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Stone & Slate



Cement production – or more accurately Portland Cement production – is the third largest worldwide CO2 producer after transport and energy generation, and its use is increasing by 5% per annum. For every ton of cement produced, a ton of CO2 is also produced due to the amount of energy required in production, and also the chemical reaction that takes place in the cement itself.
Cement is not hygroscopic, which creates many problems if applied in mortar or plaster to old walls for example, as moisture cannot escape the wall. This property, together with its strength, does make it good to use in wet conditions however, and it has become lazily over-used in modern construction as a result.
Cement can be substituted with up to 50% of Pulverised Fuel Ash (PFA) or Ground Granulated Blast-Furnace Slag (GGBS), which are by-products of power stations and iron blast furnaces respectively.
Cement cannot be reused as such, although it can be crushed and recycled as an aggregate for use in concrete or roads etc. Cement mortar/plaster/render use means that the bricks it is applied to cannot be reused, because it cannot easily be removed.


Defined as ‘a building material made from a mixture of broken stone or gravel, sand, cement, and water’, concrete wouldn’t be a problem if it weren’t for the embodied energy of cement, the aggregates, and its prolific and ubiquitous use – it is after all the most widely used man-made material in the world. Cement is one of the top CO2 producers, largely because of the amount that is used in concrete.
In order to minimise the environmental impacts of concrete, we need to re-learn how to use less of it. The Romans were great users of the stuff, but after the fall of the Roman Empire it wasn’t really used again until the mid 18th Century. Since this time,  its use has grown exponentially - it is simple, inexpensive, durable, waterproof, fireproof, soundproof, and can be poured into any shape.
Concrete also has a very high thermal mass, which means it can soak up and store heat until such a time when the air around it cools again. In the case of concrete, this is particularly useful for office buildings, which it will help keep cool in the summer and pre-warm in the winter.
Because of the energy intensity of cement production, concrete has a reasonably high embodied energy, although it is often produced locally to site using local materials with relatively little production energy. As cement production is made more efficient and aggregates are from recycled sources, its embodied energy is gradually falling.
On the health side, its dust can lead to silicosis if inhaled, and demolition can be a major source of dangerous air pollution. Additives in concrete to enhance plasticity or setting time are also often toxic. However, it is generally benign in its normal state.
Concrete cannot be reused as such (although concrete buildings can), but it can be crushed and recycled for use as hardcore or aggregate. Often concrete structures will have reinforcing steel bars within them; this is taken out at the crushing stage and completely recycled separately.


Un-fired clay is one of the oldest, and one of the most sustainable, building materials that there is - depending upon where your project is. It can be dug by hand straight out of the ground and used to build without additives. If extra strength is required, other natural low-impact materials such as hemp or straw can be mixed in.
It is estimated that between 30 and 50% of the world’s population live and/or work in a building at least partly made with clay. It can be used very simply as in adobe or cob construction, whereby damp clay is flung straight onto a timber formwork, or used as the binder between small stones respectively. It can also be formed into roof tiles or bricks and dried naturally to create a strong, durable material. 
Although relatively impermeable to water, clay remains hygroscopic and also has good thermal mass – it is therefore ideal for use as a means to regulate internal fluctuations in temperature and humidity.
As well as those above, modern uses include clay plasters, clay floor products, clay board (like plasterboard), clay renders, and clay paints.
Un-fired clay is also completely re-usable – or can be simply returned to the soil.


Much like clay above, earth is an ancient, versatile, and very sustainable building material that can be dug straight from the ground next to the site and is still widely used in much of the world – rammed earth buildings can be found on every continent of the world, except Antartica. 
It has the same basic properties as clay, and in many forms of building (e.g. adobe and cob)  is used in conjunction with clay. As well as being used to form blocks and bricks, which may or may not be fired, earth (containing about 30% clay) can be rammed into formwork to form very solid load-bearing walls – the Alhambra in Granada is 1,000 years old and made from rammed earth.
The process of ramming earth inspires a physical reaction within the earth structure causing it to set as solid as concrete. Rammed earth walls are produced in shallow ‘lifts’ (layers) of about 100mm which results in a very attractive finish. Due to the aesthetic of the finished wall, and because of its low-impact and sustainability, rammed earth walls are once again gaining popularity amongst architects, for use as feature walls at least.
A rammed earth wall will have all the properties associated with concrete – fireproof, soundproof, good thermal mass, etc, but unlike concrete it is hygroscopic and so ideal for regulating internal atmospheres.
Often the earth is stabilised by adding cement or lime to make it more weather-proof, or plasticisers are added like animal blood traditionally (to increase workability). Even with these additives, like un-fired clay, earth is completely re-usable once demolished, or can simply be returned to the soil.


Strictly speaking, lime is calcium oxide (CaO) or calcium hydroxide (CA(OH)2) - an inorganic material occurring naturally as a product of coal seam fires. However, the lime widely used is manufactured by altering limestone or chalk - burning limestone or chalk (calcium carbonate, CaCO3) converts it into highly caustic quicklime (CaO), which when water is added becomes the less caustic hydrated Lime (CA(OH)2) or Lime putty.
Lime has a whole myriad of traditional uses in many industries, and even within construction there are several applications. Lime can be used as a mortar, plaster, or render, as a stabiliser in earth, a binder in concrete, or a general whitewash.
Lime is hygroscopic and flexible – this means that it is able to both move with a structure, and allow that structure to dry out, thus avoiding cracking and frost damage. Lime pointing was previously seen as sacrificial – the soft lime would erode but save the stone or brick from doing so. The modern use of cement, which is often harder than the brick or stone around it, has seen much less durable walls being built.
Lime mortars and plasters are much softer than their cement cousins and do not stick to the stone or brick so vehemently – this means that they can easily be removed, so that stone or brick can be reused. Indeed unlike cement, the lime can also be recycled by passing back through the same process again.
Since less heat is required in the production of lime, its embodied energy is approximately two thirds that of cement. Lime is also a natural fungicide and anti-bacterial.

Stone & Slate

You don’t get a much more natural and durable building material than rock – it occurs naturally in abundance (the Earth is made of it!), can be worked and shaped, is very durable, needs little maintenance, is weatherproof, resistant to frost damage (except for the more porous rocks), and can be reused or recycled to an nth degree.
Because of quarrying it does have an embodied energy, but this can be minimised by using locally mined stone or slate, and most good stone and slate will be reused anyway.
If there are concerns with the all-round sustainability of stone and slate, it is with the quarrying itself. Perhaps because they are so obvious, quarrying operations have cleaned up their act tremendously over the last few decades, and the actual act of mining stone, in Europe at least, is clean, safe, and efficient.
But quarrying does still require that hills be dismantled and the local geography irrevocably changed. However, quarrying companies cannot just leave once the mine is exhausted without any attempt at restoration, and old quarry sites offer fantastic opportunities for wildlife habitat as well as a variety of leisure and sport activities.


In many ways wood is the most sustainable building material that there could ever be – naturally occurring, infinitely replenishable, incredibly versatile, and not only provides shelter but heats it too.
However there are many issues with timber production and supply, and it cannot be assumed to be a low-impact material. The destruction of the world’s rainforests may not have been driven by the need for timber (mostly they are cleared for cash crops or beef), but having a market for the timber certainly bankrolls the process and speeds up the destruction. The loss of rainforest cannot be underestimated – indeed, perhaps we have yet to fully appreciate the full impact of felling ancient forests on local weather patterns, climate change, and biodiversity.
Even locally supplied timber in the UK can be controversial  – our ancient woodlands, although scarce, are also important bio-diverse places, whereas softwood plantations may be easy to plant and to manage, but are notoriously barren of other wildlife, and the occasional clear-felling of whole hillsides is a psychological upheaval for local communities. The timber isn’t even considered to be of very good quality.
We currently have thousands of acres of small mixed woods that are in various states of use, from well-managed to completely abandoned. Many of these are becoming better cared for and better promoted, and represent a very large and sustainable resource in the UK.
Acetylated wood is relatively new to the market, although the technology is not new. Timber is essentially soaked in vinegar (acetic acid) which changes the wood’s ability to soak up water, rendering the timber more stable and much more durable.
In a nutshell, we would recommend that you source your timber products as locally as possible. Otherwise make sure the timber has been certified by the Forest Stewardship Council (FSC) or the Programme for Endorsement of Forest Certification (PEFC). Best of all, grow your own!
The Forestry Commission website is a very useful resource for all things timber related -
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