Ordinary Portland Cement (OPC) is the most widely used type of cement in construction due to its excellent binding properties and availability. Hardened OPC concrete, formed by the hydration of OPC, is the backbone of modern infrastructure, providing the necessary strength and durability for buildings, bridges, roads, and other structures. The hardened state of OPC concrete is characterized by a complex microstructure comprising various hydration products that impart mechanical strength and stability. Understanding the behavior of hardened OPC concrete under severe environmental conditions is crucial for ensuring the longevity and safety of structures. These conditions include extreme temperatures, aggressive chemical environments, and mechanical stresses, which can significantly impact the performance and durability of OPC concrete. This section will delve into the fundamental aspects of hardened OPC concrete, its formation, and its significance in the context of cement and concrete technology.