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Osmotische Prozesse in mineralischen Baustoffen
Citation Link: https://doi.org/10.15480/882.9144
Publikationstyp
Doctoral Thesis
Date Issued
2024
Sprache
German
Author(s)
Schulenberg, David
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Examination Date
2023-11-17
Institute
TORE-DOI
Damage due to the formation of bubbles in coating systems on mineral building material substrates has
been known since the 1970s. Previous publications on this topic have mainly dealt with the causes of bubble
formation. It remains controversial whether osmotic processes are the cause of such damage. This thesis
investigates whether a mineral building material itself forms a semipermeable membrane and leads to the
build-up of osmotic pressures. Furthermore, it should be clarified where and under what conditions the
membrane is created.
Apparatuses (so-called osmometers) were developed for the investigations in which mortar disks are used
as a membrane between two pressure-tight chambers. One of the two chambers is filled with a concentrated
solution and the other with deionized water. Before the experiments start, the samples are sealed with
epoxy resin, except for defined areas for the mass transport. With the help of pressure, temperature and
conductivity sensors as well as subsequent XRF measurement, the changes in the two solutions are measured
over time. The cement mortars are characterized in terms of their properties as green mortar and hydrated
mortar. From the results of the osmometer experiments and the characteristic properties of the samples,
conclusions should be drawn about the formation and the functionality of a semipermeable membrane and
the conditions for the formation of osmotic pressures.
The investigations have shown that even with simple cement mortars (CEM I cement; CEN Normsand;
w/c = 0,45) and 2M NaCl or KBr solution, pressures up to 0,1 N/mm2 formed after only a few months. In
tests on cement mortars with higher w/c ratios and in a test with a commercial polymer silicate mortar,
these pressures were reached after a shorter time. After finishing the experiment, ion concentrations of
the elements from the concentrated solution increased in the chamber with pure solvent, while the ion
concentration in the chamber of the concentrated solution decreased. Since the ions Cl− and Br− are not
part of the sample, they can only derive from the concentrated solution. In addition to the diffusion of
water molecules in the direction of the concentrated solution, there is also a (significantly slower) transport
of dissolved ions in the opposite direction through the sample. This does not correspond to the classic
definition of a semipermeable membrane, which is only permeable for the solvent. Nevertheless, the results
confirm that the transport processes in the experiments are osmosis.
been known since the 1970s. Previous publications on this topic have mainly dealt with the causes of bubble
formation. It remains controversial whether osmotic processes are the cause of such damage. This thesis
investigates whether a mineral building material itself forms a semipermeable membrane and leads to the
build-up of osmotic pressures. Furthermore, it should be clarified where and under what conditions the
membrane is created.
Apparatuses (so-called osmometers) were developed for the investigations in which mortar disks are used
as a membrane between two pressure-tight chambers. One of the two chambers is filled with a concentrated
solution and the other with deionized water. Before the experiments start, the samples are sealed with
epoxy resin, except for defined areas for the mass transport. With the help of pressure, temperature and
conductivity sensors as well as subsequent XRF measurement, the changes in the two solutions are measured
over time. The cement mortars are characterized in terms of their properties as green mortar and hydrated
mortar. From the results of the osmometer experiments and the characteristic properties of the samples,
conclusions should be drawn about the formation and the functionality of a semipermeable membrane and
the conditions for the formation of osmotic pressures.
The investigations have shown that even with simple cement mortars (CEM I cement; CEN Normsand;
w/c = 0,45) and 2M NaCl or KBr solution, pressures up to 0,1 N/mm2 formed after only a few months. In
tests on cement mortars with higher w/c ratios and in a test with a commercial polymer silicate mortar,
these pressures were reached after a shorter time. After finishing the experiment, ion concentrations of
the elements from the concentrated solution increased in the chamber with pure solvent, while the ion
concentration in the chamber of the concentrated solution decreased. Since the ions Cl− and Br− are not
part of the sample, they can only derive from the concentrated solution. In addition to the diffusion of
water molecules in the direction of the concentrated solution, there is also a (significantly slower) transport
of dissolved ions in the opposite direction through the sample. This does not correspond to the classic
definition of a semipermeable membrane, which is only permeable for the solvent. Nevertheless, the results
confirm that the transport processes in the experiments are osmosis.
Subjects
osmosis
osmotic pressure
mineral building material
semipermeable membrane
DDC Class
624: Civil Engineering, Environmental Engineering
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