Enhancement of strength and durability of fly ash concrete in seawater environments: Synergistic effect of nanoparticles

Construction and Building Materials 187 (2018) 448–459

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

Enhancement of strength and durability of fly ash concrete in seawater environments: Synergistic effect of nanoparticles Sudha Uthaman a, Vinita Vishwakarma a,⇑,1, R.P. George b,1, D. Ramachandran a, Kalpana Kumari c, R. Preetha c,1, M. Premila d, R. Rajaraman d, U. Kamachi Mudali e a

Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology, Chennai 600 119, India Corrosion Science and Technology Division, IGCAR, Kalpakkam 603 102, India Civil Engineering Group, IGCAR, Kalpakkam 603 102, India d Materials Science Group, IGCAR, Kalpakkam 603 102, India e Heavy Water Board, Mumbai 400 094, India b c

h i g h l i g h t s
Fly ash concrete specimens were fabricated using TiO2 and CaCO3 nanoparticles.
Year-long studies in seawater showed synergistic effect nanoparticles.
Nano-TiO2 showed faster hydration with more filler effects and antibacterial.
Nano-CaCO3 contributed to high pH and compressive strength.
1:1 ratio of nano TiO2 and CaCO3 emerged with superior concrete properties.

a r t i c l e

i n f o

Article history: Received 26 March 2018 Received in revised form 24 July 2018 Accepted 27 July 2018

Keywords: Fly ash Nanoparticles Strength Durability Antibacterial

a b s t r a c t Fly ash is used in concrete industry to reduce the amount of cement and to enhance the durability of concrete. Nuclear industry recently adopted fly ash concrete as a government initiative to reduce carbon footprint and also to provide an impermeable skin for the several structures in the cooling water system exposed to marine environment for enhanced durability. Earlier detailed studies on fly ash concrete has shown that despite its superiority over conventional concrete with respect to strength and durability, some concerns such as delayed setting and hydration, low early-age strength, and higher carbonation may pose a problem for its wider application. The present work is an attempt to overcome these deficiencies by nanophase modification of concrete with the incorporation of nano-titania and nano-calcium carbonate at 2% by weight of cement. Four different types of concrete mix were arrived and specimens of different sizes were cast in order to explore the various properties of concrete. After 28 days of curing in potable water, the samples were exposed to seawater at Nuclear Desalination Demonstration Plant (NDDP) sump at Kalpakkam and were withdrawn for testing at different ages like 56, 90, 180 and 365 days. Nanophase modification increased the pozzolanic activity resulting in faster hydration, early-age strength and long-term compressive and split tensile strength, permissible electrical resistivity for corrosion, lower chloride ion penetration, carbonation depth biofilm formation and higher internal pH. Among the different mixes, the synergy of 1% NC and 1% NT emerged superior with excellent concrete properties related to mechanical properties, pore structure, durability and optimum antibacterial activity. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Fly ash, a waste material from combustion of coal is used in concrete as a partial cement replacement to upgrade the strength ⇑ Corresponding author. 1

E-mail address: [email protected] (V. Vishwakarma). Equal contribution to this paper.

https://doi.org/10.1016/j.conbuildmat.2018.07.214 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

and durability by pozzolanic action and filler effects [1]. The desired properties can be achieved using high volume (>40%) of fly ash in concrete [2]. Nuclear industry has a new mandate to plan future nuclear power plants with a design life of 100 years to make nuclear power more economical [3]. Hence, the durability of concrete structures especially under marine exposure assumes great relevance. The sulphates and chlorides from the aggressive

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seawater environment can penetrate and deteriorate the concrete leading to its failure [4]. Many workers have reported that interaction of seawater with cement paste leads to carbonation, sulphate attack and chloride ion attack. It is reported that addition of mineral admixtures assures low permeability to aggressive anions and offer excellent durability for concrete structures in seawater [5]. Sandberg have studied the chloride binding of concrete exposed in a marine environment and reported that the transport of chloride ions in the concrete structure depends on the amount of alkali hydroxide [6]. Permeability is the most important property that determines the durability of concrete structures in seawater [7]. When a structure is located in a marine environment, chloride ions can penetrate the porous structure of concrete and reach the steel leading to deterioration [8]. The holistic study conducted by Lindvall [9] revealed that the concrete structures exposed to marine environment showed attachment of foulants on its surface and the extent of marine growth facilitate the ingress of chloride ion penetration. Nuclear Power Corporation of India Limited (NPCIL) has also recently adopted fly ash concrete as a government initiative to reduce carbon footprint and also to provide an impervious skin for the concrete structures [10]. Ramachandran et al have observed that flyash concrete exhibits superior strength and durability properties as compared to normal and modified concrete by superplasticizer in sea water environments [11]. However, their studies also brought into focus some drawbacks of fly ash concrete like slower hydration, low early age strength and higher carbonation depth [11,12]. The use of fly ash as a partial replacement of cement is reported to reduce the tensile strength of concrete [13]. Utilization of fly ash in concrete exposed to water involves the potential leaching of some elements into water. This creates a problem of secondary environmental pollution [14]. Recently, many researchers have started addition of different nanoparticles to improve physical and chemical properties of concrete structures [15–26]. It has been reported that nanoparticles can act just like fillers and densify the microstructure of concrete or they can also form heterogeneous nuclei for accelerating cement hydration; all contributing to reduction in porosity [24]. Several nanoparticles were incorporated by different researchers, SiO2 [15–18], Al2O3 [19,20], TiO2 [21], Fe2O3 [22] and CaCO3 [23,24] in the range of 0.5–5% of cement replacement which enhanced the strength and other properties of the modified concrete compared to conventional concrete without nanoparticles. It is found that SiO2 nanoparticles up to 4% by wt. of cement acted as fillers and also improved pore structure by decreasing harmful pores [16]. Ali Nazari et al., has also conducted an extensive study on the effect of nano-Al2O3 and found that the 2% nano-Al2O3 particles blended concrete, emerged with significantly higher compressive strength [20]. The inclusion of 1% nano-Fe2O3 into the concrete matrix at the early ages increased the percentage of water absorption and later the mechanical strength [22]. The increased rate of water absorption increases the number of capillary pores in concrete which in turn reduces the desired performance of concrete [27]. Jayapalan et al. [25], investigated the effect of chemically nonreactive anatase TiO2 nanoparticles on early-age hydration of cement and found that addition of 5% TiO2 accelerated the rate of

cement hydration by the heterogeneous nucleation effect. The addition of TiO2 to cement is increased the heat of hydration, accelerated the rate of reaction at early stages of hydration and enhanced the antimicrobial activity by the destructing the microbes [26]. The CaCO3 nanoparticles was first considered as only filler in cement to replace OPC, but later showed positive effects in terms of strength and acceleration of hydration rate [28]. All these studies refer to the nanoparticle modification conventional concrete (100% OPC). The main objective of this work was to overcome the drawbacks of fly ash concrete by nanophase modification. In the fly ash concrete containing 40 wt% fly ash and 60 wt% OPC, 2 wt% of OPC is replaced using nano-titania and nano-calcium carbonate individually and in combination. Based on our earlier screening studies reported elsewhere [29] where a nanophase replacement of OPC was studied in the range of 0.5–3 wt%, it was found that 2 wt% replacement of OPC is the optimum level of nanoparticle substitution. Optimum workability and strength, very low RCPT values and lowest reduction of pH was achieved with 2 wt% nanoparticle replacement. Nazari et al. [30] and Xu et al. [31] have also reported that enhancement of concrete properties by 2 wt% of CaCO3 TiO2 nanoparticle modification respectively. Hence it was decided to select 2 wt% substitutions for the long-term exposure studies in seawater environment. The various concrete properties related to strength, durability and antibacterial characteristics were evaluated by casting specimens of different shape and sizes for all the four types of concrete mixes. The specimens were cured in potable water at laboratory conditions for 28 days and thereafter exposed to seawater for a year. Advanced characterization tools like SEM, XRD and IR spectroscopy were used to get an insight into the role of nanoparticles in improving the integrity of concrete. 2. Experimental methods and materials 2.1. Materials and mix proportion Ordinary Portland Cement (43 Grade) conforming to IS 8112 – 1989 [32] was used in this study. Black granite (Hard Blue Granite Rock Aggregate - Machine Crushed) was used as coarse aggregate, with maximum size of 20 mm and 12.5 mm meeting the requirements of IS 383 [33]. High range water reducer type superplasticizer containing sulphonated naphthalene formaldehyde as base was used as chemical admixture. Crushed sand and river sand with a maximum size of 4.75 mm and meeting zone II requirements were used as fine aggregate. The partial replacement of cement was 40% by siliceous type fly ash conforming to IS 3812 [34]. The commercial grade anatase phase titanium dioxide (TiO2) and calcium carbonate (CaCO3) with size ranging from 100 to 600 nm was purchased. To attain nanosized particles, TiO2 and CaCO3 powders were ground using ball mill (SPEX 8000M-230 dual mixer mill, 230 V/50 Hz) to achieve the size of the particles within the range of 50–80 nm. The total powder content which includes cement, fly ash and nanoparticles for all the mixes was 375 kg/m3 and OPC was replaced with fly ash (40% by wt. of cement). The nanoparticles were replaced at 2 wt% by cement. Concrete cube specimens of 150 mm size were used for compressive strength test and cylindrical specimen of 100 mm diameter and 200 mm height were used for RCPT and split tensile strength test. Resistivity analysis was done on prism specimens of 350 mm length and 100 mm depth with a thermo mechanically treated (TMT) rebar of 10 mm diameter embedded in it. Mortar specimens of 35 mm diameter and 10 mm thick were prepared to visualize the biofilms under epifluorescence microscope and 100 mm cube mortar specimens were cast to perform carbonation studies. The mix proportion and casting details of the specimens are given in Table 1.

Table 1 Mix proportion of concrete (kg/m3). Sample designation

FA FAT FAC FATC

Cement

225.0 220.5 220.5 220.5

Fly ash

150

Fine aggregates River sand

Crushed sand

549

235

Coarse aggregates

Superplasticizer

Nano-TiO2

Nano-CaCO3

1130

4.5 4.8 4.8 4.8

– 4.5 – 2.25

– – 4.5 2.25

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S. Uthaman et al. / Construction and Building Materials 187 (2018) 448–459 2.4. Durability properties

2.2. Specimen preparation Four different types of concrete specimens were fabricated and named viz. 1. Fly ash concrete-FA (60 wt% OPC + 40 wt% fly ash) 2. Fly ash concrete with nano-TiO2 in which nanoparticles are replaced with 2 wt% of OPC among 60 wt% (58 wt% OPC + 40 wt% fly ash + 2 wt% nano-TiO2) 3. Fly ash concrete with nano-CaCO3 in which nanoparticles are replaced with 2 wt% of OPC among 60 wt% (58 wt% OPC + 40 wt% fly ash + 2 wt% nano-CaCO3) 4. Fly ash concrete with 1:1 ratio of nano-TiO2 and nano-CaCO3 in which nanoparticles are replaced with 2 wt% of OPC among 60 wt% (58 wt% OPC + 40 wt% fly ash + 1 wt% nano-TiO2 and nano-CaCO3) The fabricated specimens were demoulded after 24 h of casting and cured for 28 days in a curing tank at the concrete laboratory, Civil Engineering Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam under ambient conditions (temperature of 27 ± 2 °C). After curing for 28 days, all the specimens were exposed to sea water at Nuclear Desalination Demonstration Project (NDDP) sump and withdrawn at 56, 90, 180 and 365 days of casting. The concrete specimens were used for thermogravimetry, pH measurement, compressive and split tensile strength tests, chloride ion penetration test and microstructural characterization analysis. For epifluorescence study, the cylindrical mortar specimens were withdrawn at monthly frequency for a period of one year. The 100 mm cube mortar specimens were withdrawn at 1 and 6 months of exposure to marine atmosphere for carbonation test.

2.3. Characterization of concrete properties 2.3.1. Thermogravimetry and differential thermal (TG-DTA) analysis Thermogravimetry and differential thermal (TG-DTA) analysis [35] was done for 28 days cured concrete specimens using Thermal Analysis SDT Q600 (USA make). In this TGA test, proper weight loss was measured while the specimens were gradually subjected to increasing temperatures. Specimens were broken into pieces and ground into fine powder using agate mortar and pestle. The powder samples were dried at 100 °C in oven and cooled at room temperature and kept in desiccator before TG-DTA analysis. The powder sample of FA (17.67 mg), FAC (21.45 mg), FAT (22.91 mg) and FATC (13.06 mg) were taken in a ceramic crucible and Al2O3 powder was used as reference material. The experiment was performed by heating the samples from room temperature to 1000 °C at the heating rate of 10 °C per minute under nitrogen gas dynamic atmosphere.

2.3.1.1. Calcium hydroxide content. The amount of calcium hydroxide (CH) was calculated directly from the TG curves following the equation [36].

CH ð%Þ ¼ WLðCHÞ 

MWðCHÞ MWðHÞ

ð1Þ

where WL(CH) represents to the weight loss at the temperature range of 400–500 °C attributable to CH dehydration and MW(CH) is the molecular weight of CH (74 g/mol) and MW(H) is the molecular weight of water (18 g/mol).

2.3.1.2. Calcium carbonate content. The amount of calcium carbonate was calculated directly from the TG curves following the equation [37].

CaCO3 ð%Þ ¼ WLðCaCO3 Þ 

MWðCaCO3 Þ MWðCO2 Þ

ð2Þ

where WL(CaCO3) represents to the weight loss at the temperature range of 501–900 °C attributable to decarbonation of CaCO3 and MW(CaCO3) is the molecular weight of CaCO3 (100.0869 g/mol) and MW(CO2) is the molecular weight of CO2 (44.01 g/mol).

2.3.2. Crushed pH measurement The crushed pH values of 28 days cured and sea water exposed (56, 90, 180 and 365 days) concrete specimens were measured. The specimens of FA, FAT, FAC and FATC were crushed and powdered. 5 g of finely ground concrete powder was dissolved in 50 ml of distilled water and the suspension was mixed thoroughly for 15 min using a magnetic stirrer [38]. The pH of this suspension was measured by portable flat surface electrode pH meter (SENTIX 3110, WTW GmbH). Minimum three readings were measured to get the average value of the results.

2.3.3. Mechanical properties The concrete specimens were withdrawn at different ages (56, 90, 180 and 365 days) of exposure in seawater and the various properties were analyzed. The compressive strength test (IS 516) [39] and split tensile strength test (IS 5816) [40] were performed using Servo Controlled Automatic Compression Testing Machine (AIMIL make) of 3000 kN capacity. Three specimens from each mix were analyzed to get an average value of the results.

2.4.1. Rapid chloride penetration test (RCPT) RCPT was conducted by using 8 cells RCPT Apparatus-DAQ system (ASTM C 1202-97) [41] for the 28 days cured and sea water exposed (56, 90, 180 and 365 days) concrete specimens. A cylindrical specimen (200  100 mm) was typically cut as a slice (50  100 mm) and was used for this test. The RCPT apparatus consists of two reservoirs. The specimen was fixed between two reservoirs using an epoxy bonding agent to make the test setup leak proof. One reservoir (connected to the positive terminal of the DC source) was filled with 0.3 N NaOH solution and the other reservoir (connected to the negative terminal of the DC source) with 3% NaCl solution. The terminals were then connected to the 60 V DC power supply and the current reading in mA was recorded for every half an hour upto 6 hrs. The total charge passed during this period was calculated in terms of coulombs using the trapezoidal rule as given in ASTM C 1202-97 [41]. 2.4.2. Resistivity analysis The resistivity test was conducted on 28 days cured and sea water exposed concrete prism samples. The resistivity test was carried out to know the electrical resistance of the concrete [42]. This test was used to assess the probability or likelihood of corrosion of the reinforcement bar. The equipment used for this test was a battery operated, portable, four probe device which measures concrete resistivity. The set of four probes were fitted with super conductive foam tips to ensure full contact on irregular surfaces. The probes were kept in contact with the concrete surface and the LCD display indicated the resistivity value directly on the screen. 2.4.3. Carbonation test The carbonation test was done for all the four types of mortar specimens exposed to marine atmosphere for 1 and 6 months. After exposure, the mortar specimens were cut into two halves and sprayed with 1% phenolphthalein in 70% ethyl alcohol solution to a freshly fractured surface of concrete [43]. Researchers have reported that the dark aggregates in concrete creates an obstacle in the measurement of carbonation depth by making a visual illusion, when sprayed over by the phenolphthalein solution in the carbonation affected areas [44]. Hence, in order to assess the depth of carbonation, mortar specimens were used as it does not contain aggregates [45,46]. Non-carbonated areas turn purple while carbonated areas remain colorless. Then the depth of penetration was measured using Vernier caliper (Mityuto Digital Vernier Caliper). 2.5. Microstructural properties 2.5.1. X-ray diffraction studies The powder samples of 28 days cured and 365 days sea water exposed FA, FAT, FAC and FATC specimens were used for XRD analysis. The analysis was carried out using Inel – EQUINOX 2000 diffractometer with a X-ray source of Co- Ka radiation (k = 1.7889 Å) over a 2 h range of 10°–80° at a scan step size of 0.05°. The unknown crystalline compounds were analyzed and identified by Brag Brentano method. The standard JCPDS database for XRD pattern was used for crystalline phases and quantification of cementitious materials [47]. 2.5.2. Scanning electron microscope (SEM) studies The SEM analysis of 28 days cured and 365 days sea water exposed FA, FAT, FAC and FATC specimens was carried out for comparative morphological studies. All the specimens were coated with gold for electrical conduction for SEM imaging [48]. The specimens were examined under Desktop mini SEM, Korea to observe the morphological changes. 2.5.3. Infrared spectroscopy (IR) studies The concrete specimens cured for 28 days and exposed to seawater for 365 days are used for this study. IR measurements were carried out using Bruker make (Model- Vertex 80 V) IR spectrophotometer in the range 400–5000 cm1 with a resolution of 4 cm1 [49] using a combination of globar source, KBr beam splitter and DLaTGS detector. Measurements were done on powdered samples dispersed in KBr matrix in the transmission mode. 2.6. Evaluation of antimicrobial properties 2.6.1. Total viable count technique (TVC) The antibacterial studies were carried out by TVC for FA, FAT, FAC and FATC specimens for the cubical concrete specimens which were exposed in sea water for 56 and 365 days. TVC was performed to evaluate the bacterial density of aerobic bacteria using sea water agar (SWA) (Hi Media-M592). Pseudomonas agar (PSA), manganous agar (MnA), cyanophycean agar (CA), Czapek Dox Agar (CDA) to know the density of different types of microbes such as Pseudomonas, Manganeseoxidizing bacteria, algae and fungi. The withdrawn concrete specimens from the sea water were gently washed and the attached biofilm were removed using sterile brush and dispersed into 500 ml of sterile phosphate buffer (0.0425 g KH2PO4, 0.19 g MgCl2 per litre). Serial dilutions of the bacterial cell suspension were prepared.

S. Uthaman et al. / Construction and Building Materials 187 (2018) 448–459 0.1 ml of each dilution was plated onto respective media. The plates were incubated for 24–48 h at 37 °C and the bacterial density was estimated according to APHA standards [50]. 2.6.2. Epifluorescence microscopic studies Mortar specimens (35  10 mm) of all the mixes which were cured for 28 days and exposed in sea water from 1 day to 365 days were withdrawn for microscopic analysis. It was used for visualizing biofilms under epifluorescence microscope (Nikon Eclipse E600, excitation filter BP 490; barrier filter O515). The specimens were stained with 0.1% acridine orange (AO) which is a fluorescent dye. This stain differentiates the single stranded RNA and double stranded DNA. The stained samples were observed under the epifluorescence microscope [51].

3. Results and discussion 3.1. Thermogravimetry analysis Thermogravimetric analysis of 28 days cured specimens gave details on the mass loss due to free moisture, decomposition of calcium silicate hydrate (CSH), decomposition of CH and decarbonation of CaCO3 at 25–123 °C, 123–420 °C, 400–500 °C and 501–900 °C respectively. Fig. 1(a–d) shows the TG results of weight loss percentage. In the first endothermic reaction at 25–123 °C, the evaporation of surface absorbed water was found to be more on FA followed by FATC, FAC and FAT (Fig. 1a). In the second endothermic reaction which was at 123–420 °C, the weight loss due to the dehydration of CSH was found to be more on FAT (Fig. 1b) followed by FA, FATC and FAC specimens. The third endothermic reaction attributed to the decomposition of CH

451

(420–600 °C) was more on FAT (Fig. 1c). FAC followed by FATC and FA showed minimum weight loss attributed to the decomposition of CH. The final endothermic reaction explaining the decarbonation of CaCO3 at 600–730 °C shows least weight loss on FATC specimen and maximum was on FAT (Fig. 1d). 3.1.1. CH content Fig. 2 shows the percentage of CH content present in FA and modified FA concrete specimens cured for 28 days. These results show that at 28 days of hydration, CH content was found to be more on FAT and least on FAC. However, in the case of FA and FATC concrete specimens, the CH content was found to be in-between FAT and FAC. At early age two predominant reactions are pozzolanic activity where CH is consumed to form additional CSH gel, while second is nucleation effect, where hydration is accelerated leading to additional CH formation. Hence net effect depends upon the kinetics of these reactions, which need larger data base. 3.1.2. CaCO3 content The amount of CaCO3 calculated from TG curves of 28 days fresh water cured concrete specimens is shown in Fig. 3. The percentage of CaCO3 was observed in the decreasing order of FAT (14.98) > FAC (11.14) > FA (10.91) > FATC (8.86). The CaCO3 content was found to be more on FAT and it was least on FATC concrete specimens. Results showed that hydration products like CH, CSH and CaCO3 were highest in FAT showing faster hydration in the presence of TiO2 nanoparticles which is also reported by earlier workers [25].

Fig. 1. TG results of weight loss percentage on 28 days cured FA, FAT, FAC and FATC concrete specimens (a) Dehydration of surface absorbed water (b) Dehydration of CSH (c) Decomposition of calcium hydroxide (d) Decarbonation of calcium carbonate.

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Fig. 4. Crushed pH of powdered concrete specimens. Fig. 2. CH content calculated from TG curve of FA and nanophase modified FA concrete specimens cured in fresh water for 28 days.

is found to be still alkaline even after exposure to an aggressive environment for 365 days. The pH reduction was seen in the order of FA (8.57) > FAT (9.75) > FAC (9.89) > FATC (10.25). Among all the nanophase modified concrete specimens, FATC showed least reduction in pH value under sea water exposure. All the nanoparticles added FA concrete, showed higher pH for 28 days cured specimens, which confirmed nucleation effects of nanoparticles wherein additional CH is formed. The FAT specimen with highest CH content showed the highest pH [52]. During seawater exposure pH of the all the specimens decreased due to the environmental effects, i.e. ingress of free carbon-di-oxide, chloride ions and hydrogen sulphide present in the seawater [53]. However after 365 days exposure all nanophase modified FA specimens showed higher pH compared to FA. FAC showed consistently higher pH upto 180 days indicating acceleration of hydration reaction of Portland cement to form CH in the presence of NC [54]. After 180 days exposure, pH of FAT and FAC decreased whereas pH of the FATC specimens stabilized and was highest among the modified concrete specimens.

3.3. Mechanical strength Fig. 3. CaCO3 content calculated from TG curve of FA and nanophase modified FA concrete specimens cured in fresh water for 28 days.

In FAC concrete the CH content was least due to the higher reactivity of CaCO3 nanoparticles leading to the consumption of CH by pozzolanic reaction [24]. However, CSH content was lesser in FAC compared to FAT confirming the combined effect of nucleation and pozzolanic activity by TiO2 nanoparticles. FATC with 1% NT and 1% NC showed optimum hydration with values in between FAT and FAC. The content of CaCO3 was also least on FATC compared to FAT and FAC showing better concrete properties. 3.2. Crushed pH measurement pH is an important parameter for studying the properties of concrete. Fig. 4 illustrates the crushed pH of different concrete mixes (FA, FAT, FAC and FATC) before (28 days cured) and after exposure to seawater for 56, 90, 180 and 365 days. Nanophase modified FA concrete specimens showed higher crushed pH compared to FA concrete. FAC showed highest pH. With exposure pH value decreased in all the concrete specimens. However, when compared to FA, all the modified concrete of FAT, FAC and FATC

The compressive strength results of 7 and 28 days cured and 56, 90, 180 and 365 days seawater exposed concrete specimens of FA, FAT, FAC and FATC are given in Table 2. In order to get a clear picture about variations in compressive strength among the different mixes after long term exposure in seawater, the compressive strength of 365 days seawater exposed FA, FAT, FAC and FATC specimens are separately plotted in Fig. 5. Comparing the early age for all the mixes, FATC and FAC showed highest values at 7 and 28 days respectively. After seawater exposure upto 90 days, all the specimens showed progressive increase in strength and FAC showed the highest strength. However, FAC specimens showed a decline in compressive strength after six month exposure. Among all the specimens, FATC specimens showed a progressive increase in strength even after exposure to seawater for 365 days. Many workers have reported improvement of mechanical properties by addition of nanoparticles [55,56]. Compared to FA concrete, the compressive strength of the 7 days cured FATC and FAT increased by about 12.4% and 7.2% respectively. FAC showed comparable strength with respect to FA. Thus early age strength improved by addition of NT. At 28 days, the compressive strength of FAC increased marginally by 2.6% compared to FA. Further,

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S. Uthaman et al. / Construction and Building Materials 187 (2018) 448–459 Table 2 Compressive strength of unexposed and seawater exposed concrete specimens. Type of specimen

FA FAT FAC FATC

Compressive strength (N/mm2) 7 days cured

28 days cured

56 days seawater exposed

90 days seawater exposed

180 days seawater exposed

365 days seawater exposed

18.99 20.36 18.85 21.35

36.98 30.18 37.95 34.94

43.97 39.40 43.98 40.62

48.52 47.40 50.45 42.76

48.27 42.46 52.10 45.70

54.48 56.27 50.50 57.25

Fig. 5. Compressive strength of concrete specimens exposed to sea water for 365 days.

Fig. 6. Resistivity of seawater exposed concrete specimens in comparison with unexposed concrete specimens.

during exposure to seawater FAC maintained highest strength upto 180 days compared to other modified concrete showing the enhanced pozzolanic effect of NC with time. At 365 days of exposure the strength gain of FAC ceased whereas FATC and FAT showed 5% and 3.2% higher strength respectively compared to FA concrete. Table 3 illustrates the split tensile strength of concrete specimens before and after exposure to seawater. After 28 days of curing, FATC and FAC specimens showed more tensile strength compared to FA and FAT specimens. After exposure to seawater, tensile strength of FA, FAC and FAT increased upto 90 days and then started declining. However, FATC specimens did not show this decline split tensile strength. Variation of split tensile strength with respect to nanophase addition for 28 days cured and 365 days exposed specimens showed an increase compared to FA concrete. FATC followed by FAC showed the highest values of split tensile strength, indicating Fig. 7. RCPT of 28 days cured and seawater exposed concrete specimens. Table 3 Split tensile strength of unexposed and seawater exposed concrete specimens. Type of specimen

FA FAT FAC FATC

Split tensile strength (N/mm2) 28 days cured

56 days seawater exposed

90 days seawater exposed

180 days seawater exposed

365 days seawater exposed

2.64 2.40 2.93 2.95

3.78 3.87 3.47 3.75

3.92 3.30 3.9 3.67

3.84 3.42 3.6 3.69

3.28 3.33 3.56 4.06

contribution of NC. Optimum substitution levels of NC need to be further investigated with respect to split tensile strength as it decreased at 2% level. 3.4. Resistivity analysis Resistivity depends on the porosity of the concrete specimens. Generally addition of fly ash increases the density and thereby increases the resistivity. Resisvity measurements were conducted for an year on prism specimens and the results are shown in

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Fig. 6. In this study all the nanophase modified and FA concrete showed more than 30 KXcm which is well above the limit of 12 KXcm below which corrosion is likely [57]. The resistivity of 28 days cured specimens showed the same trend of TGA results where FAT with faster hydration showed the highest resistivity (52.47 KXcm). FAC with lowest CH and CSH content showed lower resistivity (36.73 KXcm). With exposure in seawater resistivity of all the specimens increased. It is to be noted that after 180 days of exposure resistivity of FA (106.65 KXcm) and FAT (107.95 KXcm) appears to be stabilized. In contrast FATC (142.41 KXcm) and FAC (142.24 KXcm) showed exponential increase in the resistivity clearly indicating the evident role of NC in enhancing the hydration and thereby providing a dense microstructure. 3.5. Rapid chloride penetration test (RCPT)

Fig. 8. Depth of CO2 penetration into the mortar specimens exposed to marine atmosphere for 6 months.

RCPT results give us an insight into the performance of concrete in chloride environments. If the total charge passing through the specimen is less than 2000 Coulombs the concrete is considered as having low chloride penetrability [41]. The chloride penetrability values of 28 days cured and 365 days seawater exposed

Fig. 9. Mineralogical changes of unexposed (28 days cured) and 365 days seawater exposed (a) FA (b) FAT (c) FAC and (d) FATC concrete specimens SiO2-Quartz, CAS- Calcium aluminium silicate, CSH- Calcium silicate hydrate, CASHH- calcium aluminum sulfate hydroxide hydrate (Ettringite).

S. Uthaman et al. / Construction and Building Materials 187 (2018) 448–459

concrete specimens are shown in Fig. 7. After 28 days of curing, all the specimens showed RCPT value less than 500 Coulombs confirming low chloride penetrability. Nanophase modification of FA concrete enhanced its resistance to chloride penetrability. FATC followed by FAC showed least RCPT values. After exposure in sea water, RCPT values further decreased due to higher pozzolanic activity. However, after 180 days RCPT value started increasing for all specimens though the values were still far below 2000 Coulombs. This trend needs further investigation. FATC showed least chloride penetrability even after 365 days seawater exposure. 3.6. Carbonation depth measurement The depth of CO2 penetration into the mortar specimens exposed to marine environment for 6 months is shown in Fig. 8. The depth of CO2 penetration was found to be higher on FA (0.53 mm) compared to nanophase modified FA mortar specimens. Among all the nanophase modified mortar specimens, FATC (0.17 mm) showed least depth of CO2 penetration further confirming the role of NC in providing the dense microstructure. 3.7. Microstructural properties 3.7.1. Mineralogical characterization studies Fig. 9 shows the X-ray diffraction spectra of the 28 days cured and 365 days seawater exposed FA, FAT, FAC and FATC specimens. After 28 days of curing all the FA and nanophase modified FA concrete specimens were found with conventional peaks of hydration [58] like SiO2 (2h = 24.1°, 31.09° and 50.01°; JCPDS 01-072-1088), CSH (2h = 32.6°, 37.7°, 42.7°, 46.2° and 71.4°; JCPDS 01-0897639) and calcium aluminium silicate (CAS) (2h = 10.1°, 34.4°, 59.27°, and 64.38°; JCPDS 01-083-1278). However, FA showed comparatively lesser number of hydration peaks and intensity.

455

FAC showed highest intensity peaks of SiO2 and CAS. FAT showed highest intensity peaks of CSH. FATC had good intensity peaks of SiO2, CAS and large number of CSH peaks. After seawater exposure for 365 days, the intensity of quartz peaks increased in FA, FAT and FATC specimens. FATC and FAT showed more number of CSH peaks with high intensity. However in FAC, intensity of CAS decreased with sea water exposure and a predominant peak of ettringite (calcium aluminum sulfate hydroxide hydrate) was found corresponding to a decrease in compressive strength as well. 3.7.2. Morphological analysis Figs. 10 and 11 shows the comparison morphology of different concrete specimens cured for 28 days and exposed to seawater for 365 days respectively. Scanning electron microscopic analysis of the nanophase modified concrete specimens at the age of 28 days, showed a comparable morphology with FA concrete. However, after exposure to seawater for 365 days, SEM micrographs showed distinct morphology for each type of specimen. Surface deterioration was exhibited by FA concrete in the form of microsized pits and grooves. In the case of FAC needle shaped crystals of ettringite formation corresponded with reduction in calcium aluminate phase decrease in XRD spectra. It is well known that gypsum and other sulphate in concrete or in the environment can react with calcium aluminate phase to form ettringite [59]. This ettringite formation can explain some detrimental properties of FAC like decrease in compressive strength and increase in chloride penetrability compared to other nanophase modified FA concrete specimens. Literature also shows that ettringite is more soluble in the presence of chlorides [60]. However, FATC clearly showed colloidal CSH morphology as reported by earlier workers [61] and was free of ettringite, voids and cracks. Though FAT also showed homogeneous morphology of hydration products, it did not show colloidal CSH morphology.

Fig. 10. SEM surface micrographs of 28 days cured concrete specimens (a) FA (b) FAT (c) FAC (d) FATC.

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Fig. 11. SEM images of concrete specimens exposed to seawater for 365 days (a) FA (b) FAT (c) FAC (d) FATC.

3.7.3. Infrared spectroscopy studies The structural changes in the main hydrated phases of all the concrete specimens were recorded with IR spectroscopy technique. Fig. 12 shows the infrared (IR) spectroscopy of as cast 28 days cured FA concrete along with FAT, FAC and FATC. IR measurements revealed expected phonon features for FA concrete. The most intense feature at 1091 cm1 corresponds to the SiAO stretching frequency of the major cementitious product, the CSH gel. The mode at 462 cm1 can also be attributed to the SiAO bending mode of the CSH gel, while the mode at 1464 cm1 arises due to

the CAO stretching of the carbonate species in the concrete. The broad absorption feature between 3000 and 4000 cm1 arises due to the presence of bound water in cement. The absence of sharp features 3616 cm1 characteristic of Portlandite (crystalline CH – the major hydration product of OPC concrete) in FA concrete provides excellent proof of the fact that alumino-siliceous rich pozzolans (fly ash) participates in reactions with CH producing various calcium aluminate and calcium alumino-silicate hydrates. Since the IR spectra of all the four 28 days cured and nanophase modified

Fig. 12. IR spectra of 28 days cured FA and nanophase modified FA concrete specimens.

Fig. 13. IR spectra of 28 days cured and 365 days seawater exposed FA and FATC concrete.

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samples are identical, it can be readily inferred that nanophase modification of FA concrete does not significantly affect the overall chemical composition/structure of the concrete samples. Fig. 13 shows the area normalized set of IR spectra of the as cast unexposed samples (FA and FATC) along with the 365 days sea water exposed samples. It can be observed from the figure that FA and FAT (not shown in Fig) concrete samples remain more or less unaffected even after 365 days of exposure to sea water, while both FATC and FAC (not shown in figure) are observed to exhibit significant changes with respect to the IR peak at 1091 cm1 corresponding to the CSH mode. It is also very surprising to note that the modified structure of both FAC and FATC samples after 365 days of sea water exposed are identical. In particular, the intensity of the major CSH mode at 1091 cm1 is drastically reduced while the shoulder at 1020 cm1 evidently picks up on sea water exposure. This clearly indicates a change

in the calcium/silica (C/S) ratio of the CSH gel that could have caused the above shift in CSH modes [62]. Since both FAC and FATC contain NC, we immediately infer that the observed structural change could be associated with NC. According to Scherer [63], CSH paste of cement is a colloidal precipitate of sol–gel nature. The condensation reaction that drives the sol–gel transition can continue with time leading to polymerization of silicate network due to ageing [64]. According to Kumar et al. [65], nanoparticles can increase polymerization of silicate chains in silica gel leading to formation of colloidal CSH and thereby accelerate the effects of ageing. Campillo et al. [61] reported that colloidal silica is more effective than agglomerated silica and contribute to strength. Thus this study shows that NC contributed to this colloidal formation thereby increasing gel porosity and reducing capillary porosity which can explain the resistivity increase in FAC and FATC even after 365 days exposure [66].

Table 4 TVC of concrete specimens exposed to seawater. Total Viable Count (cfu/cm2) Type of agar used

56 days seawater exposed concrete Type of specimens FA

Seawater agar Pseudomonas agar Manganese agar Czapek Dox agar Cyanophycean agar

FAT 5

3.9  10 2.7  105 0.9  105 1.7  103 0.4  101

365 days seawater exposed concrete Type of specimens FAC

3

6.2  10 4.7  103 1.2  103 2.5  101 –

FATC 6

5.7  10 6.9  105 1.1  105 3.9  103 –

FA 4

7.2  10 5.7  104 2.2  104 2.6  103 –

FAT 9

2.7  10 6.8  108 2.1  106 7.5  105 5.1  104

FAC 7

5.3  10 2.7  105 5.2  103 7.1  103 3.1  103

Fig. 14. Epifluorescence images of seawater exposed FA and nanophase modified FA mortar specimens.

FATC 9

1.5  10 8.7  107 8.5  105 5.7  105 1.9  104

6.1  107 3.8  106 6.7  103 2.1  104 4.2  103

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3.8. Biofilm characterization studies 3.8.1. Bacterial enumeration The TVC of aerobic bacteria on the 56 days exposed cube concrete specimens was highest on FAC (5.7  106 cfu/cm2) and least on FAT (6.2  103 cfu/cm2) whereas by 365 days bacterial density was lesser on all nanophase modified concrete compared to FA concrete specimens. FAT and FATC showed least TVC values (Table 4). 3.8.2. Epifluorescence micrographs of biofilms Epifluorescence micrographs (Fig. 14) showed gradual increase in biofilm formation as indicated by increase in the intensity of orange fluorescence of RNA on all the specimens. After 365 days of exposure FAC showed maximum intensity of orange fluorescence indicating thick biofilm formation. FAT and FATC showed least fluorescence indicating antibacterial activity.

troscopy also gave confidence by confirming absence of any new deleterious phases and nanoparticles was found to enhance only the amount of useful hydration products like CSH and CAS. Titania nanoparticles helped in early hydration by filler effects and pozzolanic activity and also enhanced antibacterial activity. However calcium carbonate nanoparticles by providing more C/S ratio contributed to several advantages like high pH, both early age strength and increase in compressive strength with seawater exposure, denser microstructure and higher resistivity. However, after 180 days of exposure, the higher concentration of CAS in FAC with 2% NC turned into deleterious ettringite that resulted in increase of chloride penetrability and reduced strength. Thus, Synergistic combination of 1% NC and 1% NT helped FATC to emerge superior with least decarbonation, reduction in pH, penetrability of chloride, highest early age and post exposure strength, split tensile strength, resistivity and optimum antibacterial activity.

Acknowledgments 4. Conclusions 1. Thermogravimetric analysis of 28 days cured specimens showed weight loss due to the dehydration of CSH was more on FAT and decarbonation of CaCO3 was least on FATC due to the enhanced rate of hydration provided by the synergistic effect of 1 wt% nano-TiO2 and 1 wt% nano-CaCO3. 2. All the nanophase modified FA concrete specimens showed higher crushed pH values compared to FA concrete with highest values for FAC. 3. FATC specimens showed least reduction in pH even after 365 days exposure in seawater. 4. Early age compressive strength of 7 and 28 days cured specimens was highest for FATC and FAC respectively due to the strong accelerating effect of nano-CaCO3 at early ages. 5. Post exposure compressive strength of FATC specimens progressively increased and was the highest at 365 days seawater exposure. 6. Split tensile strength of FATC was the highest for the 28 day cured and 365 days seawater exposed specimens. 7. Resistivity of FAC and FATC specimens continuously increased upto 365 days of seawater exposure. 8. RCPT values showed least chloride penetration for FATC specimens even after 365 days of seawater exposure. 9. All the nanophase modified FA concrete showed lesser carbonation depth compared to FA concrete with least on FATC. 10. Mineralogical phase identification in nanophase modified concrete by XRD showed large number of high intensity CSH peaks in FATC and transformation of CAS into deleterious ettringite in FAC at 365 days of seawater exposure. 11. SEM micrographs confirmed absence of detrimental phases, voids and cracks and presence of colloidal CSH in FATC specimens. 12. Infrared Spectroscopy analysis clearly confirmed that overall structure of FA concrete remained unaffected by nanoparticle substitution. Further the spectra indicated a change in C/S ratio of CSH gel in FATC and FAC probably signaling colloidal CSH formation. 13. Evaluation of antibacterial activity of nanophase modified concrete by culture and microscopic techniques clearly showed enhancement in FAT and FATC. Thus, for the first time a systematic study on holistic effects of nanophase modification of fly ash concrete on properties, durability and antibacterial activity brought out the ability of nanoparticles to eliminate the concerns regarding FA concrete and enhance its superiority for practical applications. Studies using IR spec-

Financial support from Board of Research in Nuclear Sciences, Mumbai (2013/36/33-BRNS/2355) is greatly acknowledged. Sincere thanks to Chancellor Sathyabama Institute of Science and Technology, Chennai and Director, IGCAR, Kalpakkam for guidance, encouragement and motivation.

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