Geopolymers

Forming geopolymers is a process that is very similar to cementation. Geopolymers are inorganic ceramic polymers made from aluminosilicates and cross-linked with alkali metal ions [126-128]. During fabrication, a low water content is used (H2O/M2O ~ 10-25 wt%) so that an amorphous geopolymer forms instead of crystalline zeolites which would technically form hydroceramic waste forms discussed below. A nominal composition of 4SiO2^Al2O3^M2O is used to represent the geopolymer matrix, although the Si:Al ratio varies according to the application from 1 to 3. For cements and concrete-like applications, a ratio of 2 : 1 is nominally used [129] . The alkali can be Na, K, or Cs. Geopolymers appear to be excellent low tem­perature binders and environmentally more acceptable than cement waste forms as the starting materials only need to be heated to ~700°C instead of clinkering at 1,400-1,500°C.

Geopolymers and geopolymeric cements, including but not limited to fly ash-based geopolymeric concretes, are ideal for environmental applications, such as the permanent encapsulation of radioactive species [130, 131] and other hazardous wastes [132]. Geopolymers can be used as sealants, capping, barriers, and other structures necessary at containment sites. Pilot-scale demonstrations have been performed in Europe on both mining wastes and uranium mill tailings [133-135]. Geopolymers were investigated for the disposal of radioactive wastes in Europe in the mid to late 1990s [136, 137] and the following applications have more recently been investigated.

• Geopolymers with Si:Al ratios of 1 : 1 and 2 : 1 for the stabilization of hazardous Resource Conservation and Recovery Act (RCRA) metals such as Ni, Se, Ba, Hg, Cd, Cr, Pb. A simulant RCRA spike was made that contained the RCRA components at 60x the concentration of the RCRA treatment standards known as the Universal Treatment Stand­ards or UTS limits [ 138] . The mixture was very acidic (pH < 1). The RCRA simulant was substituted for half of the 10 wt% water in the geopolymer formulation and the geopolymers met the Environment Protection Agency Toxicity Characteristic Leaching Procedure (EPA TCLP) test limits at less than the UTS limits even though the geopoly­mer contained 60x the UTS concentrations. It is not known whether or not the RCRA components interacted with the geopolymer, i. e. whether this was encapsulation or embedding (Table 6.9).

• Geopolymers derived from metakaolin and alkaline silicate solutions and having nominal Na/Al and Si/Al molar ratios of 1 and 2 were studied at ANSTO for the stabilization of [37Cs and [°Sr [139]. These geopolymers were studied by transmission electron microscopy and found to be amorphous on the ~1 nm scale after curing at 40°C. The Cs inhabited the amorphous phase, whereas Sr was incorporated only partly, being preferentially partitioned to crystalline SrCO3. This study implies that the geopolymer components do interact with some species and not with others, providing both encapsulation and embedding (Table 6.9 ).

• Special geopolymer formulations, marketed under the name DuraLith, have been patented [140] for stabilization of 129I and 99Tc. Testing [141] showed great promise for retention of technetium with rhenium used as a surrogate for the Tc, but not for iodine.

• Removal of radiolytic H2 production (and freeze-thaw problems) can be carried out by heating geopolymers at ~300°C without any serious effects on strength or leachability [142] .

• Geopolymers have demonstrated excellent fire resistance [142] .

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