Characteristics of the carbonation resistance of recycled fine aggregate concrete

Construction and Building Materials 49 (2013) 814–820

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Characteristics of the carbonation resistance of recycled fine aggregate concrete Jian Geng ⇑, Jiaying Sun Research Center of Green Building Materials and Waste Resources Reuse, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, PR China

h i g h l i g h t s  Effect of RFA on the carbonation resistance of RFAC.  Fly ash addition favors the carbonation resistance of RFAC, especially at 20% cement replacement ratio.  The microstructure of RAF and RFAC.  The self-cementing ability of RFA was analyzed.

a r t i c l e

i n f o

Article history: Received 12 June 2013 Received in revised form 29 August 2013 Accepted 30 August 2013 Available online 29 September 2013 Keywords: Recycled fine aggregate Carbonation Durability Fly ash Microstructure

a b s t r a c t This study investigates the effects of the minimum recycled fine aggregate (RFA) particle size, RFA amount, and fly ash addition on the carbonation resistance of RFA concrete (RFAC). The results reveal that the carbonation depth of RFAC increases with decreased minimum RFA particle size and increased RFA amount. At >40% RFA amount, water significantly affects RFAC carbonation. Fly ash addition favors the carbonation resistance of RFAC, especially at 20% cement replacement ratio. In this study, the self-cementing ability of RFA is proved by the microstructural analyses of RFA and RFAC but is found to have a negligible effect on RFAC carbonation for a few carbonizable hydrated products. The poor microstructure of RFAC and the interfacial zone between the new cement paste and RFA result in easier CO2 ingression for RFAC. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The reuse of construction waste is important for saving resources, protecting the environment, and realizing sustainable development in the construction industry. The use of construction waste as recycled aggregate (RA) has become increasingly common worldwide. However, given the poor quality of RA, its application is severely limited in most countries and is confined to low-grade use, such as for unbound roads [1,2]. RA is mainly obtained from waste concrete by machine crushing, and leading to sharp corners and cracks and old cement paste adhering to the surface of RA, which lead to high water adsorption. Under this condition, fresh recycled aggregate concrete (RAC) requires more water than natural aggregate concrete (NAC) for mixing, which probably lead to high porosity in concrete. Otherwise, because of the old cement paste adhered, the interfacial zone between new cement mortar

⇑ Corresponding author. Address: School of Civil Engineering and Architecture, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, PR China. Tel.: +86 13780080829. E-mail address: [email protected] (J. Geng). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.08.090

and aggregate in RAC is usually weaker than that in NAC. As a result, hardened RAC has poor durability, low strength, and low elastic modulus [3–8]. Chloride penetration and carbonation are two major problems of RFAC durability. Many differences exist between them, although the basic mechanisms are the same and both of them are mainly controlled by the pore characteristics of concrete. Regarding the anti-chloride permeability chloride penetration, most researchers agree that it increase with increased RA amount and can be improved by the addition of fly ash [9–13]. Regarding the resistance to carbonation, no unanimous conclusion has been drawn from previous reports. Sagoe-Crentsil et al. [14] reported a 10% increase in the RAC carbonation depth when RA is used, as well as a parabolic rate law of the relationship between the carbonation depth and square root of the exposure time that applies to RAC and NAC. Limbachiya et al. [15] noted that the carbonation depth and rate increase with increased amount of recycled coarse aggregate (RCA). Lovato et al. [16] mentioned that using both RCA and recycled fine aggregate (RFA) can lead to increased carbonation depth in direct proportion to their amounts. Evangelista and de Brito [17] investigated the effect of 30% and 100% RFA amount on

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the resistance to carbonation of RAC, and found 40% and 100% increases in the RAC carbonation depth compared with NAC. Zega and Di Maio [18] studied recycled fine aggregate concrete (RFAC) carbonation after a 310 and 620 day exposure test, and found that the carbonation depth of RFAC is similar to that of NAC because of the lower effective water/cement ratio of RFAC. Levy and Helene et al. [19] suggested that both RCA and RFA favor the resistance to carbonation of RAC and also proposed that the carbonation depth strongly depends on the chemical composition of concrete and not only on the physical aspects. The effect of fly ash on RAC carbonation is also unclear. Corinaldesi and Moriconi [20] investigated the effect of fly ash on the carbonation of RAC made with 100% RCA, and found that the addition of fly ash favors the resistance to carbonation of RAC. Abbas et al. [21] measured the carbonation depth of recycled coarse aggregate concrete (RCAC) with fly ash after a 140 day exposure test, and found that the resistance to carbonation of RAC declines with prolonged time because of the addition of fly ash. Limbachiya et al. [15] found that fly ash is not good for the resistance to carbonation of RAC, but also noted that fly ash favors its long-term anti-carbonation ability. Sim and Park [22] showed that the use of <30% fly ash leads to increased carbonation depth, but >60% fly ash does not affect carbonation. Kou and Poon [23] noted that the carbonation depth of RCAC increases with increased amount of fly ash regardless of the addition method, such as by weight cement replacement ratio and by weight cement addition. These results indicate that many factors affect the resistance to carbonation of RAC and RFAC. RFA is a byproduct of RCA production, and its resource utilization degree is far lower than that of RCA because of the higher amount of old cement paste in RFA. However, an increase in the number of carbonizable particles that exist in old cement paste [19,24] probably benefits the resistance to carbonation of RFAC. Fly ash also has an effect on concrete carbonation, but there is still no unified conclusion about it, especially for its amount <30% [25]. Thus, the effect of fly ash addition on RAC carbonation, particularly on the improvement of concrete performance, warrants further study. In this paper, the effects of the minimum RFA particle size, RFA amount and effect of low fly ash addition on the resistance to carbonation of RFAC are studied. To explore the carbonation mechanism at the microscopic level, Xray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and microhardness analyses are performed to analyze the microappearance and microcomposition characteristics of RFA and RFAC.

2. Experimental procedure 2.1. Materials Ordinary Portland cement equivalent to GB 175-2007 type 42.5 grade and fly ash were obtained from the Guohua Power Plant in Ningbo, China. Natural coarse aggregate and river sand were used. The chemical composition and physical properties of these materials are shown in Tables 1 and 2, respectively. The grading curves of river sand and recycled fine aggregate are shown in Fig. 1. RFA was obtained from a professional manufacturer of RA in Shanghai, China. Old cement paste adhered onto RFA is the major reason for the poor performance of RAC; a smaller RFA particle size indicates a higher content of old cement paste. Thus, RFA from the same parent concrete and with different minimum particle sizes (<0.16, 0.16, and 0.36 mm) were used to study their effect on the resistance to

Table 1 Chemical composition of cement and fly ash. Materials

Cement Fly ash

Chemical composition (%) SiO2

CaO

MgO

Fe2O3

Al2O3

SO3

Loss

20.4 56.9

61.6 4.07

0.97 2.87

3.49 4.60

7.26 26.5

3.55 0.20

3.36 2.08

Table 2 Physical properties of river sand and recycled fine aggregate. Fine aggregate

Fineness module

Water absorption (%)

Sediment percent (%)

River sand RFA1 RFA2 RFA3

2.8 2.7 2.9 3.3

1.6 7.2 6.8 5.9

1.0 34.0 31.8 13.8

carbonation of RFAC, in which RFA1 was directly obtained from Shanghai, and RFA2 and RFA3 were sieved from RFA1 according to experiment requirements in laboratory. The corresponding samples were designated as RFA1, RFA2, and RFA3. The physical properties of the RFA samples are shown in Table 2. The amount of sediment percent of RFA is tested by thermal treatment [26].

2.2. Concrete mix design The concrete mixtures were divided into two series. Series I was designed with a fixed water/blend ratio (w/b = 0.40). The replacement ratio of river sand by RFA1 was 0%, 20%, 40%, 60%, and 80% by weight. For RFA2 and RFA3, only 40% replacement ratio was considered. Series II was designed such that all concrete mixtures had similar workabilities with an 180 ± 10 mm slump. In all concrete mixtures, a poly-carboxylic acid water reducer (JS) was used. In series I, fly ash was also used with 10%, 20%, and 30% cement replacement ratios by weight. The proportions of the concrete mixtures are shown in Table 3. The properties of fresh and hardened concrete are shown in Table 4.

2.3. Test methods All concrete mixtures used for the compressive strength test and accelerated carbonation test were 100  100  100 mcm cube models. After 26 days of curing in water at 20 ± 2 °C, the samples were dried for 48 h at 60 ± 2 °C to accelerate the carbonation test. All surfaces of the dried samples were sealed by paraffin, except for two opposing side surfaces. The accelerated carbonation test was conducted at 20 ± 5 °C and 70% ± 5% RH with 20% ± 3% carbon dioxide concentration in the testing chamber, whose reference was GB/T50082-2009 (standard for test methods of long-term performance and durability or ordinary concrete). After the test, the samples were split and 1% phenolphthalein solution was sprayed on the broken surfaces. The carbonation depth was measured after 7, 14, and 28 days of carbonation exposure. All results are the mean of three samples with the same proportion. RFA1 used for X-ray powder diffraction (XRD), was prepared as a fine powder by grinding and passing through sieve 100#. XRD data were collected using a D8 Advance instrument of Bruker AXS with a Cu Ka radiation generated with 40 kV and 30 mA. LC0 and LC14 were cured for 28 days. Next, fracture surface mortars were taken from concrete that previously had been subjected to the compressive strength test. The mortars used for examination under the SEM (HITACHI S-4800) by secondary electron imaging (SEI) were placed directly in an evaporator and maintained under high vacuum overnight. The accelerating voltage was 20 kV. At the same time, the EDX-detector equipped HITACHI S-4800 was used to obtain energy dispersive X-ray analysis data (EDXA) of RFA1 for identifying its composition. The microhardness of the interfacial zone (ITZ) of both LC0 and LC14 is measured by a Leitzs microhardness tester (HVT-1000). In this test, the ITZ between new cement paste and fine aggregate is set as 0 distance point, and microhardness is tested every 20 lm from fine aggregate to cement mortar. The microhardness of every sample was tested three times and represented by different color lines.

3. Results and discussion 3.1. Minimum RFA particle size Fig. 2 shows the relationship between the minimum RFA particle size and concrete carbonation depth. The results indicate that the carbonation depth of RFAC is higher than that of control concrete (LC0) at the same exposure time, and also increases with prolonged exposure time. Thus, RFA is unfavorable to the concrete anti-carbonation ability. The carbonation depth of RFAC is also related to the minimum RFA particle size. A smaller minimum RFA particle size results in higher carbonation depth at the same exposure time. In this case, the carbonation depth of LC14 (prepared from RFA1 with the smallest minimum particle size of <0.16 mm) is the highest and

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Fig. 1. The grading curves of rive sand and RFA. Fig. 2. Relationship between the minimum RFA particle size and carbonation depth. Table 3 Proportions of concrete mixtures (kg m3). Mixtures Cement Fly ash Coarse Sand RFA1 RFA2 RFA3 Water JS LC0 LC12 LC14 LC16 LC18 LC24 LC34 LCF1 LCF2 LCF3

388 388 388 388 388 388 388 349 310 272

/ / / / / / / 39 78 116

WC12 WC14 WC16 WC18

388 388 388 388

/ / / /

1102

1102 1102 1102 1102

735 588 441 294 147 441 441 441 441 441

/ 147 294 441 588 / / 294 294 294

/ / / / / 294 / / / /

/ / / / / / 294

588 441 294 147

147 294 441 588

/ / / /

/ / / /

155

5.4

167 179 194 234

5.4 5.4 5.4 5.4

reaches 1.5 cm after the 28 day exposure test. One major difference between RFA and RCA is that the old cement paste not only remains on the RFA surface but also exists as fine aggregate. The old cement paste on the RFA surface forms a weak, porous, crackly layer, whereas the fine aggregate further increases the porosity of RFA and weakens its quality. The porous characteristic of RFA increases its water absorption, leading to many microflaws formed inside the concrete (as proven in the following microanalysis). These microflaws weaken the anti-permeability of concrete and make it easy to be carbonated. The quantity of the old cement paste increases with the reduction in the minimum particle size, as shown in Table 2. 3.2. RFA amount Fig. 3 shows the variation in concrete carbonation depth with the amount of RFA1. Similar to the results of the RCA effect on concrete carbonation, the RFAC carbonation depth also increases with increased river sand replacement ratio by RFA1, and the resistance to carbonation of concrete gradually worsens. In this study, the carbonation depth of RFAC is similar to that of the control concrete when the replacement ratio is 20%, but this value is lower than the results obtained by Evangelista and de Brito [17] and Zega and Di Maio [18] because of the smaller minimum RFA particle size used in this investigation. They suggested that 30% is the best replacement ratio for the quality of RAC. With increased replacement ratio from 40% to 80%, the increase in the carbonation depth becomes very obvious. The carbonation depths of LC14, LC16, and LC18 are 1.5, 2.5, and 4.9 cm, respectively, after the 28 day exposure test, which are far higher than that of LC12.

Fig. 3. Relationship between the RFA amount and carbonation depth at a fixed water/blend ratio.

The relationship between carbonation depth and exposure time is presented in Fig. 4. Generally, the carbonation depth of RFAC is proportional to the square root of the exposure time, i.e.,

X ¼ K c t1=2

ð1Þ

where X is the carbonation depth (mm), Kc is the carbonated coefficient, and t is exposure time (days). Similar to NAC and RCAC [14,17], the carbonation rate of RFAC increases with increased replacement ratio. In this series of experiments, the w/b ratio is fixed at 0.40 and RFA1 has a higher water adsorption. Thus, the workability of fresh RFAC is very poor and its slump decreases with increased replacement ratio, as shown in Table 4. Under this condition, microflaws can be observed on the side and surface of RFAC. Consequently, the carbonation depth and carbonation rate notably increase with increased RFA1 amount, especially for LC16 and LC18. The exposure test results of the WC series samples with different w/b ratios and the same slump are shown in Fig. 5. Compared with the LC series samples, the WC series samples have better workability because of more water used, as shown in Table 4. However, the resistance to carbonation of RFAC is not improved as expected; in fact, it worsens compared with that of the LC series samples. Lovato et al. [16] reported a similar result (for RCAC) and believed that increased water leads to increased concrete porosity because of evaporation and easier entry of CO2 into concrete. Zega and Di Maio [18] and Corinaldesi and Moriconi [20] also found that a lower w/b ratio benefits the RCAC anti-carbonation ability

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Fig. 4. Relationship between carbonation depth and exposure time: (a) at a fixed water/blend ratio, and (b) at the same workability condition.

Table 4 Properties of fresh and hardened concrete. Properties

LC0

LC12

LC14

LC16

LC18

LC24

LC34

Slump (mm) Compressive strength at 28 days (MPa)

183 46.7

154 44.5

89 38.2

50 31.2

/ 21.5

105 39.6

182 42.1

Properties

LCF1

LCF2

LCF3

WC12

WC14

WC16

WC18

Slump (mm) Compressive strength at 28 days (MPa)

122 42.1

146 38.6

160 35.8

193 43.3

186 34.6

181 29.8

174 20.2

because it can reduce concrete porosity and compensate for the use of a more porous aggregate. According to these studies and the present one, the use of a reasonable amount of water is very essential for the resistance to carbonation of RFAC, especially when the amount of RFA exceeds 40%. 3.3. Fly ash The relationship between the fly ash amount and RFAC carbonation depth is shown in Fig. 6. Results show that the carbonation depth of RFAC is lower than that of RAC but still higher than that of the control concrete. Thus, the addition of fly ash favors the resistance to carbonation of RAC. Analysis of the effect of the cement replacement ratio by fly ash on the carbonation depth reveals that the carbonation depth initially decreases and then increases with increased replacement ratio from 10% to 30%, and then reaches the minimum at 20%. Fly ash is a material with potential hydration ability and very slow hydrating speed. Chen et al. [27] measured the hydration rate of fly ash in cement paste under the different curing temperatures and times. Their results show that the hydration ratio decreases with increased replacement ratio. The hydration ratios are about 7% and 8% for 20% and 30% replacement ratios after 1 month of curing at 20 °C, and only about 14% and 15% with prolonged curing time to 5 months. In this research, the longest exposure time and highest amount of fly ash are 28 days and 30%, respectively. Consequently, the effect of the second hydration reaction of fly ash on the alkalinity of the pore solution is not very notable. In addition, the workability of RFAC is well improved at a low w/b ratio (Table 4) because the use of fly ash that helps RFCA obtain a more compact structure. Fly ash addition can also enhance the microstructure of RFAC. Therefore, in this study, the resistance to carbonation of RFAC is improved by adding fly ash. On the other hand, given the partial cement replacement ratio used in the fly ash method, the amount of cement decreases with increased replacement ratio. This phenomenon leads to decreased alkalinity of the pore solution, which is disadvantageous for the concrete anti-carbonation ability. Thus, a

Fig. 5. Relationship between the RFA amount and carbonation depth at the same workability condition.

Fig. 6. Relationship between the amount of fly ash and carbonation depth.

higher carbonation depth is obtained when the replacement ratio is higher than 20%, and the amount of fly ash used in RFAC should be lessened.

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100 Fig. 7. SEM images of RFA1.

Fig. 8. EDX pattern of RFA1.

1000

5000

Fig. 10. SEM image of LC0 after 28 days of curing.

Fig. 9. XRD pattern of RFA1.

3.4. Microappearance and composition characteristics of RFA Fig. 7 shows the SEM images of one particle of the machinecrushed RFA1. The RFA1 surface is very rough with many irregular particles adhered onto it, and numerous pores as well as microcracks are observed, which are unfavorable to the RFA quality. The SEM image magnified 500 to 5000 times clearly show the absence of a close connection among these particles, i.e., a loose and porous structure. The SEM analysis of RFA1 indicates that the concrete designed with RFA could favor permeability due to many parameters related to the properties of the RFA. Identifying the composition of RFA particles using only SEM is difficult, so these particles are also subjected to EDX analysis. The results in Fig. 8 shows that the major elements of these particles are C, O,

Si, Ca, Al, and Mg, which are also the major elements of the hydrated products of Ordinary Portland cement. Thus, these particles may be the old cement paste. The XRD pattern of RFA1 is shown in Fig. 9. SiO2 and CaCO3 are detected in RFA1. The former is from the natural aggregate of the parent concrete RFA1, and the latter is most likely obtained from the carbonated parent concrete. A small amount of C2S is also found in RFA1 because of its low hydration level. Poon et al. [28] and Shui et al. [29] examined the composition of RFA from different parent concretes by XRD and found C2S. Thus, C2S is a common mineral material in RA. Poon et al. [28] also found that the presence of C2S gradually increases with decreased RFA particle size. C2S has a low hydration level and its hydration can proceed for a long time. Thus, the amount of carbonizable particles probably increases in RFAC, which is favorable to the anti-carbonate ability of concrete. However, the results in Figs. 2–6 show that this effect is weak and negligible.

J. Geng, J. Sun / Construction and Building Materials 49 (2013) 814–820

1000

5 000

Fig. 11. SEM image of LC14 after 28 days of curing.

1000

(a)

1000

(b)

819

A comparison between the C–S–H gels in LC14 and LC0 reveals that some particles with larger sizes (30–80 lm) are adhered onto C–S–H gel in LC14. Considering that no supplementary cementing material is used in this test and based on the results in Figs. 3 and 6, these particles may be old cement paste. The magnified (5000) SEM image of these particles shows a few needle-shaped particles whose length and width (1 lm to 3 lm) are smaller than those of ettringite crystals (3 lm to 5 lm). Ettringite crystals are also found among the hydrated products of LC14. According to the results of Diamond and Lachowski [30], these particles should be type I C–S–H gel, which is only formed at the early stage of the cement hydration reaction. Thus, the hydration of old cement paste has obviously occurred, which may be related to the existence of C2S in RFA. This finding also proves that RFA has a self-cementing ability as reported by Poon et al. [24]. The self-cementing ability of RFA should favor the resistance to carbonation of RFAC. However, the exposure test results in this study reveal that RFA does not exert any beneficial effect on the resistance to carbonation. Two reasons can account for this phenomenon. First, the hydration of C2S proceeds for a long time, so the effect of this hydration on the concrete anti-carbonation ability is not obvious even after 28 days of curing. Second, the key to this effect is the amount of carbonizable hydrated products from the C2S hydration reaction. Thus, this effect would be negligible if the amount of these particles is below a certain value, which indicates that a threshold by weight of the total fine aggregate for the carbonizable hydrated products exists.

Fig. 12. ITZ of (a) LC0 and (b) LC14.

3.6. Interfacial zone (ITZ) characteristics 3.5. Microappearance characters of RFAC The SEM images of LC0 and LC14 after 28 days of curing are shown in Figs. 10 and 11. A densification of the internal structure of RFAC can be observed from the results in Fig. 6, and no obvious flaws are seen aside from a few pores. C–S–H gel is the main hydrated product and is distributed as a layered structure. A few calcium hydroxide crystals with hexagonal flakes are also clearly observed, which indicates that the degree of hydration of LC0 is better. Conversely, the results in Fig. 7 shows many pores and a few calcium hydroxide crystals in the internal structure of LC14, which indicate that the compactability of LC14 is poorer. These results can be attributed to two reasons. First, the evaporation of water from RFA leads to the formation of larger pores. Second, less water is used for the hydration of the blend materials in RFAC because the higher water adsorption of RFA leads to a decline in its hydration speed and the formation of an immature microstructure. These flaws are favorable to CO2 ingression into RFAC.

The ITZ is a very important part of the three-phase system of concrete; the other parts are coarse aggregate and paste matrix with fine aggregate. In concrete, the ITZ has a critical function, and most performance deteriorations are related to its poor structure. The ITZ between new cement paste and river sand of LC0 is shown in Fig. 12a. Calcium hydroxide crystal enrichment in the ITZ of LC0 is clearly observed, and a denser structure of cement paste around the river sand is formed. The entire structure of the ITZ of LC0 is good, except for a 10 lm-long and 150 lm-wide crack. The ITZ between the new cement paste and RFA1 of LC14 is shown in Fig. 12b. Compared with LC0, no calcium hydroxide crystal enrichment is observed in the ITZ of LC14, and more voids and cracks are found in it. The length and width of these cracks are also larger than those of LC0 because the higher water adsorption of RFA leads to less water for hydration at the ITZ. SEM analysis reveals some loose conjunctions between RFA and the hydrated products of cement paste in the ITZ.

Fig. 13. Microhardness of the ITZ of (a) LC0 and (b) LC14.

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The microhardness results of the ITZ of both LC0 and LC14 in Fig. 13 reveal some differences between LC0 and LC14. First, the microhardness of RFA is lower than that of river sand because of the old cement mortar adhered onto it. Second, the microhardness of the cement mortar of LC14 is lower than that of LC0 because of a looser structure. Third and last, the ITZ of LC14 (80 lm) is wider than that of LC0 (60 lm). These results indicate that a poor ITZ is formed in RFAC and it accelerates the carbonation process of RFAC.

4. Conclusions This study aims to evaluate the effect of the minimum RFA particle size, RFA amount, and the addition of fly ash on the resistance to carbonation of RFAC. The microappearance and composition of RFA and RFAC are also discussed. From the test results, the following conclusions are drawn. (1) The carbonation depth of RFAC increases with decreased minimum RFA particle size because a smaller minimum particle size indicates a higher amount of old cement paste in RFA. (2) The resistance to carbonation of RFAC declines with increased RFA amount under a fixed w/b especially for it >40%. Although, increased amount of water can improve the workability of RFAC and make it easy to formation, the carbonation depth of WC series samples worsens compared that of the LC series samples. Thus, water is still a very important factor affecting RFAC carbonation, and a reasonable w/b ratio should first be considered, especially when the amount of RFA exceeds 40%. (3) Generally, the addition of fly ash is favorable to the resistance to carbonation of RFAC. The carbonation depth initially decreases and then increases with increased the cement replacement ratio by fly ash from 10% to 30%, and then reaches the minimum at 20%. Thus, the use of fly ash especially at 20% cement replacement ratio is favorable to the carbonation resistance improvement of RFAC in this research. (4) SEM and XRD analyses on RFA and RFAC indicate that RFA has a self-cementing ability because of C2S. However, given the small amount of carbonizable hydrated products, the resistance to carbonation of RFAC is not improved as expected. This result signifies the presence of a threshold by weight of the total fine aggregate for carbonizable hydrated products. (5) The higher water adsorption of RFA leads to less water for cement hydration reactions. Consequently, the microstructure of RFAC, especially the ITZ between new cement paste and RFA, worsens. As a result, the porosity of RFAC increases and CO2ingression becomes easier.

Acknowledgement This work was fully supported by a Grant from the Innovative Team Plan of Ningbo Government, China (Project Ref. No. 2011B 81005).

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