Effective factors for choosing building stone (technical characteristics of stones)

Technical characteristics of stones

The technical characteristics of stones, which are obtained by chemical, physical and mechanical tests, are very different due to the variety of texture, construction and formation conditions of stones. By chemical analysis of rocks, their chemical composition can be obtained to some extent and it can be used as a supplement along with microscopic mineralogical studies. In addition, the presence of harmful compounds such as pyrite, mica pieces, talc, etc. can be determined. For stones that are used in the vicinity of salt water, chemical tests must be done.

Physical and mechanical tests have been widely used to classify rocks, including specific gravity, water absorption, porosity, compressive strength, flexural strength, tensile strength, abrasion resistance, hardness, impact resistance, thermal properties, frost resistance, and composition. The chemistry of rocks is the most important.

A – Specific weight

Sometimes the specific weight of the stone can be important. For the construction of anchorages and coastal walls, it is important to have high-density walls; Because the stone is immersed in water and loses an important part of its effective weight. On the other hand, it is necessary to use light stones in the construction of the arch or dome.

In a classification, stones are divided into two groups based on specific weight: light with a specific weight of less than 1.8 tons per cubic meter and heavy with a specific weight of more than 1.8 tons per cubic meter. Therefore, the use of light stones for building walls in areas close to mines and for heavy stones for various engineering applications such as foundations, floors, retaining walls, facades, etc. are mentioned.

In another classification, stones are divided based on specific weight as follows:

1. Very light stones (floating in water) with apparent specific gravity less than 1
2. Light stones with an apparent specific gravity of 1 to 1.5 tons per cubic meter
3. Medium stones with apparent specific gravity of 1.5 to 2.5 tons per cubic meter
4. Heavy stones with apparent specific gravity of 2.5 to 3 tons per cubic meter
5. Very heavy stones with an apparent specific gravity of more than 3 tons per cubic meter

The specific weight is important in the polishability of the stone, so that the stones with a specific weight less than 1.5 do not accept a suitable polish and a specific weight above 2.2 are well polished. The specific gravity of building stones can be measured according to the ASTM C:97 standard.

B – The amount of water absorption

The amount of water absorption is the amount of water that the stone absorbs in percentage of its dry weight. The amount of absorption in stones depends on many factors, including their type, density, presence of joints, etc. Is different. All stones are hygroscopic to some extent and always contain some water; But in general, the amount of water absorption in igneous rocks is very low, but sedimentary rocks have a higher ability to absorb moisture. Limestones and sandstones show different values. In general, the amount of water absorption should be low for building stones. ASTM C: 97 standard can be used to measure rock water absorption.

The moisture of the stones reduces their strength; Because as a result of mineral dissolution, the surface through which water passes (levels of microcracks and joints) increases and thus leads to a decrease in rock strength. If the water contains acid, it increases the solubility of the surfaces and chemical disintegration occurs, and the water containing carbon dioxide is also effective in the intensity of the dissolution of carbonates, and some gas will be produced as a result of the reaction, and the increase of the produced gases will lead to their expansion and the disintegration of the rocks. Hijazi, 1374).

The ratio of the compressive strength in the state saturated with water to the compressive strength in the state without water is called the softening coefficient. In the case of materials that do not change in their compressive strength in the above-mentioned two cases, the value of this coefficient will be equal to one. Materials with a softening factor of more than 0.8 are called water-resistant, and materials with a softening factor of less than 0.8 are called water-resistant. If a stone has a softening factor between 0.6 and 1, it is used to build a building (Komar, 1987).

C – Porosity

Porosity is a sign of the release of trapped gases in the forming rock, or the decomposition of some minerals during and after formation, or the lack of compaction of sediments during formation, etc. The fine and capillary cracks and crevices created due to rapid cooling or the influence of tectonic factors are suitable spaces for the accumulation of water and other environmental pollutant solutions, and as a result, it makes it easier to destroy the rock due to freezing and chemical erosion (Hejazi, 2004). ; Therefore, the ability to absorb water from rock with low porosity is low; But a rock with high porosity may absorb and retain large amounts of water. Of course, the remaining water in the stone also depends on the size and shape of the pores. Small pores absorb more water due to their capillarity, and on the contrary, if they are large, water comes out more easily. In some cases, despite the high porosity, the amount of water absorption is low, and there is not necessarily a direct relationship between the two.

The ratio of the volume of void spaces to the total volume of the rock is called porosity, which is usually expressed as a percentage. Rocks are divided into the following groups based on the percentage of apparent porosity:

• – Rocks with very high porosity (above 20%)
• – Rocks with high porosity (10 to 20 percent)
• – Rocks with relatively high porosity (5 to 10%)
• – Rocks with medium porosity (2.5 to 5%)
• – Rocks with low porosity (1 to 2.5 percent)
• – Very dense rocks (porosity less than one percent)

Stones with high porosity are suitable for use in panels and thin walls; Because in addition to being light in terms of heat and sound, they are more suitable and better than the types with low porosity. However, their use for preparing concrete is not allowed due to the presence of holes in concrete (especially in environments with water influence) (Hijazi, 2014).