I just submitted a Treasury Board of Canada Secretariat questionnaire, intended for individuals in the consulting and contracting industry, on the Government of Canada’s (GC) decision to set minimum standards for the carbon footprint of concrete used in federal construction projects. The questionnaire provoked many reflections which I would like to share, not only in the interest of furthering the worthy message of the GC, but also to address the collective concern of all of us who appreciate the environmental ramifications of modern-day urbanization and industrialization.
It is noted that cement production accounts for more than 8% of all carbon dioxide emissions annually. GC is mandating, that starting 2021, cement specified by GC will be Portland Limestone Cement (PLC) or equivalent. This is deemed to achieve a 10% lower-carbon emission. The objective can be met through the use of PLC or by the use of substitute cement material (SCM) to replace part of the cement in concrete. GC is further intent on mandating Type III environmental product declarations (EPD) which are third-party verified reports that describe the environmental performance of the supplied concrete.
SCMs include fly ash and ground granulated blast furnace slag (GGBFS) These materials are bi products of steel production and thus do not require the intense use of energy involved in the clinker production. Many engineers would be familiar with the use of these SCMs, particularly when seeking LEED® points under the MR 4.1 and 4.2 credits. SCMs also reduce the concretes heat of hydration and are therefore often specified to mitigate shrinkage cracking. Some SCMs are naturally occurring such as volcanic pozzolana, which I am quite familiar with when practicing in Jordan, where pozzolana cement has been produced for more than twenty years, even before Lafarge took over those plants.
While acknowledging the current limitations on the percent replacement with more environmentally friendly cementitious material, significant carbon emissions can be reduced with a paradigm shift in the industry. PLC use in North America is lagging behind that in Europe. For example, at the time when ASTM regulated the use of 5% limestone in cement in 2004, 31% of the cement used in Europe was already PLC, see Paul Tennis. CSA allowed 5% use in CSA A3001 in 2006 and later increased it to up to 15%.
The second stage of regulations by the GC is to raise the bar by combining the use of PLC with that of SCMs content. It is about time that we start specifying more environmentally friendly concrete, even for non-LEED® projects, which incidentally may come with cost-saving as the SCMs are less expensive than Ordinary Portland Cement. It is worth noting that SCMs tend to attain higher strength over time than equivalent OPC. This begs the question that our practice and standards may be better served if we move away from the 28 strength as the standard yardstick and have a combination of a longer term, perhaps 56-day strength, to reflect the lifetime condition, together with an earlier specified strength for construction process requirements. This will result in even more embodied carbon reductions.
It must be said that many other technologies are being explored to deal with this source of GHG emission more profoundly such as the use of wind power to produce the clinker as was piloted in Vattenfall, Sweden. There is the even more exciting possibility of the development of living bacterial concrete that can grow and heal itself, as reported in the New York Times January 15, 2020, to name a few.
A final point in defense of concrete is the process of carbonation continues to sink carbon from the atmosphere into the concrete structure for its entire life, and even its demolished components after its demise, as articulated by the Portland Cement Association potentially making it fully carbon neutral in longer timeframes.
Hassan Saffarini, Manager, Structural Engineering
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