PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 208

The C - S - H phase is initially formed. C 3 A
forms a gel fastest.

Water

C 3 S

C 2 S
C 3 A
C 4 AF

(a)

The volume of cement grain decreases as
a gel forms at the surface. Cement grains
are still able to move independently, but as
hydration grows, weak interlocking begins.
Part of the cement is in a thixotropic state;
vibration can break the weak bonds.

(b)

The initial set occurs with the development
of a weak skeleton in which cement grains
are held in place.

(c)

Final set occurs as the skeleton becomes
rigid, cement particles are locked in place,
and spacing between cement grains
increases due to the volume reduction of
the grains.

(d)

Spaces between the cement grains are
filled with hydration products as cement
paste develops strength and durability.

(e)

FIGURE 6.3 Development of structure in the cement paste: (a) ini-
tial C-S-H phase, (b) forming of gels, (c) initial set--development of weak
skeleton, (d) final set--development of rigid skeleton, (e) hardening
(Hover and Phillco 1990).

Tw o types of voids are formed during hydration: the interlayer hydration
space and capillary voids.
Interlayer hydration space occurs between the layers in the C-S-H. The
space thickness is between 0.5 nm and 2.5 nm, which is too small to affect
the strength. It can, however, contribute 28% to the porosity of the paste.
Water in the interparticle space is strongly held by hydrogen bonds, but