Sustainable materials

No matter how you look at it, using materials costs energy. Let us put that fact aside and examine the degree to which the materials themselves are sustainable. To be so, a material must be drawn from a source that is renewable, either because it grows as fast as we use it or because it reverts to its original state on natural decay and does so in an acceptable time span. For this to be true, the resource and the material must form part of a cycle, like the nitrogen, carbon, or hydrological cycles of the natural world: closed loops that run at steady state, recycling the elements N and C and the compound H2O such that the resource remains constant.

Which materials meet these constraints? Not many. The obvious examples are wood and natural fibers such as jute, hemp, and cotton. Provided that the total tree stock is constant such that wood is harvested at the same rate as it is grown, wood could be seen as a renewable resource. But today wood is harvested (or simply burned) much faster than it is replaced, making it a diminishing resource, and wood in the form we use it for construction has been cut, dried, chemically treated, and transported, all with some nonrenewable consequences. Similar reservations apply to natural fibers, natural rubber, and leather. Very few of the mater-ials we use today qualify as truly renewable in the sense that those of early man, him­self part of a natural ecosystem and its closed-loop cycle, were.

A number of materials do, however, have ingredients that are, for practi­cal purposes, recyclable. The construction industry uses materials in greater quantities than any other, and much construction is in less developed countries where steel, concrete, and fired brick are not easily come by. Architects with concern for the environment and a love of pre-industrial building materials design buildings using an interesting range of near­sustainable materials. Here are some examples:

■ Rammed earth and adobe. Soil is available almost everywhere. Mix it with straw or hair and a little lime cement, stomp it down between

wooden shuttering, let it set, and remove the shuttering and you have a wall that will support a light roof. Earth walls have high heat capacity, useful in a climate that is hot in the day and cold at night.

If the wall is thick enough, its thermal mass keeps the room cool as the outside temperature rises but warms it at night when the outside cools. Adobe, in the form of rammed earth bricks, is the traditional building material of Mexico and parts of Africa. Adobe bricks, mechanically mixed and extruded, are commercially available today.

■ Straw and reed. Straw, a by-product of agriculture, has long been used in building, but straw bales are a product of modern technology. Straw bales are like building blocks: stack them up and you create

a wall. Surface them with earth plaster or wood and they become durable. The walls have low thermal conductivity and low heat capacity (quite different from rammed earth), so they insulate and have low thermal mass. Thatch is reed, one with a long history of use as a roofing material. The reed grows with its base in water so it has evolved to resist it. Like straw, it insulates and is surprisingly durable; a well-thatched roof has a life of 80 years.

■ Hemp and flax. Hemp and flax are fast-growing grasslike plants containing fibers of great strength. The fibers have been used since Roman times for rope and sails, clothing, and construction. The use of a mix of industrial hemp and lime ("hempcrete") for infill in wood frame buildings is growing in Europe but is held back elsewhere (the United States, for instance) because of its mistaken association with marijuana, derived from a different variety of hemp. The hemp content of hempcrete, 75% by volume, is truly sustainable, grown

as fast as it is used and requiring no fertilizer. Hempcrete sequesters 0.3 kg of carbon per kg—up to 20 tonnes of carbon in a typical house.

■ Stone and lime. Stone may not be renewable, but one might think the resource from which it is drawn is near infinite. True in general, but not in particular: carrera marble, Sydney sandstone, Portland stone, Welsh slate—all are now scarce, even stockpiled, driving up their price. But more generally, stone is an ecofriendly material, durable and reliable. "Dressed" stone, the material of city banks, venerable universities, and corporate head offices, is expensive. Much labor and quite some energy go into the dressing. Fieldstone is there to be picked up. "Dry stone" walls are made by skilled stacking of stones as found. They work well as field boundaries but they are not load bearing. Stone bonded with a lime mortar, however, is robust and durable.

■ Quasi-sustainable materials. The materials just described are such a limited set that we will have to cast the net wider and consider quasi­renewable materials—those drawn from a resource base so large that, even allowing for exponential growth, there is no risk of exhaustion. Table 10.2 lists the 12 most abundant elements in the Earth’s crust, and here we are more fortunate. The list includes many of those that we use in the largest quantities: iron, aluminum, magnesium, and titanium; silicon, sodium, and oxygen, the components of glass and of silicates that form the basis of many ceramics; and carbon and hydrogen, the starting point for all polymers (Figure 2.1). Extracting them economically, of course, requires deposits in which they occur with high enough concentration, but when the total quantity is as enormous as it is for each of these, such deposits are plentiful. It just takes energy.

The "All other" category at the bottom of Table 10.2 is important, too.

It includes copper, zinc, tin, lead, cobalt, zirconium, and all the precious

Table 10.2

Abundance of elements in earth’s crust

Element

Abundance in earth’s crust by weight (%)

Oxygen

46.7

Silicon

27.7

Aluminum

8.1

Iron

5.1

Calcium

3.6

Sodium

2.8

Potassium

2.6

Magnesium

2.1

Titanium

0.6

Hydrogen

0.14

Phosphorus

0.13

Carbon

0.09

All others, total

<1

metals and rare earths. Some of these are important in their own right— copper, for instance; others because they are the vitamins, so to speak, of alloy design: small additions to the composition that have a big influence on behavior. For these, rich deposits are leaner and less widely distributed, and as they are depleted we are forced to extract them from ever-leaner ores. If we go lean enough, the total quantities again become large. But that, too, takes energy.

So, the bottom line is energy. With enough of it we could continue to use materials as we now do for a long time to come—provided we could afford it and could generate it without poisoning the planet. That is as far as we need to go for now. What we can do about it is the subject of Chapter 11.

10.6 Summary and conclusion

"Sustainable development is development that meets the needs of the pre­sent without compromising the ability of future generations to meet their own needs." This much-quoted definition, from the Brundtland Report of the World Council on Economic Development (WCED, 1987), captures what most people would agree is the essence of sustainability.

Sustainable development, at the time of writing, is a vision, a grand ideal. Nature achieves it through balanced cycles—the carbon, nitrogen, and water cycles are examples—in which materials are used and, at the end of life, recycled to replenish the source from which they were drawn. That requires a closed loop in which the flows at each point match in rate, for if they do not, waste accumulates somewhere in the cycle. We (the industrial "we") fail to achieve this because the ways in which we use materials either do not form a closed loop, or if they do, the units of the loop do not function at equal rates.

It is clear that we are reaching limits in the way we use fossil fuels for energy and the Earth’s resources for materials, both because these become depleted and because the way we use them damages the environment. Sustainable sources of energy exist, but all have the characteristic that the power density is low, so harvesting them requires the dedication of a very large area of land mass or ocean surface. The resources of many of the mater­ials we use in the largest quantities are sufficiently abundant that we can draw on them for a long time to come—provided that we have enough cheap, pollution-free energy to do so. And that, as of now, is what we don’t have.

What can we do about it? Forces for change and future options are the subjects of the next chapter, Chapter 11.

10.7 Further reading

Azapagic, A., Perdan, S. and Clift, R. (Eds.) (2004), "Sustainable development in practice", John Wiley. ISBN 0-470-85609-2.

Coulter, S., Bras, B. and Foley, C. (1995), "A lexicon of green engineering terms", Proc. ICED 95, pp. 1-10. (Coulter presents the nested-box analogy of green design, here used as Figure 10.1.)

Guidice, F., La Rosa, G. and Risitano, A. (2006), "Product design for the environ­ment", CRC/Taylor and Francis, ISBN 0-8493-2722-9. (A well-balanced review of current thinking on ecodesign.)

Lovelock, J. (2000), "Gaia: a new look at life on Earth", Oxford University Press. ISBN 0-19-286218-9. (A visionary statement of man’s place in the environment.)

MacKay, D. J.C. (2008), "Sustainable energy—without the hot air", UIT Press, Cambridge, UK. ISBN 978-0-9544529-3-3. (MacKay brings a welcome dose of common sense into the discussion of energy sources and use: fresh air replacing hot air.)

Nielsen, R. (2005), "The little green handbook", Scribe Publications Pty Ltd,

Carlton North. ISBN 1-9207-6930-7. (A cold-blooded presentation and analy­sis of hard facts about population, land and water resources, energy, and social trends.)

Schmidt-Bleek, F. (1997), "How much environment does the human being need? Factor 10: the measure for an ecological economy", Deutscher Taschenbuchverlag. ISBN 3-936279-00-4. (Both Schmidt-Bleek and von Weizsacker, referenced below, argue that sustainable development will require a drastic reduction in material consumption.)

von Weizsacker, E., Lovins, A. B. and Lovins, L. H. (1997), "Factor four: doubling wealth, halving resource use", Earthscan Publications. ISBN 1-85383-406-8; ISBN-13: 978-1-85383406-6. (Both von Weizsacker and Schmidt-Bleek, refer­enced above, argue that sustainable development will require a drastic reduction in material consumption.)

WCED (1987), "Report of the World Commission on the Environment and Development", Oxford University Press. (The so-called Bruntland report launched the current debate and stimulated current actions on moving toward a sustainable existence.)

Woolley, T. (2006), "Natural building: a guide to materials and techniques", The Crowood Press Limited. ISBN 1-861-26841-6. (A well-illustrated introduction to traditional building materials and their present-day modifications.)

10.8 Exercises

E.10.1 Distinguish pollution control and prevention (PCP) from design for the environment (DFE). When would you use the first? When the second?

E.10.2 What is meant by the ecological metaphor? What does it suggest about ways to use materials in a sustainable way?

E.10.3 What are the potential sources of renewable energy? What are the positive and negative aspects of converting to an economy based wholly on renewable sources?

E.10.4 The land area of the Netherlands (Holland) is 41,526 km2. Its population is 16.5 million, and the average power consumed per capita there is 6.7 kW. If the average wind power is 2 W/m2 of land area and wind turbines operate at a load factor of 0.5, what fraction of the area of the country would be taken up by turbines to meet the country’s energy needs?

E.10.5 The land area of New York state is 131,255 lcm2. Its population is

19.5 million, and the average power consumed per capita there is 10.5 kW If the average wind power is 2 W/m2 of land area and wind turbines oper­ate at a load factor of 0.25, what fraction of the area of the state would be taken up by turbines to meet its energy needs?

E.10.6 The combined land area of the state of New Mexico (337,367 km2) and the state of Nevada (286,367 km2) is 623,734 km2. The population of the United States is 301 million, and the average power consumed per capita there is 10.2 kW. Mass-produced solar cells can capture 10% of the energy that falls on them, which, in New Mexico and Nevada, is roughly 50W/m2. What fraction of the area of the two states would be taken up by solar cells to supply the current needs of the United States?

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