Bricks and Mortar

The development of a strongly mortared rubble construction (usually but misleadingly called ‘concrete’) is commonly held to be one of the greatest technical achievements of ancient Rome. ¹ The secret lies in the careful mixture of the local volcanic sand (now called pozzolana) and a pure high-quality lime, to produce a hydraulic mortar of great strength easily comparable to the best of modern mortars. Without it, it can be argued, the great domed and vaulted structures of such architectural and engineering masterpieces as the Pantheon and the imperial thermae would not have been possible. On the other hand, Roman ‘concrete’ has been characterized as a quick and economical substitute for ashlar masonry, being both cheap in terms of the cost of the basic materials and also far less dependent on highly skilled labour. Its relative cheapness is thought to be what made it feasible also for everyday domestic and commercial construction where solidity and fire resistance were sought after, as demonstrated so vividly in the warehouses and apartment blocks of imperial Ostia. Indeed, Coarelli (1977, 16–17) has argued that it was largely the upsurge in private building in Rome during the second century BC that led to the development of the technique in the first place.