Results of interrelated tests for physical-chemical and microstructural properties of four synthetic pozzolanas evaluating the hydration rate in blends with calcium hydroxide are presented. The pozzolanic activity of silica fume, classified fly ash, raw fly ash and commercial metakaolin was estimated. Results showed a clear relationship between the calcium hydroxide consumption and the reactive phase content, specific surface area and porosity of the pozzolans. The calcium hydroxide-silica fume blend consumed 98% of portlandite after 21 days, whereas the system with metakaolin also presented an effective pozzolanic activity with 94% depletion. The fly ashes were the less effective pozzolans of the study.