Conversion of ettringite

In the hydration of C3A in the presence of gypsum and Na lignosulfonate (NLS), the conduction calorimetric curve indicates that, in the absence of lignosulfonate, ettringite is formed rapidly and converted to the low sulfoaluminate form. In the presence of NLS, the conversion of ettringite to the low sulfoaluminate form is retarded.1 1... 

Thermal investigations have shown that the rate of hydration of C3A and the inter-conversion of ettringite to the low monosulfoaluminate in the C3A-CaS04 2H20-CLS-H20 system, and hydration of individual cement phases C4AF, C3S, and C2S are all retarded by lignosulfonates. Thus, it is expected that the rate of hydration of cement should also be retarded by the addition of lignosulfonates. 

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. 

A high early strength calcium sulfoaluminate cement can be formed from the raw materials slag, clay, fly ash, silica, etc.I Hydration of the system C4 A3S-CS-C4AF was carried out by DTA to investigate the rate of conversion of ettringite (high calcium sulfoaluminate phase) to the monosulfoaluminate phase. 

The subsequent conversion of the ettringite to monosulfate by reaction with C3A is also accelerated by TEA. 

Many cements show a shoulder or more definite peak (3) at about 16h. This has often been associated with the conversion of AFt into AFm phase, but comparison with the microstructural evidence shows that this is incorrect (P31) it is associated with the renewed formation of ettringite. A further, less distinct shoulder (4) has been associated with hydration of the... 

In cements in which the calcium sulfate content has been too low for complete conversion of all the AI2O3 present in the original cement to ettringite, a formation of the AFm phase may get under way in a reaction between ettringite and AH3, after all the calcium sulfate has been used up ... 

If a hardened ordinary Portland cement paste is in contact with a solution that contains ions in combination with Na or K, two principal reactions take place, a conversion of the monosulfate into ettringite and the formation of gypsum ... 

The mechanism of the sulfate attack on Portland-slag cement is broadly similar to that observed in pure Portland cement pastes. In an attack by alkali sulfates the dominant reactions are the partial decalcification of the C-S-H phase and the conversion of the existing monosulfate to ettringite. The amount of ettringite formed is lower than in the case of Portland cement, as monosulfate is the only phase that can serve as a direct source for ettringite formation, whereas neither Al incorporated in the C-S-H phase nor that present in hydrotalcite are available for such a reaction, jnst like Al in ettringite already formed independently of snUate attack (Gollop and Taylor, 1992-1996). 

The conversion of the concrete into gypsum and ettringite reduces the mechanical strength of the concrete, which is followed by a reduction of the structural integrity of the concrete and may result in total failure and collapse of the structure. 

Ramachandran followed the hydration of CjA (with and without gypsum) containing triethanolamine. It was found that TEA accelerated the hydration of C3A to the hexagonal aluminate hydrate and its conversion to the cubic aluminate hydrate. The formation of ettringite was also accelerated in the C3A-g q)sum-H20 system. 

Massazza and Costat l applied the DSC, XRD, and conduction calorimetric techniques to investigate the effect of a SNF type of superplasticizer on the hydration of CjA-gypsum mixtures. The results indicated that SNF retards the formation of ettringite and its conversion to the monosulfate form. 

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