Studies on performance enhancement of recycled ...

Construction and Building Materials 181 (2018) 217–226

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Studies on performance enhancement of recycled aggregate by incorporating bio and nano materials L.P. Singh a,⇑, Vishakha Bisht a,b, M.S. Aswathy a, Leena Chaurasia a, Sanjay Gupta c a

CSIR - Central Building Research Institute, Roorkee 247667, India Uttarakhand Technical University, Dehradun 248001, India c Swami Rama Himalayan University, Dehradun 248016, India b

h i g h l i g h t s RA and RAC were improved using bacteria induced precipitation and nanosilica. Bacteria and nanosilica treated RA/RAC showed reduction in water absorption, volume of voids (%) and enhancement in density parameters. Bio and nano modified RA/RAC visualized by FESEM which showed improvement in both, old as well as new ITZs.

a r t i c l e

i n f o

Article history: Received 10 April 2018 Received in revised form 24 May 2018 Accepted 28 May 2018

Keywords: Recycled aggregate (RA) Nanosilica Bacteria ITZ Water absorption Volume of voids Density

a b s t r a c t This paper investigates the modification effect of nanosilica and ureolytic/non-ureolytic bacteria at the interfacial transition zone (ITZ) of RA and recycled aggregate concrete (RAC).The improved RA showed reduction in water absorption (43% in non-ureolytic bacteria, 64% in ureolytic bacteria and 21% in nano modified RA) and increase in specific gravity (29% in non-ureolytic bacteria, 30% in ureolytic bacteria and 18% in nano modified RA). The studies also proved that properties of RA and RAC on the macroscale were highly improved by the soaking approach than the direct mixing approach. ITZ studies were carried out using Field Emission Scanning Electron Microscopy (FESEM) and Energy-dispersive X-ray spectroscopy (EDX) analysis, which revealed that, biogenic calcite and nano mediated accelerated hydration products densified both the ITZs (old and new), leading to substantial improvement in macroproperties of RAC. Thus, the proposed modification techniques, seems promising for the performance enhancement of recycled aggregate. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Increasing demand and dependence on natural aggregate (NA), has posed the need for a sustainable alternative to replace a fraction or all of the NA in concrete mixture. Abundance of recycled aggregate (RA) from construction and demolition waste makes it a suitable substitute for replacement with NA, however; high water absorption and weak bonding to the new matrix are the limiting factors that restricts its utilization in fresh concrete. Various methodologies have been adopted to combat the issue of water absorption related to RA, for instance, to remove the adhered old mortar by chemical treatment, heat treatment and mechanical grinding [1]. High energy consumption and cost are some disadvantages of these methods, which have motivated various researchers around the world to explore methods which focus on ⇑ Corresponding author. E-mail address: [email protected] (L.P. Singh). https://doi.org/10.1016/j.conbuildmat.2018.05.248 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

strengthening the adhered old mortar rather than removing it. Therefore methodologies like sodium silicate treatment [2], polymer impregnation [3], carbonation treatment [4] have been adopted to improve the quality of adhered old mortar. The sodium silicate treatment might introduce alkalis which increases the risk of alkali silica reaction. Likewise, in polymer impregnation method, the longer time period and compatibility of polymer with concrete are some major concerns. In concrete, made of recycled aggregate, the number of ITZs are more, as it contains old ITZ between old mortar and aggregate as well as new ITZ between old mortar and new mortar [5]. Modification of these ITZs and specifically old ITZ having adhered old mortar, can therefore be targeted to substantially decrease the water absorption of RA, to make it comparable to NA in recycled aggregate concrete (RAC). To overcome the challenge of high water absorption value of RA in RAC, fly ash and volcanic ash have therefore been incorporated and durability properties of mixes have been studied [6]. In order to improve the microstructure of both ITZs in RAC, silica fume

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has also been used [7]. The authors proposed two-stage mixing approach (TSMA) to treat RAC, which resulted in significant strengthening of ITZs between RA and new mortar. A novel triple mixing method based on TSMA has been proposed which when compared with TSMA, could further improve the properties of the RA, the microstructure of ITZs and the RAC [8]. Besides, the incorporation of pozzolanic mineral admixtures, modification of RAC with nanomaterials, has also been accomplished by surface treatment of RA. The studies reveals reduction in 7 and 28 days compressive strength of fully RAC by 12–14%, while addition of 3% nanosilica in RAC improves the strength which is comparable with NAC [9,10]. Addition of nano silica is found to significantly improve compressive strength of concrete containing 25% RCA at all ages; however, no such improvement is observed in concrete containing 50% RCA [11]. The better efficiency of ITZ due to presoaking (for 24 h) in nanosilica incorporated RAC (RAC replacement of 25% and 50% of NAC) has been experimentally verified through compressive strength, durability properties and microstructure analysis [12]. Treatment with two different nanoslurries, resulted in macroscale property improvement of both RA as well as RAC. The width of the new ITZs reduced in modified RAC and the elastic modulus obtained from nanoindentation tests proved to improve significantly [13]. The effects of nano-silica and nano-limestone on the crack propagation and microstructure properties of RAC have been investigated using SEM and MIP. Nanosilica is found to be more effective in improving the microstructure as well as reducing the water absorption (20%) and porosity (24%) in comparison to RAC [14]. Moreover, studies were done on surface treatment of recycled aggregate using nanoslurry (nSi + nCa) and the nanoindentation results reveal enhanced new ITZs between old and new cement mortars as well as surface strengthening of old mortars. Furthermore, the macroscale properties such as compressive strength and resistance to chloride diffusion were also improved [15]. On the contrary, to enhance the property of RA/RAC on the microscale, biodeposition technique has been exploited by researchers to strengthen the bonding of adhered mortar on RAC. Ureolytic bacteria, S. pasteurii modified RA of particle size 6/8 mm showed reduction in water absorption of 14–20% and 13% decrease for particle size 12/16 mm when a bacteria concentration of 107–108 cells/ml was applied [16]. Enhanced calcite precipitation on RA by increasing factors like bacterial (S. pasteurii) concentration, temperature, pH have been investigated and water reduction of 16% was observed in treated RA when application conditions were 35 °C, pH 9.5, 16.8 g/l of calcium content and108 cells/ ml bacterial concentration [17]. Two times immersion treatment with B. spahericus for surface treatment of two different types of RA [18], proved the efficiency of MICP in improving the properties of RA/RAC by biogenic calcite deposition. The treated RA using 108 cells/ml showed an improvement in compressive strength of crushed concrete aggregate (CA) and mixed aggregate (MA) by 40%and 16%; reduction in water absorption by 27% for CA concrete, 20% for MA concrete. After using B. subtilis (106 cells/ml) treated RA, the compressive strength found to increase by 20% and a substantial decrease in capillary water absorption as well as drying shrinkage has been observed. The reason of this enhancement in durability properties is proved by morphological analysis which showed denser RAC formed by bacterial activity [19]. ITZ is relatively porous and contains voids and therefore it is attributed as the main reason for higher porosity than the other two components in concrete namely, cement paste and aggregate. Since, bacteria and nanomaterial mediated microstructure modification at ITZ in both RA and RAC is not studied in depth through electron microscopy till date, this area needs further exploration. In the present work, efforts are therefore, made to explicitly focus on the microstructure changes at ITZ of RA and RAC, densified by

bio and nano material. A comparative study of ITZ of bio and nano treated and untreated RA and RAC is done. Moreover, the consequence of soaked and unsoaked RA on the water absorption, volume of voids and microstructure properties is also investigated. Thus in this study, the direct relation of microstructure modification at ITZ is established with the property improvement of RA/ RAC. The past work on bacteria modified RA/RAC were done using ureolytic bacteria, namely, B. pasteurii and B. sphaericus. An attempt is therefore taken in the present work to study the effect of non-ureolytic bacteria (B. cohnii) in RA/RAC. Moreover, comparative study with ureolytic bacteria (B. megaterium) is done so as to identify the one having better biomineralization potential. 2. Experimental programme 2.1. Materials 2.1.1. Cement The ordinary Portland cement (OPC) with Blaine fineness 390 m2/kg, confirming to IS 8112:1989 [20,21] was used for the present study and its physical properties and chemical composition are shown in Table 1. 2.1.2. Fine and coarse aggregate Local river sand with 4.75 mm maximum size was used for concrete studies and for coarse aggregates, crushed stone with maximum 16 mm graded aggregates (nominal size) was used. The physical characterization of aggregates, shown in Table 2, was carried out in accordance with IS 2386:1963 [22,23] and was found to be satisfying the criteria of IS 383:1970 [24,25]. 2.1.3. Recycled aggregates Recycled aggregates (RA) were obtained from IL & FS, Burari, India. The RA was screened by a 20-mm sieve to exclude any big particles. The properties of RA used in this research were determined as per ASTM C127-12 [26] and are given in Table 2.

Table 1 Chemical and Physical properties of ordinary Portland cement (OPC). Composition

Mass %

SiO2 Al2O3 Fe2O3 CaO MgO Alkalis (K2O+Na2O) Others Loss of ignition Physical Properties

19.3 5.8 5.0 64.3 0.8 0.9 1.5 4.0

Specific gravity Fineness (m2/kg) Consistency (%) Initial Setting time (min) Final Setting time (min)

3.13 315 27 170 240

Table 2 Physical properties of aggregates used in this study. Property

Fine aggregate

Coarse aggregate

Recycled aggregate

Water absorption (%) Specific gravity Fineness modulus Grading zone Crushing value (%) Impact value (%)

0.57 2.57 2.98 Zone-II – –

0.65 2.62 7.34 – 20.06 20.32

1.4 2.2 7.52 – 26.51 26.89

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2.1.4. Bacteria and culture media For the present study two different bacterial strains, calcite precipitating ureolytic bacteria, B. megaterium and non-ureolytic bacteria B. cohnii were procured from Microbial Type Culture Collection (MTCC), Chandigarh, India. Non-ureolytic and ureolytic bacteria were cultured in nutrient broth, incubated aerobically in a shaker incubator at 30 °C and 37 °C at 120 rpm for 24 and 48 h respectively. Bacteria were incubated until culture obtains an optical density 1.5 at 600 nm wavelength. 2.1.5. Nanosilica The powdered nanosilica (NS) having particle size 30–70 nm, is amorphous in nature with 99.8% SiO2. As reported earlier, the density and specific surface area of NS are 1.4 g/cc and 116 m2/g respectively [27]. 2.2. Methods The present studies were carried out in two stages wherein the first part of the study aims to show the modification effect of bacteria induced mineral precipitation and nanosilica on recycled aggregate, immersed for a period of 7 and 14 days in bacterial culture having 107 cells/ml and 3% nanosilica to determine the time period at which optimum deposition occurs. In the second part of the study, M20 and M40 concrete discs were cast using recycled aggregate. Two different approaches were employed for concrete casting namely, soaking approach wherein, 14 days bacteria/nanosilica soaked RA was used and direct mixing approach in which, the bacterial culture/nanosilica were added at the time of concrete casting. Moreover, the effect of two different bacillus species (Ureolytic and Non-Ureolytic) was investigated on recycled aggregates for comparative study.

crete. The 3% NS content used in fifth series in this study was based on a recent study on the effect of NS on the compressive strength development of concretes, where 3% NS was found as an optimum content. Similar series of mixture proportion were cast for M40 mix design. To study the effect of soaking, the RA were soaked in bacterial culture (ureolytic/non-ureolytic) and nanosilica solution for 14 days and these soaked RA were used for casting of M20 and M40 concrete mix design. These two mixes constitute, the sixth, seventh and eighth series of concrete. Cement, fine aggregate and coarse aggregate were taken in the ratio of 1:2:3.13 and 1:1.58:2.6 in M20 and M40 mix design respectively. Water to cement ratio w/c = 0.55, was used to cast concrete discs having a diameter of 100 mm and height 50 mm. Discs were prepared and stored in the manner described in IS 4031: Part 6 (1988) [30,31]. After curing for 28 days, the concrete discs were tested for water absorption, bulk density, apparent density and volume of permeable voids (ASTM C642-13) [32]. 2.2.3. Characteristic techniques: FESEM-EDX analysis The morphological features and mineralogical composition of RA, bio and nano-modified RA, NAC, RAC, bio and nano-modified RAC with and without soaking were analyzed using Field Emission Scanning Electron Microscopy (FESEM - make: TESCAN and model: MIRA 3) and Energy-dispersive X-ray spectroscopy (EDX- model Xflash detector 5010, Bruker, Nano GmbH, Germany). The analysis was done principally for studying the modification effect of bio and nano mediated deposition on Interface Transition Zone (ITZ) of RA/RAC. Samples were gold coated with a JFC-1200 fine coater prior to examination. 3. Results and discussion 3.1. Water absorption and specific gravity of aggregate

2.2.1. Water absorption and specific gravity Water absorption and specific gravity factors of natural aggregate, recycled aggregate and bio and nano modified recycled aggregates were determined according to IS 2386 (Part III) – 1963 [28], as these are the key features to determine physical characteristic of aggregates. According to Appendix A of ACI 211.2-98 [29], the specific gravity index of bio-modified and nano-modified recycled aggregates was also determined to check the changes in the specific gravity of RA due to bacterial and nanosilica mediated filler effect. 2.2.2. Mixture proportion of concrete A total of eight series of mixes for two mix designs, namely M20 and M40 were considered in this study. The first series was a control mix contained 100% natural aggregate (NA). The second series was similar to the first series in every aspect, but in this series 100% RA was used instead of NA. The third and fourth series contained 100% RA, wherein non-ureolytic and ureolytic bacterial culture, having 107cells/ml was incorporated during casting of concrete. In the fifth series, 3% NS was premixed with water in ultrasonic mixer and then added directly during mixing of con-

The results of study on water absorption of NA, RA, bio and nano modified RA is presented in Table 3. It was observed that with increase in number of days the water absorption of NA & RA increased, on the contrary, in the bio and nano modified RA, the water absorption is found to decrease substantially. The optimum deposition was observed at a period of 14 days. The percentage decrease in water absorption was found to be 42.9% and 64.3% in Non-Ureolytic (B. cohnii) and Ureolytic bacteria (B. megaterium) modified RA respectively. Meanwhile previous studies show 13% decrease in water absorption for particle size 12/16 mm when bacterial (B. pasteurii) concentration of 107–108 cells/ml was applied [16]. Nano modified RA was found to improve the water absorption of RA, with a percentage decrease of 21.4%. The water absorption potential of recycled aggregate gets appreciably decreased after incorporation of bio and nano material. This observation suggests the effectiveness of bio and nano materials, which acts at micro and nano levels, consequently reduce the pore spaces of recycled aggregates, leading to a decrease in water absorption. Therefore, such treatment method(s) enhance the durability of RA, making it an appropriate replacement of natural aggregate.

Table 3 Water absorption and Specific Gravity of NA,RA, bio/nano modified RA after 7days & 14 days soaking. Water Absorption

Specific Gravity

Apparent Sp. Gravity

Aggregate Type

7 days

14 days

14 days

14 days

Natural aggregate Recycled aggregate Recycled aggregate+ 3% Nanosilica Recycled aggregate+ Ureolytic bacteria Recycled aggregate+ Non-ureolytic bacteria

0.8 1.4 1.1 0.9 0.9

0.8 1.4 0.9 0.5 0.5

2.62 2.22 2.70 2.87 2.89

2.64 2.28 2.79 2.96 2.98

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The effect of biomineralization potential of non-ureolytic/ ureolytic bacteria and nanosilica in terms of specific gravity and apparent specific gravity of RA is shown in Table 3. After 14 days immersion period, the specific gravity index of non-ureolytic and ureolytic bacteria modified RA was 29.3% and 30.2% respectively. For the same parameter, the nano-modified RA showed an increase of 18.4%. The bio and nano modification method of RA resulted in the significant densification of adhered old mortar in treated RA, which leads to considerable increase in the specific gravity values. 3.2. Water absorption of concrete series Water absorption % was found to increase after boiling therefore, the comparison of all the results was done, taking the later as reference. Table 4 presents water absorption of M20 NAC, RAC and nano modified RAC (NRAC) and ureolytic/non-ureolytic bacteria modified RAC (URAC & NURAC) which indicates that NAC has lowest water absorption followed by bacteria and nanosilica soaked/ unsoaked RAC. The RAC was found to show maximum water absorption due to high porosity of adhered old mortar that facilitates water uptake. The bacteria and nanosilica observed to change the water absorption behavior of old mortar by accelerating the formation of hydration products thereby reducing the ingress of water through old mortar. Nanosilica soaked RAC, decreased water absorption by 6% whereas the unsoaked RAC decreased upto 2%, indicating better potential of soaking RA in nanosilica prior concrete casting. On the other hand, ureolytic bacteria soaked and unsoaked RAC reduced the water absorption by 22% and 9% respectively. In comparison to ureolytic bacteria, both non-ureolytic bacteria soaked and unsoaked RAC reduced the water absorption by 16%. The results of this study indicates that non-ureolytic bacteria (B. cohnii) shows better deposition, if incorporated at the time of casting whereas, ureolytic bacteria (B. mega-

terium) found to work efficiently after soaking RA into its culture for 14 days prior casting. Overall, the bacteria were found to appreciably decrease the water absorption by densifying the old mortar with mineral deposition. Similarly, Table 5 presents water absorption of M40 NAC, RAC and nano modified RAC and ureolytic/non-ureolytic bacteria modified RAC. As obvious, NAC showed minimum % water absorption; followed by bacteria and nanosilica soaked/ unsoaked RAC and RAC showed maximum water absorption. Nanosilica soaked RAC, decreased water absorption by 31% and the unsoaked RAC reduced the water absorption by 24%, indicating heightened effect of soaking RA in nanosilica. On the other hand, ureolytic bacteria soaked and unsoaked RAC reduced the water absorption by 18% and 12% respectively. In comparison to ureolytic bacterial strain, nonureolytic bacteria soaked and unsoaked RAC reduced the water absorption by 17% and 5%. The results thus indicates efficiency of ureolytic bacteria over non-ureolytic bacteria in M40 mixture proportion, though both the bacteria are calcifying in nature and showed appreciable reduction in water absorption in comparison to untreated RAC. The enhanced effect of soaking could be the result of better penetration of bacteria/ nanosilica into the adhered old mortar, filling the micro-pores and thereby strengthening the RA. 3.3. Density parameters of concrete series The densification of ITZ in concrete matrix is supposed to increase the bulk as well as apparent density. RA contains the adhered old mortar, when RAC is cast then it contains not only adhered old mortar but also the fresh new mortar, as a consequence of it, the bulk and apparent density values of RAC are relatively more in comparison to NAC. Table 4 presents density parameters of M20 NAC, RAC, nano modified RAC and ureolytic/

Table 4 Water absorption, bulk density, apparent density, volume of voids of NAC, RAC, nano modified RAC and ureolytic/non-ureolytic bacteria modified RAC (soaked /unsoaked). Tests Performed

Water Absorption after immersion (%) Water Absorption after immersion and boiling (%) Bulk density, dry (g/cc) Bulk density, after immersion (g/cc) Bulk density, after immersion and boiling (g/cc) Apparent density (g/cc) Volume of voids, %

M20 Concrete Samples NAC

RAC

NRAC w/o soaking

NRAC with soaking

URAC w/o soaking

URAC with soaking

NURAC w/o soaking

NURAC with soaking

4.97 5.34

5.98 7.40

5.93 7.26

5.92 6.93

5.96 6.72

5.06 5.81

5.94 6.23

5.22 6.18

1.61 1.69 1.70

1.62 1.72 1.74

1.64 1.74 1.76

1.65 1.75 1.77

1.62 1.72 1.73

1.64 1.72 1.74

1.64 1.74 1.74

1.65 1.74 1.76

1.77 8.62

1.84 12.01

1.86 11.93

1.86 11.44

1.82 10.91

1.81 9.54

1.83 10.23

1.84 10.22

Where, NAC – Natural aggregate concrete, RAC – Recycled aggregate concrete, NRAC – Nano modified recycled aggregate concrete, URAC – Ureolytic bacteria modified recycled aggregate concrete, NURAC – Non-ureolytic bacteria modified recycled aggregate concrete.

Table 5 Water absorption, bulk density, apparent density, volume of voids of NAC, RAC, nano modified RAC and ureolytic/non-ureolytic bacteria modified RAC (soaked /unsoaked). Tests Performed

Water Absorption after immersion (%) Water Absorption after immersion and boiling (%) Bulk density, dry (g/cc) Bulk density, after immersion (g/cc) Bulk density, after immersion and boiling (g/cc) Apparent density (g/cc) Volume of voids, %

M40 Concrete Samples NAC

RAC

NRAC w/o soaking

NRAC with soaking

URAC w/o soaking

URAC with soaking

NURAC w/o soaking

NURAC with soaking

6.46 7.06 1.62 1.73 1.74 1.83 11.45

7.83 8.18 1.63 1.76 1.76 1.88 13.33

5.12 6.19 1.67 1.76 1.78 1.87 10.36

5.39 5.62 1.69 1.78 1.78 1.87 9.49

6.16 7.22 1.62 1.72 1.74 1.84 11.71

5.53 6.70 1.64 1.73 1.75 1.84 10.97

6.45 7.76 1.61 1.71 1.73 1.84 12.48

5.78 6.80 1.65 1.75 1.77 1.86 11.24

Where, NAC – Natural aggregate concrete, RAC – Recycled aggregate concrete, NRAC – Nano modified recycled aggregate concrete, URAC – Ureolytic bacteria modified recycled aggregate concrete, NURAC – Non-ureolytic bacteria modified recycled aggregate concrete

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cps/eV (a)

12 10 O 8

Mg Al Si

Ca

Ca

6 4 2 0

2 keV

1

3

4

cps/eV 18

(b) 16

14

12

10

8

O

Si

Ca

Ca 6

4

2

0

2 keV

1

(c)

4

3

4

cps/eV

O Si

3 Ca

Ca

2

1

0 1

keV

2

3

4

Fig. 1. FESEM and EDX showing ITZ of (a) Recycled Aggregate, (b) Nanosilica modified Recycled Aggregate (c) Non-Ureolytic Bacteria modified Recycled Aggregate (d) Ureolytic Bacteria modified Recycled Aggregate.

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cps/eV (d)

6

4

(d)

O

Al Si

Ca

Ca

2

0

keV 2

1

3

4

Fig. 1 (continued)

cps/eV (a)

(b)

6

O Ca

Si

Ca

Aggregate

4

2

0 1

keV2

3

4

cps/eV (b)

Aggregate

12 10

Old ITZ

Ca O

Al Si

Ca

8 6 New ITZ

Old Mortar New Mortar

4 2 0 1

2keV

3

Fig. 2. FESEM and EDX of ITZ in (a) Natural Aggregate Concrete (b) Recycled Aggregate Concrete.

4

223


cps/eV 14

(a) 12

10

8 O

Si

Ca

Ca 6

4

2

0 1

keV

2

3

4

cps/eV (b)

12

Old ITZ 10

Aggregate 8

O

Si

Ca

Ca

Old Mortar 6

New Mortar 4

New ITZ 2

0 1

2 keV

3

4

Fig. 3. FESEM and EDX of ITZ in (a) Unsoaked Nano modified RAC (b) Soaked Nano modified RAC.

non-ureolytic bacteria modified RAC which indicates that NAC has lowest bulk as well as apparent density followed by RAC. Bacteria and nanosilica soaked/ unsoaked RAC was found to show maximum density values attributed to greater compaction in concrete matrix, as a result of its densification brought about by biocementation activity or accelerated formation of hydration products by nanosilica. Though the increase in terms of % of density is not very high, but it is comparatively more than NAC/RAC. Similar results were observed in M40 concrete series shown in Table 5. It is also noticed that, the nano and bacteria soaked RAC shows relatively high density values than unsoaked specimens, indicating the effect of soaking. Although, the difference in density parameters of treated and untreated RAC is not very pronounced but slight increase (2%) in bio and nano modified RAC can be seen. 3.4 Volume. of voids (%) in concrete series Bacteria/nano material work at micro/nano level respectively due to which pores in the concrete matrix get filled with hydrated products, making the microstructure compact. The overall volume of voids become very low and therefore, the porosity get reduced

appreciably. Table 4 presents % volume of voids in M20 and Table 5 shows % volume of voids in M40 of NAC, RAC and nano modified RAC and ureolytic/non-ureolytic bacteria modified RAC. The M20 concrete series results shows minimum voids % in NAC, followed by bio and nano modified RAC. It was observed that volume of voids (%) was highest in RAC, due to the porous/less compact adhered old mortar. Moreover, the bio and nano soaked RAC showed significantly less volume of voids (%) in comparison to un-soaked specimens. In nano modified RAC, the % voids reduced by 5% in M20 specimens and 29% in M40 specimens in comparison to untreated RAC. On the other hand, non-ureolytic bacteria soaked RAC (M20) showed 15% less volume of voids and M40 specimens of same series showed 16% reduction in volume of voids. The results shows better efficiency of ureolytic bacteria, with 21% reduction in soaked RAC (M20) and 18% reduction in case of M40 ureolytic bacteria soaked RAC. Thus, ureolytic bacteria (B. megaterium) observed to show better calcification potential. The bacterial cells serve as nucleation sites for calcite precipitation. The cell wall of bacteria carry negative surface charge (zeta potential), due to which positively charged calcium ions gets attracted and bind to it. The negatively charged carbonate ions form a precipitate on calcium ion carrying bacterial cell wall, as a

224


tent in Nano modified RA. On the other hand, bacterial incorporation is found to enhance calcium content in the microstructure due to the formation of calcium carbonate crystals at ITZ.

consequence of this phenomenon; the voids at ITZ get occupied with mineral deposition. This indicates physiochemical cell surface interactions are involved in the process of mineral precipitation and major reason behind bacteria mediated filler effect. On the other hand, silica nanoparticles reacts with Ca(OH)2 to produce more quantity of C–S–H gel which fills in the voids present in ITZ, the porosity is reduced by the incorporation of NS.

3.5.2. FESEM-EDX analysis of RAC Fig. 2 shows the microstructure and surface morphology of concrete wherein the distinct ITZ of NAC having a width of 580 nm was observed. On the contrary, RAC shows a width of 170 nm between aggregate and old mortar. In bio and nano soaked concrete series, the thick deposition of additional hydration products seems to merge the adhered old mortar with aggregate as shown in Figs. 3, 4 and 5. However, the unsoaked RAC also shows the calcite/ hydration product deposition across the width of ITZ, but the pronounced effect of soaking was visualized. Thus, the microstructural densification in soaked RAC validate the results of parameter (s) discussed earlier. The deposition on ITZ of bio and nano modified RAC was focused and EDX showed enhanced calcium and silica %. From the morphology of the RAC, the existence of two ITZs in RAC, namely the old ITZ between aggregate and adhered old

3.5. Characteristic techniques: FESEM-EDX analysis 3.5.1. FESEM-EDX analysis of RA The effect of soaking RA for a period of 14 days in bio and nano material on ITZ and corresponding EDX mapping are shown in Fig. 1. The result shows a clear demarcation of ITZ between the adhered old mortar and aggregate, whilst in bio and nano modified RA, a thick deposition of calcite/nanosilica was observed at ITZ. The bio and nano mediated deposition at ITZ seems to densify the adhered old mortar, thereby decreasing the water absorption and enhancing the specific gravity of recycled aggregate. The elemental composition from the EDX analysis indicates enhanced silica con-

cps/eV (a)

12 10

O

Al Si

Ca

Ca 8 6 4 2 0 1

2

keV

3

4

cps/eV (b)

8

6

O Ca

Si

Ca

4

2

0 1

2

keV 3

4

Fig. 4. FESEM and EDX of ITZ in (a) Unsoaked Non– Ureolytic bacteria modified RAC (b) Soaked Non– Ureolytic bacteria modified RAC.

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cps/eV 10

New ITZ

O

8

Aggregate

Si

Ca

Ca

6

Old ITZ Old Mortar

4

New Mortar

2

0 2 keV

1

3

4

cps/eV

6 O

(b)

4

Al Mg

Ca

Si

Ca

2

0 1

2

keV

3

4

Fig. 5. FESEM and EDX of ITZ in (a) Unsoaked Ureolytic bacteria modified RAC (b) Soaked Ureolytic bacteria modified RAC.

mortar and the new ITZ between adhered old mortar and new mortar was observed [5]. After effects of strengthening of RA by bio and nano materials were clearly visible on both old and new ITZs of concrete cast with modified RA. The microstructure reveals the improvement of both old and new ITZ and thick bio/nano mediated deposition throughout the length and width of ITZ was observed. The micrograph shows bio-mineralization/hydration products at old ITZ and new ITZ, i.e old adhered mortar and new mortar seem to integrate/coalesce. Meanwhile it is reported through nanoindentation that RAs’ soaking in nanoslurries, resulted in strengthening the adhered old mortar [13].From the present study it is verified that the macro property improvement is a consequence of old as well as new ITZs consolidation, due to bio-calcification/ accelerated hydration phenomenon. Thus, from the results, a distinct relation between the micro properties and macro properties can be deduced. 4. Conclusion This paper studies surface treatment of RA with bio and nano materials as well as effect of such modified RA on concrete properties. The previous work done on RA modification focused

on generally used ureolytic species (B. pasteruii and B. sphaericus) but in the present study, RA was improved using two different bacillus species, namely B. cohnii (non-ureolytic sp.) and B. megaterium (ureolytic sp.).The efficiency of bacterial treatment was evaluated by changes in parameters like water absorption and specific gravity using soaking approach. Soaking of RA in 107 cells/ml of bacterial culture for a period of14 days, significantly reduces the water absorption of RA by 42.9% in non-ureolytic and 64.3% in ureolytic bacteria. The reduction in water absorption is attributed to bacteria mediated calcification on its cell wall (biodeposition), which consecutively acts as a barrier in ingress of water or other ions. The specific gravity index of non-ureolytic and ureolytic bacteria soaked RA was 29.3% and 30.2%, indicating the filler effect on the surface and inside the pores of RA, due to bacteria induced calcite precipitation. The consolidating effect of bacteria on RAC results in reduction of water absorption and relative increase in density (bulk as well as apparent). The soaking approach not only reduced the water absorption of RA but also found effective when such soaked RA was cast in concrete. It reduced the water absorption of ureolytic and non-ureolytic bacteria soaked concrete specimens by 22% and 15% (M20 concrete specimens) and 18% in M40 concrete

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specimens. Moreover, the bacteria soaked RAC showed significantly less volume of voids (%) in comparison to unsoaked specimens. Non-ureolytic bacteria soaked RAC (M40) showed 16% reduction in volume of voids whereas ureolytic bacteria soaked RAC of same concrete design showed18% reduction in comparison to untreated RAC. Thus, both the bacterial species seems to improve the performance of RA but amongst the two, the ureolytic species of bacillus (B. megaterium) found to show better calcification potential. Likewise, nanosilica modified RA showed 21.4% reduction in water absorption and specific gravity index of 18.4% indicating deposition of additional hydration products within or on the surface of RA, which leads to decrease in uptake of water and increase in specific gravity due to densification of RA. In 3% nanosilica soaked RAC, a decrease in water absorption by 6% (in M20) and 31% (in M40) was observed. Furthermore, a considerable reduction in % voids by 29% (M40 concrete specimens) was found in comparison to untreated RAC. The microstructural study reveals the modification at both ITZs (old and new) wherein the bio and nano mediated deposition fills the pores of adhered old mortar and also result in integration of old with new mortar, giving rise to a homogeneous RAC matrix. The mineralogical study carried out through EDX verified the results of FESEM i.e enhanced weight % calcium and silica in bacteria and nanosilica treated RAC respectively. The calcite/ accelerated hydration products deposition decreases the permeability of modified concrete (RAC), which could restrict the progression of water or other substances across the concrete matrix, thereby increasing the durability of RAC. Over all, the soaking approach can be adopted over direct mixing approach for improved micro and macroproperties of bio and nano modified RAC. It was observed that application of bio/nano soaked RA in concrete, not only improve physical properties of RAC but also enhance workability which decrease the demand of water or eliminates the need of adding superplastisizer in concrete. Amongst the two mixture proportion considered for the studies, bio/nano modified M40 concrete mix was found to be more workable and showed pronounced effect in terms of durability properties. On the basis of the outcome of the present work, there is a strong probability for a greater utilization of bio and nano modified RA in low and medium strength concrete. Moreover, the bacteria and nanosilica used for improving RA are environmentally safe and energy efficient solution over heat treatment or chemical treatment method(s), generally used for RA modification. In addition, the utilization of modified construction and demolition waste may address the issues of depletion of natural resources and space required for landfill disposal, directly reducing the impact of waste material on environment.

5. Conflict of interest None.

Acknowledgements The authors gratefully acknowledge the financial support of the CSIR- Central Building Research Institute, Roorkee and Uttarakhand Council for Biotechnology, Uttarakhand, India. Any opinions, findings and conclusions or recommendations expressed in this material are those of the writer(s) and do not necessarily reflect the views of the funding bodies.

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