Effective Factors For Choosing Building Stone (stone durability)

Effective Factors For Choosing Building Stone (stone durability)

Stone Durability

The durability of a building stone is its stability against chemical attack and weathering agents. The durability of stones depends on their physical properties, which, of course, are also a function of the physical and chemical structure of the stone. The physical structure of the stone, such as the form of the layers forming the cracks, joints and empty spaces inside it, has a great influence on the durability and resistance of the stone.

Over time, several natural factors cause stone deterioration, the most important of which are listed in the table below.

The most important natural factors leading to stone deterioration

Factor	How to influence stone deterioration
Rain	Rainwater affects rocks physically and chemically. The physical effect caused by its erosive power and the chemical effect caused by the decomposition, oxidation and hydration of the minerals in the stone cause the deterioration of the stone.
Physical factor	If wetting by rain and drying by the sun happen consecutively, internal stresses will be created in the rocks and will bring about its collapse.
Chemical factor	In industrial areas, acidic waters from rain react with stone and lead to stone deterioration.
dew	In cold places, dew penetrates into the pores of the stone and the stone freezes and expands, these dews cause cracks.
Wind	The wind carries dust particles, the wear caused by the dust particles causes the deterioration of the stones.
temperature changes	The expansion and contraction resulting from intermittent temperature changes (especially if the rock contains minerals with a different volumetric expansion coefficient) causes the deterioration of the rock.
Plant grow	The roots of trees and weeds that grow around the stones keep the stones moist and they also secrete organic and acidic substances that cause the stone to deteriorate. Dust particles of organic or inorganic origin also sit on the surface and penetrate into the pores of the rock. When these particles come in contact with moisture caused by plants, the bacteriological process begins and the microorganisms that lead to acid production attack the stones and cause the stone to rot.
Reciprocal decay	Mutual decay occurs when different types of stones are used together. For example, when sandstone is used under limestone, chemicals that enter the sandstone through rainwater from the limestone will cause the deterioration of the sandstone.
Polluted air	Atmospheric pollution, steam, acids and acidic particles in the atmosphere cause stone deterioration, and stones containing calcium carbonate and magnesium carbonate are severely affected.
Mudstones	Mudstones destroy limestone; But for other stones, they act as a protective cover and prevent their destruction. By creating a series of parallel vertical holes in the limestone, mud gradually weakens them and eventually destroys them.

Any non-uniformity in the stone not only weakens it, but also causes the stone to perish sooner. For example, if water penetrates into the rock and then freezes, the stresses caused by the freezing will cause the rock to crack and break. In this way, the porosity of the stone increases and it becomes easier to repeat such a situation, and as a result, it causes the deterioration of the stone. Sometimes the water causes the cement that makes the different layers of the stone stick together, and the stone becomes structurally weak.

The chemical composition of the stone is very effective in its durability. Limestones are dissolved in acids and moisture and carbon dioxide in the air can dissolve these types of stones. Sandstone, whose particle binding material is calcium carbonate, disintegrates in the vicinity of air and moisture. While silicate stone is more resistant in such a state.

The chemical composition of the stone is effective in its resistance to fire. For example, carbonate rocks are less resistant to fire than silicate rocks. The high expansion coefficient of the stone reduces its durability, especially if it is in the vicinity of heat or fire. Because such a stone expands by creating heat and having a higher expansion coefficient, it expands more and when it cools down, it shrinks more, and as a result of this expansion, more space is occupied and it puts pressure on the nearby stones and causes the stones to crumble.

If the building stone is placed in a humid environment, this factor may reduce the porosity and protect the stone surfaces against the influence of air. The size and shape of the layer texture is also important. In general, a stone that has uniform and good layers is stronger and more durable than another stone that does not have such properties.

Unfortunately, stone durability, which should be the most important factor in stone selection, is often overlooked. There are many expensive buildings that did not pay attention to this issue in their stone selection and suffered from this area. Among the natural factors that affect this issue, we can mention the construction, texture and composition of mineralogy. The location of the stone installation in the building and its use is another factor that affects the durability of the stone.

In addition, weather conditions are also very important. A suitable building stone must be compatible with the weather conditions of the environment to ensure the durability of the structure. In hot and humid weather, chemical weathering is more effective and in cold and dry weather, physical weathering is more effective. Chemical weathering is the product of hydration, dissolution, hydrolysis, oxidation, or the reaction of acidic waters with rock-forming solutes; But physical weathering is without any chemical changes and causes rocks to crumble and change their shape into smaller grains. This phenomenon is formed by factors such as freezing, temperature changes and as a result expansion and contraction, gravity, plant growth, wind, water flow, animal activity, etc.

A – Construction

Any weakness in the rock structure, including joint surfaces, layering surfaces, fault surfaces or any type of gap caused by fault or folding (including shear cracks), create suitable conditions for the effect of weathering and freezing factors.

B – Texture

In terms of texture, stones are divided into two main groups of the same size (coarse-grained or fine-grained) and profiry (uneven grains), and in terms of the size of the stone grains, they are divided into clear crystal, hidden crystal and glass. The texture of the stones depends on the shape, size, arrangement, roundness and sphericity of the crystals. From the point of view of the reaction against the force, in addition to the mentioned factors, it also depends on the cement containing the rock crystals. Coarse-grained rocks break earlier than fine-grained rocks, especially rocks that are sensitive to temperature changes. This type of rupture is at least partially related to the difference in the expansion coefficients of the minerals that make up the rock. Rocks may be dense or porous. Dense rock disintegrates later than porous rock. Dense rocks are almost impermeable, so weathering agents cannot act quickly. On the other hand, rocks with open and capillary porosity easily absorb water and crack due to freezing. The most important textures seen in building stones are listed in the table below.

The most important visible textures in building stones

Texture Type	Specifications
Porphyry texture	Large grains of one or more minerals are embedded in a fine textured paste. In this texture, coarse crystals are located in a field without crystals or microcrystals. The presence of this texture indicates that the stone was cooled in two stages. In the first stage, it starts to cool in the depths and large crystals are formed. In the second stage, in the path of movement and close to the surface of the earth, due to relatively rapid cooling, small crystals are formed, and finally, due to the rapid cooling of the magma, crystals are not formed on the surface of the earth, and the glass forms the stone paste.
found contiguous	The grains forming the stone are interwoven in irregular bands.
Mosaic texture (granular)	The grains that make up the stone are packed together in irregular bands or with little order and overlap is not seen in them.
debris texture	Grains of pre-existing rock are joined together in a non-interlaced and mosaic manner.
Granulitic tissue	It is a type of mosaic texture with a large area in which the grains are completely condensed and minerals of different types are remarkably the same size and form irregular bands.
Oolitic texture	The stone consists of spherical grains.
Pegmatite texture	A type of texture in igneous rocks in which the grains that make up the rock are very large, and due to the uniformity of the changes in the size of the grains, it is difficult to distinguish the main grains of fine paste.
Linear or graphic texture	Minerals or crystals are crystallized parallel or almost parallel and resemble a cuneiform line.
Honeycomb tissue	The stone crystals are stalactite and beehive shaped, like Araguaite marble.
Texture or strip construction	The stone is seen as relatively narrow bands and the bands may be of one or more different colors.
Texture or mass construction	The stone has taken a lumpy state and has a dense state.
Porous texture	The porosity of the stone is relatively high and, like travertine, it can be seen even with the naked eye.

C- mineralogical composition

Since different minerals have different resistances against the attack of weathering factors, it is natural that rocks have different resistances against weathering due to the presence of different constituent minerals, and those that have minerals with lower resistance are damaged earlier by different factors.

Some specific minerals (iron sulfides) can be considered as harmful minerals in all conditions, while some other minerals like mica are harmful only in high amounts and in some rocks like sandstones and marbles. One of the causes of color change in stones that is less noticed is the presence of unstable minerals that easily transform into another mineral. These minerals are often iron sulfides, they are oxidized in the vicinity of moisture and turn into hydroxide, which results in staining of the stone.

The most important harmful minerals in building stones and how they work

Connie	How to effect
Flint	Flint or chert is present as amorphous silica in many limestones. This mineral is much harder than Minerals are associated and cause problems during cutting. In addition, the door is resistant to weathering, so it is effective It creates a small-pox appearance on the weathered surface of the stone. When the rock is exposed to weathering, probably from The part that replaces the nap takes a gap.
Mika	This mineral is present in many granites, gneisses, sandstones and marbles. For the granite group, mica is nothing but harmful unless it has created a local accumulation, in which case it will give a bad appearance to the stone. In gneisses, mica is rarely harmful, unless its quantity is so great that it forms a schist structure and interferes with cutting it to proper dimensions. In sandstones, if the amount of mica is small and has a scattered distribution, it is not harmful, but if its amount is high and concentrated along the layering, it will cause the rock to crumble in these places due to successive frosts. Mica is present as a minor mineral in many crystalline limestones or marbles. In these rocks, mica may exist in scattered, concentrated or banded forms. The scattered grains, if few, do not cause much trouble, but in the other two cases the mica not only causes difficulty in polishing the stone uniformly, but often weakens the stone against the attack of weathering, so that the stone becomes badly pitted or even flaked.
pyrite	Many building stones contain at least small amounts of this mineral. Pyrite turns into limonite as a result of weathering due to oxidation. Small amounts of this mineral with a scattered distribution do not harm the stone, but if it is in large quantities or it is concentrated, the change of pyrite to limonite will cause a soft yellow color and eventually holes in the stone. In addition, limonite on the visible surface of the stone is often washed with water and they create smallpox on the surface of the stone. In addition, with the decomposition of pyrite, some sulfuric acid is formed, which, if the rock contains carbonates, is attacked by the released acid. When pyrite changes to iron sulfate, the produced sulfate is easily soluble and is transferred to the surface of the stone by evaporation of moisture and causes scaling on the surface of the stone. Based on the above, you should avoid using stones that have significant amounts of pyrite as building stones.
Thermolite	It is an amphibole type mineral with a white to pale green color depending on the amount present in it, which is observed in some impure metamorphic limestones and usually in the form of fibrous sets with a silky luster, and when exposed to weathering, it turns into green clay materials. It turns yellow. These secondary materials can be quickly washed off the stone, as a result, there are holes on the surface of the stone. The size of tremolite pieces in some rocks reaches several centimeters.

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