<p>Calcium silicate hydrate (C-S-H) and calcium aluminum silicate hydrate (C-A-S-H) are the primary binding phases in Portland and blended cementitious systems, commonly retaining an amorphous to semi-crystalline structure. The longevity as well as the mechanical performance of such cementitious composites largely rely on the characteristics of C-S-H. On the other hand, ancient Roman concrete contains the crystalline analog of this phase – tobermorite, which is often attributed as the primary source of the extraordinary durability of this cementitious composite. With a goal of understanding the root cause of the enhanced durability of ancient and modern cementitious composites, this study investigated how amorphous and crystalline forms of C-S-H/C-A-S-H phases are affected by decalcification-induced degradation. Two different decalcifying environments were evaluated – using accelerated carbonation and NH₄NO_3. In-situ Fourier-Transform Infrared (FTIR) spectroscopy was utilized to evaluate the decalcification of these phases for 18 h. The findings indicate that during this period, C-S-H was completely decalcified under accelerated carbonation, as reflected by the decrease in Q² silicate species in the network and the formation of silicate species with a higher degree of polymerization (Q³ and Q⁴). Conversely, the crystalline version of these phases (i.e., tobermorite) showed minimal impact from decalcification, with only a 10-15% decrease in Q² under the same exposure condition. This decalcification observation was corroborated using NH₄NO₃, yielding similar results. Optimization of low-carbon alternative binders can facilitate favorable conditions for the crystallization of C-S-H and C-A-S-H, thereby fostering the development of a sustainable and durable cementitious composite.</p>