Company Announcement - Concrete, and more specifically cement, has become a key ingredient of economic growth, and cement sales are often used as an indicator thereof. Having been part been part of society for thousands of years, concrete has progressed from an elementary construction material to a modern material used in almost every structure. Today, concrete is more often than not the construction material of choice. A prime example is the Burj Khalifa skyscraper in Dubai, United Arab Emirates. At 830 metres, it is the tallest building in the world with reinforced concrete as its main structural material.

The reasons for the success of concrete are clear: it is cheap, robust and relatively easy to use. The final product is a rock-like structure that typically lasts for decades. It can be prepared by hand with limited skills and still produce a reliable product, but it can also be optimised using expert knowledge of advanced mix designs and, with the aid of modern chemical admixtures, produce a superior product with almost limitless possibilities.

Although concrete seems like the perfect construction material, there is, however, one large, unavoidable problem: it has a significant negative effect on the environment.

It is estimated that cement manufacturing alone contributes to more than 5% of world CO2e emissions with this figure to increase to more than 10% by 2050 if current trends continue. When determining the environmental impact of concrete, the contribution of cement is by far the most significant. The sand and stone, as well as admixtures do have an impact, but this is typically overshadowed by the impact of the cement.

The manufacturing process of cement is key to understanding its impact on the environment.

It all starts with the grinding of the limestone and shale, which is then added to a preheater that heats up the material to about 900˚C before it enters a long inclined kiln. About 90% of the calcination  ̶  the decomposition of limestone to calcium oxide and carbon dioxide  ̶  takes place in the preheater, while the rest occurs in the kiln up to a temperature of 1100˚C. This reaction releases large quantities of CO2.

When the temperature exceeds 1250˚C, the process of clinkering starts, during which the cement is actually formed. Full clinkering occurs at around 1500˚C. The product, the clinker, is then mixed with gypsum and milled to a fine powder which we call cement. Cement can be further extended/blended by mixing it with other materials before distribution, as explained later in this article.

It has been estimated that about 870 g of CO2 is released for every 1 kg of cement manufactured. This explains the significant impact on the environment. This is further complicated by the fact that about 60% of the CO2 emission is due to calcination, and not the energy required for the kiln. So, even if 100% renewable energy is used for heating the kiln, there still will be 530 g of CO2 released for every 1 kg of cement manufactured. To put this in perspective, it is estimated that around 4 billion tonnes of cement were produced in 2013, of which 59% was produced in China alone. Cement production in Europe declined by about 10% since 2001, but increased by almost three times in Asia and 2.5 times in Africa over the same period. With this growth in cement production, the outlook for the reduction of carbon emissions by cement manufacturing is bleak.

While it is acknowledged that many other environmental impact indicators have to be considered, the so-called carbon footprint does give a good indication of the direct and indirect impact of concrete on the environment. Although there is no current technology that could help replace concrete in a sustainable and robust manner, certain short and medium term solutions may reduce its environmental impact.

The key for the near future lies in using supplementary cementitious materials (SCMs), optimising mix designs and improving durability. Firstly, SCMs are vital in producing more sustainable concrete structures. Cement must be replaced as much as possible using waste materials such as fly ash and ground granulated blast furnace slag. The good news is that adding SCMs improved the durability and typically results in a more economical concrete. The one downside of using high large replacements of SCMs is that strength development can be retarded. However, if longer time required for strength development can be taken into account in construction projects, the environmental impact can be reduced even further.

The mix design of concrete should be improved. Mix designs of typical ready-mix suppliers are already optimised economically, but the environmental impact is not considered. Cement content can be reduced further using super plasticisers, and better graded (typically more expensive) sand and stone. Another aspect hindering the reduction of the cement content is the minimum cement content specification for durability. It is time for the industry to move completely away from prescriptive durability specifications (e.g. minimum cement content) towards performance-based specifications that will allow the use of concrete containing less cement, but still meeting the durability requirements.

Durability should form a more integral part in structural design and in construction itself. The durability of structures can be improved significantly at both the design stage and in construction. Many durability issues are a result of poor workmanship, which could be
avoided with proper staff training and supervision. Both researchers and engineers should see to it that these solutions are implemented. Universities should expose future engineers to new technology, but, even better, teach students the skills to investigate, understand and implement new technologies that have not even been developed yet. Although we simply cannot do without concrete, we have to start using it more smartly and  minimise its environmental impact. Concrete is our friend, but the friendship needs attention.