Durability of composite cements

Transcription

1 LC 3 1 st International Conference on Calcined Clays for Sustainable Concrete Durability of composite cements Paweł Durdziński EPFL Lausanne,

2 We are and will be using more and more SCMs. What are therefore the.. IMPLICATIONS FOR DURABILITY

3 Generic approach Durability determined by microstructure: CHEMISTRY: Phase assemblage PHYSICAL: Pore structure 3

4 Main components CaO-SiO 2 -Al 2 O 3 SiO 2 gel Silica Fume Natural pozzolan Blended cements: Less calcium hydroxide Lower Ca/Si C-S-H C/S 0.83 Fly Ash F C/S 1.7 C-S-H C-A-S-H C Metakaolin Portland Cement Slag C 3 ASH 4 strätlingite Ca(OH) 2 Limestone Al(OH) 3 C 3 AH 6 C 3 A.xx Aft AFm 4

5 Composition of C-A-S-H after 4-5 years Very similar C/S ratios stabilised by presence of Portlandite Al usually higher, approaching Al/Si = 0.2 (except SF no Al) Thesis J.Rossen, EPFL, 2014

6 Pore structure Antoni et al. CCR 2012 With careful specimen preparation (do not dry at 105 C) MIP can be a reliable characterisation technique Blended pastes have higher overall porosity but lower size of connected pores 6

7 Durability mechanisms Importance of pore structure Chloride ingress Carbonation Sulfate attack ASR Importance of phase assemblage

8 Impact of SCMs degradation 1 st mechanism 2 nd mechanism General Impact SCMs Chloride Transport Binding Positive Carbonation Binding Transport Negative Sulfate Conversion of AFm embedded in C-S-H to ettringite Transport ASR Pore solution ph Alumina Positive Claimed positive

9 Effect of SCMs on durability against chloride Slides from: Mathieu Antoni PhD thesis EPFL Professor Mike Thomas University of New Brunswick Canada

10 Causes of concrete degradation 1% 4% 5% Others Freeze / Thaw Alkali Silica Reaction 90% Corrosion I am sorry I do not know the source or veracity of these figures, but they are probably pretty close to the truth

11 LCC, Chloride ponding, 2 years Total chloride content [%] PC MK30 MK-B Depth (mm) M. Antoni, PhD thesis, EPFL, 2013

12 Chloride migration test ASTM C : too high voltage, self heating Modified version of the test, from SIMCO Lower voltage, run for at least 10 days Current and Voltage monitored daily Use in combination with model to estimate diffusion coefficient of ions in cementitious materials 12 M. Antoni, PhD thesis, EPFL, 2013

13 Migration test: summary Current [ma] PC U=10V D OH =7.8E-11 Sample 1 Sample 2 Model Time [hours] Current [ma] MKB45 Sample 1 Sample 2 Model U=30V D OH 7.5E Time [hours] Model shows an improvement of diffusion coefficient by factor 3 between PC and MK30 factor 10 between PC and MK-B45 13 M. Antoni, PhD thesis, EPFL, 2013

14 Effect of SCMs on chloride transport Transport is dominant Binding is secondary, but important C-S-H (a lot which binds a little) Freidel s salt (a little which binds a lot) overall play roughly equal roles in binding. SCMs with high available alumina content will give significant increase in binding capacity

15 Effect of Fly Ash on Permeability Permeability (m 2 ) Lab. Concretes W/CM = Fly Ash OPC

16 Harmon G.S. Ontario Deck Concrete Mass Exterior OPC (kg/m 3 ) Fly Ash (kg/m 3 ) - 63 W/CM d strength (MPa) Built in 1962/5 Sampled in 1993

17 Effect of fly ash on permeability long term effect Permeability (m 2 ) Lab. Concretes W/CM = Fly Ash OPC Field Concretes W/CM = Source Mike Thomas

18 Effect of slag on RCPT W/CM = 0.50 Control (no slag) 25% Slag 50% Slag Source Mike Thomas

19 Effect of SCM on RCPT Values W/CM = 0.40 RCPT (Coulombs) PC Age (days)

20 Effect of SCM on RCPT Values W/CM = 0.40 RCPT (Coulombs) PC 25 FA Age (days)

21 Effect of SCM on RCPT Values W/CM = 0.40 RCPT (Coulombs) PC 8 SF 25 FA Age (days)

22 Effect of SCM on RCPT Values W/CM = 0.40 RCPT (Coulombs) PC 8 SF 25 FA 4 SF & 20 FA Age (days)

23 Ternary Blend Fly Ash & Slag 4000 W/CM = 0.50 RCPT at 1 Year (Coulombs) Fly Ash (%) Slag (%) 30

24 Example of influence of fly ash on binding

25 CORROSION OF STEEL IN OPC & 30FA CONCRETE 35 N/mm 2, 11 years FAMCET Exposure OPC Source: CSIR Contract nr:bb OPC/30FA Slide from

26 Carbonation Slides from Professor Mike Thomas and from Mathieu Antoni

27 Carbonation CO 2 CO H O CO H + ph < 9 ( OH ) CaCO H O 2 + CO3 + 2 H + Ca ph > 13 CO 2 from the atmosphere reacts with cement hydration products such as calcium hydroxide Ca(OH) 2 and reduces the ph of concrete from above 13 to less than 9 Note: other hydration products (C-S-H) also carbonate

28 High ph (non-carbonated) Low ph (carbonated) Steel in pristine condition Steel corroding

29 Carbonation-Induced Corrosion on Building Facade Courtesy P. Harwood

30 Rate of Carbonation X carb X carb = depth of carbonation at time, t = k t m m = constant typically between 0.4 to 0.6, usually assumed m = 0.5 k = rate of carbonation, which depends on: CO 2 in the environment (typically to % in air) W/CM SCM Content - increases with increasing fly ash and slag Limestone content increases with increasing limestone content Duration of moist curing Exposure condition of the concrete Rates of carbonation are negligible in well-cured, good quality concrete!

31 Preventing Carbonation-Induced Corrosion d = k t k increases as: Depth of Carbonation, d SCM increases Limestone increases W/CM increases Strength decreases Curing decreases SCM content - one of many factors t

32 Carbonation Reducing calcium content - reduces buffer to carbonation Mg S Na K rest +H 2 O CaO Ca(OH) 2 O Reduce Ca Ca CO 2 +CO 2 Si CaCO 3 Fe Al All CaO content can react with CO 2, not just Portlandite CH + CO 2 CaCO 3 + H 2 O C-S-H + CO 2 various intermediates CaCO 3 + SiO 2 nh 2 O + H 2 O Aluminate hydrates + CO 2 CaCO 3 + hydrated alumina Ferrite hydrates + CO 2 CaCO 3 + hydrated alumina + iron oxides

33 Reduction of buffering can be offset with good curing From BRE via MDA Thomas, UNB

34 Longer term carbonation, In long term diffusion of gas through carbonated layer dominates rate M.D.A. Thomas Supplementary cementitious materials in Concrete 34

35 Calcined Clays vs Curing Time after 6 months High-grade calcined clay Worst carbonation resistance for LC 3-50 blends Similar carbonation depth for PC 1D and LC D Must be taken into account for future protocol EPFL Indoor 1 day 3 days 28 days PC PPC30 LC 3-50

36 Carbonation depths, phenolphthalein test 2.5 Natural carbonation (230d) 18 3%CO2 carbonation Carbonation depth [mm] OPC B45 Carbonation depth [mm] B45 OPC Time [sqrt(months)] 36 M. Antoni, PhD thesis, EPFL, 2013

37 XRD investigation Atmospheric carbonation, 0.04% CO2 In both cases, all hydrates tend to carbonate, not only portlandite as often assumed Only Calcite forms for PC, small amounts of Aragonite and Vaterite additionally form in B45 37 M. Antoni, PhD thesis, EPFL, 2013

38 XRD Investigation Accelerated carbonation, 3% CO2 All hydrates show again carbonation, to a much larger extent. Anhydrous phases also show partial carbonation In PC, mostly calcite forms with carbonation, with small amount of vaterite In B45, mostly aragonite forms, then calcite and vaterite, 38 M. Antoni, PhD thesis, EPFL, 2013

39 Accelerated tests: Change in carbonated phases, changes in microstructure Formation of higher density phases (aragonite) higher porosity than in natural conditions Therefore pore structure will be coarser than in natural carbonation conditions. Major factor in long term carbonation is the diffusion of gas through the CARBONATED layer 39 Danger of too pessimistic outlook from accelerated tests

40 Practical risk of carbonation corrosion? 100 Corrosion rate, high density concrete Corrosion rate, low density concrete Concrete carbonation rate Carbonation takes place in environments which are too dry for active corrosion Relative intensity [%] Relative humidity [%] Conversely conditions with enough humidity for active corrosion will only carbonate very slowly Can be dealt with by correct design and cover depths 40

41 Durable concrete? Is this concrete vulnerable to carbonation corrosion? 41

42 Effect of SCMs on carbonation #1 - Capacity to bind CO 2 is most important. Cement with less chemical CO 2 inevitably has less capacity to bind CO 2 #2 - Transport (through carbonated layer) is secondary. Good curing can partially offset effects of lower binding capacity The balance between these effects needs to be further explored Important for reinforced concrete, but there is no obstacle to using low CaO binders in non reinforced applications: blocks, bricks, pavers roof tiles

43 43 Durable concrete?

44 Sulfate Attack Slides from Cheng Yu PhD thesis South East University, China and EPFL

45 Why worry about sulfate attack Others Rarely a problem in the field Freeze / Thaw Alkali Silica Reaction Corrosion 1% 4% 5% WHY Because we use sulfate resisting cements? Because concrete has w/c < 0.45? 90% Because exposure conditions not the same as in tests? Causes of degradation in reinforced concrete Also an excellent example of how performance tests designed for Portland cements can be totally misleading for blended cements

46 First a word about DEF (delayed ettringite formation) In this talk I will not speak about DEF Heat induced internal sulfate attack Only occurs due to high temperatures (>70 C during curing) If you have problems due to DEF, you almost certainly have problems due to temperature gradients If you apply good engineering practice to avoid thermal cracking, you should not have DEF.

47 What was the motivation for these studies 1. Conventionally sulfate resisting cements are those with a low content of C 3 A 2. To improve sustainability we now see an increasing amount of cements containing supplementary cementitious materials (SCMs). Can such cements be qualified as sulfate resisting? Wide diversity of prescriptive approaches throughout Europe. CEN TC 51 charged with finding a performance test >10 years of round robin testing failed to find a reproducible test 2004 approach to NANOCEM to help on understanding fundamentals.

48 Sulfate attack 4x4x16 mortar bar - cut walls to get 2x2x16 bar Na 2 SO 4 3/10/30 g/l (renewed after each measurement) measurement every 2 weeks expansion mass cut section for SEM analysis every 4 months (or according to interesting results) 48 foreseen duration: 3 years or until damage reference: modified ASTM C1012/ C1012M-10

49 Main findings of present studies Formation of zones, but difficult to relate to expansion and damage. AFt [after Gollop and Taylor, 1992] Expansion curves show take-off point corresponding to the onset of cracking.

50 Thesis Aude Chabrelie Link cracking-expansion Stage #1: induction with surface cracking Stage #2: stable expansion with deeper cracking Stage #3: unlimited expansion with bulk cracking Length changes correlate well with changes in elastic modulus & strength

51 Ettringite and Expansion TEXT BOOKS: Formation of ettringite during hydration, concrete still soft; no expansion Formation of ettringite later in hardened concrete gives expansion It is possible to have extensive formation of ettringite in hardened concrete without any damage: remember there are lots of pores Recent systematic study shows no relation between amount of ettringite formed and expasion

52 Expansion theories Crystallization pressure theory Scherer, Supersaturated solution, 2. Confined crystal growth of solid product For pressure > 2MPa, r < 100nm Scherer, 1999 This is only plausible theory

53 PC paste before sulfate attack 1. Large crystals of AFm/AFt in pocket 2. Fine AFm crystals in C-S-H 3. Pore solution?

54 SEM-EDS Mapping (SO 3 profiles) BUT, mainly reflects the sulfate uptake by solid phases, not the sulfate content in pore solution. Amount of sulfate in solid phase (Ettr. & Gypsum) Expansion

55 How to measure the sulfate content in pore solution Relationship between sulfate bound to C-S-H (S/Si) and [SO 4 2- ] in pore solution Barbarulo et al, 2008

56 Experiment and results: OPC Water cement ratio Sample size Sulfate concentration M cm 3g/L M cm 3g/L M cm 3g/L M cm 10g/L M cm 30g/L 3 month curing Surface removed T = 20 C V solution / V sample = 28 Renew every 2 weeks (every week at first 4 weeks)

57 Effect of concentration Expansion w/c= cm

58 120d Very similar penetration profiles What cause the different expansion at 120d?

59 5mm 120d

60 4mm 120d

61 3mm 120d

62 2mm 120d

63 EDS of C-S-H 120d 2mm 3mm 4mm The reaction between SO 2-4 and solid phase buffer the [SO 2-4 ] in pore solution, depth by depth. Sample in 30g/L has more sulfate bound in C-S-H than low concentration ones, probably can explain the difference of expansion

64 Explanation of expansion As sulfate ions penetrate in the cement paste, they react with unconstrained aluminate phases, mainly AFm in the pockets, this buffers the increase of [SO 2-4 ] in pore solution. When all freely transformable Al 2 O 3 has reacted, the [SO 2-4 ] in solution will increase, constrained AFm within C-S-H can then react to ettringite and exert expansion force. Once cracking occurs, SO 2-4 can entry freely and, react even with Ca 2+ to form gypsum in cracks.

65 Slag blends Expansion SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 K 2 O Na 2 O TiO 2 Slag w/c= S cm 3g/L 10g/L 30g/L Surface spalling

66 Effect of SCMs on sulfate resistance Complicated Interaction of: buffering effects, amount of constrained AFm and perhaps transport effects Cannot say that blended cements have chemical resistance to sulfate attack if they contain alumina. Need for test methods more representative of reality where surface loss is more important than macroscopic expansion

67 Alkali - Silica Reaction Slides from Theodore Chappex PhD thesis EPFL

68 Effect of blended pastes on ASR expansion SCMs are effective in reducing deleterious ASR (empirical additions): Field & Lab experience: Samples in alkaline solution PC 5% 10%SFQ 10%MK 15%SFQ 15%MK Silicon and Aluminium addition are involved in the reduction of expansion Aluminium rich SCMs are more effective against ASR The exact mechanism by which it happens is unclear!

69 Systems Studied Experimental systems: Si SF 7.65, % 5, 10, 15% MK w w Al Q - Filler 7.35, % w 95, 90, 85% w OPC OPC EDS Pore solution extraction TGA Paste sample Pore solution Piston

70 C-S-H EDS analysis 300 days: The Si/Ca increase with increasing substitution for both systems (MK and SF-Q) The Al/Ca is constant for SFQ at all substitutions levels and increase with MK substitution Pastes can be compared in term of pore solution concentration

71 Pore solution analysis 5MK 5SFQ 10MK 10SSFQ 15MK 15SFQ - The silanol binding capacity is confirmed - No improve of fixation is observed up to 2 years in Al rich systems Al doesn t increase the alkali fixation capacity of C-S-H in blended pastes! Another phenomena is involved to control ASR in presence of Al!

72 C-S-H fixation capacity estimation 90 days: 300 days: Comparatively, the fixation capacity of SFQ and MK are similar. Aluminium has no influence on the fixation capacity of alkalis New approach: focus on the aggregates

73 Pore solution composition of MK pastes MK systems provide aluminium ions in the pore solution A peak of aluminium appears during the first 90 days

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76 76 There is a clear influence of aluminium ions on aggregates gel formation!

77 Effect of SCMs on ASR First effect is lowering of ph of pore solution Lower C/S C-S-H absorbs more alkalis SCM high in alumina also inhibit directly dissolution of amorphous silica

78 Concluding remarks Future cements will be based on Portland cement clinker with increasing amounts of SCMs Need to be able to use divers range of materials, generic approach to understanding durability Durability is not an intrinsic materials property, but a result of interaction of material with its environment Effect of SCMs of durability: Mostly positive: Chloride, ASR Sulfate attack is colmpex and has to be better understood Faster carbonation? Yes, but low risk of carbonation corrosion in most concrete. If we are serious about more sustainable concrete we need to use cements with lower CO 2 emissions e.g. LC 3 clinker/ calcined clay / limestone blends

79 No concrete is sustainable without being durable Sustainable use of concrete should adapt the composition to the application Use only what you need 79

80 80 Questions? THANK YOU