John started his talk by introducing us to concrete as a man-made rock. Many of us are familiar with mortar, which is a mix of cement sand and water, used to construct masonry walls. Mortar is not a particularly strong construction material but when the sand is replaced with aggregates we get concrete, which of course is.
Concrete has a very high compressive strength but is weak in tension, and this is why steel reinforcing is used in constructions such as motorway bridges. Steel is strong in tension, and the combination of concrete and steel reinforcing produces a structure with sufficient compressive and tensional strengths.
John ran us through very quickly the plethora of applications of concrete, from roof tiles and beams in construction to motorways, although he did have to admit that nowadays tarmac is placed on top of concrete motorways to reduce noise from the joints…
Concrete has a long history: we know for example of the Roman opus caementicium. Like its modern equivalent, Roman concrete was based on a hydraulic-setting cement added to an aggregate; usually consisting of a mixture of crushed rock, lime and volcanic ash, although unlike most modern concrete it could include bulls’ blood.
The Roman Pantheon constructed in AD125, has an unreinforced dome roof 43.3 metres in diameter, or 150 Roman feet. The Oculus, a central hole at the top of the roof, has a ring beam around it which distributes forces over the entire roof structure, which does mean it lets in the rain.
John next described a material called “tabby”, used in the Southeastern USA from the mid 18th to mid 19th centuries, essentially a mix of ubiquitous oyster shells and local clay.
Modern Portland cement has its origins in the first decade of the 19th Century, pioneered by Vicat In France and Aspdin in Britain. One very significant example of the use of concrete based on Portland cement is the Hoover Dam on the Colorado River. This used 4.5 million cubic yards, or 100 million tonnes, of concrete and was constructed between 1931 and 1936 under the direction of the US Army. Concrete though is not only a brute force material; John described its use in proton beam therapy suites where its strong absorption of protons allows it to be used for shielding and screening this highly targeted radiotherapy. Of course concrete is also used universally nowadays in nuclear power station containment vessels and in Three Mile Island and Fukushima, although unfortunately not Chernobyl, the concrete containment vessel prevented more significant radiological leakage.
It is interesting that the world cement market has been used as an indicator of economic activity, with more dynamic economies using more cement per person per year. China for example uses about a thousand kilogrammes of cement per person per year; Europe and the US use perhaps 200 to 400 kilogrammes and third world countries something like 75 to 150 kg per person per year.
Turning to what concrete actually is, John described it as “a complex mix of eutectic or alloyed oxides of calcium aluminium iron and silicon”. The primary targets in cement preparation are dicalcium silicate, tricalcium silicate, tricalcium aluminate and calcium aluminoferrite. The ratios of these targets can be adjusted to taste.
The input ingredients for cement are 80% calcium carbonate, coming from limestone, chalk cement rock, oyster shells or seasand, which is a marine limestone sediment, and about 20% silicon aluminium and iron. The aluminium. Iron also can come from millscale or pyrites. The silicon content comes from shales, marls and clays. Until recently pulverised fuel ash from coal-fired power stations also was used as a source of minerals.
Trace elements are added to tailor the characteristics of cements, including magnesium oxide up to 3% and sodium or potassium oxides up to 0.7%. There have been high alumina cements produced, but the physical properties of these may not always be as desired. Quantities of sulphur, chlorine, alkalis, chromium, phosphorus and vanadium are also carefully controlled.
To turn this mineral mix into cement it is calcined In a kiln at 1500 celsius. The fuel of choice for the kiln is coal, both because it is easily available and also because its ash is a potentially valuable source of minerals and can be sold as a commodity or used in the cementation process. The output from the kiln is a mixed-size clinker going from dust to the size of a tennis ball. This is a relatively stable product which can be sold and transported as it is, and even stored. as long as it is kept dry. To make what we know as cement from the clinker it is milled to powder, bagged and dispatched.
One issue with cement production is its carbon footprint. The calcining process transforms every 100 tonnes of calcium carbonate Into 56 tonnes of calcium oxide and 44 tonnes of carbon dioxide. This component of carbon dioxide emission is unavoidable in the preparation of cement because it is simply part of the chemistry of the calcining process. Further carbon dioxide emissions are associated with the burning of coal or other fuel to heat the furnace. Overall, the carbon dioxide footprint of cement is around 800 kilograms of carbon dioxide per tonne of clinker. UK cement production is responsible for about 12 million tons of carbon dioxide per year. The UK’s legal target is for all carbon dioxide emissions to be below 210 million tons by 2035. It is worth noting that unlike lime mortar, Portland cement does not absorb significant amounts of carbon dioxide as it hardens, or afterwards. The carbon dioxide footprint of cement production can be somewhat mitigated by using fuels other than coal; some plant in UK burns waste solvents and fuels, and in other countries even surgical waste has been used to fuel cement kilns.
The other major environmental issue apart from carbon dioxide emissions is the quarrying and blasting required to procure limestone.
Concrete, and cement, are certainly not without their environmental impacts but it is hard to imagine modern life in their absence.
Phil Chadwick
