Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 52

80

70

60

50

40

30

20

10 9

8

7

6

0

5

Mo 4

3

Ill

Kao Mus

2

1

Chl

Figure 3.8 Water reducer adsorption of different proprietary products for different
types of clay.

3.1.5 Chemical impurities
The question of more exotic chemical impurities is left to others, but the
two questions of salt and organic impurity must be addressed. There is an
extensive literature on chloride contents and their capacity to promote the
corrosion of reinforcing steel. Beach sand is liable to have very high salt
levels owing to the deposition of salt by evaporation. Sand dredged from
the sea may be less of a problem but without washing with fresh water may
still exceed a fully safe level. Salts can also cause efflorescence and higher
shrinkage and affect setting and hardening rates. Although it is useful to
know the chloride content of the aggregate at the source, compliance test-
ing for chloride content should be conducted on the concrete to capture
all sources of chloride. However, the limit used should be realistic so that
otherwise acceptable materials are not excluded or expensive pretreatment
required for no technical benefit. Another solution to chloride in the aggre-
gate is the use of a corrosion inhibitor.
Organic impurities are quite frequently encountered in pit sands. The
authors' practice is to combine the colour test (BS 812, 1960) for organic
impurity with the settling test for clay content by using sodium hydroxide