Three different metakaolins processed by a Portuguese factory were studied aiming to assess the influence of chemical and mineralogical composition of metakaolin on mortar characteristics. Mineralogical analysis of both the fine and clay fractions was carried out by X-ray diffraction. Chemical composition (major elements) was analyzed by X-ray fluorescence. Mineralogical composition reveals some significant differences between the studied samples, in both analyzed fractions, metakaolin 2 being richer in kaolinitic minerals. From a chemical point of view, siliceous content is always high (aroun d 60 %) as well as Al2O3 content (around 30 %); once again metakaolin 2 is the one richer in Al2O3 and poorer in sodium and potassium. Metakaolin from the three different batches was incorporated in lime mortars in order to produce a pozzolanic reaction and hence enable them to harden in high relative humidity conditions or, when access to CO2 is limited, as in the case of mortars supporting glazed tiles. Mortars with the same volum etric ratio were tested for flexural and compressive strength at ages of 28 and 90 days, and the module of elasticity was dete rmined. A relation between the mechanical strength achieved by lime mortars with the addition of metakaolin from different batches and metakaolin mineral and chemical composition was assessed. The knowledge of the influence of the metakaolin com position on the strength of the mortars is an important step in improving their performance and extending their application., Ana Luisa Velosa, Fernando Rocha and Rosário Veiga., and Obsahuje bibliografické odkazy