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 Roman concrete

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PostSubject: Roman concrete   Wed 05 Jul 2017, 10:35

Scientists have discovered new information about why Roman concrete used in harbours has proved so resilient.  It contains aluminium tobermorite, which crystallises when exposed to seawater.  Rather than decaying, over time the crystals have continued to spread and reinforce the concrete.  As well as explaining the durability of the ancient building material, it has been suggested that switching to a Roman-style concrete could reduce the environmental impact of certain building projects:

http://www.msn.com/en-gb/news/world/mystery-of-2000-year-old-roman-concrete-solved-by-scientists/ar-BBDLPqT?li=AAmiR2Z&ocid=spartandhp
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PostSubject: Re: Roman concrete   Thu 06 Jul 2017, 12:07

This isn't really news ... the science of Roman concrete is hardly a "2000 year old mystery", having been well understood for many decades (I distinctly remember tobermorite being mentioned in relation to concrete, ancient and modern, in materials science lectures in about 1980). Nevertheless Roman concrete was amazing stuff,  although of course they were fortunate in having access to large quantities of the key ingredient, volcanic ash. That's not to diminish their technical sophistication by any means as they put a lot of time, money and effort into improving the mix and developing its use in construction. But the best Roman saltwater-setting concrete was still dependent on them having large supplies of the right sort of volcanic ash. It should be noted that elsewhere in the Empire where the required pozzolana was not readily available (eg in Britain) most of their concrete structures had to be made with a simpler burnt lime and sand cement, unless the project was important enough to warrant the importation of volcanic ash from Italy (an alternative of using finely ground-up ceramic pots, bricks and tiles was sometimes used, but again it wasn't cheap).

Simple burnt lime mortar doesn't last as long as good Roman pozzolana-type concrete, and is certainly not at all suitable for use in seawater or indeed any water-saturated situation, but it is not inherently greatly inferior in general construction so long as it can be protected from water penetration. This sort of lime mortar is basically the type that was used to built all the medieval cathedrals and castles of Europe, and so long as it is protected from water penetration both from above and rising by capillary action from the ground, (the saying was that it needed only "a good hat and good shoes"), and repointed every few decades or so, it will easily last many hundreds of years. Simple lime mortar actually never completely sets and so it can accommodate movement and cracks, and will slowly self-heal over time in a way that modern Portland-type cement/concrete doesn't do. Nevertheless as long as Portland cement/concrete is protected from water infiltration it will last a very long time ... certainly there are buildings still standing firm that were constructed in the very first years after Portland cement was developed over two hundred years ago, and harbour walls still intact from the 19th century. What often reduces the life of modern concrete is the use of steel reinforcing bars which can chemically corrode the concrete from within as well as providing a route for water penetration.

Roman type concrete may well be the best option for constructing modern sea walls but I thought pozzolanic material (either natural volcanic ash or as fly-ash produced as the by-product of burning coal for electricity generation or iron smelting) was already added to Portland cement when building sea walls or where a less porous concrete is needed, ... or even sometimes just to reduce cost where fly-ash is cheaply available and would otherwise be a waste product. But I'm not sure how extensive are the deposits of the correct type of volcanic ash.
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