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Admixtures used in concrete have been produced for conventional concrete and the behavior of these additives in geopolymer/alkali‐activated systems is still complicated. In this study, the effects of different types of water‐reducing admixtures on the fresh and mechanical properties of Class F fly ash‐based geopolymer mortars and alkali‐activated slag mortars were investigated. For this purpose, flow diameter, rheological examination, compressive and flexural strength tests were carried out. The data obtained showed that the admixtures used in the study did not have a positive effect on the fresh properties of geopolymer/alkali‐activated mortars, and the improvements in the flow properties were due to the extra water contributed by the admixture. The flow diameters of the mixtures containing the same amount of water with the mixture containing plasticizer were generally higher than those of the mixtures prepared with the admixture up to 10.3% in the fly ash‐based mortars and 15.4% in the slag‐based mortars. Addition of plasticizers reduced the compressive strength of fly ash‐based geopolymer and slag‐based alkali‐activated mortars up to 31.7% and 29.7%, respectively.