STONES -QUALITY DETERMINANTS
Post 267 ⇒ by Gautam Shah →
Quality of a stone determines the Quality of stonework. Other factors include quarrying method, seasoning, dressing technique, and masonry or cladding style. The important properties of building stones are: durability, hardness, toughness, porosity and strength under various types of stresses. The structural properties are, mainly governed by: the class of stone (sedimentary, calcareous, igneous, etc.), mode of occurrence (surface, shallow depth, very deep mines), type and condition of the rocks (horizontal or inclined mass). The structural properties do get substantially altered by the subsequent handling of stones, such as: exposure, action of environmental factors (moisture, salt, contaminants, temperature) mechanical pressure, vibration, etc.
Most of the stone materials are extracted from a depth. Such materials have remained buried under heavy over loads, devoid of oxygen and other gases, and free of moisture, or with entrapped moisture or flooded in subsoil water, and without light or radiation exposure, possibly since their age of formation. On extraction the stone material is brought to a completely different environment. Mineral lattices begin to alter due to lessening of pressure and dehydration. Entrapped gases escape and the space begins to get filled in with moisture and atmospheric gases. In sedimentary deposits the excess moisture begins to escape, allowing minerals react, causing expansion or contraction stresses in the mass. Rain water which is slightly an acidic one causes calcium carbonate to change into bicarbonate. In industrial areas sulphur and carbon dioxide enhance the acid action on the freshly quarried stone. Chlorides on sea front and in industrial areas can get converted into weak hydrochloric acid and dissolve the carbonate rocks. Nitric acid produced from oxide or Nitrogen also corrodes stone faces. All corrosive mediums depend on supply of water or moisture, so care of fresh-cut stone is essentially a moisture management exercise.
Freshly quarried and exposed stones are affected by: 1 Temperature variations cause differential expansion and contraction of the mass causing cracking or splitting. 2 Wetting and drying cycles of rain or dew, and frost may disintegrate the stone. Other moisture associated, problems are of living organisms that flourish in wet grounds. Stones stored here are affected by algae, lichens destroy lime stones, and worms destroy all stones except granite. Vegetation growth is a problem in tropics and heavy rainfall areas. 3 Polluted atmosphere around cutting centres (industrial areas, dumping sites and sea shores) affects the surface quality. 4 Stones need to be stored in same position as they were in natural state. Layered stones such as sedimentary rocks must never be stored in vertical position before sufficient seasoning.
Seasoning is both drying and wetting cycles on one hand, and airing of stone, on the other hand. There is an increase in strength due to the re-deposition of percolated minerals, surface carbonation, transmission and deposition of minerals on the surface of a stone, by both the evaporating moisture, and addition of water. Dehydration, during seasoning, is more or less an irreversible process, Seasoning applies mainly to soft limestone; hard limestones seem to be less affected by it.
Stones have natural moisture, known as quarry sap. This quarry sap renders the stone blocks comparatively soft and makes them easily workable. Immediately after quarrying, a stone is soft and it is easy to work with it so sized and primarily dressed, but polishing requires a fully seasoned and hardened stone.
Weathering denotes both desirable and undesirable changes. It is concurrent process of seasoning. It enhances certain qualities, like colour, texture, strength, etc. and reveals micro fissures deep pores on the surface. Weathering is very effective for Lime-based stones, which absorb atmospheric Carbon-dioxide to produce hardened mass.
Porosity has no direct relation to the weathering resistance of stone materials, such as limestone. It is the shape, size, depth, and nature of pores, that plays an important role in weathering. The action of carbonic acid, increases with greater micro-porosity, capillaries prolong the dissolving action because the water is retained longer in small cavities. Sandstones containing colloidal minerals as a cementing medium has the most pronounced expansion due to porosity.
Efflorescence (wall white, stack white or wall cancer), is the appearance of crystallized salts on the stone’s surface. ‘The crystallization of the salts, and their re-crystallization from a lower to a higher hydrate within the range of mineral stability, may develop stresses of high magnitude with quite an appreciable qualitative change.’