Page 23 - ICJ Jan 2026

TECHNICAL PAPER

0.9 1200
f = 0.0088p = 16.97(100 − p)
0.8 R² = 0.9662 1000 R² = 0.95
Fraction of enclosed pores (f ) 0.6 Bulk density kg/m 3 800
0.7

0.5
600
0.4
0.3
400
0.2
0.1 200
0 0
0 20 40 60 80 100 0 10 20 30 40 50 60 70
Porosity (%) 100 − porosity [100 − p(%)]
Figure 23: Estimated fraction of enclosed pores for aerated autoclaved Figure 24: Measured bulk density against (1 − p) (%) for AAC
concrete
Average conductivity of solid, of all samples given in Table 16, is Corresponding V p (relative) is 1.621 for w/c = 1.3. For mass basis
1.11 W/m.K, with standard deviation of 0.19, which corresponds proportion of 1 sand to 1 cement, relative volume of paste V p
to CV of 17.2 %. However, there are two clusters of AAC blocks, is given as 0.811. The relative volume of sand is 1 − 0.811 =
one with higher sand content in terms of mass proportion and 0.189. for other sand to cement ratio the relative volumes can be
another with lower sand content in terms of mass proportion. obtained similarly. The average value of 1.11 W/m.K is assumed
Their respective conductivities of solid varies within ± 10 % of as conductivity of solid k s for validation of model using reported
the average of two clusters. Hence, for approximate estimation data. The measured maximum permeable porosity of AAC in
one single k s is assumed for complete range of AAC material, the study referred is above 86 %, however remaining 14 % or so,
ignoring ± 10 % variation. To predict thermal conductivity is apparently solid and still seems to have significant proportion
with the help of the model presented earlier, one needs three of impermeable pores. Apparent bulk density is related linearly
inputs: porosity, conductivity of solid, and fraction of enclosed with porosity with a negative slope. A plot of bulk density
pores. To validate the model by prediction from data reported against apparent solid fraction in percent for all 8 samples,
by others, availability of these three parameters in the data set including 0 bulk density at 100 % porosity is given in Figure 24.
is a necessity. Generally, for AAC and FC, bulk density values
and their empirical correlations with thermal conductivity are Thus, apparent specific gravity of this type of material is about
3
reported in literature. Permeable porosity and bulk density ρ 1700 kg/m , for this specific case it can be taken as 1697 kg/
3
are related thorough apparent specific gravity of solid (G) as ρ m . Reliable data on thermal conductivity of AAC are available
[50]
= G (1 − p). Average apparent specific gravity of fully hydrated with a moisture content of 3 % and not on dry state. The dry
cement paste including gel pores as a part of impermeable and saturated conductivity can be calculated as it is done for
solid component is approximately 1.8, considering inherent concrete. Thermal conductivity of soil cement system and bricks
[7]
28 % gel porosity . While the specific gravity of sand commonly etc., can be calculated in the same manner and reported in
[36,51]
varies from 2.6-2.7. Complete hydration of OPC is likely with literature .
w/c of 1.3 after comprehensive curing through autoclaving.
5.5 Moisture content and thermal conductivity
Table 16: Porosity, solid conductivity and fraction of Thermal conductivity of these materials, strongly dependent on
enclosed pores for AAC moisture content, was recognized earlier and factors (F) relating
SERIAL POROSITY CONDUCTIVITY OF FRACTION OF dry thermal conductivity to conductivity of moist porous building
NUMBER (%) SOLID k s (W/m.K) ENCLOSED PORES (f) materials as a function of volumetric moisture content were
AAC1 38.5 0.94 0.49 proposed by Jakob ; k(moist) = F × k(dry). These factors are not
[31]
AAC2 57.3 1.29 0.62 valid for all materials universally and errors may be of the order
AAC3 58.7 1.49 0.56 of 100 % or more [36,39] . However, the factors capture the general
AAC4 75.8 0.99 0.42 behavior that the rate of increase of thermal conductivity is
AAC5 81.7 1.08 0.73 higher in the lower range of moisture content and progressively
AAC6 82.3 1.13 0.80 reduces near saturation.
AAC7 86.0 0.95 0.67
Water conducts heat nearly 25 times more than that of air, hence
AAC8 86.3 1.01 0.79
when water partially replaces the air in the pores, effective

28 THE INDIAN CONCRETE JOURNAL | JANUARY 2026