Rajkumari Ratnavati Girls School. Diana Kellogg Architects.
One of the latest materials touted as an alternative to the carbon-intensive dominance of concrete and steel comes as something of a surprise: stone. Despite its ancient properties and its use as shelter over millennia, stone has only recently begun to be viewed as a viable sustainable option. And while the stone comeback is negligible compared to the skyrocketing use of mass timber, it represents a promising hope for a greener future.
"We've only just rediscovered what architects have known for 7,000 years," said London architect Amin Taha in an interview with Dezeen. "Stone is versatile, has strength, longevity, is plentiful, cheap and, with zero embodied carbon, well placed for a renaissance."
The environmental repercussions of concrete and steel have long been documented; simply put, they account for a significant amount of global greenhouse gas emissions. Manufacturing concrete and steel requires three energy-intensive steps: extraction, processing, and transportation. Of these three steps, processing is by far the most damaging to the environment.
Stone, by comparison, requires only extraction (quarrying) and transportation, giving it far less of a carbon footprint than either concrete or steel. In fact, some estimates calculate that processing stone produces half the carbon of concrete or steel manufacturing. Other estimates claim astronomical differences between the use of stone and concrete and steel. (There is also the paradox that stone is a necessary element for producing concrete, albeit in a diminished capacity. "To make concrete, you take a stone like Portland stone that has a compressive strength of about 200 Newtons per millimetre square, you crush it, you burn it, you do all these processes that are really energy-intensive, and you end up with the material that is about 40 Newtons per millimetre square, ACAN Natural Materials Group coordinator Aurore Baulier explained to Dezeen.
For architects, the battle against carbon emissions and climate change has often been characterized by roundabout strategies. Stone, as well as mass timber, can help change that. Instead of hailing dubious concepts such as carbon credits or carbon offsets, architects can effect change, in some cases, simply by the choice of materials. Stone is durable, nonflammable, requires little energy for manufacturing, needs no cladding, and has the added benefit of having high thermal-mass, allowing for cooler spaces at night. It is also reusable and demands less maintenance than other materials, giving it an extended life cycle. An abundant natural resource, stone has an edge on mass timber when it comes to sustainability. While timber is, in fact, renewable, it takes years for trees to grow and replenish the supply of lumber used for construction. Theoretically, at least, stone is inexhaustible, although areas for quarrying are limited by population density.
Although stone has not taken off yet in America, several recent projects across the globe have highlighted the possibilities of stone as a sustainable and aesthetically appealing replacement for concrete and steel.
In India, the elegant Rajkumari Ratnavati Girls School, designed by New York City-based Diana Kellog Architects, stands out in an austere desert landscape as a symbol of hope and progress. It was built with locally sourced hand-carved sandstone.
Rajkumari Ratnavati Girls School. Diana Kellogg Architects.
In France, Gilles Perraudin, one of the pioneers of the recent uptick in stone construction, produced a three-story housing development in Cornebarrieu made from limestone. And Swiss architecture firm Atelier Archiplein developed an impressive 68-unit social housing block in Geneva using solid stone. Both projects have a primal elegance and alluring textures. The unfinished facades reveal the craftsmanship involved (including sedimentary seams and quarrying marks) as well as the varying shades of color, in some ways giving the structures the air of an Amish quilt. This raw look, elegant and austere, would be a welcome change from the default glass curtain walls of so many cities.
Commune de Plan les Ouates. Atelier Archiplein.
While some may not find this deliberately imperfect aesthetic appealing, natural stone provides what most glass facades cannot: character, as well as a sense of presence and solidity, aspects of the built environment that provide not just visual appeal but also some of the positive effects of biophilic design.
Like mass timber and other materials gaining popularity due to their potential for carbon savings, stone has its shortcomings, however. Because quarrying is a natural process, the quality of the stone may be variable, and must be inspected for internal fissures to ensure structural stability. Concerns about the tensile strength of structural stone are addressed by post-tensioning—drilling holes in a stone beam, for example, and inserting steel wire rods through it. As Steve Webb, director of London-based Webb Yates Engineers, noted in Architectural Review: “The steel content of a reinforced stone beam is a fraction of that of a steel beam: to produce a steel beam requires iron ore to be excavated in Brazil or Australia, a blast furnace and a rolling mill; an equivalent stone beam can be produced at a quarry near you with a diamond saw, a drill and a strand jack.”
Then there is the question of money. As stone vanished as a large-scale construction material during the age of high modernism, it found its role in architecture reduced to a decorative element; as a result of this shift, only high-quality stone is quarried for ornamental purposes, giving stone the aura—and the price tag—of a luxury resource. But using stone for construction is not necessarily cost-prohibitive. While stone is ubiquitous, quarries are not: finding, opening, and maintaining them is expensive and, in some instances, subject to municipal legislation.
Together with mass timber—and the potential advances of green cement and CO2 bricks—stone can be part of a multi-faceted approach to ensuring that construction reduces its staggering environmental impact. Because widespread use of stone in the United States is unlikely, for a variety of reasons, its future depends on being part of hybrid structures, incorporating other eco-friendly materials, where it can minimize both carbon footprints and monotonous design.