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This paper presents the results of tests of physical-mechanical, durability and microscopic properties of geopolymer mortar mixtures based on fly ash and ladle slag. The mixtures are alkali-activated using sodium silicate and sodium hydroxide solution. Firstly, the effects of different fly ash (class „F“) particle sizes on the characteristics of the mortar mixtures were examined when binder and alkali activator were cured at 95 ºC for 24 h, also pozzolanic activity and strength activity index were investigated. After that, fly ash ground of optimal particle sizes (0,09 mm) was replaced with ladle slag, 0 to 20 % of the mass, the replacement steps being 5 %. The specimens having dimensions 4x4x16 cm were then cured in ambient conditions, and the effects of replacement of a part of fly ash with ladle slag were determined by testing water absorption, flexural and compressive strength, freeze-thaw resistance, sulfate attack, ultrasound velocity, FT-IR spectroscopy and leaching of heavy metals. According to the test results of compressive strength resistance of geopolymer mortars exposed to sulfate solution, the mortar made with fly ash and ladle slag showed better resistance to sulfate attack than the mortar made with fly ash only. © 2020, International Institute for the Science of Sintering (IISS). All rights reserved.