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MECHANICAL PROCESSES

This includes the processes of frost action, thermal expansion, wetting and drying, salt decay.
These mechanical processes generally lead to a weakening of the stone, increasing its permeability providing greater penetration of water and increasing the areas for the chemical weathering processes to take place.

Frost Action: Frost action or commonly called freeze/thaw cycles occur when water within the pore structure or cracks freezes to ice. It has been estimated when water freezes it expands between 8 to 11 percent, with a force of 2,000 pounds per square inch (....kgs/ …. M2) to 150 tons per square foot. This increase of internal pressure combined with repeated freeze/thaw cycles produces micro-fissures, cracks, flaking and spilling.

Thermal Expansion: Different minerals expand and contract at different rates, this is  known as the coefficient of heat expansion and contraction.  It is known that the temperature of stone can vary between 30 and 50 percent higher than the average air temperature.  Some of the darker minerals, absorb heat more readily, and also give  it up more quickly than some of the lighter ones. The daily and seasonal heating and cooling of stone can cause stresses and micro-fractures in and along mineral grains. Water in the pores makes thermal stressing more effective. This can eventually produce surface flaking. The marble is particularly affected by this.

Wetting & Drying: Coefficients of expansion and contraction are also involved in volume change. Stone expands when it absorbs water and shrinks as it dries. This expanding and contracting produces internal stresses at the grain boundaries. When the stone heats up a "baking effect" occurs, which will eventually lead to surface flaking.  The marble is affected by this "baking effect."

Salt Decay: Salts are some of the most damaging agents to stone. Salt manifest themselves in a process commonly referred to as Efflorescence. There are several different types. The most common form of efflorescence is the appearance of salts at the surface in the form of whitish to gray powdery fluffy blooms. This occurs when the stone, substrate or other sources of soluble salts are in contact with moisture and move to the surface by capillary action. As the moisture moves to the surface these soluble salts are deposited at the surface to recrystallize into these blooms. This form is generally harmless. Sub-florescence is similar to efflorescence, however instead of the salts being transported to the surface they crystallize and buildup within the pore/capillary structure beneath the surface. As the salts accumulate internal pressures, it develop generating spilling and flaking and may eventually lead to deep deterioration of the stone. Numerous varieties of salts have been identified in the efflorescence process. Some of these varieties by themselves or when combined with others will form a "hard and glassy skin" adhering rather strongly to the surface. This type is Crystalline efflorescence.

 

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