STUDIES ON SMALL IONIC DIFFUSIVITY CONCRETE

Transcription

1 319 STUDIES ON SMALL IONIC DIFFUSIVITY CONCRETE He Xingyang, 1 Chen Yimin, 1 Ma Baoguo, 2 Li Yongxin, 1 Zhang Hongtao, 1 and Zhang Wensheng 1 1 China Building Materials Academy, Beijing, 124, PRC 2 Wuhan University of Technology, Wuhan, 437, PRC Abstract Many results of systematic analyses of concrete deterioration in marine environment show that nearly all deteriorations are concerned with the ionic diffusion. In order to increase its life-serving, concrete materials in marine environment should satisfy the mechanics qualities required by structural design, but also be a kind of small-ionic-diffusivity concrete (SIDC). In this paper, one kind of SIDC had been manufactured by means of the compounding of mineral admixture and high effective water reducer. The results of experiments indicate that the addition of mineral admixture decreases greatly the speed of ionic diffusion. The effective chloride diffusion coefficient of the SIDC can be lowered two orders of magnitude compared to control concrete. Furthermore, the relative expansion ratio of concrete with the compound of fly ash and micro silicon is only 53.9% of that of control concrete in ASTM C112-95A at the 54th week. The composition and microstructure of SIDC had been studied in this paper. It is shown by tests that the addition of mineral admixture leads to the thinning of pore size of cement paste, reduction of unfavorable crystal phase and increase of chloride ion binding, which result in the improvement of anti-ion diffusion character of SIDC.

2 32 International Workshop on Sustainable Development and Concrete Technology 1. Introduction The engineering quality and theoretical study of marine and coastal concrete materials are the foundation to improve service life of the marine engineering projects as well as coastal buildings. Many results of systematic analyses of concrete deterioration under multiple corrosion-factor effect show that nearly all deteriorations are concerned with the ionic diffusion. Chloride diffusion and sulfate attack are without question two main deteriorating factors concerned with the ionic diffusion. The chloride-induced corrosion can cause significant deterioration of reinforced concrete structures, resulting costly repair [1]. On the other hand, in many regions of the world, soil and water contain adequate sulfate to cause deterioration of structure concrete [2]. For this reason, the development of high-performance concrete with capability of resisting chloride diffusion and sulfate attack has been the subject of research for many years. In order to enlarge its service life under multiple corrosion-factor effect, concrete materials should satisfy the physical mechanics qualities required by structural design, but also have a good resistance to all kinds of ions [3]. This is to say, it should be a kind of small-ionic-diffusivity concrete (SIDC). In former papers, mineral admixtures such as fly ash, slag, and silica fume had been incorporated into the mixes to increase concretes resistance to chloride diffusion or sulfate diffusion. However, the information on its influence on chloride diffusion and sulfate diffusion in concrete is scarce. In the present paper, the resistance to chloride diffusion and sulfate diffusion for different concrete mixes with and without mineral admixtures was studied. The composition, structure and the influence on durability of SIDC had been studied by means of SEM, MIP, etc.

3 He Xingyang et al Raw Materials and Experimental Procedure 2.1. Raw materials Cement used in this test was ASTM Type 1 portland cement (OPC) with a relative density of 3.15 and fineness of 35 m 2 /Kg. The loose density of silica fume adopted was.2 g/cm 3 with average size of.4 µm. The chemical admixture adopted in the study was FDN. The coarse aggregate used was crushed limestone with a maximum size of 2 mm. The fine aggregate used was natural river sand with a fineness modulus of 2.8. The compositions and specific surface area of cement and mineral materials were showed in Table 1. The spectrum of fly ash A and B were shown in Figs. 1 and 2. Table 1: Chemical component and specific surface area of cement and mineral admixtures Chemical BET surface Blaine specific SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 R 2 O IL component area (m 2 /Kg) surface area (m 2 /Kg) Cement Fly ash A Fly ash B Slag / 43 Silica fume / Fig. 1: XRD spectrum of fly ash A Fig. 2: XRD spectrum of fly ash B

4 322 International Workshop on Sustainable Development and Concrete Technology 2.2. Experimental procedure The studies designed nine mixes. Details of mixes were showed in Table 2. Every mix contained nine 1*1*1-mm cubic specimens, three Φ1*1-mm cylindrical specimens, and three 75*75*28-mm prism specimens. Compressive tests were carried out on 1*1*1-mm cubic specimens at age of 7, 28, and 9 days. The chloride diffusion test was carried out according to the diffusion tank test [4]. Three cylindrical specimens were used to prepare Φ1*2-mm slice specimens for the chloride diffusion test by cutting out the central part of the specimens. The resistance to chloride diffusion was evaluated with the amount of chloride penetrating through concrete slice. The 75*75*28-mm specimens immersed in 5% Na 2 SO 4 solution were used to do the sulfate attack experiment. The expansion values of specimens are used to assess concretes resistance to sulfate attack according to ASTM C112-95A. The scanning electron microscope (SEM) image was observed by SX-4 SEM. Cumulative pore size distributions were obtained by mercury intrusion using an automatic scanning porosimetry (942) Table 2: Mixture of concrete Sample OPC Water Fly ash A Fly ash B Slag MS F-agg C-agg FDN A % B % C % D % E % F % G % H % I %

5 He Xingyang et al Experimental Results and Discussion 3.1. Compressive strength The compressive strengths of concrete at ages of 7, 28, and 9 days were shown in Fig. 3. It can be observed from the figure that there is some decrease in the early age strengths of concretes mixed with mineral admixtures. But at the age of 9 days, the compressive strengths of mixes with mineral admixture nearly reach the compressive strengths of the control mix (mix A) except mixes with the addition of 5% fly ash, even exceed that of the control mix. compressive strength(mpa) R7 R28 R9 A B C D E F G H I Fig. 3: Compressive strengths of concrete at various stages 3.2. Resistance to chloride diffusion The amounts of chloride penetrating through various concrete slices were shown in Figs As shown in Fig. 4, the chloride-ion concentrations of mix B and mix C are much lower than those of mix A during the test period of 1 year. Except the inclusion of fly ash A, all other ingredients in mix B and mix C and mix A are the same. This indicates that some amount of fly ash A in concrete can improve obviously the concrete resistance to chloride diffusion. Results shown in Fig. 5 indicated that some addition of fly ash B can also improve the resistance to chloride diffusion. But the effect of fly ash B is lower than that of fly ash A. It may be relate to high content of aluminum of fly ash A.

6 324 International Workshop on Sustainable Development and Concrete Technology Con. of Chloride ( mol) A: cont r ol B: 25% FA A C: 5% FA A 2 4 weeks 6 Fi g. 4 Compar i son of mi xes B, C and A Con. of Chloride (mol) B: 25% FA A D: 25% FA B E: 5% FA B 2 4 weeks 6 Fi g. 5 Compar i son of mi xes D, E and B Con. of Chloride (mol) B: 25% FA A F: 25% SL G: 5% SL 2 4 weeks 6 Fi g. 6 Compar i son of mi xes F, G and B Con. of Chloride (mol) B: 25% FA A H: 25% FA A+1% SF I : 25% FA A+5% SF 2 4 weeks 6 Fi g. 7 Compar i son of mi xes B, H and I Concentrations of chloride ions penetrated through species containing various additions of slag and fly ash A were shown in Fig. 6. As slag was added into the concrete, the chloride-ion concentrations decreased. It shows that addition of slag can improve concrete resistance to chloride diffusion. In the range of 25%-5% by mass of cement, the improvement of slag increases with the dosage of slag increasing. The influence of microsilica (MS) in combination with fly ash on chloride diffusion can be evaluated by comparing chloride-ion diffusion concentration of set H, I, and B, shown in Fig. 7. Fig. 7 indicates that the mixes resistance to chloride diffusion is improved by addition of microsilica, and this improvement is increased as more microsilica is incorporated. As shown in Figs. 4-7, the curve of chloride concentration became linear after a certain time about 33 weeks. That is to say, the chloride diffusion had reached a steady state at that time. According to Fick Rule, the diffusion coefficient of concrete

7 He Xingyang et al. 325 can be calculated in steady state [3]. The calculated diffusion coefficients of mixes were shown in Table 3. Table 3 indicates that the diffusion coefficients of mix B and D are, respectively, 7.7% and 11.1% of that of mix A, which means 25% addition of fly ash by mass of cement improves greatly resistance to chloride diffusion. As fly ash combined with microsilica, the improvement is more obvious. The diffusion coefficient of mix H is only.84% of that of mix A, reduced two orders of magnitude. Table 3 also shows that the improvement of slag isn t higher than that of fly ash. Table 3: Chloride diffusion coefficients Mix Linear equation of 33rd-54th week chloride concentration R 2 D eff 1-1 (cm 2 /s) A Y= X B Y= X C Y= X D Y= X E Y= X F Y= X G Y= X H Y= X I Y= X Resistance to sulfate attack In this study, the relative expansion values of all specimens immersed in 5% sulfate solution were measured for one year. In Table 4, the measured results indicate that 25% addition of fly ash A, fly ash B and slag by weight of cement all improve sulfate resistance of concrete. The improvement of slag is the best among three mineral admixtures, and the relative expansion value of mix F with 25% addition of slag is only 65.4% of the relative expansion value of mix A at the 54th week. As the dosage of admixtures reach 5% weight of cement, the improving effect of all mineral admixtures have a certain degree of decline. The mix with slag has the least decline among all mixes with 5% mineral admixture by weight of cement. The reason is

8 326 International Workshop on Sustainable Development and Concrete Technology maybe that the expansion of concrete is related to the strength of concrete at a certain degree. As the mixes have upper strength and same permeability, mixes of upper strength can t easily display their expansion relative to mixes of lower strength. Table 4: Expansion ratios of mixes under sulfate attack at 54th week A B C D E F G H I Expansion ratio (%) In addition, the addition of 25% of fly ash in combination with 5%-1% of microsilicon reduces greatly the relative expansion ratios. Mix H compounded with 25% of fly ash and 1% of microsilicon has the least expansion values at every measuring time in one year. At the 54th week, the relative expansion value of mix H is only 53.9% of the relative expansion value of mix A. It is necessary to point out that the mixes with the incorporation of fly ash and microsilicon also have upper resistance to chloride diffusion. It can be concluded that incorporation of combination of fly ash and microsilicon is one of the best ways to achieve a superior resistance to ion diffusion Analysis of micropore structure The volume of pores in hardened cement paste decreased greatly with the hydration of clinker. During the course, the structure of the paste becomes more and more dense; however, the various sizes of pores exist at all hydration ages, such as large spherical pores, capillary pores, micro pores and gel pores. The effect of large spherical pores and capillary pores on the strength and permeability of hardened cement paste is higher [5]. In this study, the incorporation of mineral admixture into mixes obviously improved the resistance to ion diffusion. To study the mechanism of the improvement, the mercury intrusion porosimeter test was carried out with samples from mix A and B at the age of 3 and 9 days. The results are shown in Figs. 8 and 9. In Fig. 8, it can be observed that the volume of pores whose diameters vary from 2 nanometers to 9 nanometers in sample B is lower than that in sample A at the age of 3 days. It

9 He Xingyang et al. 327 demonstrates that the addition of fly ash increases the pile compaction of cement particles and fly ash particles in mix B. In Fig. 9, with the hydration of clinker, we can see that the structure of the paste becomes more and more dense. Another observation is that the pore ratio and diameter in group b are lower than those in group A at the age of 9 days. The addition of fly ash thins the pore size of cement paste. It seems that the change of pore ratio and diameter caused by the addition of mineral admixtures leads to the improvement in the resistance to ion diffusion. 5 4 C 1C+.. 25FA A C+. 25FA A C cumulative intrusion,(ml/g) cumulative intrusion,(ml/g) Pore Diameter(micron) Pore Diameter(micron) Fig.8: MIP results for the different pastes at 3 days Fig.9: MIP results for pastes at 9 days 3.5. SEM analysis of hydrated structure and raw materials SEM images of hydrated structure Figs and Figs are, respectively, SEM images of pure cement and cement in combination with 25% fly ash A at the age of 9 days. It was shown in Figs that main hydrate productions of pure cement are C-S-H (gel) and a few Ca(OH) 2. From Figs. 1 and 12, it can be observed that they don t have obvious difference between micro structures of sample A and sample B except Fig.1: SEM images of Sample A(1 ) that there are some fly ash particles in various diameters in sample B and some of them had reacted with Ca(OH) 2. Fig. 13 shows the inner hydrate productions and outer hydrate productions of fly ash. The

10 328 International Workshop on Sustainable Development and Concrete Technology pozzolanic reaction increases the permeability of concrete and decreases the content of unfavorable crystal phase. What s more, the second hydration reaction improves the bond of particles. Especially, the decrease in the content of unfavorable crystal phase directly reduce reagent of sulfate attack. All of those are helpful to improve the resistance of concrete with mineral admixture to ion diffusion. Fig. 14 is SEM image of slice specimen of sample B, which was taken after the chloride diffusion test. In Fig. 14, it s demonstrated that C-A-H had reacted with chloride ion and formed Friedel salt (chloroaluminate crystals). The chloride had been bound into hydrate productions, which decreased the pace of chloride ion diffusion. It is maybe the reason why the improvement of fly ash A in the resistance to chloride ion diffusion is better than that of fly ash B. Fig.11: CSH and Ca(OH) 2 in sample A (1 ) Fig.12: SEM images of sample B (1 ) Fig. 13: Hydrating surface of fly ash Fig. 14: Micrograph of chloroaluminate particle in sample B (2 ) crystals (68 )

11 He Xingyang et al. 329 Fig.15: Air pores and absorption products Fig.16: Inner structure of fly ash on the surface of fly ash particle (18 ) particle (7 ) SEM images of fly ash In the current paper, the addition of fly ash obviously improved the resistance to chloride ion diffusion. The effect of fly ash is even better than that of slag. To study the mechanism of the phenomena, the SEM test was carried out with the microstructure of fly ash A. Figs are SEM images of fly ash A. It s demonstrated in Figs that the particles of fly ash are mostly spherical particles and have complex inner surface structure. That BET surface area of fly ash is far larger than that of cement (in Table 1) also proves that fly ash has complex inner surface structure. In Fig. 8, it s shown that much mercury was remained in concrete in MIP test. It also testifies that there are many non-connected pores in fly ash. The complex inner surface and non-connected pores have the function of absorbing chloride ion. It s helpful to decrease the speed of chloride ion diffusion. 4. Conclusions The following conclusions could be obtained based on the test results of this study: The addition of mineral admixtures decreases the compressive strengths of concrete at the early age. But at the age of 9 days, the compressive strengths of mixes with mineral admixture nearly reach that of the control mix, even exceed that of the control mix.

12 33 International Workshop on Sustainable Development and Concrete Technology The additions of mineral admixtures such as fly ash, slag and microsilicon, improve greatly the concrete resistance to ion diffusion. Especially, the addition of 25% fly ash in combination with 1% microsilicon by weight of cement can reduce the chloride diffusion coefficient two orders of magnitude compared to control concrete, and also decrease the relative expansion ratio to 53.9% of control concrete in ASTM C112-95A. It can be concluded that incorporation of combination of fly ash and microsilicon is one of the best ways to achieve a superior resistance to ion diffusion. The addition of mineral admixtures leads to the thinning of pore size of cement paste, compact of hydration productions phase and reduction of unfavorable crystal phase. All of those result in the increase of the resistance to ion diffusion. The improvement of fly ash in the resistance to chloride diffusion is better than that of slag, while the improvement of slag in the resistance to sulfate diffusion is better than that of fly ash. The inner surface structure and the non-connected pores of fly ash have the function of absorbing chloride ion, which is helpful to decrease the speed of chloride ion diffusion. C-A-H can react with chloride ion and form Friedel salt (chloroaluminate crystals). It is maybe the reason why the improvement of fly ash A in the resistance to chloride ion diffusion is better than that of fly ash B. Acknowledgments The authors gratefully acknowledge the financial support of National 973 Plan (21).

13 He Xingyang et al. 331 References 1. Berke, N.S., D.W. Pfeifer, and T.G. Weil. Protection Against Chloride-Induced Corrosion. Concrete International 1(12), December 1988, pp Metha, P.K. Effect of Fly Ash Composition on Sulfate Resistance of Cement. ACI Materials Journal 83(6), November-December 1986, pp He Xingyang. Research on small ionic Diffusivity Concrete. Dissertation, Wuhan University of Technology, Wuhan, 21 (in Chinese). 4. Zongjin Li, Jun Peng, and Baoguo Ma. Investigation Chloride Diffusion for High-Performance Concrete Containing Fly Ash, Micro silica and Chemical Admixtures. ACI Materials Journal 96(3), May-June 1999, pp Powers, T.C., and T.L. Brownyand. Studies of physical properties of hardened portland cement paste (nine parts). Proceedings ACI, Vol. 43, October 1946.