A section of the First Century BC Roman wall of Empuries (Ampurias) in Spain.
The base of the wall was made of calcareous rock while the upper portion is of Roman concrete (opus caementicium). Photo by Mark Cartwright, Creative Commons
Cement is a human-made conglomerate comprised of sand and gravel aggregates with calcined lime and clay. It is mixed with water to form mortar or mixed with sand, gravel, and water to make concrete. Concrete is a mixture of broken stone or gravel, sand, and cement. In ancient times concrete often contained crushed seas shells.
Cement-matrix composites include concrete (containing coarse and fine aggregates), mortar (containing fine aggregate, but no coarse aggregate), and cement paste (containing no aggregate, whether coarse or fine).
The ancient Romans built extremely durable sea walls using a concrete made from lime and volcanic ash to bind with rocks. Rather than eroding in the presence of sea water, the material gained strength from the exposure. Scientists have discovered that elements within the volcanic material reacted with sea water to strengthen the construction.
“Contrary to the principles of modern cement-based concrete, the Romans created a rock-like concrete that thrives in open chemical exchange with seawater,” reports Marie Jackson (University of Utah) in the journal American Mineralogist.
Mixing the concrete with limestone-producing bacteria allowed for cracks to self-heal. The bacteria, either Bacillus pseudofirmus or Sporosarcina pasteurii, are found in highly alkaline lakes near volcanoes, and are able to survive for up to 200 years without oxygen or food. They are activated when they come into contact with water. They then use the calcium lactate as a food source, producing limestone that closes up the cracks.
Related reading: Ancient Roman Concrete was Incredibly Strong