In order to walk around your garden, or to have open spaces for table and chairs, you might be seeking to install a surface that is firm, attractive and needs little care. Or you may want something with similar characteristics to use as a mulch alternative. There are many suitable materials available from garden centres. But how do the different products stack up with respect to their sustainability? There are many factors to consider in addition to their appearance. As sustainable gardeners we may wish to use natural materials. However, natural ecosystems may be devastated simply to create paths or provide mulch for garden beds. Additionally, there are a range of environmental costs in their production and bringing them to your front or back door.
It is difficult to determine which product has the least impacts because there are so many factors to consider. Since most products, or the materials that go into them, come from the ground, they must be mined from a quarry of some type. So, first, let’s look at the impacts of quarrying.
Land disturbance and biodiversity loss
A quarry is a hole in the ground so quarrying will destroy vegetation and habitat of the land which is occupied by the hole as well as by any surrounding processing and transport infrastructure. Depending on the location this may, particularly in some developing countries, also destroy farming land, cause collapse of river banks and even deforestation.
Air and water pollution
Fine particles are generated by digging, blasting, crushing and grading operations, trucking and storage piles. Wind will carry such dusts considerable distances posing health impacts on nearby settlements. Rain may wash dust and other waste materials into streams where the resulting increase in concentration of suspended particles inhibits aquatic life because of reduced penetration of sunlight. Particulates may also affect the quality of water supplies.
Blasting, digging and transportation of quarried material are noisy operations affecting those nearby or on transport routes.
Quarrying is not one of the most energy-intensive industries, being outranked by manufacturing and electricity production. However, transport of materials to the end-user uses lots of fossil fuel.
All the products below are durable and will not need to be replaced frequently. They will have the impacts described above – and for some, many more.
Gravel, Pebbles and Crushed Rock
These come from rocks that are not connected to each other and are generally found where movement of water in lakes, oceans or rivers prevent them fusing. They are commonly used for driveways or paths but may be used as inorganic mulch because of their good water penetration, drainage, weed suppression and erosion control properties.
Lilydale, Dromana or Tuscan toppings are mixtures of fine aggregates 10 – 40 mm in size. Lilydale toppings come from limestone so when wet they have a tendency to bond together and make a firm surface. Tuscan toppings come from sandstone and have a pink/terracotta colour. Because crushed stone products are in plentiful supply it is cheap with the main cost of selling being transport. Therefore, they are usually taken from land close to urban areas.
Gravel is generally larger and mostly used in roadways, but sometimes in gardens.
They come from 4 main sources:
1. Active streambeds and beaches: Removing rocks, pebbles and sand from waterways is outlawed in Australia under state legislation. This is not necessarily so in the rest of the world. Some landscaping pebbles sourced from overseas have been mined from active streams and beaches, damaging these landscapes. These pebbles are supplied in their natural state after washing but are not polished.
2. Mining by-products: manufactured from the remains of natural stone quarrying overseas. These are usually tumbled to take off the sharp edges, but are not polished.
3. Sand mining refuse: These are picked out of a stockpile of stone that is sieved after sand mining operations. The stones are tumbled for about 6 hours and wax dipped prior to bagging
4. Land quarries of old stream routes: Over time there is a change in the routes that streams take as they flow to the sea. This leaves behind deposits of sand, rocks and pebbles that can be quarried from dry land without damaging existing streams.
Usually, if pebbles are available in bulk, they will have been quarried in Australia without damaging an active stream and some companies stock local and imported pebbles that are mined in low impact ways.
The invention of concrete by the ancient Romans revolutionized surface paving. It may be poured or formed into pavers. Concrete consists of cement, some sort of rock e.g. sand or gravel and water.
Cement is made from limestone mixed with other materials such as clay and heating it to very high temperatures. This process causes limestone to react with the other materials releasing carbon dioxide. So concrete has impacts derived from quarrying limestone and other added material plus a whole lot more. Cement contributes around 5% of human-derived carbon dioxide emissions with most coming from the chemical process but significant amounts from the fuel used to heat kilns. Additionally, gases such as sulphur dioxide, carbon monoxide and hydrocarbons released during the chemical reaction cause air pollution. There are large amounts of water used in the process producing alkaline waste water which must be disposed of safely.
So concrete has impacts of both quarrying and gravel mining as well as those of producing cement.
These make attractive paths and terraces and are easy to install. They may be laid on a simple sand base or set in mortar (Portland cement) when their impact will include that of cement. Bricks are made by heating clay, often mixed with other materials, to 1000o C so that, alone, their impact is mainly from firing them in kilns in the production process. Extruded bricks (those with holes) have less impact than solid dry pressed or moulded bricks because they use less clay.
There are now so-called “Carbon neutral bricks” which are fired in kilns using an alternative fuel to coal or gas. Such fuels may be waste sawdust from nearby timber mills or fly ash (fine particles which are expelled as waste from the boiler). Across the brick making industry there are also efforts to reduce carbon emissions in transport, clay extraction, waste and packaging.
There is a large range of pavers made from a variety of rocks including limestone, slate, granite, Travertine, bluestone and sandstone. There are also products such as ceramic and porcelain tile made from firing clay. Porcelain is harder and requires firing at higher temperatures. All have the impacts of quarrying. If set on a concrete slab a tiled/paved surface has the additional impacts of concrete.
Many types of stone and tile are imported from overseas which means they require considerable inputs of fossil fuel to bring them to your door. Even if they are quarried or manufactured in Australia, there can be long distance transport involved. So choose locally sourced products where possible.
Once installed, the main impacts of different types of paving relate to their permeability and durability and, of course, to aesthetics and safety. Highly durable products such as bricks and concrete do not need to be replaced as frequently as gravel, crushed rock and pebbles which tend to migrate through foot or vehicle traffic into the surrounding garden beds.
This important in managing and maximizing use of heavy rainfall. Any paving material set on concrete will cause water run off at the edges, perhaps causing erosion and loss of water from the site. Surfaces bedded on sand or where gaps between pavers are filled with gravel or ground cover plants allow rainfall to enter the soil and surrounds more gently and avoid offsite losses.
There are products which consist of pebbles or coarse toppings which are bonded to a firm surface with special glue which still leaves tiny spaces for water penetration. These will have the added small impacts of glue production.
Gravel, stones and pavers set with gaps between or without cement will allow seeds, often weed seeds, to germinate between them. If you don’t like weeding, that may lead you to rip it all up and put in something that has no gaps – and you will have almost doubled the environmental impacts!
Deciding what to use
It is not easy to decide what surface is most sustainable. Negative impacts of production or transport may be counterbalanced by the product’s longevity, ease of maintenance, permeability and capacity to be recycled.
Using Recycled and Waste Materials
Possibly the most sustainable option is to choose from the large range of recycled materials which can make sturdy and attractive paths. These avoid impacts of production and, because they are usually locally sourced, those of transport too. Recycled bricks are possibly the most common but other possibilities are:
You might smash up your own old concrete path into interesting shapes and lay it elsewhere as crazy paving. Commercially recyclers crush concrete which can be used as a base for paths or incorporated into slabs using added cement. Even the latter use has less impact that preparing fresh concrete.
While you probably wouldn’t choose these for a driveway, wood chips obtained from local tree-pruning operations can make useful surfaces between garden beds. They are permeable and soft to kneel on as you plant your veggies.
And More . . . .
We have seen surfaces which incorporate recycled glass pebbles, the bottoms of glass bottles, old ornamental metal gratings, tiles, all sorts of recycled pavers and recycled hardwood. The range is enormous – your imagination and trips to recyclers will create your next pathway or non-degradable mulch!
Willis. A-M. 1998. Concrete and not so concrete impacts Information Ecology, EcoDesign Foundation. http://www.changedesign.org/Resources/EDFPublications/Articles/Papers/Concrete.pdf
Banez J, Ajon S M, Bilolo J R, Marollano J, Nivero D. 2010. Quarrying and its Environmental Effects. https://www.scribd.com/doc/37181927/Quarrying-and-Its-Environmental-Effects
Skinder B M, Sheikh A O, Pandit A K, Bashir, Ganai B A. 2014. Brick kiln emissions and its environmental impact: A Review Journal of Natural Ecology and the Environment 6(1) 1 – 11. (http://www.academia.edu/6426675/Brick_kiln_emissions_and_its_environmental_impact_A_Review)
PGH Bricks and Pavers. Sustainability and the Environment. http://selector.com/media/documents/pgh-sustainability-brochure.pdf