Page 151 - Handbook of Thermal Analysis of Construction Materials

134 Chapter 3 - Formation and Hydration

presence of C AH was indicated by a series of inflections below 300°C.
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4
Calcium carbonate exhibited an endotherm at about 800°C. Ettringite,
Ca(OH) , and α-quartz gave endothermal effects at 140, 500, and 573°C,
2
respectively. Ettringite was absent in a severely carbonated concrete.
Solubility of Friedel’s salt increased with the degree of carbonation.
Thermal peak values of calcium hydroxide and calcium carbonate were
found to be ideally suited for estimating the degree of carbonation in
concrete elements.

9.6 Chemical Attack

Exposure of concrete to natural elements or to industrial chemicals
may result in its deterioration. In many chemical industries the chemicals
produced may react with concrete that is used as a construction material. For
example, the influence of formic acid on concrete was studied by applying
thermal techniques. [60] DTA curves of samples showed that as the exposure
of concrete to the acid is increased, the peaks due to lime and calcium
carbonate are decreased. This gave an indication of the severity of attack by
acids. Another example pertains to a fertilizer plant. Ammonium nitrate
fertilizer contained 75% NH NO and 25% CaCO as a filler. Concrete used
4
3
3
in this plant was attacked by nitrate. The lime was found converted to
calcium nitrate. Also there was a reaction between calcium aluminate
hydrate and ammonium nitrate resulting in the formation of calcium nitrate
and calcium aluminate nitrate complex. These compounds could be iden-
tified by endothermal effects appearing in DTA. [89]
Another example is related to two damaged chimneys at a power
[5]
station. Wesolowski, applying thermal techniques, found that in addition
to the sulfate attack there was evidence of incomplete hydration of cement
due to the heating effects at the early curing periods. The chemical
corrosion potential cement mortars subjected to organic chemicals can be
studied by thermal methods. The interaction between phenols and lime in
cement were established through the application of DTA/TG by Kovacs. [5]
In many countries, de-icing salts consisting of calcium chloride and
sodium chloride are applied on roads in winter. Such concrete roads are
vulnerable to attack by these chemicals. A series of mortars containing
mineral admixtures was exposed to 30% calcium chloride solution
at 5–40°C. [90] Mortars containing mineral admixtures were found to be
effective in reducing the chloride penetration. XRD and DTA indicated that
the deterioration was primarily due both to the dissolution of lime and