Features and benefits of shale mineral stone

Features and benefits of shale mineral stone

Product diversity is the result of different raw materials used by factories. In Iran, bricks have long been produced with clay. But suitable clay is not found everywhere, and the clay soils used usually contain large amounts of lime. That’s why the resulting bricks are not in accordance with the national and international standards and do not have the appropriate quality in the facade. Gradually, in the brick industry, other minerals, such as shales, have replaced clay. The characteristics of shale and shale bricks are as follow:

Shale and its constituent compounds:

Shale is a type of sedimentary rock that consists mainly of fine grains. The shale has parallel plates and layers that are easily laminated. The thickness of each layer is less than one centimeter. Shale is mainly a combination of clay minerals and quartz grains, and clay is their most important component. It is usually gray, but the amount of shale constituents determines its color. Black shale, for example, contains more carbon compounds. Being red is due to the presence of iron oxide (hematite). Iron hydroxides, including goethite and limonite in shale, produces brown and yellow shade, respectively. Chlorite, biotite, and illite minerals in shale create green shade.

The differences between shales and mineral rocks

The differences between shales and other mineral rocks are in its size and the way it is formed. Shale is a sedimentary rock with different layers. These layers can be separated easily. Shales also come in a wide range of colors.

Some features of shale bricks

Studies show that shale bricks have high physical and chemical resistance, durability and stability, thermal resistance, and moisture repulsion, some of which are described below.

Color: natural color is one of the most important advantages and features of shale bricks. Therefore, the color of these bricks is stable and does not change over time due to temperature, rain or sunlight. Shales are initially gray, or black. But after being heated in the kiln, they gradually turn to light orange and eventually black.

 No efflorescence:

 Shale bricks have low water absorption due to differences in their chemical composition compared to traditional bricks (clay bricks). They do not retain moisture and expel it quickly. So the possibility of efflorescence in these bricks is less.

Physical and chemical resistance:

 Shale bricks are completely resistant to acids, bases, moisture, heat, extreme cold and freezing. Their colors don’t change. They are also resistant to gases and pollutants in the air.

This study aims to compare some characteristics of clay and shale bricks. It also investigates the effect of the properties of the raw material used on the characteristics of these two types of bricks.

Materials and methods:

In order to study the chemical properties of the raw material, the percentage of SiO2, Al2O3, Fe2O3, CaO, K2O, SO3 were measured in clay and shale soils using the XRF method. In order to investigate the physical properties of the raw material used to produce bricks, the distribution of shale and clay particle size (after milling) in the diameter range (1.2 to 0.001 mm) was determined by sieve and hydrometer. The distribution of soil particle size (components smaller than 1.2 mm) was performed by the hydrometric method as follows: First, organic materials were removed by oxygenated water. The samples were then treated with 5% Calgon and an electric mixer and poured into standard cylinders. The result were read at 0.67, 1, 2, 4, 8, 15, 30, 60, 120, 240, 480, 720 and 1440 minutes. The readings were then corrected according to the temperature and Calgon. To reduce the error, the hydrometer reading was repeated three times at 0.67, 1, and 2 minutes, and the average readings were used. The diameter of the particles, when the hydrometer was read, was calculated by the following equation.

In the above relation, d is the diameter of the particle in millimeters, L is the effective depth of the hydrometer in centimeters, v is the viscosity in centipoises, the RD is the specific gravity of the soil in grams per cubic centimeter, the density of water and t is the settling time in minutes. Then, 30 samples of clay bricks and 30 samples of shale bricks were selected and some of their features including water absorption percentage, compressive strength, percentage of soluble materials, tolerance of dimensions and frost cycle tolerance were determined for each sample based on the methods of Iranian Institute of Standards and Industrial Research (2008).

Quality Index shale brick Clay brick
Water absorption % 16 23
pushing resistance(kg/cm2 ( 350 230
Solution Materials %
4/0 6/0
Scale tolerance (mm)
9/0 1/1
Signs of frostbite
Over 50
Less than 50

Table 1- Comparison of the average percentage of water absorption, compressive strength, soluble materials, and frost cycle tolerance in clay and  shale bricks

Table (2) The soil from shale rocks is more than clay.

Chemical analysis XRF 
Clay soils %
mine and shale rocks
AL2O3 SiO2  Fe2O3 60 80
CaO K2O SO3 Over 9% Less than 9%
LOI 15 9

Discussion and Conclusion

The results showed that shale bricks had a significantly lower water absorption percentage and higher compressive strength than clay bricks. Also, shale bricks can withstand more than 50 frost cycles, while clay bricks are not able to withstand this cycle.

Table 2 - The amount of silica, aluminum, and low LOI (Table 2) are the most important reasons for the superiority of the quality of shale bricks over clay bricks

Comparison of the size distribution of clay and shale particles showed that particles with a diameter (1.2 to 0.063) mm (sand and silt particles) had the highest frequency in shale and particles with a diameter smaller than 0.005 mm (clay particles) had the highest frequency in agricultural clay. Figures 1 and 2 show the size distribution of clay and shale particles, respectively.