Chemistry of cement:
Non-hydraulic cement such as slaked
limes (calcium hydroxide mixed with water), harden due to the reaction of carbonation
in presence of the carbon dioxide naturally present in the air. Calcium oxide
is produced by lime calcination at temperatures above 825 °C (1,517 °F) for
about 10 hours at atmospheric pressure:
CaCO3 → CaO + CO2
The calcium oxide is then spent
mixing it to water to make slaked lime:
CaO + H2O → Ca(OH)2
Once the water in excess from the
slaked lime is completely evaporated (this process is technically called setting),
the carbonation starts:
Ca(OH)2 + CO2 → CaCO3 + H2O
This reaction takes a significant
amount of time because the partial pressure of carbon dioxide in the air is
small. The reaction of carbonation requires the air be in contact with the dry
cement, hence, for this reason the slaked lime is a non-hydraulic cement and
cannot be used under water.
Conversely, the chemistry ruling the
action of the hydraulic cement is the hydration. Hydraulic cements (such as the
Portland cement) are made of a mixture of silicates and oxides, the four main
components being:
rotary Kiln
Belite (2CaO·SiO2);
Alite (3CaO·SiO2);
Celite (3CaO·Al2O3);
Brownmillerite (4CaO·Al2O3·Fe2O3).
The reactions during the setting of
the cement are:
(3CaO·Al2O3)2 + (x+8) H2O
→ 4 CaO·Al2O3·xH2O + 2 CaO·Al2O3·8H2O
(3CaO·Al2O3) + 12 H2O +
Ca(OH)2 → 4 CaO·Al2O3·13 H2O
(4CaO·Al2O3·Fe2O3)
+ 7 H2O → 3 CaO·Al2O3·6H2O + CaO·Fe2O3·H2O
And during the hardening (the
chemistry of the reaction of hydration is still not completely clear):
(3CaO·SiO2)2 + (x+3) H2O →
3 CaO2·SiO2·xH2O + 3 Ca(OH)2
(2CaO·SiO2)2 + (x+1) H2O →
3 CaO2·SiO2·xH2O + Ca(OH)2
The silicates are responsible of the
mechanical properties of the cement, the celite and the browmillerite are
essential to allow the formation of the liquid phase during the cooking. The
chemistry of the above listed reactions is not completely clear and is still
the object of research.
No comments:
Post a Comment