8.0 WEATHERING OF NATURAL MATERIAL

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Categories of weathering

Normal weathering: The weathering is not influenced by human activity. Occur when the environment is not polluted. Pure, distilled water has a neutral pH of 7. Normal, unpolluted rain is slightly acid, with a pH of 5.6, because the carbon dioxide in air combines with water to form small amounts of a weak acid called carbonic acid. Rock like limestone is attacked by normal carbonic acid with the pH of rainfall less than 5.6.

Accelerated weathering: It is directly influenced by human activities, such as mining, industrial pollution, emission of gases from vehicles, etc. Weathering process is accelerated with the addition SO_2. CO₂ and NO₂ gases in the atmosphere causing acid rain that attack stones.

Factors influencing weathering

i)     Climate

●Temperature: Temperature determines the rate of chemical weathering, expansion and contraction of the rock, decomposition of organic matter in the ground, etc.

● Rainfall: Govern the rate of chemical weathering: solution, hydration, carbonation, hydrolysis, Redox and cation exchange capacity

●Wind: Detachment of particles by bombardment and transportation from one place to another. 

ii)   Nature of parent material:

● Mineralogical composition: Soluble minerals (e.g. carbonates and evaporates) and less resistant minerals (eg mafic minerals) will weather easily than insoluble and resistant minerals (eg. quartz). Presence of salts will affect rocks with carbonate minerals.

●Texture: Porous rocks will weather easily than nonporous rocks.

● Structure: Jointed rocks are easy to fragment than intact rocks.

iii) Topography: relief controls water movement/infiltration rate. On steep slope the rate of weathering is minimum as compared to gentle slope area. Time water is in contact with the rock is less in steep slope than in gentle slope.  In steep slope the rate of infiltration is very low hence less weathering process.

iv)    Bio-deterioration (Organic activity): Stones can be damaged by biological action. Specialised microbes can grow on surfaces and in minute fissures, their by-products causing flaking and discoloration. Lichens and plants may also develop causing further damage. Decomposition of organic matter, generation of CO₂ gas and penetration of roots in the ground all assist weathering process.

v)      Human activity: Blasting creates fractures in rock mass and hence become easy for the rock to weather. When the rock is exposed to the surface it is subjected to surface condition and is susceptible to chemical and physical weathering. The emission of gases such as sulphur dioxide and carbon dioxide from industries, mines and vehicles assists weathering process of exposed rock through generation of acidic rains.

vi)    Time: Time determines the effectiveness of the above factors.

Weathering of building stones

Building stones are sometimes known as dimension stones. These are rock fragments properly shaped and used for construction of buildings, ornamental structures, retaining structures, etc. Weathering of building stones occur in a number of ways. The common ones are as follows:

Attack by pollutants: These are mainly SO_2, CO₂ and NO₂ gases that combine with rainwater to produce acids that attack the stones. These acids are H⁺ + HCO3^- resulting from CO₂;  2H⁺+SO₄^- from SO₂ and H⁺+NO₂^- + H⁺ +NO₃^- from NO_2.  When the surface of the stones is moist or there is high humidity there is a tendency of SO₂ gas to attack under the presence of Fe₂O₃ as a catalyst.

Frost weathering: This is common for stones with large porous like scoria basalt or pumice where water crystallizes and causes hydro-fracturing of the rock due to increase in specific volume. Normally the volume increases by 9% and pressure exerted may be up to 2100kg/cm². This type of weathering is possible in area where water can freeze to form ice.

Salt weathering: Involves crystallization of salts in pores and fissures. Failures are due to growth of salt crystals from solution, thermal expansion of salts and hydration of salts with the pores. The end result is spalling and fragmentation of stones. This is less effective in fine-grained stones and not common in humid environments where salt solutions are not saturated. Salts that cause weathering are brought from outside either through contact with ground water or mortar. Some are remobilised within the stones. Salts tend to cause crumbling, flaking, scaling and blistering of the stones. More aggressive salts are sodium sulphate, magnesium sulphate, calcium sulphate and sodium chloride. Impact of weathering depends on the nature of the salt and properties of stones such as presence of micro-porosity and water absorption capacity.

Solution: It is possible when you construct using stones with soluble minerals especially when they are in contact with water. Soluble minerals will slowly be dissolved and removed with time.

A)    Sources of salts for stone weathering

i) Rock weathering

It involves transformation of primary minerals in rocks to secondary minerals that are more stable at the earth’s surface. The constituents of salts are released during weathering and made soluble. The weathering is assisted by the presence of atmospheric water, oxygen and carbon dioxide. The reaction involved includes carbonation, Redox, solution, hydration, and hydrolysis. The increase of levels of carbon dioxide and organic matter enhances weathering.  Organic matters promote weathering and cation migration through complexation.

ii) Deposition from atmosphere

Salts enter the atmosphere from oceans. It occurs when droplets of water from the oceanic sprays and turbulence produce atmospheric aerosols of suspended salts crystals or highly saline droplets. Salts are removed from the air through “dry fall-out” between storms or  “a wash-out/rain-out” (Mnkeni, 1996). The salts are known as cyclic salts. The composition of the salts varies with distance from the source. Closer to the coast is predominantly NaCl-type. It is dominant Ca²⁺ and SO₄^2- ions as the air moves inland because of addition of salts from terrestrial sources.

iii) Ingress of seawater

This is through tidal waves or contamination of underground aquifer.

iv) Fossil salts

These are remnant from former marine and lacustrine lakes. When sediments or rocks of former marine and lacustrine lakes are exposed through erosion or uplift, tends to be a major source of salts in the area. Presence of Cl^- ions in local water is automatic to be of marine or former lacustrine origin. Cations cannot be used to predict the source of salt as can be derived from mineral weathering too.

v) Human activities

A number of human activities can contribute to salinity. These activities are as follow:

Industrialisation: It causes increase of gaseous nitrogen and sulphur compound in the atmosphere.  These in turn cause increase of salt concentration in the atmosphere. This results into acidic fall-out that accelerates weathering.

Mining activities: Brought to the surface saline and sodic materials. Chemical used for processing can also contribute in causing salinity in the ground or accumulation of salts.

Irrigation: Almost all irrigation water contains salts in varying degree and types. During evapotranspiration essentially pure water is removed by plant and salt is left behind to accumulate.

Domestic wastes: Cover all wastes containing salts including septic water from pit latrines.

Car pollution:

Use of pesticides:

Assessment of stone weathering

i) Petrologic approach: This includes preparation of thin sections and analyse them using petrologic microscope. Also it is done by evaluation of imagery from scanning-electron microscopy and XRD.

ii) Schimdt hammer: Instrument used to assess the strength of the rock based on rebound number. Hardest (fresh rock like granite) have rebound number of 57.5 and softest (highly weathered) is about 12.8. These rebound numbers can be converted to compressive strength using empirical formula.

iii) Comparing the chemical composition of rainfall incident upon a drainage area and the run-off derived from it. Its objective is to assess the nature and magnitude of chemical change.

iv) Direct measurement using fixed sensitive micrometer gauge. Measurement is done repeatedly of the height of rock surface above an arbitrarily established datum.

v) Laboratory accelerated weathering test: This is sometimes called slake durability test. Stone are subjected to repeated rotation in perforated drums immersed in water for given time. Loss in weight is thereafter determined to give durability index expressed in percentage. Less durable stones will give high percentage of loss in weight.

vi) Field monitoring of climatic conditions and stone characteristics. Discoloration of stones and loss in strength in a particular climate imply the rock is not stable in that area.  Monitoring should also cover the damage on known material placed in hazardous area. This will help you to assess the rate of weathering and precaution that need to be taken in advance. However, this type of assessment is time consuming

Management of building stones

Management strategies against weathering of building stones as recommended by Cooke and Doornkamp, (1990) are as follows:

i) Site assessment:  Survey and classify the ground based on the damage observed on existing structures. Both soil and water samples are collected for laboratory analyses. Sometimes electrical conductivity survey is employed for quick zonation of the area based on salt content. This will assist in giving advice whether to abandon the area or to look for appropriate measures.

ii) Design consideration

●Design should minimize access of water and degree of exposure of the building stones. Improve drainage using geotextile, drainage ditches or raise the ground by using fill.

●Material selection according to micro-environment in which stones will be placed.

●Use stones with low-saturation coefficient and micro-porosity prevent frost action.

●Use a damp course to prevent penetration of ground water and related saline solution.

iii) Regular cleaning: Removal of salts and unsightly encrustations, soot and other soiling particles, micro-organisms, parasitic vegetation, bird and animal droppings. All of these do accelerate stones weathering. In doing so it improves surface appearance of structure. Periodic inspection is essential if preventive measures are to be taken soon and effectiveness assessed. Cleaning is done using water jets, abrasive blasting using compressive air or water jet with abrasives, mechanical cleaning with brushes and chemicals such as absorbent clay packs or solvent jelly coating. Cleaning should be done carefully otherwise may cause undesirable water penetration into the building or remove protective weathering rinds and expose fresh surface to decay. Sometimes chemical cleaners may etch stones.

iv) Waterproofing or consolidation: This is surface treatment to building to seal the surface and restrict the access of water and solution using organic and inorganic products for small-scale work and adhesives and fillers for larger gaps and cracks.

v) Replacement of the damaged stones is both essential and desirable. If it is not replaced it will weaken the structure.

vi) Climatic protection: Improve air quality by control of air pollution. Set tough regulations to those causing air pollution.

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