The chemistry of ‘alkali-aggregate’ reaction

CEMENT and CONCRETE RESEARCH. Vol. I I , pp. I - 9 , 1981. Printed in the USA. 0008-8846/81/010001-09/$02.00/0 Copyright (c) 1981 Pergamon Press, Ltd.

THE CHEMISTRY OF 'ALKALI-AGGREGATE' REACTION

L.S. Dent Glasser and N. Kataoka 9 Department of Chemistry, U n i v e r s i t y of Aberdeen Meston Walk, Old Aberdeen AB9 2UE, Scotland. (Communicated by G.M. Idorn) (Received Sept. 15, 1980) ABSTRACTTo c l a r i f y ideas about the a l k a l i aggregate r e a c t i o n , the reaction between sodium hydroxide and various forms of s i l i c a has been studied. Variations with time in the sodium, hydroxyl and s i l i c a concentrations in s o l u t i o n have been followed and the volume change of s o l i d s i l i c a immersed in s o l u t i o n has been measured. I f the s i l i c a is an absorbant and reactive form such as s i l i c a gel, there is an immediate drop in both sodium and hydroxyl concentrations. Subsequently, as the s i l i c a d i s s o l v e s , sodium concentration r i s e s again. Both the f i n a l concentration of dissolved s i l i c a and the volume expansion of s o l i d s i l i c a t e pass through a maximum at an intermediate t o t a l SiO~/Na20 mole r a t i o .

-~'In cm I ~ , ~ ¢

L,~o ~ © t k ' ~ ,

;KM~X~ ~ - ~ - ~ , ¿ T - ~

Introduction In an a l k a l i - r i c h Portland cement c l i n k e r , some aluminate or sulphate ions are balanced by Na+ or K+ can be undesirable, because the pH of the pore f l u i d c l i n k e r s tends to be abnormally high. Two types of Permanent address:

of the s i l i c a t e , instead of Ca2+. This in pastes made from such reaction may c o n t r i b u t e

Central Research Laboratory, UBE I n d u s t r i e s L t d . , Ube, Japan. 1

2

Vol, I I ,

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L.S. Dent Glasser, N. Kataoka

to t h i s e f f e c t : (i)

Hydrolysis of anions of weak acids (e.g. s i l i c a t e s ) Xn-

(ii)

+

H20

~

Hx(n-i)-

+

OH-

Formation of i n s o l u b l e calcium s a l t s (e.g. sulphate) r a t h e r than calcium hydroxide: n

Xn-

+

Ca(OH)2

~

The hydroxyl ions produced react containing poorly c r y s t a l l i s e d s i l i c a attack of a l k a l i on w e l l - c r y s t a l l i s e d poorly c r y s t a l l i s e d hydrous s i l i c a in

(A)

l~,e

C>e

CaX

2/n

+

20H-

with c e r t a i n types of aggregate, those being p a r t i c u l a r l y vulnerable. The s i l i c a is contrasted with that on Fig. I .

(B)

~>e

--0--

5

--0-) "--'0--

--0--

I L.

•

Si

o 0

• No (or K)

OH

FIG. 1 The attack of a l k a l i on w e l l - c r y s t a l l i s e d s i l i c a (A) and on poorly c r y s t a l l i s e d hydrous s i l i c a (B). The s t r u c t u r e s shown are h i g h l y diagrammatic, p a r t i c u l a r l y that shown in (A). Rings of four tetrahedra are used s o l e l y for convenience in drawing. Attack on w e l l - c r y s t a l l i s e d or other r e l a t i v e l y dense forms of s i l i c a takes place mainly at the surface. I t is rather slow and produces discrete s i l i c a t e ions which pass into the f l u i d phase. Poorly c r y s t a l l i s e d hydrous s i l i c a , on the other hand, permits penetration of hydroxyl and sodium (or potassium) ions i n t o i t s i n t e r i o r . Rupture of Si-O-Si bonds through hydroxyl attack loosens the network, and produces a c r o s s - l i n k e d p o l y e l e c t r o l y t e containing a l k a l i metal ions. This type of s t r u c t u r e , in contact with water, sets up an i m b i b i t i o n pressure (1,2) (related to osmotic pressure), and because the silicon-oxygen framework is p a r t l y disrupted the material swells. To throw more l i g h t on these processes, the reaction between sodium hydroxide and various forms of s i l i c a has been studied.

Vol. I I , No. l

3 NaOH-SILICA REACTIONS, ALKALI-AGGREGATE, MECHANISMS Experimental

l.

Silica Table l summarises the characteristics of the silic~used in this study. TABLE l The characteristics of the silicas

Silica

ignition loss

Silica Gel Quartz Fused Quartz Beltane Opal Calcined Flint (i) (2) , 2.

density

particle size

(%)

(g/cm)

(mesh)

specific surface area

(i)

7.8 0.0 0.0 6.5 0.4

2.65 2.19 2.21 2.50

60 - 120 lO0 - 200 lO0 - 200 lO0 - 200 -200

770 0 0 0 0

(m2/g)

(2) 0.23* 0.084 0.078 0.13 0.57

determined by t i t r a t i o n with sodium hydroxide (3) determined by Blaine a i r permeability apparatus This method underestimates the surface area of porous materials.

Dissolution of s i l i c a in sodium hydroxide solution

Weighed quantities of various silicas were mixed with 200 mL of sodium hydroxide solution at 25°C in polyethylene bottles f i t t e d with lids and glass s t i r r i n g rods. At intervals, 2 m~ of solution were removed for analysis.

3.

Volume change of solid s i l i c a immersed in sodium hydroxide solution

Fig. 2 shows the apparatus that was devised to measure volume change of solid s i l i c a when sodium hydroxide solution is added. I t consists of the bulb of a separating funnel sealed onto the stem of a 50 m~ burette. STOPPER

NaOH SOLUTION

'4

~i

STIRRING BULB ( 25O nW)

z_

GRADUATEDTUBE (5Oral)

SILICA RUBBERPLUG FIG. 2 Apparatus to measure volume change of solid silica immersed in sodium hydroxide solution.

After adding the silica and solution, the tube is inclined so that the mixture f a l l s into the bulb, where i t is stirred with a magnetic stirrer. At intervals, the tube is turned back to the vertical position. The silica settles back into the graduated portion and i t s volume is measured: at the same time 2 m~ of solution are removed for analysis. All measurements are made at room temperature. The i n i t i a l volume of silica was measured after s t i r r i n g i t with lO0 m~ of water, and the appropriate quantity of sodium hydroxide solution was then added. Cementsat the high alkali end of the normal range may generate pore solutions of the order of 0.7 M OH- or even higher within a month or so (4). In both this and the previous test, therefore, O.Ol 0.7 M NaOH solutions were used.

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L.S. Dent Glasser, N. Kataoka

4.

Analysis of [Na+], [OH-] and [Si02] in s o l u t i o n

[Na +] and [Si02] in s o l u t i o n were analysed using a Perkin-Elmer Model 305 atomic absorption spectrophotometer. [OH-] was measured using a Beckman pH meter with a Radiometer type G202A electrode and a calomel reference electrode. Results and Discussion I.

D i s s o l u t i o n of s i l i c a

in sodium hydroxide s o l u t i o n

( I ) Dissolution with s t i r r i n 9. When sodium hydroxide s o l u t i o n is s t i r r e d with s i l i c a gel, there is an immediate drop in both [Na +] and [OH-]. Subsequently, as the s i l i c a gel d i s s o l v e s , [Na +] in the s o l u t i o n rises again. Fig. 3 shows t h i s for a t y p i c a l experiment. Fig. 4 shows the d i s s o l u t i o n of some d i f f e r e n t s i l i c a s in sodium hydroxide s o l u t i o n under conditions s i m i l a r to Fig. 3. 0 I(]

040

ISl021

i ,

% %

O [NO~OI A

[S,02 ]

0=o6

4~

I ~20 ]

o~ 0

~

O......... A

0

BELTANE OPAL

00J

~4[

9

~

QLtPJRTZ

z

oo; ~'"T]~ E}__-

...............................

[OH')

~) u

43

0

10

20

TIME (HOURS}

30 TIME (DAYS)

40

50

FIG. 3

FIG. 4

Dissolution of s i l i c a gel in sodium hydroxide s o l u t i o n : 200 m~ of 0.I M NaOH (0.05 M Na20) s o l u t i o n s t i r r e d with 1.2 g of s i l i c a gel.

Dissolution of d i f f e r e n t s i l i c a s in sodium hydroxide s o l u t i o n : 200 m~ of O.IM NaOH (0.05 M Na20) s o l u t ion s t i r r e d with 1.2 g of s i l i c a .

All of these s i l i c a s have much less s p e c i f i c surface area than s i l i c a gel, so that the d i s s o l u t i o n is much slower. The i n i t i a l drop in [Na+] is not observed, and d i s s o l u t i o n is not complete even a f t e r 50 d a y s . When sodium hydroxide s o l u t i o n is mixed with s i l i c a , reactions occur together :

the f o l l o w i n g

(i)

The s i l i c a dissolves by hydroxyl attack.

(ii)

Silanol groups on the surface of s i l i c a react with OH- of the s o l u t i o n : ~ / Si-OH

(iii)

+

OH

~

/

--Si-O-

+

H20

Na+ in s o l u t i o n binds at tile negative charge ~n the surface of s i l i c a .

Let us apply f o l l o w i n g assumptions to the above reactions and estimate the change of [Na+], [OH-] and [Si02] in the s o l u t i o n of Fig. 3.

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5 NaOH-SILICA REACTIONS, ALKALI-AGGREGATE, MECHANISMS

(i) The d i s s o l v i n g rate of s i l i c a gel is proportional to the q u a n t i t y of silica gel S remaining at any time t (ignoring the e f f e c t of changes in ION-I), dS dt

=

-kS

where k is constant. Since S = 0.0184 mole at t = O, S = 0.0184 exp ( - k t ) = 0.0184 exp ( - T ) , where T is in a r b i t r a r y u n i t s . The f i n a l [Si02] is low enough in Fig. 3, so that s i l i c a t e species in the solution are monomeric, i . e . Si(OH)~, H3Si04- and H2SiO, 2The proportion of each is determined by [OH-] in the s o l u t i o n , and the respective e q u i l i b r i u m constants (5) : Si(OH), H3Si04-

_

~

H3Si04-

+

H+

pKI

=

9.8

-

H2Si042-

+

H+

pK2

~

11.8

(ii) There are about 4.5 ~ S i O H groups per nm2 of surface area of amorphous s i l i c a (6). Since the s p e c i f i c surface area of the s i l i c a gel is 770 m2/g, there are about 0.38 mole o f ~SiOH groups per mole of the s i l i c a gel. The f i n a l [OH-] is high enough in Fig. 3 (pKa of ~SiOH group on the surface of amorphous s i l i c a is 6.5 - 9.2 (7), that most of ~ S i O H groups react with OH- during the experiment. (iii)

Na+ binds to most of the negative charge on the surface : [OH-] reacted ~ [Na +] adsorbed ~ 0.38 S

Estimated values for [Na20],[OH-] and [Si02] calculated in t h i s way are p l o t t e d in Fig. 5 against an a r b i t r a r y time scale. The shapes of the curves agree well with those for experimental data in Fig. 3. In the case of the other s i l i c a s , the s p e c i f i c surface area is so small that d i s s o c i a t i o n of Si - OH and adsorption of Na+ are n e g l i g i b l e , even i f t h e i r surfaces carry the same number of ~SiOH groups per u n i t area as s i l i c a gel.

0.1C

[$~2 ) o .,,:I O.OE o

G'

z

~

0.OE

[NazOI

b

pp........... ..o. . . . . . . - o -

....... o -

.

.

.

.

.

.

-o

0.02

8 (:}"-""

"~-,' - . . . - O . -. .

2

~)- . . . . . . .

4 TIME (T, orbitory units )

FIG.

~[ O. H. - .] . . . . . 6

5

Dissolution of s i l i c a gel in sodium hydroxide s o l u t i o n , estimated according to above assumptions, f o r c o n d i t ion as Fig. 3.

-o

8

(2)

Dissolution without s t i r r i n g .

Fig. 6 shows the changes that occur when various q u a n t i t i e s of s i l i c a gel are mixed with 200 m~ of 0.05 M NaOH (0.025 M Na20) s o l u t i o n without s t i r r i n g . In t h i s e x p e r i ment, the s i l i c a gel was f i r s t mixed with I00 m~ of water, and a f t e r I0 minutes I00 m~ of 0.I M NaOH solution was added. These conditions are perhaps nearer to those in a concrete than were those of the s t i r r e d experiments.

Since the reaction c o n t r o l l e d by d i f f u s i o n through the mass of s i l i c a gel, d i s s o l u t i o n is slower. For small of s i l i c a gel, the trend of the reaction is s i m i l a r to Fig. 3.

rate is here of s o l u t i o n quantities However, f o r

6

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L.S. Dent Glasser, N. Kataoka

I

006

'_ . . . . [Na20] j

large quantities such as I0 g of s i l i c a gel, i t takes a long time for the solution to reach the bottom of the mass of s i l i c a gel: before the reaction of OH- with ~SiOH groups is complete, unreacted OH- in the super2g natant solution attacks the top layer of s i l i c a gel and excess s i l i c a gel dissolves. As the reaction of OHwith ~SiOH groups proceeds, this excess s i l i c a reprecipitates.

~

9,,, z° o.o~ <

ff

004

SILICA GEL

u. 0.0.3 o -~ 0.02 ~

kJA

g

001

........................ A

L)

TIME

I

I

(3)

Final concentration of dissolved silica

( DAYS )

Fig. 7 shows details of the simultaneous changes in [Si02] and pH FIG. 6 as s i l i c a dissolves, the l a t t e r decreasing as the former increases. Dissolution of s i l i c a gel in sodium These dissolution paths approach a hydroxide solution without s t i r r i n g : l i m i t which almost coincides with the various quantities of s i l i c a gel border line between the s t a b i l i t y and mixed with I00 mL of water and I00 m~ i n s t a b i l i t y regions of s i l i c a t e of 0.I M NaOH solution added a f t e r solution (8). When a dissolution I0 minutes. path reaches this l i m i t , the d i s s o l u t ion of s i l i c a ceases, and the path thus represents the s o l u b i l i t y curve of s i l i c a at f a i r l y high pH. I f enough s i l i c a gel is not taken, the dissolution path never reaches the s o l u b i l i t y curve (see path for 1.2 g/ S'L 0.I M, Fig. 7). On the other hand, i f a large quantity of s i l i c a gel is C u~ taken, excess s i l i c a gel removes OHfrom solution and the dissolution path reaches the l i m i t at lower pH and hence lower final [Si02]. Thus maximum final concentration of dissolved s i l i c a ~ i(]'3 is obtained when the optimum quantity \~2g/oo,, of s i l i c a gel is taken. This point can be made clearer by referring back ~o~ ~ ~ ~ G L to Fig. 6 which shows that as the pH euantity of solid s i l i c a gel taken increases, the final quantity in FIG. 7 solution passes through a maximum and then decreases. Up to about 1 g, the The final concentration of dissolved s i l i c a dissolves completely: above s i l i c a against pH (heavy line) and that quantity, excess solid remains. the paths by which individual Excess of s i l i c a gel thus reduces the solutions approach i t ( l i g h t l i n e s ) . [OH-] of the supernatant solution to such an extent that dissolution ceases (Fig. 7). Fig. 8 shows this dependence of final concentration of dissolved s i l i c a on the total Si02/Na20 mole r a t i o . Dissolved s i l i c a passes through a maximum at a mole r a t i o of 3 - 4, depending on NaOH concentration. The final concentration of dissolved s i l i c a is almost independent of whether solution and s i l i c a gel are s t i r r e d or not and whether s i l i c a gel is Though the path of the reaction is mixed with water at f i r s t or not.

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7 NaOH-SILICA REACTIONS, ALKALI-AGGREGATE, MECHANISMS

16

) 16

o20g O'TM NoOH

1 2"

4

O~

SILICA GEL / log d

,

159/

X

i

changed under such c o n d i t i o n s , the f i n a l concentration of dissolved s i l i c a is determined mainly by the q u a n t i t y of s o l i d s i l i c a gel taken and concentration of sodium hydroxide s o l u t i o n added.

312

I%

/

~30g ~ 008

lg

075

2. Volume change of silica gel immersed in sodium hydroxide solution

~ 2g~

05

~

°~

E 5

0g i

2

i 3

i

i

I

I

4 5 10 20 SiO2/No20 MOLE RATIO

I

30

[

Fig. 9 shows the volume change with time per mole of silica gel originally taken: that is total volume change, including loss through dissolution.

FIG. 8 The r e l a t i o n s h i p of the f i n a l concentr a t i o n of dissolved s i l i c a with t o t a l Si02/Na20 mole r a t i o solid line:

d i s s o l u t i o n in 0.05 M NaOH s o l u t i o n without stirring.

dashed l i n e : d i s s o l u t i o n in 0.7 M NaOH s o l u t i o n with stirrinng. In a l l cases, there is an i n i t i a l small increase in volume. For the smaller q u a n t i t i e s of gel, the e f f e c t is overtaken by the e f f e c t of dissolution. For the larger q u a n t i t i e s of gel, the volume does not change a f t e r the i n i t i a l increase. For the intermediate q u a n t i t i e s of gel, however, the volume increases continuously but s l i g h t l y . Fig. lO shows the volume as a function of the undissolved s i l i c a gel, solution.

200

07~4

005M NoOH

NoOH

d o ..%

~150

~ I00

SILICA GEL 2g ''"~

10g

30g

, ~ - T ~--~" %

-.-

A --... lg _j 50

20g

0"5g

l 20 STIRRING TIME

3O

MINUTES )

FIG. 9 Volume change of s i l i c a gel immersed in sodium hydroxide s o l u t i o n : various q u a n t i t i e s of s i l i c a gel were s t i r r e d with I00 m~ of water to determine the i n i t i a l volume, and I00 m~ of 0.I or 1.4 M NaOH s o l u t i o n was added.

i.e.,

after correcting for [Si02] in

This diagram shows the trend in volume change mentioned above more c l e a r l y than Fig. 9. Here the decrease in volume a f t e r i n i t i a l increase for smaller q u a n t i t i e s of s i l i c a gel is not caused by the e f f e c t of d i s s o l u t i o n , because t h i s has been allowed f o r . We may suppose that d i s i n t e g r a t i o n process mentioned in the i n t r o d u c t i o n continues, the network becoming progressively looser as more S i - O - S i bonds rupture. There are then two possible explanations f o r the decrease in volume of the residual s i l i c a gel: (i)

The less dense parts of the gel dissolve p r e f e r e n t i a l l y .

(ii) Once the frame work of the gel is t o t a l l y destroyed, the remaining material collapses to a denser form, and the volume decreases. On the other hand, for l a r g e r q u a n t i t i e s of s i l i c a gel, enough hydroxyl ion is consumed

8

Vol. I I , No. 1 L.S. Dent Glasser, N. Kataoka on the way so that distintegration and the consequent volume change ceases. O05M NoOH

0 7M NaOH

z5o

o

.

~

.o- "-- 20g "'Q

j,.

~220£

SILICA GEL

30g

°;--%]

4--

8

/ /' o

Fig. II shows the volume of s i l i c a gel after 30 minutes (from Fig. lO) plotted against total Si02/ Na20 mole r a t i o . Data for mortar bar expansion (9) plotted against

i./

1SC

8

"~' "~" "

0Sg

z

10g .:

~. = . =

o

~'00t

,= o

-

~

"

"

.

lOg o

~ STIRRING TIME

25( 07M NoOH/ ~ ." "'...

MINUTES )

,

FIG. lO

j.

-

Volume change of undissolved s i l i c a gel immersed in sodium hydroxide solution.

reactive s i l i c a Na20 equivalent mole r a t i o are also shown on t h i s diagram. All data pass through a maximum at intermediate Si02/Na20 mole r a t i o . I t can r e a d i l y be seen that Fig. II (present work) is similar to Fig. 8: undissolved s i l i c a gel causes maximum volume increase when s i l i c a gel reacts with sodium hydroxide solution most effectively.

005M NoOH

/

...-~

',

"...

'....

q,

~,

,,.

v 12 Z~

~1 )'8

i

150 ~4

~ 100

d S,02/~20 ~ E

RAT~

FIG. I I Expansion of s i l i c a . The volume of s i l i c a gel in sodium hydroxide solution after 30 minutes (present work) plotted against Si02/ Na20 mole r a t i o , is compared with the results of mortar bar expansion (9), plotted against reactive s i l i c a / Na2Oequivalent mole r a t i o .

The maximum expansion of mortar bars occurs at larger Si02/Na20 mole r a t i o than that of s i l i c a gel. This difference is not r e a l l y surprising in view of the very different conditions used in the two experiments. The particle size of the aggregate used in the mortar bar test is larger, and moreover there is a considerable quantity of lime present. B o t h these factors might tend to lower the effective SiOJNa20 mole ratio in the mortar bar tests. Conclusions

I. Whens i l i c a gel is s t i r r e d with sodium hydroxide solution, there is an immediate drop in both [Na+] and [OH-], due to adsorption on and reaction with solid s i l i c a gel. Subsequently, as the s i l i c a gel dissolves, [OH-] decreases continuously but [Na+] rises again. When other s i l i c a s whose surface area is much less than s i l i c a gel are s t i r r e d with sodium hydroxide solution, the dissolution is much slower and the i n i t i a l drop in [Na+] is not observed.

Vol. I I , No. 1

9 NaOH-SILICA REACTIONS, ALKALI-AGGREGATE, MECHANISMS

2. Under the conditionswithout s t i r r i n g , perhaps nearer to those in concrete, the reaction is controlled by d i f f u s i o n of solution. 3. There is s o l u b i l i t y curve to l i m i t the dissolution of s i l i c a . When s i l i c a gel is mixed with sodium hydroxide solution, s i l i c a gel dissolves and the pH of solution simultaneously decreases. When the dissolution path reaches the s o l u b i l i t y curve, dissolution of s i l i c a gel ceases. I f a large quantity of s i l i c a gel is taken, the excess s i l i c a gel removes OH- from solution and the dissolution path reaches the s o l u b i l i t y curve at lower pH and final concentration of dissolved s i l i c a is also lower. Thus, maximum final concentration of dissolved s i l i c a is obtained at intermediate total Si02/Na20 mole r a t i o . 4. The volume expansion of s i l i c a gel in sodium hydroxide solution passes through a maximum at intermediate total Si02/Na20 mole r a t i o . This r a t i o is near to that of maximum final concentration of dissolved s i l i c a . This phenomenon suggests an explanation for observations about "pessimum" proportions of reactive aggregate in mortar and concrete. References I.

T.C. Powers, Proc. 4th I n t . Symp. Chem. Cement, Washington (1960); NBS Monograph 43, Vol. I I , 788-794 (1963).

2.

L.S. Dent Glasser, Cement and Concrete Research

3.

G.W. Sears, Analytical Chemistry, 28, 1981-1983 (1956).

4.

S. Diamond, Cement and Concrete Research 5, 329-346 (1975).

5.

N. I n g r i , "Aqueous s i l i c i c acid, s i l i c a t e s and s i l i c a t e complexes" in G. Bendz and I . L i d q v i s t , "Biochemistry of Silicon and Related Problems", Plenum, New York (1977), p. 29.

6.

R.K. l l e r , "The Chemistry of S i l i c a " , Wiley - Interscience, New York, (1979), p. 633.

9, 515-517 (1979).

7.

Ibid,

8.

W. Stumm, H. H~per and R.L. Champlin, Environmental Science and Technology I , 221-227 (1967).

p. 182-3

9.

H.E. Vivian,

CS IRO

Bulletin

256, 13-20 (1950).