Conference, Proceedings

Relationship between relative permittivity and thermal
conductivity moisture dependence of calcium silicate boards
Matus Holubek 1 , Peter Mihalka 2 , Peter Matiasovsky 2
1 Slovak Technical University, Faculty of Civil Engineering, Radlinskeho 11, 813 68
Bratislava, Slovakia, e‐mail: holubek@svf.stuba.sk
2 Institute of Construction and Architecture, Slovak Academy of Sciences, Dubravska
cesta 9, 845 03 Bratislava, Slovakia, e‐mail: usarmat@savba.sk
Abstract: The relative permittivity of calcium silicate as a function of the moisture content was
determined using a standard TDR equipment. The obtained results were compared with available results
of thermal conductivity measurements for the same material. A similarity between both material
properties as the functions of their moisture content was found. The moisture dependences of both
analysed parameters are determined by the pore volume structure of the material. The parallel power
function models of effective thermal conductivity and relative permittivity based on the information on
moisture contents relevant to the transport and on the regularity of pores filled with water enabled the
analysis of the performed measurements.
Keywords: Relative permittivity, thermal conductivity, moisture content, calcium silicate, material pore
structure
1. Introduction
The moisture content of a porous building material can be determined from the measurement
its relative permittivity. The relative permittivity is the parameter analogous to the thermal
conductivity and dependent on the moisture content too. As both these parameters depend on
the portion and configuration of particular – solid – gaseous – liquid phase components, both of
them can be determined from the pore structure parameters of a given material. The Mercury
Intrusion Porosimetry (MIP) is the standard testing method for pore structure parameters
determination. The pore structure based models of thermal conductivity and relative
permittivity are based on expression of transport properties as the power functions of particular
transport relevant component volumes. The exponents of these functions are given by the
fractal dimensions of relevant component volumes, expressing their regularity. The measured
thermal conductivity and the relative permittivity values for calcium silicate boards were
analysed with use of the proposed pore structure parameters based models.
2 Pore structure analysis
The mercury intrusion porosimetry is based on the premise that a non‐wetting liquid will only
intrude capillaries under pressure. The relationship between the pressure and capillary radius is
described by Washburn as [1]:
− 2σ
cosθ
P =
r
where: P is the pressure, is the surface tension of the liquid, is the contact angle of the liquid,
r is the radius of the capillary.
(1)
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