Introduction
Aereated concrete is a versatile building material, widely used in the construction industry due to its light weight and good insulating properties. Its structure consists of fine air pores, generated by chemical processes during production. Aereated concrete is often used in the form of blocks, plates or elements. Thanks to its thermal insulation, Aereated concrete is particularly well suited for energy-efficient buildings. It is also easy to process, making it a popular material in the building industry.
Knowledge of the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity of aereated concrete is crucial to evaluating its insulation properties for energy-efficient buildings and achieving minimized heating and cooling . This enables building designers to select suitable materials for meeting legal requirements for energy efficiency and improving living comfort.
Laser or Light Flash Analysis (LFA) is a recognized method for determining Thermal DiffusivityThermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.thermal diffusivity; in turn this, along with DensityThe mass density is defined as the ratio between mass and volume. density and Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity, allows for calculation of the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity. Actually, ideal samples for an LFA measurement consist of solid, non-porous materials. By selecting the appropriate analysis model (here, the Penetration model), partially porous materials such as aereated concrete can also be characterized.
The advantage of LFA over the frequently used platetype devices (heat flow meter and guarded hot plate) is the small sample size. Even small quantities, which are often used in research and development, can be examined without any difficulty.
Experimental
An LFA sample (ø 12.7 mm; thickness: 4 mm) was tested at 25°C, 50°C and 75°C in the LFA 717 HyperFlash®. The DensityThe mass density is defined as the ratio between mass and volume. density was determined via mass and volume at room temperature and the Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity (Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.cp) by means of the DSC method.
Results and Discussion
Figure 1 shows the thermophysical properties of aereated concrete between 25°C and 75°C. The Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity shows a slight increase with temperature. This is typical behavior for porous materials, as radiative heat transfer increases at higher temperatures.
The LFA signals were evaluated in the Proteus® software using the Penetration model.This model assumes that energy penetrates the sample through the pores. This is particularly evident at the beginning of the signal; see figure 2. The Penetration model is a better fit to this increase than the Standard model, which assumes that energy is only absorbed at the surface of the sample.
Summary
The measurements with the LFA 717 HyperFlash® demonstrate that it is also possible to characterize the thermophysical properties of samples with a porous surface when applying the appropriate model. This is beneficial for the development of new thermal insulating materials such as aereated concrete and helps increase the efficiency of thermal insulations.