Cement (what am I using? II)

Cement is widely used in construction, being a third of the ingredients for mortars, renders, and so on.  It is used to make breeze blocks, drains and many more besides.  Cement sets under water (making it ‘hydraulic’).  It has been applied to many historic buildings, traditionally made with lime, not cement, with mixed – usually bad – results.  Lime is also preferred over cement by ecological builders (although even lime, being quarried, is not that low-impact).

Composition and manufacture:  Cement is based on limestone (CaCO3), which is ground and mixed with various calcium silicates and a calcium sulphate (generally gypsum).  If you like chemical shorthand, the EU says that the composition of cement is such:  two-thirds 3CaO·SiO2, 2 CaO·SiO2; one-third Al or Fe clinker.  Wikipedia has more information.

I’ve used Lafarge’s site for information about the manufacture of cement, but the MPA has a good ‘virtual works tour‘.

  • Limestone, chalk, shale and clay are mined or blasted out of the ground.  There are pictures of a cement mine here.  I give you a railway enthusiast’s pictures because cement mines are really not pretty.  In fact, they are great scars on the land.
  • They are crushed in a big factory mill, and various other minerals are added (some of these are waste products, e.g. paper ash).
  • The ‘raw meal’ (powder) is cooked in a kiln at about 1,500°C for half and hour.  This turns the raw materials into cement clinker, with the all-important hydraulic calcium silicates.
  • Gypsum (3-5%) is added to the clinker to help it set.  The mixture is re-ground,with other additives (“of natural or industrial origin”).
  • The clinker is cooled, stored in silos and then packed in bags and transported to your local DIY store.

Manufacturers/ Brands:  the MPA is the cement manufacturers’ association.  Britain has 4 major cement manufacturers:  LafargeCemex; Hanson; Tarmac.

How environmentally friendly is cement?  Like a lot of things, cement itself isn’t too bad, but the scale on which it’s manufactured and used is.  Large-scale mining is never ecologically sound, and it requires a lot of water and machinery which itself uses a lot of diesel/ petroleum and electricity.  Heating a kiln takes a lot of fuel.  The fuels used include fossil fuels (coal) and industrial waste. The latter has problems, like incinerators, concerning dioxins; on the other hand, the waste has to go somewhere, and a secondary use like this is surely quite sensible. The MPA says that the industry burns 1.4 million tonnes of waste. Waste plasterboard is now sent to cement factories to provide gypsum for the mix and cardboard for fuel. (Builders and the public, incredibly, are not given any money for supplying their waste plasterboard.  Rant.) Even so, waste products only make up a quarter of fuels used.

The carbon footprint of the four major cement manufacturers is hard to ascertain.  The companies are very keen to flaunt their sustainability and green credentials, and it seems they are trying hard.  Even so, one can’t help feeling a bit cynical. Each company has a different way of publishing its figures.

CO2 emissions for 2011
Tarmac:  1.44 million tonnes: 1.27 million tonnes direct emissions, 170,000 tonnes indirect emissions; 11,000 tonnes from their landfill sites
Hanson: 1,635,902 from cement production (they also make bricks etc.).

Hope?  Imperial College engineers are launching a new cement, based not on limestone but on magnesium silicates.  This, they hope, will be carbon negative, as it absorbs carbon and takes half the fuel needed for furnaces.  Of course, this isn’t making the splash that it perhaps should – it was sold last year owing to its British owner’s insolvency.


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