Page 42 - Handbook of Thermal Analysis of Construction Materials

26 Chapter 1 - Thermoanalytical Techniques

3.4 Conduction Calorimetry

The reactions of various types of cements and their components
with water is an exothermic process. The intensity of heat liberated with
time depends on the type of chemical, surface area, reactivity, etc. Measure-
ment of the total heat and rate of heat development provides information on
the kinetics of hydration, degree of hydration, mechanism of hydration, the
effect of additives, setting phenomenon, etc. Conduction calorimetry finds
extensive applications in concrete technology.
Development. The first conduction calorimeter was developed in
1923. [68] Subsequently, Carson applied this technique to the investigation
of cement hydration. [69] A systematic investigation of the effect of gypsum
on heat evolved during the hydration of cement was carried out by Lerch in
1946. [70] Important conclusions were drawn by Stein who applied the
conduction calorimetric technique to studying the effect of organic and
inorganic additions on cement hydration. [71] A highly sensitive conduction
calorimeter known as a Wexham calorimeter was developed by Forrester
to investigate cement hydration. [72] Bensted [73] applied a Setaram heat flux
calorimeter to a study of oil well cements. This equipment permitted in situ
mixing of cement with water and enabled recording of the initial reactions
that occur soon after water comes into contact with the cement. In this
calorimetric technique, the mixing cell withstands internally generated
pressures up to 1.03 MPa (150 psi). Experiments could be carried out at
temperatures up to 180°C. A calorimeter containing six cells has also been
developed. This equipment consists of a water tight block with six cham-
bers, a constant temperature bath, a thermopile, and a recording system.
Each chamber in the block contains a teflon coated sample holder into
which a known amount of sample and water are mixed. The heat developed
from the instant water comes into contact with the sample is registered by
injecting water through a syringe. The heat given off by the reaction at
different times is carried by thermopiles, and the signals are amplified and
registered by a computer system.
Applications. Conduction calorimetry has been widely used for a
study of the hydration reactions of various cementitious systems. Tricalcium
silicate, being the dominant compound in portland cement, determines to a
large extent the strength and other properties of concrete. Conduction
calorimetric curves of tricalcium silicate and portland cement show five
steps during the hydration process (Fig. 10). [74] In the first stage, as soon as
the silicate or cement comes into contact with water, Ca and OH ions are