Civil Engineering Reference
In-Depth Information
CHAPTER 3
Fly Ash, Slag, Silica Fume,
and Natural Pozzolans
conservation standpoint but also for the technical benefits
they provide concrete.
Supplementary cementitious materials are added to
concrete as part of the total cementitious system. They may
be used in addition to or as a partial replacement of portland
cement or blended cement in concrete, depending on the
properties of the materials and the desired effect on concrete.
Supplementary cementitious materials are used to
improve a particular concrete property, such as resistance to
alkali-aggregate reactivity. The optimum amount to use
should be established by testing to determine (1) whether the
material is indeed improving the property, and (2) the correct
dosage rate, as an overdose or underdose can be harmful or
not achieve the desired effect. Supplementary cementitious
materials also react differently with different cements.
Traditionally, fly ash, slag, calcined clay, calcined shale,
and silica fume were used in concrete individually. Today,
due to improved access to these materials, concrete produc-
ers can combine two or more of these materials to optimize
concrete properties. Mixtures using three cementitious
materials, called ternary mixtures, are becoming more
Fig. 3-1. Supplementary cementitious materials. From left
to right, fly ash (Class C), metakaolin (calcined clay), silica
fume, fly ash (Class F), slag, and calcined shale. (69794)
Fly ash, ground granulated blast-furnace slag, silica fume,
and natural pozzolans, such as calcined shale, calcined clay
or metakaolin, are materials that, when used in conjunc-
tion with portland or blended cement, contribute to the
properties of the hardened concrete through hydraulic or
pozzolanic activity or both (Fig. 3-1). A pozzolan is a
siliceous or aluminosiliceous material that, in finely
divided form and in the presence of moisture, chemically
reacts with the calcium hydroxide released by the hydra-
tion of portland cement to form calcium silicate hydrate
and other cementitious compounds. Pozzolans and slags
are generally catergorized as supplementary cementitious
materials or mineral admixtures. Table 3-1 lists the appli-
cable specifications these materials meet. The use of these
materials in blended cements is discussed in Chapter 2 and
by
Detwiler, Bhatty, and Bhattacharja (1996)
.
The practice of using supplementary cementitious
materials in concrete mixtures has been growing in North
America since the 1970s. There are similarities between
many of these materials in that most are byproducts of
other industrial processes; their judicious use is desirable
not only from the national environmental and energy
Fig. 3-2. Scanning electron microscope (SEM) micrograph
of fly ash particles at 1000X. Although most fly ash
spheres are solid, some particles, called cenospheres,
are hollow (as shown in the micrograph). (54048)
