Natural pozzolans were used as pozzolanic cement in concrete mixtures to increase the long-term strength, the concrete durability, and other material properties of Portland cement many centuries ago. The most common pozzolan is the volcanic ash, a siliceous or aluminosiliceous material, which is highly vitreous, coming from volcanic eruptions. In the Greek territory, 39 reactive volcanoes have been recorded both in insular and in terrestrial areas. The reactivity of pozzolans, in the case of lime-based mortars, is attributed to their content in amorphous silica which reacts with Ca(OH)2 from lime, in environmental conditions, and forms C–S–H compounds responsible for the strength gain. Their use in building materials was diachronic. The significant properties of mortars containing pozzolans derive from the mechanism of its gradual strengthening attributed to the reaction of silicates with lime to form secondary cementitious phases which increase the durability and the dense structure of the mortars. In the present paper, two natural pozzolans from Greece are analyzed in order to record their morphological and analytical microstructure as well as their thermal and physical properties. The results revealed that the pozzolans tested, are materials of high quality and can be used for the production of compatible repair mortars. Also, valuable criteria could be instituted for the selection of reactive pozzolans which could be used for conservation purposes. Among others, crucial parameters for compatibility between old mortars and new ones are the surface features (color, texture, and roughness), the composition (type of binder, granulometry of aggregate), and the pore size distribution.
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