Page 148 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Durability Aspects 131

In addition to the physical effects that cause frost damage, the
possibility of chemical changes playing a role has been considered by
Ludwig and Stark. [78] The influence of low temperatures (freeze-thaw
cycles) on the rate of formation of AFm and AFt phases in cements
containing low C A (1.7%) or high C A (10.7%) was examined by DTG.
3
3
At 150 freeze-thaw cycles, cement with a low C A content was found to
3
contain 53% AFm phase and 17% AFt phase and the corresponding figures
for the C A rich cement were 34% and 38%, respectively. It is not clear,
3
however, the extent to which the rate and amount of formation of these
sulfoaluminates influence the damage due to freezing and thawing process.

9.5 Carbonation

Carbonation involves the reaction of CO with the hydrated cement
2
components. This will result in shrinkage. In addition, carbonation also
decreases the pH of the system making the reinforcing bars more prone to
corrosion. It is not easy to assess the extent to which the C-S-H phase
undergoes carbonation and shrinkage on exposure to CO because it is not
2
easy to remove CH from the paste without affecting the C-S-H phase. In a
method involving continuous leaching and monitoring of the lime remain-
ing in a hydrated tricalcium silicate paste by DTA, the time to terminate
extraction was determined. Ramachandran [38] was, thus, able to prepare a
lime-free C-S-H. The resultant C-S-H exhibited a much higher shrinkage
than calcium hydroxide (Fig. 42). [84]
The carbonation effect on synthesized tobermorite has been exam-
ined by XRD / DTA. [83] An endothermal effect at about 180°C indicated the
dehydration of tobermorite and an exothermal effect at about 800°C was
caused by the formation of wollastonite. The carbonated samples exhibited
a broad endothermal effect in the range of 450–600°C and a sharp peak at
about 600–650°C. The former indicated the presence of vaterite and the
latter, calcite. The carbonation decreased the peak due to wollastonite.
Thermal methods can also be used to estimate the amount of carbonation
that has occurred at different depths in concrete. These techniques estimate
the amount of calcium carbonate formed and also the amount of calcium
hydroxide remaining in an uncarbonated form.