The interview by Global Cement Magazine’s editor Peter Edwards with the president and CEO of the US Portland Cement Association on page 56 (Global Cement Magazine - May 2018) throws up some interesting points. Among them is the suggestion that cement and concrete are battling other building materials - notably wood - on a new front: that of sustainability. If you accept that we need to reduce our impact on the planet (who doesn’t?), then we really need to improve our sustainability and to ‘go green.’
Wood looks ‘green.’ In its former life, as a tree, it actually was green, at least in colour. During its lifetime, a tree takes CO2 from the atmosphere and by the near perfect process of photosynthesis (only recently ‘improved’ by scientists1), using sunlight, makes water, sugar and oxygen,2 sequestering CO2 in its cells. There are around 3 trillion trees on the planet, or around 420 for each person.3 If each tree weighs 10t on average, or 5t on a dry weight basis, that’s a lot of carbon sequestration in total. However, that’s strictly a temporary sequestration, since the carbon sequestered is rapidly given up when the tree dies and rots. Wood used in construction is, again, just a temporary sequestration of carbon. Wood used in construction looks nice, but it’s not the whole story.
Cement, on the other hand, is guilty of negative sequestration: The CO2 that was formerly bound up in the CaCO3 that makes up limestone has to be liberated during the production of Portland cement, to make the CaO that is the ultimate basis of clinker. CO2 liberation from Portland cement clinker manufacturing is something that cannot be avoided.
However, that is not the whole story. Clinker is only one of the ingredients in cement, and cement is only one of the ingredients of concrete - the ultimate product of our industry. To reduce the amount of CO2 that is produced during the production of concrete, we can reduce the amount of clinker in the cement and/or reduce the amount of cement in the concrete. For people who are in the business of selling cement to make concrete, this does not sound like a great choice.
The cement industry and the concrete industry are really in the business of sticking things together and making things that last (and that have a wide variety of other desirable attributes as well, including resilience, strength, low labour cost, low material cost, attractiveness, durability, high thermal mass etc). The cement industry would rather use cheaper ingredients in its cement than costly (and polluting) clinker, if the cement has the same properties. So, to come to the theme of this ‘Last Word,’ is a reducing clinker factor (the amount or percentage of clinker that is used in cement) a threat or an opportunity for the global cement industry?
Of course, there are threats and opportunities of reducing the clinker factor, but I think that the opportunities vastly outweigh the threats. For instance, threats from a decreased clinker factor might include:
- Scarcity of alternative materials with cementitious properties (granulated blastfurnace slag, quality flyash, natural pozzolans);
- Transport costs for supplementary cementitious materials (SCMs) compared to local materials;
- SCM preparation costs (primarily grinding);
- Any loss of performance and possible reputational damage for subsequent material failures
... but opportunities might include:
- Lower costs of SCMs compared to clinker;
- Better grindability of SCMs compared to clinker;
- Improved cement performance (particularly when using additives) compared to OPC;
- Vast stockpiles of potentially usable SCMs;
- New SCMs on the horizon (Feeco’s suggestion of using red mud, for example4);
- Greater sustainability in cement production.
As seen on page 53 of this issue, a cement plant in Cuba is to start to produce LC3 (‘limestone calcined clay cement’) cements. LC3 cements5 have a clinker factor of only 50%, with the rest being 30% calcined clay (metakaolinite), 15% limestone and 5% gypsum. As the promoters of the technology state, producing LC3 cement ‘does not require capital intensive modifications to existing cement plants’ and produces binders with the same performance as OPC. There are other options out there for lowering the clinker factor and every possible replacement material for clinker is cheaper than clinker. What’s not to like?
The environmental impact of concrete - already one of the ‘greenest’ building materials by weight - can be further reduced as well. As seen on page 68 of this issue, CO2 can be recycled back into concrete, increasing its strength and reducing the requirement for cement. Discuss!
1 https://www.technologyreview.com/s/608535/to-feed-the-world-improve-photosynthesis/
2 https://en.wikipedia.org/wiki/Photosynthesis
3 http://www.bbc.co.uk/news/science-environment-34134366
4 http://feeco.com/sustainability-in-industry-employing-red-mud-in-construction-materials/
