200 M. Kutschera, L. Nicoleau, and M. Bräu
• Crack bridging by gypsum crystals, acting locally behind the crack tip
• Secondary cracking operating in a macroscopic process zone around the
crack tip
• Substantial macroscopic crack branching that contribute to wake effects.
[Saâdaoui Caspar 2000]. The compressive strength of set plaster becomes signifi-
cantly lower by wetting or even high relative humidity. Generally speaking, this
can be explained with layers of water on the different surfaces acting as lubricant
in between the dihydrate crystals, as proven by AFM methods [Finot Goudonnet
1997]. The fracture behavior of set plaster results mostly from the weakening of
forces at the interface between gypsum crystals rather than a crystal
break down. Macroscopic crack propagation occurs by linkage with micro cracks
originating from weak regions [Saâdaoui Fantozzi 2005]. The sustainability of
gypsum-based materials is determined not only by the strength but also largely by
their deformability. Under permanent load, elastic and plastic deformations occur
in gypsum, which primarily depend on raw materials quality, on placing and hard-
ening conditions, on the hardened materials shape and on the applied load [Sattler
1974; Odler Roessler 1989].
Besides control at the nano level, adjustments may be performed at a bigger
scale. In order to control the rheology of gypsum paste, additives like superplasti-
cizers contribute to the flow of the paste. Foaming and defoaming agents control
the macro pore structure.
6 Summary
In 2010 it is not new to affirm that nanotechnologies are one of the greatest vec-
tors of innovation in cementitious systems. Never the less, there was a real break-
through of these technologies into the real life of consumers only in the past few
years. Despite its historical existence, we illustrated that with new developments
in material analysis, the key parameters in cement hydration can be identified and
hence new molecular architectures can help to tune the parameters that govern
concrete properties.
It has been shown that two key starting points for nano-engineering and nano-
modification of construction materials exist: the nucleation step and the crystalli-
zation period of the materials. Both steps substantially determine the transition of
the material from a fluid suspension to a hardened structural material. Active
components which change those steps are either supramolecular or colloidal (par-
ticle) nano-seeding-additives. But it is not easy to find suitable substances as they
considerably change the subtle balance of dissolution, precipitation, surface en-
ergy, critical seed size, etc. Only the combination of mechanics, thermodynamics,
polymer chemistry and material engineering areas results in more and more ad-
vanced, top-performing and highly sustainable solutions. And every step down in
nano-size technology, nano-analytics and understanding of those mechanisms
happening on the nano-scale will still open up the opportunity to improve those
processes leading to more and more advanced, efficient and powerful construction
materials.