Why was Roman concrete so durable?

Massive Roman structures made from concrete thousands of years ago still stand today.

When we think of concrete we also think about cracks and chips, but concrete from the Romans has stood the test of time, but why?

The monuments have stood up against severe weather and even earthquakes and water.

In modern times, concrete is a mixture of cement, air, water, sand, and gravel.

An MIT study found that the Roamn concrete has Pozzolana, a material that, when finely divided, forms and in the presence of moisture will chemically react with alkalis to form cementing compounds.