The hydration process of alkali-activated slag cement, especially at early-age, was studied by FT-IR, compared with that of Portland cement. The results show that during the hydration of alkali activated slag cement, two processes have taken place, the dissolution of Al3+ from the slag and then the recombination of Al3+ ions with the silicate anions. The former associated with the break of the glass network of slag and the later with the re-polymerization of the silicate and aluminosilicate anions. The rate of break of the glass network increases with the dosage of water glass, which completed in 24 h when the dosage of the water glass solution equivalent to Na2O exceeds 6% of the slag powder. It is confirmed by the IR study that Ca(OH)2 is absent in the hardened alkali activated slag cement paste.