Durability of composite cements

Paweł DurdzińskiEPFL

Lausanne, 01.07.2015

LC3 1st International Conference on Calcined Clays for Sustainable Concrete

IMPLICATIONS FOR DURABILITY

We are and will be using more and more SCMs.

What are therefore the..

Generic approach

Durability determined by microstructure:

3

CHEMISTRY:Phase assemblage

PHYSICAL:Pore structure

Main components CaO-SiO2-Al2O3

4

C3A.xxAftAFm

Portland Cement

Slag

Fly Ash

C

Natural pozzolan

SilicaFume

Limestone

Metakaolin

F

C3ASH4

Ca(OH)2 Al(OH)3

SiO2 gel

C3AH6

strätlingite

C/S 1.7

C-S-H

C/S 0.83

C-A-S-H

Blended cements:• Less calcium hydroxide• Lower Ca/Si C-S-H

Composition of C-A-S-H after 4-5 years

Very similar C/S ratios – stabilised by presence of PortlanditeAl usually higher, approaching Al/Si = 0.2 (except SF no Al)

Thesis J.Rossen, EPFL, 2014

Pore structure

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

Antoni et al.CCR 2012

Durability mechanisms

Chloride ingress

Carbonation

Sulfate attack

ASR

Impo

rtan

ceof

pha

se a

ssem

blag

e

Importance

of pore structure

Impact of SCMs

degradation 1st mechanism 2nd mechanism GeneralImpact SCMs

Chloride Transport Binding Positive

Carbonation Binding Transport Negative

Sulfate Conversion of AFm embedded in C-S-H to ettringite

Transport Claimed positive

ASR Pore solution pH Alumina Positive

Effect of SCMs on durabilityagainst chloride

Slides from:Mathieu Antoni

PhD thesisEPFL

Professor Mike ThomasUniversity of New Brunswick

Canada

Causes of concrete degradation

1%

4%5%

90%

Others

Freeze / Thaw

Alkali Silica Reaction

Corrosion

I am sorry I do not know the source or veracity of these figures, but they are probably pretty close to the truth

LCC, Chloride ponding, 2 years

11

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35 40

PC MK30MK-B45

Tota

l chl

orid

e co

nten

t [%

]

Depth (mm)M. Antoni, PhD thesis, EPFL, 2013

Chloride migration test ASTM C1202-97: too high voltage, self heating

⇒ Modified version of the test, from SIMCO

Lower voltage, run for at least 10 daysCurrent and Voltage monitored daily

Use in combination with model to estimate diffusion coefficient of ions in cementitious materials

12M. Antoni, PhD thesis, EPFL, 2013

Migration test: summary

13

0

2

4

6

8

10

12

14

16

0 200 400 600

Curr

ent [

mA]

Time [hours]

MKB45 Sample 1Sample 2Model

U=30VD OH 7.5E-12

0102030405060708090

100

0 200 400 600

Curr

ent [

mA]

Time [hours]

PC

Sample 1Sample 2Model

U=10VD OH =7.8E-11

Model shows an improvement of diffusion coefficient by factor 3 between PC and MK30 factor 10 between PC and MK-B45

M. Antoni, PhD thesis, EPFL, 2013

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

Effect of Fly Ash on Permeability

Lab. ConcretesW/CM = 0.60 - 0.63

OPC

Fly Ash

10-17

10-18

10-19

10-20

10-21

Perm

eabi

lity

(m2 )

DeckConcrete

MassExterior

OPC (kg/m3) 236 147

Fly Ash (kg/m3) - 63

W/CM 0.57 0.59

28-d strength (MPa) 29 28

Built in 1962/5Sampled in 1993

Harmon G.S. Ontario

Lab. ConcretesW/CM = 0.60 - 0.63

Field ConcretesW/CM = 0.57 - 0.59

OPC

Fly Ash

10-17

10-18

10-19

10-20

10-21

Perm

eabi

lity

(m2 )

Effect of fly ash on permeability – long term effect

Source Mike Thomas

W/CM = 0.50

Control (no slag)

25% Slag

50% Slag

Effect of slag on RCPT

Source Mike Thomas

Effect of SCM on RCPT Values

100

1000

10000

10 100 1000Age (days)

RC

PT

(Cou

lom

bs)

100 PC

W/CM = 0.40

Effect of SCM on RCPT Values

100

1000

10000

10 100 1000Age (days)

RC

PT

(Cou

lom

bs)

100 PC25 FA

W/CM = 0.40

100

1000

10000

10 100 1000Age (days)

RC

PT

(Cou

lom

bs)

100 PC8 SF25 FA

Effect of SCM on RCPT Values

W/CM = 0.40

Effect of SCM on RCPT Values

100

1000

10000

10 100 1000Age (days)

RC

PT

(Cou

lom

bs)

100 PC8 SF25 FA4 SF & 20 FA

W/CM = 0.40

0 10 20 30

010

2030

0

1000

2000

3000

4000R

CP

T at

1 Y

ear (

Cou

lom

bs)

Fly Ash (%)

Slag (%)

W/CM = 0.50

Ternary Blend – Fly Ash & Slag

Example of influenceof fly ash on binding

CORROSION OF STEEL IN OPC & 30FA CONCRETE35 N/mm2, 11 years FAMCET Exposure

OPC OPC/30FASource: CSIR Contract nr:BB078 5600 5671

Slide from

Carbonation

Slides from Professor Mike Thomasand from Mathieu Antoni

CO2

CO2 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 9Note: other hydration products (C-S-H) also carbonate

pH > 13

pH < 9+− +→+ HCOOHCO 22322

( ) OHCaCOOHCaHCO 23223 22 +→++ +−

Carbonation

High pH (non-carbonated)

Steel in pristine condition

Low pH (carbonated)

Steel corroding

Courtesy P. Harwood

Carbonation-Induced Corrosion on Building Facade

k = rate of carbonation, which depends on:

• CO2 in the environment (typically 0.030 to 0.035 % 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!

mcarb tkX ⋅=

Xcarb = depth of carbonation at time, t

m = constant typically between 0.4 to 0.6, usually assumed m = 0.5

Rate of Carbonation

t

Dept

h of

Car

bona

tion,

dtkd ⋅= k increases as:

• SCM increases• Limestone increases• W/CM increases• Strength decreases• Curing decreases

Preventing Carbonation-Induced Corrosion

SCM content - one of many factors

Reducing calcium content - reduces buffer to carbonation

Mg

S

AlFe

KNa

rest

Ca

Si

O

Reduce Ca

CaCO3

CaO Ca(OH)2

+H2O

CO2⇑ +CO2

C-S-H + CO2 → various intermediates → CaCO3 + SiO2nH2O + H2OCH + CO2 → CaCO3 + H2O

Aluminate hydrates + CO2 → CaCO3 + hydrated aluminaFerrite hydrates + CO2 → CaCO3 + hydrated alumina + iron oxides

All CaO content can react with CO2, not just Portlandite

Carbonation

Reduction of buffering can be offset with good curing

From BRE via MDA Thomas, UNB

Longer term carbonation,

34

M.D.A. Thomas Supplementary cementitious materials in Concrete

In long term diffusion of gas through carbonated layer dominates rate

Calcined Clays vs Curing Time after 6 months

High-grade calcined clay

Worst carbonation resistance for LC3-50 blends

Similar carbonation depth for PC 1D and LC3-50 3D

Must be taken into account for future protocol

1 day 3 days 28 days

PC

PPC30

LC3-50

EPFL Indoor

Carbonation depths, phenolphthalein test

36

Natural carbonation (230d) 3%CO2 carbonation

0.0

0.5

1.0

1.5

2.0

2.5

OPC B45

Carb

onat

ion

dept

h [m

m]

02468

1012141618

0 1 2 3Ca

rbon

atio

n d

epth

[mm

]

Time [sqrt(months)]

B45

OPC

M. Antoni, PhD thesis, EPFL, 2013

XRD investigationAtmospheric 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

37M. Antoni, PhD thesis, EPFL, 2013

XRD InvestigationAccelerated 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,

38M. Antoni, PhD thesis, EPFL, 2013

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 outlookfrom accelerated tests

Practical risk of carbonation corrosion ?

0

20

40

60

80

100

0 20 40 60 80 100

Corrosion rate, high density concreteCorrosion rate, low density concreteConcrete carbonation rate

Rela

tive

inte

nsity

[%]

Relative humidity [%]

Carbonation takes place in environments which are too dry for active corrosion

Conversely conditions with enough humidity for active corrosion will only carbonate very slowly

Can be dealt with by correct design and cover depths

40

Durable concrete?

41

Is this concrete vulnerable to carbonation corrosion?

Effect of SCMs on carbonation

#1 - Capacity to bind CO2 is most important.

Cement with less chemical CO2 inevitably has less capacity to bind CO2

#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

Durable concrete?

43

Sulfate Attack

Slides from Cheng YuPhD thesis

South East University, China and EPFL

Why worry about sulfate attack

1%

4% 5%

90%

OthersFreeze / ThawAlkali Silica ReactionCorrosion

Causes of degradation in reinforced concrete

Rarely a problem in the field

WHY• Because we use sulfate resisting

cements?• Because concrete has

w/c < 0.45?• Because exposure conditions not the

same as in tests?

Also an excellent example of how “performance tests” designed for Portland cements can be totally misleading for blended cements

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.

What was the motivation for these studies

1. Conventionally “sulfate resisting” cements are those with a low content of C3A

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.

Sulfate attack

4x4x16 mortar bar - cut walls to get 2x2x16 bar

Na2SO4 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)

foreseen duration: 3 years or until damage

reference: modified ASTM C1012/ C1012M-1048

Main findings of present studies

• Formation of “zones”, but difficult to relate to expansion and damage.

• Expansion curves show “take-off” point corresponding to the onset of cracking.

[after Gollop and Taylor, 1992]

AFt

Link cracking-expansion Stage #1: induction with surface cracking

Stage #2: stable expansion with deeper cracking Stage #3: unlimited expansion with bulk cracking

Thesis Aude Chabrelie

Length changes correlate well with changes in elastic modulus & strength

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

Crystallization pressure theory Scherer, 2002 2004

1. Supersaturated solution,

2. Confined crystal growth of solid product

For pressure > 2MPa, r < 100nm Scherer, 1999

Expansion theories

This is only plausible theory

1. Large crystals of AFm/AFt in “pocket”

3. Pore solution?

2. Fine AFm crystals in C-S-H

PC paste before sulfate attack

SEM-EDS Mapping (SO3 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

How to measure the sulfate content in pore solution

Relationship between sulfate bound to C-S-H (S/Si) and [SO42-] in pore

solution

Barbarulo et al, 2008

Experiment and results: OPC

Water cement ratio Sample size Sulfate concentration

M1 0.55 1×1×16 cm 3g/L

M2 0.55 2×2×16 cm 3g/L

M3 0.55 4×4×16 cm 3g/L

M4 0.55 2×2×16 cm 10g/L

M5 0.55 2×2×16 cm 30g/L

V solution / V sample = 28Renew every 2 weeks (every week at first 4 weeks)

3 month curing Surface removedT = 20°C

Effect of concentration

Expansion

w/c=0.55 2×2×16cm

Very similar penetration profiles

120d

What cause the different expansion at 120d?

5mm

120d

4mm

120d

3mm

120d

2mm

120d

4mm2mm 3mm

120d

The reaction between SO42- and solid phase buffer the [SO4

2-] 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

EDS of C-S-H

As sulfate ions penetrate in the cement paste, they react with unconstrained aluminate phases, mainly AFm in the pockets, this buffers the increase of [SO4

2-] in pore solution.

When all freely transformable Al2O3 has reacted, the [SO4

2-] in solution will increase,constrained AFm within C-S-H can then react to ettringiteand exert expansion force.

Once cracking occurs, SO42- can entry freely and,

react even with Ca2+ to form gypsum in cracks.

Explanation of expansion

w/c=0.55 55S5 2×2×16cm

Slag blends

Expansion

3g/L

10g/L

30g/L

Surface spalling

SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2

Slag 31.53 15.85 0.32 36.87 9.69 2.76 0.44 0.332 1.76

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

Alkali - Silica Reaction

Slides from Theodore ChappexPhD thesis

EPFL

Effect of blended pastes on ASR expansion

SCMs are effective in reducing deleterious ASR (empirical additions):

Field & Lab experience:

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!

5%

15%SFQ15%MK

10%SFQ

10%MK

PC

Samples in alkaline solution

Systems StudiedExperimental systems:

MK

OPC

Q - Filler

SF

OPC

Si

Al

95, 90, 85%w

5, 10, 15%w7.65, …%w

7.35, …%w

Paste sample

Pore solution

Piston

EDS Pore solution extraction TGA

C-S-H EDS analysis

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

300 days:

5MK

5SFQ

10MK

10SSFQ

15SFQ15MK

Pore solution analysis

- 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!

Comparatively, the fixation capacity of SFQ and MK are similar. Aluminium has no influence on the fixation capacity of alkalis

C-S-H fixation capacity estimation

New approach: focus on the aggregates

90 days: 300 days:

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

74

75

76

There is a clear influence of

aluminium ions on aggregates gel

formation!

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

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 CO2 emissions

e.g. LC3 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”

THANK YOUQuestions?

80