Synthesized cement systems made with variable C₃A/C₄AF ratios, containing C₃S, gypsum and, optionally, calcite, were stored long-term at humid conditions at 5 or 20 °C, without any protection against atmospheric carbonation. Analytical techniques able to assess both the crystalline and amorphous phases were used. Experimental results were compared with thermodynamic simulations. The systems with C₃A/C₄AF < 1 better preserved the soundness of the C[sbnd]S[sbnd]H phase, which hosted iron, and prevented thaumasite formation. The addition of calcite in these systems inhibited carbonation. When occurred (mixtures without calcite), the carbonation was significantly more intense at ambient temperature. In the systems that underwent extensive deterioration, cross-linking of silicate structures, AFt decomposition, and iron release from the deteriorating C[sbnd]S[sbnd]H, occurred, while Al-incorporating amorphous silica, calcium carbonate polymorphs and hydrous iron oxide formed. The presence of unreacted C₃A in the systems with C₃A/C₄AF = 1, suggested that C[sbnd]S[sbnd]H decomposition was contributed by available sulfates.