Final answer:
Pozzolana is the siliceous material that reacts with slaked lime to form a slow-hardening cement, crucial to the construction of Roman concrete.
Step-by-step explanation:
The siliceous material that reacts chemically with slaked lime (calcium hydroxide, Ca(OH)2) in the presence of moisture to form a slow-hardening cement is called pozzolana. This material, volcanic ash coming from around the area of Mt. Vesuvius, near ancient Pozzuoli, contributes to what is historically referred to as Roman concrete.
When mixed with slaked lime and water, it undergoes a chemical reaction that results in a durable cementitious material. This discovery enabled the Romans to build structures that were both strong and could set underwater, revolutionizing construction and architecture in their time.