Embodied carbon and cement replacement: what are the alternatives?
Over recent years, the UK has taken important steps to become a world leader in cutting greenhouse gas emissions. In 2019, the Government declared a climate emergency and set a legally binding net zero target by 2050 and new interim targets to reduce emissions by 78% (relative to 1990 levels) by 2035. Last year the Net Zero Strategy was released with detailed actions for each industry across the economy, including targets on gradually phasing out fossil fuels.
The built environment is responsible for about 50% of carbon emissions in the UK. More scrutiny is now given to the whole lifecycle of buildings to see where emissions can be further reduced.
This article explores the importance of embodied carbon, traditional strategies to cut emissions and potential alternatives to be considered.
As highlighted in LETI (London Energy Transformation Initiative)’s guidance Embodied Carbon primer, the embodied carbon represents an increasing proportion of the whole life carbon of a building to become significant between 40-70% depending on the use of the building.
With the decarbonisation of the electricity grid increasingly relying on renewable energy, the proportion of operational carbon, associated with the in-use operation of the building, is steadily decreasing compared to the embodied carbon (CO2 and other GHGs associated with product, construction, use, end of life). With a fabric first performance approach and energy efficient measures put forward and encouraged, plus the use of renewable technology such as air source heat pumps and PV panels, new buildings have the potential to become ultra-low energy/zero carbon in operations by 2030.
It is shown, when analysing the breakdown of the whole life carbon stages, that the product stage (A1-A3) / and overall upfront carbon phase (A1-A5) account for the largest proportion of carbon over the life cycle of the building. Whereas operational emissions represent only about 25-30% in ultra-low energy buildings.
This analysis shows that embodied carbon is crucial to consider when assessing buildings’ emissions.
The UK is reliant on fossil fuels (gas, oil, and coal) for its production of electricity, with gas being the most exploited (40% in 2021). In the current context of climate emergency, the UK government has committed to phase out coal and to have all new homes banned from installing gas and oil boilers by 2025.
Coal contributed to only 2% of the UK electricity mix in 2020 and 2021, down from 25% five years ago, which shows the progress made by the UK on this front.
Coal production is relevant to this study, as one common cement replacement material, Pulverised Fuel Ash (PFA), or fly ash, is a by-product of coal-fired power plants.
When coal power plants were widely used in the UK, it was an innovative process to reuse the waste from the industry into the construction industry, introducing circular economy concepts. The pollution impact extent was however not noticeable yet, and today, the much-needed coal phase-out has put a strain on PFA supply.
UK Steel Production
Another common cement replacement material used to improve cement properties and reduce embodied carbon is Ground Granulated Blast Furnace Slag (GGBS). GGBS is a by-product of steel manufacturing and has been used as partial replacement for Portland cement throughout the 20th century. Nowadays it is still the go-to measure recommended to reduce the embodied carbon of concrete elements.
In September 2017, the UK Government published research on Cement manufacturing: use of fly ash and blast furnace slag, reporting the future availability of fly ash and granulated blast furnace slag for UK cement manufacturing. The paper states that the UK primary steel production has declined significantly in the past two years, reducing the amount of available slag. UK stockpiles of granulated blast furnace slag are small and imports from Asia have steadily increased, driven by an economic price.
Moreover, DSG Consultants reported in July 2021 that India and Turkey have become major sources of blast furnace slag to Western Europe (52% imports in 2020). Demand from these countries is set to increase, as slag demand is on the rise and the European production is under carbon constraints.
Far-distance sourcing goes against the original aim to use slag from steel manufacturing, which is to reduce the embodied carbon of concrete. Besides, the impact on logistics is significant, with increasing size of storage facilities and unloading equipment. The supply chain needs to account for the increase in shipments, which would contribute to further increased embodied carbon emissions.
Cement replacement supply shortages
Both UK coal phase-out and decline in steel production have led to cement replacement supply shortages – materials would need to be imported from other countries, depleting other areas of the world, and significantly increasing transport distances – adding more carbon to the atmosphere. Therefore, advising PFA or GGBS as cement replacement cannot be considered as the main sustainable measure anymore in the UK, as it defeats the purpose of a green economy towards reaching net zero and phasing out fossil fuels as main targets.
Nowadays architects and structural engineers still widely rely on concrete for numerous benefits including speed of construction, strength, and low costs. Building green and reducing carbon emissions is perceived as not compatible with ‘business as usual’, traditional construction and out-of-the-box design is becoming necessary.
Alternative options need to be reviewed at an early stage as part of the Life Cycle Assessment and during Circular Economy discussions between design team members, to see what can be done to minimise embodied carbon without relying on cement replacement.
What are the alternatives to reduce embodied carbon emissions?
As described earlier, upfront carbon (modules A1-A5, from the production of raw materials to the construction) is often the most carbon intensive step in the life cycle of a building. This accounts for materials extraction, manufacturing, transport, construction, and installation process (Figure 1).
When focusing on this phase of the life cycle, it is useful to look at big ticket items (Figure 3) that indicate where to focus on when reducing materials impact and therefore carbon emissions.
Materials with the highest embodied carbon content are typically found in the superstructure (upper floors, frame), substructure (foundation, piling) and façade (envelope). Internal walls, finishes and external works also have an embodied carbon impact though not as significant.
Figure 3: Big ticket items study for a typical mixed-use development (Source: LETI, Mirko Farnetani)
Considering this, what can already be done in 2022?
In line with the circular economy principle “Conserve resources, increase efficiency and source sustainably”, working on a lean design will help to reduce the quantities of materials used. This is done by the design team at early stage (typically RIBA Stage 2) through an assessment of different options for the structure and big-ticket items including the façade design. The embodied carbon of each option is then reviewed to choose the most appropriate materials and design.
Lean design is already widely implemented in major projects and encouraged by structural engineers. Measures include reduce grids and floor slabs, optimise member utilisations, rationalise, avoid basements and minimise foundation sizes by designing a lightweight superstructure and façade.
Working on lean design in all new buildings would help reduce part of the embodied carbon emissions. It could be developed further with new innovative techniques emerging, such as adoption of new shape of floors that could cut concrete usage by 75%.
Low Carbon Materials
There are several alternatives on the market and fresh from the innovation lab that can be explored as a contributor to embodied carbon reduction.
Even low embodied carbon finishes, though representing a small part of the total, can have a positive impact when adding up to the rest of the improvement measures, and help to further reduce emissions. Considering exposed concrete for floors and ceilings is also an effective option to reduce finishes and therefore embodied carbon.
A list of 10 examples is given below to show what is possible to reduce carbon emissions as alternative or additional measures:
- Recycled aggregates (RA). Adding RA into concrete can help reduce embodied carbon, however this should be only specified when locally available otherwise the benefits are outweighed by the additional transportation. A low percentage (about 20%) can be used to retain the concrete properties.
- External walls recyclable or low embodied carbon blocks: Gablok, clay blocks (for example Ziegel), zero waste and reusable (e.g. Polycare innovation). Mostly used for low rise residential properties, these blocks can be a solution for future high rise in a few years’ time.
Clay blocks build-up (versus concrete build-up with bricks) can reduce embodied carbon emissions by 30% (Greengage calculations).
- New concrete: ultra low carbon concrete, wood concrete developed in France last year. Low carbon concrete solution Align.
- Engineered bamboo for structural or decorative elements.
- Cladding insulation: recycled polystyrene, Foamglas (minimum 60% recycled and new glass with mixed carbon).
- Mushroom materials: ongoing innovative research around fungus for insulation and bricks.
- Recycled aluminium. A kilogram of recycled aluminium displaces only 3.5 kilograms of virgin material compared to the 85 kilograms required for virgin aluminium. (Source: Sustainable Building Materials for Low Embodied Carbon | Architect Magazine).
- Low embodied carbon plasterboard such as Ecosmart, Fermacell. A drywall made from absorbed carbon dioxide has been launched this month.
- Composite window frames instead of aluminium
- Timber: timber studs instead of steel, subject to acoustic requirements. Cross Laminated Timber (CLT) instead of concrete.
Moreover, the Environmental Science & Technology journal published an article on Material Diets for Climate-Neutral Construction last month (April 2022) that identify low-carbon and climate-negative materials based on their net Global Warming Potential (GWP). This further helps to understand what materials to use in construction to minimise GHG emissions (see Figure 4 below).
Figure 4: Material classification according to the net-GWP value (Source: Environ. Sci. Techno. 2022, 56, 5213-5223)
Transport and methods of construction
Sourcing locally materials (less than 15-20km) will help to further reduce embodied carbon emissions (A4 module “Transport to site”).
In addition, on construction site, using EV vehicles for transport and construction will allow to cut A5 emissions.
The Case of Timber
It is shown that a cross-laminated timber (CLT) structure can emit about 30% less carbon than reinforced concrete structure (independent study).
Considering this significant reduction and the UK green agenda, you may wonder why there are not more timber buildings in the UK?
In the light of the Grenfell tragedy, the UK government decided to review the building regulations on fire and produced an amendment banning combustible materials (including timber) on all residential buildings over 18 metres high.
Even though feasible for low rise residential developments, the fear and lack of knowledge of timber properties make it difficult to adopt this natural material as a main element of the structure. This is a reason why structural engineers and architects are usually reluctant to use timber as part of the design.
With the emphasis around safety in the UK, and developers having to follow building regulations, timber buildings designers must disclose all facts to the insurer that are material to risk, which is a long process that could result in a non-agreement. Time allocated to this could therefore be lost.
Besides it is believed that rebuilding and repair costs can be increased when timber is used, therefore insurers develop a fear of increasing costs and interrupting timescales if they agree to assure timber buildings, which is why many decide against.
A small revolution is needed in the construction world, where new materials need to be introduced and adopted as best practice throughout the industry to reduce carbon emissions. The necessary change from traditional steel and concrete to more innovative design face several limitations. These include timescales, costs and fire safety, and a general inflexibility, ‘building as usual’ preference in the construction sector mostly due to safety and efficiency concerns from developers, clients and planners. Discussions with contractors also showed the difficulty encountered in supplying innovative construction elements such as low carbon concrete or other green products that cannot be easily found in the UK marketplace.
Research on innovative materials is very active and a switch in mindset is required from professionals and insurers to try new techniques, designs, and materials, to set a new standard for the UK as a leader in zero carbon construction. During projects, more time could be given for research to explore different options at early design stage and select low embodied carbon, cost-effective, and locally sourced materials where feasible.
All team members need to engage in the process of finding non-traditional materials and come together to commit to reduce embodied carbon emissions. With major developments, there would be room to test different options and choose the most carbon efficient path.
Construction professionals need to be bolder and get out of the traced path of concrete with GGBS content and steel ‘business as usual’ design for all buildings.
Finally, a change of policy, along with educational sessions on timber properties, will be required if the UK wants to move forward and build high rise CLT buildings, whereas more timber structures arise in other corners of the Earth, contributing to cutting GHG emissions worldwide.
Written by Manon Dangelser, reviewed by Liz Grove.