Calcium Borate Binders - A summary of experiments by Mark Tyrer and Essie Ganjian
Ca2+ + 2 B(OH) 30 + 2OH- + 2 H 2O →
Ca[B(OH)4]2.2H 2O
Initial experiments showed that the borate slag is
hydraulic: it reacts with water and sets hard in a
matter of 30 minutes. It has a modest resistance to
organic acid attack and was therefore used as the
cementitious binder in one of the field trials of
these liner systems constructed near Risley,
Cheshire. It readily leaches residual borate ions to
solution. The slag comprises alternating layers of
zinc oxide (light phases in the figure below) and
glassy borax (dark, fractured phase).
Calcite structure
Calcite
reference
pattern
Count rate
Limex -70
{104}
The Limex is
confirmed by
XRD to be
dominantly
calcite
{110}
{119}
During the MIRO/ENTRUST project "Novel
Composite Landfill Liners", such a phenomenon
was investigated in which calcium (from byproduct calcium carbonate) and borate (from a
metallurgical borate slag) react to form calcium
diborate dihydrate (which has the mineral name
"hexahydroborite"). The reaction scheme is a
simple precipitation:
for pure calcite. The lattice structure is shown to
the left of the figure.
{012}
Reactive, self-sealing binders are attractive
materials for many waste containment systems. The
concept is simple; as a material degrades, the ions
released react with those from an adjacent material
to form a new phase, which has a greater molar
volume than either of the original components. If
this reaction product is highly insoluble, its
potential for use in a self-sealing barrier is high.
Degrees 2θ CuKα
Figure 2 Theoretical calcite lattice and reference
pattern compared with that measured for Limex-70
(inset, centre).
Considering next the thermodynamics of the
system, the leached slag released borax and a small
quantity of zincite to solution. If these two phases
are equilibrated with water, we may expect zinc
borate Zn(BO2)2 to precipitate, further limiting the
availability of zinc ions. Considering these three
phases at equilibrium with water, the resulting
solution would be expected to contain the
following dissolved elements:
Element
B
Na
Zn
pH
Concentration / mol.dm-1
3.90 e-1
2.42 e-1
1.38 e-7
9.096
Table 1 Solution composition following aqueous
equilibration of Na2B4O7 , ZnO and Zn(BO2)2
[PHREEQCI(v2.6) Lawrence Livermore database]
Similarly, leaching of the Limex-70 is calculated to
release 1.39e-4 M [Ca and CO3] at pH 9.3.
Figure 1 Backscattered SEM micrograph of
polished borate slag. Scale bar = 50 µm
The source of calcium ions is a by-product calcium
carbonate ("Limex-70"), produced by sparging
gaseous CO2 through a calcium-rich alkaline
solution. The precipitate is finely divided, but
contains well crystalline calcite and residual
organics; principally sugars. X-ray diffraction
shows strong reflections for calcite and figure 2
compares the measured pattern with that calculated
Considering a solution which contains a mixture of
the two pore solutions it is possible to calculate the
quantity of Ca[B(OH)4]2.2H2O which is predicted
to precipitate from 1 dm-3 of solution. Figures 3 to
5 show respectively, the change in pH, solution
composition and yield of Ca[B(OH)4]2.2H 2O as the
volume fraction of the mixture changes from
chemistry dominated by borax (on the left) to a
calcite-dominated solution (on the right).
This suggests that the greatest yield of
hexahydroborite will be close to the limex and this
is indeed observed in the column experiments
described below.
sealed and remained impermeable for three months.
At the end of the experiment, these columns were
split and the reaction product at the interface was
analysed by x-ray diffraction and electron
microscopy.
Mixing of Limex and borate slag pore solutions
10
pH
9.9
9.8
Hydrated borate slag
ZnO
Elemental Concentration /
mol.dm -3
1.00E+00
1.00E-02
1.00E-04
1.00E-06
1.00E-08
1.00E-10
1.00E-12
1.00E-14
1.00E-16
Bx
C
Ca
Na
Figure 6 X-ray diffraction pattern of material from
the interface formed in the column experiments.
Zn
B
0
0.2
0.4
0.6
0.8
1
Volume fraction Limex solutions
Figure 4 Composition of a mixed pore solution of
variable composition between borax slag
dominated (left) and Limex dominated (right)
Mixing of Limex and borate slag pore solutions
-3
Moles of precipitate from 1dm pore solution
Given confidence that the reaction product is
indeed hexahydroborite, the next concern must to
for its stability as a function of pH. Again, recourse
was made to thermodynamic calculations. Figures
7 and 8 show the effect of changing solution pH on
the stability of this phase. By calculating its
solubility in solutions where the pH is adjusted by
addition of sodium hydroxide or hydrochloric acid,
it can be shown that the phase is likely to be stable
in any alkaline environment.
1.6E-04
Solubility of Hexahydroborite
as a function of pH
8.0E-05
4.0E-05
0.0E+00
0
0.2
0.4
0.6
0.8
1
Volume fraction of Limex solutions
Figure 5 Yield of Ca[B(OH)4]2.2H2O precipitated
from a mixed pore solution of composition varying
between borax slag dominated (left) and Limex
dominated (right)
Column experiments in which ground borate slag
and Limex-70 were packed as two layers, were
subjected to low flow rates of both deionised water
and a synthetic landfill leachate (a mixed
electrolyte of Na, K, Cl, SO4 and organic acids).
The eluting solution rapidly formed an expansive
reaction product between the two layers which
effectively reduced the columns' permeabilities.
The pure water columns were subject to a 1.3m
head and remained impermeable for the duration of
the experiment (four months). The leachate
columns were subject to more aggressive testing
(3.0m head) and whilst a little solution eluted over
the first few days, the columns satisfactorily self-
Solubility / moles.dm-3
1.2E-04
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
0
2
4
6
8
10
12
14
pH
Figure 7 Solubility of Hexahydroborite as a
function of pH, adjusted by NaOH or HCl.
Dissolution of Hexahydroborite in NaOH or HCl
Concentration / mol.dm-3
Moles of
hexahydroborite
{101}
Inset: reference spectra calculated for
borax (left) and anhydrous borax (right)
Bx
Mixing of Limex and borate slag pore solutions
Bx
Figure 3 Variation of pH in a mixed pore solution
of variable composition between borax slag
dominated (left) and Limex dominated (right)
Bx
1
{002}
0.8
Bx
0.6
Bx
0.4
Volume fraction Limex solution
Bx
0.2
Bx
0
Bx = Borax, Na2B4O7.10H2O
9.6
{100}
Bx
9.7
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
Ionic Strength
Ca
B
Na
Cl
0
5
10
15
pH
Figure 8 Solution composition on dissolving
hexaborohydrite in NaOH or HCl
In conclusion, the use of two industrial by-products
in combination shows the potential for producing a
reactive barrier composition, which will promote
self-sealing of preferential flow paths.
10cm
Lx
Bx
10cm
Lx
Bx
Figure 9 Pure water columns.(Imperial College)
Figure 11 Synthetic leachate columns (Coventry
University)
Borate
slag
200mm
Limex-70
Figue 10 Pure water colmumns (detail)
Figure 12 Synthetic leachate columns (detail)