hydration of c3s and c2s

Hello,

In Cement Chemistry by Taylor (table 7.5), the heat of full hydration of c3s and c2s are given as respectively 517 kJ/kg(c3s) and 262 kJ/kg(c2s).

I would be interrested to understand the physical reason for this rather large difference.

Thanks,

Michel

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Re: hydration of c3s and c2s

lalbatros:

In Cement Chemistry by Taylor (table 7.5), the heat of full hydration of c3s and c2s are given as respectively 517 kJ/kg(c3s) and 262 kJ/kg(c2s).

I would be interrested to understand the physical reason for this rather large difference.

Hello Michel,

Very basically, this difference is due to the fact that C3S is formed from C2S plus an additional mole of CaO. During the hydration of C3S this extra mole of CaO must also effectively hydrate, releasing heat. (1135.5kJ/kg CaO.)

More specifically, if the hydration products of C3S and C2S are the same, (except of course for the quantities of Ca(OH)2 formed), the heat of hydration of C3S should equal the sum of the heats of reaction for (1) the formation of C3S from C2S and CaO, (2) the heat of hydration of C2S and (3) the heat of hydration of CaO. eg;-

C2S + CaO    -->  C3S    +  13.4 kJ/mole
C2S + 2H2O ---> CaO.SiO2.H2O + Ca(OH)2  + 45 kJ/mole
CaO + H2O   -->  Ca(OH)2  + 63.7 kJ/mole

Therefore, theoretically, the total heat of hydration for C3S is (13.4 + 45 + 63.7) = 122.1 kJ/mole or 534.6kJ/kg C3S, which agrees well with the experimentally obtained values which incidentally range from 504 to 530 kJ/kg C3S.

Regards,

Ted.

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