 The Romans made durable concrete strong enough for load-bearing walls. Roman concrete contains a rubble of broken bricks and rocks set in mortar. The mortar included lime and pozzolana, a volcanic material that contributed significantly to its strength. Roman concrete structures such as the Colosseum, completed in 80 AD, still stand. Their longevity may be explained by the fact that the builders used a relatively dry mix of mortar and aggregate and compacted it by pounding it down to eliminate air pockets. Although derived from earth products, concrete structures would not usually be considered earth structures.
