ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 2, pp. 182!185. +
Pleiades Publishing, Inc., 2006.
Original Russian Text + Yu. P. Gladkikh, V. I. Zavrazhina, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 2, pp. 184!187.
AND INDUSTRIAL INORGANIC CHEMISTRY
Hydroxylation of the Surface of Calcium Sulfates
in Aqueous Media and Processes of Their Solidification
Yu. P. Gladkikh and V. I. Zavrazhina
Shukhov State Technological University, Belgorod, Russia
Received January 26, 2005
Abstract-Mechanisms of hydroxylation of the surface of calcium sulfates and their formulations in aqueous
media and processes of their solidification are considered.
Various calcium sulfates, such as calcium sulfate
O, calcium sulfate dihy-
O, anhydrite CaSO
, estrich gypsum
. CaO, and formulations with a number of
inorganic and organic compounds, are gypsum bind-
ers, which are widely used in building.
The chemistry of calcium sulfate hemihydrate is
the best studied. It is known  that the structure of
the gypsum stone is formed in aqueous media through
dissolution of the binder to give calcium and sulfate
ions and then calcium sulfate dihydrate, which is less
water soluble than calcium sulfate hemihydrate.
O 6 CaSO
According to other data , the solidification of
gypsum binders occurs by the topochemical mech-
anism, i.e., water is added directly to solid particles.
The solidification reaction in a solidifying mixture of
calcium hemihydrate sulfate with water is described
by the following scheme:
O + 1.5H
O 6 CaSO
Thus, there are different explanations of the solid-
ification of gypsum binders in terms of the solution
and topochemical mechanisms. However, there are
examples  of solidification of calcium sulfate
dihydrate and its formulations with various sub-
stances, which cannot be understood in terms of re-
actions (1) and (2). This indicates that these binders
harden by a different mechanism. No other concepts
of the solidification of calcium sulfate dihydrate have
been formulated. Furthermore, it is impossible to ex-
plain satisfactorily the role of some additives of or-
ganic compounds in the solidification of gypsum bind-
ers and the mechanism of their action on the basis of
the available concepts of the processes of solidifica-
tion of gypsum binders.
Thus, despite the large body of theoretical and ex-
perimental data on the solidification of gypsum bind-
ers, the existing mechanisms require rethinking.
In this paper, we discuss the hydroxylation of
the surface of mixtures of calcium sulfates with water
and the condensation mechanism of their solidifica-
tion. The known procedures for production of gypsum
materials, including their preparation from calcium
sulfate dihydrate and mixtures of gypsum binders
with inorganic and organic additions are considered
in terms of this mechanism.
It is known [6, 7] that finely dispersed particles of
inorganic solid substances, including gypsum bind-
ers, obtained by a mechanical milling and thermal
treatment have free unsaturated valence bonds on
the surface and possess a free surface energy. When
water molecules are adsorbed on surface defects of
calcium sulfates these defects are hydrated to calcium
sulfate dihydrate and are hydroxylated similarly to
defects on the surface of mechanically activated sil-
icon dioxide .
When other particles of this kind, with surface de-
fects, are placed in a humid atmosphere or in water,
there occurs dissociative adsorption of water molec-
ules on electron-acceptor and electron-donor centers
on the surface, formed upon cleavage of crystals. For
example, a separate fragment of the surface of calcium