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CO2 emissions from cement production - a new net-zero ready mixed product

CO2-emissions-from-cement-production CO2 emissions from cement production

 A new net-zero ready mixed cement product

Concrete is the most important structural material in the world. A composite material made from cement, aggregates such as gravel, sand and rock, along with water, humankind has been using it since the times of the ancient Egyptians. However, behind concrete's many benefits lurk massive dangers to both the planet and the people who live on it.

While concrete poses multiple threats to the environment, the most urgent of these is its effect on climate change and global warming. Cement, concrete's primary ingredient, is produced using an energy-intensive process that emits massive quantities of greenhouse gas. We will look at this in some detail.

The Paris agreement on climate change called for a 16% reduction in annual carbon emissions from the cement industry. In 2018 Chatham House proposed a rethink on cement manufacture including increased use of renewables in the production process, higher energy efficiency, alternative source materials, and carbon capture.

Researchers and businesses have made substantial progress towards these goals. We will look at the current state-of-the-art, including the launch of UK's first carbon-neutral ready mixed concrete product. There is a twist, however - achieving carbon neutrality relies on offsetting some of the carbon emitted in its manufacture.

Cement manufacture and carbon emissions

Manufacturing cement is a complex and energy-intensive process. The most common type used in concrete is Portland cement. To make this, the raw materials, mainly clay and limestone, must be mined and then ground to a fine powder. The mix is then fired in a kiln at around 1450 °C to produce clinker nodules typically 1 mm to 25 mm across. The clinker is ground to a fine powder and mixed with gypsum to produce the cement. The gypsum controls the cement setting time. During the heating cycle, the raw materials react to produce a mixture of various calcium silicate and aluminate minerals along with carbon dioxide (CO2).The firing process emits 0.55 tonnes of CO2 for each tonne of cement produced. If the heat generation is from a non-recyclable source such as coal, an additional 0.23 tonnes of CO2 is produced. Overall, the production of 1-tonne cement produces 0.78 tonnes of CO2.

If that sounds a lot, then that's because it is. Cement production accounts for 4.8% of global anthropogenic greenhouse gas, a massive 2.8 billion tonnes a year, and is the third most significant contributor after transport and energy generation. It doesn't stop at CO. Cement manufacture produces a wide range of additional pollutants including nitrogen oxides (NOx), sulphur dioxide, dioxins, polychlorinated biphenyls (PCB) plus many other chemical toxins. 

Low carbon cement 

Many organisations are exploring alternative technologies that promise substantial reductions in cement's massive carbon footprint. There are two approaches: (1) reduce the carbon emissions from the firing process and (2) change the chemistry to create less CO2.

Cement kiln fuels 

Reductions to the carbon footprint of cement could be readily achieved by switching over from non-renewable energy sources such as coal to renewables. Cement kilns are usually heated using hot gasses produced by combustion of kiln fuels, typically coal, coke, heavy fuel oil, and natural gas. Alternative fuels (AF) such as used car tyres, sewage sludge, chemical waste, and other unpleasant materials are also used. While none of which could be considered environmentally friendly and all of which contribute to global warming, they produce less CO2 than pure fossil fuels.

Alternative cement chemistry

Many alternative cement chemistries have been investigated, though the most promising of these appears to be geopolymer cement. Geopolymer types of cement are aluminosilicate based. They are manufactured from industrial waste such as slag from blast furnaces and fly ash from coal-powered power stations. Extensive research is being carried out into the production of safe geopolymers that promise a massive decrease in CO2 emissions. Several such products are now on the market. 

One example is "Vertua – low Carbon Concrete" developed by Cemex. The product uses geopolymers to achieve dramatic CO2 reductions of up to 70%. The company is marketing it as "the UK's first net-zero ready mixed concrete" but make net-zero carbon only by offsetting the 30% CO2 that they cannot remove from the process. CEMEX provides carbon offsetting by investing in projects that remove CO2 from the atmosphere, for instance, tree planting and fighting deforestation. This tactic means the company can provide clients with a carbon-neutral certificate.

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