STAINING TECHNIQUES FOR GEOMATERIALS PETROGRAPHY - Geomaterials: Under the Microscope - page 172

ETCHING AND STAINING TECHNIQUES FOR CEMENT MINERALS AND SLAGS

Subject Method summary* Observation

Portland cement Aluminates
Hydrofluoric acid vapour 1 : hold a few millimetres Tricalcium aluminate (C 3 A) turns grey.

above acid and etch in vapour for 2-10 s at room Tetracalcium aluminoferrite (C 4 AF)

temperature. turns white. Also stains silicates (see

below).

5% potassium hydroxide in ethyl alcohol solution 1 :

Tricalcium aluminate (C 3 A) turns blue.

immerse for 20 s, wash with 1:1 ethyl alcohol, water,
then isopropyl alcohol buff (with microlap cloth) for
15 s then wash with isopropyl alcohol.
Warm (40°C) distilled water 1 : immerse 5-10 s. Aluminates turn blue to brown.

Tricalcium silicate (alite, C 3 S) light tan.
Dicalcium silicate (belite, C 2 S) is
unchanged. Calcium hydroxide is
multicoloured.

Silicates
Dilute salicylic acid stain 1 : solution comprises 0.2 g Tricalcium silicate (alite, C 3 S) and

salicylic acid + 25 ml isopropyl alcohol + 25 ml dicalcium silicate (belite, C 2 S) turn

water. Immerse for 20-30 s. blue-green. Alite stains faster so may

be deeper coloured.

Salicylic acid etchant 1 : solution comprises 0.5 g

Alite and belite visibly etch. Belite

salicylic acid + 50 ml methyl alcohol. Immerse shows lamellar structure.

for 45 s.
Nital 1 : nital solution comprises 1.5 ml nitric acid +

Alite turns blue to green. Belite turns

10 ml isopropyl alcohol. Immerse for 6-10 s. brown to blue.

Hydrofluoric acid vapour 1 : hold a few millimetres

Alite turns brown and belite turns blue.

above acid and etch in vapour for 5-10 s at room
temperature.
Hydrates
Napththol green B 2 : immerse thin section for 12 h

Calcium hydroxide turns green.

in an alcoholic solution of the dye containing
a few percent water.

High-alumina Aluminates

cement Boiling 10% sodium hydroxide Monocalcium aluminate (CA) turns blue

solution 1 : immerse for 20 s.

or brown.

1% borax solution 3 : immerse for 30 s.

Dodeca-calcium hepta-aluminate (C 12 A 7 )
turns grey.

Boiling 1% borax solution 3 : immerse for

Pleochroite (approximately C 22 A 13 F 3 S 4 )

unknown time. is etched.

Silicates
As for Portland cement, although HAC has no
tricalcium silicate but does contain limited amounts
of dicalcium silicate.