Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 51

that siliceous stone dust can have pozzolanic properties if it is ground suf-
ficiently fine. Also calcareous stone dust (e.g., limestone) will act to some
extent like superfine calcium carbonate, as discussed in Chapter 2, Section
2.10. However Day's practice is to use the settling test to allow for the
extra water requirement of the fine dust but to neglect the possible strength
increase.
Murdock studied the influence of fines and specific surface on the water
demand on concrete, and concluded that the water demand of particle sizes
that could be attributable to particle size alone disappeared for sizes lower
than 100 microns.
Those who have used fly ash in concrete know that adding fine particles
into a concrete mix can actually increase the workability of concrete. It
is important not to confuse the change in water demand with the change
in admixture demand (the amount of admixture required to get the same
workability between two sands).
As each cement has its own performance characteristics, so does each
sand, especially if the sand has fine materials (under 75 microns), as these
small particles cease to be mere inert fillers and have the ability to influence
the concrete mix performance from a chemical perspective. This is particu-
larly true for clays.
If clays are present, these clays will have a major impact on how the con-
crete performs in its plastic state. This does not refer to clay lumps, but clay
particles that have been either liberated from hard rock through crushing,
or fine clay particles in natural sands that have not been removed by wash-
ing or classifying
Simplifying the complex chemistry between clays and the range or types
of admixtures that are used in concrete, there are five types of clays that
are found in aggregates.
As previously discussed, there is a misconception that when there are
fine materials in sands, even clays, there will be a corresponding increase in
water demand. And, not all clays are deleterious to concrete. Typically, the
only deleterious clays for concrete are montmorillonite, smectite, and illite;
all others are basically inert. Of course, these particles are the smallest of
the small, typically a particle size less than 3 microns.
The deleterious clays actually attract admixtures to their surface, depend-
ing on the admixture and clay. Figure 3.8 shows the admixture adsorption
rate at one hour with the different pure clays. This graph shows that each
clay type (pure form of the clay) adsorbs different admixtures at a differ-
ent rate. Some admixtures (polycarboxylate ethers, PCEs) have a higher
adsorption rate than the lignin-based admixtures. This can be a major
concern for using these types of water reducer compared with the older
fashioned lignin admixtures if the aggregate contains clays. An alternative
procedure is to use an anticlay admixture to coat the clay particles and
limit adsorption problems.