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Microstructural studies on recycled aggregate concrete 429

14.2.3 Transmission electron microscopy

Transmission electron microscopy (TEM) is used to overcome the insufficient reso-
lution provided by SEM to cement and concrete research at the nanoscale. A TEM
image is produced from the beam of electrons transmitted through a sample after
interaction with sample atoms (Fultz and Howe, 2007) due to differential absorption
of electrons caused by differences in phase composition or thickness (Azarsa and
Gupta, 2018). Resolution around 1 nm is easily achieved (Azarsa and Gupta, 2018).
TEM is an especially powerful technique because besides the image (information
on sample microstructure and morphology), it can supply electron diffraction pat-
terns from the same sample area, rendering compositional identification and crystal-
lographic information (Eades, 1993). Concrete sample preparation for TEM can be
a complex procedure depending on the type of information to be obtained, since the
material is impervious to the electron beam. Powdered materials with dimensions
small enough to be electron transparent can be quickly prepared by the deposition
of a dilute sample onto support grids. However, this fails to preserve the complex
spatial relation between hydration products (Azarsa and Gupta, 2018). To allow
electrons to transmit through it, a bulk sample must be polished to less than 100 nm
thick (often by ion etching or ion milling) so that some portions of the specimen is
thin enough (Azarsa and Gupta, 2018). TEM observation of properly prepared sam-
ples allows studying the morphology, crystallisation and elemental composition of
cement hydration products in early hydration, in mature cement, mortar and con-
crete, and to illustrate the effect of nanoparticles addition on concrete (e.g.,
Scrivener and Pratt, 1983; Plank et al., 2006; Gallucci et al., 2010; Han et al., 2012;
Gu et al., 2016; Scho ¨nlein and Plank, 2018).

14.2.4 Atomic force microscopy
Atomic force microscopy (AFM) is a high-resolution scanning probe microscopy
technique, providing quantitative topographical representation of surfaces (Morris,
1993). Images are obtained using a probe with a sharp tip that is moved across the
surface of the sample. Tests are carried out in room atmosphere and without sample
preparation requirements (Peled et al., 2013). As the tip is brought into contact with
the sample, the relative position of the surface is raster scanned and the height of the
probe corresponding to the probe-sample interaction renders surface topography
(Morris, 1993). The image formed is plotted as a map of the intensity of the mea-
sured value at each coordinate, expressed as a colour hue. The useful magnification
7
3
range is from 10 to 10 3 , with resolution up to 0.1 0.2 nm on smooth surfaces,
and vertical resolution below 1 nm (Morris, 1993). Topographic AFM can, thus, pro-
vide high-resolution surface texture characteristics of cement-based systems, includ-
ing at the nanoscale (Yang et al., 2003; Peled and Weiss, 2011; Peled et al., 2013;
Bentz et al., 2015; Horszczaruk et al., 2015; Cascudo et al., 2018; Zhu et al., 2018)
coupled to the possibility of real-time imaging during hydration (Yang et al., 2003).