Point Arising

This is the general term used by the Romans for their concrete, i.e. binding agent, water and aggregate. What made their binding agent different from regular lime mortar (sand and quicklime i.e. Calcium Oxide) was the addition of a volcanic powder/dust called Pozzolona instead of the sand. This turned regular lime mortar, which hardens due to the Carbon Dioxide in the air, into something truly remarkable - a hydraulic cement (sets under water). Because regular lime mortar needs the presence of air it can only be used in thin layers, the CO₂ can't get far inside the mix to set. The addition of this volcanic powder meant that the setting took place without the need for air, and Carbon Dioxide, to get to all parts of the concrete due to a chemical reaction between the volcanic ash and the slaked lime; it therefore set under water too. More importantly, large volumes could be cast which would set. This was the Roman secret. The crowning glory of Roman concrete technology was the cast concrete Pantheon dome in Rome. [Present Portland cement still contains lime and substances chemically similar to the Pozzolan ash and works in the same fashion.]

was another kind of concrete that did not use Pozzolona volcanic ash but instead, originally, had a type of a pinkish orange kaolin clay from Signia in Latium added to it, hence its name. This too was a hydraulic concrete and waterproof. Some time later the Romans found that a type of concrete could be made with sand, lime and crushed terracotta bricks/tiles as a substitute for the kaolin clay from Signia. This is the concrete we often see in Britain since there was no volcanic ash or pink kaolin clay to be had in Britain. They may have imported small quantities, of course, although I doubt it. It is the pinkish (crushed brick) concrete we see in UK hypocaust systems. Vitruvius wrote extensively on it like many other architectural subjects.

Opus reticulatum - uses shaped blocks of tufa set in opus caementicium . Looks like a diagonal net.

Opus spicatum is a type of paving made from small bricks laid in a zigzag pattern like a parquet floor.

Kentish Ragstone

Much of the Roman stone building in Kent used ragstone and most of it came from the Maidstone area. There are several quarries around the town. The lack of suitable stone for building in East Kent may point to the reason why not so many villas, being made of wood, have been found in this area; while at west Kent, nearer Maidstone and Sevenoaks, rock is available for building and villas survive. Ragstone was available too on the escarpment overlooking Romney Marsh. The ragstone bearing bed runs along the line from Hythe to Maidstone to Westerham. Here it can be seen as ashlar blocks on the north wall at Richborough with flint cobbles below.

Ironstone

Used here merely for a decorative chequerboard effect - alternate black and white blocks in the Richborough north wall. Not sure where this comes from in Kent although it is generally associated with chalk. I read somewhere it may have been found in Folkestone and Petham near Canterbury. On the Sussex border it was sometimes found and used for smelting iron. Note White yardstick.

Tufa

This is a lightweight kind of limestone rich in carbonates often found in volcanic areas or mineral springs and it too is found in Kent at Westingbury near Maidstone. It was used for infilling walls in Roman times, it was also a good thing to add to concrete. This image shows cut tufa blocks (with plants) in the north wall at Richborough. Unusual for wall building since the Romans favoured it for its lightness (see holes) for ceilings and vaulting. Its presence here along with pieces of roofing tile below show that old buildings on the site were robbed to build the Saxon Shore fort walls.

Tiles

Often called lacing or levelling or bonding courses, tiles are found in many Roman walls and are not only decorative but structural was well. Sometimes even roofing tiles (tegulae) are incorporated in walls when old structures are torn down. He is a inverted tegula from Richborough

Flint

Flint cobbles are found all over the place especially in the Hythe and Folkestone areas on the beach. Any flints that might have been in fields have probably long since been removed (not too good for the ploughshare I guess). All the forts used flint cobbles in their walls

Quenin Gate Photo & Diagram (after Marian Day?)

Greensand

The Hythe Beds (sic) at Lympne (and elsewhere I imagine) provide some large Greensand blocks like the ones used in the Quenin Gate at Canterbury, some robbed greensand blocks can be seen in the arches of the church at Reculver too.