Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 162

TABLE 7.5
Summary of Effects of Organic Contaminants on
Cement Hydration

Effect on

Organic Compound Setting Hardening

Phenol Strong retarder 49

Oil Strong retarder 49

Grease Strong retarder 49

Hexachlorobenzene Minor retarder 49

Trichloroethylene Minor retarder 49

Toluene Minor retarder 48

Cyclooctane Moderate retarder 48

Hexanol Strong retarder 48

for C 3 A hydration, and will cause a flash setting at high concentrations. 3,9 Nestle et
al. 48 reported that polar solvents delay hydration to a much greater degree than non-
polar solvents; findings for specific contaminants studied by them and others 49 are
summarized in Table 7.5. Many organic compounds also result in progressive dete-
rioration of cement products over time. 6,8
Most organic compounds are retarders of calcium aluminate cements. 8 Citrate
salts, in particular, are commonly used retarders. However, Bier et al. 44 present a
way of using Li and citrate together to optimize the setting characteristics of calcium
aluminate cements such that retardation does not necessarily result, whereas Baker
and Banfill 42 use trilithium citrate as an accelerator, and trilithium citrate had almost
no effect in experiments conducted by Damidot et al. 45

7.2.4 O THER E FFECTS

Although pozzolans are accelerators of C 3 S hydration, 50 they themselves hydrate
slowly, and pozzolanic cements tend to gain strength more slowly than portland
cement, unless an activator, such as CH or an alkali, 51,52 is used. Pozzolans may
mitigate the effects of other impurities, e.g.,53-55 and are known to reduce the hydraulic
conductivity and improve the strength and durability of the final product in the longer
term. When added to calcium aluminate cements, pozzolans help to form C 2 ASH 8 ,
which resists conversion to less voluminous C 3 AH 6 . 56-58
Finally, hydration of a cement is also affected by the physical properties of
the cement and any impurities, i.e., crystallinity, crystal defects, and the particle
size and surface area of the different mineral phases. Reactivity of binders
increases with fineness, but fine impurities, such as clay or colloidal matter, can
retard setting. 9