Concrete - Geomaterials: Under the Microscope - page 78

168

a higher proportion of the brown-coloured C 4 AF
interstitial phase than normal Portland cement. However,
thin section examination alone is not a reliable indicator
of the use of SRPC, it must be confirmed by reflected
light examination of a highly polished and etched
specimen. The method is described in BS 1881: Part 124
(British Standard Institution, 1988) and by Grove (1968).
For reflected light examination of cements, the choice of
etchant reflects the constituent to be identified and a
range of appropriate etching techniques are described in
Appendix B. For differentiating between normal and
sulfate-resisting Portland cements the two most useful
etching methods are those that use either HF vapour or
10% KOH solution. These two different etching
techniques produce the following colours that are seen in
reflected light:
Clinker phase

168 Unhydrated Portland cement clinker after etching
with HF vapour. C 3 S is brown, C 2 S is blue (very little
in this example), the interstitial matrix is differentiated
into C 3 A (light grey) and C 4 AF (white); PPR, ×300.
(Courtesy of Barry J Hunt.)

HF vapour 10% KOH

C 3 S (alite) Brown Does not etch

C 2 S (belite) Blue Does not etch

169

C 3 A (aluminate) Light grey Bluish grey

C 4 AF (ferrite) White White

The method for distinguishing between SRPC and normal
Portland cement is based on the ratio of C 3 A to C 4 AF in the
interstitial phases of residual cement clinker grains. For
normal Portland cement the ratio does not usually exceed
1:2 (aluminate:ferrite) and in contrast the ratio is typically
more than 1:5 for SRPC. A least 20 residual cement grains
>20 μm in size should be examined for each concrete or
cement sample. Figures 168-170 show residual cement
grains within concrete polished specimens that have all
been etched with HF vapour for 2-5 seconds. Figure 168

169 Unhydrated Portland cement clinker after etching
with HF vapour. C 2 S is blue and exhibits lamellar
twinning, C 3 S is brown. The interstitial matrix is
differentiated into C 3 A (light grey) and C 4 AF (white);
PPR, ×300. (Courtesy of Barry J Hunt.)

170

170 Unhydrated sulfate-resisting Portland cement
clinker after etching with HF vapour (centre). C 3 S is
brown, C 2 S is blue (very few in this example), the
interstitial phases consist almost entirely of C 4 AF
(white); PPR, ×300. (Courtesy of Barry J Hunt.)