The cast-in-place concrete in the IL Centre contains 50 percent “supplementary cementing materials,” meaning that it replaces a large percentage of cement with other materials, particularly two types of industrial waste: slag & fly ash. It is estimated that the IL Centre will stop 300,000 kg of carbon dioxide from being released into the atmosphere through this measure alone.

  • Slag: A by-product of iron production, it is ground to fine powder to make slag cement. It can replace up to 50 percent of the cement in concrete. The largest benefit of slag is that it requires almost 90 percent less energy to produce than traditional cementing materials.
  • Fly ash: Created by coal-fired electric power generation, it can typically replace 20 to 30 percent of the cement in concrete.

Both these substitutions reduce the environmental impact of manufacturing an equivalent amount of cement and allow industry wastes to be safely recycled into a permanent structure rather than into a landfill site.

This non-traditional method of making concrete saves energy and decreases carbon dioxide emissions. For every tonne of these supplementary materials used, approximately one tonne of carbon dioxide (that would have been created making the cement) is not being released into the atmosphere.

Concrete containing slag cement and fly ash even has some additional, desirable benefits over ordinary concrete. As a liquid, high-slag concrete has reduced water demand and improved plasticity, making it easier to pump and finish. It also has a lower heat of hydration — as concrete hardens by chemical reaction, heat is given off that can cause concrete to crack in large pours. As a solid, the concrete is more durable and less susceptible to degradation, and has increased strength.

The drawback, however, is that the concrete cures more slowly. This increases the amount of time it has to be supported by forms. The longer cure time can make slag and fly-ash concrete unsuitable for high-rise construction and rushed projects, but it is extremely practical for broad low-rise buildings.

Since these types of cement substitutes are still new products, not all structural engineers are comfortable using them in buildings. But as engineers are exposed to supplementary cementing materials more frequently, the environmentally friendly choice will hopefully become increasingly popular.

Concrete Column