TRICALCIUM ALUMINATE-GYPSUM SYSTEM

The rate of hydration of C4AF is retarded in the presence of superplasticizers.In an investigation involving conduction calorimetry, Gagne found that, in the presence of SNF and SNF + SO/ , the hydration of tetracalcium aluminoferrite was retarded during the 60 minutes of hydration. 

Monocalcium aluminate, a major component of high alumina cement, hydrates to metastable products CAHjo, C4AHj3, and C2AHg which eventually convert to the stable cubic aluminum hydrate, CjAH. In the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA results have shown that the degree of conversion of CAHjo and C2AHg phases to the cubic phase is marginally retarded. 

The role of the superplasticizer on the rate of hydration of CjA-gypsum mixtures is not clear. All the possibilities, viz., retardation, acceleration, and ineffectiveness have been reported. The variation in results reported from different sources may be due to the differing conditions of hydration such as the w/s ratio, the type of superplasticizer, its molecular weight, the type of cation associated with it, the proportion and particle size of the C3A and gypsum, the type of sulfate, and the temperature. There is, however, general agreement that the conversion of ettringite to monosulfate is retarded in the presence of a superplasticizer. 

The effect of 0, 1,2, and 4% SMF on the hydration of C3A + 25% gypsum may be followed by conduction calorimetry (Fig. 4).Iri An exothermic peak appears at 20 hours with 1 % SMF, at about 23 hours with 2% SMF, and at about 15 hours with 4% SMF. A continuous hump is registered for the sample containing no admixture. The retardation effect of C3A-ettringite reaction to produce low sulfoaluminate may be caused by the adsorption of SMF on the hydrating C3A surface. 

Slanickal used XRD and DTA techniques to follow the hydration of C3A-gypsum mixtures containing 1-4% SMF. The mixture was hydrated for 2 hours to 24 hours, and the products were estimated by DTA and XRD. Table 1 shows the amounts of ettringite and monosulfate hydrate formed at different times of hydration in the presence of 4% SMF. It is obvious that the formation of ettringite is almost nil at 2 hours in the 

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Ramachandran, V. S., Adsorption and Hydration Behavior of Tricalcium Aluminate-Water and Tricalcium Aluminate-Gypsum-Water Systems in the Presence of Superplasticizers, J. Am. Concr. Inst., 80 235-241 (1983)... 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO and the tricalcium aluminate keeps the SO level near zero concentration for the first 10-20 min causing rapid stiffening and quick set (approximately 15 min). It is also reported that chemical... 

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