Atomistic Simulation studies of the Cement Paste Components

Atomistic Simulation studies of the Cement Paste Components
calcium leaching, so that the porosity increases, in detriment of the mechanical
properties [48, 96, 97]. On the other hand, the corrosion of metals embedded in concrete
(conductions, pipes or reinforcements) is usually prevented by the high pH of the pore
solution. The presence of large portlandite amounts ensures the high alkalinity levels of
the cement paste, and protects the metals [48].
• Ettringite
Ettringite, CASH 6 32 , is the result of the hydration processes of aluminate and ferrite
clinker phases in the presence of gypsum. When all the gypsum content is consumed
during the hydration reactions, the continuous dissolution of aluminate and ferrite
phases consumes the ettringite to form a second product with lower sulphate content.
However, ettringite crystals may persist in mature cement pastes [98]. In such cases, they
present a rod-like morphology and may reach lengths up to 10 μm [4]. Ettringite belongs
to the AFt family of compounds (Al 2 O 3 -Fe 2 O 3 -trisulpho). As can be seen in figure
1.6.1, the ettringite structure is based on columns arranged parallel to the z axis. The
columns are formed by aluminium atoms octahedrally coordinated to hydroxyl groups
alternated with three calcium atoms in a plane. The Ca atoms share and are coordinated
to the hydroxyl groups of aluminium and to four water molecules placed in the space
between columns. The stoichiometry of the columns leads to a charge excess, which is
neutralized by sulphate ions placed in the channels between these columns [99, 100].
A secondary formation of ettringite may occur in mature cement pastes. Sulphate ions
coming from the environment could react with portlandite and monosulphoaluminate to
form new ettringite [48]. Furthermore, monosulphoaluminate is not stable at
temperatures higher than 70º. If the cement paste is exposed to those temperatures, the
monosulphoaluminate dissolution releases Al +3 and SO -2 4 ions. The first ions are likely
to enter the silicate structure of the C-S-H gel, and therefore, when the temperature
decreases again, the Al +3 to SO -2 4 ratio is lower, and ettringite is formed [48, 98]. Both
processes, known as sulphate attack and delayed ettringite formation, are associated to
an expansion of the cement paste and the apparition of micro-cracks, although the
mechanism is not fully understood [48, 98].
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