The Romans knew how to make concrete. The Pantheon (http://en.wikipedia.org/wiki/Pantheon,_Rome), a building with a concrete dome 4,535 metric tons in weight and 43.3 meters across with supports only on the edge, has been standing for around 2000 years. Try that with a bag of ‘quickset’!
Recently there has been media hype about how the ‘mystery’ of Roman concrete had been solved. Well, that’s interesting… Vitruvius wrote down the ingredients and method in ~25BC. For the last hundred odd years variations of the mix have been used for all sorts of civil structures where extra strength was needed (http://www.romanconcrete.com/docs/spillway/spillway.htm). However, what really happened recently was Researchers studied and Explained HOW Roman concrete works so well. (http://newscenter.lbl.gov/news-releases/2013/06/04/roman-concrete/ and http://onlinelibrary.wiley.com/doi/10.1111/jace.12407/abstract)
The trick is in the molecular structure of the mix. In normal Portland cement, the chemistry is called (in concrete chem terms ) C-S-H. In highschool chem thats lime + various hydroxyls. Roman concrete on the other hand has a concrete structure of C-A-S-H… the A is aluminium. The form of that change in structure is now understood as crystal ‘fibres’ that occur in the cured concrete, effectively creating a type of rock (Al-tobermorite). The added Al (which substitutes for some of the Silicone) also result in chemical stability, the concrete is less reactive so doesn’t rot like Portland cement in the presence of seawater or other reactive chemicals. Incidentally the photo of the Al-tobermorite crystals makes me wonder what effect salting concrete with carbon nanotubes, or even some other type of crystal, may have.
The ingredient that makes this change is Pozzolan Ash – a volcanic ash found originally in Pozzuoli (Naples), that is rich in aluminium. More conveniently it has been found that the ash that rises from coal burning (called Fly Ash) has similar qualities to pozzolan ash, so makes a suitable substitute for the creation of romanesque concrete – also called Green Concrete because it is sequestering CO2 as a byproduct rather than requiring extra energy (coal power) to make.
Up next – how to make Roman Concrete, without using vast amounts of energy, and using ingredients that are easy and inexpensive to make…
24th December 2013 – Merry Christmas, here’s some more concrete for those stockings!
Okay, here’s my thinking – and please remember! I’m not a chemist, so what follows is quite probably a load of rubbish (if you Are a chemist who understands this stuff, please tell me what went wrong here!)
Lets look at the fly ash, and whether it can be substituted with something that does not require the burning of fossil fuels.
Fly ash provides its qualities to roman concrete via ‘cenospheres’ (http://www.wvcoal.com/Research-Development/australia-and-california-synthesize-coal-ash-microspheres.html)(http://en.wikipedia.org/wiki/Cenosphere), which are ‘spherical inorganic hollow microparticles comprising the lightest component of fly ash’. They provide immense strength and low reactivity with light weight and react with lime (with heat) to form crystals.
It is standard practice to enhance the quantity of cenospheres in fly ash (for commercial use) by baking it with sugar and sawdust (or various other mixes of carbohydrates). But making these things from scratch is an energy intensive process.
BUT fly ash is used because it is Similar to Pozzolan Ash – not identical! Now I jump back to Pozzolan Ash. There are a few types (http://en.wikipedia.org/wiki/Pozzolan) that may be helpful – lets look at zeolite.
Zeolites are microporous aluminosilicate minerals, observed by Axel Fredrik Cronstedt in ~1756. They are great for water purification and nitrogen extraction, and as catalysts in various processes. And they can be made to order (http://www.lookchem.com/Chempedia/Chemical-Technology/Inorganic-Chemical-Technology/3012.html)(http://materials.binghamton.edu/labs/zeolite/zeolite.html).
Thanks to Mr Takeshi Mitsuda of the Material Research Laboratory, Nagoya Institute of Technology Japan (https://www.jstage.jst.go.jp/article/minerj1953/6/3/6_3_143/_pdf) there may be easier ways to produce the type of zeolite that Roman Concrete could be made from. Here’s my first take on the recipe:
1. Mix sodium aluminate and sodium silicate to produce aluminium rich zeolite.
2. Add lime.
3. Use resultant mixture as cement!
Unfortunately sodium aluminate and sodium silicate are generally manufactured in energy intensive processes…
Recipe IIa – Sodium Aluminate:
1. In a steel or nickel vessel heated by steam, bring a mixture of aluminium hydroxide (gibbsite) with sodium hydroxide to boiling point. Let cool.
2. Crush the resultant sediment.
(could one use raw bauxite and sodium hydroxide to directly produce the sodium aluminate?)
Recipe IIb – Sodium Silicate (Waterglass):
1. Mix seawater with quartz powder.
2. React with an alkali.
Combine IIa with IIb, add lime and voila! Cement.
But can all this be boiled down to:
1. Quartz + water + bauxite + NaOH = Zeolite (react in solar oven?)
2. Zeolite + Limestone = Roman Concrete
3. RC + seawater + aggregate + labour = really hard long lasting building material?
One day I’ll try to mix all this and see what happens 🙂
Sometime soon I’ll have a go at working out if the chemistry makes any sense. Wish me luck!