Previously we’ve spoken about improving your home internally. A fresh lick of non-toxic paint, or some reclaimed timber shelves being just two ways you can spruce up your space with the environment in mind. This week however we take a slightly different stance, as we delve into the ever-evolving world of building materials, past, present and future.

Rammed Earth
A rammed earth theatre lecture. Courtesy of CAT

Our main focus will be the Centre for Alternative Technology (CAT), an educational charity in South Wales, ‘committed to researching and communicating positive solutions for environmental change’. CAT use natural materials throughout their site and hold workshops on green building. For a comparative contrast we will also examine just how damaging some of our beloved building materials really are, concrete being a primary example.

The construction industry
The built environment is responsible for around 40% of the UK’s overall carbon footprint, According to the UK Green Building Council. This staggering figure includes the energy used to maintain existing buildings, through heating and electricity, but also the overall ‘embodied carbon’ of buildings.
‘Embodied carbon’, or ‘life cycle impact’ are useful starting points when understanding the environmental repercussions of a given material. They assess the environmental impacts of extracting, manufacturing and transporting construction materials, as well as the building’s maintenance footprint and what happens to all the material once it’s demolished.

Stages that enable construction. Courtesy of the Irish Green Building Council
Stages that enable construction. Courtesy of the Irish Green Building Council

The cost of concrete

Concrete is the most prolific man-made material on the planet and has enabled cities to develop at an exceptional rate. Unfortunately though, it has the highest embodied carbon rate of all materials used in construction, producing around 550kg of CO2 per cubic metre used. To understand how this figure is calculated, it’s helpful to unpack the embodied carbon definition a bit further.

Concrete is comprised of cement, water and aggregates (sand and gravel). The excavation process to produce Portland cement involves quarrying, which causes airborne pollution as dust particles and releases CO2. Extracting aggregates and sourcing water also requires intensive machinery and transportation which contributes to its embodied carbon status.

The removal of gravel and sand at such a rapid rate for concrete production is also stripping riverbeds and beaches of sediment, thus increasing flooding and causing conflict within these coastal regions. Such information isn’t necessarily relevant to the embodied carbon equation, so whilst the term is useful, it is worth also considering the ‘life cycle impact’ of a material. This holistic approach recognises the environmental disturbances of construction that aren’t exclusively related to carbon emissions.

In order to make cement and concrete, industrial sized kilns are needed which are energy intensive. The chemical process of making cement also produces CO2 and is responsible for around 8% of global CO2 emissions.

Centre for Alternative Technology (CAT)

Centre of Alternative Technology

Cabins for guests and students. Courtesy of CAT

The subject of concrete can be a distressing one. Fortunately though, there are organisations who are developing solutions. CAT was founded in 1973 by engineers, architects, builders and growers, who wanted to steer away from a fossil fuel dependent society, and provide viable alternatives. The educational charity specialises in ‘sharing practical solutions for sustainability’ and offers an array of short courses and workshops including natural building, ecologically informed food growing, renewable energy and wastewater management.
They also offer various master’s degree programmes, including an MSc in Green Building, and in Sustainable Food and Natural Resources. These are renowned for their practicality and hail people from all over the UK and Europe.
To accompany their courses, CAT have released a series of reports with guidance for governments to achieve a carbon neutral society. These resources have existed long before the Net Zero Britain target was announced in 2019, yet they provide an invaluable guide for policy makers to reach this goal. Net Zero Britian is a legally binding contract to decarbonise Britain by 2050, a topic we will cover in more depth in another article.
So now that you have an insight into CAT, let’s see what they have to say about concrete and cement.

CAT’s alternatives

CAT argues that in many cases, cement can easily be replaced with more environmentally friendly alternatives. Lime-based render, mortar and clay plasters are perfectly sufficient are much healthier and environmentally friendly options. Instead of concrete block construction, CAT uses various materials including unfired earth, hempcrete and straw bales. Some of which are abundant and reusable, such as earth. Whilst others are renewable as they can be regrown. They can also be sourced more locally, thus reducing CO2 emissions through transportation. Hempcrete is a popular alternative to concrete and it is considered a ‘better-than-zero-carbon material’, as it stores more atmospheric carbon for the lifetime of the building than is emitted during extraction, construction and transportation.

Centre of Alternative Technology
CAT's main site. Courtesy of CAT

So, what exactly is hempcrete?

The life of hempcrete begins in the field. Hemp is a tall thin plant that can grow up to 3-4 metres in four months without the use of fertilisers. Whilst it grows it improves local biodiversity and also rapidly absorbs CO2 from the atmosphere.
Hempcrete is made through mixing the ‘shiv’ of the hemp plant (the inner core) with a lime-based binder. This binder is similar to cement as it is produced through intensive heating in a kiln. The high embodied carbon content of the binder is then offset by the atmospheric carbon that the hemp sequesters (when referring to the embodied carbon equation). The finished product is a durable, insulating, reworkable material, appropriate for both new builds and renovation projects.

Healthy breathable buildings are the future

Hempcrete buildings are breathable buildings. This means they buffer humidity and moisture, thus preventing damp and naturally regulating the temperature of the building. Concrete and brick, due to their density, aren’t very breathable or insulating, meaning such buildings require more carbon consumption through maintenance alone. It is important that architects and engineers take this on board when drawing up plans for new builds. Equally relevant too is retrofitting existing homes to improve their energy efficiency. CAT have information and resources on retrofitting with natural materials, so you don’t need to move to Dorset and build a hempcrete house just yet!

Centre of Alternative Technology
CAT under construction. Courtesy of CAT


Unfortunately, the construction sector is dominated by a small number of major producers who are often unwilling to adapt their business models and experiment with alternatives. Concrete is reliable and cheap, despite its colossal environmental impacts. Very few sustainable alternatives have reached commercialisation and therefore aren’t being applied on a large scale. In order to change this governments must support low carbon construction initiatives with funding and apply pressure on the industry to change. This also requires scrutinising the embodied carbon and lifecycle impact of materials and incorporating adequate regulations into policy that reflect this model.
Following on
This article only scratches the surface of such a broad and complex subject. We think it’s important to start somewhere though and to acknowledge that the construction sector, as the largest single consumer of resources in the world, needs addressing.
We hope that you feel inspired to use natural building materials after reading this and we encourage you to have a look at CAT’s website for more ideas. Many of the methods they promote, such as building with hempcrete, straw and timber-frame techniques, can all be learned relatively easily and quickly. We will also be discussing our own experiences of green building and retrofitting in future articles, so stay tuned to find out more.

Centre of Alternative Technology
Older footage of CAT. Courtesy of Arte Útil
August 18, 2021

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