Cement is something that the majority of people do not spend a large proportion of their day thinking about, but it does, rather literally, hold our society together.
Cement has been used in the construction of buildings since Roman times, but modern cement as we know it today was first utilised on a large scale during the Industrial Revolution. Cement is a mixture of minerals, predominantly calcium carbonate from limestone, which can bind together other materials, such as sand, once hardened. The main type of cement used in modern construction is Portland cement, which is a mixture of 90% fine limestone and clay (known as ‘clinker’), and a small amount of calcium sulphate and other minor ingredients such as magnesium oxide.
World production of cement is currently around 3.6 billion tons per year and this is set to rise to around a billion tons by 2050 as the standard of living in developing countries continues to accelerate. In fact, in terms of volume used, it is the most important substance on Earth bar water.
The benefits of cement are simple, yet effective. Cement is cheap, easily pourable and becomes extremely rigid. Therefore it is used extensively in producing concrete, and hence many of the world’s building and roads, as it is added together with water and sand as a binding agent. It is also used in other common construction materials such as mortar, stucco and grout.
A familiar concrete barrier, by the side of the road, made with cement. Image Credit: www.fhwa.dot.gov
Issues with Traditional Cement
Though cement is extremely useful and an integral part of modern society, it is not environmentally friendly.
To create cement, the constituent limestone must first be heated, and this is undertaken on an industrial scale using fossil fuels, which inevitably leads to CO2 being poured into the atmosphere. Cement production is one of the worst polluting industries in USA, and accounts for 5% of all carbon dioxide emissions globally. In general, every ton of cement produced leads to a ton of carbon dioxide gas being released into the atmosphere. Other harmful emissions from the Portland cement manufacturing process include nitrogen oxides, sulphur oxides and carbon monoxide.
There is also the inherent environmental impact of quarrying the limestone for production and transportation of the finished product. The mining of the constituent minerals needed for cement can cause large amounts of subsidence and various types of pollution.
Traditional cement production.
How to Make Green Cement
Large cement producers have realised the environmental impact of cement for quite some time and have trialled eco-friendly alternatives since the early 00’s.
Early attempts to produce ‘green’ cement included adding by-products to reduce the amount of limestone needed: steel by-products (slag), ash and more magnesium oxide have all been trialled with varying levels of success.
Furthermore, by adding more magnesium oxide to Portland cement it was hoped that the temperature of production could be lowered to a temperature that was achievable by heat produced using biofuels. Unfortunately, as the heating of magnesium oxide produces substantial CO2 the use of MgO did nothing to reduce emissions of the overall process, sometimes doubling emissions relative to traditional production.
Other temperature reducing minerals were also tried, but the exact chemistry of Portland cement is not well known and it is hard to predict how the chemical reaction which hardens the cement would be affected by the addition of extra minerals.
However, there are many companies that are still working with different blends of Portland cement to try and create a sustainable alternative.
Companies Involved in Green Cement Production
Since the realisation that it is difficult to modify traditional Portland cement to be eco-friendly, many new companies have sprung up, trying to rewrite the rule book on cement production.
Nikolaos Vlasopoulos, an environmental engineer at Imperial College, London, decided to move away from Portland cement altogether and base a new cement around magnesium silicates. The benefits of this are that magnesium silicates are in plentiful supply, and most importantly the chemical process used to produce the cement is actually carbon negative-it takes in more CO2 than it emits. His company, Novacem, strives to offer cost and performance parity with traditional cement and is on the Global Cleantech 100 list for the second year running (20011, 2012).
An Australian company, Calix, uses superheated steam to produce cement, and carbon dioxide emissions from the process can be captured using their ‘Endex Reactor’. Their ‘Catalytic Flash Calcination Reactor’ can also be used in other industries besides cement.
Engineers at Drexel University are working on cement that is based on ancient ideas from Roman and Egyptian times. The mixture, which does not require heating, mixes limestone along with slag and reduces CO2 emissions by around 97% relative to Portland cement.
Meanwhile, Celera in California is using the carbon dioxide emissions of power plants to make cement, via chemical reactions with salt water.
Given the modernity of the green cement sector, it is difficult to judge which, if indeed any, of these new methods of cement production will be successful. All have benefits to the environment relative to Portland cement, but all equally have other issues, such as production costs or material integrity. On top of this, not enough research has yet been undertaken to understand what the potential environmental issues with these new production methods could be. The green cement industry is certainly one to watch in the coming years.