Assessment of simple methods of determining the free chloride ion content of cement paste

CEMENT and CONCRETE RESEARCH. Vol. 17, pp. 907-918, 1987. Printed in the USA. 0008-8846/87 $3.00+00. Copyright (c) 1987 Pergamon Journals, Ltd.

ASSESSMENT OF SIMPLE METHODS OF DETERMINING THE FREE CHLORIDE ION CONTENT OF CEMENT PASTE

C. Arya, N.R. Buenfeld and J.B. Newman Concrete Studies Section Department of Civil Engineering Imperial College of Science and Technology London, SW7 2BU

(Communicated by G.M. Idorn) (Received June 26, 1987)

ABSTRACT A range of possible techniques for assessing the "free" chloride ion content of hydrated cement pastes dosed with sodium chloride or calcium chloride have been investigated. The techniques are based on mixing powdered samples with a solvent and measuring the amount of chloride passing into solution. The values obtained have been compared with the chloride ion content of pore solution expressed from parallel specimens. The results indicate that in the range of chloride additions considered several extraction techniques may be used to estimate the free chloride ion content of cement paste; the total chloride content will dictate the most appropriate extraction technique(s) to adopt. Introduction The high pH of cement paste pore solution enhances the formation of a passivating oxide layer around steel embedded in concrete. However, in sufficient quantity, chloride ions may disrupt this layer allowing corrosion to occur. Chlorides are often present as contaminants of concrete mix ingredients and, until recently, were the principal constituent of most accelerating admixtures. Dissolved chloride ions also may penetrate unprotected hardened concrete in structures exposed to marine environments or to deicing salts . Chlorides in concrete may be present in a number of states: (I) "bound" (a) chemically bound with the hydration products of the cement such as the C A or CaAF phases; (b) loosely bound with t~e C-S-H gel [I]; and (2) "free" ions within the pore solution. It is generally recognised that only the free chloride ions influence the corrosion process and, consequently, an accurate assessment of the free chloride ion content is essential when attempting to determine the risk of corrosion of embedded reinforcement. 907

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Vol. 17, No. 6 C. Arya, et al.

At present the only reliable method of determining free chloride ion content is by expressing the pore solution from core samples using a high pressure device and analysing the extract [2]. The use of such specialised apparatus is clearly impractical as a routine part of investigative surveys of concrete structures. Thus, it was with this limitation in mind that this project was initated with the aim of assessing, and where necessary developing, more practical methods for determining the free chloride ion content. The methods adopted are based on mixing powdered samples with a solvent and measuring the amount of chloride passing into solution. ExDerimental Ordinary Portland cement (OPC) pastes were prepared with a water/cement ratio of 0.5 and various levels of NaC1 or CaClp dissolved in the mixing water. Refer to Table I for cement composition and-Table 2 for mix details. Batches consisting of 15kg of each of the mixes were prepared. After thorough mixing, 10 small specimens (45 dia. x 58mm) suitable for pore pressing and 7 large specimens (108 dia. x 98mm) for producing powdered samples, were cast in cylindrical plastic moulds and the moulds sealed. To minimise segregation the specimens were rotated at approximately 6 r.p.m, for 24hrs. The specimens were then stored in the sealed condition at 21 2°C for a further 27 days. Three small specimens from each batch were heated to constant weight I05°C and then I000°C to determine the evaporable water and cement contents of the specimens. Due allowance was made for ignition losses.

at

Pore solution was expressed from five small specimens from each batch using a cylinderical press similar in construction to that described by Barneyback and Diamond [3]. Each specimen was subjected to two loading cycles up to 100

TABLE I Composition of OPC Chemical

SiO2

AI203

Fe203

19.7

5.8

3.2

Analysis ( % by weight ) CaO

64.6

MgO 1.2

SO3 2.7

loss on

free

K20

Na20 i g n i t i o n

llme

0.47

0.23

1.4

1.7

Bogue Compositions ( % by wel~ht ) C3S C2S C3A C4AF 55.2

TABLE 2 Mix Details

Mix Reference

Cl-,by weight of cement from: CaC12

0.5% 1.0% 2.0%

1.o~

14.8

9.9

9.7

Vol. 17, No. 6

909 FREE CHLORIDE ION, DETERMINATION, METHOD

tonnes. A measured volume of the expressed solution was diluted and analysed for chloride ion content by potentiometric titration with silver nitrate solution using a chloride ion selective electrode, in accordance with the method described by Berman [4]. The pore solution analyses, together with the evaporable water and cement content values, were used to calculate the percentage of "free" chloride ion by weight of cement. Concurrently with the pressing operation, the large specimens were powdered by a combination of drilling and crushing. The powdered pastes were then dried to constant weight at I05uC and the dried powder sieved into four fineness ranges, namely 600-300 m, 300-150 m, 150-75 m and <75~m. The samples were then transferred into air-tight plastic bags and stored in vacuum desiccators over silica gel and sodium hydroxide until required. The following parameters were thought to be of significance in developing appropriate techniques to determine the free chloride ion content of cement paste based on testing powdered samples:a) particle size {fineness) b) temperature of solvent c) solvent/solid ratio d) extraction time e) agitation f) solvent type Generally, two replicates of each test were undertaken, although in some cases this was increased to five where it was felt necessary to carry out a statistical analysis of the results. The chloride ion content of the solution was determined by pipetting a 20ml aliquot into a beaker and potentiometrically titrating with silver nitrate as previously described. ~esults and Discussion pore Solution E~pression Fig. I shows the percentage of free chlorides by weight of cement as a function of the total chloride. The results show that the percentage of bound chlorides remains approximately constant in the range of sodium chloride additions considered and, therefore, that the percentage of free chlorides increases in direct proportion to the total chloride content. Results published by Ramachandran et al. [5] using a similar pressure vessel to extract pore solutions from 1.0 w/c OPC pastes containing 0.8, 1.5 and 3.0 % calcium chloride (fig. I) show that the bound chloride content increases as the total chloride content is increased. This difference in behaviour is almost certainly attributable to the associated cation. Finenes~ Two extraction techniques were adopted to investigate the effect of fineness on the quantity of chlorides leached from a sample. The first involved extraction at room temperature in which 20g samples of the powdered paste were added to 400ml of distilled water; the mixture was stirred for 5mins and then allowed to stand for 55mins prior to analysis. The second procedure involved boiling a 20g sample of the paste in 400ml of distilled water for 5mins and

910

Vol. 17, No. 6 C. Arya, et al°

2.0

l

f

l / //

g E o

0

1.8/

; ,6 -

C

/,/////~

NoCI

1,41 .2 -

~e

,'~'~'""

0.8-

CcCI,

17.6-

5

RAMACHANDRAN [5]

0 •4. 0.20.0

//

//// ,"

o.o

o~s

,Jo

1:s

zlo

2Js

3.0

Total Chloride (% by wt. of cement)

FIG. I Free chloride ion content as a function of total chloride content.

TABLE 3 Effect of Fineness.

Extraction Technique

5min stirring 55min standing

Fineness

% C1 leached by wt of cement

standard deviation

<75

0.211

0.012

75-150

0. 186

0.014

150-300

0.179

0.011

300-600

0.185

0.003

<75

0.857

0.017

M

75-150

0.759

0.005

I!

150-300

0.844

0.007

N

300-600

0.837

0.002

w

I!

5min b o i l i n g 55min standing

All tests on mix B Total C1 = 1.0% Free Cl = 0.578%

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911 FREE CHLORIDE ION, DETERMINATION,

METHOD

allowing the mixture to stand for 55mins. The mixture was filtered and then allowed to cool to room temperature before titrating. Due allowance was made for the volume of water which had evaporated during the boiling operation. Five replicates of each test were carried out. The results are presented in Table 3 which indicates that fineness has very little effect on the quantity of chlorides removed using the room temperature extraction procedure. By boiling the samples in distilled water a similar trend is observed although the results obtained for the 75-150~m samples show a reduction of around 10 % in the quantity of chlorides extracted. No explanation has been found to account for this discrepancy. Whilst the results presented here suggest that the ranges of fineness considered do not have any significant effect on the quantity of chlorides extracted from OPC pastes, Guenther [6] presents data which shows fineness is an important factor in the case of mortar specimens. Temperature The effect of solvent temperature on the volume of chlorides passing into solution was also investigated. The results are presented in fig. 2. As expected, the higher the temperature the greater the proportion of total chlorides passing into solution. It would also appear that bound chlorides are released at temperatures higher than 50°C, although results of similar tests carried out on the remaining mixes, (refer to fig. 7), show that this does not apply universally. Work carried out by others [7] on the pure calcium chloroaluminate compound, 3CaO.AI^O~.CaCI^.IOH^O, indicates that in cold water it is only slightl~ ~olubl~ butLthat in hot water it decomposes.

1.0

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solvent

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t..

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5 5 m i n ~andin(:J

(_] 0.0

lb

2b

3b

4b

5b

6b

7b

8b

Temp. of Solvent (°C) FIG. 2 Effect of Temperature.

9b 1do 11'o

120

912

Vol. 17, No. 6 C. Arya, et al.

1.0

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mix fineness

: B : <7~n

solvent temp.

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5rain stirring 55min standing

".C 0

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lb

zb

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Solven/Solid Ratio FIG. 3 Effect of Solvent/Solid Ratio.

Solvent/Solid ratio Three extraction procedures were used to determine the effect of the solvent/ solid ratio on the quantity of chlorides leached from 20g samples of the powdered paste as follows : a) 5min stirring 55min standing b) stirring mixture for 6hr c) stirring mixture for 15hr The results from the last two techniques (fig. 3) show that the percentage of chlorides passing into solution reaches a maximum at a solvent/solid ratio of about 1:20. The initial increase in the percentage of leached chloride with increasing solvent/solid ratio can be explained in terms of the increased chloride ion concentration gradient between the paste and the solvent causing more rapid dissolution/ diffusion. However, as the solvent/solid ratio is increased still further, the percentage of chlorides leached decreases; it is thought that this occurs as a result of a slowing down of the diffusion process due to less efficient agitation of the extra volume of solvent. Standing Time Samples (20g) of each of the four mixes were dispersed in 400ml of distilled water and the mixtures allowed to stand. Aliquots (20ml) of the solvent were removed after 24, 48, 72 and 168hrs and their chloride contents determined. An equivalent volume of solvent was added to the mixture so as to maintain a constant solvent/solid ratio. Fig. 4 shows the percentage of chlorides passing into solution as a function of standing time; the bracketed percentages in fig. 4 are the amount of chloride expressed as a percentage of the free chloride ion content. It can be seen that, for all mixes tested, the concentration of chlorides in the solvent increased steadily over the first 48hrs and levelled off after 72hrs. Fig. 5 shows that as the total chloride

Vol. 17, No. 6

913 FREE CHLORIDE ION, DETERMINATION, METHOD

2.0

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fineness

: 75-15~uzn

E O0

solvent

: water

E 1.6-

solvent solid : 1:20

(3

temp.

-8

1.4-

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: 20"C

(6s~)

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s c

"~ 1 . 2 ..Q 1,0-

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144

166

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S t a n d i n g Time (hrs)

FIG. 4 Effect of Standing Time.

content of the mix increases, the ratio of leached/ pore solution chlorides decreases. The results also show that some of the bound chlorides are released from the mix containing 0.5% CI- whilst a proportion of the free chlorides take longer than 168hrs to be leached from mixes containing 2 % CI . ~gitation The effect of continuously stirring the mixture during room temperature tests is shown in fig. 6. Comparing these results with those obtained by allowing the mixture to stand for up to 168hrs shows that, in all cases, the percentage of chlorides removed is greater if the mixture is continuously agitated. The likely explanation for this behaviour is that if the mixture is not agitated a local equilibrium is established between cement particles and the layer of water immediately surrounding them which inhibits further dissolution. However, when the mixture is agitated no such equilibrium can occur and the unbound chlorides readily pass into solution until such time as their supply is exhausted or they reach equilibrium with the entire solution. Solvent After a careful survey of solvents which could be used to leach out the free chlorides, methanol and ethanol were selected as two possible alternatives to distilled water. The methanol molecule has similar physical properties to that of water and i8 generally regarded to be chemically inert in reactions with hardened cement pastes at room temperature [8]. Ethanol also has similar properties, although it was considered likely to be less effective than methanol, because of its less polar structure and larger molecular size.

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Vol. 17, No. 6 C. Arya, et al.

I

400

.---. 350 o

r" C)

300-

I

I

I

: 75-15~1~n

fineness

k k k

aolvent solvent

: solid

temp

woter

: 1:20 : 20°C

250-

LL "~ t- 200O 0 _j~ 150~______I

-o •r-

~

TOTALCHLORI[

100-

0 r

,2,.][

500

0.0

i

0.5

i

i

1.0

I .5

i

2.0

lime

/

2.5

Totel Chloride (N by wt of cement) FIG. 5 Effect of Standing Time.

Table 4 summarises the percentages of chlorides leached using four fairly aggressive extraction techniques on the mixes containing 1 % C1 from sodium or calcium chloride. As can be seen, both methanol and ethanol proved to be extremely ineffective at leaching out the free chloride. However, it can also be seen that in the case of mix B and methanol, increasing amounts of chloride were leached as harsher extraction techniques were applied. This trend is also apparent in the case of mix D and methanol, although the results from the soxhlet extraction are slightly higher than expected. With ethanol this effect is less pronounced although difficulties were experienced in accurately determining the very low levels of chloride leached. To understand why certain solvents are more effective than others in extracting chlorides it is necessary to consider the form of chlorides in the powdered samples and the capacity of the solvents to dissolve these chlorides. The chemical analysis of pore solution extracted from OPC specimens shows that it is mainly composed of Na +, K +, OH- and Ca ~+ ions. The solution is saturated with respect to Ca(OH) 2. If NaCI is dissolved in the mix water I the most significant change is an increase in the levels of dissolved Naand CI- ions in the pore solution. However, if CaCI 2 is added, because the pore solutio~ is already saturated with respect to calcium hydroxide any additional Ca L+ is precipitated as Ca(OH)p [9]. Thus it can be seen that irrespective of the associated cation, t~e CI- ions eventually coexist in solution predominantly with OH-, Na + and K + ions. As the powdered samples dry, Ca ~+ is precipitated as Ca(OH) 2 and then, KCI and NaCI are formed as the solubilities of the respective compounds are exceeded. As can be seen from Table 5, NaCI and KC1 are highly soluble in water but only slightly soluble in the two organic solvents. Nevertheless, calculations show that in theory it should be possible to leach out all the free chloride known to be present in the samples of paste, using methanol or ethanol, assuming

Vol. 17, No. 6

915 FREE CHLORIDE ION, DETERMINATION, METHOD

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1,0

1

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1

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c"

g

0.9

E 0.8

"6

0.7

>., 0 . 6

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

free

CI-

.El

0.5 0.4 tO 0 Q~ J "0

o t(D

mix

: B

0.3

finsness

: < 75,Mm

0.2

solvent

solvent

: woter solid

temp.

: 1:20 : 20"C

O.1 0.0

24

0

Stirring Time (hrs)

FIG. 6 Effect of Agitation.

TABLE 4 Extraction of chlorides with methanol or ethanol.

Extraction Technique

Solvent

Boil mixture for 20hrs.

Methanol

Soxhlet for 20hrs [12]

Methanol

Ethanol

Ethanol

Methanol Standing for 168hrs

Ethanol

Methanol Stirring for 168hrs

Ethanol

Mix

Cl (% by wt. of cement)

B D B D

0.311 0.129 0.051 0.018

B D B D

0.218 0.186 0.066 0.021

B D B D

0. 183 0. O59 0.061 0.021

B D B D

0.209 0.090 0.130 0.073

fineness : 75-150~m solvent/solid: 1:20

Free chloride (% by wt of cement) Mix B (NaCI)

0.578%

Mix D (CaCl 2)

0.297%

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Vol. 17, No. 6 C. Arya, et al.

TABLE 5 Solubilities of chloride salts in various alchols. Solubility Water

Solute

Temp. (°C)

NaCI

20 50 100

35.7

20 50 100

23.8

20

74.5

KCI

CaCI 2

(g solute per 100ml solvent) Methanol Ethanol 1.127 1.028

0.115 0.114

1.537 0.765

0.162 0.107

23.1

20.3

39.1

56.7

that the free chloride now all exist as NaC1 or KC1. It is thought, therefore, that the cause of the low results is kinetics rather than thermodynamics and that more chloride would be removed if the extraction period were prolonged. This explanation casts doubt on the existence of the loosely bound chloride state as suggested by Ramachandran [I]. During his investigations to remove the free chloride from samples of C~S paste dosed with CaCl_ he found that 3. significantly higher levels of chlorlde were leached by was~ing samples of the powdered paste in water rather than in ethanol. The choice of these solvents was based on the fact that CaC12 is highly soluble in both (Table 5). The difference in the percentages of chloride leached by the two solvents was attributed to the existence of the so-called loosely bound chloride state; whilst water was capable of removing both the free and loosely bound states, ethanol was effective in removing only the free. From the foregoing, however, it would appear that a more likely explanation for the low level of chloride leached was that the pore solution does not contain CaC12, as had been assumed, but rather KC1 and NaC1. The latter chloride compounds are only slightly soluble in ethanol; furthermore, the leaching technique applied was too mild to solvate much of the free chloride. Assessment of extraction te~h~ieues Fig. 7 shows the level of chlorides leached, as a percentage of the free chlorides in the pore solution, from the three sodium chloride mixes, using a selection of the extraction techniques. Techniques involving extraction at room temperature are attractive in that chloroaluminate compounds are known to be only slightly soluble in cold water and thus, theoretically, the bulk of the chlorides passing into solution should be the free component. The extraction procedure involving boiling the sample in distilled water for 5mins and standing for 55mins is rather similar to the method prescribed by the Ontario Ministry of Transportation and Communication [11] for the determination of water-soluble chlorides in concrete, which recommends boiling the sample for 5mins and standing for 24hrs. Although there appears to be no reason why boiling the sample for 5mins should remove only the free chloride, the method does correlate well with pore solution chloride, especially at the higher total chloride levels.

Vol. 17, No. 6

917 FREE CHLORIDE ION, DETERMINATION, METHOD

I

400

t

fineness

~350

o(.-

I

I

: 75-15~

::;?°Jo

300

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L. b-

to o

250

200 150

"V_. 100

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Total Chloride (% by wt of cement) Extraction technique and approx, range o! applicability

I ,to..i~ 8hrs

l

J ~ h r s standing 1 6hrs s t i r r i n g . _ _

FIG. 7 Comparison of selected extraction techniques.

No single technique studied is sufficiently accurate over the range of chloride additions considered to be used universally. Nevertheless, certain techniques can be used over a limited range, as shown in fig. 7. The limits of each range have been defined so as to enable an estimate of the free chloride content to be determined to an accuracy of within ~20% of the true value. Further research being carried out is based oN the notion that the total chloride content of the sample will dictate the most appropriate extraction procedure to adopt. Work is in progress to assess the applicability of these techniques to mortar and concrete specimens containing admixed or externally derived chloride. ~onclusions A range of possible techniques for assessing the "free" chloride ion content of hydrated cement pastes, dosed with sodium or calcium chloride, has been investigated. The techniques are based on mixing powdered samples with a solvent and measuring the amount of chloride passing into solution. The values obtained have been compared with the chloride ion content of pore solutions expressed from companion specimens. The quantity of chloride leached was found to be independent of powder fineness up to the maximum particle size tested of 600~m but increased with solvent temperature and agitation time of the powder/solvent mixture. Solvent/solid ratio was found to have only a marginal influence on the leaching process. Methanol and ethanol proved to be ineffective as leaching media due probably to the low rates of dissolution of NaCI and KCI salts.

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Vol. 17, No. 6 C. Arya, et al.

The results indicate that, in the range of chloride additions considered, several extraction techniques may be used to estimate (to within ±2~%) the free chloride ion content of cement paste. The total chloride content of the sample will dictate the most appropriate extraction technique(s} to adopt. Acknowle~gemen~ Dr. N.R. Buenfeld was supported by an SERC Postdoctoral Research Fellowship. Reference~ I. 2.

V.S. Ramachandran, Materiaux et Constr., 14, 3 (1971}. K. Tuutti, Corrosion of Steel in Concrete, Report fo 4.82, p. 261 The Swedish Cement and Concrete Association, Stockholm {1982). 3. R.S. Barneyback Jr. and S. Diamond, Cem.Concr.Res. 11, 229 (1981). 4. H.A. Berman, Journal of Materials, I, 330 {1972). 5. V.S. Ramachandran, R.C. Seeley and G.M. Polomark, Materiaux et Constr., ~ , 285 (1984). 6. F. Guenther, "Chloride Corrosion of Reinforcing Steel CT 140/160", Proceedings of the International Conference on the Problems of Accelerating Concr. Hardening in the Preperation of Precast Reinforced Concr. Constr., Moscow, p.61 {1964} published 1968. 7. Handbook of Chemistry and Physics, Editor R.C. West, 66, p. B-82, The Chemical Rubber Co., Florida {USA), (1985-86). 8. L.J. Parrot, Cem.Concr.Res., 13, 65 (1983). 9. C.M. Hansson, Th. Frolund and J.B. Markussen, Cem.Concr.Res., 15, 65 (1985}. 10. Solubilities of Inorganic and Organic Compounds, Editors H.&T. Stephen, Vol. I Part I, p. 108-9, 654-5, Pergamon Press Ltd., Oxford {1963). 11. Ontario Ministry of Transportation and Communication, "Method of Test for Determination of Water Soluble Chloride Ion in Concr., No LS-411 (1983). 12. A.I. Vogel, A Textbook of Ouantitative Inorganic Analysis including Elementary Instrumental Analysis, p. 905, Longmans, London {1961).