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Cemtech Conference
Roma, Italia
17-20 September 2006
The release mechanism of hexavalent chromium
Davide Padovani & Matteo Magistri
The release mechanism of hexavalent chromium
Why Mapei is interested in Cr(VI) ?
Because we are a cement consumer and a chemical
(grinding aids, adhesives, materials for building) producer;
we are involved in both sides
What is the Mapei approach to the “Cr (VI) problem” ?
We decided to launch a long term research program: from
one side we are studying the best way to reduce Cr(VI) by
redox reactions (from Cr(VI) to Cr(III)), from the other side
we are looking for a wider approach: why not find the way of
avoiding the Cr(VI) release during cement hydration?
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1.400 Million €
turnover 2006
46 Plants
in 23 different countries
4.500 Employees
12% of workforce dedicated to R&D
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R & D Laboratories
Vinavil SpA
Villadossola
Verbania - Italy
Mapei S.p.A.
Milan - Italy
Rescon Mapei AS
Sagstua - Norway
Mapei France SA
Saint Alban (Tolosa)
Francia
Mapei Corp. (Headq.)
Deerfield Beach
USA
Mapei Inc.
Laval (Montreal)
Canada
Sopro Banchemie GmbH
Wiesbaden
Germany
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The release mechanism of hexavalent chromium
CONTENT OF THE SPEECH
A) We are interested in Cr(VI) released into solution
B) C3A and C4AF are able to “capture” Cr(VI)
C) The role of ettringite in Cr(VI) release
D) The case of cement: what happens before ettringite?
- different hydrated products are able to “capture” Cr(VI)
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The release mechanism of hexavalent chromium
We are interested on Cr(VI) released in solution
Not all the Cr(VI) contained in cement goes into solution.
Considering that our target is to avoid having Cr(VI) in solution:
- Produce clinker without Cr(VI)
- Reduce Cr(VI) to Cr(III) through redox reactions (iron/tin
salts)
Theoretically there is another opportunity:
- Avoid that Cr(VI) goes in solution (immobilisation…)
That’s why we are studying the release mechanism of
hexavalent chromium
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The release mechanism of hexavalent chromium
C3A and C4AF are able to “capture” Cr(VI)/1
1. In previous works has been demonstrated that C3A can
immobilize Cr(VI)
2. In this work we have repeated the same experiment by
using C4AF: the result is similar
3. In practice:
- we have synthesised pure C4AF
- we have hydrated the C4AF in a solution already
containing Cr(VI), with and without gypsum
- we have measured the amount of Cr(VI) after C4AF
hydration
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The release mechanism of hexavalent chromium
C3A and C4AF are able to “capture” Cr(VI)/2
TABLE 2
C4AF
+ gypsum
C3 A
C3 A
+ gypsum
Cr(VI) – mg/l
Cr(VI) – mg/l
Cr(VI) – mg/l
Cr(VI) – mg/l
(ppm)
(ppm)
(ppm)
(ppm)
0
9,74
9,85
9,49
9,32
1
0,00
8,03
0,00
4,31
5
0,00
5,67
0,00
3,53
15
0,00
5,42
0,00
3,59
20
0,00
5,35
0,00
3,47
40
0,00
4,72
0,00
3,61
Time (min)
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C4AF
The release mechanism of hexavalent chromium
C3A and C4AF are able to “capture” Cr(VI)/3
GRAPH 1 - Immobilisation of Cr(VI) by systems C3A/C4AF/Gypsum
12,00
10,00
It appears that C3A and C4AF have the same behaviour:
they can immobilize soluble chromates in their hydration
products, but in the presence of gypsum the
immobilisation of Cr(VI) is not total. This can be
explained by considering that in the presence of gypsum,
C3A and C4AF react with sulphates forming ettringite.
Cr(VI) g/l (ppm)
8,00
6,00
4,00
2,00
0,00
0 min
1 min
5 min
C3A
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C3A + gypsum
15 min
C4AF
20 min
C4AF + gypsum
40 min
The release mechanism of hexavalent chromium
C3A and C4AF are able to “capture” Cr(VI), but…
At this point it becomes very interesting to verify if the
immobilisation of chromates by C3A and C4AF hydration
products is reversible or not when adding gypsum.
We repeated the previous experiment by adding gypsum
after that Cr(VI) was captured by hydrated aluminates.
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C3A; C4AF
Gypsum
Water + Cr(VI)
Addition
t0
t1
Time
The release mechanism of hexavalent chromium
The role of ettringite: reversibility of the reaction
GRAPH 2 - Immobilisation of Cr(VI) by systems C3A/C4AF/Gypsum - Effect of the post-addition of gypsum
12,00
Gypsum
addition
10,00
Cr(VI) g/l (ppm)
8,00
6,00
It is clear that the chromates immobilised by hydration of
C3A and C4AF can be released if the concentration of
sulphate in solution increases.
4,00
2,00
0,00
0 min
1 min
1 min after the add. of
gypsum
C3A
10
5 min
C4AF
10 min
15 min
The release mechanism of hexavalent chromium
The role of ettringite in Cr(VI) release
What we suppose is:
In real cement hydration, with multimineral clinker grains,
both mechanisms
what we see
- hydrated
aluminateshappen
are ablecontemporarily;
to capture Cr(VI)
when we measure Cr(VI) according to EN 196-10 is the
- when gypsum is added the equilibrium of the reaction shifts
final result of this complex process
toward ettringite, so part of Cr(VI) is released in solution
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The release mechanism of hexavalent chromium
The case of cement: what happens before ettringite?
To summarize, the release mechanism of Cr(VI) can be
synthesised this way: immediately after water addition
we have the release of Cr(VI); then the aluminates start
to hydrate and to immobilize part of the Cr(VI), while
sulphates become available and contribute to the
ettringite formation; at this phase part of Cr(VI) is once
more released in solution.
In the final part of this speech we well deepen our
knowledge of aluminates hydration, by synthesising them
in a water solution and confirm that they are still able to
capture Cr(VI).
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The release mechanism of hexavalent chromium
Hydrated aluminates and Cr(VI) immobilisation
While ettringite has a structure
and characteristics that have
been already studied and defined,
the hydration of aluminates leads
to a family of compounds that can
be very different. If we only
consider pure C3A, the most
stable hydration product is the
C3AH6 (Ca3[Al(OH)6]2), that can
crystallize in various cubic forms,
of which at normal temperatures
icositetrahedra is the most
stable.
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Figure 1 – ESEM image of a C3AH6 crystal
The release mechanism of hexavalent chromium
Hydrated aluminates and Cr(VI) immobilisation
Another way to obtain hydrated aluminates (or even ettringite) is
by direct synthesis in aqueous solution of calcium salts (CaO,
Ca(NO3)2) and sodium aluminate (Na2OAl2O3) (or aluminium
sulphate Al2(SO4)3).
By controlling the conditions of synthesis (e.g. temperature,
concentration of reagents, time of reactions, ...), single types of
hydrated aluminates can be obtained.
Their aptitude to immobilise hexavalent chromium has been
verified simply by conducting the synthesis in aqueous chromate
solutions.
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The release mechanism of hexavalent chromium
Hydrated aluminates and Cr(VI) immobilisation
Figure 2 – XRD analysis of synthetic C2AHx
Figure 3 – ESEM image of synthetic C2AHx
This is an example of this synthesis performed in our lab; the
results of the XRD analysis on the precipitate demonstrate that we
obtained an hydrated aluminate of general formula C2AHx. The
images collected with the ESEM-FEG show hexagonal plates, that
are the typical shape of crystals of this type of hydrated
aluminates.
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The release mechanism of hexavalent chromium
Hydrated aluminates and Cr(VI) immobilisation
As far as Cr(VI) is concerned, IC results show that of the initial 50 ppm of
Cr(VI), only 3,4 ppm remained in solution: this means that a great part of
the chromates has been captured by the C2AHx and immobilised in its
structure.
Cr(VI) during C2AHx synthesis
60
50
ppm
40
30
20
10
0
0
Syhthesis
time
Time
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The release mechanism of hexavalent chromium
Conclusions
1) Both C3A and C4AF are able to immobilize chromates during
their hydration. In the presence of gypsum, the ettringite that is
formed has a lower tendency to capture the chromate ion.
2) The immobilisation of chromates by the C3A and C4AF is
reversible: during the formation of ettringite the chromium
captured can be released in solution.
3) The hydration of C3A in the absence of gypsum may form
several types of products. Using a direct synthesis in aqueous
solution it has been demonstrated that the C2AHx type is able to
immobilize Cr(VI). We can suppose a similar behaviour for the
other forms.
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The release mechanism of hexavalent chromium
The reduction of Cr(VI) by immobilisation
And to conclude: even if the path to reduce Cr(VI) by
immobilisation seems full of difficulties, we believe in
it and we’ll continue our studies….
Even during the last world football cup nobody would
bet on the Italian team (except Mapei) and this was
the result…….
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Cemtech Conference
Roma, Italia
17-20 September 2006