By Frédéric Gagnon Eng., M.Sc.,
The notion of green concrete has nothing to do with its color or even the color of leprechauns! The term refers to concrete that has constituents or manufacturing processes which have a smaller environmental footprint compared to conventional concretes.
Conventional concrete is a construction material that unfortunately can be detri mental to the environment. The manufacturing process of cement, which is the main component of concrete, represents between 5% to 7% of the worldwide carbon dioxide emissions from industrial processes, which is in itself 1/3 of all emissions originating from human activity. However, concrete presents advantages over other available construction materials. Among these, the possibility to produce it locally, which limits transport efforts and CO2 emissions. Concrete is recyclable since it can be crushed and reused. Furthermore, concrete has a high thermal inertia compared to other materials and this allows for better insulating properties, which also helps to reduce emissions.
Motivated by public opinion, as well as political will, and by green movements such as the LEED certification program (Leadership in Energy and Environmental Design), the world cement producers are always looking for ways to improve their product with respect to the environment. The following presents some of these initiatives taken by the world concrete producers.
Supplementary Cementing Materials
By adding different elements, mainly industrial residues, in the fabrication process of concrete, a considerable reduction in the use of cement powder is achieved, which in turn, allows for a significant reduction in air pollution. These supplementary cementing materials can however modify the overall chemical and mechanical properties of concrete. The full impact of these products on the behavior of concrete is more often than not unknown even though they are used in new constructions. This, in turn, can lead to defects for the structures where these supplementary cementing materials are used.
The principal supplementary cementitious materials (recycled products) are: fly ash, ground granulated blast furnace slag, silica fumes, and limestone fillers. Other products can occasionally be used for specific applications in the construction industry.
Fly ash, as the name implies, is ash that is recovered from electrostatic dusters or from carbon fired thermal plants. Blast furnace slag is a residue obtained from the casting of cast iron. Silica fume is a byproduct of the manufacture of alloys made of silicium and ferro silicium. Finally, limestone fillers are fine powders, which are released during the crushing of limestone in quarries and mines. More often than not, more than one supplemental product is used in the fabrication of concrete.
The ternary binders, regularly used in Eastern Canada, are composed of three basic elements, which are Portland cement (conventional cement) and a combination of two of the above-mentioned materials. Besides providing a reduction in the quantity of cement powder, concretes made with ternary binder provide structures with reduced shrinkage, a higher durability and lower heat of hydration. However, some of these concretes, when exposed to freeze-thaw cycles, as well as de-icing agents, can show increased signs of scaling or spalling earlier in their lifecycle with respect to structures built with conventional concretes. Much of the research underway in North America aim for a better understanding of these phenomenons.
Not only is it important to reduce the amount of cement powder when manufacturing concrete, it is also important to make sure that the these new "concretes" are also durable. In fact, all the efforts to reduce the environmental footprint are in vain if the new concretes have a shorter lifespan. As an example, anhydrous calcium sulfate is an industrial by-product that comes from aluminum smelting and it is used as a supplementary material in specialized concrete products. This material has demon strated that it in creases the overall quality of the concrete in parti cular usage.
However, concretes or mortars, which contain anhydrous calcium sulfate, have proven to be vulnerable to water and humidity, and this results in sulfatic attack and swelling of the material. The use of such concrete exposed to the elements or in a humid surround ing must be barred, since there can be a large increase in volume. Figure 1 illustrates this phenomenon in the case of a slab on grade in a basement.
The green approach towards concrete is not only a passive one. In fact, newly developed products actively and/or through chemical processes will clean the air as a scrubbing filter would.
Titanium dioxide (TiO2) is a photocatalyst that is added to concrete and which allows a reduction of gases that are returned to the atmosphere. Nitrogen oxides (NOx) are reduced through a succession of chemical reactions, which are ini tiated by ultraviolet rays from the sun. These types of concrete can be used in a conventional fashion for the construction of roads or building veneers. As the concrete absorbs solar energy, it will clean the air polluted by industrial activity as well as fuel fossil burning vehicles.
Other research projects on concrete, underway right now by a society of the Imperial College of London, would have the capacity to absorb large quantities of CO2 as it cures and ages. This is not the only advantage of this material, since the cement based on magnesium oxide, the main component of this concrete, is a derivative of silicate minerals. This material requires to be heated at a much lower temperature than regular Portland cement when it is manufactured. Since the temperature in the kiln is much lower, the overall emission of CO2 is less than 0.5 ton of CO2 per ton of concrete, in comparison to nearly a ton for conventional cements. As a result, this material has practically a negative carbon footprint.
Permeable or porous concrete is a material, which offers the advantage of allowing surface water to percolate into the support subsoil. This type of concrete typically contains 15% to 25% of voids and a one square meter surface will allow approximately 200 liters of water per minute to percolate. Figure 2 shows an example of
water passing through a sample of such a concrete.
By allowing the water to percolate through the concrete, these slabs allow the soil to be recharged with water and this reduces the amount of water sent to the municipal drainage systems and in the waterways when it rains or there is snowmelt. The U.S. Environmental Protection Agency (EPA) has recognized the environmental impact of such concretes.
Concrete is directly behind water on the usage scale, among all substance in the world. Every year, close to a ton of concrete is produced for each inhabitant. In Canada, ready mix concrete is the most used construction material and it represents approximately 70% of the volume of all construction materials. It is therefore not surprising that there is a lot of efforts and money funneled in research and development of concrete to limit its carbon footprint. Given its widespread use, each incremental reduction in concrete's carbon footprint has an important global impact.