Early age, hydration, mechanical and microstructure properties of nano-silica blended cementitious composites

Construction and Building Materials 233 (2020) 117212

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Early age, hydration, mechanical and microstructure properties of nano-silica blended cementitious composites K. Snehal, B.B. Das ⇑, M. Akanksha Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, Karnataka 575 025, India

h i g h l i g h t s  Blended cementitious mixes are optimized through particle packing theory.  Optimum dosage of nano-silica is found to be 3%.  Hydration characteristics of blended mixes are determined through a TGA.  WH found to be more for all blended mixes as compared to control mix.  WH and Ca/Si ratio relationship is found to be linear and inversely related.

a r t i c l e

i n f o

Article history: Received 18 June 2019 Received in revised form 1 October 2019 Accepted 10 October 2019

Keywords: Nano-silica Microstructure Thermogravimetry Hydration Calcium to Silica Ratio

a b s t r a c t This study was carried out to understand the influence of nano-silica on hydration properties of binary, ternary and quaternary blended cement paste and mortar containing micro to nano sized admixtures including fly ash (FA), ultrafine fly ash (UFFA) and nano-silica in colloidal form (CNS). Characterization methods such as thermogravimetric analysis (TGA), X-ray diffraction studies (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) was employed to quantify the hydration products. Further, early age and mechanical properties were also investigated for binary, ternary and quaternary cementitious system blended with nano-silica. The optimized proportions of blended paste and mortar are designed through modified Andreasen and Andersen particle packing model. The experimental test results revealed that the optimum dosage of CNS in binary blended cement composites is 3%. The presence of nano-silica in cementitious system amplified the hydration and pozzolanic activity, thereby promoting densified microstructure at nano scale. The flow test indicated the intensified demand for water absorption and reduced workability with the rise in level of incorporation of CNS particles in cement paste. Quaternary blended mix performed superior hydration along with strength properties amongst all the blended samples. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Concrete is a composite material which consists of a binding material known as cement. Ordinary Portland cement (OPC) is favourably and widely consumed for the production of concrete. However, with the consumption of each ton of OPC, there is an equal amount of CO2 released to the atmosphere [1–3]. With increasing level of industrialization and rapid urbanization, huge amount of industrial wastes or by-products are generated, which become challenging for disposal. It is observed that researchers have successfully utilized these industrial by-products in concrete [4–7]. The most prominent ones in this regard are fly ash, ground ⇑ Corresponding author. E-mail address: [email protected] (B.B. Das). https://doi.org/10.1016/j.conbuildmat.2019.117212 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

granulated blast furnace slag (GGBS), silica fume, rice husk ash etc., popularly known as secondary cementitious materials (SCMs). From the available literature it is observed that when SCMs are used in conjunction with OPC it contributes to the engineering properties of cementitious composites by means of its pozzolanic activity [4–7]. For instance, fly ash, an industrial by-product from coal based thermal power plant is extensively used pozzolanic ingredient for cement-based materials [8]. Another widely used SCM in construction industry is ground granulated blast furnace slag (GGBS) produced from pig-iron industries [9]. Silica fume, an ultrafine pozzolanic material from silicon industry with average particle size ranging from 0.1 mm to 1 mm i.e. 100 times smaller as compared to cement particles, is identified as the supreme pozzolanic material [7,10]. In addition to silica fume, it is observed that researchers have classified/processed the ultrafine particles

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

from industrial by-products such as fly ash and GGBS and utilized in the cement matrix [11–12]. However, most of these SCMs initially behave only as filler and remain inert in the hydration process leading to lower early compressive strength in concrete [4,11–14]. Moving ahead from micron sized SCMs over the past decades of development, use of nano-additives in the field of construction has opened up new avenues in the cementitious systems [15–17]. Amongst a number of nano additives, nano-silica (nano-SiO2) incorporation in cement composite is finding pronounced interest by the researchers owing to its superior and hasty pozzolanic reactivity [18–26]. It is reported that implementation of nanotechnology in cementitious system by introducing nano-silica significantly improved the properties and performance of cement composites [27]. Further, the problem of lower initial strength gain was mitigated and ‘‘high performance concrete” could be effectively produced with a greater service life [28,29]. Nano sized silica particles are capable of filling the voids between the particles of CS-H gel by functioning as nano filler and thereby enhancing the pozzolanic reactivity, which would result in greater densification of micro-structure followed by improved mechanical and durability properties of concrete [24,25,30]. However, literature says that particle size and quantity of nanosilica has an impact on drying shrinkage [31,32], workability [33,34] and heat of hydration [35,36]. In this perspective, it is very much necessary to use SCMs in cement composites with the relevant choice of size and type of SCMs to produce a sustainable concrete. With the advancement in concrete technology from a single binder base to binary, ternary and quaternary blends, the selection of right combination of material plays vital role in enhancing the performance of concrete [37–39]. Choosing suitable combinations of replacement materials by adopting trial and error technique becomes a tedious job. Furthermore, performance of concrete is directly influenced by the type of ingredients and their packing characteristics [40–42]. Thus, knowledge of particle packing concept and its influence on cementitious system is necessary to enable a mixture designer to select apt proportion of replacement materials from a pool of SCMs [43]. The particle packing of concrete is measured in terms of its packing density, which can be defined as ratio of the solid volume of the particle to total volume occupied by the particles [43]. For achieving the optimal particle packing of cementitious matrix with multiple ingredients, modified Andreasen and Andersen model is found to be one of the appropriate and best-known packing models [44]. It is reported that blended cement mortar designed using modified Andreasen and Andersen model showed optimistic properties like improved hydration, compressive strength, filler effect and dense microstructure, that aids for better durability of the material [42,45–47]. At this point of time, it is understood that utilization of SCMs in conjunction with nano-silica in blended cement composite is essential to counter balance the negative effects of SCMs (low initial compressive strength) and nano-silica (drying shrinkage, workability and high heat of hydration) to obtain a high

performance and sustainable material. The integration of particle packing concept would help in understanding the influence of binary, ternary and quaternary blended mixes which includes 10 6 to 10 9 m matrix sized admixtures including fly ash (FA), ultrafine fly ash (UFFA) and nano-silica in colloidal form (CNS) on the early age, hydration, mechanical, microstructure and durability properties of cementitious paste and mortar. 2. Materials and experimental methodology 2.1. Materials Commercially available ordinary Portland cement (OPC) confirming to ASTM Type 1 [48], low calcium class F fly ash (FA) confirming to ASTM C 618 [49], class F ultra-fine fly ash (UFFA, 5–10 mm) and colloidal nano-silica (CNS, 20 nm), polycarboxylate ether based superplasticizer were used in this study. Natural river sand confirming to zone-II (IS: 383-2016) [50] was used for mortar mixes. The properties of the materials used in this study are summarized in Table 1 and the particle size distribution of these materials is presented in Fig. 1. The basic properties of colloidal nano-silica provided by the manufacturer are listed in Table 2. 2.2. Experimental methodology 2.2.1. Particle packing by modified Andreasen and Andersen model In this study modified Andreasen and Andersen packing model [44] was employed to design the mortar mixes of multiple blends (i.e. binary, ternary and quaternary blend). Seventeen mortar mixes were prepared with different combination of OPC, FA, UFFA and CNS. The mix designations for different combinations of binary, ternary and quaternary mixes are represented in Table 3.

CNS UFFA OPC FA Sand

100 80

Percentage Finer

2

60 40 20 0 1E-3

0.01

0.1

1

10

100

1000

10000

Particle Size (µm) Fig. 1. Particle-size distribution characteristics of materials used in the study.

Table 1 Properties of the cement, fine aggregate, UFFA and FA used in present study. Material

G

F(m2/kg)

SC (%)

OPC-53 Fine Aggregate Ultra-fine fly ash (class F) Fly ash (class F)

3.15 2.56 2.2 2.2

300 – 670 265

32 – – –

Setting time (min)

q (kg/m3)

IST

FST

Loose

Dense

110 – – –

170 – – –

– 1524 – –

– 1780 – –

FM

– 3.14 – –

Compressive strength (MPa) 3 days

7 days

28 days

56 days

22.1 – – –

36.1 – – –

44.1 – – –

49.5 – – –

* G-specific gravity, F-fineness, SC-standard consistency, q-density, FM-fineness modulus, IST-initial setting time, FST-final setting time.

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the CNS (60% liquid phase), addition of water content was adjusted accordingly. The mixture proportion of multi blended cement paste mixes were kept same as mortar mixture proportions excluding the sand content. The paste mixes were designated with symbol ‘‘P” such as CP, CNS-0.5P, FN-15P and so on.

Table 2 Physical properties of CNS. Properties

Value

Average particle size Dispersion medium Solid content pH Viscosity SiO2 content

20 nm Water 40% (w/v) 9 25 cps 99.5%

2.2.2. Preparation of blended mortar specimens Mixing of blended cement mortar was performed by means of an automatic mortar mixer designed in accordance with EN 1961 [52]. Mixes of blended mortars were cast in cubical specimens of size 70.6 mm  70.6 mm  70.6 mm for compressive strength test and cylindrical mortar specimens of 100 mm diameter and 50 mm height were cast for rapid chloride ion penetration test. Further, the specimens were stored in a humid condition (27 ± 2 °C temperature and 95% relative humidity) and were demoulded after 24 hrs of storage. Subsequently, these samples were stored in a curing tank (water submerged condition, 27 ± 2 °C temperature) until the day of testing.

The software ‘‘EMMA” which works on the principle of modified Andreasen and Andersen packing model was adopted to design the mortar mixes of multiple blends. The material proportions for a particular mix were kept on modified until the particle packing curve reaches the optimum fit with respect to the target curve generated based on modified Andreasen and Andersen packing model [51]. A distribution coefficient (q) value of 0.30 suitable for medium workability was considered for all mix proportions. The mixture recipe of multi blended mixes are listed in Table 4. The mortar mixes were designated with symbol ‘‘M” such as CM, CNS-0.5M, FN-15M and so on. The blended mortar specimens were prepared with a fixed binder-sand ratio of 1:3 and water-binder (w/b) ratio of 0.5. Considering the quantity of water present in

2.2.3. Testing methods for blended cement mortar 2.2.3.1. Compressive strength. Compressive strength of all blended mixes was determined at 3, 7, 28 and 56 days in accordance with

Table 3 Mix designations for binary, ternary and quaternary blended mixes. Blends

Different combination

Control Binary blends

OPC OPC + CNS

Ternary blends

OPC + FA + CNS

OPC + UFFA + CNS

Quaternary blends

OPC + FA + UFFA + CNS

General Designation

C CNS-0.5 CNS-1 CNS-1.5 CNS-2 CNS-2.5 CNS-3 CNS-3.5 FN-15 FN-25 FN-35 UN-15 UN-25 UN-35 FUN-15 FUN-25 FUN-35

OPC (%)

100 99.5 99 98.5 98 97.5 97 96.5 85 75 65 85 75 65 85 75 65

Percentage of replacement (by weight of OPC) Total replacement (%)

FA (%)

UFFA (%)

CNS (%)

0 0.5 1 1.5 2 2.5 3 3.5 15 25 35 15 25 35 15 25 35

– – – – – – – – 13 23 33 – – – 9 16 20

– – – – – – – – – – – 13 23 34 4 7 14

– 0.5 1 1.5 2 2.5 3 3.5 2 2 2 2 2 1 2 2 1

Table 4 Mix recipe for blended mixes used in the present study. Mix designation

OPC (kg/m3)

FA (kg/m3)

UFFA (kg/m3)

CNS (kg/m3)

Sand (kg/m3)

Water (kg/m3)

W/C ratio

SP (% bwob)

CM/CP CNS-0.5 M/P CNS-1 M/P CNS-1.5 M/P CNS-2 M/P CNS-2.5 M/P CNS-3 M/P CNS-3.5 M/P FN-15 M/P FN-25 M/P FN-35 M/P UN-15 M/P UN-25 M/P UN-35 M/P FUN-15 M/P FUN-25 M/P FUN-35 M/P

568.35 565.51 562.67 559.82 556.98 554.14 551.30 548.46 483.1 426.35 369.4 483.1 426.35 369.4 483.1 426.35 369.4

– – – – – – – – 73.89 130.72 187.56 – – – 51.15 90.94 113.67

– – – – – – – – – – – 73.89 130.7 193.2 22.7 39.8 79.6

– 2.84 5.68 8.53 11.37 14.21 17.05 19.89 11.4 11.4 11.4 11.4 11.4 5.7 11.4 11.4 5.7

1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1 1705.1

284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2 284.2

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

– – 0.1 0.15 0.2 0.22 0.25 0.27 – – – – – – – – –

* bwob: by weight of binder material, * M stands for mortar mix, * P stands for paste mix.

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

IS 4031-Part 6:1988 [53] at a loading rate of 35 N/mm2/min. Three specimens of each mix for all curing ages were considered and the average was noted as compressive strength value for corresponding mix. 2.2.3.2. Rapid chloride-ion penetration test (RCPT). Rapid chlorideion penetration test (RCPT) is a method used to figure out concrete durability and the test was carried out for three identical specimens as per ASTM C 1202 [54]. In this study, RCPT was conducted at three curing ages i.e. 7, 28 and 56 days for all blended mixes. 2.2.4. Preparation of blended paste specimens The mix proportions for various blended cement paste mixes are listed in Table 4. Mixing of blended cement paste was performed by means of an automatic mixer designed in accordance with EN 196-3 [55]. Setting time, surface temperature variation during the period of setting, workability (flow diameter and flow value) and chemical shrinkage of all mixes were measured. To be used in microstructural characterization, blended cement paste samples of each mixes were cast in cylindrical silicon mould (30 mm diameter and 20 mm height) for 24 hrs at 27 ± 2 °C and then placed in a saturated water condition for certain curing ages (i.e. 7 and 28 days). After the accomplishment of appropriate curing ages samples were subjected to solvent exchange technique using isopropanol for 24 hrs in order to stop the hydration process for characterization [56]. Then, samples were oven dried at 40– 60 °C up to attainment of constant weight and were preserved in a desiccator containing pellets of silica gel to avoid any moisture contact. 2.2.5. Test methods for blended cement paste 2.2.5.1. Setting time and temperature. Initial setting time and final setting time test for all blended paste mixes were conducted in accordance with IS 4031-Part 5: 1988 [57]. The surface temperature was measured at an interval of 10 min with the aid of infrared thermometer from the mixing stage to final setting. 2.2.5.2. Workability. The mini flow table test was carried out in accordance with EN 1015-3 [58,59]. The average flow diameter at four perpendicular directions was measured and flow value was reported as percentage of increased flow diameter to the original base diameter of mini slump cone. 2.2.5.3. Chemical shrinkage. Chemical shrinkage was determined by means of dilatometry method as specified in ASTM C 1608 [60]. The drop-in water level in a hydrating cement paste was considered as chemical shrinkage i.e. ml/100 gm. Chemical shrinkage of each sample was periodically recorded at 1, 3, 7, 14, 28 and 56 days. 2.2.5.4. Thermo gravimetric analysis (TGA). Thermogravimetric analysis was carried out by employing a TG/DTA analyser from Seiko (EXSTAR 6000 TG/DTA 6300). Hardened paste samples were grinded to powder, passed through 75 mm sieve and were characterized at temperature range of 50–900 °C in nitrogen purge atmosphere. Heating rate and purge rate was 10 °C/min and 20 ml/min, respectively. From TG results, calcium hydroxide (CH) content for all blended mixes was determined at 7 and 28 days of curing. Decomposition of CH was considered as weight loss between the temperature range of 400–500 °C [61]. Temperature boundaries were identified from derivative thermogravimetric curve (DTG). On the basis of CH decomposition, the percentage of CH content was calculated [60–63].

Further, from the total weight loss percentage for temperature range of 35–600 °C, amount of water associated to CH content was subtracted and the resulted value is expressed as WH, i.e. the amount of water associated to hydration products excluding CH content [64]. 2.2.5.5. X-Ray diffraction (XRD). The hardened paste samples at curing ages of 7 and 28 days were grinded, passed through 75 mm IS sieve and was analyzed for mineralogical composition. XRD patterns of powdered samples were obtained by employing Jeol-JPX 8P with Cu Ka radiation (40 kV/40 mA) at deflection angle ranging from 10° to 80° and at a scanning speed of 2°/min. The obtained patterns were then analyzed using X’Pert High Score Plus software. 2.2.5.6. Scanning electron microscopy – energy dispersive X-Ray Spectroscopy (SEM-EDX). From the stored samples as explained in Section 2.2.4, chunks from the core of the paste samples were collected and then gold sputtered for microstructural analysis with aid of scanning electron microscope (SEM). Images were obtained through scanning electron microscope (Jeol, JSM-638OLA) in secondary electron mode and elemental analysis was conducted through an EDX analyser to know the change in elemental composition within the boundary of image. 3. Results and discussions 3.1. Particle packing model by modified Andreasen and Andersen model The optimized particle packing curves for multi blended mortar mixes is presented in Fig. 2. Particle size distribution of binary, ternary and quaternary blended mortar mixes were made to relate with modified Andreasen and Andersen model curve i.e. ideal curve for the arrival of better packing density. It can be observed from figure that actual gradation curve of all mixes found to be fit with ideal gradation curve with q value of 0.3. This is to be noted that for attaining optimum particle packing it is essential to have q value less than 0.36 [51]. The closest matching between gradation curve of recipe and ideal curve is obtained through altering material quantity inputs by trial and error. In this investigation, sand content is kept fixed for all blended mortar mixes. The gradation curves were statistically fit and it is found that exponential relation gives the best correlation. It can be observed

140 120

Percentage Passing

4

100 80

Ideal Curve (q=0.3) 2 2 CNS-0.5M; R = 0.9465, CNS-1M; R = 0.9466 2 2 CNS-1.5M; R = 0.9468, CNS-2M; R = 0.9468 2 2 CNS-2.5M; R = 0.9469, CNS-3M; R = 0.9471 2 2 CNS-3.5M; R = 0.9471, FN-15M; R = 0.9552 2 2 FN-25M; R = 0.9565, FN-35M; R = 0.9574 2 2 UN-15M; R = 0.9576, UN-25M; R = 0.9595 2 2 UN-35M; R = 0.9608, FUN-15M; R = 0.9621 2 FUN-25M; R= 0.9654, FUN-35M; R = 0.9722

60 40 20 0 0.01

0.1

1

10

100

1000

10000

Particle Size (mm) Fig. 2. Particle packing curve for blended mortar mixes optimized by modified Andreasen and Andersen particle packing model.

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K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

from Fig. 2 that with incorporation of blended admixtures possessing different particle sizes, improvements in particle packing curves were noted. It can be noted that compared to particle packing curve of binary blended CNS mixes ternary blended mixes integrated with CNS (i.e. FN and UN) showed better fit. However, quaternary blend (intermixed with three different sized particles, i.e. micro to nano scale) represented the best fit amongst all mixes. 3.2. Blended cement mortar 3.2.1. Compressive strength Compressive strength results of binary blended mortar admixed with the combination of OPC and CNS at the curing age of 3, 7, 28 and 56 days is plotted as histogram and is presented in Fig. 3. It can be observed from Fig. 3 that among all CNS binary blended mortar mixes CNS-3M mortar mix exhibited the highest compressive strength with respect to control mortar. The percentage of strength gain for CNS-3M mix was observed to be 73%, 39%, 21% and 14% at the curing age of 3, 7, 28 and 56 days, respectively in comparison to control mortar (CM). It can also be understood from the graphical representation that influence of CNS on development of compressive strength is more effective at the age of 3 days compared to 7, 28 and 56 days of curing age. This can be

Compressive Strength (Mpa)

60

attributed to the implication of early age hydration process. Rapid improvement in early age strength could be due to the reason that, presence of nano sized silica particles may promote the early hydration reaction of cement particles acting as a nucleation site for C-S-H gel formation [65]. In addition to that superior pozzolanic reactivity of nano-silica amplified the C-S-H gel formation by consuming Ca(OH)2 crystals thereby promoting the improvement in compressive strength. On the other hand, beyond 3% replacement of CNS, it was observed that there is a noticeable drop in compressive strength at all the curing ages. The possible reason for the drop in compressive strength after certain level of CNS replacement may be due to the presence of excessive nano-silica, which suppressed the hydration process by engrossing the water necessary for hydration of cement particles in production of C-S-H gel [66]. Further, the presence of excess nano-silica content may lead to silica leaching [67] and also it might have agglomerated generating a weak porous zone, which hampers the development in compressive strength [61,68]. Fig. 4(a–b) represents the compressive strength plot for ternary blended cement mortar at the curing age of 3, 7, 28 and 56 days. It can be noted from figure that presence of CNS particles in ternary blended mixes (FN and UN) improved the early as well

CM CNS-0.5M CNS-1M CNS-1.5M CNS-2M CNS-2.5M CNS-3M CNS-3.5M

50

40

30

20

10

0

3

7

56

28

Curing Age (days)

60 50 40

a

CM FN-15M FN-25M FN-35M

30 20 10 0

3

7

28

Curing Age (days)

56

Compressive Strength (MPa)

Compressive Strength (MPa)

Fig. 3. Compressive strength of binary blended CNS based cement mortar mixes.

60 50 40

b

CM UN-15M UN-25M UN-35M

30 20 10 0

3

7

28

Curing Age (days)

Fig. 4. Compressive strength of ternary blended cement mortar mixes a) FN mixes b) UN mixes.

56

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

as later age compressive strength. This might be attributed to optimum particle packing of mortar mix along with the influence of CNS in hydraulic and pozzolanic reaction. Nevertheless, UN mortar mixes displayed slightly higher compressive strength than control mortar even in 3 days of curing age. However, the strength gain in UN mortar mixes is observed to be better compared to that of FN mortar mixes. Compressive strength results of quaternary blended cement mortar presented in Fig. 5 illustrates that gain in compressive strength is more compared to control mix. This may be ascribed to better particle packing in addition to the pozzolanic reactivity of FA, UFFA and CNS in different scale. The reason could also be attributed to finer particles of UFFA add faster reactivity along with the presence of CNS and better pore filling effect at micro to nano scale [23,25]. 3.2.2. Rapid chloride-ion penetration test (RCPT) Results of RCPT for blended mortar mixes at the age of 7, 28 and 56 days are presented in Table 5. It can be observed from the table that incorporation of CNS in mortar mixes significantly reduces the penetration of chloride ions (charge passing) compared to control mix. It can also be noticed that reduction in charge passing is more pronounced after curing age of 28 days. The reduction in charge passing was observed to be more prominent for 3% CNS admixed mortar and the values for CNS-3M were 206 C and 101 C at the age of 28 and 56 days, respectively. Whereas, the percentage of reduction in charge passed for CNS intermixed ternary and quaternary blended FN, UN and FUN mortar mixes were in the range of 52%-68%, 58%-75% and 62%-78%, respectively at the end of 56 days with respect to control mortar. The substantial reduction of charge passed with the addition of nano-silica or pozzolanic materials may be attributed to the significant effect of secondary cementitious materials on pore solution chemistry affected by alkalis, level of replacement and age has a lot to do with RCPT results [69].

3.3.1. Setting time and temperature The setting time of binary, ternary and quaternary blended cement pastes integrated with colloidal nano-silica are presented in Figs. 6 and 7. It can be observed from Fig. 6 that there is a significant decline in initial and final setting time with increase in the content of nano-silica. Flash setting was observed after the replacement level of 3.5% CNS and hence not reported here. This accelerating effect in

Compressive Strength (MPa)

60

40

CM FUN-15M FUN-25M FUN-35M

30 20 10 0 3

Blends

Specimen

Control Binary blends

CM CNS 0.5M CNS 1.0M CNS 1.5M CNS 2.0M CNS 2.5M CNS 3.0M CNS 3.5M FN-15M FN-25M FN-35M UN-15M UN-25M UN-35M FUN-15M FUN-25M FUN-35M

Ternary blends

Quaternary blends

Charge Passed (coulombs) 7 days

28 days

56 days

1148 1031 1003 984 811 763 608 688 1001 956 930 978 935 912 955 900 845

521 498 456 402 384 316 206 215 355 320 236 335 295 216 310 250 215

401 298 222 161 106 109 101 105 240 210 165 220 175 145 215 160 129

350

298

300

Initial setting time 278

250

Final setting time

266 222

204

200 150

118

100

110

106

98

50 0

CP

93

179

69

163

45

160

30

CNS-0.5P CNS-1P CNS-1.5P CNS-2P CNS-2.5P CNS-3P CNS-3.5P

Mixes

3.3. Blended cement paste

50

Table 5 RCPT results for binary, ternary and quaternary blended cementitious mortar.

Setting Time (min)

6

7

28

56

Curing Age (days) Fig. 5. Compressive strength of quaternary blended FA, UFFA and CNS cement mortar.

Fig. 6. Setting time of binary blended CNS cement paste mixes.

setting time of cement paste incorporated with nano sized silica particles can be attributed to the reduction in dormant period, faster rate of pozzolanic reactivity and hydration [33,59]. Fig. 7(a) shows that CNS admixed FA and UFFA based ternary blended paste (i.e., FN and UN) exhibited the tailoring effect in setting time by balancing the retardation effect of FA and UFFA. The delay in initial setting time of FN and UN cement blends compared to control paste were seen to be between 8.5%–22% and 2.5%–22.8%, respectively. However, quaternary blended paste also showed similar trend of setting time as CNS admixed ternary blends i.e. percentage of delay in setting time ranged between 5.9%–23.7% (Fig. 7b). The results of surface temperature (T) variation during setting time of binary blended cement pastes integrated with CNS are presented in Fig. 8. It can be noted from figure that with increase in CNS content, surface temperature of cement paste increased substantially from the initial stage itself. For instance, initial rise in temperature of control paste is about 28 °C while, for CNS-3P mix, rise in temperature was found to be 31 °C. It can be observed from the figure that pattern of rise in surface temperature is similar in correspondence to all binary CNS mixes. The reason behind this may be due to superior pozzolanic reactivity of CNS promoting to faster hydration reaction, which may lead to higher heat liberation [33,65]. The variation in surface temperature for ternary and quaternary blended cement paste during setting is depicted in Fig. 9. Amongst

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K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

500

500

Final Setting time

Setting Time (min)

400 350 300

298

250 200 150

118

334

130

352

345

339

150

140

150

336

130

345

142

100

Final Setting time

400 350 300

298

250 200

155

118

150

342

333

326

160

168

FUN-25P

FUN-35P

100 50

50 0

Initial Setting time

450

Setting Time (min)

Initial Setting time

450

CP

FN-15P FN-25P FN-35P UN-15P UN-25P UN-35P

0

CP

FUN-15P

Mixes

Mixes

(a)

(b)

Fig. 7. Setting time of a) ternary (FN and UN) and b) quaternary blended (FUN) cement paste mixes.

38

34

CP CNS-0.5P CNS-1P CNS-1.5P CNS-2P CNS-2.5P CNS-3P CNS-3.5P

37 36

CP FN-15P FN-25P FN-35P UN-15P UN-25P UN-35P FUN-15P FUN-25P FUN-35P

33 32

35 31

T ( C)

T (°C)

34 33 32 31

30 29 28

30 27

29 26

28 0

50

100

150

200

250

300

Time (min) Fig. 8. Surface temperature of binary blended CNS cement pastes during the period of setting.

all the CNS integrated blended paste, FN showed lower peak temperature. This could be due to slow reactivity of FA, nevertheless presence of CNS played a role in slight increment in temperature. However, in case of CNS integrated ternary blended UN and quaternary blended FUN mixes the increase in surface temperature is noticeable compared to the ternary blended FN mixes. The peak temperature for ternary blended UN and quaternary blended FUN paste was found to be approximately similar to control paste. However, it is important to note that at replacement level of 35% for both UN and FUN mixes surface temperature was found to be lesser than that of control paste and the same could be attributed to the dilution effect and slow rate of hydration. 3.3.2. Workability Influence of CNS on flowability of binary blended cement paste is depicted in Fig. 10. It can be noticed from figure that increase in content of CNS in binary blended cement paste significantly reduced the flow diameter/flow value. The reason could be attributed to filling of voids by finer nano-silica particles which lead to increased water demand as well as cohesion between the particles [33,59].

0

50

100

150

200

250

300

Time (min) Fig. 9. Surface temperature of ternary and quaternary blended cement pastes during setting.

Further, a subsequent drop in slump flow especially after CNS replacement level of 0.5% was modified by appropriate addition of superplasticizer to maintain stability of flow value with respect to control paste. The modified flow diameter and flow value of binary blended CNS cement paste with the aid of superplasticizer are represented in Fig. 11. It can be seen from the graphical representation that flow diameter of binary blend CNS cement paste modified with superplasticizer is maintained between 156 mm and 157 mm. The flow performance of ternary and quaternary blended cement paste is displayed in Fig. 12. It can be noticed from Fig. 12(a) that flow diameter/flow value for ternary blended pastes intermixed with CNS (i.e. FN and UN mixes) reduced marginally compared to that of control paste. This could be attributed to the larger surface area of nano-silica which imbibes the water used for mixing thereby lowering the workability of cement paste [33,66]. However, it is noted that flow diameter of these mixes rests beyond 150 mm which is closer to control paste without any addition of superplasticizer. Furthermore, quaternary blended FUN mixes also showed the flow value nearby to the control paste in the range of 158 mm–160 mm (Fig. 12b). Comparing Figs. 10 and 12 it can be understood that when the binary blend of CNS

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

Flow Value (%) Flow Diameter (mm)

160

Flow Value (%)

140

160 140

120

120

100

100

80

80

60

60

40

40

20

20

0

0

0.5

1

1.5

2

2.5

3

3.5

Flow Diameter (mm)

8

0

CNS %

0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0

160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 CP

CNS-0.5P

CNS-1P

CNS-1.5P

Flow Diameter (mm)

CNS-2P

CNS-2.5P

Flow Value (%)

CNS-3P

SP (% bwob)

Flow Diameter (mm)/Flow Value (%)

Fig. 10. Flow diameter and flow value of binary blended CNS cement paste.

CNS-3.5P

SP (%)

Fig. 11. Modified flow diameter and flow value of binary blended CNS cement paste.

mixes are combined with FA/UFFA to form ternary and quaternary blends, the hindrance in workability of binary blends are stabilized. This can be attributed to ball bearing action of FA and UFFA particles [11,70]. 3.3.3. Chemical shrinkage Chemical shrinkage results of binary blend cement paste admixed with different dosage of CNS at the age of 1, 3, 7, 14, 28 and 56 days are presented in Fig. 13. Increased level of chemical shrinkage values was found for all binary mixtures of cement paste comprising of nano-silica. For instance, chemical shrinkage for CNS-3P was measured as 1.9, 2.8, 3.6 and 3.9 ml/100 gm at the age of 1, 3, 7 and 14 days, respectively and for control paste it is measured as 0.2, 0.7, 1.5, 2.1, 3.5 and 3.6 ml/100 gm at the age of 1, 3, 7, 14, 28 and 56 days, respectively. It is noticeable from this figure that after 28 days of curing age, the chemical shrinkage value for CNS-3P and CNS-3.5P is not reported. This is due to self-desiccation which led to cracking and failure of flask (Fig. 14). This may be attributed to presence of nano-sized silica particles having higher surface to volume ratio, where larger number of atoms are available at surface level, which makes the cementitious system hyper-reactive in terms of hydration leading to increase in the value of chemical shrinkage [71].

Fig. 15(a–b) illustrates the response on chemical shrinkage for ternary and quaternary blended paste at the age of 1, 3, 7, 28 and 56 days. It can be observed from figure that ternary blended pastes containing nano-silica (i.e. FN and UN mixes) shows higher rate of chemical shrinkage as compared to control paste. Maximum level of shrinkage was noted for FN-25P among FN mixes, volume change was measured to be between 2.5 and 4.5 ml/100 gm at the age of 1–56 days. Whereas, similar trend of increase in chemical shrinkage was observed for UN mixes, but after 56 days, crack in flask was noticed for both UN-25P and UN-35P mixes. The chemical shrinkage values measured for all replacement levels of quaternary blended cement paste integrated with FA, UFFA and CNS is potted in Fig. 16. Increased level of chemical shrinkage with time was observed up to the total replacement percentage of 25% (i.e. FUN-15P and FUN-25P) and then reduced for FUN-35P mix. The chemical shrinkage value measured for quaternary blended composite cement paste found to be similar to FN mixes, however, lesser compared to the UN ternary blended mixes. 3.3.4. Thermogravimetric analysis (TGA) Thermogravimetric analysis was carried out at the age of 7 and 28 days for all blended mixes. For brevity, TG/DTG curves for binary, ternary and quaternary blended cement paste specimens at

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

(a) 80

175

flow diameter

Flow value 170

60

165

50 160 40 155 30 150

20

Flow Diameter (mm)

Flow Value (%)

70

145

10 0

140 CP

FN-15P FN-25P FN-35P UN-15P UN-25P UN-35P

Mixes

(b) 80

175

flow diameter

170

60

165

50 160 40 155 30 150

20

Flow Diameter (mm)

Flow Value (%)

70

Flow value

145

10 0

140 CP

FUN-15P

FUN-25P

Mixes

FUN-35P

Fig. 12. Flow diameter and flow value of a) ternary (FN and UN) and b) quaternary blended (FUN) cement paste.

Chemical Shrinkage (ml/100 gm)

6.0 5.5

1 day 14 days

3 days 7 days 28 days 56 days

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

% CNS Fig. 13. Chemical shrinkage for binary blended cement paste integrated with CNS.

the age 28 days are presented in Fig. 17. Thermogravimetric mass loss takes place at specific temperature boundaries when hydrated cement paste samples are exposed to an elevated temperature, owing to loss of free water, dehydration, de-hydroxylation and de-carbonation of hydration products [65,72]. From TGA results, it can be observed that significant weight losses took place between temperature ranges of 110 °C–300 °C, 400 °C–500 °C and 600 °C–800 °C. Temperature boundaries of various decomposition phenomenon of hydration products were identified by means of

9

differential thermogravimetry curve (DTG). Weight loss seen in the range of 110 °C–300 °C signifies the dehydration of water molecules associated to calcium silicate hydrate gel (C-S-H) [64,73]. Further, the weight loss observed between the ranges of 400 °C–500 °C represents the de-hydroxylation of calcium hydroxide (CH) [73] and thermal degradation happens between the temperature ranges of 600 °C–800 °C relating to de-carbonation of calcium carbonate (CC). The quantification of hydration products was done on the basis of this observed mass loss at particular temperature ranges. The values of CH and WH content for all blended mixes at the curing age of 7 and 28 days were determined and is presented in Table 6. For binary blended CNS mixes, with the increase in dosage of CNS significant reduction in CH percent and increase in WH percent was observed as compared to control mix. It is important to note that, after 3% CNS dosage there was no further deviation in CH and WH quantity, which means that incorporation of nanosilica content exceeding 3% may not be effective in hydration activity. It can be noticeable from the table that with intrusion of CNS in ternary blended FN and UN mixes, there found to be an intensified reduction in CH% and pronounced increase in WH% compared to that of control mix. This could be attributed to the more active pozzolanic reactivity of FA and UFFA in presence of highly reactive nano-silica particles. Quaternary blended paste samples also showed reduction in CH content and increase in WH content as like CNS incorporated ternary blended samples, but in a higher rate. The CH and WH values obtained are observed to be closer to that of binary blended CNS samples. 3.3.5. Mineralogical characterization (XRD) Mineralogical characteristics of hydrated binary blended cement paste incorporated with different proportions of CNS were studied by means XRD analysis at the age of 7 and 28 days and the XRD pattern is shown in Fig. 18. It can be seen from the figure that most prominent peaks obtained from the analysis are calcium silicate hydrate (C-S-H) and calcium hydroxide (CH). The intensity of peak for CH was found to decrease with increase in amount of CNS content compared to that of control paste at 7 and 28 days, whereas, the intensity of C-S-H peak increased with increase in CNS content. The intense peak of CH was positioned at 2h angle of 18.2° and 34.171°, whereas C-S-H was at 29.47°. Figs. 19–21 represent XRD pattern for ternary and quaternary blended mixes, which shows the peaks of calcium hydroxide (CH) in the form of portlandite, calcium silicate hydrate (C-S-H) in the form of tobermorite, pure silicon dioxide (SiO2) in the form of quartz low syn, calcium silicates (unhydrated cement particles) and calcite. Strong peaks were observed for all the blended paste samples are CH (2h angle: 18.2° and 34.216°) and C-S-H (2h angle: 29.766° and 32.282°). In addition, quartz peaks were also noticed for samples containing FA and UFFA. It can be observed from Figs. 19 and 20 that compared to control samples, presence of nano-silica in both FN and UN samples amplified the peak of C-S-H and dramatically reduced the peak of CH. However, among FN and UN ternary blended samples, UN showed higher intensity of C-S-H and lower CH peak. It is important to note that suppressed peak of CH and extra peak of C-S-H was seen at the age of 28 days. Quartz peak was also observed and found to be intensified with the enhancement in replacement level in both FN and UN mixes. Similar trend of peaks like other CNS integrated ternary blended samples were also noticed in quaternary blended paste samples (Fig. 21). Extra peak of C-S-H at the age of 7 days itself is detected, however a greater number of C-S-H peaks are found at 28 days.

10

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

6.0

6.0

1 day 14 days

5.5 5.0

3 days 7 days 28 days 56 days

a

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 CP

FN-15P

FN-25P

FN-35P

Mixes

Chemical Shrinkage (ml/100 gm)

Chemical Shrinkage (ml/100 gm)

Fig. 14. Test specimen of chemical shrinkage resulted in failure of flask.

5.5 5.0

1 day 14 days

3 days 7 days 28 days 56 days

b

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 CP

UN-15P

UN-25P

UN-35P

Mixes

Fig. 15. Chemical shrinkage for ternary blended cement paste integrated with CNS (a) FN (b) UN paste mixes.

Reduced peak of CH was also noticed in both the ages. Among ternary and quaternary blended paste samples, quaternary blended paste samples indicated larger formation of C-S-H and were seen to be more noticeable at the age of 28 days.

Chemical Shrinkage (ml/100 gm)

6.0 5.5 5.0

1 day 14 days

3 days 7 days 28 days 56 days

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 CP

FUN-15P

FUN-25P

FUN-35P

Mixes Fig. 16. Chemical shrinkage for quaternary blended cement pastes integrated with FA, UFFA and CNS.

3.3.6. Microstructural analysis The SEM-EDX analysis was carried out for all the hydrated paste samples at curing age of 7 and 28 days. The Ca/Si atomic ratio was determined for all samples on the basis of elemental compositions obtained from EDX analysis. In view of conciseness, SEM-EDX image of control and CNS-3P samples at the age of 28 days is presented in Fig. 22. It can be noticed from SEM images that compared to control sample, CNS-3P showed a denser matrix. The atomic ratio of Ca/Si, calculated from EDX study was used to analyse the chemistry of C-S-H formation in the matrix of hydrated cement paste. It is reported that chemistry of C-S-H formation greatly relies on the chemical activity of existing calcium (Ca) and silicate (Si) ions in pore solution which are produced during the process of hydration [74]. Literature states that lower value of Ca/Si ratio characterizes the compact and densified microstructure of cement matrix due to the development of stronger network of C-S-H [75].

11

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

Fig. 17. TG/DTG curves for a) CNS mixes b) FN mixes c) UN mixes and d) FUN mixes at 28 days of curing age.

Table 6 CH and WH content of blended mixes at 7 and 28 days of curing age. Blends

Specimen

Control Binary blends

CP CNS 0.5P CNS 1.0P CNS 1.5P CNS 2.0P CNS 2.5P CNS 3.0P CNS 3.5P FN-15P FN-25P FN-35P UN-15P UN-25P UN-35P FUN-15P FUN-25P FUN-35P

Ternary blends

Quaternary blends

CH (%)

WH (%)

7 days

28 days

7 days

28 days

11.9 9.01 7.99 7.02 6 5.68 5.21 5.23 9.32 8.52 7 8.06 7.42 6.98 8.02 7.11 6.87

19.98 14.06 12.89 10.54 9.56 9.13 8.69 8.69 15.88 14.32 12.02 14.12 13.33 11.96 14.001 12.99 11.96

13.98 14.82 14.95 16.38 16.99 17.1 18.99 18.88 14.98 15.32 15.99 15.06 15.79 16.39 15.12 15.859 15.22

16.915 17.73 18.14 19.27 20.135 20.85 21.835 21.7 18.03 18.69 19.12 18.78 19.072 19.32 18.85 19.15 19.49

The resulted Ca/Si values for all samples at the curing age of 28 days arrived from the EDX analysis are summarized and are presented in Table 7. The EDX analysis demonstrates the reduced value of Ca/Si ratio for all the blended cement paste samples

incorporated with various combinations of FA, UFFA and CNS compared to that of control paste. The reduction of Ca/Si ratio could be due to accelerated reactivity of nano-silica and the additional formation of C-S-H [73]. The lowest value of Ca/Si ratio for ternary

12

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

- CH

C-S-H

- CH CNS-3.5P

a

C-S-H CNS-3.5P

b

CNS-3P

CNS-3P

CNS-2.5P

CNS-2.5P

CNS-2P

Intensity (a.u)

Intensity (a.u)

CNS2P

CNS-1.5P

CNS1.5P

CNS-1P

CNS-1P

CNS-0.5P

CNS-0.5P

CP

10

20

30

40

50

60

CP

70

10

20

30

40

2

2 (degrees)

50

60

degrees

Fig. 18. XRD pattern of binary blended and CNS admixed paste samples at the age of a) 7 days and b) 28 days.

CH -C-S-H

- Calcite Q- Quartz SiO2 Calcium silicate

CH C-S-H

FN-35P

FN-35P

b +

a

-Calcite Q-Quartz -SiO2 -Calcium silicate

Q

Q FN-25P

FN-12P

Q

FN-15P +

Q

Intensity (a.u)

Intensity (a.u)

+

FN-25P

Q

Q CP

+

CP

10

20

30

40

50

2 (degrees)

60

70

10

20

30

40

50

60

2 (degrees)

Fig. 19. XRD patterns of FN ternary blended hydrated cement paste samples at (a) 7 days (b) 28 days.

70

70

13

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

CH - Calcite C-S-H SiO2

Q

UN-35P

b

UN-35P

a

- Calcite Q- Quartz SiO2 Calcium silicate

CH C-S-H

Veterite Q- Quartz Calcium silicate

Q

UN-25P

UN-25P

Intensity (a.u)

Intensity (a.u)

Q

UN-15P

Q

UN-15P

Q

Q

CP

CP

10

20

30

40

50

60

70

10

20

30

40

50

60

70

2 (degrees)

2 (degrees)

Fig. 20. XRD patterns of UN ternary blended hydrated cement paste samples at (a) 7 days (b) 28 days.

CH - Calcite Q- Quartz SiO2 C-S-H Calcium silicate FUN-35P

Q

FUN-25P

+Q

Q

-Calcite Q-Quartz -Calcium silicate -SiO2

b

FUN-35P

+

+Q

FUN-25P

+

FUN-15P

+Q

FUN-15P

Intensity (a.u)

Intensity (a.u)

+

a

CH -C-S-H

Q CP

10

20

30

40

50

60

70

2 (degrees)

CP

10

20

30

40

50

60

70

2 (degrees)

Fig. 21. XRD patterns of FUN quaternary blended hydrated cement paste samples at (a) 7 days (b) 28 days.

blended FN and UN mixes were observed for FN-25P and UN-25P mixes. The quaternary blended cement paste showed comparatively lower Ca/Si ratio than control as well as ternary blends.

However, it is noticed that there is an increase in Ca/Si ratio for all ternary and quaternary blended samples having higher percentage of cement replacement i.e. 35%.

14

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212

CNS-3P- 28 days

Fig. 22. Image of SEM-EDX analysis for control paste and CNS-3P sample at curing age of 28 days.

Table 7 Ca/Si atomic ratio for control, binary, ternary and quaternary blended cement paste at the age of 28 days. Specimen

Ca/Si atomic ratio (28 days)

Control Binary blends

CP CNS 0.5P CNS 1.0P CNS 1.5P CNS 2.0P CNS 2.5P CNS 3.0P CNS 3.5P FN-15P FN-25P FN-35P UN-15P UN-25P UN-35P FUN-15P FUN-25P FUN-35P

2.186 1.903 1.694 1.593 1.382 1.276 1.051 1.185 1.768 1.628 1.815 1.656 1.446 1.761 1.554 1.247 1.697

Ternary blends

Quaternary blends

Ca/Si, atomic ratio (%)

Blends

WH% v/s Ca/Si Linear Fit of B

3.0

2.5

y= -0.2541x+6.481 2 R = 0.9385

2.0

1.5

1.0 14

16

18

20

22

WH% Fig. 23. Variation of Ca/Si ratio versus water related to hydration products (WH) excluding CH content.

3.3.7. Relationship between Ca/Si ratio and water related to hydration products (WH) Quantified C-S-H from TG analysis in the form of water related to hydration products (WH) was correlated with Ca/Si ratio and the same is presented in Fig. 23. It can be seen from figure that Ca/Si ratio found to be inversely proportional and is fitting linearly with WH% with a precision value of ‘‘R2 = 0.9385”. It is to be noted that higher WH% relates to lower Ca/Si atomic ratio which signifies solid microstructure of cementitious system on account of larger formation of C-S-H.

4. Conclusions On the basis of experimental results following conclusions can be drawn.  Implementing the theory of particle packing i.e. modified Andreasen and Andersen model, the right proportions of mineral admixtures could be proportioned for the production of

K. Snehal et al. / Construction and Building Materials 233 (2020) 117212









multi blended cement paste and mortar. The multi blend composite could achieve the best possible particle packing and chains in improving the mechanical, durability and microstructural properties. In binary blended cement mortar the highest compressive strength and lowest chloride ion permeability was corresponded to the optimum nano-silica dosage i.e. at 3%. While, compared to control and ternary blended samples, quaternary blended samples performed the best at all the curing ages owing to its better particle packing and pozzolanic effect. It can be concluded from TGA results of blended paste samples that the presence of supplementary materials with nano-silica combination showed the potential reduction in CH amount and improvement in WH quantity. The maximum consumption of CH was observed in binary blended sample containing 3% nano-silica and closer result was seen for quaternary blended sample. XRD analysis of binary blended paste samples revealed the significant increase in C-S-H peak and drastic reduction in CH peak with the increase in nano-silica content even at the early age of 7 days. In ternary and quaternary blended samples presence of nano-silica had an immense effect in C-S-H and CH peak, but the additional C-S-H peaks are much stronger after 28 days. Ca/Si atomic ratio attained from the results of SEM-EDX analysis was found to be lower for binary blended samples at all the ages indicating denser formation of C-S-H. However, all the composite samples showed the lower value of Ca/Si ratio compared to control sample. Quaternary blended samples executed the lowest Ca/Si ratio compared to control and other ternary blended samples.

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