Cementitious materials

Cementitious products comprise the glue that holds concrete together. These materials include traditional Portland cement and other cementitious materials, such as fly ash, ground granulated blastfurnace slag (ggbs), limestone fines and silica fume. These materials are either combined at the cement works (to produce a composite cement) or at the concrete mixer when the concrete is being produced (the cementitious product is called a combination in this case). 
 
Fly ash and ggbs are the most commonly used of these materials in the UK. These secondary materials are useful by-products of other industrial processes, which would potentially otherwise be sent to landfill. Apart from the obvious attractions of creating a use for these materials, their incorporation in factory made cements or as concrete mixer additions translates into direct reductions in quarrying, energy consumption and carbon dioxide emissions.
 
Ggbs is a useful by-product recovered from the blast-furnaces used in the production of iron. It can be used un-ground as a coarse aggregate or as a supplementary cementitious material (where it can replace up to 70% of cement in a concrete mix). Fly ash is a useful by-product of coal-fired power stations and is environmentally beneficial. If it were not used in composite cements or as an addition at the concrete mixer then the material would be wasted and sent to landfill.
 
Together, fly ash and ggbs account for around 15% of total UK consumption of cementitious material. The UK uses approximately 2 million tonnes (Mt) of ggbs and 600,000 tonnes of fly ash as cementitious replacement every year, with the following environmental benefits 1:
  • Reduction in annual CO2 emissions of approximately 2.5Mt
  • Reduction in primary energy use by 2,000 million kilowatthours
  • Saving of 4 Mt of quarry material
  • Saving of 2.5 Mt of landfill
Using ggbs or fly ash in concrete, either as a mixer addition or through a factory made cement, significantly reduces the overall greenhouse gas emissions associated with the production of concrete. The reduction in overall greenhouse gas emissions can be as high as 60%, depending on the concrete mix design and the application2.
 
The advantages of these materials do not stop there. Their beneficial chemical properties in conjunction with Portland cement produce extremely workable and durable concretes that are highly resistant to penetration by chloride ions, sulfates and other chemicals 3.
 
The Spinnaker Tower in Portsmouth is a prestigious example, where a 50% ggbs replacement concrete mix was used in its construction 4.  Fly ash on the other hand was extensively used in the channel tunnel rail link (120,000 tonnes) and Heathrow Terminal 5 (135,000 tonnes) 5.
 
For more information on the different cementitious materials please follow one of the links below:

Spinnaker Tower, Portsmouth. 50% ggbs was used in its construction.


References and further information

1. Higgins D D, Sustainable Concrete - How can additions contribute?, The Institute of Concrete Technology Yearbook: 2006-2007.   
 
2. Draft-in-proof The Concrete Centre study, Embodied CO2 of various concrete mixes, Unpublished work
 
3. Swamy, R, Designing concrete and concrete structures for sustainable development, CANMET/ACI International Symposium on Concrete Technology for Sustainable Development, Vancouver, Canada, 2000
 
4. Figure from the Cementitious Slag Makers Association
 
5. Technical data sheet 1.3: applications for PFA in concrete, UK Quality Ash Association (UKQAA), 2006