Wednesday, 30 January 2013

Fly ash - adding it to cement and china clay waste could save the Pebble Beds

The Devon Stone Federation advised DCC, Local Aggregate Assessment (LAA, 5.15), that as an alternative to the "qualities of the Pebble Beds resource" china clay waste has a number of issues including "increased water demand and proportion of cement required in concrete". However, as posted earlier, this did not stop china clay waste being used in the construction of the Olympic Park or One Coleman Street in London.

What the Devon Stone Federation, whose five quarrying members include AI, failed to mention was that any extra cement required can be offset cost effectively with 'fly ash'. Fly ash was not referred to in DCC's otherwise comprehensive LAA, yet it "has been in common use as a cementitious component in concrete in the UK for several decades". The construction of One Coleman Street used "30-40% fly ash in place of Portland cement" to reduce its environmental impact.

Fly ash is a residue from coal-fired power stations. Worldwide, more than 65% of fly ash produced from power stations is disposed of in landfills and ash ponds. However, "as a very fine, inert and lightweight material, fly ash is ideal when used in the concrete mix as a substitute for cement". "As well as reducing the amount of water required in the mixture, fly ash can limit demand for natural resources. Studies conducted by the UKQAA and partnership bodies have shown that fly ash can be included in the concrete mix at rates of up to 80% of the cement content, significantly reducing the need for both natural aggregates and water, while reducing the concrete’s carbon footprint. While [each tonne of] cement creates approximately one tonne of carbon in the manufacturing process and contains 913kg/tonne of embodied carbon, fly ash contains 4kg/tonne".

Cement production worldwide "produces more than 5% of mankind's carbon dioxide emissions" - more than the entire aviation industry; using fly ash at Coleman Street alone saved 500 tonnes of CO2. AI itself does not produce cement, it sources it from UK producers and a Holcim plant overseas. AI does however supply fly ash, and it is used in 25% of Holcim's cement output.

So, DCC should recognise (at least for concrete applications, which is where two thirds of sand and gravel end up) that with the help of cement additives there are sustainable, environmental, economic and viable alternatives to the Pebble Beds piled up all across Cornwall and Devon, that should be used before plundering more virgin resource. It may not be in the Devon Stone Federation's interests to promote the use of waste china clay aggregates, but surely it must be in the interest of the county, its people and the environment, and this should be reflected in Devon's mineral policies.