Environmental Engineering Reference
In-Depth Information
three aspects: (a) chemical fixation of contaminants — chemical interactions between
the hydration products of the cement and the contaminants; (b) physical adsorption of
the contaminants on the surface of hydration products of the cements; and (c) physical
encapsulation of contaminated waste or soil (low permeability of the hardened pastes).
3,4
The first two aspects depend on the nature of the hydration products and the contami-
nants, and the third aspect relates to both the nature of the hydration products and the
density and physical structure of the paste. If the leaching and other performance of the
cement-solidified waste forms meet appropriate environmental criteria, the cement-
based waste forms may be suitable for controlled construction applications.
The selection of cementing materials for S/S of waste must consider the follow-
ing aspects, based on the characteristics of the waste: (1) compatibility of the cement
and the waste; (2) chemical fixation of contaminants; (3) physical encapsulation of
contaminated waste and soil; (4) durability of final waste forms; (5) waste form
leachability; and (6) cost-effectiveness of S/S. In practice, a variety of additives are
often used with cementing materials to address all of these aspects. Some commonly
used additives are described in detail in Chapter 8 as well as in previous publica-
tions.
1,5
This chapter focuses mainly on the various hydraulic cement systems used
for S/S of wastes.
4.2
PORTLAND CEMENT-BASED MATERIALS
Portland cement is the most widely used cement because of its commercial avail-
ability and low cost. However, for construction use, supplementary cementing mate-
rials, such as granulated/pelletized blast furnace slag, coal fly ash, volcanic ashes,
condensed silica fume, rice husk ash, and natural pozzolans, are often used to replace
portland cement to reduce the cost or to improve the performance of concrete. For
the same reasons, these supplementary cementing materials are very often used to
replace portland cement in S/S of wastes. This section will discuss portland cement-
based cementing materials used for S/S of wastes, which include portland cement,
portland blast furnace slag cement, portland pozzolan cement, portland
cement-sodium silicate mixture, polymer-modified portland cement, and masonry
cement.
4.2.1
P
ORTLAND
C
EMENT
4.2.1.1
Characterization and Hydration Chemistry
Portland cement is a hydraulic cement produced by pulverizing clinker and calcium
sulfate (usually gypsum — CaSO
4
.2H
2
O) as an interground addition. Cement clinker
consists mainly of tricalcium silicate (3CaO.SiO
2
), dicalcium silicate (2CaO.SiO
2
),
tricalcium aluminate (3CaO.Al
2
O
3
), and tetracalcium aluminoferrite
(4CaO.Al
2
O
3
.Fe
2
O
3
). Different types of portland cement are manufactured to meet
various normal physical and chemical requirements for specific purposes. ASTM
C150 specifies five types of common portland cement produced by adjusting the
proportions of their minerals and finenesses as shown in Table 4.1.
6
Canadian CAS-
A5 cement Types 10, 20, 30, 40, and 50 are essentially the same as ASTM C150
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