Post 676 –by Gautam Shah
Stones are procured through collection off the surfaces, by extraction (mining) from depths of the earth and reused (spolia) from existing or demolished buildings. The stones of these types are abundantly available. Major problems with sustainable stone exploration are the economics of transportation. Other issues are cost of size conversion, surface preparation and quality equalization. In future greater attention will have to be for management of stone-wastes at locations of mining and processing.
Stones are used for their surface quality and structural properties. And in spite of technologically greater capacity to search over wider and deeper terrains, stones always remain scarce or unviable at many places. At use-points natural stones must arrive in optimum mass-units and in forms that are viable for transport, storage and usage.
Stone resources are of basic two types: Surface Stones and Extracted Stones.
Surface Stones show many, but qualitative and size variations. Over a geographic region, though the quality is fairly consistent. Quality equalization can only be enforced through region-based sourcing, selection and separation. Surface-collected materials are naturally formed (boulders, pebbles, gravel, sands, etc.) or wastes of stone processing. Such materials are fractured along the plane of shearing force or across the weakest plane, and so show varied structural properties, colour and grain structure (texture) on different faces. These stones are equally weathered on all faces.
Extracted Stones materials are loaded (buried) with varying depths of overburden, of the same or different nature of materials. The over burdening mass, protects as well as contaminates the stones. The water passing through the organic soil burden is nominally acidic, and so affects the alkaline stone mass. Fresh lime stones are soft and porous, but when exposed to Carbon dioxide begin to change, harden due to the aeration.
Typically, igneous (granite, trap) and metamorphic rocks (marble, schist, slate) have nearly crystalline compounds, and are not stratified so do not present any layers or strata. Sedimentary rocks (lime stone, sand stone, soap stone, travertine) are formed of uniform constitution, though stratified, often in inclined and curved formations due to movements in the earth mass. Sedimentary rocks show grains intervened by a cementing medium.
All stones collected from the surface or mined, must go through some primary processing.
■ Subtractive processes remove excess mass for surface cleaning, sizing, cleaving and pattern sculpting. The processes are, chipping, splitting, cutting, dressing, sculpting, engraving, grinding, polishing etc.
■ Formative processes do not add any mass but change the spatial or physical characteristics of stone such as its sensorial, structural and environmental behaviour. The treatments include impregnation, edge reinforcing, various types of chemical treatments through acid, alkali, solvent and other oxidative compounds, heat and flame treatments, sintering, spluttering, dying, bleaching etc.
■ Additive processes add to the stone mass. Till very recently technologies involved were of Surface layering by way of coating or cladding. But now ceramic formation, metal alloying and deposition, surface synthesis, surface molecular treatments are being used.
The sustainability of stone is dependent on basic three aspects: 1 Minimum mass for largest possible surface extent, 2 Reuse of all waste products, 3 New uses for very small sized materials (sand, gravel, pebbles).
1 Stones are valued for their surface qualities, and we need to extend the Surface area. The extended surface reduces the mass / weight of the stone. This can be done by thin sectioning, and by techniques of amalgamation of bits and pieces.
2 Stones have certain structural properties which we can be altered and reinforced. This process starts with new ways of excavation, extraction and conversion of the material. And can be extended to new forms of usage.
3 A new field is emerging on materials’ technology front. This is about creating new materials combinative formations. The formations include various types of composites, geometrical or spatial compositions and combining or ‘synthesizing’ materials of diverse nature. These reconstructive processes include using particulate matter (various grades of fineness such as dust, sands, gravels, pebbles, chips and lumps) as fillers with a matrix of resin or cement. Forming layered composites with sheets or slabs of stone and other materials (polymer sheets, fabrics). Forming amalgamated materials by lamination, co-extrusion, sheet forming, metalizing, ceramic forming, etc. and chemically converting stones into byproducts like minerals and chemicals.
Spolia (Latin, ‘spoils’) are repurposed- recycled building stone used for new construction, or decorative sculpture reused in new monuments. These stones are from existing or demolished buildings or building workshops.
Sustainable Strategies for Stone
Stone is the least of bio-degrading materials, so not a ‘recoverable or ecological’ material. It can be recycled through reuse processes. Sized blocks of stones for masonry and flooring, have been reused since Egyptian and Roman times. But stone-waste dumps at mine heads and workshops are causing environmental problems.
Stones are broken or crushed from larger stocks for many purposes like roads, embankments etc. which is an avoidable practice. Stones like gravel and boulders (from river beds and old glaciers’ paths) are some of the toughest stones, left over after natures’ processes. But these rounded stones are not used in masonry work, or broken down to smaller sizes. River and seacoast sands are becoming scarce in supply, and could easily be replaced with ground stone, at least in mass concrete plants.