. The influence of gibbsite on pozzolanic reactivity of the calcination products was evaluated. Raw materials were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and thermogravimetric analysis techniques. The three samples were calcined at 600, 750, and 900 °C for 1 h. The assessment of the pozzolanic reactivity of calcined clays was made from alkali-soluble aluminum and silicon monitored by Atomic Absorption Spectrometry (AAS). Compressive strength of LC3 blends was also carried out to assessing the reactivity of calcined clays. The kaolinite is identified as the main clay mineral in the samples with similar content for three sample, while the associated minerals are hematite, goethite, and gibbsite in samples R1 and R2. The amount of alkali-soluble aluminum at 600 and 750 °C seems to be influenced by the presence of gibbsite in the sample. This effect decreases with increasing temperature up to 900 °C. For the gibbsite content studied, no influence on the compressive strength was observed.