Page 29 - openaccess

TECHNICAL PAPER

science, and technology by Ramachandran and Beaudoin [84] ,
and finally the microstructural guide for cementitious materials
by Scrivener et al. [85] .
Apart from microscopy, microanalytical tools are used in
petrography to assess the inherent properties, performance in
different exposures, and forensic assessment of degradation of
concrete materials. Table 7 provides a quick glance of commonly
used petrographic techniques in concrete research with their
relevance [6-11,74-88] . Petrography is a collective name given to
the characterization methodologies based on microanalytical
techniques provided in Table 7. The petrographic interpretations
Figure 22: Evolution of porosity in aggregates, Xing et al. [73]
based on mineralogy have to be made carefully, otherwise may
lead to misleading conclusions. A minimum of three techniques
rock forming minerals by Deer et al. . Other epic references shall be employed before reaching a specific inference. For
[7]
are petrographic atlas by RILEM TC 219-ACS [51] , petrographic example, igneous siliceous rock granite exhibits excellent
manual by Walker et al. [77] , colour atlas of minerals by thermogravimetric properties at elevated temperatures in TG/
Mackenzie, and Adams [11] , a guide to thin section microscopy DTA analysis. However, the microstructural alterations unraveled
by Raith et al. [78] , colour guide of geomaterials under the by the thin section petrography answer their poor performance
microscope by Ingham [79] , petrography of carbonates rock in strengths after a fire of 550°C, compared to calcareous
by Peter, and Dana [80] , concrete microscopy by Roy et al. [81] , aggregate like limestone [21,39,63,68] . In this regard ASTM C856,
microscopy applied to geological and construction materials by Clause 3 [89] specifies a minimum five-year experience for the
Jana [82] , scanning electron microscopy of cement and concrete professional who involved in petrographic interpretations of
by Winter [83] , handbook of analytical techniques in concrete hardened concretes

Table 7: Summary of some common petrographic techniques and their relevance
TECHNIQUE RELEVANCE
Optical microscopy Reflected mode: To study morphology, phase distributions and crack patterns in stereo microscope at relatively smaller
magnifications up to 100X. Study of cement phases, ITZ and aggregates in concrete with polished specimens at lower or
higher magnifications up to 1000X [21, 39, 63, 68] .
Transmitted mode: To study bulk mineralogy of aggregate grains, powder mineralogy and thin section imaging in
crossed polarized and plane polarized light. In thin section petrography mineral species and assemblage can be
identified and quantify. Mineralogical modification upon thermal, physical or chemical triggers can be assessed.
Characterization of bubbles in foam concrete is possible in transmission mode microscop y [21, 39, 63, 68].
Florescence microscopy: Used to identify the diagnostic properties of some mineral based on the florescence.
properties. Possible in polarized microscope with fluoresce setup or thin section prepared with standard fluorenes dyes
[86] .
Scanning electron Secondary electron mode (SE): Morphology, topography, crystal structure and habit of aggregates and cement phases
microscopy (SEM) can be identified on fractured surfaces.
Back scattered electron mode (BSE): Images with density-based contrasts enable to identify different phases in
aggregate and binder phase; on finely polished representative specimens [85] .
Energy dispersive spectroscopy (EDS): Equipped with SE or BSE modes. Used to perform chemical characterization of
phases with the finger print X-ray characteristics of element present in the phases
Transmission electron Used in a similar way as SEM. However, more resolution of can be obtained even at higher magnification of 10000X.
microscopy (TEM) Sample preparation is laborious [85] .
X-ray diffraction (XRD) Used to identify the crystalline phases in concrete materials-based Bragg’s law of diffraction [88] .
X-ray fluorescence (XRF) Chemical characterization of materials. XRF measurements provide elemental composition and can be converted into
oxide compositions [21, 39, 63, 68]
X-ray tomography Provides 3D information of materials. Useful techniques in material science to study any alteration upon exposures [89].
Thermogravimetric Provides insights in to the phases present in the system and evaluate their thermal decomposition as function of time
differential thermal and temperature [21, 38, 64, 68]
analysis (TG-DTA)
Fourier -Transform Infrared Can be effectively employed to study the type and extend of bonds present in the minerals. Effectively used to
Spectroscopy (FTIR) characterize the rock or concrete mineralogy after exposure to severe environmental conditions [84].

30 THE INDIAN CONCRETE JOURNAL | AUGUST 2022