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Metakaolin, AS2 (s), is an important pozzolanic additive for the
production of concrete. It can be prepared during water
decomposition by heating the clay mineral kaolinite, AS2H2 (s)
which at approx. 550 ° gives off crystal water. In
Mčedlov-Petrosyan: Thermodynamics of Silicates, the following
values are given for heat capacities, cp (AS2H2 (s)) = 247.4 J /
mol · K, cp (AS2 (s)) = 13.0 J / mol · K, and cp (H ( g)) = 34.9 J
/ mol · K. In addition, the following equations, (1), (2) and (3),
are given with values for standard reaction enthalpy, and an
alternative formation reaction for metakaolin is given in (4). Note
that cement chemical abbreviations are used in this
assignment.
(1) 3S3A2 (s) → 2A3S2 (s) + 5S (s) ΔH298⦵ = 226.4 kJ / mol
(2) 2AS2H2 (s) → 2AS2 (s) + 4H (g) ΔH298⦵ = 511.0 kJ / mol
(3) A3S2 (s) + 4S (s) + 6H (g) → 3AS2H2 (s) ΔH298⦵ = -449.5 kJ /
mol
(4) 2S3A2 (s) + 2S (s) → 4AS2 (s)
At 800 ° C the ΔG⦵ for reaction (c) is -1283.8 kJ / mol
Indicate the highest water vapor pressure which at 800 ° C
prevents reaction (3) from proceeding!
The answer needs to be in Pa