Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 75

such combinations require continuous agitation to avoid settling out. It
also tends to be more sensitive to cement type and can result in greater
workability loss at intermediate workability.
PCE and naphthalene sulphonate admixtures are totally incompatible
with each other. Therefore, the premix supplier should use the same type of
admixture for all concrete on a particular project. Incompatibility may also
effect production if both admixture types have a common delivery path.
4.3.3 Retarders
Set retardation to any desired extent is readily available with no deleterious
effects, with or without water reduction.
Sugar is a powerful retarder and very small quantities can produce a
dramatic effect. It should be noted that set retardation is not the same thing
as workability retention. Mixes containing water-reducing retarders may
lose slump more rapidly than plain concrete in some circumstances.
Delayed addition may be very important because a greater effect is obtained
by a delay of the order of 5 minutes after the water has been in contact with
the cement. When retarding admixtures are added with the mixing water,
the retarder can retard the release of gypsum, which is added to cement dur-
ing manufacture to control rapid setting resulting a more rapid set. It is not
usually practicable to actually delay addition in ready-mix operations, but
the same effect may be obtained if the undiluted admixture is added at the
end of the batching process and takes some time to disperse through the mix.
This problem appears to have diminished and most producers add retarding
admixtures to the mixing water with apparent impunity.
4.3.4 Accelerators
Set acceleration, unlike retardation, is only obtainable within limits and
with some risk (or certainty) of deleterious side effects. Most accelerators
tend to increase shrinkage. The trend is for high early strength to be
achieved by the use of nonretarding HRWR in many applications rather
than chemical accelerators to avoid any detrimental effect. The field of
accelerators in particular is one in which development work is occurring
and details are not readily available. The information given next is likely to
prove outdated. Purchasers will need to carry out their own trials.
Calcium chloride is by far the most economical and effective accelerator.
However, it has the severe disadvantage that it promotes the corrosion of
reinforcement (and any other embedded steel), particularly if not properly
dispersed. Many, but not quite all, authorities claim that it also increases
shrinkage quite substantially. Calcium formate and calcium nitrite, which
is primarily used as a corrosion inhibitor, produce almost similar strength
gains but less effect on setting times. Both are substantially more expensive