Page 49 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

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430m /g. Particle size distribution ranges from 0.01 Materials such as colloidal silicas (0.5 µm to
µm to 0.3 µm with 70 percent of the particles less 0.0005 µm in size) behave like pozzolans and their
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than 0.10 micron . pozzolanic nature is now well established. Usually,
the percentage of CH consumed by silica fume has
Silica fume has been used in mortars and concretes been used as an index of the pozzolanic activity of
as a partial replacement for Portland cement (from silica fume.
5 percent to 40 percent by weight of cement). It can
be either blended with cement or added directly to Contradictory data have been obtained with
concrete. In most cases an increase in compressive respect to the percentage of CH consumed by silica
and flexural strengths is obtained, and in some fume. Traetteberg, one of the first to study silica
cases substantially so. For example, Buil and co- fume, has reported that, in a Portland cement
workers have reported almost 100 percent increase paste with 10 percent silica fume by weight of
in compressive strength after 2 days of curing for cement, only about 2 percent of the liberated CH
mortars with 40 percent by weight of cement silica was consumed by the silica fume . In contrast,
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fume and superplasticizer . Buck and Burkes have reported high pozzolanic
activity in Portland cement-silica, fume pastes in
Although there is no doubt that the pozzolanic which all the CH liberated was consumed by silica
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reaction caused by incorporation of silica fume fume at about 28 days . Sellevold and co-workers
particles to Portland cement pastes, mortars, or have studied pastes of cement with silica fume by
concretes, leads to improvement in strength, the X-ray diffraction quantitative analysis (QXRD).
mechanisms by which this improvement takes
place are not fully established. The purpose of this Thermogravimetric analysis (TGA), mercury
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paper is to discuss the current understanding of porosimetry and other methods . They have
the pozzolanic reaction process and to examine reported a very reactive pozzolanic reaction; an
the mechanisms by which this reaction is thought addition of 24 percent of silica fume by weight of
to contribute to strength enhancement. Strength cement being enough to consume all the CH.
enhancement by non-pozzolanic activities will also
be examined. Nelson and Young have studied the pozzolanic
activity of colloidal silica added to C S and Portland
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cement (ASTM I) pastes . The colloidal silica had a
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POZZOLANIC REACTION PROCESS 99.8 percent content of amorphous SiO , while their
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Pozzolans are siliceous and aluminous materials, surface areas ranged from 175m /g to 430m /g.
which in the presence of moisture and at ordinary The addition of amorphous silica to the C S
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temperature, can react with Ca(OH) (or CH*) pastes varied from 25 percent to 75 percent of the
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to form strength contributing cementitious theoretical amount required to convert all the CH
products, such as calcium silicate hydrates (C-S-H), to C-S-H according to the following two equations :
calcium aluminate hydrates (C-A-H), and calcium
sulphoaluminate hydrates (C-S-A-H). The CH is one
of the products of hydration of Portland cement
and constitutes about 20 percent of the volume of *General abbreviations: C=CaO, S=SiO , H=H O. A=Al O .
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the hydration products . F=Fe O , S=SO 3
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50 The Indian Concrete Journal | November 2018