The use of building stones in the facade 

The most used stone in the building is its facade. The facade of the building plays an effective role in the formation of the initial mentality of a visitor to the building, that is why there is a special sensitivity in choosing the stone facade. The exterior of the building may give a special grandeur to a building and stand out like a luxurious dress on a building. This eye-catching beauty causes that building to be known and famous with the same facade, as if an identity or a nickname has been assigned to that building through the choice of stone facade. 

The facade of the building is of particular importance due to exposure to atmospheric conditions (rain, frost, sun, pollutants). Recognizing the beauty, cleanliness and purity of the environment in the first stage of dealing with any animate and inanimate phenomenon is observed in the appearance of that phenomenon, which in the case of a building goes beyond the painting and layout of its interior architecture to the role of stone in its facade. Due to being exposed to the open air and individual and natural sensitivities, the material of the facade of the building is usually considered an important and significant topic and it focuses an important part of the mind of the designer and user. The type and type of these materials are selected according to the needs of the spaces and the amount of use. Building stone is one of the items that should be considered in all buildings and spaces. In terms of technical issues, building stone should be considered from two aspects: 

1.    In terms of beauty of appearance 

2.   In terms of protecting the underlying materials 

The surface of architectural spaces, including building stones, are usually exposed to the attack of purulent and dirty factors. From a natural point of view, these factors are dust and dust in the air, in terms of heat-generating materials, smoke from vehicles, and from a human point of view, contact with contaminated hands and other greasy and polluting materials on the building stone in the exterior and interior of buildings. To solve these problems, the facade of buildings and the materials used for infrastructure must be selected, which can be washed, have durability against detergents, and have appropriate physical and chemical resistance. In the case of classroom spaces, he used colors that increase the concentration power of the scholar or student. In the dormitories, he used samples that affect the level and feeling of relaxation, and in recreational and high-traffic spaces, he used more happy and durable samples, and in other spaces, such as amphitheatres, cinemas, terminals, stores, residential buildings, management and headquarters spaces, support spaces and relief and hospitals and clinics, special examples of those conditions, to use and even to use its diversity and with different correct and compatible arrangements in similar or different environments, in which the imagination of an architect should be mixed with the engineering judgment of a lithologist and It comes down to image or computer modeling. 

Stone is basically the final coating on the surfaces, which on the one hand prevents the corruption of the underlying materials and, on the other hand, prepares the working and living environment in a more favorable way. As a result of the passage of time, the possible corruption of the underlying materials and other damaging factors and external erosion are transmitted to the stone, and lateral pressures and these factors affect the wear and shortening of the stone’s life; Therefore, it is necessary that public, welfare places, dormitories and educational spaces, teaching aids, classrooms and libraries every 6 months, hospitals and medical centers every month and sports spaces every 12 months should be reviewed and washed technically and appropriately, and in case of damage of some samples The damaged sample should be replaced by a healthy sample. When using stone on the sides of the shelter walls, around the roof edges in the roof and insulation and the sides of the walls, around the salt floors in the basements, according to the connection of the insulation of the seats and the walls and observing the cleanliness of the materials and knowing the absence of rotten materials and piercing vulnerable minerals and the layers below. And it is necessary to consume it on the upper level. 

Stone face must have special features to be able to last against these factors; Therefore, to choose stone for use in the facade of the building, special attention should be paid to the following points: 

1.     The type of weather in the region should be taken into consideration and a suitable stone should be selected according to the climatic conditions. For example, in cold and humid areas, stone with very low porosity should be used, otherwise the stone may crumble due to freezing, or in places where there is a possibility of sandstorms, it is subject to wear. 

2.  It can be polished and cut.

3.    It has relatively good tensile and bending resistances. 

4. Its mineralogical composition should be considered. Because harmful minerals (iron minerals) are oxidized against the weather and thus cause premature wear. The presence of impurity as a complex in one point or one stretch is a weak point and may cause destruction of the stone. 

5.  It is better to estimate the useful life of the building and choose the stone facade according to it. 

6.   If the stone is installed in a different way, it should have good adhesion with the mortar.

7.   Its color should be stable.

8.   To be stable against vapors, gases and sunlight.

9.  The softening coefficient should be between 0.7 and 0.9. 

Compared to other stones, travertine facades have good resistance to cold, heat, sunlight and air humidity. Of course, most granites also have these characteristics; But sometimes the humidity of the air may cause the iron minerals in them to oxidize and make the stone look ugly, and in some dark types of granite, sunlight causes the stone to lose its natural color. 

In large and industrial cities, with the growth of air pollution, acid rains are observed, and for this reason, the resistance of masonry to acid rains has become very important. This means that in case of rain, the constituent elements of the stone facade should not mix with water and destroy the shine and polish of the stone facade. 

This defect is mostly noticed in stones with high porosity and high percentage of lime, which can be considered a large group of marble stones as part of this type of stones, and for this reason, the use of marble stone in large and industrial cities as a facade stone is not recommended. The type of composition of minerals and constituents of marble stones is such that they undergo weathering on the surface and depth very quickly, and in a short period of time, first the surface and then the influence of water in the veins of these stones, we will witness the corrosion and dissolution of limes on the sides of the seams, and after installation in the season In winter, when it freezes, its surface gradually gets deep cracks and loses its beauty and polish. No matter how slow and gradual the effect of rain or surface water on the facade; But finally, it happens over time and unfortunately this effect mostly occurs in a completely non-uniform way on the facade. If a part of the used marble facade stone contains silica compounds, these parts resist the dissolution process more than the carbonate parts, and the appearance of the facade stone is destroyed in a non-uniform way, and as a result, a blistered appearance is created. The effect of this kind of destruction is the flaking of the surface on the stone; Therefore, marble is not a suitable option for facade stone unless it has a very thick thickness (more than 4 cm) and that is in the siliceous and strong type of marble. The marble stone used for the facade must not be eroded; For example, it is not shiny or polished in any other way. Stone facade with such a surface, even in case of weathering and surface damage, there is no problem. The famous example of using marble as facade stone is the Azadi Tower building in Tehran, where Shahiyadi marble is used, but with a non-glossy surface and an uneven and very thick surface, it has good resistance. 

In recent years, with the advancement of mining and processing machinery technology, the use of travertine stone has become widespread in Iran. Travertine stone used to be one of the ugliest stones; But with the change in the method of its production, luck has turned to this stone and with the advancement of technology, it has reached the height of dignity from the bottom of humiliation; But how long this situation and the general acceptance of travertine will continue is an important question and we must know that although the history of using resin in travertine stone processing is only 20 years; But the life span of a facade is not more than this time and even after this period, the travertine stone facade is not destroyed, but only the face of the stone facade no longer has the beautiful effect of the previous resin and as if the stone facade was not resined and polished from the beginning.

A very disturbing appearance, of course, this is also a style of stone facade, and in addition, it is possible to restore it by spending money, and as a result, in recent years, the use of travertine as a stone facade has been very well received. 

Due to its great variety in color and design, travertine has been able to take a special place as a facing stone, and considering the cost, it can be said that travertine is the best choice for facing stone. Unfortunately, travertine stone is notorious among the common people as a type of fragile stone; But in the historical buildings of Rome, which are more than 300 years old and travertine stone has been used as facade, this wrong hypothesis has been completely rejected. Travertine has a porous surface by nature, and this porosity together with limestone has a good relationship with cement and makes it stick more firmly to the wall during installation and thus has a good durability as a facade stone. White, light cream, and chocolate travertine colors are more popular in the world market.

  It should be noted that the stone facade installation method is different from the rest. Due to the difference in ambient temperature during the day and night and the changes of the seasons and as a result of the expansion and contraction of the stone, if the cement mortar method is used to install the facade stone, there is a possibility that the stone will be removed from the facade; Therefore, it is necessary to fix the facade stone by scooping the back of the stone on the facade, otherwise, after installing the facade stone, to prevent falling, one by one, they should be screwed to the facade, which does not look good for the facade of the building, so it is better to spend which is supposed to be used for this work after installing the stone facade, before installing it should be used for scoping it. 

The strength in the installation of facade stone becomes more important, especially when they are supposed to be placed at a height; Therefore, the face stone should have a suitable gap so that the mortar and grout that is poured behind the stone adheres firmly to the face stone and gives double strength to this composition; Because according to the mentioned factors, the possibility of stone separation from the facade wall is high and the risk of stone falling threatens passers-by and cars; Therefore, travertine stone is one of the best choices for facing stone, keeping this feature in mind, and the main weakness of marble stone for use as facing stone is the same. 

Masonry is another suitable option for facade. The selection of stone for the facade is completely technical and correct. Masonry is between granite and travertine in terms of water absorption and grout adhesion, and it is neither as absorbent as travertine nor as non-sticky as granite. 

In Iran, especially in tropical regions, the use of this stone for facades has been common since ancient times, because its light color does not absorb much heat from the facade of the building, and the building is protected from excessive heat. The resistance of this stone against surface and deep weathering is relatively good, and in some cases, masonry can compete with travertine and granite stones in this regard.