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@PHDTHESIS{Winkels:1023957,
author = {Winkels, Bernd},
othercontributors = {Raupach, Michael and Lohaus, Ludger},
title = {{C}arbonatisierung von {P}orenbeton},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2025-10879},
pages = {1 Online-Ressource : Illustrationen},
year = {2025},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2026; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2025},
abstract = {Autoclaved aerated concrete is a hydrothermally cured,
porous building material. Its high pore volume fraction
results in low bulk density and thermal conductivity, while
the closed-cell microstructure can nevertheless provide
substantial load-bearing capacity. Consequently, autoclaved
aerated concrete is suitable for use in monolithic masonry
structures. During hydrothermal curing, the strength-giving
mineral 11 Å tobermorite, a calcium silicate hydrate (CSH)
phase, is formed. The structure of tobermorite is
susceptible to alteration through carbonation: in the
presence of moisture, carbon dioxide induces the
transformation of CSH phases into the calcium carbonate
polymorphs calcite, vaterite, and aragonite. Depending on
the phase assemblage and exposure conditions, carbonation
can affect the mechanical properties to varying extents.
However, the relationship between these phase
transformations and the mechanical and physical properties
of autoclaved aerated concrete remains insufficiently
understood. Within a research project funded by the German
Research Foundation (DFG), the phase assemblage and its
transformation, as well as the changes in
mechanical-physical properties induced by carbonation, were
comprehensively investigated on model autoclaved aerated
concretes produced under realistic conditions. The study
systematically examined variations in the sulphate content
of the raw material mixture and the effects of different
autoclaving and environmental conditions. Across the
investigated sulphate range (approximately 1 to 2.8
$wt.\%),$ no significant influence on the morphology or
spatial distribution of tobermorite within the solid matrix
was observed. Contrary to expectations, model autoclaved
aerated concretes with lower sulphate contents exhibited, in
some instances, greater resistance to phase transformation
than those with sulphate contents near the presumed optimum.
More influential than sulphate content, however, were the
autoclaving parameters and the corresponding resulting phase
assemblage. As anticipated, a more crystalline phase
assemblage displayed higher resistance to carbonation
compared to structures with limited crystallinity. A
comparison of two bulk-density classes demonstrated that the
carbonation behaviour of the matrix in the near-surface
region is comparable regardless of bulk density. Under
natural CO₂ concentrations and a relative humidity of 65
$\%,$ carbonation progressed primarily within the amorphous
CSH phases, whereas tobermorite remained largely intact. In
contrast, exposure to a twelvefold increase in CO₂
concentration under the same humidity conditions resulted
initially in a continuous phase transformation that
subsequently transitioned into accelerated transformation
kinetics. This induced a complete decomposition of
tobermorite and a disproportionately high formation of
vaterite, as elevated CO₂ concentrations modify both the
reaction mechanisms and the reaction kinetics. Furthermore,
it was shown that this process leads to enhanced shrinkage
deformations associated with carbonation. Pronounced
carbonation gradients across the specimen cross-section,
together with the resulting strain and stress gradients
between surface and interior regions, give rise to multiple
cracking, which indirectly affects load-bearing and
deformation behaviour. In addition to this cracking, the
phase transformation itself exerts a direct influence on
mechanical properties, as demonstrated, among other methods,
through tensile testing.},
cin = {311310},
ddc = {624},
cid = {$I:(DE-82)311310_20180808$},
pnm = {DFG project G:(GEPRIS)445500601 - Mechanismen der
Carbonatisierung von Calciumsilikathydrat-Phasen in
hydrothermal gehärteten porosierten Baustoffen (445500601)},
pid = {G:(GEPRIS)445500601},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2025-10879},
url = {https://publications.rwth-aachen.de/record/1023957},
}
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