Page 191 - Handbook of Thermal Analysis of Construction Materials

Section 6.0 - Mineral Admixtures 175

High calcium fly ash and granulated blast furnace slag are cementi-
tious. Low calcium fly ash is termed a normal pozzolan. Condensed silica
fume and rice husk ash are highly pozzolanic. Slowly cooled blast furnace
slag, bottom ash, and field burnt rice husk ash are weak pozzolans.

6.1 Fly Ash

Fly ash contains about 15% crystalline material, the rest being an
amorphous material and carbon. The major crystalline materials in fly ash
are quartz, mullite, hematite, and magnetite. The reactivity of fly ash
depends on the loss on ignition, fineness, and mineralogical and chemical
composition. The pozzolanic activity also depends on the mineralogy of
coal and on the glassy or non-crystalline structure of fly ash.
Fly ash may be used as a replacement of cement or fine aggregate
or as an additional component at the concrete mixing plant. Fly ash has been
used up to 60% replacement of cement. Addition of fly ash reduces the
water requirement for a particular consistency or flow. Low calcium fly ash
acts largely as a fine aggregate initially, but with time will react to
pozzolanic compounds. High calcium fly ash participates in the early
cementing reactions. Partial replacement of fly ash results in the reduction
in temperature rise in fresh concrete due to the reduction in the heat of
hydration.
The rate of strength development in fly ash concrete depends on the
type of fly ash, temperature, and curing conditions. Concrete containing
50% low calcium fly ash replacement and a superplasticizer is capable of
developing 60 MPa compressive strength at 28 days and 20–30 MPa in 3
days. Beyond 7–8 weeks, all concrete mixes containing fly ash show
strength comparable to concrete mixes having an equal cement content. At
early ages, concretes containing 50% replacement are more permeable than
concrete without them. The trend reverses after about 180 days because of
the pozzolanic activity of the fly ashes. [83] The sulfate resistance of concrete
is enhanced by adding fly ash to concrete.
In 1985, Malhotra and others initiated studies on structural con-
crete incorporating more than 50% low calcium fly ash. [84][85] This was
achieved by using a higher than normal dosage of superplasticizer. The high
volume fly ash exhibits a 1-day strength between 5 and 9 MPa. The later age
compressive strength reaches 60 MPa at one year. Compressive strength of
more than 90 MPa have been reported on test cylinders and drilled cores
from various concrete monoliths. [86]