PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 207

ultimate strength of the concrete is enhanced by increasing the content of
relative to the amount of Furthermore, calcium hydroxide is sus-
ceptible to attack by sulfate and acidic waters. Increasing the proportion of
relative to reduces the quantity of calcium hydroxide and, therefore,
improves the durability of the concrete.
hydrates more rapidly than contributing to the final set time
and early strength gain of the cement paste. The rate of hydration is accel-
erated by sulfate ions in solution. Thus, a secondary effect of the addition of
gypsum to cement is to increase the rate of development of the C-S-H.

C 2 S C 3 S.

C 2 S C 3 S

C 3 S C 2 S,

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6.5.1 Structure Development in Cement Paste

The sequential development of the structure in a cement paste is summarized
in Figure 6.3. The process begins immediately after water is added to the ce-
ment [Figure 6.3(a)]. In less than 10 minutes, the water becomes highly alka-
line. As the cement particles hydrate, the volume of the cement particle
reduces, increasing the space between the particles. During the early stages
of hydration, weak bonds can form, particularly from the hydrated
[Figure 6.3(b)]. Further hydration stiffens the mix and begins locking the
structure of the material in place [Figure 6.3(c)]. Final set occurs when the C-
S-H phase has developed a rigid structure, all components of the paste lock
into place and the spacing between grains increases as the grains are con-
sumed by hydration [Figure 6.3(d)]. The cement paste continues hardening
and gains strength as hydration continues [Figure 6.3(e)]. Hardening devel-
ops rapidly at early ages and continues, as long as unhydrated cement parti-
cles and free water exist. However, the rate of hardening decreases with time.

C 3 A

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6.5.2 Evaluation of Hydration Progress

Several methods are available to evaluate the progress of cement hydration
in hardened concrete. These include measuring the following properties
(Neville 1981):

1. the heat of hydration
2. the amount of calcium hydroxide in the paste developed due to hydration
3. the specific gravity of the paste
4. the amount of chemically combined water
5. the amount of unhydrated cement paste using X-ray quantitative analysis
6. the strength of the hydrated paste, an indirect measurement

6.6 Voids in Hydrated Cement

Due to the random growth of the crystals and the different types of crystals,
voids are left in the paste structure as the cement hydrates. Concrete
strength, durability, and volume stability are greatly influenced by voids.