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Stone Porosity

All natural stone is porous, which means it has interconnected capillaries through which liquids and gases can move. In fact, porous materials act like a hard sponge and actually suck in liquids over time, along with any dissolved salts and other minerals. More porous stone, such as sandstone will absorb liquids relatively quickly, while denser volcanic stones such as granite are significantly less porous and may take an hour or more to absorb oils and water-based liquids.

Stone as a Building Material

Since ancient times, stone has been popular as a building and decorating material. Its strength, durability, excellent insulation properties, ability to be shaped, and the variety of stone types and colors make it an exceptionally versatile building material.

The porosity and makeup of most stone does, however, leave it prone to certain types of damage if unsealed. Staining is the most commonly occurring damage, which occurs when oils and other liquids are sucked deeply into the material through its system of capillaries, where it can be impossible to remove. Salt Attack occurs when salts dissolved in water are carried into the stone. The 2 most common types of salt attack are efflorescence and spalling.

Acid Attack. Calcite-based stone such as marble, limestone and travertine react with acidic substances on contact, breaking down the surface and leaving dull marks or even deep furrows over time. This is known as acid etching. Even mild household acids, including cola, wine, vinegar, lemon juice and milk, can damage these types of stone. The milder the acid, the longer it takes to etch calcite-based stone; stronger acids can damage the stone in seconds.

Spalling in mechanical weathering

Spalling is a common mechanism of rock weathering, and occurs at the surface of a rock when there are large shear stresses under the surface. This form of Mechanical weathering can be caused by freezing and thawing, unloading, thermal expansion and contraction or salt deposition.

Freeze thaw weathering is caused by moisture freezing inside cracks in rock. Upon freezing its volume expands, causing large forces which cracks spall off the outer surface. As this cycle repeats the outer surface repeatedly undergoes spalling, resulting in weathering.

Unloading is the release of pressure due to the removal of an overburden. When the pressure is reduced rapidly, the rapid expansion of the rock causes high surface stress and spalling.


Exfoliation (or onion skin weathering) is the gradual removing of spall due to the cyclic increase and decrease in the temperature of the surface layers of the rock. Rocks do not conduct heat well, so when they are exposed to extreme heat the outer most layer becomes much hotter than the rock underneath causing different thermal expansion. This differential expansion causes sub-surface shear stress, in turn causing spalling. Extreme temperature change, such as forest fires, can also cause spalling of rock. This mechanism of weathering causes the outer surface of the rock to fall off in thin fragments, sheets or flakes, hence the name exfoliation or onion skin weathering.

Salt spalling

Salt spalling is a specific type of weathering which occurs in porous building materials, such as brick, natural stone, tiles and concrete. Dissolved salt is carried through the material in water and crystallizes inside the material near the surface as the water evaporates. As the salt crystals expand this builds up shear stresses which break away spall from the surface. Porous building materials can be protected against salt spalling by treatment with penetrating sealants which are hydrophobic (water repellent) and will penetrate deeply enough to keep water with dissolved salts well away from the surface.


In corrosion, spalling occurs when a substance (metal or concrete) sheds tiny particles of corrosion products as the corrosion reaction progresses. These corrosion products are not soluble or permeable, but, unlike passivation, they do not adhere to the parent material's surface to form a barrier to further corrosion. This happens as the result of a large volume change during the reaction.

In the case of actinide metals (most notably the depleted uranium used in some types of ammunition), the material expands so violently upon exposure to air that a fine powder of oxide is forcibly expelled from the surface. This property, along with these elements' inherent toxicity and (often to a lesser extent) radioactivity, make them very dangerous to handle in metallic form.

Natural Stone Pavers does not in anyway warranty or guarantee any stone pavers and does not endorse any sealer products.

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