pore solution analysis as a tool for studying early –age...

R.D. Hooton, T. Ramlochan, and M.D.A. Thomas

Pore Solution Analysis as a Pore Solution Analysis as a Tool for Studying Early Tool for Studying Early ––Age Age

Hydration & Predicting Future Hydration & Predicting Future DurabilityDurability

Cement Hydration Summit, Quebec, July 2009

ContentsContents

Obtaining Pore Solutions

Studying Early-age Hydration

Portland cements

SCM Effects

Studying Durability

Effects on ASR

The role of Ca/Si and alumina on alkali and chloride binding

IntroductionIntroduction

Much of the mixing water is not used for hydration

Remains in capillary pores and as physically adsorbed water in “gel”

and interlayer pores of cement paste

Provides a transport medium for aggressive ions

Composition of pore ‘fluid’

or ‘solution’

in the capillary pores can be affected by:

Binder type and composition

(e.g., blended cements)

Mix proportions

(e.g., w/cm)

Duration and type of storage/exposure

(e.g., leaching, carbonation)

Aggregate

type (e.g., feldspars release alkalis)

0

10

20

30

40

50

60

70

8 9 10 11 12 13pH

Dis

solv

edSi

lica

(mm

ol/L

)

Pore solution compositionsPore solution compositions

Portland cement concretes are naturally alkaline:

cement phases react to produce solutions that are saturated with CH

small percentages of Na2

O and K2

O in the cement phases as alkali

sulphates (e.g., arcanite; aphthitalite, calcium langbeinite)

alkalis in SCMs

Solubility Curve of Amorphous Silica(Tang & Su-fen, 1980)

The principal dissolved ions in solution are K+, Na+, and Ca2+

cations, and SO4

2–

and OH–

anions; to a much lesser extent silicate and aluminate species.

Charge balance

between cations and anions activity vs. concentration

Pore solution expressionPore solution expression

Longuet, et al. (1973); Barneyback

and Diamond (1981)

Pore solutions can be expressed under pressure

(> 400 MPa), or by centrifugal force (and/or displacement with a heavy liquid) and analyzed.

Extractions of pore fluids from paste/mortars helps our understanding

of hydration chemistry, mass transport, and mechanisms such as DEF and ASR.

Criticisms that the technique is not representative possibility of concentration gradients from discontinuities in pore structure

Analyses of pore solutions can be done by ICP-

AOES, IC, titration, flame photometry, etc.

Used to speciate

ions in solution

Small sample sizes

Obtaining Pore SolutionObtaining Pore Solution A partial list of A partial list of ““trickstricks””

Obtaining satisfactory samples of pore solution from pore squeezing is not trivial.

High w/c

samples (at least 0.5) of paste will provide more solution.

Samples should be sealed to prevent carbonation, leaching, or dilution from outside water.

While maximum pressures of 80,000 psi are typical, slow cycling between 50,000 and 80,000 psi will typically increase yield.

Compressed gas or vacuum can be used to get all expressed fluid from the device (design of the base with 2 holes having threaded connections is improvement over original Barneyback

& Diamond design).

Samples obtained are usually small and need to be diluted for analysis

After analysis, need to check for electro-neutrality of the sum of anions vs

cations. Ie. If not neutral, then some ion has been missed.

Other considerationsOther considerations

CO2

goes into solution to give CO32−

ion, which react with Ca2+

to produce CaCO3

. OH−

and Ca2+

ions are provided by dissolution of CH and reduction in Ca/Si of C-S-H simultaneously. OH−

(and alkalis) also removed from solution, resulting in reduction in pH (below 10).

Leaching of alkali hydroxide will slowly reduce the pH

The reduction in OH−

concentration results in an increase in Ca2+

concentration (buffered solution).

Therefore, need to prevent carbonation of samples and carbonate exposure of collected solutions prior to analysis. Analysis should be completed as soon as possible.

Check for ElectroCheck for Electro--neutralityneutrality

After analysis, need to check for electro-

neutrality of the sum of anions vs

cations. Ie. If

not neutral, then some ion has been missed.

Bleszynski, 2002

Pore Solution During EarlyPore Solution During Early--age Hydrationage Hydration

The initial stage of hydrationThe initial stage of hydration

Alkali sulphates present in the cement (clinker) dissolve within

seconds, due to their high solubility, contributing SO4

2–, Na+, and K+

~Balance most SO42–

present as alkali sulphates

Syngenite (K2

SO4

·CaSO4

·H2

O)

The alkali concentrations may vary from ~5-50 mmol/L for Na+

and ~20-

400 mmol/L for K+

depending on the alkali sulphate content of the clinker and the w/cm.

Typically [K+] > [Na+] because most alkalis are present as K2

O.

The initial

SO42–

concentration (up to ~200 mmol/L) is set by the solubility of the alkali sulphates present (supersaturated with respect to gypsum).

Dissolution time of the calcium sulphate depends on the form of calcium sulphate present, in the order: hemihydrate/ -CaSO4

, dihydrate, and anhydrite.

The initial stage of hydrationThe initial stage of hydration

Upon contact with water both calcium and silicate ions go into solution.

The relatively high silicate concentrations that initially occur

quickly fall to < 0.05 mmol/L. Silicate ions continue to enter the pore fluid but their concentration remainsremains

low

throughout.

Concentration of Ca2+

continues to increase

and may exceed 20 mmol/L, which is the saturation of Ca(OH)2

. Ca2+

ions are also supplied by the free lime.

O2−

ions derived from the calcium silicates enter the fluid phase as OH−:O2−

+ H+

→ OH−

The concentration of hydrous alumina (Al(OH)4−) in the fluid phase is low

throughout; below ~0.1 mmol/L.

The induction periodThe induction period

Little

change in concentration of ions in solution during dormant period

does not mean there is nothing occurring.

Indicates an approximate balance

between the dissolution of the cement phases and precipitation of product.

Diamond, 1983.

The induction periodThe induction period

Ettringite

begins to form almost immediately on mixing.

Formation of ettringite consumes Ca2+

and SO42−

from solution.

Sulfate ion level is maintained by concurrent dissolution of the calcium

sulphate (gypsum and anhydrite), which dissolves to add Ca2+

and additional SO4

2−

As long as calcium sulphate is still present, concentration of SO4

2−

in the pore fluid changes only slightly.

Diamond, 1983.

The acceleration periodThe acceleration period

Calcium sulphate (syngenite?) becomes completely dissolved during the acceleration phase.

SO42−

concentration starts to decline due to continued formation of AFt, as well as adsorption of SO4

2−

by the C-S-H

K+

and Na+

are also taken up by the C-S-H.

When the calcium sulphate is depleted, the concentration of SO4

2−

subsequently declines to values less than ~5 mmol/L by 1 day.

Diamond, 1983.

The acceleration periodThe acceleration period

Reduction in SO42−

ion concentration not accompanied by corresponding reduction in cation

concentration.

Electrical neutrality is maintained by ‘replacement’

of sulfate ions with OH−

ions.

OH−

ion concentration is much higher after ‘replacement’

(pH > 13).

Portlandite (calcium hydroxide) also precipitates from the fluid phase

The concentration of Ca2+

declines gradually (to values less than the solubility of CH).

Diamond, 1983.

What if there is no calcium sulphate?What if there is no calcium sulphate?

Alkali sulphates dissolve quickly

Without calcium sulphate SO42−

concentration begins to decrease immediately

As long as there is sufficient C3

A to consume the extra SO4

2−

as ettringite, ‘replacement’

of SO42−

by OH−

ions will take place.Diamond, 1983.

What if there is too much calcium sulphate?What if there is too much calcium sulphate?

Extra calcium sulphate does not increase the SO4

2−

concentration

Sulphate concentration maintained for longer period

‘Replacement’

of SO42−

by OH−

ions does not occur

The result is the OH−

concentration is suppressed.

Ca2+

concentration does not decrease

Diamond, 1983.

Long term changes in pore solutionLong term changes in pore solution

Beyond about 1 day the only ions in solution above concentrations of a few mmol/L are K+, OH−, and Na+.

40-60% of the Na+

and 50-70% of the K+

are present in the pore fluid (some in major cement phases or in C-S-H).

Ultimate concentrations typically range from 5-250 mmol/L for Na+

and 75-700 mmol/L for K+.

Concentrations tend to rise

slightly approaching a limit after about 28-90 days (some studies show concentrations passing through a maxima and then decreasing slightly).

Primarily due to consumption of the fluid phase (from ongoing hydration).

Additional amounts of alkali will enter the pore fluid as the major cement phases hydrate and they are released (does not seem to have a great influence on the pore solution).

AlkalisAlkalis

Alkalis accelerate hydration at early age.

Attributed to an increase in the permeability of the layer of hydration product surrounding the alite grains after reaction has become diffusion controlled.

Correlation between OH−

concentration and Na2

Oe at 28 days (w/cm 0.5).

Ultimate concentrations, will therefore, depend on cement

alkalis.

Alkali-aggregate reactions. Nixon and Page, 1987.Diamond and Penko, 1988.

Na2Oe, wt. %

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

[OH

– ], m

ol/L

0.0

0.2

0.4

0.6

0.8

1.0Canham, 1986Longuet et al., 1973Page and Vennesland, 1983Diamond, 1981Ramlochan, 2000Struble, 1987Kollek et al., 1986Barneyback, 1983Diamond, 1983

r 2 = 0.93

y = 0.722x – 0.004

Effect of temperatureEffect of temperature

The ‘solubility’

of ettringite increases significantly with temperature.

Does not form ettringite

As a consequence:

Sulphate concentration does not decrease significantly during the heat curing.

OH−

concentration is suppressed.

Effects of Effects of SCMsSCMs

The net result with use of SCMs

is they lower

the concentration of alkalis and hydroxyl in the pore fluid (more than if they act as

an inert diluent of the Portland cement).

pH < 13 depending on replacement levels and SCM.

‘Secondary’

C-S-H deficiency in Ca2+

(alkali substitution)

Effects of Effects of SCMsSCMs

With blast-furnace slag, initial alkali levels are not much below an inert diluent; reaching a constant value beyond 28-90 days.

With silica fume a marked decrease in alkali concentration occurs at early age.

Some increase in alkalinity over time has been observed; possibly due to release of alkalis.

Bleszynski, 2002.

Effects of Effects of SCMsSCMs

Similar observations have been reported with fly ash in spite of the often higher total alkalinity of the binder.

Fly ashes unusually high in alkali can increase alkali concentrations in the pore fluid (above low-alkali cement alone).

Metakaolin significantly reduces alkalinity at higher concentrations.

Shehata, 2001.

Ramlochan, 2000.

0.0

0.1

0.2

0.3

0 6 12 18 24

Age (months)

Expa

nsio

n (%

)

025355065

Slag (%)

ASR Expansion and cracking can be controlled by limiting alkalies

or using

SCMs—but why?

•Cement pastes -

W/CM = 0.50 using

cements with 0.61, 0.76, and 1.09%

Na2

OE

.

• Sealed and cured at 23oC

•

Pore pressed at range of ages from

1 to 730 days.

•

Solution analyzed by titration (OH) &

flame photometry (Na & K)

ASR: Role for Pore Solution Analyses

Bleszynski, Hooton & Thomas, 2002

Effect of Slag and Silica fume on Pore Solution Alkalinity @ 91 Days

Summary of 2-Year Paste Specimen Pore Solution Alkalinity

A 200-250 mM/l threshold had been suggested by others (eg. Diamond 1983)

Relationship between ASR Expansion and Pore Solution Alkalinity

Relationship between Expansion and Pore Solution Alkalinity

This data suggests that a suitable threshold to control expansion is 320-365 mM/L

Percent Reduction in Pore Solution Alkalinity wrt Portland Cement Control

Conclusions from SlagConclusions from Slag--SF ASR StudySF ASR Study

•

The effectiveness of silica fume or blast- furnace slag in controlling ASR expansion

is related to the ability of the SCMs

to reduce pore solution alkalinity and maintain its depressed levels over time.

•

These binders, as shown by Glasser, have lower Ca/Si C-S-H which promotes alkali binding

•

Slag and Slag-SF binders also have more Al in the C-S-H, which also promotes alkali binding and appears to prevent its release over time.

• Cement pastes -

W/CM = 0.50

• High-alkali cement (= 1.02% Na2

Oe

)

• Sealed and cured at 23oC

• Pore pressed at range of ages

• Solution analysed

by titration (OH)

& flame photometry (Na & K)

• 12 fly ashes

• Ternary mixes with silica fume

Fly Ash Pore Solution and Alkali Binding Fly Ash Pore Solution and Alkali Binding Studies: Studies: Thomas & Thomas & ShehataShehata

• CSH analysis by SEM/EDS

• Ca(OH)2

analysis by TGA

Alkali BindingAlkali Binding

“The hydration products of systems containing Portland cement (PC) and SCM have relatively low Ca/Si atomic ratio and this enhances the ability of the hydration products to bind alkalis and hence reduce their availability in the pore solution. This high alkali-binding capacity of hydrates of low Ca/Si ratio has been attributed to the hydrate's surface charge. As the Ca/Si decreases, the surface charge becomes less positive, or more negative, and attracts the alkali cations

(Na+ and K+) from the surrounding pore solution.”

Refs:

S.-U. Hong, F.P. Glasser, Alkali binding in cement pastes: Part I. The C–S–H phase, Cement and Concrete Research 29 (1999) 1893–1903.

F.P. Glasser, J. Marr, The alkali binding potential of OPC and blended cements, Il Cemento

82 (1985) 85–94.

Effect of 25% Fly Ash on Pore Solution Composition

High-Alkali Cement Paste with 25% Fly Ash

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600 700 800Age (days)

OH

Con

cent

ratio

n (M

/L)

Fly AshCaO / Na2 Oe

17.5 / 1.68

6.38 / 1.4113.6 / 3.77

27.7 / 1.65

Control

Effect of % Fly Ash on Pore Solution CompositionHigh-Alkali Cement Paste with ‘F’ & ‘C’ Fly Ash

0.0

0.2

0.4

0.6

0.8

1.0

0 200 400 600 800Age (days)

OH

Con

cent

ratio

n (M

/L)

70%

50%

25%

Control

27.7% CaO, 1.65% Na2 Oe 6.38% CaO, 1.41% Na2 Oe

0.0

0.2

0.4

0.6

0.8

1.0

0 200 400 600 800Age (days)

OH

Con

cent

ratio

n (M

/L)

70%

50%

25%

Control

Hydrate CompositionHydrate Composition• SEM/EDX analysis of paste samplesused in pore solution studies

• Composition of ‘inner’ C-S-H (effectof fly ash)

• Differences in pore solutioncomposition for different SCMscannot be explained onthe basis of increased binding by‘inner’ C-S-H

• Role of outer & ‘secondary’

C-S-H

?

Ca/Si

(Na+

K)/S

i

Portland cement

25% Low-CaO Fly Ash

Effect of 25% Fly Ash on Pore Solution CompositionHigh-Alkali Cement Paste with 25% High-Alkali Fly Ash

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 100 200 300 400 500 600 700 800Age (days)

OH

Con

cent

ratio

n (M

/L)

Fly AshCaO / Na2 Oe

15.9 / 8.46

12.3 / 8.45

18.9 / 8.73

Control

Increasing CaO

Less binding as C/S rises

Effect of Silica Fume on Pore Solution CompositionHigh-Alkali Cement Paste with Silica Fume

0.2

0.4

0.6

0.8

1.0

0 200 400 600 800 1000 1200Age (days)

OH

Con

cent

ratio

n (M

/L)

5% Silica Fume

10% Silica Fume

Control

Bound alkali is released slowly

Concrete ASR expansions were not controlled by 8% Silica Concrete ASR expansions were not controlled by 8% Silica Fume alone (confirmed by longFume alone (confirmed by long--term outdoor exposure term outdoor exposure studies)studies)

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

Ave

rage

Exp

ansi

on (%

)

0 200 400 600 800Age (days)

Control

8% SF

35% Slag

Ternary mixes

Effect of Fly Ash on Pore Solution CompositionHigh-Alkali Cement Paste with 5% Silica Fume & ‘F’ Fly Ash

0.2

0.4

0.6

0.8

1.0

0 200 400 600 800 1000 1200Age (days)

OH

Con

cent

ratio

n (M

/L)

5% Silica Fume

5SF / 15FA

Control

5SF / 10FA

‘F’ Ash = 6.38% CaO, 1.41% Na2 Oe

FA stabilizes bound alkali

In summary, alkali concentration in the pore solution is dependent on:

•

Na2

Oe•

CaO

•

SiO2

In the cementitious

system

(i.e. including portland

cement and all supplementary cementing materials)

Concentration of Na, K & OH

in pore solutionas Na2

OeCaO

SiO2&

Cement Composition & Pore Solution AlkalinityCement Composition & Pore Solution Alkalinity

But there is also a role of alumina in C-S-H in stabilizing bound alkali

0.0

0.5

1.0

1.5

2.0

0.00 0.05 0.10 0.15 0.20 0.25 0.30(Na2Oe x CaO)/(SiO2)2 of CM

OH

at 9

0 da

ys (M

ol/L

) .

Shehata, 2001 Unpublished Bleszynski, 2002 Ramlochan, 2000

R2 = 0.913

79 blends of:• Portland cement• Fly ash• Slag• Silica fume• Natural pozzolan

Cement Composition & Pore Solution AlkalinityCement Composition & Pore Solution Alkalinity

The role of alumina in alkali bindingThe role of alumina in alkali binding

S.-U. Hong, F.P. Glasser, Alkali sorption by C-S-H and C-A-S-H gels Part II. Role of alumina, Cement and Concrete Research 32 (2002) 1101–1111.

This explained the beneficial effects seen for fly ash, slag, & metakaolin

in alkali binding, as well as big improvements with

low Ca/Si C-S-H

C/S = 0.85 C/S

C/S = 1.5

Role of Alumina in Chloride BindingRole of Alumina in Chloride Binding

Similarly, pozzolans

or slag with higher alumina

contents also tend to bind more chlorides due to formation of increased quantities of chloro-

aluminates.

This was shown by Zibara

(PhD thesis supervised by

Hooton & Thomas 2002)

Cement pastes were cast and cured for 28d, then exposed to chlorides using the equilibrium method

Time to corrosion is extended by chloride bindingTime to corrosion is extended by chloride binding

Initiation period, tiPropagation period, tp

End of service life

Dam

age

Time

Cl , CO2 penetration

O2 diffusion,resistivity

(Tuutti, 1982)

Simplified Service-Life Model

Chloride BindingChloride Binding

Ion exchange reactionsIon exchange reactions

PhysiPhysi--sorptionsorption

Experimental method for Chloride BindingExperimental method for Chloride Binding

Chloride binding isothermsChloride binding isotherms

Effect of cement compositionEffect of cement composition——CC33

A contentA content

CC33

A and CA and C44

AF addition to cementAF addition to cement

C3A C4AF

Pure phases supplied by Lafarge

Phase transformation in CPhase transformation in C33

A pasteA paste

Effect of AlEffect of Al22

OO33

in in SCMsSCMs

on chloride bindingon chloride binding

w/cm = 0.5 @ 56d

25% Slag or Fly Ash increases binding

8% Silica Fume decreases chloride binding

8% Metakaolin

improves binding the most

Effect of carbonationEffect of carbonation

Desorption isothermsDesorption isotherms

Questions?Questions?