Figure 2
sorption of cations. When GGBS-blended binders are used, c/s ratios of <0.8, favour the sorption of cations (Harris et al., 2002).
Isomorphic substitution of calcium ions for other divalent cations of similar size including strontium, barium and lead is possible. Metals may also substitute for silicon and become chemically incorporated into the C-S-H.
Contaminants can become incorporated into newly formed solid, crystalline phases and compound-specific interactions are possible. Portlandite (calcium hydroxide)
Table 2: Summary of cement hydrates and their properties Phase
Key properties Calcium silicate hydrate (C-S-H)
Gel-phase, high surface area, highly hydrated, variable c/s ratio, fills pores
Portlandite Ettringite (or Aft) Monosulphate (or AFm) |108| ENVIRONMENT INDUSTRY MAGAZINE Plate-like crystals, fills pores Prismatic crystals
Plate-like crystals (behaves similar to ettringite)
buffers the pore solution pH to 12.5, but its interaction with metallic compounds is limited. Ettringite can however, incorporate metals in its crystalline structure. The trivalent metals: Fe3+, Cr3+, Mn3+, and Ti3+ can substitute for Al3+, and Ca2+can be replaced by Mg2+, Zn2+, Mn2+, Fe2+, Co2+, Pb2+, Cd2+ and Cu2+. The replacement of SO42- by a range of oxyanions such as CrO42-, AsO43-, SeO42-, IO3-, CO32-, NO3- can also occur and small organic molecules may occupy the channel sites within the ettringite structure. Monosulphate and tetracalcium aluminate hydrate behave similarly to ettringite. The anions SO42- and OH- can enter interstitial and interlayer sites of this phase.
Significance to s/s
Sorptive capability. Can incoporate divalent and trivalent ions in its structure. C-S-H can be modified to regulate the redox environment of a waste form
The key buffering phase, maintaining pH ≈ 12.5
Can incorporating metals, anions and organic molecules in its structure
Similar to ettringite, as can substitute metals and anions into its structure
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