Ettringit Formation and Destruction of Concrete

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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

www.nrc.ca/irc/ircpubs

NRCC-45995


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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 CouncilOttawa, ON, Canada, K1A 0R6

+Universit Laval, Ste. Foy, Qubec, Canada, G1K 7P4

ABSTRACT

A surface energy-based mechanism for expansion in cement systemssusceptible to Delayed Ettringite Formation (DEF) is proposed. The volumechange behavior of nanoporous materials filled with impregnants and exposed to

water vapor (considered analogous to the DEF situation) is described. Therelevance of expansion due to Bangham swelling and dissolution phenomena is

explained. The role of microcracking in expansion due to DEF is discussed. It issuggested that it is not necessary to invoke classical crystal growth theory toexplain expansion due to DEF. It appears that both AFtprecipitation and surface

effects are occurring concurrently and influencing the volume stability of thematerial.


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Introduction

The deleterious effects of delayed ettringite formation (DEF) in concrete that has

experienced temperatures above about 70C have been documented bynumerous investigators [1]. Taylor et.al. have described an expansion

mechanism that suggests that substantial pressures from crystal growth are mostlikely to be generated in confined spaces at high saturation [2]. In theirdescription, monosulfate is intimately mixed with C-S-H at the end of the heat

treatment. The ettringite subsequently formed under conditions giving rise toexpansion is also closely intermixed with C-S-H. Although the descriptorintimately 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 highsurface 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 thepore radius is below ~100 nm. Arguments based on surface energyconsiderations 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 toDEF. It will be further argued that surface energy considerations for explaining

DEF expansion are consistent with the generation of significant levels of stress atmicrocrack tips.

Surface Energy-Length Change Phenomena in Porous Materials

The charged nature of the calcium silicate hydrate (C-S-H) surface and itscharacteristics 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 canbe modified by various phenomena: gas or molecular adsorption; ionic

adsorption; chemical reactions including dissolution phenomena. The C-S-Hcompounds 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 layerscomposed of chains of silica tetrahedra bound to CaO polyhedral sheets. Thesilicate 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]. Thespecific adsorption of positively charged calcium ions modifies the charge of the

surface (as the Ca/Si ratio increases). The net charge ultimately becomespositive.

The pure C-S-H phases investigated by the authors had nitrogen surface areasranging 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 silicaglass. Length change due to surface energy changes of these relatively highsurface area materials can be appreciable [7].


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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 indistilled 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 ionsfrom 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 andhydrated Portland cement paste, are useful models for demonstrating the

potential deleterious effects of surface energy phenomena due to wetting. Vycorglass impregnated with elemental sulfur, polymethylmethacrylate (PMMA) orcalcium 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 avery 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 watervapor molecules and the interaction energies are of the normal adsorptive type,

then the swelling forces created by the decrease in surface free energy could bevery high. Depending on the irregularity of the interfacial boundary and theextent of interfacial bonding, high local stresses acting at specific sites along the

interface may develop causing ultimate destruction of the matrix. The drivingforce possibly emanates from differences in surface energy release (a net

Bangham effect). Adsorption occurs on both the glass and sulfur (or calciumhydroxide) surfaces. Large expansion of both solids due to the Bangham effect

should result. One possibility is that the sulfur (or calcium hydroxide) expandsmore than the glass due to differences in the free energy changes with respect toeach solid. It is unlikely that there is any chemical interaction especially in the

glass-sulfur system. This explains why sulfur and calcium hydroxide are expelledor extruded from some of the pores (Figure 1). Similar length change results areobserved for the PMMA cement paste composite although no disintegration of

the specimen is observed. The adhesion or interfacial bond between thepolymer and the hydrated Portland cement may be responsible for maintaining

the integrity of the high surface area matrix. The process is diffusion controlledand 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 andettringite) with C-S-H suggests that the sulfate phases are either microcrystallineor mixtures of microcrystalline and amorphous material. These phases can be

considered to be finely divided with a high surface area. The surface area of themixture 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


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can potentially result in large decreases in surface free energy. The principal

requirements of the DEF mechanism include a pre-heating treatment = 70C andsubsequent moist curing. The transformation of AFmto AFtinvolves the

consumption of an additional 20 moles of water. The heat treatment is also likelyto drive moisture out of a specimen. Therefore subsequent exposure to water

vapor is likely to involve readsorption. Expansion due to DEF does not occur inabsence of moist conditions. Mass transfer of water into fully impregnatedporous bodies occurs more effectively in the vapor phase. For example sulfur

impregnated porous glass, fully immersed in liquid water does not undergo thesame deleterious behavior, at least within a similar time frame. It is suggestedthat intimately mixed C-S-H and AFm/AFtphases (at the nanoscale) can be

modelled as a mixture of finely divided solids at the nanoscale. Mass transfer ofwater 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. Itshould also be noted that in industrial environments heat treatment is oftencarried out under non-ideal conditions. The resultant Bangham swelling or

dissolution can result in deleterious volume changes analogous to those thatoccur when impregnant-filled porous silica glass is exposed to water vapor. It is

therefore not necessary to invoke a crystal-growth theory to explain theexpansive behavior of DEF.

As stated previously adsorption of sodium ions appears to be associated withexpansion. An association of the desorption of sulfate ions with expansion would

require a decrease in surface free energy on desorption. Desorption is usuallyaccompanied by an increase in surface free energy. Desorption of sulfate ionsfrom 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 essentiallynanoparticulates. Water vapor can likely permeate into the interfacial zonesgenerating the expansive forces related to surface free energy. It is therefore not

necessary to invoke a crystal-growth mechanism to account for expansion dueto DEF. Similar length change phenomena can occur due to the presence of

microcrystalline or mixtures of microcrystalline and amorphous phases in thevicinity of microcrack tips. The population of microcracks may be sufficient togenerate damage via the mechanism suggested. The response of the C-S-H-

sulfate phase to water vapor ingress is likely to occur in coincidence withexpansive processes occuring in the vicinity of crack tips.


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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 Suspensionsduring Early Age Hydration, Journal of Colloid and Interface Science 202,

261-268 (1998).6. H. Viallis-Terrisse, Ph.D. thesis, Universit de Bourgogne,Interaction des

Silicates de Calcium Hydrates,principaux constituants du ciment avec les

chlorures dalcalins.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-Hon C/S Ratio, Proc. 8 thInt. Cong. Chem. Cem., Rio de Janeiro, Brazil, 1-6(1986).

8. R. Kondo and M. Daimon, Phase Composition of Hardened Cement Paste, inProc. VI Int. Conf. Chem. Cement, Moscow, 1974.

9. R.A. Helmuth, Dimensional Changes and Water Adsorption of HydratedPortland 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. 5

th

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. 7 thInt. Congr.Chem. Cem., Paris, Vol III, Paper VII-46 Vll-50 (1980).

13. J.J Beaudoin, S. Catinaud and J. Marchand, Volume Stability of CalciumHydroxide 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, Universit Laval, Durabilit a Long Terme de

Matriaux Cimentaires Avec ou Sans Fillers Calcaires en Contact avec deSolutions Salines, Dec. 2000, Chapter 7, pp 202-361.

17. R.F. Feldman and J.J Beaudoin, Some Factors Affecting the Durability ofSulfur-Impregnated Porous Bodies, Cem. Concr. Res., 8, 273-281 (1978).


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18. V.S. Ramachandran, R.F. Feldman and J.J Beaudoin, Concrete Science,

Heyden & Son Ltd., London, 1981, p 264.

(a

(a


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Figure 1. Impregnated porous glass exposed to 100% RH(a) the porous glass sulfur system showing extruded sulfur rods

(b) the porous glass calcium hydroxide system showing nodules of calcium hydroxide extruded from the pores

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Ettringit Formation and Destruction of Concrete