Pore Solution Analysis as a Tool for Studying Early Age Hydration & Predicting Future Durability R.D. Hooton, T. Ramlochan, and M.D.A. Thomas Cement Hydration Summit, Quebec, July 2009
Contents 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
Introduction 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)
Pore solution compositions Portland cement concretes are naturally alkaline: cement phases react to produce solutions that are saturated with CH small percentages of Na 2 O and K 2 O in the cement phases as alkali sulphates (e.g., arcanite; aphthitalite, calcium langbeinite) alkalis in SCMs Dissolved Silica (mmol/l) 70 60 50 40 30 20 10 0 Solubility Curve of Amorphous Silica (Tang & Su-fen, 1980) 8 9 10 11 12 13 ph The principal dissolved ions in solution are K +, Na +, and Ca 2+ cations, and SO 4 2 and OH anions; to a much lesser extent silicate and aluminate species. Charge balance between cations and anions activity vs. concentration
Pore 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 Solution A partial list of tricks 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 considerations CO 2 goes into solution to give CO 3 2 ion, which react with Ca 2+ to produce CaCO 3. OH and Ca 2+ 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 Ca 2+ 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 Electro-neutrality After analysis, need to check for electroneutrality of the sum of anions vs cations. Ie. If not neutral, then some ion has been missed. Bleszynski, 2002
Pore Solution During Early-age age Hydration
The initial stage of hydration Alkali sulphates present in the cement (clinker) dissolve within seconds, due to their high solubility, contributing SO 2 4, Na +, and K + ~Balance most SO 2 4 present as alkali sulphates Syngenite (K 2 SO 4 CaSO 4 H 2 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 K 2 O. The initial SO 4 2 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/ -CaSO 4, dihydrate, and anhydrite.
The 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 remains low throughout. Concentration of Ca 2+ continues to increase and may exceed 20 mmol/l, which is the saturation of Ca(OH) 2. Ca 2+ ions are also supplied by the free lime. O 2 ions derived from the calcium silicates enter the fluid phase as OH : O 2 + H + OH The concentration of hydrous alumina (Al(OH) 4 ) in the fluid phase is low throughout; below ~0.1 mmol/l.
The 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 period Ettringite begins to form almost immediately on mixing. Formation of ettringite consumes Ca 2+ and SO 4 2 from solution. Sulfate ion level is maintained by concurrent dissolution of the calcium sulphate (gypsum and anhydrite), which dissolves to add Ca 2+ and additional SO 4 2 As long as calcium sulphate is still present, concentration of SO 4 2 in the pore fluid changes only slightly. Diamond, 1983.
The acceleration period Calcium sulphate (syngenite?) becomes completely dissolved during the acceleration phase. SO 2 4 concentration starts to decline due to continued formation of AFt, as well as adsorption of SO 2 4 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 SO 4 2 subsequently declines to values less than ~5 mmol/l by 1 day. Diamond, 1983.
The acceleration period Reduction in SO 2 4 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 Ca 2+ declines gradually (to values less than the solubility of CH). Diamond, 1983.
What if there is no calcium sulphate? Alkali sulphates dissolve quickly Without calcium sulphate SO 4 2 concentration begins to decrease immediately As long as there is sufficient C 3 A to consume the extra SO 4 2 as ettringite, replacement of SO 4 2 by OH ions will take place. Diamond, 1983.
What if there is too much calcium sulphate? Extra calcium sulphate does not increase the SO 4 2 concentration Sulphate concentration maintained for longer period Replacement of SO 2 4 by OH ions does not occur The result is the OH concentration is suppressed. Ca 2+ concentration does not decrease Diamond, 1983.
Long 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).
Alkalis 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. [OH ], mol/l 1.0 0.8 0.6 0.4 Canham, 1986 Longuet et al., 1973 Page and Vennesland, 1983 Diamond, 1981 Ramlochan, 2000 Struble, 1987 Kollek et al., 1986 Barneyback, 1983 Diamond, 1983 Correlation between OH concentration and Na 2 Oe at 28 days (w/cm 0.5). Ultimate concentrations, will therefore, depend on cement alkalis. 0.2 y = 0.722x 0.004 r 2 = 0.93 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Na 2 Oe, wt. % Alkali-aggregate reactions. Nixon and Page, 1987. Diamond and Penko, 1988.
Effect 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 SCMs 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 Ca 2+ (alkali substitution)
Effects of SCMs 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 SCMs 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). Shehata, 2001. Metakaolin significantly reduces alkalinity at higher concentrations. Ramlochan, 2000.
0.3 Slag (%) ASR Expansion and cracking can be controlled by limiting alkalies or using SCMs but why? Expansion (%) 0.2 0.1 0.0 0 6 12 18 24 Age (months) 0 25 35 50 65
ASR: Role for Pore Solution Analyses Cement pastes - W/CM = 0.50 using cements with 0.61, 0.76, and 1.09% Na 2 O E. Sealed and cured at 23 o C Pore pressed at range of ages from 1 to 730 days. Solution analyzed by titration (OH) & flame photometry (Na & K) 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 Slag-SF SF ASR Study The effectiveness of silica fume or blastfurnace 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.
Fly Ash Pore Solution and Alkali Binding Studies: Thomas & Shehata Cement pastes - W/CM = 0.50 High-alkali cement (= 1.02% Na 2 O e ) Sealed and cured at 23 o C Pore pressed at range of ages Solution analysed by titration (OH) & flame photometry (Na & K) 12 fly ashes Ternary mixes with silica fume CSH analysis by SEM/EDS Ca(OH) 2 analysis by TGA
Alkali 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 OH Concentration (M/L) 1.0 0.8 0.6 0.4 0.2 High-Alkali Cement Paste with 25% Fly Ash 0 100 200 300 400 500 600 700 800 Age (days) Control Fly Ash CaO / Na 2 O e 27.7 / 1.65 17.5 / 1.68 13.6 / 3.77 6.38 / 1.41
Effect of % Fly Ash on Pore Solution Composition High-Alkali Cement Paste with F & C Fly Ash 1.0 27.7% CaO, 1.65% Na 2 O e 6.38% CaO, 1.41% Na 2 O e 1.0 OH Concentration (M/L) 0.8 0.6 0.4 0.2 Control 25% 50% 70% OH Concentration (M/L) 0.8 0.6 0.4 0.2 Control 25% 50% 70% 0.0 0 200 400 600 800 0.0 0 200 400 600 800 Age (days) Age (days)
Hydrate Composition SEM/EDX analysis of paste samples used in pore solution studies Composition of inner C-S-H (effect of fly ash) (Na+K)/Si 25% Low-CaO Fly Ash Portland cement Ca/Si Differences in pore solution composition for different SCMs cannot be explained on the basis of increased binding by inner C-S-H Role of outer & secondary C-S-H?
OH Concentration (M/L) Effect of 25% Fly Ash on Pore Solution Composition High-Alkali Cement Paste with 25% High-Alkali Fly Ash 2.0 1.8 1.6 1.4 1.2 1.0 0.8 Increasing CaO Fly Ash CaO / Na 2 O e 18.9 / 8.73 15.9 / 8.46 12.3 / 8.45 Control 0.6 0 100 200 300 400 500 600 700 800 Age (days) Less binding as C/S rises
Effect of Silica Fume on Pore Solution Composition 1.0 High-Alkali Cement Paste with Silica Fume OH Concentration (M/L) 0.8 0.6 0.4 Control 5% Silica Fume 10% Silica Fume Bound alkali is released slowly 0.2 0 200 400 600 800 1000 1200 Age (days)
Concrete ASR expansions were not controlled by 8% Silica Fume alone (confirmed by long-term outdoor exposure studies) 0.25 Average Expansion (%) 0.20 0.15 0.10 0.05 Control 8% SF 35% Slag 0.00-0.05 0 200 400 600 800 Age (days) Ternary mixes
Effect of Fly Ash on Pore Solution Composition High-Alkali Cement Paste with 5% Silica Fume & F Fly Ash 1.0 F Ash = 6.38% CaO, 1.41% Na 2 O e OH Concentration (M/L) 0.8 0.6 0.4 0.2 Control 5% Silica Fume 5SF / 10FA 5SF / 15FA 0 200 400 600 800 1000 1200 Age (days) FA stabilizes bound alkali
Cement Composition & Pore Solution Alkalinity In summary, alkali concentration in the pore solution is dependent on: Na 2 Oe CaO SiO 2 In the cementitious system (i.e. including portland cement and all supplementary cementing materials) Concentration of Na, K & OH in pore solution as Na 2 Oe CaO & SiO 2 But there is also a role of alumina in C-S-H in stabilizing bound alkali
Cement Composition & Pore Solution Alkalinity OH at 90 days (Mol/L). 2.0 1.5 1.0 0.5 0.0 Shehata, 2001 Unpublished Bleszynski, 2002 Ramlochan, 2000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 (Na 2 Oe x CaO)/(SiO 2 ) 2 of CM R 2 = 0.913 79 blends of: Portland cement Fly ash Slag Silica fume Natural pozzolan
The 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. C/S = 0.85 C/S C/S = 1.5 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
Role 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 chloroaluminates. 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 binding Simplified Service-Life Model Damage End of service life O 2 diffusion, resistivity Cl, CO 2 penetration Initiation period, t i Time Propagation period, t p (Tuutti, 1982)
Chloride Binding
Ion exchange reactions
Physi-sorption sorption
Experimental method for Chloride Binding
Chloride binding isotherms
Effect of cement composition C 3 A content
C 3 A and C 4 AF addition to cement C3A C4AF Pure phases supplied by Lafarge
Phase transformation in C 3 A paste
Effect of Al 2 O 3 in SCMs on 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 carbonation
Desorption isotherms
Questions?