Page 51 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

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dissOlve and the .silica fume particles will react with at 55°C and 90°C The pastes had a water/cement
CH liberated from the hydration of the Portland ratio = 0.50. The paste hydration was performed in
cement’s calcium silicates, to form additional closed polyethylene bottles under saturated steam
C-S-H. The result of this process is the formation of conditions from 2.5 h to 24 h. The silica fume used
a rigidly cemented C-S-H network. had 92.6 percent content of SiO , and a surface
2
area of 29.9m /g.
2
Grutzeck and co-workers in another work have
tested the above model for Portland cement-silica The following results were obtained : After 2.5h
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fume pastes . The tested mixes had the following and at 90°C as much as 68 to 95 percent of the CH
composition : Class H cement 69.4 percent, silica (depending on the initial quantity) had reacted
fume 15.3 percent, 30 m quartz 15.3 percent, with silica fume. On the other hand, after 2.5h at
superplasticizer 1.4g/30.2g deionized water, 55°C only 25 to 55 percent of the CH (depending
and water/cement ratio=0.43. The pastes were on the initial quantity) had reacted with the
examined with SEM. From the observations, it was silica fume. After 24h, the quantity of unreacted
concluded that the model of pozzolanic reaction CH was 3 to 8 percent and it was independent
proposed for silica fume-CH aqueous solution is of both temperature and initial quantity of CH.
applicable to Portland cementsilica fume pastes as Therefore, it can he concluded that the reactivity
well. of silica fume with CH at early ages depends
strongly on temperature, but not at later ages,

In particular, the model can explain the high for example 24 h.
water demand, the early strength gain, the dense
micro-structure, and the reduction in CH content MECHANISMS OF STRENGTH
observed in Portland cement-silica fume pastes. ENHANCEMENT
There were, however, some noticeable differences. The addition of silica fume to Portland cement
For instance, the reaction process was much pastes, mortars, or concretes improves the strength
slower for the Portland cement-silica fume pastes. development of the Portland cement mixes. It is
Agglomeration of the silica fume particles occurred, on this basis of strength improvement, that silica
but not in all cases. Moreover, C-S-H formation did fume has been claimed to he highly pozzolanic.
occur on the surfaces of these agglomerations,
but it was difficult to distinguish between the C-S-H There are three mechanisms suggested to explain
formed by the pozzolanic reaction of silica fume the strength enhancement capability of silica
particles and the C-S-H formed by the hydration of fume due to pozzolanic reaction process and they
the calcium silicates. are discussed in the following sections. Strength
enhancement by non-pozzolanic activities have
Despite these differences, it was concluded that the also been suggested and will be discussed as well.
silica fume-silica rich, Ca-poor “gel” C-S-H model
fits the hydration process of Portland cement-silica Strength enhancement by pore-size refinement and
fume pastes and explains the related observed matrix densification: Mehta studied the strength
phenomena. development characteristics of Portland-pozzolan
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cement pastes . A volcanic ash (Santorin Earth)
Kurbus and co-workers have studied the reactivity was used as the pozzolanic material. This material
of silica fume with CH in Portland cement pastes

52 The Indian Concrete Journal | November 2018