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Carbonation from an environmental perspective

World Cement,


Revisions to Part L of the Building Regulations over the last decade have steadily cut the operational CO2 emissions from new buildings, largely through greater insulation and airtightness requirements. Sadly, there are practical limits on how much further these regulations can be enhanced in the future, as the law of diminishing returns applies. So, in the ongoing drive to cut the CO2 footprint of buildings, more attention is now being paid to the embodied impacts of the fabric itself. This is also being driven by the shifting balance between operational and embodied CO2 in new buildings, which has shifted from a traditionally accepted split of around 90% operational, 10% embodied to the current level of around 70% and 30% respectively, meaning that the embodied component has become more significant as operational emissions have fallen. Whilst the physical amount of embodied CO2 remains about the same, this shift has nevertheless increased its significance within the field of sustainable design.

Of all the major suppliers of construction materials, the timber sector is one of the most vocal when it comes to embodied impacts, with much attention given to the perceived benefit of sequestered CO2 in timber. Less well known is the fact that concrete also absorbs CO2 during its life cycle. This is through a chemical process known as carbonation, which has the benefit that the CO2 remains locked in and is not released at the end of its life, whereas in the case of timber it ultimately decomposes or is burned and the CO2 sequestered (stored) is all released back as CO2 or methane. As a consequence, the relevance of concrete carbonation is now broadening to include its positive contribution to the environmental performance of concrete, particularly at the end of a building’s life when the concrete is typically crushed and recycled.

To get an idea of how much CO2 is absorbed during the life cycle of concrete; the British Cement Association (now MPA Cement) undertook a carbonation study1 in 2007, with the objective of investigating its significance from an embodied CO2 perspective. The study adapted a methodology for calculating uptake that was developed by the Nordic countries.2 This was applied to the UK cement/concrete market, taking account of the volumes produced for each of the main applications,3 along with typical mix designs and strengths. A service life of 60 years was assumed, followed by a secondary life of a further 100 years as a recycled material. These periods were agreed with the BRE Group, who went on to use the study to update the way carbonation is accounted for in its Green Guide ratings. Results from the study showed the extent to which the initial embodied CO2 of cement leaving the plant gate will, on average, be reduced by the end of the 160-year period as a consequence of carbonation. This reduction was found to be approximately 20%, or to put it in terms of concrete, RC 40 made with CEM1 will have its initial embodied CO2 reduced by about 18%, from around 160 – 131 kg CO2/t after 160 years.

As you would expect, the study showed that carbonation during the service life period is relatively low for ready-mix and precast, which were assumed to be relatively high strength. However, it is much more significant at the end-of-life stage, when a high proportion of concrete is crushed and recycled. The shift is a consequence of the crushing process, which hugely increases the surface area, greatly increasing CO2 uptake even when used in ground works, albeit at a slower rate than when it remains above ground. Using figures from WRAP4 and government research into construction waste,5 an average crushing rate of 61% was assumed for UK concrete. However, the reports containing this data were already a few years old at that time, and anecdotal evidence suggests that the current rate is likely to be higher. This is significant as it means that the average carbonation rate calculated in 2007 is likely to be an underestimate by today’s standards.

The crushing and recycling aspect of the original study has recently been looked at in more detail by Mineral Products Association (MPA) to see how much CO2 is likely to be absorbed specifically during deconstruction and demolition. This represents a distinct phase in the life cycle of a building as set out in BS EN 15804,6 the standard that provides the core set of Product Category Rules for the creation of Environmental Product Declarations (EPDs), which will soon appear for a broad range of materials and products. A common EPD scenario is where a building is demolished and the various materials are sorted and crushed onsite, remaining there for a number of weeks or months prior to reuse. Although in lifecycle terms this may seem a short period, the exposure of crushed concrete to air for even a few weeks results in a relatively rapid uptake of CO2. This carbonation, along with that which occurs during the service life of the building and following recycling of the concrete, will influence the overall CO2 figure reported in an EPD. The actual amount of CO2 absorbed during the deconstruction and demolition phase is dependent on a number of factors including time; but initial calculations suggest that it will be in order of 4% or more of the initial embodied CO2 of the concrete, which is not an insignificant amount.

Work in the field of carbonation will continue alongside the development of Products Category Rules that fully reflect the influence this process has on the embodied CO2 of concrete. Ultimately, this will help ensure that the environmental performance of concrete is accurately accounted for in the carbon profiling of buildings.

References

  1. Recarbonation scoping study, C. Clear, T. De Saulles, British Cement Association, November 2007.
  2. Uptake of carbon dioxide in the life cycle inventory of concrete, K. Pommer, C. Pade, Danish Technological Institute, October 2005.
  3. 2005 Market for Cementitious Material, Leading Edge Management Consultancy, March 2006.
  4. Low-strength concrete ground engineering applications for recycled and secondary aggregates, C. Sowerby, DTI/WRAP Aggregates Research Programme STBF/13/8C, June 2004.
  5. Survey of arisings and use of construction and demolition waste, ODPM, 2001.
  6. BS EN 15804:2012 Sustainability of construction works. Environmental product declarations. Core rules for the product category of construction products.

Written by Tom De Saulles, MPA – The Concrete Centre, UK, and adapted from press release by Louise Fordham.

Read the article online at: https://www.worldcement.com/europe-cis/19092013/mpa_concrete_centre_uk_carbonation_concrete_enviornmental_performance_193/


 

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