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3. Results
3.1 Concrete
(M) log P CO2 = -2.65
Carbonatation is one of the most relevant processes in concrete degradation. Presence of calcite is
greater in the hydration area (Fig. 2a); whereas the rest of precipitates are found in the pores close
the interface with the bentonite. Portlandite begins to precipitate in this area until cover all the sample
surface (Fig 2b, 2c).
6 μm
Figure 2. (a) Crystals of Ca CO3 on the sample surface (b) Ca (OH)2 covering concrete surface(c)
detail of crystals
As well, gypsum crystals were observed (Fig.3). They come from the reaction of substitution between
portlandite and sulphate present in the hydration solution.
40 μm 40 μm
Figure 3. Gypsum crystals in a micro fracture.
(a) (b) (c)
3.2 Interface concrete/bentonite
A homogeneous white precipitate is formed at the interface. The thickness of this layer is around 2
μm. The SEM-EDX analysis of a cross-section of this area shows the formation of a continuous
layer of portlandite. Other secondary mineral phases were found at the interface: calcium silicate
hydrates or CSH gels tobermorite-type with Ca/Si molar ratio of 0.6, magnesium hydroxide (brucite)
and ettringite (Aft) (Ca6Al2(SO4)3(OH)12 26(H2O)) (Fig. 4). Also, dissolution of quartz was
detected .This process enables the precipitation of CSH minerals such as tobermorite.
Tobermorite
6�m 6�m
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