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Early hydration (up to four weeks) of five ordinary Portland cement pastes with different water-to-cement ratios (0.3, 0.4, 0.5, 0.6, and 1.0 wt) was investigated through positron annihilation lifetime spectroscopy. Measurements were performed at four different hydration times (1 day and 1, 2, and 4 weeks) and compared with results obtained from thermogravimetric analyses. Deconvolution of positron spectra allowed us to separate orthopositronium annihilations in the evacuated gel pores from those in the pores containing water, and therefore, to monitor the pores’ evolution with aging time. It is found that the concentration of gel pores increases with aging time, as well as by increasing the content of water in the paste; on the other hand, the typical sizes of the pores do not show significant variations, either with aging time or with the water-to-cement ratio.© 1999 American Institute of Physics.