Page 48 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

Mechanisms of hydration and strength

developments in Portland cement composites

containing silica fume particles

M. D. Cohen and M. Klitsikas

The principal source accountable for the improvement in strength when silica
fume is added to Portland cement is the pozzolanic reaction process occuring
between silica fume particles and Ca(OH) that result from the hydration
2
of Portland cement. The mechanisms by which this reaction contributes to
strengthening of Portland cement paste, mortars, and concretes are not fully
established. This paper is based on a review of selected papers and its objective is
to present the current understanding of the pozzolanic reaction process and the
subsequent mechanisms, by which strength enhancements are achieved, such as
pore-size refidement and matrix densification, reduction in content of Ca(OH) ,
2
and cement-aggregate interfacial refinement. Strength enhancements by non-
pozzolanic activities are also examined and experimental observations related to
each category are presented and discussed.

I 1

n recent years, many organizations have become involved in research aimed at finding

less energy-intensive materials for use in the cement and concrete industry. One such material is
silica fume .

Silica fume is the by-product of the production of silicon metal or ferrosilicon alloys in electric arc
furnaces. Reduction of quartzitic silica in the presence of carbon at temperatures of the order
of 2,000°C results in the production of silicon. About 10 to 15 percent of the quartz is lost in the
2
form of SiO . After cooling, the gases condense into extremely small spheres of amorphous SiO .
2
2
A typical chemical composition of silica fume produced in North America is 93.7 percent SiO ,
2
0.3 percent Al O , 0.8 percent Fe O , 0.2 percent MgO, 0.2 percent CaO, 0.2 percent Na O, 0.5
2
2 3
2 3
percent K O, 2.6 percent carbon, 0.1 percent sulphur, and 2.8 percent loss on ignition (LOI). The
2
2
surface area of silica fume particles (determined by nitrogen adsorption) ranges from 17m /g to
The Indian Concrete Journal | November 2018 49