One kaolin and two kaolinitic claystones were characterized (XRD, XRF, FTIR, TG, SEM, MIP), dehydrated at 750 °C, finely ground and mixed with an alkali activator in weight ratios of 55:80, 55:75, 60:70, 65:65, 70:60 and 75:55. Geopolymer binders were then prepared by water addition up to the invariable content of 34 wt%. The binders were finally mixed with quartz sand in a weight ratio of 40:60. Differences in the properties between the two types of clay materials, changes in these properties due to dehydration and milling, and the impact of the observed differences on the properties of geopolymer binders (dynamic viscosity, pore size distribution, colour shade) and filled geopolymers (flexural strength, compressive strength, elastic modulus) were investigated. The kaolinitic claystones had a morphology different from the kaolin, characterized by smaller kaolinite plates, significantly lower pore volumes with average pore diameters and a higher impurities content that caused a more intense colour compared to kaolin. The above mentioned differences remained unchanged after thermal and mechanical treatment of the starting clay materials. The geopolymer binders prepared from the calcined and milled kaolinitic claystones were darker in colour and had a significantly lower dynamic viscosity (approximately three times) compared to geopolymer binders prepared from kaolin. Dynamic viscosity decreased with decreasing activator content, while volume and pore size increased. The mechanical properties of all the prepared geopolymers were excellent (compressive strength >50 MPa, bending strength >7.9 MPa and elastic modulus >15 GPa) up to a ratio of metakaolinite component/alkali activator of 1:1.