w
2
2w2g
Es2g ISIST ei2yp2rhesyx
The reaction of compound C 3 A with water is very fast and is responsible for flash setting of
cement (stiffening without strength development) and thus it will prevent the hydration of C 3 S
and C 2 S. However, calcium sulphate (CaSO 4 ) present in the clinker dissolves immediately in
water and forms insoluble calcium sulphoaluminate. It deposits on the surface of C 3 A forming
a colloidal membrane and consequently retards the hydration of C 3 A. The amount of CaSO 4 is
adjusted to leave a little excess of C 3 A to hydrate directly. This membrane in the process breaks
because of the pressure of the compounds formed during hydration and then again C 3 A
becomes active in the reaction.
The hardening of C 3 S can be said to be catalyzed by C 3 A and C 3 S becomes solely responsible
for gain of strength up to 28 days by growth and interlocking of C-S-H gel. The increase in
strength at later age is due to hydration of C 2 S.
xX I 2"
2
2 2
2 2
2
2
2
2 2(
22
2
(
2p
2SI
P 22 22
22 22"
22
22 22
22
22022
22
2222
2
(
22
2 2
(
2
2
X
gQeDgQD2gRepD2gP
Q 22 22
2
2
2
222
2
2
2
2 2r
("D2
2
"
"2
2 2 22 2
2
2
2 2
2
2
2
(2
2p
2SP
2(
22
2 2
(
2
2
X
gRepD2gQeD2gQD2gP
SU ei2i siwix2py2rhesyx
About an average 23 per cent (24 per cent C 3 S, 21 per cent C 2 S) of water by weight of cement
is required for complete hydration of Portland cement. This water combines chemically with
the cement compounds and is known as bound water. Some quantity of water, about 15 per cent
by weight of cement, is required to fill the cement gel pores and is known as gel water.
Therefore, a total of 38 per cent of water by weight of cement is required to complete the
p 2S I g!22g 2g!
2
22g p 2S P 22r&22!
g 2g!v2D2