A cement is any substance which binds together other materials by a combination of chemical processes known collectively as setting.1 Cements are dry powders and should not be confused with concretes or mortars, but they are an important constituent of both of these materials, in which they act as the 'glue' that gives strength to structures. Mortar is a mixture of cement and sand whereas concrete also includes rough aggregates (Figure 1).2,3
Because it is a major component of both of these building materials, cement is an extremely important construction material. It is used in the production of the many structures that make up the modern world including buildings, bridges, harbours, runways and roads. It is also used for facades and other decorative features on buildings. The constant demand for all of these structures, increasingly from the developing world, means that cement is the second most consumed commodity in the world after water.
Modern cement has come a very long way from its origins. Archeological evidence has been found that a form of crude concrete was used in hut construction in the areas now covered by Serbia, dating from around 5600BC and Israel (7000BC).7 Although the materials were never sufficiently strong to form monolithic structures, they provided some elements of stability to otherwise loose structures. The ancient Greeks also had access to lime plaster,8 which was often used for decorative purposes and crude cements derived from calcined lime were also used by the Roman empire,9 notably in the construction of the Pantheon in Rome.10
The progress of cement development was slowed in the Middle Ages, with significant information being lost, but by the industrial revolution, cements (and the concretes that they could be used to make) were increasingly being researched as an alternative to wood and stone. In the late 1700s and early 1800s a great number of novel cement patents and formulations were devised by those keen to exploit the need for new building materials.11
Among the first to realise the great potential of cement and concrete for modern engineering was James Smeaton, who was charged with the task of rebuilding the Eddystone Lighthouse on the English Channel in 1755 following a devastating fire. Smeaton researched the properties of lime from many regions of the UK and tested them systematically for their resistance to water. After much research he determined that lime obtained from Aberthaw in Wales was the most suitable and remarked that he had found a cement that could, "equal the best merchantable Portland stone in solidity and durability."11
A string of others were awarded patents for novel cementitious materials in the coming years, including James Parker (1796), Edgar Dobbs (1810), Louis Vicat (1818), Maurice St. Leger (1818), James Frost (1822) and Joseph Aspdin (1824). The most successful of these were Parker, who termed his invention 'Parkers Cement' then 'Roman Cement', Frost who coined the term 'British Cement' and Aspdin, who (possibly inspired by the earlier work of Smeaton) termed his invention 'Portland Cement.'11
For much of the 1800s, Roman cement dominated, but in the latter part of the century developments into the use of rotary kilns for cement production in the US led to Portland cement becoming the dominant cement type. By 1900, Atlas Cement Company, based in the Lehigh Valley, had 29 rotary cement kilns producing cement 10 times more rapidly than earlier kilns. By this point Portland cement had undergone a large number of incremental improvements, but its place at the top of the global cement pile was far from assured.11
Writing in 1882, British construction consultant Henry Faija wrote, "I do not wish to frighten manufacturers, but my own impression is that in a few years Portland cement will be superceded by another material. Portland cement, as at present made, is a chemical combination carried out in the crudest way."12 His prophecy did not come true, however, and Portland cement is still the industry standard cement. It represents around 90% of British cement production13 and is by far the most common cement type in the world.14
Production and setting chemistry
Modern Portland cement is termed 'Ordinary Portland Cement' (OPC) and is made by combining limestone or chalk (CaCO3) with sand (SiO2), clay and other materials (eg: Al2O3, Fe2O3, TiO2) in a kiln at temperatures in the region of 1400-1500°C.15 The mixture first undergoes decarbonation in which CO2 is lost from the limestone to produce calcium oxide (CaO). Continued heating and reaction causes calcination in which varying calcium silicates are produced. The majority of these are alite (Ca3SiO5) and belite (Ca2SiO4).
Calcination produces 'clinker' (Figure 2), nodules of mixed calcium silicates, which is then ground into a fine powder and mixed with up to 10% gypsum (CaSO4·2H2O) to produce cement.15 Gypsum is added because it stops cement from setting too rapidly. By adjusting the proportion of gypsum it is possible to give rise to cements with different setting times. A number of different classification schemes are in operation for cement around the world including ASTM and the European EN-197 standards.
When cement is mixed with water, a variety of exothermic (heat-producing) chemical reactions begin (Figure 3).15
The sulphates and some of the gypsum dissolve almost instantly, which gives rise to a highly alkaline sulphate solution. The most reactive non-soluble components then start to react with the water. These are usually minerals that contain aluminium, which react to form aluminate gels. This process normally occurs in a few minutes and produces a lot of heat (I) (Figure 3).
There is then a dormant period with little change (II) (Figure 3). Then the calcium- and silicon-containing minerals begin to react (III) (Figure 3). This forms crystalline hydration products such as calcium silicon hydrate and calcium hydrate that interlock to give high strength to the finished product. If sand or aggregate is present this will be bound strongly to the cement. Typically cement takes in the region of 40 – 50 hours to set but some cements continue to harden for years.
Global cement production was 1.53Bnt in 2010, excluding Chinese statistics that claim production of 1.8Bnt/yr in 2010.14 Although China represents a major manufacturer and consumer of cement, there is well-informed skepticism surrounding its claim to produce as much cement as the rest of the world combined.17
After China, which is undoubtedly the world's largest cement producer, the most significant producers of cements in 2010 were India (~21% of global production excluding China), the USA (~6%), Turkey (~6%) and Brazil (~6%).14 Cement is produced in far more countries than many other commodities. This is due not only the high demand, but also because the process uses readily available raw materials and has low associated production costs.
Figure 4 shows how demand has increased in the past, with massive increases in production around the turn of the century. The first year in which 1Bt of cement was produced was 1986. The first year that over 2Bnt was produced was 2003, just 13 years later. The first year in which over 3Bnt was produced was 2009, a tripling of the total global cement capacity in just 23 years.14
These recent increases have mainly been fueled by developing nations such as China and India, with significant increases in production also seen in the rest of Asia, the Middle East and South America. The African market is still comparatively small, but should contribute to increasing cement demand in the coming decades.
In comparison, European and North American cement markets have been stagnant since the start of the global financial crisis, due to fiscal belt-tightening and a drop in spending by governments. Indeed US cement production contracted massively in 2010 compared to 2009, dropping by 26% from 86.3Mt to 63.9Mt.14 US consumption was down by a similar amount, dropping from 96.8Mt in 2009 to just 71.5Mt in 2010. Consumption peaked at 128.2Mt in 2005, when production was also at its highest-ever value of 99.3Mt.
Companies and prices
The main multinational cement producers are Lafarge (based in France), Cemex (Mexico) and Holcim (Switzerland). Other major players (some with interests in several continents) include HeidelbergCement (Germany), Anhui Conch Cement Company (China), Italcimenti (Italy), Eurocement (Russia), ACC (India), UltraTech (India), Cimpor (Portugal) and Taiheiyo Cement (Japan). There are thousands of smaller companies concerned with the production of cement. Constant mergers mean that cement manufacturing is an ever-changing business landscape.
Wholesale prices for cement are closely guarded by suppliers. They are often supplied only on a case-by-case basis to individual customers. Prices are affected by a large number of factors such as the type of cement, supply and demand in the region and the quantity required. As a result of such variations the price of OPC varies between around US$35/t (close to the marginal cost of production in an efficient plant) up to US$180/t or more.17 Because cement has such a low value to weight ratio, transport costs are often the largest single contributor to the overall cost along with electrical and fuel costs.
The desire for improved living standards across the globe and ongoing development will ensure that cement remains an extremely important commodity for construction.
Global cement production and consumption is forecast to rise from the estimated 3.3Bnt seen in 2010 to around 4.2Bnt by 2015 and then to almost 5.9Bnt by 2025. This represents overall forward expansion to 2025 of approximately 78%. Half-decade expansion is expected to be approximately 27.5% for the period from 2010 to 2015, slowing to 20% in the next half decade and slowing further to 16% between 2020 and 2025.18
Within this outlook there are very different forward patterns of growth for individual regions, ranging from 35% for Oceania to over 70% for North America, Africa and east Asia, over 80% for the Middle East and south-east Asia and more than 114% for south-west Asia. Of the total 2589Mt forecast increase in world cement consumption, over 76% is set to occur in Asia, with volume increases of 1561Mt for east Asia (Excluding China), 285Mt for south-west Asia and 139Mt for south-east Asia. The Middle East is set to record consumption growth of almost 150Mt, with expansion of 137Mt for Africa and 108Mt for South and Central America.18
The total for the 27 EU Member States is set to rise by approximately 68.6Mt to 2025 with the 17 Eurozone countries contributing 54.8Mt. Other non-EU European nations will contribute an additional 72Mt, while an increase of 4Mt is anticipated for Oceania. The North American total for 2025 is forecast at 63Mt above the 2010 level.18
Cement production accounts for 5-6% of all man-made CO2 emissions. Around 50% of emissions are from decarbonation and around 40% from the fuel used to fire the kiln.19 Considering the predicted increase in production there is pressure on the cement industry to maintain or reduce CO2 emissions. Carbon-neutral biomass and other alternative fuelse are increasingly being used to reduce specific cement-related CO2 emsissions.
Increased plant efficiency is therefore desirable from both an economic and environmental perspective. Several manufacturers are involved in carbon-trading schemes and the industry is represented by intergovernmental panels such as the World Business Council for Sustainable Development, (WBCSD). In 2009 the WBCSD stated that it may be possible to use carbon capture and sequestration technologies to reduce CO2 emissions from cement manufacture by 18% by 2050.20 In addition there is significant research into new cement types, which produce less CO2 during manufacture, including cements which sequester significant quantities of CO2 during setting.21 If these can be made economically viable they may enable significant reduction in CO2 output from the cement industry. Novacem22, TecEco23, Cenin24, Calix25 and numerous others26 have made significant progress towards low, zero or negative carbon cements, based on a variety of chemical and processing approaches.
7. Brown, G.E.; 'Analyses and history of cement.' Gordon E Brown Consultants, Ontario, Canada, 1996.
8. Vitruvius, (translated by Gwilt, J). 'The architecture of Marcus Vitruvius Pollio,' Priestly and Weale, London, 1826.
9. Blake, M. E.; 'Ancient Roman construction in Italy from the Prehistoric period to Augustus,' Publication 570, Carnegie Institute of Washington, Washington DC, 1947.
11. Bhatty, J. I.; Miller, F. M.; Kosmatka, S. H.; Bohan, R. P. (Eds.) Portland Cement Association, 'Innovations in Portland Cement Manufacturing,' Skokie, Illinios, USA, 2011.
12. Middleton, R. E.; 'Portland Cement,' Army and Navy Co-operative Society, London, 21 December 1882.
17. Global Cement Magazine, PRo Publications, February 2010.
18. Ocean Shipping Consultants Ltd, 'Major growth ahead for cement sector' (and sources therein), Global Cement Magazine, March 2011.
19. The Cement Sustainability Initiative: Progress report, World Business Council for Sustainable Development. Published 2002-06-01
20. INTEF Civil and Environmental Engineering, Cement and Concrete, N-7465 Trondheim, Norway. 'Mechanism of performance of energetically modified cement EMC.'
26. Edwards, P. 'Future Cement – Looking beyond OPC' – Global Cement Magazine, January 2011, pp.12-22.