366 ANNUAL REPORT 
Efflorescence.—A great source of annoyance is frequently the ap- 
pearance of soluble salts on the surface of cement work, giving rise to 
disagreeable patches of so-called “whitewash.” These salts are prin- 
cipally sulphates of the alkalies, lime, magnesia and iron, and can to a 
large extent be prevented by the addition of fine barium carbonate to 
the mortar, which takes up the available sulphuric acid and forms the 
insoluble barium sulphate according to the reaction: 
CaSO,+ BaCO,=CaCO,+ BaSO,,. 
Excess of the barium carbonate should be used. The barium chloride 
is more efficient, a smaller amount being sufficient owing to its solubility 
in water, but any excess will in itself produce a white efflorescence. Jor 
decorative purposes cement with a silicious clay base free from gypsum 
should be used. | 
Another method of preventing efflorescence is the application of a 
solution of silicon fluoride of magnesium or lead. This solution reacts 
with the lime of the cement forming an insoluble double silicon fluoride. 
Dr. Michaelis* advocates the use of a dilute water glass solution 
which is applied several times and finally the surface is washed thor- 
oughly with water to remove the alkalies which have become free by the 
combination of the silicic acid with the lime. . 
For preparing a cement surface for painting Dr. F'ruehling** recom- 
mends an ammonium carbonate solution applied upon the hardened 
cement. 
Adulteration.—Sometimes it is found that Portland cement is adul- 
tered with ground slag and it becomes necessary to detect the presence of 
the latter, ‘or again natural cement may be the adulterant. Such con- 
stituents may be detected by means of the following differentiation : 
rt. Portland cement has a higher specific gravity than either the 
natural cement or slag. 
2. The loss on ignition of natural cement is much greater than that 
of Portland cement. 
3. The amount of carbon dioxide absorbed by natural cement may 
be greater than that taken up by Portland cement. 
4. Owing to the high content of sulphides in slag the amount of 
potassium permanganate decolorized by slag is much greater than that 
consumed by Portland cement, and of course the amount of sulphide 
sulphur is much larger in amount. 
Mechanical Separation.—The difference in specific gravity is em- 
ployed to separate the admixed substances from the Portland cement. 
This is carried out as follows: By means of the 20, 30 and 40 mesh 
*Dr. Michaelis, Die Hydraulischen Moertel, 313. 
** Polytech, Centralblatt, 1871, p. 261. 
