Roman concrete

I can remember when you could climb on top of the toppled ones.

They remarked that in the "old days" they hired out hammers and chisels so people could take samples of the stones home.

Can you still access the West Kennett Long Barrow? There used to be a finger-post indicating where you could walk up the hill and go inside it.

I think the Colosseum (and many other Roman buildings) is stone with a concrete infill isn't it? The Pantheon is a rare example of concrete cast in shutters.

They used the infill method for medieval cathedrals, but, in this case, the infill would either be rubble or lime mortar concrete, which could take literally centuries to harden completely, as this requires CO2 to migrate in and change the Ca(OH)2 to CaCO3.

Not a civil engineer, but we used to have a "geometra" (roughly = chartered building surveyor) in the house.

I was told, when I was little, that "concrete" (in Italian "cemento" or "cemento armato" or even better "calcestruzzo armato") is different from "cement" ( = "calcestruzzo").

The name "concrete" is supposed to be used only for the mix of cement and steel for XX century kind of high rise concrete buildings or bridges.

You can build a 1 or 2 floor house with cement and stones without steel reinforcement.

I am sure civil engineers in UK and in Italy use different and possibly confusing terminologies.

Cement is the binding material, e.g. Portland cement which is supplied as a fine powder. The word is also used for glues generally.

Concrete is cement plus aggregate, i.e. sand and stones of various sizes, i.e. gravel. Coarse aggregate is also called ballast, from its use to stabilise unladen ships.

And then there is mortar, which in my understanding is a simple cement-sand mix of various proportions depending on application, but which doesn't contain 'aggregate' as such.

The whole point of concrete is to have stones, which are strong, and fill in the gaps with smaller stones, which are strong and then fill the gaps in those with sand, which is strong and finally to fill in the gaps in the sand with cement, which isn't very strong!

Quite. Use of aggregate, which is generally cheap, saves a lot of cement, which generally isn't.

I would tend to use "cement" for the grey anhydrous powder - and NEVER for the hydrated solid: only Americans have that meaning.

Cement plus sand (mixed with water to hydrate it) gives mortar

Cement plus sand plus gravel (mixed with water to hydrate it) gives concrete

We've known for a long time it was based on lime. The secret ingredient to longevity appears to be rice or similar.

The size and type of the aggregate determines the compressive strength of the concrete mix