Page 477 - New Trends in Eco efficient and Recycled Concrete

Microstructural studies on recycled aggregate concrete 427

embedded with low-viscosity epoxy resin. This stabilises the sample by filling pores
and cracks so that it better withstands stresses imposed during grinding and polish-
ing as well as preserving the original microstructure and distribution of components
by minimising debonding of particles from softer phases (Detwiler et al., 2001;
Jana, 2006). After resin hardening, samples for metallographic examination are
mechanically ground and polished with progressively finer abrasives. Etching or
staining follows to selectively develop characteristic colours for the different phases
of concrete, allowing their identification (Jana, 2006). Petrographic examination
requires production of thin sections through which light can be transmitted (Jana,
2006). After epoxy curing the sample is further mounted on a glass microscope
slide, cut to a thickness of approximately 1 mm and then ground to a final thickness
around 20 μm. The resolution limit in OM is approximately 0.2 μm, restraining the
practical magnification limit to B1500 3 (Holik, 1993). Petrographic examination
is preferable over metallographic examination in determining the abundance of fine
features such as fly ash particles, residual unhydrated cement grains or primary calcium
hydroxide (Jana, 2006). Overall, OM allows to examine the large-scale features and
relationships in concrete, providing information on compositional, textural, minera-
logical, and microstructural properties at the submillimetric and microscale (Jana, 2006).

14.2.2 Scanning electron microscopy
High magnification and spatial resolution higher than those produced by optical
microscopes are necessary to resolve the details of individual features of concrete
(Diamond, 2004), featuring the use of scanning electron microscopy (SEM) since
the early 1970s (Diamond, 1972, 1976; Diamond et al., 1974). SEM produces
images by probing the sample with a focused beam of electrons, which interact
with atoms in the surface to produces various signals that contain information about
the material (Goldstein et al., 1981). Depending on the specific equipment magnifi-
cation in the 100,000 3 order of magnitude is possible, which is at least two orders
of magnitude higher than that achievable with the best optical microscopes
(Goldstein et al., 1981; Pham et al., 2011). Additionally, SEM equipment can be
coupled to chemical analysis apparatus, either energy dispersive X-ray spectroscopy
(EDS) or wavelength dispersive X-ray spectroscopy, to detect the characteristic X-
rays produced by interaction of electrons with the sample material. This allows an
estimate to be made on the abundance and distribution of elements in the sample
surface. While early SEM studies were mainly carried out in secondary electron
(SE) mode, the use of backscattered electrons came into play (Scrivener and
Gartner, 1987) and were generalised in the mid-1980s. SE images result from
inelastic interaction of the electron beam with the surface (Goldstein et al., 1981)
rendering information on morphological features (namely size, shape and distribu-
tion) of constituents. SE is adequate for observation of flat surfaces and fracture
surfaces of cementitious materials, revealing a wealth of information such as types
of aggregates, mineralogical and textural properties of the paste, bond between
aggregates and paste, secondary deposits in voids and cracks, pore distribution, and
areas of interest to be further examined by compositional analysis (Jana, 2006).