PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 205

6.5 Hydration of Portland Cement

Hydration is the chemical reaction between the cement particles and water.
The features of this reaction are the change in matter, the change in energy
level, and the rate of reaction. The primary chemical reactions are shown in
Table 6.2. Since portland cement is composed of several compounds, many
reactions are occurring concurrently.
The hydration process occurs through two mechanisms: through-solution
and topochemical. The through-solution process involves the following steps
(Mehta and Monteiro 1993):

1. dissolution of anhydrous compounds into constituents
2. formation of hydrates in solution
3. precipitation of hydrates from the supersaturated solution

The through-solution mechanism dominates the early stages of hydration.
Topochemical hydration is a solid-state chemical reaction occurring at the
surface of the cement particles.
The aluminates hydrate much faster than the silicates. The reaction of
tricalcium aluminate with water is immediate and liberates large amounts of
heat. Gypsum is used to slow down the rate of aluminate hydration. The
gypsum goes into the solution quickly, producing sulfate ions that suppress
the solubility of the aluminates. The balance of aluminate to sulfate deter-
mines the rate of setting (solidification). Cement paste that sets at a normal
rate requires low concentrations of both aluminate and sulfate ions. The ce-
ment paste will remain workable for about 45 minutes; thereafter, the paste
starts to stiffen as crystals displace the water in the pores. The paste begins
to solidify within 2 to 4 hours after the water is added to the cement. If there
is an excess of both aluminate and sulfate ions, the workability stage may
only last for 10 minutes and setting may occur in 1 to 2 hours. If the avail-
ability of aluminate ions is high, and sulfates are low, either a quick set (10 to
45 minutes) or flash set (less than 10 minutes) can occur. Finally, if the alu-
minate ions availability is low and the sulfate ions availability is high, the
gypsum can recrystalize in the pores within 10 minutes, producing a flash
set. Flash set is associated with large heat evolution and poor ultimate
strength (Mehta and Monteiro 1993).
Calcium silicates combine with water to form calcium-silicate-hydrate,
C-S-H. The crystals begin to form a few hours after the water and cement are
mixed and can be developed continuously as long as there are unreacted ce-
ment particles and free water. C-S-H is not a well-defined compound. The
calcium-to-silicate ratio varies between 1.5 and 2.0, and the structurally
combined water content is more variable.
As shown in Table 6.2, the silicate hydration produces both C-S-H and
calcium hydroxide. Complete hydration of produces 61% C-S-H and 39%
calcium hydroxide; hydration of results in 82% C-S-H and 18% calcium
hydroxide. Since C-S-H is what makes the hydrated cement paste strong, the

C 3 S

C 2 S