Page 324 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Silica Fume 301

problems. Other forms of silica that have potential for use in concrete are
synthetic silica, silica gel, colloidal silica, and fumed silica. Condensed
silica acts as a filler and is also a good pozzolan. The specific surface area
2
of silica fume may be as high as 20,000 m /kg, which is almost an order of
magnitude higher than that of normal portland cement. Fineness, amor-
phous nature, and high silica contents make silica fume a very reactive
material. Depending on the amount and nature of silica fume and w/c ratio,
the concrete strength at 28 days could reach a value of 100 MPa. The action
of silica fume on the hydration of cement and cement components can be
followed by DTA, DSC, TG, and conduction calorimetry. Variability in the
hydration rates of cement in the presence of silica fume reported by
different investigators is expected because silica fumes differ in their
physical and chemical nature. Most concretes made with silica fume
contain a superplasticizing admixture. Conduction calorimetric studies
indicate that all the stages of hydration in pure components of cement, such
as C S, C A, C A + CaSO , C A + CaSO + Ca(OH) , alite and the
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4
2
4
3
3
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interstitial phase, are accelerated with a few exceptions. [4]
Addition of even a small amount of active silica accelerates the
hydration of tricalcium silicate. [17]–[21] The effect of different percentages
of finely divided SiO (Aerosil) (S) on the hydration of tricalcium silicate
2
has been studied by conduction calorimetry (Fig. 5). [17] The addition of
even a small amount of silica accelerates the hydration of C S by lowering
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the time at which the exothermic peak appears. The peak appears earlier as
the dosage of SiO increases. The degree of hydration of tricalcium silicate
2
has been determined by estimating the amount of calcium hydroxide
formed at different times, using TG. Table 3 gives the amount of lime
formed at 3, 7, and 24 hours. [17] At all times, the amount of lime is higher
in samples containing SiO . Larger amounts of lime formed, especially at
2
higher dosages, are indicative of acceleration of C S.
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The C/S ratio of the C-S-H product decreases by the addition of
silica. It is reported that in the absence of SiO at a w/s ratio of 10, C S yields
2
3
a C-S-H product with a C/S ratio of 0.8 at 24 hours, and this ratio decreases
to 0.33, and 0.36 at w/s ratios of 1 and 0.4 respectively. [19] The acceleration
of early hydration of C S is ascribed to the fineness of the particles that
3
would provide preferential nucleation sites. The pozzolanic particles also
offer a surface for the precipitation of C-S-H from the pore solution. Thus,
the removal of the Ca and silicate ions would result in a reduction in the
thickness of the relatively impermeable C-S-H growing on the C S grains. [1]
3