Time to rethink our approach to low-carbon concrete

The modern built environment is unimaginable without the widespread use of cement-based materials. Today we use about 500 kg/per capita [1] [2], more than the amount of food we consume [3]. The amount of cement has increased 25 fold in the last 60 years [2] [4], a much higher rate than other commodities such as crude steel and the population [1]. CO2 emissions from cement production has been growing in both absolute and relative terms. Today the cement industry make up between 5-8% of the total anthropogenic CO2 emissions [5] [6].

Globally, according to the Millennium Development Goals, there is a need to build decent houses for 828 million people. In addition to houses, the entire built environment and the infrastructure needs to be enlarged, maintained and improved. To cope with these social demands, we need to assure the supply of a cheap mineral binder for the entire world. Since we cannot miniaturize the build environment, there are limitations to dematerialize the construction industry [1].

The demand for cement-based materials is expected to continue growing. The 2050 forecast for global cement production is between 3.7 and 5.5 billion tonnes per year [1] [5] [7] [8]. If conventional mitigation strategies remain the same, a significant increase in total CO2 emissions from cement production can be expected.

Figure: Comparison of cement, crude steel and population growth [1].

At the same time, global CO2 emissions are expected to be reduced. In the scenario that combines the 450ppm mitigation path, which implies a 50% reduction in global CO2 emissions (1990 levels) with business as usual production of cement, by the year 2050 cement production alone will be responsible for between 20% and 30% of the anthropogenic CO2 emissions [4] [7]. This scenario will have enormous costs, both in political terms as well as in carbon taxes [8].

Concrete routinely comes up as one of the highest impact materials in buildings and infrastructure, combined volume of use with emission and resource use intensity. The focus of green rating tools and government requirements has been on replacing the CO2 intensive ordinary Portland cement ingredient in concrete with deemed lower impact materials such as blast furnace slag (BFS), fly ash from coal fired power stations or silica fumes, so called supplementary cementitious materials (SCM).

There is however reasons to question this approach to mitigate the impact from concrete use. There is simply not enough SCM to go around [8]:

  • BFS: Considering all sources, the availability of blast furnace slag will not surpass 0.3 billion tonnes per year, resulting in a maximum average clinker replacement with BFS lower than 10%. Cement consumption is growing faster than steel consumption.
  • Fly ash: Coal plants are still the world’s most important energy source at present. However, according to numerous sources, the maximum average clinker replacement content by fly ash will be 7% or so.
  • Silica fume: Total world silica fume production was estimated to be 0.9 million tonnes in 2006. This accounts for less than 0.01% of total cement production making the analysis of silica availability unnecessary.

It would seem that except for technical or localized reasons, there is no need to promote the use of higher amounts of slag or pozzolans in cement mixtures, except where a technical reason might exist (e.g. heat of hydration or durability) or in places that have localized surpluses due to logistic restrictions. BFS and fly ash must be considered scarce materials that must be explored in very efficient ways [8].

A new dilemma into this equation is the allocation of CO2 to BFS from blast furnaces and fly ash from coal fired power stations. Allocation will be a complex issue with several possible options, all giving very different CO2 intensity values for the slag and ash [9] [10] [11]. Currently BFS and fly ash are routine treated as close to burden free materials compared with ordinary Portland cement, however, this may change subject to the outcome of the allocation question for life cycle assessment (LCA) and carbon accounting purposes. BFS and fly ash may even be considered as impact intensive as ordinary Portland cement when allocation is based on avoided production principles. This scenario will contribute to the costs, both in political terms as well as in carbon taxes.

Our approach to minimising the impact of cement and concrete will be paramount for our success in tackling climate change over the coming decades. The common, convenient approach to substitute out Portland cement is not effective. The menu of options to reduce impacts include [1]:

  • Energy efficiency: A state-of-the art dry-kiln with pre-calciner consumes about 50% less energy than a long wet kiln typically used up to the 70’s. However, state-of-the-art kilns already achieve an efficiency of about 63% against the theoretical minimum limit, which makes it probably the most efficient thermal machine with large industrial use today.
  • Energy matrix: The clinker kiln is also a very flexible machine, which allows the cement industry to change the fuel in a relatively simple way. For example, the Brazilian cement industry changed from almost 100% fuel oil in the 1970’s to a mix between charcoal (~40%) and coal (~50%) in 1984 and nowadays relies almost only on pet coke. In Europe, the use of wastes as fuel can be as high as 80% of the thermal demand.
  • Carbon capture and storage: This is an expensive option. An increase in the price of cement implies an increase in the cost of housing in infrastructure, which has social implication particularly in the developing countries. Carbon capture and storage will not be a sustainable solution in most regions.
  • Binder use optimisation: There is enormous potential to supply the cement-based materials that society needs,with much lower environmental impacts. Globally, a lot of cement is wasted through both losses and overdosing of cement in concrete applications. There is significant scope to reduce CO2 emissions by simply more efficient use of cement, primarily in smaller scale construction and developing countries.
  • Increased binder use efficiency: Leading research show that it is possible to produce concretes using almost a third of the current market and lab benchmarks. This can be done using well-established packing engineering, selected fillers and commercial Portland cement and dispersants. Scaling up this solution will require substantial R&D effort, facing problems related to robustness of this systems and its long-term performance. It also might demand new technologies for production of better controlled aggregates, especially ultra-fine particles, more reactive clinkers, new admixtures, at compatible cost [8].

Since the use of blast furnace slag and fly ash are limited, and carbon capture and storage is not desirable due to its high costs (which will have important social consequences in developing countries), it is necessary to develop new routes to make it possible increase cement production at low cost and significantly low CO2 emissions. The focus of current green building and infrastructure rating tools must change, much along the lines of what has already been done for steel and plastics, where the focus has been removed from recycled content to best environmental practice along the product life cycle.

We need to evolve current cement and concrete standards and sustainability strategies. It starts with broadening the scope away from the current cement substitution focus to more efficient use of cement and cleaner production of the cement we use.


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