Green Cements

The concrete industry is the largest user of natural resources in the world and thus has a considerable environmental impact. Each ton of Portland cement requires about 1.5 tons of raw material for its production. This industry is not only energy intensive but is also a major contributor of greenhouse gases, in the form of CO₂. Each ton of Portland cement that is produced involves the release into the atmosphere of about one ton of CO2. Indeed, according to Mehta (1999), the cement industry is responsible for about 7% of global CO₂ emissions; thus, there is considerable interest now in developing cements that are more environmentally friendly. One such cement (CEMROC), based on blast-furnace slag, has recently been described by Gebauer et al. (2005). This cement, produced by Holcim in Europe, is reported to show close to zero CO₂ emission during its production (only about 100 pounds per ton of cement).

It is similar to the supersulfated cement described above and is particularly well suited for use in structures exposed to aggressive environments. Other cements of this general type will almost certainly be developed in the future. Another (and simpler) approach is to use much greater proportions of fly ash in concrete. A great deal of development is being conducted on what is referred to as high-performance, high-volume fly ash concrete (Malhotra, 2002; Malhotra and Mehta, 2002). Such concretes may be defined as:

• Containing at least 50% fly ash by mass of the cementing materials
• Having a Portland cement content of less than 200 kg/m³
• Having a water content of less than 130 kg/m³
• Having a water/cementing materials ratio of less than 0.35
• These concretes reach their full strength potential rather more slowly than conventional concretes, but the end result is a low-permeability, durable concrete.