Pore Solution Analysis as a Tool for Studying Early –Age...
Transcript of 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?