Calcium (alumino) silicate hydrate (C-(A)-S-H), the principal binding phase in cementitious systems, is typically poorly ordered at ambient conditions but transforms into mineral-like phases under subsurface high-pressure high-temperature (HPHT) conditions. Model systems such as synthesized C-(A)-S-H, prepared at controlled Ca/Si and Al/Si ratios under saturated steam pressure, are ideal for predicting thermodynamic stability. However, they do not capture the complexity of cementitious systems, where multiple hydration products and secondary phases form concurrently and coexist with unreacted or partially reacted starting materials during setting under subsurface conditions. To better represent the complexity of this evolving phase assemblage, we followed a performance-based approach and developed realistic formulations incorporating different aluminosilicate sources. We investigated the microstructural evolution as a function of the starting oxide composition, temperature, pressure, thermal cycling, age, and medium acidity. In the first study, we establish a link between macroscopic performance and early-age phase development via thorough phase identification methodologies. Second, we demonstrate that, at critical transformation temperatures, long-term phase alterations of mineral-like hydration products are governed by the ionic medium and thermal cycling rather than by age alone. These findings highlight the importance of tracking phase evolution and transformation kinetics in realistic cementitious systems to understand the long-term chemical durability of subsurface infrastructure.