Post –-by Gautam Shah
Stones like many other natural materials are abundantly available. Most of the rocks that we are likely to encounter and exploit are within the top 16 kilometres of Earth’s face. This mass is made up of 95% Igneous rocks, and rest consisting of widely spread cover of Sedimentary and Metamorphic rocks.
We today have greater capacity to search over wider terrains and also reach at sub surface locations. Exploitation of stones as collection from the surface or extraction from various depths is not a major technological problem, but economics of transportation limits its commercial usage.
There are 3 essential sources of Building Stone Materials:
● Surface collected stones
● Extracted stones: protruding and subterranean mass
● Waste and recycled stones
The stones occur in many forms and sizes:
1 Large pieces which can be further down sized or cut into smaller units,
2 Units that are used without any other processing,
3 Pieces which are crushed or disintegrated into finer particles,
4 Rejected material from mining and collection processes,
5 Wastes from stone sizing and dressing operations,
6 Debris material recovered from demolition of old buildings and other structures.
Surface stones from single geographic region show qualitative and size variations of minor nature. Further quality equalization can be done by location-based sourcing, visual selection, grading, separation. Surface collected stones can be further quality equalized through many types of ‘processes’.
● Surface collected materials are naturally formed such as boulders, pebbles, gravel, sands, etc. These are very tough materials and equally weathered on all faces.
● Other surface collected materials are broken by natural disintegrating forces like weather, chemical reactions, land mass movements, internal stresses, etc. These stones may show up with varied weathering on their faces. Such materials are fractured along the plane of shearing force or across the weakest plane, and so show unpredictable structural properties, inconsistent colour and grain structure (texture) on different faces. These stones due to their long exposure are either the toughest remains or the weaker fractures. In the first instance further dressing or downsizing is difficult, and in the second case consistent shaping is not possible.
● Such materials are found spread or located over terrain difficult to access. Collection unless manual involves a large amount of useless mass.
Extracted materials are buried (loaded) under the same or different nature of materials’ mass. The over burdening mass protects, as well as contaminates the deposit. The water leaching through the organic soil burden is nominally acidic and affects the alkaline stone mass. Typically Lime stones when freshly extracted, are not exposed to Carbon Dioxide due to the overburden, and so are soft and porous, but begin to harden on aeration.
● Igneous and metamorphic rocks are not strongly stratified and do not present distinctive layers or strata. Sedimentary rocks are stratified, generally in horizontal layers. However, due to movements in the earth mass inclined and curved formations also occur. Sedimentary rocks show grains intervened by a cementing medium.
● Igneous and metamorphic rocks are often made of many different substances, some of these components, as remnants, are nearly crystalline compounds.
● Sedimentary rocks are comparatively formed of uniform constitution though with streaked colouration due to seepage of dissolved substances and stratification.
● Extracted rocks, require dressing, and often downsizing. The cleavage or fracturing during dressing and fracturing depends not only on the basic classification of the stone and also on constituent minerals such as silica, quartz, feldspar, mica, etc. These aspects also define the types of tools used for working and the nature of surface finish possible.
Igneous rocks, such as Granite and Trap are formed with the solidification of molten materials. Mineral gases and liquids penetrated into the stone and created new crystalline formations with various colours. Sedimentary rocks such as Lime stone, Sand stone, Soap stone Travertine, are formed from the bonding of deposition under pressure and heat over a very long period. Metamorphic rocks are formed by the transformation of igneous or sedimentary rocks, due to influence of heat or chemical action. Metamorphosed form of stones: Marble (of lime stone), Schist (of sand stone) and Slate (of mud-stone). Amorphous Solid is any material which does not have its molecules arranged in a lattice, or crystalline structure. Amorphous solids make up only 10 % of solids in the world. A well-known example of amorphous solid is glass, and that is why these solids are often termed glass. Amorphous solids’ structures have similarity to liquids, and so are also called supercooled liquids. Plastic is made from polymers, long strings of molecules purposefully chained together and is technically an amorphous solid. Crystalline Solids constitute nearly 90 % of all solids in the world. Crystalline solids have a lattice of molecules. The ordered pattern repeats substantially through the mass. Stones are classified as Siliceous when silica is the principal earthy constituent, Calcareous have carbonate of lime as the predominant material, and Argillaceous have alumina is the main component.
WASTES AND RECYCLED STONES
Stone extraction or collection creates large quantity of rejected and broken mass. Site based dressing and downsizing, are mainly done to reduce the mass for transportation, These generates large quantity of wastes. As stone sites are very remote from the point of use or application, it is uneconomic to transport and use such waste materials. Downsizing and cutting workshops located near urban localities, have an advantage that the wastes originating here have consistent one face or dimension. Machines that dress a block with rotary or stripe saws create wastes with smooth finish on one or more faces. Similarly slabs’ end or edge cuts have a uniform thickness profile. Stone polishing machines provide ground particles which are used as filler media.
Angular cut wastes can be tumbled with iron bits in a rotary drum to achieve rounded edged pebbles. Stone wastes can be used to create cement and resin-based composites, and for ‘synthesizing’.
Stone buildings that are demolished in urban areas end up as debris for land fill for lack of man power required for separation and re-use. However, in rural area, it is possible to separate and reuse the material. Older stone flooring units are thicker in comparison to modern supplies. This can be split into two or more units and use the cut-face as the new face. Similarly masonry or building blocks can be cut to thinner blocks for use in cladding or surfacing. The advantage in reuse is free supply of mature (weathered-seasoned) stones..