Page 53 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

2+
similar for the two pastes, there were hardly any by the Ca ions liberated by the hydration of the
large pores in the Portland cement-silica fume Portland cement silicates.
3
3
pastes as compared with 0.22cm /g and 0.17cm /g,
respectively, for the controlled paste resulting in 77 Regourd and co-workers have investigated the
percent and 97 percent increase in strength. microstructure of a pavement in Canada built with
concretes containing silica fume . The silica fume
15
Consequently, it was concluded that the presence contents ranged from 10 percent to 40 percent by
of silica fume in the Portland cement mixes caused weight of cement. After one winter in service cores
considerable reduction in the volume of large of the concretes were taken from the pavements
pores, at all ages and was, therefore, instrumental and examined with SEM. It was observed that
in causing the observed increases in compressive the concretes were unusual because they had a
strengths. very compact microstructure with very dense and
amorphous C-S-H.

The exact mechanism by which the process of pore
refinement takes place in silica fume-Portland The microstructure was still very dense even after 3
16
cement pastes is not completely understood. years of performance under the same conditions .
However, certain conclusions can be drawn by EPMA showed the C/S ratio to be between 0.9 (for 20
observing the microstructure of these pastes. percent replacement of cement by silica fume) and
1.3 (for 15 percent replacement of cement by silica
fume) as compared to 1.6 for ordinary Portland
Regourd and co-workers have studied the cement C-S-H. Alumina, alkalis and SO were also
3
microstructure of blended cement mortars with present in the C-S-H. Therefore, the presence of
30 percent by weight additive, in which 5 percent silica fume modified the composition of C-S-H and,
of the additive was replaced by an equal weight more particularly, their calcium content.
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of silica fume . The additives used were slags, fly
ash, volcanic rock and quartz. The microstructures Strength enhancement by reduction in content of
were studied with SEM from 7 days up to 90 days. CH: The contribution to strength increase by the
It was found that the replacement of 5 percent pozzolanic reaction mechanism has been viewed
additive by an equal weight of silica fume results differently by Scrivener and co-workers . CH
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in the formation of a dense C-S-H with lower crystals in Portland cement pastes is a source of
microporosity than the blended cement mortars weakness because cracks can easily propagate
without silica fume. through or within these crystals without any
significant resistance. In the presence of pozzolanic
An electron probe microanalysis (EPMA) revealed reaction, the reduction in the content of CH leads
that the C/S ratio of the C-S-H was lower for the to an increase in strength. Thus, strength increase
blended cement mortars containing silica fume. is not due to increase in C-S-H “gel” formation or
For example, the C/S ratio for the ordinary Portland reduction of pores but simply to reduction in CH
cement mortars was 1.70 as compared to 1.20 content.
for the mortar with 70 percent Portland cement,
5 percent silica fume, and 25 percent slag. The Strength enhancement by cement paste-aggregate
decrease in the calcium content was attributed to interfacial refinement: In concrete the characteristic
the lime consumption by the silica fume, activated of the transition zone between the aggregate

54 The Indian Concrete Journal | November 2018