Carbonated aggregates

Carbonated aggregates have recently featured in the news as a result of the negative carbon footprint of Lignacite’s “Carbon Buster” concrete block – the first concrete block to achieve this. Needless to say, the block uses “manufactured” aggregates – artificial aggregates who’s manufacture converts carbon dioxide gas into solid carbonate products, giving the material its carbon negative credential – sufficient to more than offset the carbon emissions of the cement used in block’s manufacture, with a net capture of 14kg of CO2 per tonne of manufactured block.

(Carbonation is a naturally occurring chemical process where atmospheric carbon dioxide slowly reacts with the hydrated Portland cement matrix of concrete, altering its chemical composition. In doing so, it reduces the alkalinity of the concrete from a pH of 12.5 to 8 or less and exposes any underlying steel reinforcement to the risk of corrosion.)

Patented by Carbon8 Aggregates, Accelerated Carbonation Technology speeds up this process and is capable to turning waste into a material the Environment Agency have confirmed is a “product” (i.e. the carbonation process is an agreed waste recovery process) suitable for virgin aggregate replacement saying “Concrete blocks made from Carbon8 Aggregate … show no worse detriment to the environment or human health than blocks made with virgin aggregate”. The process mixes APCr (Air Pollution Control residue) with carbon dioxide, sand, cement and water to make C8Agg®, an inert carbon-negative aggregate suitable for many applications.

Carbon emissions of cement and concrete

Whilst the production of Portland cement is very energy-intensive and gives off virtually as much CO2e as the weight of product it produces (approximately 910 kg CO2e per tonne of cement), it is only one component of concretes and mortars. In addition, OPC is normally blended with cement replacement materials such as ground granulated blastfurnace slag (ggbs – 67 kg CO2e per tonne) and fly ash (pfa – 4 kg CO2e per tonne). As a result, the weighted average for all cements and blends sold in the UK is approximately 850 kg CO2e per tonne. (Cradle to factory gate)

Reference to the Sustainable Concrete website indicates that in 2012, the average carbon intensity of UK concrete was 79.4 kg of CO2 per tonne of concrete produced, a 23% reduction from the 1990 baseline, with a target reduction to 72.2 kg CO2 per tonne by 2020. Reinforcing bar typically adds a further 10 kg of CO2 per tonne of concrete used.

Carbonation & secondary concrete aggregates

Production of the cement used in concrete is recognised as one of the largest single contributors to greenhouse gas emissions, making up 5% of global CO2 emissions. However, once cast, concrete begins to carbonate, reabsorbing some of the carbon dioxide given off in its manufacture, the rate of carbonation being dependent primarily on the porosity of the concrete matrix, and to a lesser extent, its aggregates

Recent research in Japan has shown that CO2 uptake by the cement hydrate increases significantly when particle sizes are small and are subjected to alternate wetting and drying. Examination of concrete from crushing plants has indicated a typical carbonation uptake of 11kg of CO2 per tonne of crushed concrete aggregate – about 14% of the average UK production carbon dioxide emissions per tonne.

Belin has taken this further and looked at the benefits that enhancing carbonation of recycled concrete aggregates may have on its performance as a material. Recognising that porous weak cement matrices in old concrete leads to poor durability, the researchers found that by accelerating carbonation in crushed concrete aggregates, porosity was dramatically reduced, making the treated aggregates comparable to natural sands and stones commonly used in concrete manufacture.

Which seems like a win-win opportunity for the construction industry. By accelerating the carbonation of selected post-demolition crushed concrete, not only is the end product more durable and suitable for use in concrete rather than being relegated to fill, but in doing so it also absorbs a proportion of the carbon dioxide given out during its initial manufacture. It will be interesting to see if carbon sequestration through accelerated carbonation treatments of crushed concrete becomes more common over the next few years in projects looking to reduce their carbon footprint.

(This is an extract from a longer fully referenced article on this topic available from BPS Eco Ltd)

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