The influence of CaSO4-2H2O upon hydration character of 3CaOAl2O3

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Journal titleMagazine of Concrete Research
Pages185196; # of pages: 12
Subjectcement; hydration; gypsum; concrete admixtures; differential thermal analysis; Concrete
AbstractThis paper presents a quantitative interpretation of results of differential thermal analysis of hydration reactions at 2, 12, 23 and 52 degrees C of 3CaO.Al[2]O[3] mixed with 0, 0. 25, 2.5, 10 or 20% gypsum, and compares these with length changes obtained from the hydration of compacts of these mixtures. Estimates of the relative reaction rates from d.t. a. peak areas and measurement of length change during these reactions showed marked agreement, although interpretation had to be made with caution. It was concluded that the formation of ettringite (high sulpho-aluminate) had no direct effect upon reaction rate, but that the reactivity of 3CaO.Al[2]O[3] was reduced by the sorption of (SO[4])[2-] ions on active sites of its surface. Gypsum reduces the rate of conversion of the hexagonal hydro-aluminates to the cubic hexahydrate, probably also by sorption of (SO[4])[2-] ions, and thus is a second mechanism by which they can retard the normal hydration of 3CaO.Al[2]O[3]. The degree to which a given addition of gypsum retards the hydration of 3CaO.Al[2]O[3] at a particular temperature is affected by a balance between (1) the concentration of (SO[4])[2-] ions on and around the surface of the 3CaO.Al[2]O[3]; (2) the rate of reaction of (SO[4])[2-] ions with the hexagonal hydroaluminates; (3) the thickness of the hexagonal hydroaluminate layer around the 3CaO.Al[2]O[3] grain. Reactions at the various temperatures are discussed in terms of the above parameters.
Publication date
AffiliationNRC Institute for Research in Construction; National Research Council Canada
Peer reviewedYes
NRC number9535
NPARC number20375951
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Record identifier87277d69-7602-4d5a-b6cd-72047366a6cb
Record created2012-07-23
Record modified2016-05-09
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