Hydraulic cement is a type of fast-setting cement that hardens or sets when combined with water. It is specifically designed to stop water flows and is frequently used in applications dealing with active water leaks. The chemical reaction from mixing it with water can cause the cement to expand and form a tight seal, thus blocking off water, even under pressure. For this reason, hydraulic cement is often used for repair work on concrete structures such as foundations, tunnels, manholes, sewers, swimming pools, and basement walls. It is highly durable and resistant to water damage, making it ideal for both interior and exterior applications.