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In the production of concrete from cement powder and water, setting behavior of the slurry is determined by the formation of ettringite (Ca 6 Al 2 (OH) 12  · (SO 4 ) 3  · 26 H 2 O) from tricalcium aluminate (Ca 9 Al 6 O 18 , abbreviated C 3 A) and gypsum (CaSO 4  · 2 H 2 O). Due to the high reaction potential of cement and water, premature hydration can occur after unintentional exposure to moisture. Model binary mixtures of C 3 A and gypsum stored at 90% relative humidity and 35 °C produced ample amounts of ettringite, which subsequently reacted with atmospheric CO 2  to CaCO 3 , Al(OH) 3  and gypsum. Investigated were the two main polymorphs of tricalcium aluminate encountered in cement, pure, cubic C 3 A and orthorhombic C 3 A in which calcium is partially substituted by sodium or potassium. Alkali substituted C 3 A converted to ettringite faster and more completely than pure C 3 A. Ettringite from prehydration caused a seeding effect, which promotes crystal growth and accelerates bulk hydration of the C 3 A/gypsum mixtures. Set retarders commonly applied in cement were dissolved in the mixing water prior to hydration to investigate their ability to counteract this acceleration. Sodium gluconate merely delayed the crystal growth but does not prolong the hydration process overall. Potassium pyrophosphate retarded much more effectively by suppressing the seeding effect via removal of calcium ions from the hydration reaction.

Impact of aging on the hydration of tricalcium aluminate (C 3 A)/gypsum blends and the effectiveness of retarding admixtures