Mechanism of alkali-silica reaction and the significance of calcium hydroxide

CEMENT and CONCRETE RESEARCH. Vol. 21, pp. 647-654, 1991. Printed in the USA. 0008-8846/91. $3.00+00. Copyright (c) 1991 Pergamon Press plc,

MECHANISM OF ALKALI-SILICA REACTION AND THE SIGNIFICANCE OF CALCIUM HYDROXIDE

H. Wang and J.E. Gillott Department of Civil Engineering The University of Calgary Calgary, Alberta, Canada, T2N IN4

(Communicated by H.F.W. Taylor) (Received Jan. 3, 1991)

ABSTRACT Experiments indicate that Ca(OH)2 [CH] aggravates alkali-silica reaction causing increased expanslon of mortar bars. Ca(OH) 2 has two major functions: firstly it acts a "buffer" to maintain a high pH, i.e. a high concentration of hydroxyl ions in pore solutions; secondly, Ca++ ions may exchange for alkali ions on silica gel leading to further production of swelling alkalisilica complex. A mechanism of alkali-silica reaction is proposed which emphasizes the effect of Ca(OH)2 on reaction and expansion.

Introduction The mechanism of alkali-silica reaction and expansion has been of great interest since the discovery of the reaction. Although there is a wealth of information concerning the reaction many details remain obscure particularly in regard to the role of Ca(OH) 2. This paper presents data concerning the effect of Ca(OH). on expansion of mortar bars containing 2% opal as reactive component and ~iscusses the mechanism of alkali-silica reaction and expansion in the light of new findings. Previous Explanations of the Role of Ca
[1,2]

Powers and Steinour postulated that the alkali-silica complex is expansive and that the lime-alkali-silica complex is non-expansive and interpreted the function of Ca(OH) 2 in alkali-silica reaction and expansion as follows: (i) The initial attack of sodium and calcium hydroxides on reactive silica builds up a zone of non-expansive lime-alkali-silica complex which separates the unreacted silica from the lime and alkali in the concrete pore solutions. 647

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(2) During further attack on silica, lime and/or alkali diffuse through the non-expansive layer to react with silica. The relative amount of lime and alkali external to the non-expansive layer determines the type of complex produced in the silica grain. (3) If the ratio of lime to alkali outside the layer is sufficiently high, llme is delivered to the reaction site fast enough to produce a nonswelling llme-alkall-silica complex. (4) If the ratio of llme to alkali outside the layer is not sufficiently high, llme can not reach the reaction site fast enough. Thus, an alkalisilica complex is produced which can imbibe water and swell, causing expansion of the concrete. The Ca++ ion concentration in concrete controls the formation of the type of reaction products, i.e. swelling alkali-silica complex or non-swelllng llme-alkali-silica complex. (b) Explanation of Chatterji

[3]

Chatterji did not accept the non-expansive nature of a llme-alkalisilica complex. He proposed an alternative explanation based on the net amount of materials which diffuse into a reactive grain as follows: (I) The attack of OH- on silica grains is accompanied by the penetration of cations, i.e. Na+, K+ and Ca++ to the reaction sites. However, 'more of the smaller ions, i.e. Na+ and K+ will follow the penetration of OH- ions than the larger ions, i.e. Ca++ although both types of cations will penetrate reactive silica grains. (2)

Some molecules of silica will diffuse away from their original sites.

(3) The Ca++ ion concentration in the environment controls the rate at which silica diffuses out of the grains. The higher the Ca++ concentration of the environment, the lower the rate at which silica diffuses out of the grains and the higher the rate at which cations diffuse into the grains. (4) Expansion occurs when the amount of material entering a grain, Na+, Ca++, OH- and water, exceeds the amount leaving (i.e. silica).

i.e.

Therefore, the Ca++ concentration in the environment controls the relative rates of diffusion into and out of the reactive grains. Experiments To Test The Effect of Ca(OH)2 On Expansion of Mortar Bars (a)

Materials and Experimental Techniques

Opal, from Nevada, U.S.A. was used as alkali expansive component in mortar bars. The opal was crushed, proportioned in the same gradation as the aggregate required by ASTM C-227, and used as a 2% replacement for a non-alkall expansive limestone aggregate. Other constituents used in the mortar bars included silica fume slurry (SF) from W.R. Grace, type I0 low alkali cement from Lafarge Canada Inc., and chemical reagent grade Ca(OH) 9 and NaOH from Fisher Sclentific. Company. Alkali content was boosted t5 1.0% equivalent Na20 by addition of NaOH to the mixing water. The method of ASTM C-227 was used to assess the alkali-expansivity of the mortar bars. Bars were measured in triplicate and average values were calculated. b)

Results of Expansion

Fig. I shows the effect of Ca(OH) 9 on expansion of mortar bars due to Four sets of bars were cast in two groups. Bars in group A contained no silica fume whilst bars in group

alkall-sillca reaction caused by 2% op~al.

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ALKALI-SILICA REACTION, Ca(OH)2, AGGRAVATION

649

KEY~ LS'

9%

CM

0%

CH ( C O N T R O L ) ~ o ~

9%

GH

0%

CR

GROUP A ,0% SF

GROUP 8 ,20% S F ~ " "

FIG.I Effect of Calcium Hydroxide on Expansion of Mortar Bars Due to ASR

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--t-

Z

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TIME IN DAYS

B contained 20% silica fume replacement of cement. In each group one set of bars contained an addition of 9% Ca(OH) 2 and the other set contained 0% addition of Ca(OH)2. In group A, containing no silica fume, the bars containing 9% Ca(OH)2 expanded much more than the control bars containing no Ca(OH)2. When measurements were terminated at an age of 550 days the bars containlng 9% Ca(OH) 2 had expanded by about 30% more than the control bars (0% Ca(OH)2 ) . In group B, containing 20% silica fume, the bars containing 9% Ca(OH)^ showed no expansion to an age of about 5 months but significant expansion showed up later and continued until measurements were terminated at 550 days. The set of bars containing 0% Ca(OH)_ showed no expansion to the age at which measurements were terminated (550 bays). In both groups of mortar bars containing 0% and 20% silica fume, the presence of 9% Ca(OH)2 aggravated the alkali-silica reaction in terms of the increase in expanszon.

Discussion of the Mechanism of Alkali-Silica Reaction and Expansion and the Significance of Ca(OH)2 A mechanism of alkali-silica reaction, which emphasizes the effect of Ca(OH)2, is proposed and is diagrammatically illustrated in Fig. 2. The mechanlsm is described in terms of four steps as follows: (a) The Surface Structure of the Original Opal is shown Indicating the Silanol Groups on the Surface of Opal (Fig. 2a). (b) Exchange of Alkali Ions (Na+, K+ and/or Ca++) for Protons of Silanol Groups takes place at the Surface of Silica Once silica grains are placed in a solution containing Ca++ and alkali ions, the initial ion exchange of Ca++ and alkali for protons of the silanol groups on the silica surface takes place (Fig. 2b). Alkali ions can directly exchange for the H+, therefore, an alkali-sillca complex is produced. However, Ca++ ions in the solution can either directly exchange with H+ ions of the silanol groups or exchange with the alkali ions which

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were previously adsorbed on the silica surface. Therefore, a llme-alkalisilica complex is produced. In the initial stages of the exchange the lime-alkali-silica complex is concentrated mainly on the silica surface. This is because the silica grains are directly in contact with the solution containing both Ca++ and alkali ions. At this stage, the lime-alkalisilica complex is not responsible for expansion.

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(e).Exchange of calcium ions for alkali ions (formation of non-swelling lime-alkali-silica complex) •, F i g . 2 S c . h e m a t i c diagaTa,m t o s,h9w a . l k a [ i suica reacUon ann me role or cmcmrn m ~,aK

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(c)

ALKALI-SILICAREACTION, Ca(OH)2, AGGRAVATION

651

Breakdown of Internal Si-O-Si Bridges under the Action of OH-

The OH- concentration is high enough in the pore solution of Portland cement paste for attack on the Si-O-Si bridges to take place. This results in the formation of internal silanol groups (Fig. 2c). (d)

Exchange of Alkali Ions for Protons of Internal Silanol Groups

At the same time that internal silanol groups are created, alkali cations diffuse to the reaction sites and exchange for the protons of the newly created silanol groups (Fig. 2d). Alkali-silica groups have a high affinity for water and therefore, a swelling colloidal alkali-silica complex is produced capable of sorbing water so that pressure is built up leading to expansion. The fact that alkali ions diffuse to the reaction sites before Ca++ ions is attributed to the following two factors: (I)

More Alkali Ions Are Available at the Initial Stage

The solubility of NaOH and KOH is much higher than that of Ca(OH) 2 (Table i). In other words, alkali ions are more readily available for the exchange reaction. Also, when alkali and Ca(OH)2 are in the same system, the solubility of Ca(OH)2 is greatly suppressed d~e to the common ion effect (Fig. 3). The exchange of protons on the silanol groups is mainly controlled by the diffusion of cations (Ca++, Na+ and K+) to the reaction sites. The rate of diffusion is a function of the cation concentration and also high concentration of alkali ions increases the probability that they will diffuse to the reaction sites first. (2)

Hydraulic Radius of Cations

The hydraulic radius is another factor that affects the diffusion of the cations to the reaction sites. The hydraulic radius of Na+ and K+ ions ~s much smaller than that of Ca++ ions (Table 2) and this tends to increase the rate of diffusion of Na+ and K+ ions. (e)

Ca++ Exchange for Alkali Ions

Once expansion occurs due to swelling of the alkali-silica complex it will provide more space and increase the probability that Ca++ will diffuse to the sites of the alkali-silica complex. Therefore, an exchange of Ca++ for alkali ions begins to take place (Fig. 2e). This happens because Ca-O bonds have a much higher electrostatic energy than Na+(K+)-O bonds (Table 2). Greenberg [7] studied the sorption of calcium hydroxide and sodium hydroxide on the surface of silica and found that the dissociation of the (~SiO)Na group was greater than that of the (~SiO)Ca group; this was the result of the higher electrostatic energy of Ca-O bonds than of Na-O bonds.

TABLE i Solubility of Alkalies and Ca(OH) 2 in Water [4] Alkalies

Solubility at O°C

NaOH KOH Ca(OH)2

42 107
(g/100 cc) at lO0°C 347 178 0.077

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35

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FIG.3 Solubility of CaO in Alkaline Solution (5)

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Partial replacement of Na+ and K+ ions results from the ion exchange of Ca++ for Na+ and K+. The released Na+ and K+ become available for further exchange with protons of newly created silanol groups due to the continuing attack on Si-O-Si bridges within silica grains. Some Na+ and K+ will not be exchanged and will remain in the lime-alkali-silica complex. TABLE 2 Hydraulic Radius of Na+, K+ and Ca++ lons and the Electrostatic Energy between Cations and O- [6]

Cations (R)

Na+ K+ Ca++

Hydraulic radius (~)

3.3 3.1 4.2

R-O distance

4.65 4.45 5.55

(A)

Electrostatic energy ZIZ 2

(---g-) 0.21 0.22 0.36

From the above summary of steps in the mechanism of alkali-silica reaction and expansion, it can be predicted that there will be a gradation in concentration of Ca++ and alkali ions in the reaction rim of a reactive silica particle. Therefore the pattern shown in Fig. 4 is to be anticipated. The concentration of Ca++ ions should be highest at the outermost part of the reaction rim and should decrease towards the inner part of the reaction rim because ion exchange of Ca++ for alkali is inhibited by its slow diffusion rate. Alkali ion concentration should be highest somewhere within the reaction rim. This is because (I) at the outer part of the reaction rim the alkalis in the alkali-silica complex are partially exchanged by Ca++ ions; the released alkali ions migrate within the reactive grain producing more alkali-silica complex by exchange for protons on newly created silanol groups; (2) at the inner part of the reaction rim, alkalis follow the penetration of OH- into the silica grain and begin to exchange with protons of newly created silanol groups, therefore, the content of alkali ions in this region is lower; (3) in the

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ALKALI-SILICA REACI"ION, Ca(OH)2, AGGRAVATION

REACTIONRIM

Co FIG.4

~//~

/ ALKALI

Alkali (Na + K) and Ca Distribution in Reaction Rim. A

653

~~

OPAL GGREGATE

REACTION RiM

position of the reaction rim containing the highest amount of alkali, a well-formed alkali-silica complex exists and the exchange with Ca++ is about to start. It has to be pointed out that the reaction of alkali with silica and the exchange of Ca++ is a kind of dynamic balance, The reaction and exchange processes are changed momentarily, therefore, it is impossible to predict the position of the ion distribution in the reaction rim, nevertheless, the trend of the ion arrangement in the reaction rim can be shown as in Fig. 4. The predicted ion distribution discussed above in a reaction rim is supported by data obtained by Foole [8] who used electroq probe microanalysis to obtain the composition in various parts of the reaction rim within opal particles. The alkali content is the highest within the reaction rims and the calcium content is the highest at the outermost edge. The calcium content dramatically decreases towards the inside of the reaction rim. The proposed steps in the reaction mechanism can also explain the results obtained by Knudsen and Thaulow [ 9 ] . They found by energy dispersive X-ray spectrometry that the gel within reactive particles tended to be lower in calcium than the gel located in cracks which tended to be higher in calcium. It seems to the authors that the higher calcium content in the gel located outside reactive grains is due to the exchange of Ca++ for alkali ions in the expanded gel.

Function of Ca
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(2). The source of Ca++ ions Ca++ ions can exchange for alkali ions in the swelling alkali-silica complex to produce a non-swelling lime-alkali-silica complex. This process releases the alkali ions needed for further production of a swelling alkalisilica complex by exchange with protons on silanol groups. Therefore, Ca(OH)^ provides the source of Ca++ needed to release alkali ions in the z alkali-silica complex. From the viewpoint of both alkali-silica reaction and expansion, these two functions performed by Ca(OH) 2 are detrimental. In the presence of calcium hydroxide the alkali-silica reaction is expansive and deleterious to concrete or mortar. This is indeed the case in the experiments conducted in this program. Mortar bars containing extra Ca(OH)2 showed increased expansion. Conclusions (i) The presence of Ca(OH)2 aggravates alkall-sillca reaction, and increases expansion of mortar 5ars containing 2% opal. The bars containing 20% silica fume do not expand, but when 9% Ca(OH) 2 is incorporated, the presence of 20% silica fume no longer effectively controls the expansion. (2) Two functions are performed by Ca(OH) 2. Firstly, it acts as a "buffer" to maintain high pH, i.e. high OH- ion concentration in pore solutions. Secondly, Ca++ may exchange for alkali ions; the released alkali ions may produce further alkali-silica complex which is expansive. (3) The process of alkali-silica reaction and expansion is outlined in Fig. 2. Four steps are recognized to show (I) initial reaction on the silica grain; (2) the attack of OH- on siloxane groups; (3) ion exchange of Na+ and K+ for protons of the silanol groups and the formation of swelling alkall-silica complex and (4) ion exchange of Ca++ for alkali ions in the alkall-silica complex and the formation of non-swelling llme-alkali-sillca complex. Although process (4) produces a harmless lime-alkali-silica complex, it results in partial release of alkali ions which are available for further production of swelling alkall-silica complex. Acknowledgements The authors wish to thank Terry Quinn for his help in performing the experiments and Rene Kadach for secretarial assistance. Financial support provided by NSERC and The University of Calgary is greatly appreciated. Thanks are also extended to Lafarge Canada Inc. and W.R. Grace for supplying materials. References [i] [21 [3] [4] [5] [6] [7] [8] [9]

T.C. Powers and H.H. Steinour, J. Amer. Concr. Inst., 26, 497, (1955). T.C. Powers and H.H. Steinour, J. Amer. Concr. Inst., 26, 785, (1955). S. Chatterji, Proc. 8th Int. Conf. on Alkali-Aggregate Reactions, Kyoto, Japan, I01, (1989). R.C. Weast, Handbook of Chemistry and Physics, (1988), Rubber Co. Press, Cleveland, Ohio, U.S.A. H. Xu, Proc. 7th Int. Conf., Ottawa, (1986), Noyes Publ., New Jersey, USA, p. 451. Z. Ding, 1979, in Chinese. S.A. Greenberg, J. Phys. Chem., 60, 325, (1956). A.B. Poole, Effect of Alkalies on Props. of Contr., Proc. Symp. London, Cement and Contr. Assoc., 163, (1976). T. Knudsen and N. Thaulow, Cement and Contr. Res., 5, 443, (1975).