Can Concrete ever be Clean?

Concrete, by definition, is the most popular material in construction. It is also responsible for a shocking seven percent of global emissions.  It is not emission intensive to produce; the extent to which it is used, however, makes it incredibly polluting.

The raw materials that make up concrete are abundant, easy to source and have incredible structural properties which are vital in laying foundations for buildings. The production process is also relatively cheap. These characteristics make it extremely hard to give up or replace.

Why is concrete so polluting?

Cement: the binding component that holds concrete together, can have different compositions based on the purpose of its use.

The most common type of cement, Portland, is composed of 90% clinker and 10% other materials such as silica and alumina, making it one of the most energy and CO2 intensive types of cement. The production of its main component, clinker, is responsible for the biggest source of emissions in the whole chain.

Let’s talk about clinker…

Emissions associated with concrete production are linked to two processes, both of which concern the production of clinker.

The first one, responsible for around two thirds of emissions concerns the transformation of limestone - the traditional starting base for clinker - into lime through calcination. This process chemically releases large amounts of CO2 and must be carried out at temperatures between 900 and 1250 C°.

The second process sees the lime transformed into clinker in a rotary kiln that reaches temperatures as high as 2000 C° thus requiring a lot of thermal energy, generally from the combustion of fossil fuels.

Green(er) alternatives

Ongoing research aimed at developing greener alternatives to concrete target different stages of clinker production. According to a 2018 study by Lehne and Preston most of them aim to find clinker alternatives or substitutes to lower the clinker content in cement while maintaining the same structural properties.

An example is Cemex, a Mexican building material company which has developed a line of green cements, called Vertua, which achieve different grades of decarbonization. A Swiss laboratory has created a clinker free cement - certified to achieve a 70% reduction in emissions - which obtains its binding and structural properties from a Geopolymer. The company has implemented further energy reduction programs and substituted around 60% of traditional fuels with alternative ones to reduce the energetic impact of the production process.

Companies such as Siemens offer power supply optimization and plant electrification solutions to reduce the energy consumption related to the heating up of kilns.

Clinker substitutes

Many newly patented cement formulas involve the use of fly ash or furnace slag (industry waste) as clinker substitutes. An example is that of EcoCem, an Irish cement producer specializing in low impact cements based on GGBS [Ground Granulated Blastfurnace Slag] and Hanson, a UK based cement producer, part of Heidelberg cement group, which also uses GGBS.

However, the problem with such alternative materials is that they generally have much lower production rates than what would be required to satisfy the entire sector’s demand for concrete. In future they are going to be even less available as they are by-products of carbon heavy industries which are likely to diminish their production. The production rate of slag is only 10% of the potential demand for such material. Furthermore, some of the materials such as volcanic ashes and furnace slag itself, are subject to geographical availability.

Energy optimization

Another strategy that is being implemented across the industry is that of integrating carbon capture and storage technologies in cement plants. This, when combined with oxyfuel technologies (combustion in oxygen instead of air) make for up to 70% more concentrated fumes thus allowing for much easier capture. This could be extremely effective in cutting down the impact of clinker production.  An example is that of Heidelberg cement’s plants in Norway and Sweden, which plan on fully integrating CCS by 2030.

Alternative ways to reduce the carbon footprint of concrete production would be to reduce the need for maintenance and repair mid-way through the operative lifecycle of a building.

To illustrate this, a team at Cambridge University is currently researching the use of microcapsules containing healing agents - epoxy and polyurethanes - to insert in concrete mixes to allow for self-repair of microfractures that normally occur in concrete subject to various types of stress. This would noticeably reduce the need for maintenance and substitution of components ensuring a reduction in waste.

In order to further reduce the impact of concrete use, end of life strategies should be implemented. For example, OB plant has developed technologies to allow for on-site recycling of concrete which allows for the recovery of aggregates and clean water.

On the horizon…

Carbon reduction methods for concrete are numerous and diverse. Most of them are being successfully explored and the market is seeing alternatives made available. Each has their own advantages and disadvantages, and the magnitude of demand for cement certainly does not play in favor of a sector-wide transition. But if the demand for such alternatives keeps rising and research is further stimulated, large-scale use of low impact concrete might soon be a reality.