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The early and middle periods

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 [Pg.226]

Concentrations of SiOj and AljO, are low throughout, values of 0.03-0.05 mmol 1 for Si and below 5 ppm for A1 having been reported for times from 20 s to 45 min (T38). [Pg.228]

Any more detailed interpretation demands that both activity coefficients and the existence of complex species be taken into account (B90,G58). Gartner el til. (G58) showed that, for most cements, the solution is saturated in CH within 12 min and that the saturation factor (defined as the activity product divided by its value at saturation) reaches a maximum of between 2 and 3 within 2h. For gypsum, saturation is reached within 6 min, but the saturation factor never exceeded 1.3 in the cases studied. With cements high in K,0, the solution can become saturated in syngenite. Gartner el id. found that the and Na concentrations increased rapidly during the first 12 min and only slowly, or not at all, during the subsequent period up to 3 h. [Pg.228]

Other methods that have been used to follow the reactions in cement pastes during the first day include measurements of AC electrical conductivity (T39,A14) and dielectric constant (A14). [Pg.228]

The evidence from microstructure, calorimetry and other sources suggests that the hydration processes of cement and C3S are essentially similar. There are important differences in the nature of the early product and in where the C-S-H formed in the middle stage of reaction begins to deposit, but in both cases it would appear that the early reaction slows down because of the deposition of a layer of product, which either isolates parts of the anhydrous surfaces from the main solution or allows the concentrations close to those surfaces to rise to values approaching the theoretical solubilities of the anhydrous compounds. In both cases, the initiation of the main reaction and the kinetics in its acceleratory phase appear to be controlled by the nucleation and growth of C-S-H. [Pg.228]


See other pages where The early and middle periods is mentioned: [Pg.226]    [Pg.227]    [Pg.234]    [Pg.136]   


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Middle

Middlings

The Early Period

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