The value of the coefficient of thermal conductivity of expanded clay

Materials with isolated voids in the structure protect the surface well from cold. The thermal conductivity of expanded clay depends on the grain size and density. The insulation weighs a little, insulates from sounds, but is hygroscopic. The material requires additional insulation from moisture in order to effectively protect the building from heat loss.

Description of thermal conductivity

The ability of an insulation to transfer energy from heated layers to parts with a lower temperature is called thermal conductivity. The process is provided by the chaotic movement of molecular particles, its intensity depends on moisture content, compaction, pore size.

The physical process of heat conduction is accelerated when there is a large temperature difference outside and inside the building. Spontaneous energy transfer always flows from a hotter environment towards a colder environment and occurs before thermodynamic equilibrium appears.

Coefficient of thermal conductivity

To numerically express the ability of a material to transfer energy, there is a coefficient of thermal conductivity. The indicator indicates the amount of heat flowing through the material sample under specified conditions. The test standard always has the same dimensions in length, width and area and is checked at a standard temperature difference (1 K). The heat transfer coefficient is measured in W / m · K, which corresponds to the International System of Units.

The name of the coefficient of thermal resistance is used in the construction field. The thermal conductivity of expanded clay is 0.1 - 0.18 W / m · K. High-quality material is characterized by a numerical indicator of 0.12 - 0.17 W / m · K, insulation with such properties retains up to 80% of internal heat.

Factors affecting the value of thermal conductivity

Expanded clay is used in construction as a porous bulk insulation or as a filler in the production of lightweight concrete. Granules are obtained by firing shale or clays and have an oval, round shape, sometimes with sharp corners. The building material is produced in the form of sand.

The bulk density of expanded clay is in the range of 150 - 800 kg / m3, the bulk density depends on the technological regime upon receipt. The ability to conduct heat depends on the size of the granules, the porosity of the material and its moisture content.

Expanded clay fraction

When comparing the characteristics, it is concluded that the thermal conductivity decreases with an increase in the size of the granules. Medium to coarse gravel is best used for insulating unloaded roofs and wooden floors. Fine-grained expanded clay is used for lightweight floor screed.

Expanded clay fractions are set in accordance with the standards of GOST 9757 - 90:

1. A small group is defined from 5 to 10 millimeters. The material is used for the production of wall blocks from expanded clay concrete. Small granular filler is used in the concrete screed of covering or floors, since large parts increase the layer thickness.
2. From 10 to 20 mm - middle fraction. The material in the bulk material well insulates floors, attic floors from the cold, is used for insulating lawn areas and draining the earth.The fraction is rarely used in screeds and concrete floors, it is added to the mortar if the thickness of the layer does not matter.
3. From 20 to 40 mm - large granules. They insulate heating mains, basements, floors of utility rooms, and insulate the building from noise.

Interlayers of bulk insulation effectively protect from the cold if 2-3 fractions are used at the same time. In this way, voids are filled, rigidity increases, and convection of currents is prevented.

Porosity

The raw materials are placed in drums, where they rotate and at the same time are heated to high temperatures. Under such conditions, the material swells, and porous granules are obtained, which are protected from the outside by a caked clay crust. Most of the voids are closed, the partitions between them also contain voids.

The pore size is regulated by the introduction of citrogypsum and mineral impurities into the charge during production. An additive in an amount of 1 to 3% forms closed voids up to 1 mm in size. An increase in the volume of the additive to 4–9% leads to an expansion of the pores to 1.5–2 mm, while the number of closed cavities increases. The number of insulated voids increases thermal insulation properties and reduces water absorption.

Humidity

Water absorption of expanded clay ranges from 8 to 20%. When moisture gets inside the material, the surfaces of the granules are moistened, which slowly absorb the liquid. Gradually, water enters the spheres through microscopic cracks and is retained inside. Expanded clay accumulates moisture and gives it away with difficulty. The mass increases, the characteristics of the thermal conductivity of expanded clay change, and the strength decreases.

Dry expanded clay can withstand up to 25 series of freezing and thawing, wet one is destroyed by the expansion of water at negative temperatures. Expanded clay is protected from moisture and vapor barrier films.

Types of expanded clay depending on the size of the granules

Bulk insulation is classified by granule size and shape.

There are varieties of expanded clay:

• gravel;
• crushed stone;
• sand.

Coarse-grained material adds height to the room, usually the thermal insulation effect is achieved with a backfill thickness of 20 to 30 cm.To reduce the layer size, expanded clay can be combined with mineral wool, polystyrene foam, and expanded polystyrene.

The material can be compared by grades for strength. There are 13 varieties of gravel and 11 samples of expanded clay crushed stone. The tensile strength of one grade is different, for example, P100 crushed stone collapses at 1.2–1.6 MPa, and gravel of a similar grade deforms at 2–2.5 MPa.

Gravel

The material consists of rounded particles with a crust of molten clay, which contain voids inside. There are gravel fractions: 5–10, 10–20 and 20–40 mm. Depending on the density, 10 grades of insulation from M150 to M800 are presented in bulk. Gravel of M900 and M1000 grades is produced by special order.

Gravel concretes filled with medium and fine granules are lightweight, do not load structures and show improved thermal insulation properties. Wall blocks made of expanded clay concrete are used in low-rise buildings, they protect the building from cold air, have good air permeability and belong to environmentally friendly categories.

Crushed stone

Expanded clay of this type contains separate elements of irregular angular shape with sharp edges and edges. The size of the fractions is determined similarly to gravel. Due to its shape, the material has a low bulk density and is used to insulate attics and basements. Foundations and bases are insulated with expanded clay from freezing. In the ground, waterproofing is arranged with foil-clad material, polyethylene, roofing material, and protection from household and atmospheric vapors is mounted on top.

The coefficient of thermal conductivity of expanded clay depends on the size of the crushed stone, but with an increase in size, the thickness of the required layer increases. A cement-sand screed (at least 4 cm) is made on top of the bedding to increase strength.

Sand

This category includes expanded clay, containing fine particles up to 5 mm. The material is obtained by firing residues from the production of crushed stone or gravel, or by crushing large pieces. Sand is used for indoor insulation along with large species or is used in floor screed.

Bulk thermal insulation is more effective than fine granules in a cement-sand mixture. The moisture from the solution is absorbed by the granules, and they lose their protective properties. A comparative analysis of wall blocks made of expanded clay sand and gravel shows that the former conduct heat faster, but are distinguished by increased strength.

Production processes affecting the thermal conductivity of expanded clay

The expanded clay production technology provides for processes to increase porosity and obtain isolated closed circuits of different sizes. The raw material is quarry clay, which is mined in open pit mines. Swelling tests are carried out in the laboratory prior to use to determine suitability for manufacture.

The equipment includes:

• loosening machines;
• granulators;
• drying drums;
• rotating crucibles for firing;
• cooling tanks with air supply;
• transporters.

In production, dry or wet raw materials of various grinding are used. At a temperature of +1000 - + 1300 ° C, the mass swells and the surface of the particles acquires tightness due to sintering.