Metakaolin-based geopolymers are increasingly recognized as durable alternatives to Portland cement owing to their superior chemical and mechanical resilience in aggressive environments. Yet, a comprehensive thermodynamic framework for predicting their long-term stability across different pH conditions remains underdeveloped. In this study, we apply a thermodynamic modeling approach to simulate the dissolution and phase behavior of geopolymer gels over an extended pH spectrum representative of nitric acid and NaOH exposure. Model predictions were validated against published experimental datasets, with emphasis on aqueous sodium concentrations and solid–solution phase transitions in aluminosilicate gels. To address limitations in previous approaches, protonated gel and silica gel phases were introduced as essential endmembers in the proposed ideal solid-solution model. Their inclusion enables accurate representation of alkali–proton exchange processes that control sodium leaching under near-neutral to alkaline conditions, as well as the onset of dealumination under acidic attack. The model reproduces the pH-dependent release of sodium and aluminum, capturing the coupled mechanisms of alkali loss and framework destabilization. These results demonstrate the capability of ideal solid-solution N-A-S-H models to provide mechanistic insights into the chemical stability of geopolymers. Beyond fundamental understanding, the framework supports mix design optimization to enhance acid resistance and offers a predictive basis for assessing the service life of alkali-activated materials in aggressive environments.