Long-term sulfate resistance of synthesized cement systems with variable C3A/C4AF ratio at low temperature or ambient conditions: Insights into the crystalline and amorphous phase assemblage

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 degrees 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-S-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-S-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-S-H decomposition was contributed by available sulfates.

Automated summary

Synthesized cement systems with different C3A/C4AF ratios were tested under long-term humid conditions to assess the changes in crystalline and amorphous phases. The findings indicate that systems with C3A/C4AF < 1 better preserved the integrity of the C-S-H phase and prevented thaumasite formation. Addition of calcite inhibited carbonation reactions.