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A Surface energy-based expansion mechanisms associated with delayed ettringite formation
Beaudoin, J. J.; Marchand, J.
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A Surface energy-based expansion mechanism associated with delayed ettringite formation
Beaudoin, J.J.; Marchand, J.
A version of this document is published in / Une version de ce document se trouve dans : RILEM Workshop on Internal Sulfate Attack, Villars, Switzerland, Sept. 4-6, 2002, pp. 1-7
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A SURFACE ENERGY-BASED EXPANSION MECHANISM ASSOCIATED WITH DELAYED ETTRINGITE FORMATION
J.J. Beaudoin* and J. Marchand+
*Institute for Research in Construction, National Research Council Ottawa, ON, Canada, K1A 0R6
+Université Laval, Ste. Foy, Québec, Canada, G1K 7P4
ABSTRACT
A surface energy-based mechanism for expansion in cement systems susceptible to Delayed Ettringite Formation (DEF) is proposed. The volume change behavior of nanoporous materials filled with impregnants and exposed to water vapor (considered analogous to the DEF situation) is described. The relevance of expansion due to Bangham swelling and dissolution phenomena is explained. The role of microcracking in expansion due to DEF is discussed. It is suggested that it is not necessary to invoke classical ‘crystal growth’ theory to explain expansion due to DEF. It appears that both AFt precipitation and surface effects are occurring concurrently and influencing the volume stability of the material.
Introduction
The deleterious effects of delayed ettringite formation (DEF) in concrete that has experienced temperatures above about 70°C have been documented by
numerous investigators [1]. Taylor et.al. have described an expansion
mechanism that suggests that substantial pressures from crystal growth are most likely to be generated in confined spaces at high saturation [2]. In their
description, monosulfate is intimately mixed with C-S-H at the end of the heat treatment. The “ettringite subsequently formed under conditions giving rise to expansion is also closely intermixed with C-S-H.” Although the descriptor ‘intimately mixed’ is not a precise definition of the state of the monosulfate or ettringite it is apparent that these compounds would be finely divided (with a high surface area) and occupy nanospace. It is argued by them that this would satisfy the requirement that a substantial pressure can arise at a ‘pore’ wall only if the pore radius is below ~100 nm. Arguments based on surface energy
considerations will be presented in this note that suggest it is not necessary to invoke a ‘crystal growth’ mechanism to explain the source of expansion due to DEF. It will be further argued that surface energy considerations for explaining DEF expansion are consistent with the generation of significant levels of stress at ‘microcrack’ tips.
Surface Energy-Length Change Phenomena in Porous Materials The charged nature of the calcium silicate hydrate (C-S-H) surface and its
characteristics as a highly divided solid favors effects related to the adsorption of ions or molecules. It is apparent that the surface energy of C-S-H surfaces can be modified by various phenomena: gas or molecular adsorption; ionic
adsorption; chemical reactions including dissolution phenomena. The C-S-H compounds present in hydrated Portland cement compounds are generally ill-crystalline. At the molecular scale, their nanostructure is analogous to that of the natural clay tobermorite [3]. Their primary structural unit is organized in layers composed of chains of silica tetrahedra bound to CaO polyhedral sheets. The silicate chains are negatively charged [3,4]. Zetametry studies indicate that the surface charge of C-S-H is strongly influenced by their Ca/Si ratio [5,6]. The specific adsorption of positively charged calcium ions modifies the charge of the surface (as the Ca/Si ratio increases). The net charge ultimately becomes positive.
The pure C-S-H phases investigated by the authors had nitrogen surface areas ranging from 30.7 to 111.9 m2/g [7]. The C/S ratios varied from 0.68 to 1.49. These surface areas are of similar magnitude to those obtained for porous silica glass. Length change due to surface energy changes of these relatively high surface area materials can be appreciable [7].
The Bangham equation provides a basis for testing Gibbs’ equation on solids γ ∆ = ∆ 1 k L L (1) where L L ∆
is the length change of an adsorbent during the reversible adsorption process, ∆γ is the change in surface free energy.
Several authors are of the opinion that Bangham effects are at the origin of volume instability of hydrated cement systems, especially at low values of relative humidity [8,9]. The mass and length change sorption isotherm for hydrated Portland cement exhibits large primary and secondary hysteresis [10]. The secondary hysteresis is associated with both intercalation effects and surface adsorption phenomena. These effects have been separated by constructing (through appropriate use of scanning loops) reversible and
irreversible sorption isotherms [10]. The reversible expansion due to adsorption is about 20% of the total expansion along the isotherm and amounts to a
significant value of about 0.1%. This represents a substantial volume change and results from surface energy changes due to sorption of water.
Length Change Due to Dissolution
The dissolution of porous solids in various solvents has been shown to be
expansive [11-13]. Examples are: the dissolution of porous silica glass in NaOH solutions (0.1M - 3.2M); the dissolution of Ca(OH)2 in distilled water and various salt solutions; the dissolution of hydrated cement paste in 1.0N aqueous HCl; the dissolution of ettringite in various salt solutions. These results indicate that changes occurring on the surface – whether due to sorption of inert ions or dissolution of a constituent or a portion of the entire solid – produce significant expansion. This can be attributed to a change in the surface free energy of the solid.
It would appear that there may be an apparent contradiction between the two following observations : expansion resulting in the dissolution of silica glass (or AFt or Ca(OH)2) and shrinkage that results from the decalcification of C-S-H [14,15]. This can be explained as follows. During the initial leaching of the paste in distilled water there is expansion. Accompanying the expansion there is
dissolution of both the C-S-H and CH phases. Both are occurring simultaneously and both are expansive. The key is the time scale. Expansion occurs for pure C-S-H in distilled water during the first few hours. The subsequent contraction observed by Feldman and attributed to C-S-H was observed after several days. This is likely due to the eventual collapse of structure on removal of structural lime. The result is not incompatible with our observations.
It is also noted that ionic adsorption can also accentuate the volume instability of the solid. This is demonstrated by the length change behavior of C3S paste
immersed in distilled water and sodium chloride solutions [16]. Samples immersed in sodium chloride solutions (20-180 g/L) expand up to about 0.04% within 24h and continue to expand slowly over a period of several days. The expansion at all concentrations exceeds that for the C3S paste immersed in distilled water. The fact that no new chloride bearing compounds were detected indicates that the adsorption of ions could account for the additional expansion. This argument goes against the assumption that the desorption of sulfate ions from the C-S-H surface could eventually lead to swelling.
Nanoporous Materials Impregnated with a Second Phase
Nanoporous materials with high surface area e.g. porous silica glass and hydrated Portland cement paste, are useful models for demonstrating the
potential deleterious effects of surface energy phenomena due to wetting. Vycor glass impregnated with elemental sulfur, polymethylmethacrylate (PMMA) or calcium hydroxide undergoes significant expansion (>1%)when exposed to water vapor [17,18]. In order to fill the small pores the impregnant has to be in a very finely divided state. The impregnant in porous glass can have a surface area in the range of of 300-500 m2/g. If this surface can be reached by water vapor molecules and the interaction energies are of the normal adsorptive type, then the swelling forces created by the decrease in surface free energy could be very high. Depending on the irregularity of the interfacial boundary and the extent of interfacial bonding, high local stresses acting at specific sites along the interface may develop causing ultimate destruction of the matrix. The driving force possibly emanates from differences in surface energy release (a net Bangham effect). Adsorption occurs on both the glass and sulfur (or calcium hydroxide) surfaces. Large expansion of both solids due to the Bangham effect should result. One possibility is that the sulfur (or calcium hydroxide) expands more than the glass due to differences in the free energy changes with respect to each solid. It is unlikely that there is any chemical interaction especially in the glass-sulfur system. This explains why sulfur and calcium hydroxide are expelled or extruded from some of the pores (Figure 1). Similar length change results are observed for the PMMA cement paste composite although no disintegration of the specimen is observed. The adhesion or interfacial bond between the polymer and the hydrated Portland cement may be responsible for maintaining the integrity of the high surface area matrix. The process is diffusion controlled and dependent on the thickness of the test specimen.
A Surface Energy-Based Expansion Mechanism for DEF
The concept of an intimate mixture of sulfate phases (e.g. monosulfate and ettringite) with C-S-H suggests that the sulfate phases are either microcrystalline or mixtures of microcrystalline and amorphous material. These phases can be considered to be finely divided with a high surface area. The surface area of the mixture is specially high as the interfacial zone bounds both the surface of the matrix (C-S-H) and the surface of the sulfate phase. Exposure to water vapor
can potentially result in large decreases in surface free energy. The principal requirements of the DEF mechanism include a pre-heating treatment = 70°C and subsequent moist curing. The transformation of AFm to AFt involves the
consumption of an additional 20 moles of water. The heat treatment is also likely to drive moisture out of a specimen. Therefore subsequent exposure to water vapor is likely to involve readsorption. Expansion due to DEF does not occur in absence of moist conditions. Mass transfer of water into fully impregnated porous bodies occurs more effectively in the vapor phase. For example sulfur impregnated porous glass, fully immersed in liquid water does not undergo the same deleterious behavior, at least within a similar time frame. It is suggested that intimately mixed C-S-H and AFm/AFt phases (at the nanoscale) can be modelled as a mixture of finely divided solids at the nanoscale. Mass transfer of water to interfacial sites in such mixtures is more likely to occur (in a manner analogous to behavior of the porous glass/sulfur system) in the vapor phase. It should also be noted that in industrial environments heat treatment is often carried out under non-ideal conditions. The resultant Bangham swelling or dissolution can result in deleterious volume changes analogous to those that occur when impregnant-filled porous silica glass is exposed to water vapor. It is therefore not necessary to invoke a ‘crystal-growth’ theory to explain the
expansive behavior of DEF.
As stated previously adsorption of sodium ions appears to be associated with expansion. An association of the desorption of sulfate ions with expansion would require a decrease in surface free energy on desorption. Desorption is usually accompanied by an increase in surface free energy. Desorption of sulfate ions from the C-S-H surface as an explanation for expansion would not appear to be tenable.
Concluding Remarks
It is suggested that changes in surface free energy due to sorption or dissolution phenomena can account for expansion due to DEF. The sulfate phases
(monosulfate and ettringite) when intimately mixed with C-S-H can be characterized as finely divided with high surface area and are essentially nanoparticulates. Water vapor can likely permeate into the interfacial zones generating the expansive forces related to surface free energy. It is therefore not necessary to invoke a ‘crystal-growth’ mechanism to account for expansion due to DEF. Similar length change phenomena can occur due to the presence of microcrystalline or mixtures of microcrystalline and amorphous phases in the vicinity of microcrack tips. The population of microcracks may be sufficient to generate damage via the mechanism suggested. The response of the C-S-H-sulfate phase to water vapor ingress is likely to occur in coincidence with expansive processes occuring in the vicinity of crack tips.
References
1. B. Erlin (Ed), Ettringite – The Sometimes Host of Destruction, SP177, Am. Concr. Inst. Intl., Farmington Hills, MI, USA, 1999, pp265.
2. H.F.W. Taylor, C. Famy and K.L. Scrivener, Delayed Ettringite Formation, Cem. Concr. Res. 31, 683-693 (2001).
3. H.F.W. Taylor, Cement Chemistry, London, Academic Press, (1990) pp. 475. 4. R.J. Kirkpatrick, J.L. Yarger, P.F. McMillan, P. Yu, and X. Cong, Raman
Spectroscopy of C-S-H, Tobermorite and Jennite, Advn. Cem. Bas. Mat. 5, 93-99 (1997).
5. L. Nachbaur, P.C. Nkinamubanzi, A. Nonat and J.C. Mutin, Electrokinetic Properties which Control the Coagulation of Silicate Cement Suspensions during Early Age Hydration, Journal of Colloid and Interface Science 202, 261-268 (1998).
6. H. Viallis-Terrisse, Ph.D. thesis, Université de Bourgogne,Interaction des Silicates de Calcium Hydrates,principaux constituants du ciment avec les chlorures d’alcalins.Analogie avec les argiles ,Oct.2000,pp.257..
7. J.J Beaudoin, R.F. Feldman, J. Baron and M. Conjeaud, Dependence of Degree of Silica Polymerization and Intrinsic Mechanical Properties of C-S-H on C/S Ratio, Proc. 8th Int. Cong. Chem. Cem., Rio de Janeiro, Brazil, 1-6 (1986).
8. R. Kondo and M. Daimon, Phase Composition of Hardened Cement Paste, in Proc. VI Int. Conf. Chem. Cement, Moscow, 1974.
9. R.A. Helmuth, Dimensional Changes and Water Adsorption of Hydrated Portland Cement and Tricalcium Silicate, M.S. Thesis, Illinois Inst. Tech., 1965, pp 64.
10. R.F. Feldman, Sorption and Length-Change Isotherms of Methanol and Water on Hydrated Portland Cement, Proc. 5th Intl. Symp. Chem. Cem., Tokyo, Japan, Vol. 3, 53-56 (1970).
11. G.G. Litvan, Volume Instability of Porous Solids Part 2. Dissolution of Porous Silica Glass in Sodium Hydroxide, J. Matls. Sci., 19, 2473 (1984).
12. G.G. Litvan, Volume Instability of Porous Solids Part 1. Proc. 7th Int. Congr. Chem. Cem., Paris, Vol III, Paper VII-46 – Vll-50 (1980).
13. J.J Beaudoin, S. Catinaud and J. Marchand, Volume Stability of Calcium Hydroxide in Aggressive Solutions, Cem. Concr. Res., 31, 149-151 (2001). 14. R.F. Feldman, V.S. Ramachandran, Length Change in Calcium Hydroxide
Depleted Portland Cement Pastes, II Cemento, 86 (2), 87-96 (1989).
15. R.F. Feldman, P.J. Sereda, V.S. Ramachandran, A Study of Length Changes of Compacts of Portland Cements on Exposure to H2O, High. Res. Rec. No. 62, 106-118 (1965).
16. S. Catinaud, Ph.D. Thesis, Université Laval, Durabilité a Long Terme de Matériaux Cimentaires Avec ou Sans Fillers Calcaires en Contact avec de Solutions Salines, Dec. 2000, Chapter 7, pp 202-361.
17. R.F. Feldman and J.J Beaudoin, Some Factors Affecting the Durability of Sulfur-Impregnated Porous Bodies, Cem. Concr. Res., 8, 273-281 (1978).
18. V.S. Ramachandran, R.F. Feldman and J.J Beaudoin, Concrete Science, Heyden & Son Ltd., London, 1981, p 264.
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Figure 1. Impregnated porous glass exposed to 100% RH (a) the porous glass – sulfur system showing extruded sulfur rods
