Conference, Proceedings

dessicators are decreased by the perforated plates, the specimens were considered as the infinite
plates and the sorption process could be solved as 1D problem with use of solution (7).
In figures 3 – 4 there is an example of the adsorption and desorption time courses and their
logarithms for chosen lime‐cement plaster after 35 95 % and 35 95 % relative humidity
changes. All analysed time courses of sorption could be expressed as the exponential functions.
Water vapour adsorption
[kg]
Figure 3: Exponential adsorption time course – lime‐cement plaster, 32 95 % relative humidity change
Water vap our desorp tio n (k g)
0,004
0,0035
0,003
0,0025
0,002
0,0015
0,001
0,0005
0,006
0,005
0,004
0,003
0,002
0,001
0
0,0E+00 5,0E+06 1,0E+07 1,5E+07
Time [s]
0
0,0E+00 1,0E+06 2,0E+06 3,0E+06 4,0E+06 5,0E+06 6,0E+06
Time (s)
Water vapour desorption
logarithm
0,0E+00
0
-1
5,0E+06 1,0E+07 1,5E+07
-2
-3
-4
-5
-6
-7
y = -3E-07x - 1,0182
0,0E+00 1,0E+06 2,0E+06 3,0E+06 4,0E+06 5,0E+06 6,0E+06
0
-0,5
-1
-1,5
-2
-2,5
-3
-3,5
-4
-4,5
-5
Time (s)
y = -7,65E-07x - 1,96E-01
R 2 = 9,90E-01
Figure 4: Exponential desorption time course – lime‐cement plaster, 95 32 % relative humidity change
From the logarithms of the sorption courses the exponents, expressing the speed of sorption
process, were determined. The inverse values of the exponents represent time constants of the
process. The sorption time constant is defined as the value reaching the 63 % of the ultimate
moisture change. In figure 5 there is a distribution of the occurrence of the time constant values
for all analysed materials. The most frequent values lie in interval 0 ‐ 30 hours that means the
sorption experiments lasted approximately 1 – 3 months. The great values, more than 50 hours,
were observed for the gypsum and wooden materials. In a case of gypsum materials the time
constants were high during desorption. This phenomenon can be explained by the hydration
changes, typical for these materials. The woods have a large time constants in general, which is
given by their moisture capacities.
Water vapour adsorption
logarithm
Time [s]
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