Page 478 - New Trends in Eco efficient and Recycled Concrete

428 New Trends in Eco-efficient and Recycled Concrete

Sample preparation for SE observation simply consists in the sectioning or frag-
mentation of a representative portion of the material, drying and coating it with a
thin layer of electrically conductive material to prevent accumulation of electrical
charge (Diamond, 2001; Detwiler et al., 2001). Backscattered electron (BSE) mode
relies on the detection of the electron beam reflected by interaction with sample
atoms (Goldstein et al., 1981). Since elements with high atomic number BSE more
strongly than elements with low atomic number they appear brighter in the image,
allowing the detection of a reproducible contrast between areas with different chem-
ical compositions (Goldstein et al., 1981). In combination with information from
local chemical microanalysis, observation in BSE mode can be used to gather a
more comprehensive understanding of the heterogeneous nature of concrete and
also to quantify the phases present (Scrivener, 2004). BSE observation requires
preparation of a flat-polished surface, similar to petrographic examination except
that a finer polish and a conductive coating are mandatory (Kjellsen et al., 2003;
Diamond, 2004). For this purpose, the sample is again cut after resin hardening,
exposing a new epoxy-impregnated surface that is further ground and polished with
progressively finer abrasives and coated. The corresponding BSE images are ana-
lysed regarding colour intensity variations since compositional difference between
cement and concrete microstructural features is sufficiently wide to allow ranking
phases with different average atomic number via grey-scale contrast. Unhydrated
cement grains and detectable pores are readily ranked respectively at the white,
bright end and at the dark end of the grey scale; CH is somewhat slightly darker
than C S H, while the chemical variability of the C S H phase results in vari-
able grey levels (Diamond, 2001). Interpretation must, thus, be cautious (Diamond,
2004; Scrivener, 2004). A thorough guide ranking the phases in clinker and cement
is available (Stutzman et al., 2015) that renders supplemental information such as
domain shape (e.g., angular or rounded), position within the microstructure (e.g.,
framework grain, matrix, dispersed phase) and bulk chemistry to accomplish identi-
fication of individual phases. The use of BSE images for phase quantification in
concrete is more difficult and tiresome than compositional interpretation, requiring
30 to more than 100 images to derive valid quantitative information (Scrivener
2004). Micrographs for quantification purposes are most often obtained at 500 3
magnification with 512 3 512 pixel resolution, which is considered a large enough
area to represent an adequate number of the original cement grains and water-filled
spaces (Diamond, 2001, 2004). Collected micrographs are subjected to image analy-
sis. Each BSE image is segmented into a series of strips approximately 10 μm wide,
parallel and along the ITZ. Each strip is converted into a binary image by threshold-
ing segmentation based on grey level to determine the content of the three
segmentable constituents: unhydrated cement grains, detectable pores and CH.
C S H is not independently determinable by segmentation because of variable
grey levels and is estimated by difference. The percentage area of each component
is determined by dividing the number of pixels of each component by the total
number of pixels in the field, which is assumed to provide an appropriate estimate
of the volume percent of the corresponding component (Campbell, 1999; Diamond,
2001).