Page 476 - New Trends in Eco efficient and Recycled Concrete

426 New Trends in Eco-efficient and Recycled Concrete

arrangement and the centimetric random structure of mortar/coarse aggregates
arrangement (Fig. 14.1). As recycled aggregates (RA) have more irregular shapes
due to their recycling processes and their specific surfaces areas tend to be higher
than those of their natural counterparts (Solyman, 2005; Evangelista et al., 2015).
And, although at the engineering end-use the length scale is metric (with concrete
considered as a uniform material with bulk properties), concrete properties are in
fact determined by its microstructure.
Details of concrete microstructure differ depending on cement chemistry, fine-
ness and content, water-to-cement ratio (w/c), presence of supplementary materials
(e.g., fly ash, silica fume, superplasticiser), mixing and curing procedures
(Diamond, 2001). They also depend on aggregate type and properties (Diamond,
2001) and, even if all other parameters are kept constant, the total or partial replace-
ment of natural aggregates (NAs) by RAs introduces profound microstructural
alterations in the produced concrete, especially at the ITZ.

14.2 Tools for microstructural characterisation of
concrete

The heterogeneity and multi-length scale of hydrated cementitious products result
in high microstructural complexity, ensuing that none of the existing analytical and
instrumental methods can cover them all (Bonen, 2006). Microscopy, either optical,
electron or scanning probe, is the most common and comprehensive tool to assess
those features. The information in visual form rendered by microscopy observation
of concrete is most often complemented with information from other methods,
which include X-ray elemental microanalysis, X-ray diffraction of crystalline
phases, and pore size and distribution characterisation.

14.2.1 Optical microscopy

The investigation of cement-based materials by optical microscopy (OM) essen-
tially refers to the classic geologic techniques of optical mineralogy and petrogra-
phy (Campbell, 1999). Optical microscopes use visible light and a system of lenses
to magnify and resolve the structure of materials (Holik, 1993). The metallographic
microscope (reflected light) and petrographic microscope (transmitted light) varia-
tions are the most common to study cement-based materials (Campbell, 1999). In
both cases a flat polished surface is required for observation. The procedure devel-
oped for preparation of concrete samples for microscopy is well established
(Scrivener and Pratt, 1984; Scrivener et al., 1986; Kjellsen et al., 1991) and several
guides and protocols have been produced (e.g., Kjellsen and Monsøy, 1996;
Campbell, 1999; Stutzman and Clifton, 1999; Detwiler et al., 2001; Scrivener et al.,
2017). After cutting with a high-precision saw, the sample is dried to prevent the
progress of hydration reactions often by using solvent exchange procedures
(Kjellsen et al., 2003; Jana, 2006; Wang et al., 2016a). The dried sample is then