The paper presents the results of research of high-strength geopolymer mortars, prepared from two types of siliceous fly ash. Sodium hydroxide and mixtures of sodium hydroxide and sodium silicate in the form of solutions were used as activators. The activator was added in amounts ranging from 6 to 15% NaOH in relation to the ash mass. After preparation, the mortars were cured at the temperature of 60°C, 80°C or 100°C for various times, from 24 hours to 7 days. Increasing the addition of sodium or a higher curing temperature increase the flexural and compressive strength of the tested mortars. The best results - compressive and flexural strength of 100 MPa and 20 MPa, respectively, were obtained for mortar activated with 15% sodium hydroxide, cured at 100°C for 24 hours. Blended activator – sodium silicate and sodium hydroxide also allowed to obtain high strengths, but lower than sodium hydroxide alone. Extending the curing time at high temperature beyond 24 hours did not produce favourable results in most cases. Fly ash geopolymer mortar, activated with 15% NaOH by mass of ash, cured at 100°C for 24 hours, showed about 18% reduction in greenhouse gas emissions compared to common Portland cement mortar.