Page 84 - Handbook of Thermal Analysis of Construction Materials

66 Chapter 2 - Introduction to Portland Cement Concrete

with hydrated cement and concrete. As a result of the reaction of carbon
dioxide, the alkalinity of concrete can be progressively reduced, resulting
in a pH value below 10.
The process of carbonation of concrete may be considered to take
place in stages. Initially, CO diffusion into the pores takes place, followed
2
by dissolution in the pore solution. Reaction with the very soluble alkali
metal hydroxide probably takes place first reducing the pH and allowing
more Ca (OH) into the solution. The reaction of Ca(OH) with CO takes
2
2
2
place by first forming Ca(HCO ) and finally CaCO . The product precipi-
3 2
3
tates on the walls and in crevices of the pores. This reduction in pH also
leads to the eventual breakdown of the other hydration products, such as the
aluminates, C-S-H gel, and sulfoaluminates. Generally, it is found that low
w/c ratio, good compaction, and proper curing cause significant improve-
ments in concrete permeability and resistance to carbonation.

Several workers have concluded that carbonation depth is propor-
tional to the square root of time. The proportionality constant is a coefficient
related to the permeability of the concrete. Factors, such as w/c ratio,
cement content in concrete, CO concentration in the atmosphere, and the
2
relative humidity, in addition to normal factors such as concrete density,
affect the value of this coefficient.

18.0 DELAYED/SECONDARY ETTRINGITE
FORMATION

The potential for concrete deterioration as a consequence of the
delayed ettringite formation in the precast industry has recently been
recognized. One of the important factors required for this type of reaction
is high temperature curing of concrete such as that occurring in the precast
industry. The delayed formation of ettringite is attributed to the transforma-
tion of monosulfoaluminate to ettringite when steam curing is followed by
normal curing at later ages. In recent work, it was indicated that sulfate may
be bound by the C-S-H gel that is released at later ages. Increased
temperature is expected to accelerate the absorption of sulfate by the silicate
hydrate. It has also been confirmed that the ettringite crystals are usually
present in cracks, voids, and the transition zone at the aggregate-binder
interface, causing expansion and cracking. Further, ASTM Type III cement
is more vulnerable to deterioration due to the delayed ettringite formation