Page 139 - Handbook of Thermal Analysis of Construction Materials

122 Chapter 3 - Formation and Hydration

The application of DTA to various binary, ternary, and quaternary
systems of relevance to cement chemistry has been discussed in a book by
Ramachandran. [1]

9.0 DURABILITY ASPECTS

Concrete may deteriorate if adequate precautions are not exercised
to protect it from adverse effects that could result from exposure to natural
or artificial conditions. Several physical, chemical, and electrochemical
processes are known to induce cracking of concrete. Concrete can have
durability problems as a consequence of its exposure to sea water, sulfates,
chlorides, freeze-thaw action, carbon dioxide, etc., or when it is attacked by
artificially induced processes such as exposure to acids and salts in
chemical plants or to fire. In recent years, a new type of durability problem
was encountered that involved use of steam cured concrete products. The
distress was caused by the formation of delayed ettringite. If the raw
materials in concrete are not carefully controlled, there may be an eventual
failure of concrete elements, e.g., the presence of excess alkali in concrete
that promotes alkali-aggregate expansion reaction, harmful impurities in
the aggregates, or the presence of excess amounts of dead-burnt MgO.
Thermal techniques in combination with others have been employed with
success to examine the raw materials as well as the failed concrete. The
knowledge gained from such work has been applied to produce more
durable concrete.

9.1 Aggregates

Some organic and inorganic compounds present in small amounts
in aggregates may affect concrete strength and durability. In Fig. 37, the
thermograms of some of the harmful impurities that may be present in
[1]
concrete are given. Pyrite exhibits two exothermic peaks between 400
and 500°C caused by oxidation. Gypsum shows two characteristic effects
at 180 and 220°C for dehydration effects. Montmorillonitic clay mineral is
characterized by three endothermal effects at 160, 660, and 900°C due
respectively to the release of interlayer, dehydroxylation and destruction of
lattice. An exothermal effect at about 950°C is attributed to the formation
of a spinel. Humus gives strong exothermal peaks over a wide range of