OH− in the pores of cement paste

CEMENT and CONCRETE RESEARCH. Vol. 18, pp. 895-900, 1988. Printed in the USA 0008-8846/88. $3.00+00. Copyright (c) 1988 Pergamon Press plc.

CHLORIDE PENETRATION CEMENT PASTE

Civil

AND THE RATIO OF CL-/OH-

IN THE PORES OF

O.A. Kayyali Engineering Department, Kuwait University, P.O. Box 5969, Kuwait 13060 and

Civil

M . N . Haque Engineering Department, Australian Defence Campbell, A.C.T. 2600, Australia

Force Academy,

(Communicated by F.H. Wittmann) (Received March I, 1988) ABSTRACT Hardened Portland cement paste samples were immersed in sodium chloride solution. Their pore solution was expressed and analysed for C£- and OH- ions concentrations. The development of free C~- and of C£-/OH- ratio is discussed in relation to the limits previously established for the onset of depassivation of steel reinforcement. It was found that within a relatively short period penetration of the chloride ions could cause the C~-/OHratio in the pores to exceed the depassivation limits. Introduction A c c o m m o d a t i o n of chlorides by the pore solution of cement paste occurs as a result of either the introduction into the mix of a chemical additive which contains chloride or the contamination of concrete by chloride carrying agents existent in the microenvironment. Determination of the concentration of the free chloride ions caused by the first process has been performed in several research works (1-4). Fewer research has been done in relation to the chloride whose source is the microenvironment. Page et al (5) studied the diffusion process of chloride ions in hardened cement pastes and related it to the pore structure of the material. In a recent publication,Page et al (6) related the risk of chloride induced corrosion to the relative concentrations of chloride and hydroxyl ions as well as to the diffusivity of the chloride ions. Diamond (7) stressed the importance of C£-/OH- as an indicative factor in steel depassivation. Penetration of chlorides from the surrounding solutions was studied by Midgley and Illston (8) who measured the total chlori895

896

Vol. 18, No. 6 O.A. Kayyali and M.N. Haque

des at several locations in the paste and calculated the amount of free chloride as the difference between the total chloride and that calculated from the stoichometry of m o n o c h l o r o a l u m i n a t e . In the study reported here, the free chloride is measured directly with the aid of the pore solution expression technique. The development with time of C£-/OH- is discussed in relation to the limits considered critical for the onset of depassivation. Experimental

Details

Cylindrical specimens of cement paste 41 mm diameter and 100 mm high were cast. The cement was Ordinary Portland cement and the water was glass distilled. The ratio of water to cement was 0.23 which was the normal consistency of the cement. Mixing was performed in a small mixer according to ASTM method (9). The samples were compacted on a vibrating table and kept under polythene sheets inside a fog room at 23°C. After 24 hours they were demoulded. Samples whose initial curing was one day were immediately immersed in sodium chloride solution. Samples whose initial curing periods were 4 days or 7 days were kept in the fog room for further 3 days and 6 days respectively, after which they were immersed in the solution. The solution of sodium chloride was prepared by dissolving 38.574 g of pure NaC£ per litre of distilled water to produce the concentration of 23400 ppm of C£ion. This value approximates the chloride ion concentration in the Arabian Gulf (i0). The samples were kept in the solution at 23°C for periods of 28 days and 90 days. After the designated period, the samples were taken out of the solution, brought to surface dry condition, and introduced in a pore solution expression device. The device was manufactured according to the description of Barneyback and Diamond (ii) who modified the device used previously by Longuet et al (12). The extracted solution was analysed for C£by potentiometeric titration following the method described by Berman (13). The OH- determ i n a t i o n was performed by titration against standard hydrochloric acid of 0 . 0 1 N using phenolphthaleine as indicator. Results

and Discussion

The development of chloride ion concentration in the pore solution of hardened cement paste is shown in Fig. I. When it is taken into c o n s i d e r a t i o n that the hardened paste contained no chloride ions before contact with the salt solution, the following points are observed:

(a)

The diffusion of chloride ion is influenced by the initial curing period. Smaller concentrations were recorded for pastes which were normally cured for longer periods before immersion into the chloride solution. This observation is expected since the porosity of the paste decreases in the first few weeks of hydration. At this low w/c ratio, unconstricted capillaries in the range of i00 nm diameter were shown to exist in important proportions in pastes of 1 to 7 days of age (14). It has been suggested that the presence of this type of pores facilitates the diffusion of chloride ions (5).

Vol. 18, No. 6

897 CHLORIDE PENETRATION, CI/OH RATIO, PORE SOLUTIONS

(b)

Two phases of chloride ion penetration must be in operation. The first takes place shortly after contact with the solution occurs. The duration of this phase is still to be established. The second phase which is represented in Fig. 1 by lines A and B appears to be governed by the diffusion characteristics in cement paste as discussed by Page et al (5). Thus the rate of diffusion decreased as the initial curing period increased. This result emphasises the significance of the pore structure development in relation to the diffusion of chloride ions.

(c)

Within the period of tests recorded here, the initially chloride free pore solution acquired as high as 334 mmole/£ as chloride ion concentration. In fact, within a relatively short period, the concentration of chloride in the pore solution was in excess of 200 mmole/£. Thus within thirty days, the chloride ion concentration became about one third that of the surrounding solution. After 90 days the chloride concentration in the pores became" 0.5 that of the surrounding solution. It is of interest to record here that these levels of concentrations are comparable to the levels obtained in cement pastes which were mixed with water containing chloride of 0.4% to 1.0% of the cement weight (2). These levels are also comparable with levels obtained for mortars mixed with water containing 2% C£- by weight of cement (15).

7 340-1 320"1

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of

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time on concentration of chloride ions fluid of hydrated cement paste.

in the pore

898

Vol.

18, No. 6

O.A. Kayyali and M.N. Haque

Development tions

in the Ratio of Chloride

to Hydroxyl

Ion Concentra-

This aspect is presented in Fig. 2 which clearly shows that initial curing period has an important effect on the C£-/OHratio. A ratio e x c e e d i n g 3.5 was recorded for pastes which were cured for one day before immersion for 90 days in the salt solution. Pastes which were cured for 4 days or 7 days before immersion for 90 days gave the ratios of 1.66 and 0.82 respectively. Moreover, it is apparent that the development of C£-/OH- experiences two phases. An initial phase accompanies the progress of penetration until e q u i l i b r i u m is established. This phase must be dependent on several factors such as the water content of the paste, the h y d r o s t a t i c pressure at the contact surface, and the size of the specimen. In the second phase the rate of increase in the ratio of C£-/OH- depends upon factors which influence the diffusion of chloride ions in the paste such as porosity whose effect appears in these tests. Specimens which have lower number of unsegmented relatively large capillary pores showed a lower rate of C£-/OH- development. The value of C£-/OH- experienced near initial e q u i l i b r i u m were around 0.8 for pastes cured normally for 4 or 7 days. It has previously been found that the critical C£-/OHratios for the onset of steel depassivation is around 0.3(16,7). Diamond (7) found that a 1% CaC£ 2 by weight of cement or its equivalent of NaC£ (about 0.48% C£-) resulted in a C£-/OH- ratio of 0.77 in CaC£ 2 pastes, and 0.64 in NaC£ pastes, These ratios are comparable with the results obtained here which correspond to values of d e p a s s i v a t i o n and are equiva4

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40

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in the pore

fluid of hydrated

cement

paste

Vol. 18, No. 6

899 CHLORIDE PENETRATION, CI/OH RATIO, PORE SOLUTIONS

lent to the addition of C£- in the mixing water of about 0.5% by weight of cement. This level is considered above the limit for low risk of corrosion (17). Observation on the Solution Expression Process Difficulty was experienced in the process of solution expression for specimens which were immersed in sodium chloride solution for 90 days. Pressures up to 600 MPa were applied repeatedly but the amount of liquid collected was very small. In the case of the samples whose initial curing was one day before immersion in solution for 90 days, only few drops were expressed out. Difficulties were encountered in measuring and diluting the expressed fluid. Therefore only C£-/OH- related to that case could be accurately determined since it is a ratio of concentrations. However, an important conclusion arising from this experience is that the flow of solution became restricted after long immersion in the salt solution. It was observed that pore solution was much easier to be expressed from fog cured specimens. It is appropriate at this stage to draw attention to the findings of Midgley and Illston (8) who investigated Portland cement paste of 0.23 w/c immersed in sodium chloride solutions. They found that penetration of chloride ions reduced the sizes of the pores compared to specimens cured in nonchloride environment. They suggested that the reduction in the pore size is due to the formation of calcium chloride on the surface of the C-S-H. Their conclusion that the permeability to water will consequently be reduced has therefore been practically experienced in the tests reported here. Conclusions

(I)

Sufficient initial curing is needed in concrete which is expected to be exposed to chloride contamination. Long curing periods result in smaller pores where diffusion takes place at a low rate.

(2)

The pores of hardened cement paste in contact with chloride solution could acquire in relatively short period a large proportion of free chloride. The levels of chloride concentration were comparable with levels obtained from treating the original mix with 0.4% to 2.0% of C£- by weight of cement.

(3)

The ratio of C£-/OH- which is considered a principal parameter determining whether depassivation of steel reinforcement would occur has been found to exceed limits set previously for the onset of depassivation. Hence pores of dense matrix in contact with a chloride solution whose concentration is similar to that of some sea waters could accommodate fluid whose C£-/OH- is high from the point of view of corrosion risk.

(4)

There has been practical evidence that the flow of fluid in the pores was made difficult as a result of immersion in chloride solution. This confirms previous finding that permeability decreases in such a situation.

900

Vol. 18, No, 6 O.A. Kayyali and M.N. Haque

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