Expanded clay thermal conductivity and its dependence on various factors
The thermal insulation properties of expanded clay are well known and are largely determined by the raw materials from which it is made. The specific thermal conductivity of expanded clay is one of its main characteristics, which, together with its low specific gravity and strength, determines the widespread use of this material in construction.
What affects the thermal conductivity of expanded clay
For materials that perform protective functions, thermal conductivity is a particularly important characteristic. For expanded clay, as a natural material, it depends on a combination of its various qualities.
Firstly, the thermal conductivity of expanded clay depends on its fraction (granule size): the larger the granules, the more insulation will be needed. Thermal conductivity is influenced, for example, by characteristics such as moisture and porosity of expanded clay. It is not easy to determine the average coefficient of thermal conductivity of expanded clay due to many deviations. In the reference literature, the value you can find data that it ranges from 0.07-0.16 W / m.
Expanded clay with minimal thermal conductivity should be chosen. The higher the coefficient of thermal conductivity, the greater the amount of heat passes through the insulator layer for a certain time and the lower its thermal protection, respectively. Thus, the greater the porosity of expanded clay, the lower its density, as well as thermal conductivity.
Expanded clay is hygroscopic: with increasing humidity, it increases its thermal conductivity and loses its insulation properties, and with increasing weight, the load on the floors also increases. High-quality waterproofing of expanded clay is necessary to maintain properties that ensure the preservation of heat in your home.
So, expanded clay has thermal conductivity, which depends on its fraction: with decreasing size of expanded clay grain, its voidness decreases, bulk density increases and thermal conductivity increases.
Expanded clay granules are divided into expanded clay gravel, crushed stone and sand.
Expanded clay crushed stone
Obtained from expanded expanded clay mass by crushing.
Expanded clay gravel
Round or oval particles obtained in a rotary kiln by swelling of light clay. It has a strong dense surface, therefore it is often used as a concrete filler. It has the lowest coefficient of thermal conductivity. For example, expanded clay gravel of 10-20 mm grade in bulk density M350 and grade P125 in strength (3.1 MPa) has a thermal conductivity of 0.14 W / (m ° C).
Expanded clay sand
It has a fraction of up to 5 mm and is most often used for insulation.
Production processes affecting the thermal conductivity of expanded clay
According to research results, the thermal conductivity of expanded clay depends on the presence of quartz in it at a certain stage of production and, to a lesser extent, on the density and porosity of the material. The conclusion suggests itself that the quality of expanded clay is influenced by the method of its production, since vitreous quartz appears precisely during the production process.
Note that single-crystal quartz itself has a high thermal conductivity (6.9-12.2 W / m), which entirely depends on the characteristics of the raw material. From clay with good expansion, quartz is obtained in the glass formation phase, the thermal conductivity of which is higher than that of quartz from clay with worse expansion. A similar dependence also extends to the properties of expanded clay.
Manufacturing technology is also important. Silica contained in expanded clay promotes an increase in thermal conductivity, while other oxides, on the contrary, lower it.This does not apply to gases that are formed when the clay mass is heated to a swelling temperature. It was found that when the content in pores is from 55% H2 + CO, the thermal conductivity of expanded clay is twice as high as when filled with air.
The micropore size also affects thermal conductivity: the smaller the pores, the lower the thermal conductivity. Moreover, the porosity itself does not significantly affect this characteristic.
The characteristics listed above mainly depend on the production method. The usual production method, as a rule, does not allow to significantly change the quality of expanded clay. However, modern production methods (plastic method or "joint firing") can significantly increase the thermal insulation properties of expanded clay.
In the overall comparison of the characteristics of expanded clay and foam, claydite is preferred, although the thermal conductivity of the foam is very low - 0.038-0.041 W / m.
