Asbestos case studies 1: Water pipes made of asbestos cement. Erzsébet Tóth (Eötvös L. University, Budapest)

Transcription

1 Asbestos case studies 1: Water pipes made of asbestos cement Erzsébet Tóth (Eötvös L. University, Budapest)

2 Asbestos-cement water pipe production: Selyp (Hungary, 2000) Final products Raw material: chrysotile asbestos from Canada Production waste

3 Fibre-cement production: Technology is more or less independent of the fibre type used: asbestos is now replaced by cellulose and synthetic fibres. After forming the pipe/roofing tile/corrugated sheet etc., the product is kept for a while in warm and steamy environment to enhance the fastening and hydration of the cement.

4 Fibre-cement production: binding of the cement Slurry is made up of fibers, portland cement and water discussed next 60 90% wet process

5 Concrete - cement Definitions Cement is a hydraulic binder, i.e. a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes and which after hardening, retains its strenght and stability even under water ENV 197-1: CEM cement Portland cement is a hydraulic cement produced by pulverizing portland-cement clinker ASTM C Concrete is a composite material produced by using cement to bind fine and coarse aggregate (sand and gravel) into a dense coherent mass. Mortar: use of only fine aggregates (sand)

6 Cement chemical nomenclature Alite Ettringite

7 Cement mineralogy The microscopical study of Portland cement clinker to determine the mineralogy started at the end of the 19th century. Törnebohm gave the names alite, belite, celite and felite to four distinctive crystalline components + isotropic residue. Later: Alite = tricalcium silicate C 3 S Belite and felite = dicalcium silicate C 2 S Celite = Calcium Alumino Ferrite mainly C 4 AF Isotropic residue = Calcium Aluminates mainly C 3 A

8 Cement mineralogy Actually Portland cement contains four major mineral phases: Alite = tricalcium silicate C 3 S % Belite = dicalcium silicate C 2 S % Aluminate phase = mainly tricalcium aluminate C 3 A 5-10% Ferrite phase = mainly C 4 AF 5-15 % (brownmillerite) For asbestos (and fiber) cement: high C 3 S and low C 3 A is the best

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10 Cement mineralogy - Alite C 3 S - Orthosilicate monoclinic (M1 or M3) - Biaxial Negative with 2V ~ Low birefringence ~ first order gray int. Color - colourless with // polars high positive relief - 3 triclinic - 3 monoclinic and 1 rhombohedral polymorphs exist - First crystal structure determination by Jeffery (1952)

11 Alite - Mineral formula: Ca 3 SiO 5 - Chemical formula: 3CaO.SiO 2 (C 3 S)

12 Alite crossed polars

13 Cement mineralogy - Belite Larnite Ca 2 SiO 3 is the natural analogue of belite - Five polymorps exist at ordinary pressures the β C 2 S polymorph is most common. - Monoclinic - Space group P2 1 /n - Biaxial Negative with 2V ~ second order interference colors - colourless (amber) yellow with // polars high positive relief - First cristal structure determination by Midgley (1952)

14 Belite - Mineral formula: Ca 2 SiO 4 - Chemical fomula: 2CaO.SiO 2 (C 2 S)

15 Belite- crossed polars

16 Cement mineralogy - Aluminate Phase -Cubic - Typically fills interstices between crystals of belite and ferrite. - light-brown color Cement mineralogy - ferrite phase - Orthorombic biaxial negative - Composition ranges from C 5 A 2 F to C 6 AF 2 - Typically fills interstices between crystals. - light-brown yellow color - brownmillerite is a rare natural analogue.

17 Hydration of cement minerals Belite + water C 2 S + 2H CSH + CH Ca(OH) 2 ; Portlandite Calcium Silicate Hydrate

18 Cement mineralogy - portlandite Ca(OH) 2 Hexagonal - Uniaxial Negative - first-order red and second order blue int. colors - colourless, forming minute hexagonal plates - Crystal structure is identical to the Mg(OH) 2 structure Layered structure with Ca octahedrally and O tetrahedrally coordinated. Interlayers forces are weak giving good (0001) cleavage ; P-3m1 spage group

19 Portlandite - Ca(OH) 2

20 CSH - Calcium Silicate Hydrate SEM - BSE image

21 Fundamental details about the most important cement hydrates, the calcium silicate hydrates or CSH-phases (structure, hydration kinetics, bonding mechanism etc.) are still unknown. This is due to their small particle size (~20 nm), low ordering, heterogeneity and low stability. -Merlino S, Bonacorssi E, Armbruster T Am Mineral., 1999, 84: Merlino S, Bonaccorsi E, Armbruster T Eur J Mineral., 2001, 13 : E. Bonaccorsi, S. Merlino, H. F. W. Taylor, Cem. Conc. Res., 2004, 34, Occurs as a natural mineral C 300 C Tobermorite 1.4 nm Tobermorite 1.1 nm Tobermorite 0.9 nm

22 hydrated Alite Grain

23 Asbestos cement water pipes following decades of use VT: water-pipe (17 th district Budapest, age unknown) VR: water-pipe operating for 44 years (1028 Budapest, between )

24 Strong internal alteration: stronger interaction with potable water than with soil humidity potable water is typically hard water in the region

25 Carbonation Concrete will carbonate if CO 2 from air or from water enters the concrete according to: - Ca(OH) 2 + CO 2 --> CaCO 3 + H 2 O - Calcium Silicate hydrates + CO 2 -> various intermediate mineral phases Various intermediate mineral phases -> CaCO 3 + H 2 O + SiO 2.nH 2 O - Ferrite hydrates + CO 2 --> CaCO 3 + hydrated alumina + iron oxides The carbonation process requires the presence of water because CO 2 dissolves in water forming H 2 CO 3. -If the concrete is too dry (RH <40%) CO 2 cannot dissolve and no carbonation occurs. - If the concrete is too wet (RH >90%) CO 2 cannot enter the concrete and the concrete will not carbonate. Optimal conditions for carbonation occur at a RH of 50% (range 40-90%).

26 Carbonation Concrete surface Carbonated zone

27 Carbonation combined with leaching Natural water can leach carbonates, formation of soluble calcium bicarbonate, the only remaining product is a gelatinous silica product. Diagram illustrating the zones of soft water attack (St John et al. 1998)

28 Free asbestos web on the inner surface of the water pipe, in direct contact with the potable water

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31 Mg, Si, O serpentine asbestos Ca cement material VR red alteration zone, thin section

32 Mg, Si, O serpentine asbestos Ca cement material VR red alteration zone, thin section

33 XPD of the VR water pipe B: brownmillerite; C: calcite; CHR: chrysotile; G: gypsum; K: kaolinite; L: larnite; P: portlandite; Q: quartz; V: vaterite; 10Å: TOT layer silicate red rectangle indicates the region of the main clinker phases

34 Strong internal alteration: stronger interaction with potable water than with soil humidity potable water is typically hard water in the region

35 XPD of the VR water pipe B: brownmillerite; C: calcite; CHR: chrysotile; G: gypsum; K: kaolinite; L: larnite; P: portlandite; Q: quartz; V: vaterite; 10Å: TOT layer silicate red rectangle indicates the region of main clinker phases

36 Alteration zonation in water pipes 1. Free web of asbestos fibres (thickness: 1 2 mm) with a dark brown crust (thickness ~0.1 mm) Crust composition: Fe-Mn-(oxy)-hydroxides (inferred from dark brown colour), quartz, calcite, gypsum, organic material(?) 2. Red alteration zone (thickness: 1 2 mm) Composition: vaterite, calcite, chrysotile, (quartz) 3. Pale grey alteration zone (thickness: 3 mm) Composition: calcite, vaterite, brownmillerite, chrysotile 4. Dark grey (unaltered) zone (thickness: 6 10 mm) Composition: portlandite, brownmillerite, larnite (tricalciumsilicate?, mayenite?), chrysotile 5. Yellow outer mineral crust (thickness: 1 mm) Composition: calcite, gypsum, quartz, kaolinite?, 10Å-layer silicate

37 Conclusions 1. Asbestos cement water pipes are corrobated both by drinking water (inside) and soil humidity (outside). The inside interaction is much more pronounced than the outside one. 2. Interaction with drinking water: cement material (and partly also serpentine asbestos) is replaced by CaCO 3 polymorphs, calcite and vaterite. Alteration zonation develops outwards. 3. As a consequence of cement consumption, a free web of asbestos fibers develops on the inner surface of the pipe, which is a potential contaminant of drinking water. The associated health risk has to be assessed in the future (water pipes laid down in the ies may serve till !!!!!). 4. Interaction with soil humidity (outer pipe surface): cement is only a little bit attacked, and a protective mineral layer made of calcite and gypsum develops on the outer surface of the pipe. No real alteration zonation inwards. 5. The observed effects will be the same for other fibre-cement pipes as well, therefore the replacing fibres need to be tested for their possible health effects.