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Cement is fundamental for contemporary society, but what are its environmental impacts and how is it possible to reduce them?

Cement can be considered a resource that revolutionized the history of engineering and the way cities started to develop. Residences, squares, buildings, stadiums and practically any type of construction rely on this substance as one of their basic materials. But have you ever wondered how the cement production process works and how to reduce its impacts?

Cement manufacturing is not simple and requires a lot of energy and different mechanisms. Used as a binding agent, the main raw materials present in its composition are limestone and clay. Both found, still in excess, and extracted from nature.

Thus, it can be considered that the sites destined for its production are constituted by two major activities: limestone mining and cement manufacturing. Factory facilities are usually close to limestone extraction sites to facilitate the transport of heavy raw material to the industrial area.

The limestone rock mining activity is carried out in large open pit mechanized quarries. And, after being extracted, the rocks are disassembled and reduced with explosives to have an adequate granulometry.

Manufacturing steps

The technological process of cement production implemented in most Brazilian industries is known as the dry process and consists mainly of the following steps:

1. Grinding and homogenization of raw materials (obtaining raw flour)
2. Clinkerization of raw flour in rotary kilns (clinker production) and subsequent clinker cooling
3. Clinker grinding for and addition of gypsum to obtain cement
4. Bagging and shipping the final product

First, the raw materials – limestone (94%), clay (4%) and smaller amounts of iron and aluminum oxides (2%) – are ground and mixed until a fine powder is obtained (raw flour). This material is then introduced into a rotary kiln where it is heated to a temperature of 1500 °C, before being suddenly cooled by blasts of air. This is how clinker is produced, the basic material needed for the manufacture of cement. The material obtained (clinker) is mixed with gypsum (gypsum) and other additions (such as limestone, pozzolan or slag) giving rise to various types of cement that are finally bagged so that they can be sold.

This process requires a high consumption of energy, either in the form of thermal energy (heat), through fuels used to heat the rotary kilns for the production of clinker, or in the form of electrical energy, consumed throughout the industrial process to move machines, turn rotary kilns and mills. Most of this consumption, however, is related to the expenditure of thermal energy during the burning of fuels.

The fuels that feed the ovens are, in most cases, from non-renewable sources, such as oil and coal. Among the most used fuels, some solid stand out, such as petroleum coke and gasoline, and some gaseous ones, such as natural gas and other coal derivatives.

Petroleum coke is the main source of energy in the cement industry, being the main fuel used in the clinker rotary kiln. It is a black and shiny granular material made up mainly of carbon (90 to 95%), but it also usually has an expressive sulfur content (about 5%). The reason this fuel is used a lot is due to its high calorific value associated with its low acquisition cost.

In addition to these traditional fuels, industrial and biomass residues and rejects, charcoal and agricultural residues can also be used to feed the ovens.

Environmental impacts

Cement plants end up polluting the environment and are responsible for relevant environmental impacts.

And, although the manufacturing process of this material does not directly produce solid waste, since the ash from burning fuels in the rotary kiln is normally incorporated in the clinker itself, there is a high emission of gaseous pollutants and particulate material.

Thus, the main impacts are caused by the emission of polluting gases from this burning. An example is the high emission of carbon dioxide (CO2), one of the main gases that unbalance the greenhouse effect.

Under the guidance of the World Business Council for Sustainable Development (WBCSD - World Business Council for Sustainable Development), the Cement Sustainable Initiative (CSI - Cement Sustainability Initiative) commissioned an extensive research program on the impact of the cement industry worldwide and worked to develop an action plan to find ways to improve the sustainability of cement production.

Cement plants are responsible for about 5% of the global emission of carbon dioxide (CO2), from an anthropogenic source, released annually into the atmosphere. It is estimated that, in the production of one ton of clinker, one ton of CO2 is produced, contributing in large part to the increase in the greenhouse effect, according to a study.

In the cement manufacturing process, sulfur oxide, nitrogen oxide, carbon monoxide and lead compounds can also be released, all of which are pollutants.

In addition, during the first stage of extracting raw materials, physical impacts may also occur, such as landslides in limestone quarries and erosion due to vibrations produced in the ground. And the extraction of clay in rivers can deepen these watercourses, reducing the amount of water in the beds and disturbing the existing habitats, which reduces the biodiversity of several regions.

Alternatives to reduce impacts

The forecast is that cement production will continue to grow in the coming years, which would consequently increase total CO2 emissions in the world. To avoid this scenario, it is necessary that the production process undergoes changes, since the demand for cement will hardly decrease.

The CSI action plan, mentioned above, lists some options to enable sustainability in cement production:

• Alteration of industrial plants, in order to capture the emitted carbon;
• Use only the dry way in the production process, requiring less furnace feed;
• Reuse of industrial and agricultural waste to feed the oven, instead of using fossil fuels (co-processing);
• Partial replacement, in buildings, of cement by other materials;
• Change in cement formulation so that its production releases less CO2.

These attitudes would need to be taken by the producers of the material. Choosing cement models based on these practices and putting pressure on government and companies to regulate sustainable legislation for the sector are methods of trying to change the current course. Cement, as already mentioned, is fundamental for the "construction" of the society we know today. Therefore, we should not demonize it, but look for alternatives on a large scale so that its impacts are reduced and more sustainable alternatives can be developed.