J. Cent. South Univ. Technol. (2010) 17: 263-268

DOI: 10.1007/s11771-010-0040-4                                                                                                                  

Invading track of chloride ions in cemented-based materials

MA Kun-lin(马昆林),  XIE You-jun(谢友均), LONG Guang-cheng(龙广成), WU Ke-gang(吴克刚)

School of Civil and Architectural Engineering, Central South University, Changsha 410075, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: Invading track of chloride ions and chloride ion distribution rule in cement-based materials were investigated by partially soaking in 3.5% (mass fraction) NaCl solution and fully immerging in 3.5% and 5.0% (mass fraction) NaCl solution, respectively, and relevant invading mechanisms were discussed. Results indicate that under full immerging condition, the invading track of chloride ions in cement mortar is similar to beeline that is vertical to chloride ion invading direction, and chloride ion content decreases rapidly with the increase of chloride ion invading depth. Under partial soaking condition, the invading track of chloride ion in cement mortar is similar to the shape of concave parabola, and chloride ion content decreases slowly along the lengthway direction of cement mortar samples in the distance of 20-80 mm from the bottom. Lots of chloride ions accumulate in cement mortar surface layer under the effect of capillary rise and evaporation and then invade cement mortar by diffusion effect. Under partial soaking condition, cement mortar is distinguished by four areas, i.e., immerging area, wet area, crystallization area and dry area.

Key words: concrete; chloride ions; invading track; durability

1 Introduction

Chloride ion attack on cemented-based materials is a common cause of deterioration of reinforced concrete structures. Cemented-based materials may be exposed to chloride by seawater or deicing salts; chloride initiates corrosion of the reinforcement, which disrupts the cemented-based materials through expansion. Researchers paid more attention to chloride ion attack on concrete. JENSEN et al [1] studied chloride ingress into cement paste and mortar by electron probe  microanalysis, and found that the presence of fine aggregate and formation of interfacial transition zones at paste-aggregate boundaries did not significantly affect diffusion rates. MECK et al [2] believed that a simple colourimetric method of measuring the depth of chloride penetration into concrete by spraying a 0.1 mol/L AgNO₃ solution was very attractive, but its sensitivity needed further improvement. HOSSAIN et al [3] presented the performance of evaluating marine salt deposition and resistance to marine salt penetration, and also monitored the deposition of chlorides and sulfates in wet candle sensors located at different stations. LIANG and LAN  [4] paid more attention to the deterioration of concrete structure in marine envirment and provided relative remedying methods.

Generally, from the velocity of chloride ion ingress into cemented-based materials one can judge the extent of chloride ion attack on cemented-based materials. However, the process of chloride ion ingress into cemented-based materials is very complicated, including diffusion effect, capillary effect and permeability. And different external environmental factors have great influence on chloride ingress into cemented-based materials besides intrinsical factors of cemented-based materials [5-6]. Real environment that cemented-based materials are employed is complex and diversiform, and the ways of chloride ingress into cemented-based materials are more than one [7-8]. Recently, researchers have paid more attention to the rules of chloride ion ingress into cemented-based materials and have obtained many helpful results, especially in full immerging saturated water condition [9-13]. Furthermore, in many cases, that chloride ion attack on cemented-based materials under partial soaking unsaturated water condition is the main attack mode. However, the invading line and track and chloride ion distribution of cemented-based materials under partial soaking condition, which are very important aspects for people to fully understand the invading way of chloride ions are seldom reported.

In this work, different invading tracks of chloride ion in cemented-based materials and chloride ion content in different positions of cemented-based materials were investigated, especially under NaCl solution partial soaking condition, and invading mechanisms were discussed deeply.

2 Experimental

2.1 Raw materials and mixed proportion

The cement was ordinary Portland cement with a grade of 42.5 according to the Chinese standard, supplied by Xiangxiang Cement Co., Ltd, Hunan Province, China. The sand used in the experiment had a fineness modulus of 2.88 and an apparent density of 2.67 kg/m³. A polynaphthalene sulphonate superplasticizer with the trademark of Mighty 100 was supplied by the Kao Chemical Corporation, Shanghai, China. All water used in this experiment was demineralized water. All the samples had the same fluidity. The mass ratios of cement to sand to water were 1:2.50:0.35 and 1:2.50:0.65, respectively.

2.2 Experimental procedures

(1) All the cement mortar samples were molded in a stainless steel mold with dimensions of 40 mm×     40 mm×160 mm and demolded for 24 h. And then, moist curing was employed for 27 d. After moist curing was finished, all the samples were dried at 60 ℃ in oven until the mass of samples kept constant. Subsequently, samples were divided two groups (A and B). Samples of group A were subjected to partial soaking in 3.5% NaCl solution, and the soaking height of samples in solution was 20 mm. Samples of group B were epoxy-cooled, and only one surface was free of coating. The integrality of epoxy coated on surface of cement mortar was inspected periodically in order to keep the accuracy of the experiment. All samples of group B were fully immersed in 3.5% NaCl solution and soaked for 300 d. In the process of experiment environmental relative humidity was kept at (60±5)% and temperature was kept at (20±2) ℃. The schematic diagrams of partial soaking and full immerging experiments are shown in Fig.1.

Fig.1 Schematic diagrams of partial soaking (a) and full immerging (b) experiments

 (2) After being soaked for 300 d, samples were taken out. Some cement mortar samples were split in halves along lengthway, and 0.1 mol/L AgNO₃ solution was sprayed on the cross-section of split cement mortar profile. Whereafter, white and black regions with well-distinguished appeared on cross-section. The white region, which was polluted by chloride ions, was due to precipitation of AgCl (Cl^-+Ag⁺→AgCl↓) , and the black region, which was not polluted by chloride ions, was due to oxidation of AgCl in air. The colour-change boundary was called chloride ion invading track. So invading track and depth of chloride ion in cement mortar can be directly observed from profile.

(3) After being taken out from solution, other cement mortar samples in partial soaking were drilled in 30 mm steps with 10 mm diameter drill-bits along lengthway. And the granules drilled from regions a and b in transverse were collected, respectively. The schematic diagrams of sampling position of cement mortar in partial soaking can be seen in Fig.2. Other cement mortar samples in full immerging condition were drilled in 5 mm steps with 10 mm diameter drill-bits along the direction of chloride ingress and the granules were also collected.

Fig.2 Schematic diagrams of sampling positions of cement mortar in partial soaking condition (a, b: Sampling positions): (a) Split section; (b) Sampling positions

 (4) All the collected cement mortar granules were crushed, sand-eliminated, and milled to pass a sieve with a diameter of 0.08 mm according to Chinese standard GB/T50344—2004. And then, according to Chinese standard JTJ270—98, the contents of free chloride ions in cement mortar powder were analyzed.

3 Results and discussion

3.1 Invading track of chloride ion

Figs.3-4 show the pictures of invading track in cement mortar profile. As can be seen from Fig.3 that, under the partial soaking condition whether the mass ratio of water to cement is 1:0.65 or 1:0.35 the shapes of invading track of chloride ion to cement mortar are similar. Chloride ions invade underside of cement mortar completely. In upside of cement mortar, two sides of cement mortar are invaded by chloride ions, and the invading height of chloride ion on both sides of cement mortar is higher than that of central section of cement mortar. Chloride ions do not invade the central section of cement mortar, and the color of central section of cement mortar is darkened because of oxidation of Ag⁺ in air. The invading height of chloride ion in cement mortar with mass ratio of water to cement being 1:0.65 is obviously higher than that of cement mortar with mass ratio of water to cement being 1:0.35, but the shapes of chloride ion invading track in both cement mortars are very similar. All the invading tracks are similar to the shape of concave parabola. Lots of chloride ions accumulate in surface layer of cement mortar, and few chloride ions are on central section.

Fig.3 Invading tracks of chloride ions in cement mortar under partial soaking condition at different mass ratio of water to cement: (a) 1:0.35; (b) 1:0.65

Fig.4 Invading track of chloride ions in cement mortar under full immerging condition

However, as can be seen in Fig.4, under full immerging condition the invading depth of chloride ion is uniform, and the invading depth of both sides and central section are almost the same. Under full immerging condition, the invading tracks of chloride ion to cement mortar are similar to beeline that is vertical to invading direction of chloride ions.

3.2 Content of chloride ions

Fig.5 shows mass fractions of chloride ions along lengthway of cement mortar at different mass ratios of water to cement under partial soaking condition. As can be seen from Fig.5 that the mass fraction of chloride ions on the section of cement mortar immerged in NaCl solution is high, and chloride ion content decreases with the increase of lengthway height of cement mortar. In the same height, compared with that of cement mortar with mass ratio of water to cement being 1:0.65, the mass fraction of chloride ions is lower in cement mortar with mass ratio of water to cement being 1:0.35. And in the same cement mortar and at the same height, the mass fraction of chloride ions in powder drilled from region a is obviously higher than that in powder on central section drilled from region b, which accords with the color- change boundary in cement mortar profile after spraying 0.1 mol/L AgNO₃ solution (see Fig.3). Whether mass ratio of water to cement of cement mortar is 1:0.65 or 1:0.35, a change tendency can be discovered that in the range of 20-80 mm above the bottom, the mass fraction of chloride ions is large and decreases slowly with increasing height. However, after the distance from bottom is beyond 80 mm, the mass fraction of chloride ions decreases quickly with increasing height.

Fig.5 Mass fractions of chloride ions in cement mortar profile under partial soaking condition at different mass ratios of water to cement: (a) 1:0.35; (b) 1:0.65

In full immerging condition, the mass fraction of chloride ions along the invading direction is shown in Fig.6. As can be seen from Fig.6, with the increase of distance from surface layer of cement mortar free of coating, the mass fraction of chloride ions decreases quickly. Namely, whether full immerging concentration of NaCl solution is 5.0% or 3.5%, with increasing distance, the mass fraction of chloride ions in cement decreases rapidly. In full immerging condition, diffusion effect is the main mode of chloride ion ingress into saturated cement mortar, which accords with Fick’s second law [14]. Therefore, the mass fraction of chloride ions decreases obviously with increasing invading depth.

Fig.6 Mass fraction of chloride ions in cement mortar profile under full immerging condition

3.3 Discussion

Generally, diffusion effect is the main way for chloride ion ingress into cemented-based materials. Researchers take Fick’s second law to describe chloride ion ingress into cemented-based materials. However, in some environmental conditions, other invading mode may become prevalent. In this work, under partial soaking condition, capillary rise rate is very fast. Capillary rise of solution in cement mortar surface layer reaches a stable height after experiment continues for 12-24 h. In the process of experiment, the height of capillary rise keeps stable.

Fig.7 shows the photo of cement mortar under partial soaking condition in 3.5% NaCl solution for  300 d, and the white substance in middle of cement mortar is NaCl crystal. Fig.8 shows the schematic diagram of capillary and diffusion in cemented-based materials according to the tested results of mass fraction of chloride ions in cement mortar, invading track of chloride ions and appearance of cement mortar. As can be seen from Fig.8, under partial soaking condition, there are four areas after chloride ion attack on cemented-based materials, which are immerging area, wet area, crystallization area and dry area.

Fig.7 Photo of cement mortar under partial soaking condition in 3.5% NaCl solution for 300 d

Fig.8 Schematic diagram of capillary and diffusion in cement- based materials

(1) Immerging area. In this area, cement mortar is soaked in NaCl solution totally, and cement mortar soon becomes saturated because of penetration and diffusion effect. Concentration difference of chloride ions between surface layer and inner of cement mortar drives chloride ions in surface layer to diffuse to inner cement mortar. With the effect of diffusion, the mass fraction of chloride ions in cement mortar increases quickly. Therefore, in immerging area, diffusion effect is the main mode of chloride ion invading. Furthermore, in immerging area, chloride ion ingress comes from five directions.

(2) Wet area. Wet area is up to immerging area but near to soaking solution. First of all, because of the strong capillary effect, capillary rise rate of NaCl solution in surface layer in wet area is rapid. Secondly, because wet area is close to the soaking solution and evaporation quantity of water from soaking solution is great, relative humidity in this area is higher than that in environment, which causes evaporation quantity of water in cement mortar surface layer to be low in wet area. Thereby, evaporation quantity of water is lower than capillary rise quantity of solution, resulting in the fact that this area is always wet. Capillary rise rate of solution is higher than evaporation rate of water, and concentration of NaCl solution increases gradually with the development of capillary effect and evaporation effect, so in this area the surface layer is always wet. Cement mortar surface layer is always wet, evaporation raises the salt concentration in this area and the diffusion of salt back toward the source may prevent the solution from becoming supersaturated in wet area, so it is hardly to observe crystallization products.

(3) Crystallization area. Crystallization area is in upside of wet area. In cement mortar surface layer, first of all, with increasing cement mortar height, capillary rise rate of solution decreases slowly. Secondly, because in crystallization area environmental relative humidity is lower than that in wet area, evaporation rate of water in surface layer increases and exceeds capillary rise rate gradually. In addition, because of evaporation of water, concentration of solution in surface layer increases, and exceeds saturated concentration of NaCl solution gradually. Finally, crystallization products are separated. In crystallization area, white crystal can be seen.

Especially, in crystallization area, reversion of current direction takes place in surface layer of cement mortar. Water is evaporated from the pores that are open to air, resulting in the increase of mass fraction of chloride ions in pore solution. So concentration difference between the surface layer and the inner increases, which drives chloride ions in the surface layer to move to the inner. Thereby, diffusion effect takes place in unsaturated water condition. Because water in surface layer is evaporated continuously and capillary rise effect continues, NaCl solution comes to surface layer continuously, and crystal is separated. Crystallization products accumulate in surface layer, which increases concentration of solution and accelerates diffusion effect of chloride ions. Therefore, in crystallization area, lots of chloride ions accumulate in surface layer by capillary effect. Chloride ions invade inner cement mortar in unsaturated water condition by diffusion effect.

But, if crystallization products of salt solution possess strong capability of physical attack on cement- based materials, such as Na₂SO₄ solution [15-16], serious physical crystallization attack on cemented-based materials will take place in crystallization area. And this phenomenon is called as “corroded root” of cemented- based materials [17].

(4) Dry area. Dry area is in upside of crystallization area. In dry area, capillary rise cannot reach, so cement mortar samples are seldom attacked by solution in this area.

The process of capillary rise, water evaporation, concentration of solution increasing and chloride ion diffusion goes around and around and takes place in the same time, resulting in more chloride ions to invade inner cement mortar. Obviously, invading rate of NaCl solution in cement mortar is faster under the effect of capillary than that under the effect of diffusion.

4 Conclusions

(1) Under partial soaking in NaCl solution condition, chloride ion invading in cemented-based materials is by capillary and diffusion effect, which forms an invading track similar to the shape of concave parabola in cement mortar profile. Under full immerging condition, chloride ion invading in cemented-based materials is finished by diffusion effect, which forms an invading track similar to the shape of beeline vertical to chloride ion invading direction.

(2) Under partial soaking condition, lots of chloride ions accumulate in surface layer under the effect of capillary rise and evaporation. Concentration difference of chloride ions between surface layer and inner cement mortar forms. Chloride ions gradually invade the inner cement mortar under the diffusion effect. The mass fraction of chloride ions in surface layer is much larger than that in inner cement mortar.

(3) The formation of salt solution invading tracks in cemented-based materials has close relationship with unsaturated or saturated states of cemented-based materials.

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Foundation item: Project(50678174) supported by the National Natural Science Foundation of China

Received date: 2009-04-29; Accepted date: 2009-08-30

Corresponding author: XIE You-jun, Professor; Tel: +86-731-82656568; E-mail: xieyj@mail.csu.edu.cn

(Edited by CHEN Wei-ping)