Page 89 - Handbook of Thermal Analysis of Construction Materials

72 Chapter 3 - Formation and Hydration

A substance subjected to thermal treatment may undergo physical
or chemical changes as in dimension, magnetic susceptibility, weight,
crystalline transition, mechanical property, acoustic property, and heat
effects, etc. In thermal analysis these changes are followed as a function of
temperature. It has been suggested that thermal analysis should also be
extended to allow for rapid heating of the sample to some temperature
followed by a measurement of the property with time under an isothermal
condition. In the quasi-static thermal analysis method, a substance is heated
at known intervals of temperature for a few hours and a particular property
is measured. In the dynamic method, such as DTA and TG the property of
a material is followed by continuous heating at a uniform rate.
The detailed description of DTA, TG, DSC, and Conduction
Calorimetric Techniques has been presented in Ch. 1. A very brief account
of these techniques is given below.
In DTA the difference in temperature ∆T between the sample and
a reference material such as α-Al O is recorded while both are subjected
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2
to the same heating program. Generally, the temperature (x-axis) is plotted
against the ∆T on the y-axis. The exothermal effects are shown upward and
the endothermal effects downward with respect to the base line. In the DTA
of cementitious materials, thermal effects are reported in terms of the
characteristic temperature, peak temperature, temperature range of the
peak, peak width, peak amplitude or height, and peak area. By determining
the nature of the peak (endothermic or exothermic), the temperature of the
characteristic peak, and other general characteristics, it is possible to utilize
DTA for both qualitative and quantitative purposes. By heating the binary
or ternary mixtures in the DTA apparatus, the sequence of reactions during
heating may also be followed. Thermal methods are used for identifying
new compounds, estimating the products, formulating the mechanism of
reactions, synthesis of compounds, durability assessment, trouble shooting,
and quality control, etc. These methods are also used in conjunction with
other physical, chemical, and mechanical techniques. Many factors such as
the type and size of sample holder, furnace, thermocouple, rate of heating,
sensitivity of the recording system, degree of dryness of the sample, the
amount of sample, particle size and crystallinity, packing density, thermal
conductivity, and shrinkage or swelling of the sample, will affect the
results. The usefulness of DTA is further enhanced with the development
of multipurpose types of equipment which incorporate one or more types of
adjunct techniques to DTA. Examples are: DTA-Effluent Gas Analyzer,
DTA-Mass Spectrometer, DTA-DTG-TG, DTA-TG-Radioactive Emis-
sion, DTA-TG-Dilatometer, DTA-XRD (X-Ray Diffraction), etc.