Archaeological evidence in Greece proves that natural pozzolans have been used for mortar syntheses since antiquity. Mixing pozzolans with lime produces mortars with combined properties; on the one hand increasing their mechanical strength, and on the other improving their resistance to moisture. The syntheses produced this way are ideal for constructions that require resistance to water (water-tanks, aqueducts, renders) (Papayanni and Stefanidou, 2006). In the present study, research is dealing with the efficiency and reactivity of three natural pozzolans supplied from volcanic areas of the Aegean Sea, and furthermore with the effect of these materials to the physical and mechanical properties of the lime-pozzolan conservation mortars used for the restoration of archaeological monuments. The pozzolans studied are marketed volcanic products from Milos, Kimolos and Western Macedonia, and are supplied in various grain sizing (silt+clay <0.063mm, and coarser, up to 0.5mm or even up to 2mm). The practical aspect of the present study is indicated not only from the extensive usage of pozzolans on many significant restoration projects in Greece (Acropolis, Dafni Monastery, Nicopolis, Rhodes, Athos, Chania, Rhethymno), but also from the need of decoding the mechanism of reaction of such volcanic materials to conservation mortars. Sufficient amounts of the above mentioned materials were analyzed for their chemical and mineralogical composition using electron microscopy (SEM-EDX) and X-ray diffraction (XRD), respectively (Fragoulis et. al., 1997). The reactivity of the pozzolans was studied in the laboratory through their ability to react with calcium ions when dissolved in saturated solution (McCarter and Tran, 1996) and the remaining products were analysed with X-ray Diffraction (XRD). Further study investigated the effect of those pozzolans when used in lime-based mortars, relative to the ratio of the pozzolan participation within the binder, the sizing of the material and the curing period of the mixtures. The dry mortars were tested for their physical characteristics and microstructure using optical and electron microscopy (SEM), as well as for their compressive strength. In addition, the binding products of the mixtures were analyzed with X-ray Diffraction (XRD) and Thermal Analysis (DTA/TG). The outcome of the research proved that the ability of a pozzolan to react with lime depends on its sizing, as well as on its mineralogical composition. It was thus indicated that the reactivity of a pozzolan increases when the material is ground to smaller sizing, but when comparing different kinds of pozzolans, a coarser material may be of equal reactivity, or even greater, to a finer one. The latter proves that the mineralogy is the main factor that affects reactivity. The factors that contributed to the increase in strength properties were the mineralogical characteristics, the sizing, the ratio participation of the pozzolan within the mixture and the duration of curing of the mortars. The greatest strength-increase was observed in the mixture with 50% participation of pozzolan, ground below 63μm and after twelve months of curing. Compared to pure mortars, the pozzolanic syntheses achieved up to eight times higher values of compressive strength. Conclusively, when synthesising a mortar for a restoration project, the criteria that should be considered are firstly the composition of the materials, and secondly the grain sizing and their ratio participation in the binder, so that ultimately the mixture has the properties required.