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Relationship between microscopy contributions and durability of cement-based composites

Wei-Ting Lin*,1,2 and An Cheng1 1 Dept. of Civil Engineering, National Ilan University, Ilan 260, Taiwan 2 Institute of Nuclear Energy Research, Atomic Energy Council, Executive Yuan, Taoyuan 325, Taiwan

This study aimed to establish the relationship between microscopy contributions and durability indices of cement-based composites containing supplementary cementitious materials. The influences of compressive strength, permeability, and microstructure of cement-based composites containing rock wool wastes, ggbs, fly ash and silica fume were discussed. The relationship between pore structure and corrosion behavior was investigated and compared. The results demonstrate that the inclusion of supplementary cementitious materials in cement-based composites resulted in a higher compressive strength, a lower permeability, a lower critical pore size and a lower corrosion rate. The scanning electron microscope observations confirmed these findings. Moreover, the corrosion rate and the critical pore size were suitable for evaluating the durability of cement-based composites. A regression analysis found that the probability of rebar corrosion was greater than 90 %, when the corrosion rate was 11.54 μm/yr, or the critical pore size was 26.71 nm. Therefore, rock wool wastes can act as either a supplementary cementitious material or inert filler in cement-based composites, depending on the hydration or pozzolanic reaction. The use of rock wool wastes can reduce the cost of natural aggregates and minimize the environmental impact of solid waste disposal.

Keywords supplementary cementitious materials; durability index; scanning electronic microscopy

1. Introduction

Cement-based composites are among the most widely-used construction materials due to their low cost, high compressive strength, high durability, versatility, and easy-handling. Unfortunately, cement-based composites are intrinsically porous and may deteriorate and be liable to rebar corrosion as a result of exposure to harsh environments or poor construction quality. In general, proper design procedures, adequate concrete cover depth, corrosion-inhibiting admixture, and low-permeability cement-based composites can be selected for corrosion prevention and control [1-3]. However, the denser pastes can be enhanced by lowering the water cementitious ratio or through the addition of supplementary cementitious materials (SCMs). SCMs such as silica fume, fly ash, and ground granulated blast furnace slag (ggbs), are commonly used to replace a portion of the cement in cement-based composites to improve the quality and/or durability of cement-based composites [4-6]. On the other hand, the micro or medium cracks always exist in cement-based composites no matter how much efforts have been done. Cracks may lead water, chloride-ion, and carbon dioxide into composites and finally induce steel corrosion. To assure the rebar in composites free from corrosion, the composites surface could be treated with proper coating materials. In addition, the crack inhibition can be increased through the addition of fibers.

Improvement in performances of cement-based composites becomes an important topic for construction materials sector and attracts many researchers focus on this subject. Most of the performances of cement-based composites are influenced by the pore structure of cement paste phase. Mechanical property of concrete is mainly affected by meso-pores, while shrinkage and creep are affected by micro-pores [7]. Thus, it is vital to understand the characteristics of pore structures in cement-based composites i.e., porosity, pore volume, critical pore diameter and pore size distributions. Many researchers have therefore been trying to realize the pore system through microscopic techniques [8], such as Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopic (SEM). The report suggested that the SCMs incorporations can improve the compressive and splitting tensile strengths, and elastic modulus of cementitious composites [8-9]. Some reports also investigated that SCMs should meet several criteria included fluidity, impermeability, strength, corrosion protection and ion resistance [10-11]. Macro characteristics such as mechanical and physical properties can be tested according to relevant standards or specifications. Based on current technique, microstructures involving pore volume, pore size and solid particle of cement-based materials could be accessed using SEM micrographs and MIP spectrum. The aim of this study was to deepen our understanding of porestructure and durability in cement-based composites, through the evaluation of testing methods and material variables. The material variables are consisted of silica fume, ggbs, fly ash, rock wool wastes and polyolefin fibers. In addition, the degradation of cement-based composites is considered a key factor in the durability of structures and a major concern for civil engineers [12]. The durability indices could be linked with transport mechanisms that contribute to deterioration and reflect the quality of the cement-based composites quickly. In order to realize the micro-property of cementitious composites and establish the durability indices, the relationship between macro and micro characteristics is taken and discussed. The threshold of active corrosion is defined by corrosion rate, pore sizes and diffusion coefficient integrated into the durability indices. Therefore, the effect of SCMs on the corrosion

Current Microscopy Contributions to Advances in Science and Technology (A. Méndez-Vilas, Ed.)

© 2012 FORMATEX 1093

behavior, chloride diffusion and pore size distribution of cement-based composites is analyzed and a predictive model is presented that allows further optimization of the durability. In conclusion, this is important to generate the link between these durability indices and establish the essential parameters that are required for consideration in durability design.

2. Experimental program

2.1 Materials and mix proportion

Type I Portland cement conforming to ASTM C150 was used in all mixes. The diameter of the silica fume was about 0.1-0.2 μm. The specific gravity of the silica fume, fly ash, ggbs and rock wool wastes was 2.20, 2.46, 2.90 and 2.80, respectively. Rock wool wastes (RWW) obtained from thermal insulation materials were crushed and ground. The chemical compositions of cement and slag are listed in Table 1. The SEM image in Fig. 1 shows that rock wool waste has a cylindrical shape, with an average diameter of 3.5 μm. In addition, the surface area of rock wool wastes is 206 m2/kg, which is lower than other pozzolanic materials and cement.

Table 1 Chemical composition of various recycled materials and Portland cement.

Chemical composition

Rock wool wastes

Fly ash ggbs Silica fume Cement

SiO2 (wt.%) 38.7 54.0 33.5 91.5 21.2 Al2O3 (wt.%) 18.6 24.0 9.0 0.2 5.4 Fe2O3 (wt.%) 5.3 8.0 3.6 0.7 3.2 CaO (wt.%) 20.9 2.0 43.8 0.4 63.8 MgO (wt.%) 7.0 1.3 2.7 1.5 2.0

K2O+Na2O (wt.%) 2.0 0.9 0.6 1.9 0.8 Other (wt.%) 7.5 9.8 6.8 3.8 3.6

Surface area (m2/kg) 206 420 415 22500 364

(a) SEM micrograph (x100) (b) SEM micrograph (x500)

(c) SEM micrograph (x1000) (d) SEM micrograph (x5000)

Fig. 1 SEM observation of rock wool wastes.

Table 2 shows the mix properties of control mortar specimens. The water/cementitious ratios (w/cm) were 0.35 and 0.55, and the sand/binder ratios were kept at a constant of 2.75. The RWW, ggbs, fly ash (10 %, 20 %, 30 % and 40 % by weight) and silica fume (5 % and 10 % by weight), was added to different mixes to partially replace the cement, respectively. The fine aggregates had a fineness modulus of 2.51. The absorption of fine aggregates was 0.63 %, with a relative density of 2.49 under the saturated surface dry (SSD) condition. Proper mixture was achieved using a high-range water-reducing admixture (superplasticizer).



Table 1 Mix proportions of the control specimens (kg/cm3).

Mix no. w/cm Water Cement Fine aggregate Superplasticizer

A 0.35 191.8 564.1 1551.2 5.0 B 0.55 278.8 506.9 1394.0 0.0

2.2 Specimens

The experiments in this study cast a total of 540 mortar specimens for 36 different mixes. All the specimens were cured in saturated limewater until testing. For each mix, nine 50 x 50 x 50 mm cube specimens were prepared to test the compressive strength and three ψ100 x 200 mm cylindrical specimens were prepared for the corrosion test. Circular plates with a thickness of 50 mm cut from the central portion of the cylindrical specimen (ψ100 x 200 mm) were used for the rapid chloride penetration test (RCPT). Specimens with a thickness of 10-15 mm were sliced from cylindrical specimens and used in MIP. Besides, the specimens sliced from the mortar specimens of 1 x 1 x 1 mm were prepared for the SEM observation.

2.3 Testing methods

Compressive strength tests at ages of 7, 28 and 91 days were performed in accordance with ASTM C39. The RCPT was prepared according to ASTM C1202 specifications. An MIP test was conducted according to ASTM D4404 specifications. The MIP test is a widely applied method to measure the pore size distribution in cement-based composites. A spectrum can be used to determine the volume and size distribution of pores in cement-based composites by assuming circular pore cross-section. Under normal circumstances, the amount of pressure required to force mercury into a pore is inversely proportional to the radius of that pore. An Automated Mercury Porosimeter (Micromeritics Autopore IV series) was used to exert a maximum pressure of 410MPa and to estimate an apparent pore diameter, ranging between three nm and 100000 nm.

For the corrosion test, a 12.7 mm-diameter rebar was embedded in the center of the cylindrical specimen. The exposed surface area of the rebar in a 3.5 % NaCl solution was approximately 40 cm2 (Fig. 1). The rebar was connected to the corrosion cell to act as a working electrode, the saturated calomel electrode acted as a reference electrode, and the titanium mesh acted as a counter electrode. A current density of 0.5 mA/cm2 was applied and the half-cell potentials and linear polarization resistance was measured using a Nichia NP-G100/ED potentiostat at 24-hour intervals. In addition, the corrosion rate of reinforcing steel (r) was computed from corrosion current densities using Faraday’s equation r=icorra/nF, where icorr represented current density, n represented the number of equivalents exchanged, a represented the atomic weight, and F represented the Faraday constant (96500 coulombs/mol).

Fig. 2 Schematic illustration of acceleration test.

Hitachi Model S-510 SEM was used for microstructure analysis. The samples obtained 5cm from the top surface of cylindrical specimen were sliced to a dimension of 5mm(l) x 5mm(w) x 3mm(t), oven-dried for 24 hours, vacuumed up to 0.1 torr, and Au-ion-sputtered for 2~3 min.

＋

Rebar

Cylindrical sample

3.5% NaCl solution

Titanium mesh

Exposed length (100 mm)

Epoxy coating length (150 mm)



3. Results and discussion

3.1 Compressive strength

The compressive strength development curves of mortar specimens containing 10 %, 20 %, 30 % and 40 % recycled materials of cement replacement with w/cm of 0.35 and 0.55 are shown in Fig. 3. At the early age, the RWW specimens yield lower compressive strength than that of the control specimens. As expected, the incorporation of ggbs and fly ash in the composites causes a reduction in its compressive strength except ggbs specimens with lower w/cm ratio. After 28 days, the compressive strength of RWW specimens also trends to decrease with the replacement increase and gives a lower compressive strength up to 20% than the control specimens. At the later age, the compressive strength of RWW specimens trends to increase with the curing age, which is similar to the ggbs and fly ash specimens. The highest compressive strength of RWW specimens is up to 15 % higher than that of control specimens. The addition of those recycled materials in composites trends to enhance the compressive strength and the behavior is directly related to their respective pozzolanic strength activity index values [13-14]. In addition, it can be also explained that the content of SiO2 (38.7%) and the fineness particles (the particle size less than 75 μm) help to react with Ca(OH)2 to produce an addition calcium silicate hydrate, which improves the compressive strength of cement-based composites at the later age. The addition of RWW might result in quantities of Ca(OH)2 consumption and it requires for the cement particles and time to react.

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Fig. 3 Compressive strength histograms of a) lower w/cm ratio, b) higher w/cm ratio.

The relationships between compressive strength and replacement of cement at the age of 7 and 91 days are illustrated in Fig. 4. Compared to those recycled materials, the development trend of RWW specimens is similar to the fly ash specimens for various level replacement of cement. The cement-based composites with 10 % to 30 % RWW replacement of cement seems to give higher compressive strength than those of 40 %. Moreover, the use of RWW as a partial substitution of cement in cement-based composites can reduce the amount of cement required to make sustainable composites as well as reduce the quantity of waste material which results in more environmentally friendly.

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Fig. 4 Compressive strength vs. replacement of cement of a) 7 days, b) 91 days.

Fig. 5 demonstrates the development of the compressive strength for mixtures A and B. In all mixtures, the compressive strength increased with increasing age. However, the additions of silica fume and fly ash significantly improved the compressive strength. The compressive strengths of specimens with silica fume tended to increase with the increase of the replacement amount and curing age due to a pozzolanic reaction. Prior to the age of 28 days, fly ash



specimens had lower strength than plain specimens did, because of the prolonged hydration of fly ash. Furthermore, hydroxyl ions released from the hydration product of the cement to break down the glassy particles at early age can only be obtained slowly. This was probably because the fly ash provides adequate lime needed to react with the pozzolans in the hydration process.

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Fig. 5 Compressive strength development curves of a) lower w/cm ratio, b) higher w/cm ratio.

3.2 RCPT

The total charge-passed of specimens with various RWW, silica fume, fly ash is listed in Table 2. For higher w/cm, the total charge-passed decreased as the RWW content increases. For lower w/cm, the specimens containing 20% RWW content had lower total charge-passed. The total charge-passed for the AR20, AR30 and AR40 specimens is less than 2000 coulombs, which indicates low chloride ion permeability in cement-based composites. It may be due to pozzolanic reaction between calcium hydroxide and reactive silica in RRW and the denser internal structures are formed.

Table 2 RCPT results.

w/cm Pozzolanic addition Replacement of cement (%) Total charge-passed (coulombs)

0.35 - - 2584 RWW 10 2342 RWW 20 1067 RWW 30 1213 RWW 40 1985 Silica fume 5 1355 Silica fume 10 980 Fly ash 15 2043 Fly ash 25 1933 Silica fume+ Fly ash 5+10 1213 Silica fume+ Fly ash 5+20 1152

0.55 - - 7374 RWW 10 5680 RWW 20 4845 RWW 30 4032 RWW 40 2886 Silica fume 5 2075 Silica fume 10 1601 Fly ash 15 2109 Fly ash 25 1863 Silica fume+ Fly ash 5+10 1868 Silica fume+ Fly ash 5+20 1722

The total charge passed decreased with increasing silica fume or fly ash content. Incorporating silica fume and fly

ash in composites for two mixtures decreased the total charge passed below 2000 coulombs, which indicates very low chloride ion penetrability in concrete specimen. Specimens with pozzolanic had a denser internal structure providing an effective barrier against chloride-ion penetration. In conclusion, the addition of silica fumes was effective in reducing



the total charge passed. A combination of silica fume and fly ash had a more significant impact on reducing the total charge passed and thus prevents chlorides to penetrate concrete. The addition of rock wool wastes also led to reduction of permeable voids and improvement of the durability.

3.3 MIP test

Based on the RCPT results, the specimens containing silica fume and fly ash were tested by MIP. This test can be understood the microstructures involving pore volume, pore size and solid particle of cement-based materials. Pores in hydrated cement paste can be categorized as either capillary or gel pores. Capillary pores are defined as pores of sizes between 10-10000nm and usually determine compressive strength, permeability, and diffusivity in cement-based composites. Gel pores (3-10nm diameter) are considered an intrinsic part of the calcium-silicate-hydrate (C-S-H), which mainly affects concrete shrinkage and creep. The capillary pore system consists of many interconnected large pores, through which water, ions, carbon dioxide, and oxygen penetrate or diffuse into the rebar surface, deteriorate protective passive films, and cause corrosion. Fig. 6 demonstrates the relationship between gel pore and capillary pore. Capillary pores significantly decreased and gel pores significantly increased when the silica fume or fly ash was incorporated into the cement-based composites, particularly in 10 % silica fume specimens. The cumulative mercury intrusion, the pore volume ratio, and the critical pore decreased as the silica fume or fly ash increased. The reduction in the pore volume due to the addition of silica fume may be the outcome of the continuous generation of pozzolanic reaction products from the hydration of fly ash that fills the pores. In addition, the specimens with silica fume had refined pore structures. Small silica fume particles might have packed efficiently between the cement grains, and thereby subdivided the pore space. The porosity and capillary pores decreased while the gel pores increased as a result of the inclusion of silica fume and fly ash in the cement-based composites

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Gel pore

Capillary pore

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Fig. 6 Relationship between gel pore and capillary pore of a) lower w/cm ratio, b) higher w/cm ratio.

3.4 Corrosion test

According to ASTM C876, when the Open Circuit Potential (OCP) is between 0 and -127mV (SCE), the probability ere is less than 10 % that reinforcing may corrode. When the potential ranges from -127 mV to -276mV (SCE), corrosion probability is uncertain and corrosion probability may be higher than 90 % for OCPs higher than -276 mV (SCE). The OCP indicated that control specimens (specimens A and B) significantly increased exposure time span to severe corrosion, especially in higher w/cm (Fig. 7). Silica fume or fly ash replacing cement may also have refined the pore structure and reduced the corrosion probability of reinforcing steel, which is consistent with comparative results of MIP and permeability tests. Furthermore, the inclusion of silica fume in fly ash specimens could significantly reduce the corrosion probability of two mixtures.



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Fig. 7 OCP vs. Immersion time curves of a) lower w/cm ratio, b) higher w/cm ratio.

Fig. 8 demonstrates the corrosion rate of specimens. The corrosion rate decreased significantly with an increase in the pozzolanic content, especially when using silica fume. Compared to control specimens, the corrosion rate of specimens with 5 % silica fumes at w/cm ratios of 0.35 and 0.55 decreased by approximately 40 % and 38 %, respectively. When 10 % silica fumes were added, the corrosion rate significantly decreased by approximately 53 % and 43 % in specimens at w/cm ratios of 0.35 and 0.55. Silica fumes in the mixtures may have quickly transformed into dense calcium silicate hydrate, filling up the interstitial spaces between the matrix and aggregates and forming a dense, strong, and relatively impermeable composite.

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Fig. 8 Corrosion rate vs. Immersion time curves of a) lower w/cm ratio, b) higher w/cm ratio.

In order to empirically examine and understand the durability of cement-based composites containing ggbs and its corrosion behavior under sustained loadings. The beams with a size of 150 mm × 150 mm × 900 mm were subjected to various sustained loadings (37% and 75% of the ultimate load) and exposed to 3.5% NaCl solution for the acceleration corrosion test. Corrosion test results with and without sustained loading as shown in Fig. 9. It indicates that without sustained loading (i.e. without significant visible cracks in concrete), mix B (B0) and mix C (C0) significantly increased exposure time span to severe corrosion in comparsion with mix A (A0). It also appears that ggbs replacement for cement refined the pore structure and reduced the corrosion probability of reinforcing steel. Although, the pH value was 13.2 for control concrete, 12.8 for 40% ggbs replacement concrete and 12.4 for 60% ggbs replacement concrete, respectively, it seems no prominent negative effect on the corrosion behavior due to decreasing alkalinity in pore solution. With 37% loading ratio (specimens with pre-determined 0.01 mm wide cracks), the OCP of A37 specimen drops from -75 to -427 mV (SCE) after 4-day accelerated exposure. B37 specimen and C37 specimen reach sever corrosion condition after 6-day~8-day exposure, which reveals that sustained loading or cracks plays paramount role in the corrosion process of reinforcing steel in concrete. It can also be observed from Fig. 9 that the specimen (A75) with 75% sustained loading the potential drops below –427 mV (SCE) even after 2-day accelerated corrosion process and ggbs concretes having cracks wider than 0.02 mm (B75 and C75) have no better corrosion resistance. At higher sustained loading ratio, OCP does not significantly vary among the three mixes, and the corrosion process is controlled by the pre-determined cracks. In addition, the potential of –270 mV (SCE) is corresponding to current density of 0.25 A/cm2 , 0.32 A/cm2, and 0.28 A/cm2 for mix A, B and C, respectively as indicated in Fig. 10. The threshold of active corrosion for concrete may be defined by current density and is increased by slag addition



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Fig. 10 Relationship between OCP and current density.

3.5 SEM observation and durability indices

SEM observations were used to explore the microstructure of the corresponding specimens. The denser internal structures of the RWW specimens are formed as illustrated by SEM observations in Fig. 11. It may be due to pozzolanic reaction between calcium hydroxide and reactive silica in RWW. The addition of RWW may cause smaller sizes of capillary pores and RWW may decrease the pore interconnectivity because the fiber-shape RWW could bridge cracks and arrest capillary pores. The SEM observations of 20 % RWW specimens at the age of 28 days and 91 days are illustrated in Fig. 12. RWW in the composites helped to form a denser microstructure and cause improvement in strength and permeability, which is consistent with the results of compressive strength and total charge-passed. In addition, the inclusion of RWW in composites formed a denser microstructure when the PSAI value increases [13-14]. (From 82% at the age of 28 days to 103% at the age of 91 days) Thus, the addition of rock wool wastes leads to reduction of permeable voids and improvement of the durability.

a) b)

Fig. 11 SEM observation of specimens with and without RWW of a) control specimen, b) 20% RWW specimen.



a) b)

Fig. 12 SEM observation of 20% RWW specimens of a) 28 days, b) 91 days.

As shown in SEM microscopy (Fig. 13) for the specimens containing ggbs, a great number of needle-shape ettringite and plate-shape calcium hydroxide and large capillary pores (0.05~10μm) were found in OPC specimens. But few needle-shape ettringite existed in ggbs specimen and the capillary pores were less than (10 to 50nm) in which could be filled up with pozzolanic reaction product such as low density C-S-H gel. It appears that higher ggbs replacement percentage has denser structure and prevents concrete from water penetration, which is also verified by the results of compressive strength test and rapid chloride penetrating test.

a) b)

Fig. 13 SEM micrograph of a) control specimen, b) ggbs specimen.

a) b)

c)

Fig. 14 SEM micrograph of a) control specimen, b) 5 % silica fume specimen, c) 10 % silica fume specimen. SEM images of the specimens with and without silica fumes were taken at the matrix away from the transition zone and are shown in Fig. 14. While irregular crystallike structures and several large pores were found on the surface of the control specimen, smooth surfaces with no distinct pores were observed in 5 % and 10 % silica fume specimens. Those SEM images also illustrated relatively denser and more homogeneous microstructures in composites containing silica fumes. Silica fumes in the mixtures may have quickly transformed into dense calcium silicate hydrate, filling up the

Ca(OH)2

Ettringite

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interstitial spaces between the matrix and aggregates and forming a dense, strong, and relatively impermeable composite. The composite microstructure of 10 % silica fume specimens had fewer pores, more plane structures, more narrow pore size distribution and more C-S-H particles, which is consistent with the results of compressive strength and RCPT tests. Specimens with 15 % fly ash had lower corrosion rates than specimens with 25 % fly ash in two mixtures. An increased amount of fly ash may cause a partial reaction in the hydration process as illustrated in Fig. 15. The inclusion of silica fume in fly ash significantly influenced the corrosion rate, which decreased up to 45 % and 43 % in the mix of 5 % silica fume and 10 % fly ash specimens and the mix of 5 % silica fume and 20 % fly ash specimens compared to control specimens at a w/cm ratio of 0.45. At a w/cm ratio of 0.65, the corrosion rate of the mix of 5 % silica fume and 10 % fly ash specimens and the mix of 5 % silica fume and 20 % fly ash specimens decreased up to 38 % and 39 %. Silica fume is a very reactive pozzolan when used in composites due to a high SiO2 content, which could react with Ca(OH)2 to form a denser microstructure. Thus, improvements in the corrosion resistance of cement-based composites obtained by the combination of silica fume and fly ash were attributed to changes in the porosity nature and microstructure of the hardened paste The dense microstructure of composites can limit the movements of aggressive ions toward the surface of embedded reinforcing steels.

a) b)

Fig. 15 SEM micrograph in 15 % fly ash specimens of a) lower w/cm ratio, b) higher w/cm ratio.

From the corrosion test, the potential of –276 mV (SCE) corresponds to a corrosion rate of 11.54 μm/yr. The threshold of active corrosion for concrete may be defined by the corrosion rate and is set at a corrosion probability of 90 % or higher. The corrosion rate positively correlated with the porosity (Fig. 16) indicating that a smaller porosity was a response to a lower corrosion rate. Therefore, the relationship between OCP and permeability indices including corrosion rate and critical pore size could be obtained by regression analysis. Fig. 17 demonstrate the correlations and Table 3 lists the critical value under high corrosion levels. At the threshold of active corrosion, the composites exhibited a corrosion rate of 11.54 μm/yr, a diffusion coefficient of 0.62x10-12 m2/s, and a critical pore size of 26.71 nm. This reflected chloride permeability and corrosion resistance, which validates the present method to compare different mixtures.

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OCP vs. porosity

Fig. 16 Relationship between porosity, corrosion rate, and OCP.

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ze (

nm)

-800

-700

-600

-500

-400

-300

-200

-100

0

OC

P (

mV

, SC

E)

OCP vs. corrosion rate

Critical pore size vs. corrosion rate

Rebar corroson is occuring (> 90%)

Fig. 17 Relationship between corrosion rate, critical pore size, and OCP. Table 3 Critical value under a high corrosion level.

Corrosion probability OCP Corrosion rate Critical pore size

> 90% -276 mV 11.54 μm/yr 26.71 nm

4. Conclusions

The results of this study concluded that it is feasible to utilize RWW as partial replacement of cement in cement-based composites. The finer particles (less than 75 μm) played an important role and RWW possessed a cementitious capability due to the hydration or pozzolanic reaction. The improved mechanical properties and permeability of cement-based composites in the presence of RWW may be attributed to the microstructural changes of the hardened paste and as illustrated from SEM observations. From the results of mechanical properties and SEM observations, the RWW could played the role of particle and fiber. Considering the macro and micro behaviors, the RWW both had the particle parking effect and fiber arresting effect.

SEM micrographs indicated that the addition of ggbs modified the products and the pore structure in a hardened cementitious material, which is also verified by the results of permeability test and rapid chloride penetrating test. The GGBS reacted with water in alkali environment and then with calcium hydroxide to form cement hydration product through pozzolanic reaction to form extra C-S-H gel in the paste and slow down the strength development at early age. Denser microstructure or lower porosity resulted from higher C-S-H content that represented higher ggbs replacement percentage and higher durability of concrete.

The addition of silica fume lowered the porosity and critical pore size of fly ash cement pastes. The silica fume could react with Ca(OH)2 to form a denser microstructure, which was consistent with comparative results of MIP tests and SEM observations. The composite microstructures of the silica fume or fly ash specimens had fewer pores, more plane structures, and more C-S-H particles. Furthermore, the corrosion rate positively correlated with the porosity indicating that a smaller porosity was a response to a lower corrosion rate. In conclusion, at the threshold of active corrosion (corrosion probability higher than 90 % for OCP above -276 mV), the composites exhibited a corrosion rate of 11.54 μm/yr and a critical pore size of 26.71 nm.

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