From Command and Conquer Wiki
A Second Tiberium War Tiberium Silo
Tiberium Silos were prefabricated structures that were used to store excess Tiberium that Tiberium Refineries could not hold. These structures looked very different throughout the progress of the three Tiberium Wars, but generally were roughly dome-shaped.
In the First Tiberium War, silos were small structures that would hold Tiberium; if sold, it would produce a technician, and if no excess capacity in other Silos and Refineries existed, the stored Tiberium would be lost.
In the Second Tiberium War, the silos grew a bit larger, and would hold a bit more Tiberium. However, as a consequence, the silos would also be easier to destroy. Destruction of a full Silo scattered its contents in the immediate area, devastating if the silo was full of Tiberium Vinifera due to its explosive qualities. Unlike all other versions of both the Tiberium Silo and the Ore Silo, rather than storage being spread evenly across all storage buildings, one silo would be filled entirely before the next one was filled.
In the Third Tiberium War, the silos gained ventilation towers (for the Global Defense Initiative) or cooling towers (for the Brotherhood of Nod). Also, in the Third Tiberium War, silos also became harder to destroy, but could hold less Tiberium than before. The Scrin never use them because they can store an infinite amount of Tiberium. If a silo were to be destroyed, green Tiberium crystals would spawn around the silo's former location. This feature was exploited by Nod forces during the Third Tiberium War where they destroyed Tiberium silos in Washington DC in-order to seed Tiberium.
There are also some civilian silos originally constructed as part of a Tiberium farming operation by civilian companies. These can be captured for a one-off payment of 5000 credits and can also be used as functional silos for your own operations.
[edit] See Also
Global Defense Initiative First Tiberium War Arsenal 
Brotherhood of Nod First Tiberium War Arsenal 