‘Green’ Cement: The Uncomfortable Truth – Latest

‘Green’ Cement: The Uncomfortable Truth

The construction industry is eager for sustainable innovations, but when utilised without care and intelligence some solutions can do more harm than good. Elliott Wood Founder and Chief Executive, and Structural Engineers Declare co-chair, Gary Elliott, shares his thoughts on the uncomfortable truth about ‘green’ cement replacements:

At a time when we are all trying to find ways of reducing carbon generation, it's staggering to see our industry taking such a thoughtless and wasteful approach to employing alternative solutions.

As we’re all aware, the production of OPC (Ordinary Portland Cement) creates very high levels of CO2. To mitigate these emissions, GGBS (Ground Granulated Blast furnace Slag) and PFA (Pulverised Fuel Ash) - by-products of the steel and coal industries respectively - are often used as a partial, or in some cases total, replacement for cement in construction.

On the face of it, replacing a carbon-intensive raw material with an industry by-product gets a big thumbs up in the face of the carbon crisis. So, does this mean we can return to business as usual, building in concrete? Absolutely not.

Using high proportions of these cement replacements in the UK, for standard construction, makes no sense whatsoever. There is simply not enough GGBS and PFA to go around. If we use GGBS on a project in the UK, that's material that can’t be used elsewhere in the world, where it might be more useful.

Much of these by-products are currently being shipped into the UK, in some cases travelling halfway around the world from countries like Japan and China, a process which largely negates any carbon emissions saved by specifying ‘sustainable concrete’. It would make far more sense to use these materials at the location of origin rather than transporting them.

In the UK, their use is being justified in standard concrete construction on the basis that it represents a ‘low carbon’ solution. When GGBS/PFA is specified in the UK, the source of the materials is likely unknown, meaning that their actual carbon factors are also unknown and therefore often underestimated. We might all be thinking that we are doing the ‘green’ thing in using it, but we most definitely are not!

GGBS and PFA cement replacements can be very useful if used intelligently for mass pours, in hot countries; where the heat of hydration is problematic or where there are aggressive ground conditions. For the vast majority of cases this does not apply to our building construction industry.

And it gets worse; the curing times for even moderately high concentrations of GGBS/PFA are much greater than for OPC mixes. This impacts contractors’ programmes, and of course time is money. In order to speed up the programme, contractors are therefore increasing the amount of OPC in the mixes. This causes an imbalance in the specified proportions of the OPC:GGBS mix, so in order to make sure the mix is still aligned with the specification, the amount of GGBS is also increased to maintain the intended proportions.

This means that both the amount of cement and the amount of GGBS have increased in the mix, well beyond what is required to achieve the strength requirements, simply to achieve programme improvements. Taking a carbon-saving substitution and utilising it without consideration for the knock-on impact of transportation, local conditions or material scarcity, all to maintain 'business as usual', is utter madness.

While there is a place for the intelligent use of low carbon substitutes in construction, to reverse the damage already happening to our planet requires a far more drastic shift in our industry’s attitude towards materials. We cannot build our way out of the climate emergency. We need to attribute greater value to what we have already and prioritise repurposing existing buildings and reusing the materials contained within them. Anything else will be too little, too late.

Further Information

Read our helpful guide to approaching reuse: Full Circle to Reuse

Find out more about solutions to help lower carbon emissions and reduce waste in our video series: Too Little, Too Late. - Deep Dive Series