The concrete we make today is nowhere even close to what the ancient Romans made some 2000 years ago. The ancient concrete was far superior in terms of quality. It was far more durable and far more environment-friendly.
The Romans mixed volcanic rock with lime and produced the concrete. When they needed to make underwater structures, they would mix volcanic ash with lime and create mortar. The mortar and volcanic tuff were then packed inside a wooden structure. The moment the entire thing came in contact with seawater, an exothermic reaction (chemical reaction in which heat is released) was triggered. Water molecules entered into lime structure and hydrated the lime. The hydrated lime then reacted with the volcanic ash, cementing the entire structure together.
The ancient maritime concrete made by Romans was studied carefully and it was found that Romans added aluminum, resulting in a completely different type of compound. Addition of aluminum formed what is known as C-A-S-H (calcium-aluminum-silicate-hydrate) as opposed to C-S-H (calcium-silicate-hydrate) of modern day Portland cement. C-A-S-H turned out to be exceptionally stable and strong binder.
Additionally, the addition of Aluminum allowed the formation of ideal Tobermorite crystalline structure, which is extremely durable and strong compared to modern day Portland cement, which claims that C-S-H resembles Tobermorite crystals but it is not true. Finally, some other minerals were also found in Roman concrete which have the potential of encapsulating hazardous wastes.