This study investigates the effects of carbonation following short curing periods (72 hours) on CEMI and composite cement systems (30% PFA, 30% & 60% GGBS).Modifications in carbonation behaviour were observed compared to idealised/28 day lab cured samples and accelerated carbonation testing.Carbonation of Ca(OH)2 and C-S-H did not occur simultaneously, with decalcification of C-S-H only beginning once no more Ca(OH)2 was available.Decalcification and dealumination of the C-S-H phase occurred following exposure to ambient [CO2], and CaCO3 microcrystals were observed in the outer product (Op) regions only.A reduction in the Ca/Si ratio of the Ip C-S-H appears to be a result of migration of the Ca ions, driven by a concentration gradient.Furthermore, the rate and extent of carbonation and the nature of the carbonate species formed is dependent on the level of replacement, the replacement material and the degree of hydration.Predominantly calcite was the polymorph formed, however some vaterite was also observed in the samples containing PFA.STA data indicated the production of a crystalline carbonate phase resulting from carbonation of Ca(OH)2 and a metastable, poorly crystalline carbonate phase resulting from the carbonation of the C-S-H phase.