Sustainable thinking

Clay Technology magazine
,
16 Aug 2018

Aidan Bell, co-founder of Envirobuild, UK, looks at the production of clay and how modern methodologies have influenced an ancient discipline.

Clay is undoubtedly one of Earth’s most useful materials. It has been used for millennia in pottery, dishware, and even musical instruments. Its most common use today is in construction products such as bricks, wall, and floor tiles. Given the size of the global building sector today, clay production has reached unprecedented levels, with 25.5 million tonnes produced in the USA alone in 2015.

The transformation of this ancient discipline has unfortunately established practices that are not sustainable in the long-term. For example, the production of clay bricks in South Africa, from the sourcing of raw materials to preparation and firing, is responsible for 2.6 million tonnes of CO2 equivalents emitted per year, contributing to global climate change. The environmental consequences are severe – acid raid, pollutant smog, and deforestation are endangering ecosystems and damaging our health.

 If we want to preserve the natural beauty of the world, then those of us in the clay industry must act to transform it for the better of the planet. Indeed, many have already done so. Experts in the field are leading experiments to discover modern, environmentally-friendly methodologies. The following advancements in clay production are invaluable to not only the protection of the planet, but also the continued growth of the clay industry. 

Raw materials

The harvesting of clay is one area where modern practices are changing its production for the better. Like mining activities, clay harvesting removes the earth surface, affecting the soil nutrient cycle of the area, while also representing potential sources of water contamination if near to river bodies. As a result, rare and fragile ecosystems are put at risk, with both animals and plant species suffering. Moreover, the excessive extraction of clay and the removal of topsoil have resulted in an enormous depletion of virgin clay.

In response to this, many companies are paying close attention to improving the sustainability of the extraction process. Ensuring finite resources are not unnecessarily wasted and taking steps to remediate depleted extraction sites is a priority. For instance, adapting depleted clay sites for activities like agriculture or creating valuable alternative habitats for endangered animals and plants can promote biodiversity. 

The UK government has also listed certain places as sites of special scientific interest (SSSI), forcing companies to take extra measures to ensure fauna, flora and geological or physiographic properties are protected for future generations.

Resource conservation and the remediation of depleted extraction sites is a priority. For instance, many UK production sites are now listed as SSSI or are designated locally as sites of ecological and/or geological significance. These new practices put in place are encouraging biodiversity and the sustainable extraction of this raw material. 

Clay preparation

Studies have found that alongside the firing steps of clay production, preparation is the other area with the largest impact on ecosystem quality. Internal fuels, such as coal gangue, are raw materials that are added during the preparation stage of brickmaking to improve the drying performance of the brick, reducing the risk of cracking, which would worsen the appearance and quality. The downside is that not only is clay extraction expensive, but the emissions caused at the coal mine during extraction also contribute to the overall impact of clay preparation on human health. These emissions not only endanger our health, but that of entire ecosystems by contributing to global warming through greenhouse gases (GHG). 

However, the current trend in clay production focuses on using post-consumer waste and industrial by-products as internal fuels – a methodology that recycles more and wastes less. Researchers have used a variety of recycled waste, such as foundry sand, clay waste and granite sawing over things like coal to enhance brick quality. Hence, recycling the waste into brick production is a viable solution not only to reduce the environmental impact of clay preparation but also an economical way of improving its overall quality. 

These recycled-materials now mixed into clay preparation have been manufactured to improve density, strength, and thermal conductivity, to produce impressive results. For example, an earthquake-resistant material has been produced with high compressive strength thanks to mixing clay-sand bricks with varying percentages of rice husk ash. 

Nevertheless, more work remains to be done. The use of non-renewable energy and finite raw materials means that more investment is needed to find ways of producing clay in a more sustainable and eco-friendly way. Many clays are unsuitable for brickmaking, meaning those that are suitable should be extracted responsibly while more efforts should be directed towards incorporating a larger proportion of industrial waste in materials that use clay to ensure its continued availability.  

Forming methods 

Several methods of forming clay bricks exist, each one producing a product with unique characteristics. Hand moulding is the oldest method, with an individual needing to manipulate a lump of wet clay into a brick-shaped mould by hand before turning it onto a plank for drying. Handmade bricks are full of character but inefficient to produce in large quantities. 

A far more modern methodology is producing soft mud bricks, a formation method that creates very soft clay with a high percentage of water, typically 25–30%. The clay is pressed into a sanded mould box by a machine, with any excess material being automatically trimmed and re-used. Not only does this produce less waste, it is also an economical option – modern soft mud machines can produce more than 40,000 bricks an hour. 

Another modern method of clay forming is extrusion, whereby clay is fed into a vacuum chamber which extracts all of the air from the clay. It is later forced through a mouthpiece and extrusion die, where it can be given different finishes depending on the type of perforation bars used inside. The bricks are then cut before finally being placed on steel rails to be sent to the dryer. 

Both of these modern practices have vastly improved efficiencies in clay production and are responsible for turning what were once small, localised industries into interconnected global giants. 

Kilns

As already stated, the firing process of clay production is the most environmentally harmful step. The purpose of firing is to transform the relatively weak dried clay into strong, durable bricks. The process is complex varying significantly depending on the type of clay and kiln used, as well as the intended finished product. 

The most common method of firing clay bricks in the UK is through a tunnel kiln. The bricks are placed on kiln cars, before gradually progressing through the kiln and experiencing increasingly hot temperatures until they reach the soak temperature – the final temperature before cooling. Tunnel kilns are preferred over others for their energy efficiency – waste hot air is taken to heat the bricks at the front of the kiln, while the waste heat from the cooling zone is used in the driers before firing the clay bricks. Modern clay factories are thus able to produce high volumes of bricks with remarkably high levels of energy efficiency. 

Far less efficient kilns are still widely used in other parts of the world. During firing, the main environmental impacts result from the emissions of the coal added to the clay mixture as internal fuel. Studies have shown that the worst performing kiln technology for its impact on the environment is the Hoffman kiln, which uses a variety of fuel oils and large quantities of coal to vitrify the clay. It was traditionally coal fired, but modern environmental legislation means that most Hoffman kilns in the UK are now gas fired. Despite this gradual move away from coal-fired kilns, other parts of the world still use this method. 

With modern kilns now being fitted with computerised controls allowing for greater management over energy use, the clay brick industry has made significant steps toward becoming environmentally friendly. Continued efforts should be made toward reducing the use of fossil fuels, moving away from fuel-burning kilns to electric kilns and generally reducing the energy consumption of the drying and firing process. 

Product quality

Modern regulation has meant that clay production must meet certain standards to be sold commercially. These rules ensure a more reliable quality product, benefiting the consumer and the environment. Quality checks are carried out at every stage of the production process, from the extraction of clay, to firing and finally to dispatch. 

Since 2013 UK regulation has meant that manufacturers have been required to CE-mark bricks in the UK. This CE mark can be found on the product packaging, or on the delivery documentation, and assures that the product complies with the essential requirements of the relevant European health, safety,  and environmental protection legislation. 

The use of robots has also led to significant improvements in both the product quality and environmental impact of clay brick production. The more precise handling of clay bricks, for example, has eliminated handling damage resulting from manual setting. By minimising the defects in clay, less clay bricks fail to meet requirements and customers obtain a better quality end product. By reducing clay brick waste, less energy is used and less GHGs are produced.

Looking ahead

By taking a look at the production of clay and how modern methodologies have influenced this ancient discipline, we can clearly see that significant improvements have been made in making the industry more environmentally friendly. The promotion of sustainable extraction practices and the use of recycled materials in clay preparation, are helping to reduce the industry’s negative impact on the environment and the depletion of finite resources. 

Modern forming methods are increasing efficiency, while efforts are being made to lessen the use of external fuels in firing. Also, quality control rules are establishing general standards that benefit the consumer and the environment, while overall quality is increasing thanks to more precise robots. Despite this, the clay industry still has a long way to go in its long-term sustainability efforts, but it’s fair to say it has come some way