Avaliação da formação de etringita por modelagem termodinâmica do sistema CAO–Al₂O₃–CaSO₄–NaCl em ambiente com CO₂

Abstract

Portland cement is the primary material used in civil construction, accounting for approximately 90% of buildings worldwide. During hydration, clinquer forms phases such as C-S-H, portlandite, and ettringite, which are essential for the mechanical development of concrete. Ettringite plays a dual role: when formed in a controlled manner at early ages, it enhances initial strength and microstructural densification; however, its delayed or excessive formation—especially in environments with high humidity, extreme temperatures, or the presence of ions such as carbonate and chloride—can lead to expansion, cracking, and premature deterioration. This study investigates, through thermodynamic modeling using the GEMS software, the factors that govern the formation and stability of ettringite, with emphasis on the effects of minor compounds such as carbonates and chlorides. The simulations are based on Gibbs free energy minimization, enabling the prediction of stable solid phases and dissolved species. Results indicated that the highest ettringite formation (~41 g) occurs at the 2:5:4 mass ratio of the CaO–Al₂O₃–CaSO₄ system. The addition of CO₂ promotes the dissolution of carbonate species and shifts the pH toward favorable conditions for ettringite formation, although with no significant increase in its quantity. The approach based on Taylor’s (1997) stability criteria demonstrated the robustness of the model, revealing the influence of salts on the formation of new phases and on the reduction of ettringite. These findings reinforce the potential of thermodynamic modeling as a predictive and strategic tool for optimizing performance and durability in cementitious systems.