Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 139

TABLE 6.5.1
Comparison of Hydroceramic and Geopolymeric Concretes

Hydroceramic Geopolymer

Pozzolanic
substrate

Metakaolin only: no other single
aluminosilicate pozzolan or
combination of individual alumina/silica
sources works as well

GPs have been made with virtually
every conceivable combination of
alumina-containing pozzolans:
raw clay, calcined clay, fly ash,
powdered feldspar, etc.
commercial GP cements also often
contain a substantial amount of a
C-S-H forming component,
usually granulated blast furnace
slag (GBFS)

Activating
solution

Concentrated (typically 8-20 M) NaOH
solution only: choose a NaOH
concentration that simultaneously
provides enough alkali (see Formulation
guidelines) and satisfies the mixture's
water demand

Strong (typically 8 M) solution of
sodium or potassium hydroxide
solution; usually contains a good
deal (1-4 M) of soluble silica, too

Formulation
guidelines*
(approximate)

1. The mix should contain roughly equal
amounts (atom-wise) of sodium,
aluminum, and silicon
2. Emulate cancrinite; i.e., less than 25%
of the sodium should be present in forms
(salts) other than oxide, hydroxide,
aluminate, or silicate
3. At least 20% of the sodium should
initially be present as NaOH

1. The SiO 2 :M 2 O ratio of the
activating solution should be
between 4:1 and 6.6:1
2. The pozzolanic substrate must
provide some base-soluble
alumina
3. The mixture's overall Al 2 O 3 :SiO 2
ratio should be between 1:5.5 and
1:6.5

Curing
conditions

Various hydrothermal: e.g., 1 month at
95°C; 10 hours in a culinary-type
(~118°C) autoclave; or 1 hour in a 200°C
autoclave

Not hydrothermal; typically a few
weeks at room temperature or a
few hours at 50-80°C

Chemical
characteristics

Behaves like cancrinite: i.e., exhibits
substantial cation exchange capacity but
doesn't readily release alkali to pure
water

Exhibits a good deal of cation
exchange capacity but does not
resist water leaching of its alkali

Physical
characteristics

Generally chalk-like, unconfined
compressive strength typically 5-15
MPa

Hard, brittle, often glassy

Primary virtues Passes DOE's leach tests, low overall

water solubility, high cation exchange
capacity, compatibility with silicate rock
(i.e., repository environment)

Superior to ordinary portland
cement for both repository
construction and backfill/overpack
applications

* The GP “formulation rules” are taken from Davidovit's U.S. patent (# 4 349 386, 1982) - other
workers have since demonstrated that the numeric ratios quoted above are overly restrictive.