PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 219

ready-mixed concrete operations is a hazardous substance (it contains caus-
tic soda and potash) and its disposal is regulated by the Environmental Pro-
tection Agency (EPA). In addition, the high pH makes concrete wash water
hazardous under the EPA definition of corrosivity.
The conventional practices for disposing of concrete wash water include
dumping at the job site, dumping at a landfill, or dumping into a concrete
wash water pit at the ready-mix plant. The availability of landfill sites for
the disposal of concrete wash water has been drastically reduced in the past
few decades. In addition, the current environmental restrictions either pre-
vent or limit these conventional disposal practices. As a result, most ready-
mix batch plants have developed a variety of operational configurations to
manage their own wash water. The alternatives include settling ponds,
storm water detention/retention facilities, and water reuse systems. Chemi-
cal stabilizing admixture systems have been used to circumvent the neces-
sity to remove any wash water from concrete truck drums, and allow wash
water to be reused for mixing more concrete. Studies have concluded that
concrete properties are not significantly affected by the use of stabilized
wash water (Borger et al. 1994).
Currently, concrete wash water is being used as mixing water for concrete
in many places throughout the U.S. Some agencies, however, do not allow its
use due to the existence of other impurities derived from concrete admixtures.

6.11 Admixtures for Concrete

Admixtures are ingredients other than portland cement, water, and aggre-
gates that may be added to concrete to impart a specific quality to either the
plastic (fresh) mix or the hardened concrete (ASTM C494). Some admixtures
are charged into the mix as solutions. In such cases the liquid should be con-
sidered part of the mixing water. If admixtures cannot be added in solution,
they are either weighed or measured by volume as recommended by the
manufacturer. Admixtures are classified by the following chemical and
functional physical characteristics (Hewlett 1978):

1. air entrainers
2. water reducers
3. retarders
4. hydration controller admixtures
5. accelerators
6. supplementary cementitious admixtures
7. specialty admixtures

The Portland Cement Association (PCA) identifies four major reasons
for using admixtures (Kosmatka et al. 2002):

1. to reduce the cost of concrete construction
2. to achieve certain properties in concrete more effectively than by other
means