Cemtech Conference Roma, Italia September 2006 The release mechanism of hexavalent chromium Davide Padovani & Matteo Magistri

Why Mapei is interested in Cr(VI) ? 1 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? The release mechanism of hexavalent chromium

2 46 Plants in 23 different countries Million € turnover Employees 12% of workforce dedicated to R&D

R & D Laboratories 3 D.A.M. Mapei Inc. Laval (Montreal) Canada Mapei Corp. (Headq.) Deerfield Beach USA Rescon Mapei AS Sagstua - Norway Mapei S.p.A. Milan - Italy Vinavil SpA Villadossola Verbania - Italy Sopro Banchemie GmbH Wiesbaden Germany Mapei France SA Saint Alban (Tolosa) Francia

A)We are interested in Cr(VI) released into solution B) C 3 A and C 4 AF 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 4 The release mechanism of hexavalent chromium

5 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

6 The release mechanism of hexavalent chromium C 3 A and C 4 AF are able to “capture” Cr(VI) /1 1.In previous works has been demonstrated that C 3 A can immobilize Cr(VI) 2.In this work we have repeated the same experiment by using C 4 AF: the result is similar 3.In practice: - we have synthesised pure C 4 AF - we have hydrated the C 4 AF in a solution already containing Cr(VI), with and without gypsum - we have measured the amount of Cr(VI) after C 4 AF hydration

7 The release mechanism of hexavalent chromium C 3 A and C 4 AF are able to “capture” Cr(VI) /2

8 The release mechanism of hexavalent chromium It appears that C 3 A and C 4 AF 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, C 3 A and C 4 AF react with sulphates forming ettringite. C 3 A and C 4 AF are able to “capture” Cr(VI) /3

9 The release mechanism of hexavalent chromium At this point it becomes very interesting to verify if the immobilisation of chromates by C 3 A and C 4 AF 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. C 3 A and C 4 AF are able to “capture” Cr(VI), but… C 3 A; C 4 AF Water + Cr(VI) Gypsum Addition Time t0t0 t1t1

10 The release mechanism of hexavalent chromium The role of ettringite: reversibility of the reaction Gypsum addition It is clear that the chromates immobilised by hydration of C 3 A and C 4 AF can be released if the concentration of sulphate in solution increases.

11 The release mechanism of hexavalent chromium 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 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

12 The release mechanism of hexavalent chromium 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.

13 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 C 3 A, the most stable hydration product is the C 3 AH 6 (Ca 3 [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

14 The release mechanism of hexavalent chromium Another way to obtain hydrated aluminates (or even ettringite) is by direct synthesis in aqueous solution of calcium salts (CaO, Ca(NO 3 ) 2 ) and sodium aluminate (Na 2 OAl 2 O 3 ) (or aluminium sulphate Al 2 (SO 4 ) 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

15 The release mechanism of hexavalent chromium 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 C 2 AHx. The images collected with the ESEM-FEG show hexagonal plates, that are the typical shape of crystals of this type of hydrated aluminates. Hydrated aluminates and Cr(VI) immobilisation

16 The release mechanism of hexavalent chromium 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 C 2 AHx and immobilised in its structure. Hydrated aluminates and Cr(VI) immobilisation

17 The release mechanism of hexavalent chromium Conclusions 1) Both C 3 A and C 4 AF 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 C 3 A and C 4 AF is reversible: during the formation of ettringite the chromium captured can be released in solution. 3) The hydration of C 3 A in the absence of gypsum may form several types of products. Using a direct synthesis in aqueous solution it has been demonstrated that the C 2 AHx type is able to immobilize Cr(VI). We can suppose a similar behaviour for the other forms.

The reduction of Cr(VI) by immobilisation 18 The release mechanism of hexavalent chromium 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 Conference Roma, Italia September 2006