Civil Engineering Reference
In-Depth Information
THERMAL ANALYSIS
Thermal analysis techniques have been available for years
to analyze hydraulic reactions and the interactions of
cement with both mineral and chemical admixtures (Figs.
2-45 and 2-49). Traditionally, thermal analysis was not part
of a routine test program for cement. However, thermal
analysis has gained recent popularity in analyzing chemi-
cal and physical properties of cementitious materials and
raw materials for cement manufacture (
Bhatty 1993
,
Shkolnik and Miller 1996
, and
Tennis 1997
).
Thermal analysis involves heating a small sample at a
controlled rate to high temperature (up to 1000°C or more).
As compounds react or decompose, changes that occur as
a function of time or temperature are recorded. As the
sample's temperature changes, various reactions occur that
cause a change in the sample's weight, temperature,
energy, or state. A solid can melt, vaporize, decompose to a
gas with a residual solid, or react with a gas (at elevated
temperatures) to form a different solid or a different solid
and another gas.
Typical uses for thermal analysis include:
• identifying which hydration products have formed
and in what quantities
• solving early stiffening problems
• identifying impurities in raw materials
• determining the amount of weathering in clinker or
cement
• estimating the reactivity of pozzolans and slags for use
in blended cements
• identifying the organic matter content and variations
in a quarry
• quantifying the amount of carbonation that has taken
place in an exposed sample
• analyzing durability problems with concrete.
Fig. 2-47. Density of cement can be determined by (left) using
a Le Chatelier flask and kerosine or by (right) using a helium
pycnometer. (68825, 68826)
determined by dividing the cement density by the density
of water at 4°C. The density of water at 4°C is 1.0 Mg/m
3
(1.0 g/cm
3
or 1000 kg/m
3
).
A relative density of 3.15 is assumed for portland ce-
ment in volumetric computations of concrete mix propor-
tioning. However, as mix proportions list quantities of
concrete ingredients in kilograms or pounds, the relative
density must be multiplied by the density of water at 4°C
stated as 1000 kg/m
3
(62.4 lb/ft
3
) to determine the particle
density in kg/m
3
or lb/ft
3
. This product is then divided
into the mass or weight of cement to determine the
absolute volume of cement in cubic meters or cubic feet.
Bulk Density
The bulk density of cement is defined as the mass of
cement particles plus air between particles per unit
volume. The bulk density of cement can vary considerably
depending on how it is handled and stored. Portland
cement that is fluffed up may weigh only 830 kg/m
3
(52 pcf), where-
as when consoli-
dated by vibra-
tion, the same
cement can weigh
as much as 1650
kg/m
3
(103 pcf)
(
Toler 1963
). For
this reason, good
practice dictates
that cement must
be weighed for
each batch of con-
crete produced, as
opposed to use of
a volumetric mea-
sure (Fig. 2-48).
Specific thermal analysis techniques are discussed
below.
Fig. 2-48. Both 500-mL beakers contain
500 grams of dry powdered cement. On
the left, cement was simply poured into
the beaker. On the right, cement was
slightly vibrated—imitating consolida-
tion during transport or packing while
stored in a silo. The 20% difference in
bulk volume demonstrates the need to
measure cement by mass instead of
volume for batching concrete. (68970)
Fig. 2-49. Thermal analysis equipment. (69116)
