Civil Engineering Reference
In-Depth Information
Fig. 3-9. Although they can be used in general construction, silica fume and metakaolin are often used in applications such
as (left) bridges and (right) parking garages to minimize chloride penetration into concrete. (68681, 69542)
ground granulated blast furnace slag and
ACI 233 (1995)
provides an extensive review of slag.
NATURAL POZZOLANS
Natural pozzolans have been used for centuries. The term
“pozzolan” comes from a volcanic ash mined at Pozzuoli,
a village near Naples, Italy, following the 79 AD eruption
of Mount Vesuvius. However, the use of volcanic ash and
calcined clay dates back to 2000 BC and earlier in other
cultures. Many of the Roman, Greek, Indian, and Egyptian
pozzolan concrete structures can still be seen today, attest-
ing to the durability of these materials.
The North American experience with natural
pozzolans dates back to early 20th century public works
projects, such as dams, where they were used to control
temperature rise in mass concrete and provide cementi-
tious material. In addition to controlling heat rise, natural
pozzolans were used to improve resistance to sulfate attack
and were among the first materials to be found to mitigate
alkali-silica reaction.
The most common natural pozzolans used today are
processed materials, which are heat treated in a kiln and
then ground to a fine powder (Figs. 3-10, 3-11, and 3-12); they
include calcined clay, calcined shale, and metakaolin.
SILICA FUME
Silica fume, also referred to as microsilica or condensed
silica fume, is a byproduct material that is used as a poz-
zolan (Fig. 3-7). This byproduct is a result of the reduction
of high-purity quartz with coal in an electric arc furnace in
the manufacture of silicon or ferrosilicon alloy. Silica fume
rises as an oxidized vapor from the 2000°C (3630°F)
furnaces. When it cools it condenses and is collected in
huge cloth bags. The condensed silica fume is then
processed to remove impurities and to control particle size.
Condensed silica fume is essentially silicon dioxide
(usually more than 85%) in noncrystalline (amorphorous)
form. Since it is an airborne material like fly ash, it has a
spherical shape (Fig. 3-8). It is extremely fine with particles
less than 1 µm in diameter and with an average diameter
of about 0.1 µm, about 100 times smaller than average
cement particles.
Condensed silica fume has a surface area of about
20,000 m
2
/kg (nitrogen adsorption method). For compari-
son, tobacco smoke's surface area is about 10,000 m
2
/ kg.
Type I and Type III cements have surface areas of about 300
to 400 m
2
/kg and 500 to 600 m
2
/kg (Blaine), respectively.
The relative density of silica fume is generally in the
range of 2.20 to 2.5. Portland cement has a relative density
of about 3.15. The bulk density (uncompacted unit weight)
of silica fume varies from 130 to 430 kg/m
3
(8 to 27 lb/ft
3
).
Silica fume is sold in powder form but is more
commonly available in a liquid. Silica fume is used in
amounts between 5% and 10% by mass of the total cemen-
titious material. It is used in applications where a high
degree of impermeability is needed (Fig. 3-9) and in high-
strength concrete. Silica fume must meet ASTM C 1240.
ACI 234 (1994)
and
SFA (2000)
provide an extensive review
of silica fume.
Fig. 3-10. Scanning electron microscope micrograph of
calcined shale particles at 5000X. (69543)
