Cemtech Conference Roma, Italia 17-20 September 2006 The release mechanism of hexavalent chromium...

Cemtech ConferenceCemtech Conference

Roma, ItaliaRoma, Italia

17-20 September 200617-20 September 2006

The release mechanism of hexavalent chromiumThe release mechanism of hexavalent chromium

Davide Padovani & Matteo MagistriDavide Padovani & Matteo Magistri

Why Mapei is interested in Cr(VI) ?

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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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46 Plants in 23 different countries

1.400 Million € turnover 2006

4.500 Employees 12% of workforce dedicated to R&D

R & D LaboratoriesR & D Laboratories

33

D.A.M

.

D.A.M

.

Mapei Inc.Laval (Montreal)

Canada

Mapei Corp. (Headq.)Deerfield Beach

USA

Rescon Mapei ASSagstua - Norway

Mapei S.p.A.Milan - Italy

Vinavil SpAVilladossola

Verbania - Italy

Sopro Banchemie GmbHWiesbadenGermany

Mapei France SASaint Alban (Tolosa)

Francia

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)

CONTENT OF THE SPEECH

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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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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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TABLE 2 C4AF C4AF

+ gypsum C3A

C3A

+ gypsum

Time (min) Cr(VI) – mg/l

(ppm)

Cr(VI) – mg/l

(ppm)

Cr(VI) – mg/l

(ppm)

Cr(VI) – mg/l

(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

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GRAPH 1 - Immobilisation of Cr(VI) by systems C3A/C4AF/Gypsum

0,00

2,00

4,00

6,00

8,00

10,00

12,00

0 min 1 min 5 min 15 min 20 min 40 min

Cr(

VI)

g/l

(pp

m)

C3A C3A + gypsum C4AF C4AF + gypsum

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.


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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.

C3A and C4AF are able to “capture” Cr(VI), but…

C3A; C4AF

Water + Cr(VI)

Gypsum

Addition

Timet0

t1

1010


The role of ettringite: reversibility of the reactionGRAPH 2 - Immobilisation of Cr(VI) by systems C3A/C4AF/Gypsum - Effect of the post-addition of gypsum

0,00

2,00

4,00

6,00

8,00

10,00

12,00

0 min 1 min 1 min after the add. ofgypsum

5 min 10 min 15 min

Cr(

VI)

g/l

(pp

m)

C3A C4AF

Gypsum

addition

It is clear that the chromates immobilised by hydration of C3A and C4AF can be released if the concentration of sulphate in solution increases.

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The role of ettringite in Cr(VI) release

In real cement hydration, with multimineral clinker grains, both mechanisms happen contemporarily; what we see when we measure Cr(VI) according to EN 196-10 is the final result of this complex process

What we suppose is:

- hydrated aluminates are able to capture Cr(VI)

- when gypsum is added the equilibrium of the reaction shifts toward ettringite, so part of Cr(VI) is released in solution

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The case of cement: what happens before ettringite?

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).

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.

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Hydrated aluminates and Cr(VI) immobilisationWhile 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.

Figure 1 – ESEM image of a C3AH6 crystal

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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.

Hydrated aluminates and Cr(VI) immobilisation

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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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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

0

10

20

30

40

50

60

0 Syhthesistime

Time

ppm


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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.

The reduction of Cr(VI) by immobilisation

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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…….

Cemtech ConferenceCemtech Conference

Roma, ItaliaRoma, Italia

17-20 September 200617-20 September 2006

Cemtech Conference Roma, Italia 17-20 September 2006 The release mechanism of hexavalent chromium...

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