shelters, and meet basic needs. In stone structures, building stone has been utilized alongside lime and gypsum mortars. It must be acknowledged that although stone has partially yielded its place to materials like bricks due to its weight and the challenges of transportation and extraction, it still retains its significance in heavy and robust structures such as bridges, tunnels, avalanche sheds, retaining walls, riprap, and foundation courses. Examining the general properties of stones involves aspects such as tensile strength, compressive bearing capacity, permeability, wave propagation, stability, and considering the direction of bedding planes and weak surfaces during operations; building stones typically possess planes where the adhesive force and grain bonding are lower or minimal compared to other layers. Overall, the resistance of a building stone should be free of fissures, veins, cracks, excessive pores, decay, and weak layers, and it should ideally feature a uniform color. In general, natural stone possessing sufficient weather resistance and an appealing color can be used in construction operations as rubble stone, ashlar stone, load-bearing stone, veneer/slab stone, capstone, column stone, and even in the form of stone utensils and objects.

Read More: Building Stone | Flooring Stone

Classification of Building Stones

Building stones come in various types, determined based on a series of parameters. According to their specific gravity, building stones are categorized into two types as outlined below:

- Heavy-weight stones: Building stones with a specific gravity greater than 1.8 grams per cubic centimeter.

- Light-weight stones: A category of building stones whose specific gravity is less than 1.8 grams per cubic centimeter.

Based on the Compressive Strength of Building Stone

- Heavy stones: Those with a compressive strength ranging from 100 to 1000 megapascals.

- Light stones: Those with a compressive strength ranging from 4 to 200 megapascals.

Based on the Softening Coefficient of Building Stone

- If the value of this coefficient ranges from 0.6 to 1, it is used for building construction.

Use of Stones for Foundations and Walls

In building foundations and portions of walls located below ground level, igneous and sedimentary stones with a softening coefficient greater than 0.7 are utilized. The absence of clay minerals, heterogeneous pieces, fissures, and cracks is essential in such stones.

Stone Walls for Building Facades

Igneous, sedimentary, or metamorphic stones possessing the following characteristics are used in stone walls:

- Compressive strength between 0.4 and 50 megapascals.

- Specific gravity between 0.9 and 2.2.

- Softening coefficient between 0.6 and 0.7, and the stone must not be weathered.

Facade Stone and Flooring Stone

Igneous, metamorphic, and sedimentary stones possessing these traits are employed as building facade stones. Characteristics of facade building stone include the following:

- The compressive strength of the facade stone must exceed 5 megapascals.

- Softening coefficient between 0.7 and 0.9.

- Absence of fissures and cracks in the facade stone.

- Absence of clay minerals and soluble substances in the building facade stone.

- Absence of alteration and weathering states.

- Possessing desirable and high-quality color.

Stones Used as Flooring

Various types of stones can be utilized for flooring. Let's take a look at the most popular options:

Granite

Onyx (مرمر)

Travertine

Read More: Characteristics of Building Stone | Facade Stone

Stones Usable in Road Construction

Stones have multiple applications in road construction, among which are bridge building, sub-base preparation, paving, rail ballast, and more. Various igneous, metamorphic, and sedimentary stones possessing the following characteristics will be usable in road construction. Characteristics of building stones in road construction:

- Compressive strength exceeding 100 megapascals

- Softening coefficient exceeding 0.9

- Water absorption rate of less than 1 percent

- Specific gravity greater than 2.3 grams per cubic centimeter

- Absence of clay minerals, gypsum, and water-soluble substances

- Absence of alteration and weathering in the stone

Stones Resistant to Solutions and Heat

Stones resistant to acids include granite, diabase, diorite, quartzite, and basalt.

Stones resistant to alkaline solutions include limestone, dolomite, marble (مرمریت), and magnesite.

Stones resistant to high-temperature conditions include basalt, diabase, and tuff. An important feature of these stones is possessing a compressive strength exceeding 100 megapascals with a maximum water absorption rate reaching 1 percent.

Degradation in Building Stone

The primary cause of degradation in building stone is the effect of soluble salts upon them. Environmental pollution, frost, decay in metal components, the presence of weak veins, and poor execution practices also cause the destruction of stones.

Effect of Soluble Salts on Building Stone

If moisture carrying soluble salts evaporates from the stone's surface, some salt remains on its surface as efflorescence and as a layer within the pores of the stone. The continuation of the moisture-evaporation cycle causes an increase in crystal volume and the scaling/peeling of the stone surface. Therefore, more porous stones are more sensitive to soluble salts.

Environmental Pollution and Its Effect on Building Stone

Stones containing calcium carbonate minerals are particularly sensitive to acidic environments. Sulfur dioxide in a humid environment and oxygen in the air produce sulfuric acid, which affects limestones and yields calcium sulfate. Limestones and calcareous sandstones are more sensitive in this regard.

Regarding limestones, the calcium sulfate formed on the surface is washed away by water. However, on non-washable surfaces, the surface becomes blackened by soot and transforms into hard crusts and protrusions surrounded by powdery lime. In magnesian limestone types, the creation of magnesium sulfate accelerates the degradation process.

In sandstones, pores are filled by gypsum (calcium sulfate); the hard crusts created often collapse due to differences in thermal expansion. Although siliceous sandstones are not directly damaged by the attack of atmospheric acids, the gypsum produced by limestone causes damage to them, which arises due to evaporation resulting from crystallization on their surface.

Onyx, which is fundamentally calcium carbonate, is attacked by acids present in the air, and its polished surface becomes rough over time. However, due to its dense and compact texture, it is less affected by the crystallization process.

Effect of Frost

Destruction due to frost is more frequently observed in building parts such as capstones, awnings, foundation courses, and window sills. Generally, limestone and dolomite are more exposed to the attack of frost effects than sandstone. Onyx, slate, and granite are not affected by frost due to their low porosity.

Decay of Metals

Rainwater that flows from copper surfaces and its alloys onto limestone surfaces creates green stains. Rust resulting from iron and steel materials is very hard and difficult to clean from the porous surfaces of stones. The most severe damage stems from the expansion of rusting iron and steel components inside the facade stonework. For this reason, all metal components used in facade stone installation must be rust-resistant.

Fire and Building Stone

Fire rarely causes total destruction in stonework. However, the facade surfaces of granite, onyx, and sandstones may become blackened or cracked due to fire. Limestones are generally unaffected by fire, except that light-colored stones permanently turn pink due to the oxidation of the iron present within them.

Stonework Construction Operations

To construct a stone building, a series of requirements must be observed during execution. These principles, which guarantee the construction of a building with better quality, are as follows:

- The characteristics of the building stone must comply with the provisions stated in the previous section.

- During masonry work, the stone must first be moistened and then used.

- Stones must be laid as headers and stretchers in such a way that the pieces interlock well with each other, and the joints between stones must not align vertically over one another.

- Mortar between stones must be placed in a way that prevents the edges of the upper and lower stones from contacting each other.

- The stone pieces laid in the first course must be larger than the upper courses.

- The stone must be placed along its natural bedding orientation and initial direction. Thus, the direction of forces applied to each piece of building stone must be perpendicular to its natural vein or bedding. This issue is of special importance regarding layered stones.

- The stone piece must be positioned at the desired location in such a way that it does not move after contacting the mortar.

- Stonework masonry operations are not permitted in weather below 5 degrees Celsius, and following construction operations, they must be appropriately protected against impacts and weather factors.

- Depending on the type and strength of the stone, climate conditions, and design materials, proper joint pointing must be performed on the stone walls.

- For installing veneer/slab stones, appropriate metal hooks made of stainless steel or other suitable metals such as bronze must definitely be provided for better bonding of the stone to the mortar or for dry installation methods.