This chapter explores the development and properties of cementitious materials, focusing on Portland cement and alkali-activated cementitious materials (AACMs). The significance of Portland cement is attributed to its cost-effectiveness, availability, and mechanical properties. Its hydration process forms key products like calcium-silicate-hydrate (C-S-H), calcium hydrate (CH), and ettringite (AFt)/monosulfate (AFm), which influence concrete performance. AACMs, including geopolymers, offer environmental benefits by utilizing industrial by-products and exhibit superior durability through their N-A-S-H gel structure. However, further research is needed to fully understand their micromechanical properties. The interfacial transition zone (ITZ) between aggregate and cement paste is crucial for mechanical properties and durability of concrete. While ITZs in RAC are complex due to old mortar, modifications with nanomaterials show promise in enhancing performance. In contrast, geopolymer concrete has a stronger, less distinct ITZ. Ongoing research is essential to optimize these materials for sustainable construction.