Concrete was usually covered as concrete walls were considered unaesthetic. Roman builders covered building walls with stones or small square tuff blocks that would often form beautiful patterns noting that brick faced concrete buildings were common in Rome especially after the great fire of 64 AD.
There were many variations of concrete and Rome even saw the Concrete Revolution which represented advances in the composition of concrete and allowed for the construction of impressive monuments such as the Pantheon. For example, Roman builders discovered that adding crushed terracotta to the mortar created a waterproof material which could be then be used with cisterns and other constructions exposed to rain or water.
Romans mastered underwater concrete by the middle of the 1st century AD. The city of Caesarea gives us an impressive example of Roman construction. The production technique was quite incredible: the mix was one-part lime for two-parts volcanic ash, and it was placed in volcanic tuff or small wooden cases. The seawater would then hydrate the lime and trigger a hot chemical reaction which hardened the concrete.
Actually it has been argued that the concrete used by the Romans was of better quality than the concrete in use today. Recent research from US and Italian scientists has shown that the concrete used to make Roman harbors in the Mediterranean was more resistant than modern concrete (known as Portland cement).
The production process was dramatically different. Portland cement is made by heating clays and limestone at high temperatures (various additives are also added) while the Romans used volcanic ash and a much smaller amount of lime heated at lower temperatures than modern methods.
For example, Roman harbors remain intact today after 2,000 years of waves breaking on the harbors' breakwaters whereas Portland concrete begins to erode in less than 50 years of sea battering. The concrete from ancient Rome also had bending properties that Portland concrete does not have due to its lime and volcanic ash, which explains why it does not crack after a few decades.
Incredible facts about Roman concrete