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Using geopolymers can reduce significant amounts of CO2-emissions during the production compared to Portland cement. Although illite/smectite clays are very abundant on earths crust and rich in SiO2 and Al2O3, studies of their geopolymerization potential are rare. Thus, the illite/smectite clay of Friedland (NE Germany) was calcined (850 °C) and ground to form a reactive metaclay and then mixed with synthetic gibbsite (to test the effect of Al-concentration) and 6 molar NaOH or KOH, in order to study their geopolymerization at 25, 50 and 75 °C within 28 days. The raw clay, the precursors, and the geopolymers were characterized by XRF, XRD, SEM-EDX, Flame-AAS, nitrogen adsorption and compressive strength test. 25 °C was too low to initiate the geopolymerization of illite/smectite. Increasing the curing temperature increased the reactivity of meta-illite/smecite. Si and Al dissolution was confined to the first 24 h, followed by the hardening of the geopolymers within 28 days. At 50°C, KOH-activation formed amorphous and mesoporous aluminosilicates, which significantly cemented the particles and agglomerates of the metaclay. Consequently, geopolymers with high compression strength (~38 N/mm2) were formed. Adding 10 wt% Gibbsite (precursor Si/Al = 2.1) to the metaclay strengthened the formation of amorphous aluminosilicates and increased the compression strength of the geopolymer by 20 % from 38 - 45 N/mm2. At 75 °C, the reactivity of the metaclay in NaOH was higher than in KOH. NaOHactivation at that temperature formed geopolymers with high compression strength (~30 N/mm2) due to the cementation by microporous phillipsite (K-, Na-zeolite) crystals. Thus, alkali-activation of the calcined and ground meta-illite/smectite from Friedland form high strength geopolymers under hydrothermal conditions.