Residual Cement Paste (RCP) refers to the cementitious material that remains bonded to aggregate particles after the demolition of concrete structures. This material is a key component in recycled aggregate concrete, where it can affect the properties of new concrete in which the recycled aggregates are used. The mechanism behind RCP’s impact lies in its porosity and the old hydration products it contains, which can interact with new cement hydrating agents in a secondary concrete mix. These interactions can affect the hydration kinetics and the microstructural development of the new concrete, influencing its overall mechanical properties and durability.