The Effects of Alkali Sulfate on the Hydration of a C(3)A-CaSO4 center dot 2H(2)O System

The hydration mechanism of the 3CaO center dot Al2O3-CaSO4 center dot 2H(2)O system in the presence of alkali sulfates has been investigated. The early hydration rate of 3CaO center dot Al2O3 was accelerated by the addition of Na2SO4 and K2SO4. This is closely related to the formation of syngenite ( CaO center dot K2SO4 center dot H2O ), and the U-phase added K2SO4 and NaSO in the 3CaO center dot Al2O3-CaSO4 center dot 2H(2)O system, respectively. The formation of the rigid syngenite and U-phase structure led to rapid setting and decreases the sulfate content in the liquid phase of the hydrating cement to the extent that it cannot adequately retard the hydration of 3CaO center dot Al2O3. In case of the alkali sulfate not added to the 3CaO center dot Al2O3-CaSO4 center dot 2H(2)O system, the ettringite was transformed to monosulfoaluminate immediately after the consumption of gypsum. However, when the alkali sulfates were added to this system, the ettringite did not transform to monosulfoaluminate immediately even though the gypsum was completely consumed. There was a stagnation period to transform to the monosufoaluminate after the consumption of gypsum because the syngenite and U-phase remained as the sulfate source.