During a symposium on natural stone, the German Naturwerkstein Verband (DNV) showed a diagram of a study from England comparing building materials and CO2-emissions for harvesting, processing into products and construction; the embodied carbon. The diagram deviates from the well-known building materials pyramid. Natural stone stands out best in the diagram, followed by brick, wood and other materials. Natural stone is beneficial because hardly any energy is required for production. It is formed by nature and is immediately ready for use after sawing. Brick is clay that is fired at high temperatures – usually in gas ovens. Wood must be grown and, after sawing, still needs many operations before it will last a long time, such as heating or preserving with glue, oil, paint or chemicals. Wood also has to be transported regularly over longer distances.
Natural stone cannot be called a biobased material, because it does not grow back within 100 years, but over millions of years. But stone is present in almost inexhaustible quantities everywhere on earth in the kilometers thick earth’s crust. And it is also constantly replenished in the form of igneous rocks. A major disadvantage of natural stone is that nature is destroyed before extraction. By filling up mines and quarries, nature can recover over time, but there is of course damage to nature.
A major advantage of natural stone is that it is a circular material; reusing it is very easy. Stone facade panels can be used again in facades or as a floor after some sanding. Natural stone was therefore one of the first materials that Rotor’s recycling branch started working with. Finally, stone prefers to come from nearby, for example by rail from surrounding countries.
Apart from the transition to bio-based and circular construction, the construction industry has been developing towards lighter constructions for some time. The facade increasingly involves light supporting structures – whether concrete, steel or wood – with thick insulation packages, covered by plastic films, an air cavity and a weather-resistant skin. Facades consist of large packages of various materials that are glued and screwed together, with a high… embodied carbon, and are also difficult to take apart when the building is dismantled. The transition to a fully biobased facade only partially solves this problem. To prevent the facade from heating up and for fire safety, a concrete-like material often needs to be added. Then you take two steps forward and then one step back.
We are currently experimenting with the agency with 50 centimeter thick aerated concrete blocks for new facades of a transformation project in Brussels. The blocks are porous and insulate well. So you no longer need poorly scoring insulation materials, cavity structures or plastic films. The inside and outside are smoothly finished and ready. But building with solid natural stone would be much more beneficial. Houses have been built with local stones for thousands of years. Nowadays facades obviously need to insulate better, a well-insulating, porous tuff (a sedimentary rock made of volcanic material) from Germany could be an excellent solution.
To get rid of our concrete addiction, a smart mix of materials is the best method to achieve a low… CO2-footprint to realise. The bulk of materials in a building consists of well-protected supporting structures and interior walls, and can easily be made from bio-based materials. And if a facade has to last a long time – without too much maintenance and interim replacement – then a stone shell is not such a bad idea. In addition to the embodied carbon, the entire life cycle – including demolition and reuse – should be taken into account. For long-term sustainability, the operational phase of a building must be able to be extended as long as possible with a shell that does not decay and a structure with great flexibility to accommodate other functions. And this in a way that our descendants still take the trouble to reuse parts. Hey, then I am suddenly very close to our old city centers with their beloved canal houses and highly regarded nineteenth-century neighborhoods: they are largely made of wood but with stone walls and facades, and are endlessly converted from house to shop to office and vice versa. Maybe in the future we can still make robust buildings and beautiful stone facades?
Alexander Pols is architect-director of Kollhoff&Pols architects. He is an architect member of the Welstands and Monuments Committee of The Hague, a guest lecturer and committee member for the professional experience period and examination committee of the Register of Architects.
