Stone

Many myths compare stones as the 'bones' of Mother Earth. Extraction of minerals in most cultures has been accompanied by complex rituals and rites, undertaken as carefully as possible by, amongst other things, filling up the holes and passages into the mine when extraction was finished. A Sioux Indian smallholder expressed this spiritual attitude thus:

You ask me to dig in the earth. Do I have to take a knife and plunge it into my Mother's breast? You say that I must dig and take away the stones. Do I have to remove her flesh to reach down to her bones?

There are three main categories of stone:

• Igneous rocks. Consolidated areas of rock that have forced their way up through splits in the crust of the earth, these are the hardest types of rock, such as the granites, syenites and dolerites.

• Sedimentary rocks. Petrified and disintegrated stone which has combined with organic materials. In this group are sandstone, slate and limestone.

• Metamorphic rocks. Formed by exertion of pressure and the action of high temperatures on igneous or sedimentary rock types, which transforms their structure. Examples of these rock types are crystalline slate and quartzite.

None of these groups can be referred to as the oldest, since the geological processes are a continuous, cyclic process. Sedimentary rock types can be formed through hardening of gravel, sand and clay originating from the disintegration or breaking down of igneous or metamorphic rocks; igneous stones can arise through the melting of metamorphic and other types of rock and later consolidation, and metamorphic rocks can arise from changes in older sedimentary, igneous or metamorphic structures.

There may be some truth in the view expressed by Asher Shadmon of the HABITAT centre in Nairobi:

Stone is the building material of the future. We are on our way into a new Stone Age. The resources are limitless and evenly spread over the whole globe. Extraction does not require a lot of energy and does not pollute. And most important of all is that the material is durable. (Shadmon, 1983)

A differentiation is usually made between field stone and quarry stone. While field stone is found in the open, on beaches or in fields, quarry

Table 7.1 Uses of stone in the building industry

Type of stone

Minerals

Areas of use

Clay slate

Clay minerals

Roof covering; flooring

Diabase

Plagioclase; Pyroxene

Rockwool; crushed as aggregate in concretes; structures

Gabbro

Feldspar; Pyroxene

Crushed as aggregate in concretes; structures; flooring; wall cladding

Gneiss

Aluminium silicates; Quartz; Mica

Crushed as aggregate in concretes; structures; flooring; wall cladding

Granite

Feldspar; Quartz; Mica

Crushed as aggregate in concretes; structures; flooring; wall cladding

Limestone

Lime

Ground to limeflour (cement, lime binder, etc.); smaller structures

Marble

Lime/dolomite

Structures above ground; flooring; cladding

Mica slate

Quartz; Feldspar; Mica

Roof covering; wall cladding; flooring

Phyllite slate

Quartz; Feldpar; Mica

Roof covering; wall cladding; flooring

Sandstone/quartzite

Quartz, often incl. lime or feldspar

As sand (production of Portland cement, concretes, fillers); smaller structures

Serpentine

Serpentine minerals; Chlorite; Magnesite

Cladding; flooring

Steatite/soapstone

Talc; Chlorite; Magnesite

Structures above ground; cladding

Syenite

Aluminium silicates; Pyroxene

Crushed as aggregate in concretes; structures; flooring; wall cladding

Quartzite slate

Quartz; Aluminium silicates; Mica

Roof covering; wall cladding; flooring

stones are deliberately quarried. Stone is primarily used in the form of blocks, cut slabs or sheets, as slate or crushed stone (see Table 7.1). It is used to construct walls, retaining walls, edging and bridges. In arch structures or small spans it has been used since antiquity as structural roof material. Dressed stone and specially made slabs can be used for exterior or interior cladding, framing around doors and windows, fireplaces, floors and stairs. Slate can be used on floors, stairs, fireplaces, as framing around doors and windows, as roof covering and as wall cladding. Crushed stone or gravel is used as aggregate in all concrete structures.

Stone has very high compressive strength and low tensile strength. Consequently, it is possible to build high buildings of solid stone, whereas a stone beam has a very limited bearing capacity. The Egyptian and Greek temples show this very clearly: dimensions of horizontal stone slabs are immense to achieve small spans. In Roman aqueducts as in Gothic cathedrals, the principle is that of the arch; the compressive strength is thereby used at its maximum, making spans of up to 70 m possible.

Evidence remains from Upper Palaeolithic times (40000-10000 years ago) of the use of stone for curbs and low boundary walls (Wright, 2005). From earliest Neolithic times field stones have been used for mud mortar as an alternative to mud brick. Around 3000 BC, man began to quarry blocks from bedrock and dress them into regular forms. Some of these beginnings can be seen in Mesopotamia, but it was in early dynastic Egypt that the essential developments took place. Since then, stone has been used continuously, with its apotheosis during the late Middle Ages when a widespread stone industry developed throughout northern Europe. The stone villages of this period were usually built with a foundation wall and ground floor in stone; the rest of the building was in brick. By the early 1900s, the stone industry had lost its status, mainly due to a rapid rise in the use of concrete. Large quantities of stone are still quarried and sawn into slabs, including marble, in countries like Italy and China, and a reasonable amount of slate extraction still continues; but the dominant use for stone today is crushed stone for concrete aggregate.

Many in the building industry anticipate a renaissance in stone building, even if not quite as optimistically as Asher Shadmon. New technologies have made it possible to re-open disused quarries, and use for facade cladding is increasing. One reason is that natural stone - with the exception of sandstone and limestone - is less sensitive to pollution than concrete and related materials. However, all porous stones are exposed to frost damage. In northern Europe frost damage is expected to double with climate change (Noah's Ark, 2006).

Stone is ubiquitous, even if in short supply in certain regions. Extraction and refining is labour-intensive, consequently the use of energy is much lower than for bricks or concrete (see Table 7.2). Stone is, therefore, not responsible for significant energy related pollution.

Extraction and stone crushing is usually a mechanical process with no need for high temperatures. Various energy sources can be used, ranging from manual power to wind and water power, either directly or as electricity-based technology. It is also estimated that considerable energy savings are possible in many stone industries (Konstantopoulou et al., 2004).

The weight of stone suggests that the distance between quarry and building site should be short. Quarries along the coast have the potential advantage of energy effective water transport. Mobile extraction plants could be moved to small quarries near relevant building sites, employing local labourers.

Large quarries inevitably damage the landscape even if they are eventually restored or become overgrown. They can also lead to altered groundwater conditions and damage local ecosystems. To extract granite for use as crushed stone by the 'gloryhole' method, the

Table 7.2 Embodied energy in natural stone products

Products

Embodied energy (MJ/kg)

Granite, as blocks

0.5

Granite, as crushed stone

0.4

Limestone, as block

0.5

Sandstone, as block

0.5

Slate

Less than 0.5*

Ornamental stone, different types

1.5

* There are no relevant figures for slate, but we can assume that the embodied energy is lower than for other stones.

|Table 7.3 Potential material pollution during the working of stone

Final product

Potential pollution

DC

Granite/sandstone/phyllite slate/quartzite slate/gneiss

Dust containing quartz

< Û.

Diabase/gabbro/syenite/marble/limestone/soapstone/serpentine/clay slate

Dust containing no quartz

mountain or rock is drilled from the top and stones extracted through a vertical tunnel (which gets wider the deeper it goes). This leads to less visual disturbance of the landscape.

mountain or rock is drilled from the top and stones extracted through a vertical tunnel (which gets wider the deeper it goes). This leads to less visual disturbance of the landscape.

Stone often contains radioactive elements such as thorium and radium, and a quarry can increase the general level of radiation in a neighbourhood by emitting radon gas. Generally, the extraction of slate, limestone, marble and sandstone has little likelihood of causing radiation risks. Extracting volcanic or alum slate requires caution, including the measurement of radiation levels before removing stone for general use.

Environmental hazards of the industry include noise, vibration and dust. Quartz dust is the most harmful (see Table 7.3). While sandstone usually has a high content of quartz, the content in granite can vary widely from a few per cent to over 50%. More processing leads to greater damage; by using undressed stone rather than perfectly dimensioned blocks, these problems are reduced.

Unless radioactive stone is used in construction, there are no problems during the lifetime of stone in or on buildings, and demolition waste will also be inert.

Quarrying and dressing stone products result in large amounts of waste stone. This can be crushed to make useful aggregate; for example, for concrete.Waste chips from marble quarries are a valued ingredient for terrazzo flooring, and marble dust is useful in the production of lime and cements.

It is useful to reflect on the fact that resource use and waste is partly a question of design. For example, simple decisions such as choosing a floor pattern that can use small sizes of stone of varying lengths, rather than large pieces, will result in half as much waste at the quarry.

The lifespan of blocks, slabs and slates is generally higher than a building's lifespan and components assembled without mortar are especially well-suited for extensive re-use. These secondhand products are usually valuable. Crushed stone also has the potential for recycling as aggregate in concrete products.

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