MICROSTRUCTURE OF LONG TERM MARINE IMMERGED ANTI-WASHOUT CONCRETE

MICROSTRUCTURE OF LONG TERM MARINE IMMERGED ANTI-WASHOUT CONCRETE Shaowei Yang and Hengjing Ba School of Civil Engineering, Harbin Institute of Technology, Harbin, China Abstract In this contribution, the microstructure of long term marine immerged anti-washout concrete was investigated. In the experimental program, a new anti-washout agent was used as the control anti-washout concrete mixing. For other two mixtures, the content of cement was replaced by fly ash 30% and fly ash 25%+silica fume 5% (in mass), respectively. SEM observations were used in analyzing the damage of concrete immerged in sea water after 720 days. The results of Scanning Electronic Microscope (SEM) show that there are some crystals observed on the surface of the control concrete, where crystals grow thick and complete. A lot of AFt was observed inside the concrete. The surface of concrete mixed with fly ash and silica fume was compact and the crystals were tiny. Only few AFt was observed inside the concrete. The composition of the crystal on the surface of the control concrete detected by Energy Dispersive X-ray Spectrometer Analysis (EDXA) was very complex. However, the composition of the crystal on the surface of concrete mixed with fly ash and silica fume was mostly Mg(OH) 2. 1. INTRODUCTION The anti-washout concrete is a new type concrete developed in the last thirty years. The improvement of the anti-washout concrete performance is based on the viscidity increasing of the concrete by incorporating the anti-washout agent. The concrete can be cast and dropped in water directly. The disadvantages of the normal concrete in water such as poor anti-washout performance and lower strength are improved. The anti-washout concrete is a new research aspect in recent years [1]. At present, the research on anti-washout concrete is mainly focus on anti-washout agent, the workability of fresh concrete and the strength development of concrete. The influence of anti-washout agent on the process of hydration, hydration products and its microstructure is the key research issues. The emphases of most research are how to improve the mechanical properties of the concrete without decreasing the anti-washout capability. The mordern analyzing methods such as DTA (Differential Thermal Analyzer), TG (Thermal Gravimetry), XRD (Energy Dispersive X-ray Spectrometer Analysis) and SEM (Scanning Electronic 403

Microscope) were taken by Lin et. al [2] to study the hydration of anti-washout concrete mixed with anti-washout agent, such as polyacry lamide. There are only few researches on the durability of anti-washout concrete in marine environment were reported [3, 4]. This paper investigated the microstructure of anti-washout concrete mixed with anti-washout agent and mineral admixtures immerged in seawater for a long term. The influence of seawater on the long time performence of concrete and its microstructure was investigated. 2. MATERIALS AND EXPERIMENTAL PROGRAM 2.1 Chemical composition in seawater Main chemical compositions in seawater are chloride, sulfate and some undissolved substance as listed in Table1. In this paper, the simulated seawater as marine environment in the laboratory is prepared according to the chemical composition. Table 1: Chemical compositions in seawater(g/l) NaCl MgCl 2 Na 2 SO 4 CaCl 2 KCl 24.4 11.1 4.1 1.2 0.7 2.2 Raw materials The raw materials used in this experiment include P O 42.5 cement (ordinary Portland cement), fly ash, silica fume, sand, gravel, polycarboxylate reducer, MP anti-washout agent etc. The chemical compositions of cement, fly ash and silica fume are list in Table 2. The modulus of fineness of medium sand is 2.74. The main particle size of the gravel is 10~20mm. Table2: Chemical composition of P O 42.5 cement, fly ash and silica fume(%) SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 R 2 O Ignition loss Cement 21.08 5.47 3.96 62.28 1.73 2.63 0.50 1.61 Fly ash 50.62 28.98 7.15 2.77 1.15 0.33 1.81 8.19 Silica fume 92.16 0.84 0.37 1.04 1.57 1 0.99 1.63 2.3 Experimental program The mix proportion of concrete in this test is shown in table 3. The influence of the mineral admixtures on the microstructure and compressive strength of concrete immerged in seawater for a long term will be investigated. 404

Table 3: Mix proportion of concrete (kg/m 3 ) Sample Water Cement Fly ash Silica fume Sand Gravel WR AWA C 152 400 - - 758.4 1137.6 2.00 7.20 F 152 280 120-758.4 1137.6 2.00 7.20 F+S 152 280 100 20 758.4 1137.6 3.20 7.20 In the test, the new type of anti-washout agent MP was adopted to prepare the anti-washout concrete. The samples were cast according to the method mentioned in DL/T 5117-2000 testing regulations of anti-washout concrete [5]. The slump of the concrete was controlled in the range of 220-260 mm in order to meet the requirement of the workability for anti-washout concrete. The concrete specimens were cured in 20 and RH=90% environment for 28 days after demolding at 24 hours. After 28 days curing, the samples were then cured individually in standard and marine environment for 360 and 720 days. At the designed age, the compressive strength of concrete was examined. The changes of microstructure and crystal compositions were investigated by SEM and EDAX. 3. RESULTS AND ANALYSIS 3.1 Compressive strength The compressive strength of all concrete were measured at 28 days, 360 days and 720 days. Compressive strength(mpa) 44 42 40 38 36 34 32 C F SF 0 200 400 600 800 Age(d) Figure 1: Compressive strength of concrete cured in standard condition 405

Compressive strength MPa 1st International Conference on Microstructure Related Durability of Cementitious Composites 44 C 42 F 40 SF 38 36 34 32 0 200 400 Age d 600 800 Figure 2: Compressive strength of concrete cured in seawater It can be seen from Fig.1 that in standard curing, the development of compressive strength was different. The increasing rate of compressive strength of fly ash concrete was the highest among three mixtures. The strength of control concrete deteriorated with the immerged ages. It was illustrated that the ions in seawater effect the development of compressive strength. Both strength of fly ash concrete and fly ash + silica fume concrete increased with curing ages under two curing conditions. But the strength increments of concrete submerged in sea water were lower than those in standard condition. The addition of the mineral admixtures distinctly reduces the damage of the ions in seawater. 3.2 Microstructure Analysis of concrete 3.2.1 SEM observations SEM was used to analyze the microstructure of concrete immerged in seawater for 720 days. The objects of the observation include surface layer, inner microstructure and hydrated products. A B a) b) 406

1st International Conference on Microstructure Related Durability of Cementitious Composites C c) d) E D e) f) Figure 3: Microstructure of concrete immerged in sea water Fig.3 a) shows the microstructure of the surface of control concrete. The hexagonal prism crystal with diameter of 10μm can be seen from the picture. The crystallinity is high and the orientation is perfect. It is produced by the reaction between the C-S-H gel, Ca(OH)2 and the ions contained in seawater during the wetting and drying cycles. The products swell after it crystal. The crystal fills the pores or microcracks in the concrete surface, and keeps growing in the pores and cracks. Thus the crystal stress appears. If the crystal stress is larger than the tensile stress of the cement paste, the more cracks occur. The crack broadens with the growth of the crystal. This will result in the decrease of the concrete strength. Fig.3 b) is the microstructure of the surface of fly ash + silica fume concrete. The surface of the concrete mixed with mineral admixture is smooth. The crystal distributes equably on the surface. The crystal grows disorderly and unsystematic. There is no big and complete crystal produced. The later strength keeps growing because there is no high crystal stress in concrete. It can be guessed that the surface defects of concrete with mineral admixture are much less than those of the control concrete. Therefore, the numbers of crystal grown on the concrete surface will be getting less and harmful ions, consequence, this can not be easy to intrude into the concrete. One of reasons might be the weakening surface bleeding of concrete with fly ash or silica fume. Fig.3 c), d) is the microstructure of the cross section of the fly ash + silica fume concrete. It can be seen from the picture that the surface is covered by a homogeneous thin layer of 407

compact hydrated products. The thickness of the layer is about 30 μm. It can prevent the pervasion of the harmful ions in seawater. Fig.3 e) is the microstructure of the inside control concrete. Ettringites can be found in the observations of cross-section of the control concrete. When the sulfate from simulated sea water enters the concrete, it reacts with some compositions of the hydrated production of cement such as Ca(OH) 2 C 3 AH 6 etc. The indissoluble salt minerals such as Afts are formed. The volume of these products swells and the cracks of surface layer or inside concrete occur. It not only decreases the concrete strength, but also increases the permeability of the corrosive ions. The integrity of concrete was destructed gradually [3,4]. It can be seen in the picture that the crystal is perfect. The cement paste may crack if the stress of crystal exceeds the tensile strength of the cement paste. The crystal keeps growing in the cement cracks and accelerates the expansion of the cracks. Fig.3 f) is the microstructure of fly ash and fly ash + silica fume concrete. It indicates that the surface of the fly ash and silica fume is covered by the second hydrated products. The silica fume particle in the upper part of the picture is already hydrated in most parts. The whole particle is surrounded by hydrated products. There is no obvious crystal observed in the picture. The hydrated products distributed uniformly, and no visible cracks were observed. The functions of the fly ash in concrete are mainly including filling effect, activation effect and tiny aggregate effect. Therefore, the fly ash improves the workability of fresh concrete, reduces the heat of hydration, and increases the compactness of concrete. Silica fume can be highly distributed in concrete, and densifies the mortar by filling the space between the cement particles. At the same time, silica fume reacts with the free Ca(OH) 2. The react product is hydrated calcium silicate (CaO SiO 2 H 2 O), which is relatively stable hydration product of cement. The reaction changes the pore structure and the structure of interface of cement gel in concrete. Thus the compactness of the concrete is improved. The concrete with silica fume is provided with the property of high strength, high impermeability and high resistance to wear and tear. Silica fume possesses many excellent characteristics as a mineral admixture of concrete, especially used with the other admixtures. It can not only reduce the dosage of silica fume (in range of 3%-5%), but also demonstrate the superposition effect. It is shown in many examination that silica fume can reduce the consistence of chloride and the content of Ca(OH) 2. The saturation of chloride and the corrosion of sulfate is prevented effectively [6-9] 3.3 EDAX EDAX was adopted to analyze the element composition of spot A, B, C, D and E. 408

a) b) c) d) e) Figure 4: Results of EDAX It can be seen from Fig.4 a) that in spot A, there are not only the elements like Na, Mg, Cl, S contained in seawater, but also the elements contained in concrete such as Ca, Al, Si, Fe etc. The composition is relatively complex. The crystal is not made up by a single chemical compound. It is a mixed salt made up of multi chemical compounds. The crystal is perfect and the crystal stress is relative high. The leads the later strength decreases. The element composition is relatively simple in spot B and C. It can be seen that the elements are Mg, O. It can be estimated to be Mg(OH) 2 crystal. The Mg(OH) 2 crystal is hexagonal plate, looks like Ca(OH) 2 crystal. The crystal distributes equably on the surface of the concrete, and covers the whole surface. It forms a compact protective film on the surface. The film prevents the 409

diffusion of the ions in seawater such like Na + Mg 2+ Cl - SO 2-4. It reduces the generation of crystal in concrete. This benefits the later strength of concrete. As indicated in Fig.4 d) that the products of spot D is ettringite. The ettringite is produced 2- by the reaction between the SO 4 in seawater and the unhydrated C 3 A in concrete. The concrete structure loosens because the volume of ettringite expands. It indicates in Fig.4 e) that the element of spot E is the composition of the production of the second reaction between silica fume and Ca(OH) 2. The C-S-H gel is observed. 4. CONCLUSIONS After cured in simulated seawater for 720 days, the later compressive strength of the concrete mixed with fly ash and fly ash + silica fume keeps growing, compared with the control concrete. Silica fume and fly ash in concrete show not only improve the inner microstructure of concrete, but also change the formation of crystal on the concrete surface. The crystal on the control concrete surface is big and complete, and will produce biggish crystal stress. The crystals on the fly ash and silica fume concrete are a few and the crystal stress is lower. From EDAX analysis, it can be seen that the compositions of crystal on the surface of control concrete are complex. They are mixed compounds of salt. The crystal on the surface of fly ash and silica fume concrete is Mg(OH) 2 crystal. The structure of the crystal is compact and forms a layer of protective film. The film prevents the intrusion of the harmful ions in seawater. REFERENCES [1] Jiang, C.S., Chen, J. and Lv, L.N. Research on High Performcance Anti-washout Concrete (in chinese). Journal of Hubei Institure of Technology, 2004, 19(2):18-20. [2] Lin, X., Yan, P.Y. and Sha, L.H., Hydration of the Pastes of Underwater Anti-washout Concrete (in chinese). Concrete, 2001, 136(2):24-29. [3] Brown, P., Hooton, R.D. and Clark, B., Microstructural Changes in Concretes with Sulfate Exposure. Cement and Concrete Composites, 2004, 26(8): 993-999. [4] Brown, P.W, Hooton, R.D. Ettringite and Thaumasite Formation in Laboratory Concretes Prepared Using Sulfate-resisting Cements. Cement Concrete Composites, 2002, 24(3/4):361 370. [5] DL/T 5117-2000, Testing Regulations of Anti-washout Concrete. 2002, 28pages. [6] Bentz, D.P., Influence of Silica Fume on Diffusivity in Cement-based Materials II. Multi-scale Modeling of Concrete Diffusivity. Cement and Concrete Research, 2000: 30(7): 1121 1130. [7] Toutanji, H.A. and Bayasi, Z., Effect of Curing Procedures on Properties of Silica Fume Concrete. Cement and Concrete Research, 1999, 29(4): 497 501. [8] Garboczi, E.J, Berryman J.G., New Effective Medium Theory for the Diffusivity or Conductivity of a Multi-scale Concrete Microstructure Model. Concrete Science and Engineering, 2000, 2(3): 88 96. [9] Bentz, E.D.P, Garboczi J., Percolation of Phases in a Three-dimensional Cement Paste Microstructure Model. Cement and Concrete Research, 1991, 21(2/3): 325 34. 410