Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 8

j. It can be delivered from the same depot and even ground at the
same plant but be produced from a different clinker, that is,
imported clinker using different materials and produced in a dif-
ferent kiln may have been used.
k. Sometimes the mill certificate with the cement does not relate to
the actual cement delivered.
2. As it is produced:
a. Variation in raw materials.
b. Segregation at any of several stages.
c. Incorrect proportion or uneven distribution of gypsum
(CaSO 4 .2H 2 O).
d. Variable firing and grinding temperatures.
e. Unsatisfactory grinding, including overall fineness, particle size
distribution, and particle shape.
f. Deterioration (including segregation) of clinker in storage.
g. Seasonal variations.
2.1.2 Significant test results
Cement users in some parts of the world can obtain test certificates from
their cement suppliers. The following may be of assistance in interpreting
the kind of information usually provided on such certificates. Where no
test data are obtained in this way, it may be considered too expensive to
undertake routine testing on behalf of a single project or small ready-mix
plant. A solution to this problem is to take a sample either daily or from
each truck of cement (whichever is least). The sample should be kept in a
(well-labeled!) sealed container until the 28-day concrete test results are
obtained and then discarded. A sample is then available and should be
tested if unsatisfactory concrete test results are encountered for which no
other explanation can be found.
Where regular test data are obtained, it is useful to maintain graphs of
the information provided. As with concrete test data, cusum (cumulative
sum) graphs are far more effective at detecting change points. (See
Chapter 10.)
The main results likely to be provided are
1. Setting time—Initial and final set are both arbitrary stages in smooth
curve of strength development. Abnormal results can indicate incor-
rect proportion of gypsum, excessive temperature in final grinding
(which dehydrates gypsum and alters its effectiveness) or deterioration
with age.
2. Fineness, finer cement will
a. React more quickly (faster heat generation)
b. React more completely