The cement and concrete scene has been changing slowly for some time, but it has a long way to go. The history, chemistry and probable future of cement and concrete are summarized here –
Stones cut and placed together thousands of years ago still stand as impressive buildings and monuments. Ancient artisans worked out how to build with stones so as to support much more than their own weight, but their skills failed when their foundations were unstable, sometimes because of earthquake, but more often because of water. Glues – mortars or cements – made from sticky clays or crushed limestone, sometimes mixed with volcanic ash, slowed decay by water, wind and sun but never stopped it. From the ancient Romans to the present, engineers and builders have wanted a better type of glue to hold stones together to resist forces that have shaped the earth – water, wind, sun and gravity.
In 1824 Joseph Aspdin, the son of a Yorkshire bricklayer, went one better than shaping and cementing rocks. He made synthetic rocks in any desired shape and size from a mixture of limestone, clay and sand which he patented and called portland stone – the first modern type of concrete. Aspdin’s idea did not catch on for a long time because it was too expensive, but also because his concrete was not strong enough for cantilever bridges or buildings higher than three storeys.
Forty years later Joseph Monier was a commercial gardener in Paris who, with no technical training, grasped the idea that concrete and iron wire used together made a much stronger, more durable building product than either material alone. Concrete had high compressive strength, and embedded wire made up for concrete’s serious lack of tensile strength. He patented his invention in 1867, then had the vision to extend its application from flower pots to reinforced slabs and girders including railway sleepers, pipes, floors, arches and bridges. Steady improvement of concrete, and replacement of iron by steel wire, allowed the building of large bridges and high-rise buildings. Attractive stonework became the façade of buildings, not their framework, while reinforced concrete gave much better foundations and structures. Without reinforced concrete there would be no skyscrapers: cities, bridges and tunnels as we know them could not have happened.
Australia played a significant part in these developments. In the 1890s and 1900s a remarkable German entrepreneur and engineer, Dr Wilhelm Scheidel, improved methods for making high grade steel, cement and concrete at Portland, two hours west of Sydney. Scheidel’s factory produced high quality cement which was railed to Sydney in huge quantities, and Portland became known as the town that built Sydney. His work became the basis for the modern standard for portland cement written in 1909. Over the past century, with all its scientific and technological progress, it is remarkable that one material – portland cement – has retained its central role in most kinds of construction upon which our lifestyles depend.
Portland Cement and Concrete
Cement is usually a much smaller part of a concrete mixture than the other stuff which gives it strength, insulating capacity, density or whatever qualities are required. Sand and aggregate (small sharp unweathered stones) with water and reinforcements make up the bulk of concrete. After many trials it has been found that, with few exceptions, additions of just about anything to this kind of mixture degrades its properties. Reactive (amorphous or non-crystalline) silica is commonly added in the form of flyash from coal power stations, replacing 20% or more of portland cement according to strengths required. Crushed pumice, burnt rice husks and synthetic silica fume are useful sources of amorphous silica but are generally less available and/or more expensive.
Too much water is the biggest common cause of reduced concrete strength, but the optimal amount of water leaves mixtures too dry to mix well, to pour or to pump. This is commonly overcome by adding tiny amounts of special materials –
Portland cement plants are usually built close to limestone mines because that is the main material needed. Mixtures of limestone and other minerals are designed to give a composition which, when roasted at about 1450C, will yield a mixture containing oxides in about these proportions –
about 70% calcium oxide (quicklime) CaO
about 20% silica (silicon dioxide) SiO2
about 5% alumina (aluminium oxide) Al2O3
about 3% ferric oxide (one of many iron oxides) Fe2O3
Some potassium, magnesium and sulfate are also present. It is important to limit the amounts of certain materials which degrade the properties of portland cement – sodium, potassium, magnesium and phosphorus.
Nothing happens to dry cement until it is exposed to water. Quicklime quickly dissolves, forming a strongly alkaline solution which begins to react with other oxides both in the cement and in the surfaces of the sand granules and the stones. The reactions heat the watery mixture which, in its early stages, is a gel. Over hours and days a complex set of reactions creates a mass of silicates (most natural rocks are silicates) which bind the whole mixture into a very tough material we know as concrete. The main components in high grade concrete are four hydrated salts –
C3S belite tricalcium silicate Ca3SiO5
C2S alite dicalcium silicate Ca2SiO4
C3A celite tricalcium aluminate Ca3Al2O6
C4AF ferrite complex ferric salt Ca4Al2Fe2O10
Some gypsum (hydrated calcium sulfate) is also present.
New types of cement
We have such a huge debt to portland cement that it is too easy to overlook its technical disadvantages. If this were a new material looking for a foothold in today’s cement market we would dislike several features –
Some other cements have more desirable attributes than these, but this is counterbalanced by the very well known properties of portland concrete, particularly its durability for more over century.
A fifth factor – the rising energy and environmental cost for producing portland cement – seems likely to limit its future. Its manufacture is one of the world’s most energy demanding industries, consuming perhaps 5% of the world’s total energy produced. The unwanted byproduct of roasting limestone is carbon dioxide (CO2). Also a lot of fossil fuel is used to create the other oxide components of portland cement, so in total around 1.4 tonnes of CO2 is emitted into the air when making one tonne of cement. Everybody expects that some day not too far distant there will be an effective global price imposed on atmospheric discharge of CO2, and that will impose a seriously increased cost burden on portland cement.
Amongst many part replacements for portland cement which have been tried, the most generally useful is flyash, the fine solid material – black smoke – which used to be vented to the atmosphere from the burning of coal. Flyash is now collected in power station chimney stacks, and its supply for use in concrete has become an industry in its own right. This is a return to ancient Roman engineering: flyash is virtually synthetic pozzalans (volcanic ash).
However, partial replacement of portland cement can only ever be a stopgap solution for a growing cost and environmental problem, so more radical solutions have been actively sought in recent decades. Two alternative cements arising out of years of research and development include –
Geopolymers – polymers made from rocks – is a 1970s concept which arose out of a need for fire resistant building materials – see en.wikipedia.org/wiki/Geopolymer. Roasting of certain clays, particularly kaolinite, gives a mass of highly reactive aluminosilicate polymers which can then be heavily crosslinked by caustic solutions to give an extremely tough cement. For a short history of these developments in Europe and Australia see www.geopolymer.org/applications/geopolymer-cement. The concept of geopolymers is attractive because its successful application to concrete should ensure its indefinite future. In its practical form as we see its current development, it has some huge potential advantages –
This is a very different material from portland or geopolymer cement. In 1867, the same year that Monier patented reinforced concrete, a very smart Frenchman named Stanislaus Sorel patented another idea for synthetic rock, this new product based on magnesia rather than limestone. In some important respects, Sorel cement was better than portland but, as so often happens, his timing was wrong. By that time investors had begun to support portland cement, so they weren’t about to have their heads turned by Sorel. A lot of people ever since have studied magnesia cement because it has some remarkable and useful properties, but portland cement was way ahead in the race.
Sorel cement is a broad range of mixtures based around magnesia, otherwise called magnesium oxide or MgO. Magnesia concrete has been used for decades, mainly for interior flooring, but also for a lot of decorative items – billiard balls, for example – because it has high impact strength and takes a high polish. It has many other advantages over portland concrete –
A Chinese-Australian consortium has commercialized magnesia cement products under the Ubiq brand which have great water resistance using homespun Chinese technology to stop water wicking into the products. Another Australian company, Calex, has acquired the rights to British technology – see www.theguardian.com/business/2012/oct/11/mystery-firm-buys-novacem-green-technology-rights. These ventures between them would seem to have the potential to revolutionize the concrete industry through several advances –