Advance treatment by nanographite for Portland pulverised fly ash cement (the class F) systems

Composites Part B 82 (2015) 59e71

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Advance treatment by nanographite for Portland pulverised fly ash cement (the class F) systems Mehmet S. Kirgiz* Trakya University Architecture Faculty, Construction Department, Edirne 22100, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 August 2014 Received in revised form 24 April 2015 Accepted 6 August 2015 Available online 14 August 2015

This is a full article that overcomes such some negative side effects as rapid coagulation and reduced early strength in class F fly ash-substituted cement (FFA-SC) by serving nano graphite particle (nG). This study uses class F fly ash (FFA), nG, and ASTM type I cement as constituent materials to prepare proper pulverized fly ashePortland cement combinations (35% FFA þ 65% ASTM I þ 1.1% nG i.e.). Pastes include lime and/or lime þ nG, and tap water to examine ups and downs in Ca(OH)2 content. Mortars also contain these prepared cements, tap water and/or tap water þ super plasticizer (SP), and mortar sand in order to measure fluidity, flexural strength, and compressive strength according to present standard methods. Results indicate for FFA-SC system that the nano graphite particle increases the reduced early strength gain at early age, and the SP reduces the rapid coagulation. The use of nG also shows to be favorable in terms of the Ca(OH)2 content, the fluidity and the flexural strength gain, and compressive strength gain in FFA-SC system when compared to the pure Portland cement with and without nano graphite particle. © 2015 Elsevier Ltd. All rights reserved.

Keywords: A. Nano-structures A. Recycling B. Mechanical properties B. Physical properties Portland pulverised fly ash cement

1. Introduction Researches replace FFA with ordinary Portland cement in mortar, paste and concrete due to its pozzolanic activity which enhances properties of cement-based material. FFA exhibits some negative side effects such as rapid coagulation and the reduced early strength gain although it has pozzolanic effect on cement. In order to increase the use of pulverised fly ash in cement, there is a need to eliminate the negative side effects. Researches on FFA-SC system add nanoparticle (nP) for compensating these negative side effects. Thus, many comprehensive researches are carried out on treatment of the FFA-SC system with nP. Using of nP shows such some benefits as activation of cement hydration and regulation of early strength gain, which could serve to reduce the Ca(OH)2 formation. Moreover, many researches focus on nano silicon particle (nS) and demonstrate its effectiveness on enhancing early strengths of cement [1e19]. Nevertheless, a recent study carried out by Hou et al. shows that nS may have some negative effects on the later age properties of pure Portland cement [20]. Researches describe limestone powder (LP) as a filler to improve rheological

* Tel.: þ90 539 6145238. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.compositesb.2015.08.003 1359-8368/© 2015 Elsevier Ltd. All rights reserved.

properties of cement-based material. Studies show that LP can accelerate early age hydration, provide nucleation places for the Ca(OH)2, and react with tricalcium aluminate (C3A) and tricalcium silicate (C3S) to produce calcium carboaluminates (CeCeA) and calcium carbosilicate hydrates (CeCeSeH), respectively. Additionally, articles on nano calcium carbonate particle (nC) specify a potential for offsetting the aforementioned negative side effects in the FFA-SC system, even at high replacement rates [21e34]. However, proper proportioning of fly ash, the super plasticizer (SP), and a small addition of nano clay particle (nCL) in cement can increase strengths of mortar significantly after casting with little compromise to initial fluidity [35e38]. nCL reduces lateral hydraulic pressure significantly because this behavior relates to flocculation behavior which studies show that nCL increases flocculation strength and flocculation size. nCL has a high water absorption of 200% by mass, making it a possible governing factor [39e43]. It is important to recall that some previous studies were performed in the past on fly ash-blended high performance Portland cements, those cements containing nC and LP and nCL for overcoming significant drawback, like the reduced strength gain at early age. However, the aforementioned nano materials do not increase early strength gains of cement. Since using of nG increases compressive and flexural strength of Portland limestone cement system at 7d and 28d [44] author carries out this comprehensive study to add nG

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M.S. Kirgiz / Composites Part B 82 (2015) 59e71

for FFA-SC in order to overcome the aforementioned negative side effects. Moreover, this study motivates further exploration of using nG due to the rapid coagulation and strength reduction in FFA-SC system at early age. The study reported in this article aims to decrease the rapid coagulation and to increase the reduced early strength gain in FFA-SC system. It also presents some innovations provided for FFA-SC system by serving nano graphite particle in terms of the Ca(OH)2 content, the rheology, the flexural strength, and the compressive strength. The results can also provide to enhance for FFA-SC performance and reduce necessary pure Portland cement amount which ensures economic viability for engineering applications.

Table 2 Physical properties of ASTM type I cement. Code of cement

Physical properties Water demand Density Setting-time (min) (% by mass of cement) Initial Final

ASTM type I 30

This experimental study uses ASTM type I cement, nG particle, FFA particle, tap water, SP, and standard mortar sand as constituent materials to explain how to effect the nG on some properties of substituted-cement and mortar (Tables 1e4). Section Nano Graphite Particle (nG) explains detailed information for nG. Table 1 shows chemical compositions of ASTM type I cement, nG particle, and FFA particle. 2.1.1. ASTM type I cement Table 2 gives the number of physical properties of ASTM type I cement according to international present standards. 2.1.2. Nano graphite particle (nG) This comprehensive study uses the pure nG particle with primary particle size under 100 nm, thickness of 2.5 nm, and purity of carbon >99%. Table 3 gives some physical properties of nG particle. Since graphite is a metal based material composed of 2e3 mm diameter-disk-shaped, plateletless, hydrophobic, flaky particle, and pure carbon cell with atoms arranged in a regular hexagonal pattern, researches expect from the nG that can enhance the properties of FFA-SC system, similar to done by nC and carbon nano tube (CNT) [45e47]. Researches examine widely no nG particle for cement study, but studies on nC and CNT show a potential for offsetting the aforementioned negative side effects in Portland pulverised fly ash cement. Fig. 1 shows the scanning electron microscope (SEM) image of nano graphite particle. However, the nG can also act as a surfactant since it is hydrophobic, similar to how soap or shampoo can make a stain disperse in water. This characteristic is an agent provides for dispersion of insoluble material, such as fly ash. The reduced fineness caused very high surface area for nG particle is an advantage, too. This high surface area may enable to reduce rapid coagulation and increase the flexural strength and compressive strength for class F fly ashsubstituted cement (FFA-SC). Manufacturers can easily produce the nG particle in bulk quantities when compared to other nano materials, and hence the nG can resolve the strength-related issues

Table 1 Chemical compositions of ASTM type I cement, nG particle, and FFA particle. Materials

Chemical compositions (%, w/w) CaO SiO2

Al2O3 Fe2O3 MgO SO3 K2O Na2O C

ASTM type I 62.9 20.2 4.7 3.3 cement nG particle e e e e FFA particle 5.13 44.46 22.63 17.3

2.7

3.3

e

e

e

Alkali LOI e

1.1

99.9 e e 1.78

0.1 2.88

194

3-d

7-d

12

19

Codes of nanographite

Specific surface (m2/g)

Carbon content (%)

Bulk density (g/cm3)

Compressed density (g/cm3) (4.5 GPa)

(22 GPa)

nG particle

350

99

2.23

1.11

1.54

[48e51]. Nano graphite manufacturers also offer the nG particle with Blaine fineness at the range of 350e1250 (m2/g) [45e47]. 2.1.3. Pulverised fly ash (PFA) PFA appears in conventional power stations which are established around the world. Out of 270 million tons of fly ash generates each year around the world, only 20% is reused or recycled. Remaining 80% is landfilled which demonstrates potential environmental problem in close future. PFA has chemical composition as valuable as not to be landfilled. Table 1 shows chemical compositions of PFA. These chemical compositions describe the PFA as class F artificial pozzolan according to ASTM C593-95 and ASTM C618-12a for improving compounds of cement hydration and strengths of cement mortar [52,53]. The FFA is a highly fine powder including spherical particle less than 50 (mm) in size. Since FFA has valuable chemical compounds aforementioned, the FFA is the most commonly used artificial pozzolan by construction industry. Pozzolans are siliceous and alumina materials having an ability to form cementitious compounds when mixed cement, lime, and water. Table 4 gives some physical properties of FFA particle. Fig. 2 shows SEM image of the FFA. The FFA also contains inorganic glassy particle formed from the mineral matter in the coal. During burning, these minerals are in liquified state and combined chemically, and solidified with suspended exhaust gas. Electrostatic precipitators and/or bag houses then collects PFA. According to ASTM, lignite and/or subbituminous coals usually manufacture the class F fly ash that meets the requirements of construction industry. This class of fly ash displays some cementitious properties as well as pozzolanic properties. However, anthracite and/or bituminous coal combustion normally produces the class F fly ash. Canadian Standards Association also classifies the PFA in view of chemical composition. According to classification, CI type PFA has calcium oxide content varies from 8% to 20%, and Ch type PFA has more than 20% calcium oxide content. However, this association does allow no more than 5% PFA to blend cement due to the aforementioned negative side effects. FFA used

Table 4 Some physical properties of FFA particle. Bulk Soundness Strength Water Code of Moisture Fineness activity (Passing from density demand (%) fly ash content 3 index (%)a 325 sieve) (%) (g/cm ) (%) (%) FFA a

e e e e 1.03 1.68 1.69 0.67

156

Table 3 Some physical properties of nG particle.

2. Materials and methods 2.1. Materials

3.15

Compressive strength (MPa)

0.15

14.1

2.55

98

0.01

85

The strength activity index means that when fly ash is replaced with pure cement, the fly ash-cement should have 85% compressive strength of pure Portland cement according to ASTM C593-95 and ASTM C618-12a.

M.S. Kirgiz / Composites Part B 82 (2015) 59e71

Fig. 1. SEM image of nano graphite particle.

for this research has the lowest calcium oxide content in the constituent materials, less than 8% (Table 1). In a properly proportioned mix the FFA can help to improve some of the cement properties which can be enumerated reduction of water requirements and workability and self-consolidation and increasings of strength at later age. FFA also has some negative effect on properties of cement-based material aforementioned in Introduction. For instance, FFA-substituted cement system exhibits a rapid coagulation and the reduced early strength gain. This present article is interested in offsetting effort of the negative side effects. 2.1.4. Mixture proportion design and preparations of cement and mortar composites Another aim, which is different than goal presented in Introduction section, with this new FFA-SC containing nG relates

Fig. 2. SEM image of the FFA.

61

to engineering application, for instance concrete composite structures. However, in comparison with the potential cost of such structural material (the material not being marketed yet, it is impossible to discuss about the problem of cost), which may be obligatorily very high compared to “traditional” cements, it is essential to develop the mechanical performance to measure the mechanical performance/cost ratio. Bearing in this mind, it is necessary to save the matter and to use it only at useful places from a mechanical point of view. The cement avoiding the use of traditional way of blending is perfectly possible. The economy can pass through the choice of a simple and mechanically composite form. In this manner, mechanical tests are, thus, carried out on mortar composite representative of the cement function. It is consequently essential to reach characteristic mechanical behaviors that overcome the aforementioned negative side effects. Thus, this study includes three stages, mixture proportion design, preparations of cement, and mixings of mortar. First stage is mixture proportion designs for cement. Cement combination consists of four main types in view of constituent materials. One combination of these four types is 35FFAenGeASTM I containing 35% FFA and 65% ASTM type I cement and three different weighings of nG (1 g, 2.5 g and 5 g). Second combination of these types is FFAeASTM I containing 35% FFA and 65% ASTM type I cement without nG particle. Third type is nGeASTM I containing 100% ASTM type I cement and 1g-nG and 2.5g-nG and 5g-nG without FFA particle. Last type is ASTM type I cement as control cement (CC). Following procedure uses a medium planetary mixer in preparations of cement for 4 (min): (1) add FFA and ASTM type I cement or add FFA and ASTM type I cement and nG or add only ASTM I type cement and nG into bowl in order to homogenize; (2) mix them for 240 (s) at low speed; (3) and pack the homogenized cements to protect humidity. FFAenGeASTM I mortars and FFAeASTM I mortars and nGeASTM I mortars and ASTM I mortars contain these homogenized cements to measure how to effect the nG on some properties of cement in this research (Table 5) [54,55]. FFA-SC is considered starting from the same concept as fly ashPortland cement, but with some evolutions compared to the last. These evolutions are as follows: ❖ Whereas the fly ash-Portland cement contains no nG, the FFA-SC contains three different amounts of nG. ❖ Mortar composite, which is prepared by FFA-SC, can contain 0.76 water-to-cement ratio whereas the mortar composite, which is prepared by fly ash-Portland cement, contains 50% of it. Second stage deals with the preparations of mortar to monitor rheology and flexural strength and compressive strength at early age. M1, M2, and M3 mortar includes tap water:cement:sand:nG ratio of 1:1.3:6:0.0044, 1:1.3:6:0.011 and 1:1.3:6:0.022 respectively. M4 mortar also consists tap water:cement:FFA:sand ratio of 1:1.3:0.7:6. M5, M6, and M7 mortar comprises tap water:cement:FFA:sand:nG ratio of 1:1.3:0.7:6:0.0044, 1:1.3:0.7:6:0.011 and 1:1.3:0.7:6:0.022, respectively. However, control mortar (CM) takes place tap water:cement:sand ratio of 1:1.3:6 as seen in Fig. 3 (Table 5). Following procedure uses a medium planetary mixer in preparations of mortar for 4 (min): (1) add water and the aforementioned homogenized cement into bowl; (2) mix them for 30 (s) at low speed; (3) add standard mortar sand at 30 (s); (4) mix them for 30 (s) at high speed; (5) stop the mixer for 15 (s) to scrape bowl; (6) mix for 60 (s) at high speed; (7) fill each mortar mixings in prism molds 25.4  44  100 (mm) or cubic molds 50.8  50.8  50.8 (mm) as three layers; (8) collapse each layer 60 times [54,55]. Table 5 presents types of cement, codes of mortar, mixture proportions of mortar, and water-to-cement (w/c) ratio, water-to-sand (w/s) ratio, and cement-to-sand (c/s) ratio.

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Table 5 Types of cement, codes of mortar, mixture proportions of mortar, and water-to-cement (w/c) ratio, water-to-sand (w/s) ratio, and cement-to-sand (c/s) ratio [54,55]. Types of cement

ASTM I 1nGeASMT I 2.5nGeASMT I 5nGeASMT I 35FFAeASMT I 35FFAe1nGeASMT I 35FFAe2.5nGeASMT I 35FFAe5nGeASMT I

Codes of Mortar

Control M1 M2 M3 M4 M5 M6 M7

Mixture proportions of Mortar Cement (kg/m3)

FFA (kg/m3)

Water (L/m3)

Sand (kg/m3)

nG (kg/m3)

380.8 380.8 380.8 380.8 380.8 380.8 380.8 380.8

0 0 0 0 205 205 205 205

292.9 292.9 292.9 292.9 292.9 292.9 292.9 292.9

1757.8 1757.8 1757.8 1757.8 1757.8 1757.8 1757.8 1757.8

0 1.3 3.2 6.5 0 1.3 3.2 6.5

2.2. Methods 2.2.1. The Ca(OH)2 content experiments of paste In order to make a better explaining the effects of nG particle on the Ca(OH)2 content, this study mixes pastes containing 100% lime and 86.5% lime þ 13.5% nG. Vicat needle set monitors the reductions of the Ca(OH)2 content, similar to normal setting condition in cement paste without heat decomposition. Vicat needle measures the Ca(OH)2 content of paste from loss on diving (LOD). Following procedure uses a medium planetary mixer for mixing every lime paste: (1) add water and pure-lime or nG blended-lime in bowl; (2) mix them for 90 s at low speed; (3) stop mixer for 15 s to scrape bowl (4) mix for another 90 s at low speed; (5) fill fresh pastes in Vicat mold for 4 min. Pastes are mixed 1:1 water-to-lime ratio [54e57].

W/C

W/S

C/S

0.76 0.76 0.76 0.76 0.76 0.76 0.76 0.76

0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.16

0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.21

2.2.2. Rheology experiments of mortar Set of flow table specifies the fluidities of mortar. Following procedure examines for fluidity ratios of mortar: (1) wipe carefully the flow table to clean and dry; (2) place the flow mold at the center of flow table; (3) place a layer of mortar about 25 (mm) in thickness into the mold; (4) tamp 20 times by tamper, the tamping pressure is sufficiently done to ensure uniform filling of the mold; (5) fill again the mold with mortar and tamp as specified in 4th sequence; (6) cut off the mortar to a plane surface flush with the top of the mold by drawing the straightedge or the edge of the trowel with a sawing motion across the top of the mold; (7) wipe the flow table top to clean and dry, being especially careful to remove any water from around the edge of the flow mold; (8) lift the mold away from the mortar 1 min after completing the mixing operation; (9) drop the set of flow table 25 times in 15 s, unless otherwise specified [54,55,58].

Fig. 3. Graphical abstract for study sequence [54,55].

M.S. Kirgiz / Composites Part B 82 (2015) 59e71

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Table 6 Types of cement, codes of mortar, flexural strength of mortar, compressive strength of mortar, fluidity of mortar, and standard deviations for these mortar properties. Codes of mortar

Flexural strength (MPa) 1-d

Standard deviation

2-d

Standard deviation

3-d

Standard deviation

Compressive strength (MPa) 1-d

Standard deviation

2-d

Standard deviation

3-d

Standard deviation

Flow (mm)

Standard deviation

Control M1 M2 M3 M4 M5 M6 M7

1.6 1.5 1.3 1.6 1.9 2.4 2.5 1.8

0.1 0.07 0.03 0.03 0.05 0.07 0.2 0.04

1.9 2.4 2.1 2.7 3 3.6 2.8 2.2

0.1 0.09 0.1 0.06 0.08 0.09 0.03 0.01

2.5 2.7 2.8 3.1 3.3 3.8 2.9 2.5

0.08 0.1 0.07 0.05 0.1 0.02 0.08 0.1

3.4 3.7 3.6 3.5 5.7 4.9 3.7 3.7

0.6 0.1 0.2 0.2 0.4 0.1 0.03 0.06

4.6 5.1 6.2 8.1 7.8 9 6.5 6.7

0.1 0.2 0.2 0.1 0.5 0.2 0.2 0.5

7.7 7.7 8 8.4 8 10.1 7.2 6.7

0.2 0.3 0.05 0.1 0.9 0.2 0.5 0.5

165 145 135 130 160 158 140 135

0.02 0.01 0.09 0.03 0.1 0.07 0.04 0.2

Table 6 also shows flows of mortar without SP, and Table 7 specifies types of cement, codes of mortar, mixture proportions of mortar and SP content, and flow results of mortar with SP.

Fluidity

result should be divided ten. Fig. 3 presents a graphical abstract for study sequence. 3. Results and discussions

2.2.3. Flexural strength experiments of mortars TS EN 196-1 standard measures flexural strengths of prism mortar (25.4  44  100 mm) at 1-d, 2-d, and 3-d. After 1 day of casting, sample displaces from mold and places into water in humidity controlled curing closet at 22 ± 3  C and 98% relative humidity until testing. MTS hydraulic flexure set tests three samples for each mix at each ageeloading rate of the test is 0.008 (mm/s) [54,55,59], and the average value presents the descriptive flexural strength and standard deviation gives a measure to quantify for normal distribution of the flexural strength in Table 6. Equation (1) calculates flexural strength abbreviated as Rf in N/mm2.

Rf ¼ 1:5 

n . o Ff  L b3

(1)

where Rf is the flexural strength, in N per square mm; b is the side of the square section of the prism, in mm; Ff is load applied to the middle of the prism at fracture, in N; L is the distance between supports, in mm. For transforming N/mm2 to MPa, result should be divided ten. 2.2.4. Compressive strength experiments of mortars TS EN 196-1 standard also measures compressive strengths of cubic mortar (50.8  50.8  50.8 mm) at 1-d, 2-d, and 3-d. After 1 day of casting, sample displaces from mold and places into water in humidity controlled curing closet at 22 ± 3  C and 98% relative humidity until testing. MTS hydraulic compression set tests three samples for each mix at each ageeloading rate of the test is 0.008 (mm/s) [54,55,59], and average value specifies the descriptive compressive strength and standard deviation gives a measure to quantify for normal distribution of the compressive strength in Table 6. Equation (2) calculates compressive strength abbreviated as Rc in N/mm2.

Rc ¼ fðFc =2580:6Þg

(2)

where Rc is the compressive strength, in N per square mm; Fc is maximum load at fracture, in N. For transforming N/mm2 to MPa,

Table 6 summarizes types of cement, codes of mortar, flexural strength of mortar, compressive strength of mortar, fluidity of mortar, and standard deviations for these mortar properties. Following figures states how to effects the nG on properties of hydraulic cement mortar. 3.1. The Ca(OH)2 contents Fig. 4 shows some reduction of the Ca(OH)2 content accompanying with the increasing nG in the paste. Hydration of cement usually produces the Ca(OH)2 while reactions between hydration product reduces the Ca(OH)2, 40%-Ca(OH)2 is in cement paste prepared by 50% water-to-cement ratio. This calcium hydroxide mass provides the setting-time and stiffness for cement at the beginning of hydration reaction between cement and water. After that starting reaction finishes, this grand mass waits in cement paste with showing no reaction. The effect of nG particle on the Ca(OH)2 is significant for cement hydration in view of originating novel reaction between the Ca(OH)2 and carbon, like calcium carbon hydroxide (CaC(OH)2) and calcium carbon oxide (CaCO). In comparison with Fig. 4, one notes that during the first 5 (h) of hydration, the nG addition cannot decrease the Ca(OH)2 content; there is no difference in the Ca(OH)2 content between control paste and nG-blended paste. Reduction in Ca(OH)2 content is then due to the reaction mechanism between the Ca(OH)2 and carbon in nGblended paste. This effect takes place apparently after 8.5 h which the nG reaction finishes [54,55]. Fig. 4 compares difference for the Ca(OH)2 content between nG-blended paste and control paste. Two different parts can explain reduction of the Ca(OH)2 content shown in Fig. 4. Before 5 h, mechanism of the nG provides stabilization for Ca(OH)2 content. Since its activator effect almost starts after 5 h, the Ca(OH)2 content keeps on decreasing. This means that generation of Ca(OH)2 slows down. Analysis shows that

Table 7 Types of cement, codes of mortar, mixture proportions of mortar and SP content, and flow results of mortar [54,55]. Types of cement

35FFAeASMT I 35FFAeASMT I 35FFAe5nGeASMT I 35FFAe5nGeASMT I

Codes of mortar

Mixture proportions of mortar Cement (kg/m3)

FA (kg/m3)

Water (L/m3)

Sand (kg/m3)

nG (kg/m3)

SP (kg/m3)

M4 M4-SP M7 M7-SP

380.8 380.8 380.8 380.8

205 205 205 205

292.9 292.9 292.9 292.9

1757.8 1757.8 1757.8 1757.8

0 0 6.5 6.5

0 4.1 0 5.8

W/C

W/S

C/S

Flow (mm)

Standard deviation

0.76 0.76 0.76 0.76

0.16 0.16 0.16 0.16

0.21 0.21 0.21 0.21

160 220 135 160

0.1 0.03 0.2 0.05

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Fig. 4. Difference for the Ca(OH)2 content between nG-blended paste and control paste [54,55].

after 8.5 h of hydration, 4%-Ca(OH)2 reacts with carbon in the nGblended paste while control paste does display no reaction (Fig. 4). Using of nG particle shows such a benefit as activation for cement's hydration product which could serve to consume the Ca(OH)2 content [54,55]. 3.2. Rheology Flow of cement-based material deals with the rheological properties of the fresh cement paste and sand. Since sand is a rigid granulated and water-absorbing material, it bars flow of cement paste to absorb water of mixing. Water-to-cement ratio blocks this flow behavior of cement paste, too. Mechanical properties of hardened cement-based material increase if less water is used in the mix design, however, reducing of water-to-cement ratio may cause a lack of ease for mixing and application, like workability. To avoid these undesired impacts, researches use vibrator and/or superplasticizer to decrease the apparent yield stress and the viscosity of fresh cement paste typically. Understanding of this

rheological behavior depending on ratio of fluidity may provide a scientific approach for onsite workability [60e63]. Fresh state influenced by the rheological factors of mortar checks reliability of cement-based material quality. Factors include some conditions, such as temperature and humidity, mix proportion, and characteristics of constituent material, to effect the rheological properties of cement-based material mixed freshly. Furthermore, the mixing process also influences the rheological parameters, including mixer type, mixing sequence, and duration [64]. The measurements start 3.5 min after water contacts the cement, and the room is 23  C temperature and 53% humidity constantly so that the aforementioned factors do not affect these experiments. Table 6 compares fluidity of mortar in view of types of cement, codes of mortar, and standard deviations. Fig. 5 shows the effect of nG on the flow of ASTM I, ASTM I/nG, FFA-SC, and FFA-SC/nG mortar, with the 0.76 water-to-cement ratio. There is an obvious decrease in flow as nano graphite particle is increased in the FFA-SC system and the pure Portland cement system (Table 6, Fig. 5). Since nG is hydrophobic, the rapid coagulation may cause the reductions of flow in

Fig. 5. The effect of nG on the flow of ASTM I, ASTM I/nG, FFA-SC and FFA-SC/nG mortar [54,55].

M.S. Kirgiz / Composites Part B 82 (2015) 59e71

mortar. The highest coagulation is in the M7 mortar containing 35% FFA and 65% pure Portland cement and 1.1% nG. Ratio of 0.022 nG-tow leads to over 14% lower fluidity for M7 mortar when compared to M4. The highest increase at fluidity is in the CM. Blending of 35%-FFA underlies to reduce thixotropy of M4 mortar, more than over 3% (Table 6, Fig. 5). Thixotropy is a time-dependent shear thinning property. In other words thixotropy is an antonym for coagulation. Certain fluids that are thick under static conditions will flow over time when vibrating, agitated or stressed. They then take a fixed time to return much viscous state than before. Causing of the greatest coagulation in M7 may attribute high water demand and negative soundness (shrinkage) of FFA as seen in Table 4. Since nG is blended for mortar containing FFA-SC, average of fluidity is over 6% lower than M4 mortar without nG. These results reveal that nG particle decreases the thixotropy of mortar more than the thixotropy is increased by FFA particle [54,55]. However, using of nG could combine pure Portland cement and the FFA particle to show such a benefit as reducing of mortar thixotropy. As nG has flaky-shaped particle seen in Fig. 1, it serves to reduce the flow from 160 (mm) to 140 (mm) in M5, M6, and M7 FFA-SC mortar (Table 6). This flaky-shaped particle leads to an advantage for nG to bring together FFA and cement particle properly in mortar. Consequently, it is accurate that the nG is a fluidity reducer material for FFA-SC system to increase coagulation. Reducing of flow is more than over 14% in FFA-SC mortar with nG (Table 6). These flow results reveal that the nG does not separate pure Portland cement and FFA particle each other in mortar. It combines them, instead (Table 6, Fig. 5). Moreover, freshly mixed cement paste is a suspension of noncolloidal rigid particle which is suspended in an interstitial water. The deformability of cement particle is minor, but their flocculation affects the rheological behavior of the cement mortar. Flocculation is a process which undispersed particle grows in form of cluster by a combination of van der Waals and electrostatic forces [63]. A suspension with flocculated cement particle has high viscosity, but this value decreases when the particle is dispersed equally. Flocculation is main reason that cement mortar shows thixotropy. The thixotropy is reversible whereas irreversible stiffening of fresh mixture increases the viscosity monotonically and causes the setting for cement-based material finally. Coagulation contributes to the irreversible stiffening between cement particles, but its rate is no high in the period of mixing and placement. Additionally, the coagulation may affect the quasi-static response of mixture related to the construction process. Formwork pressure of

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self-consolidating cement based material [65e67] and slip formwork for pavement [68] are examples showing this relation. Researches reports indirect evidence of this relation both the pressure response for estimating the formwork pressure and the shape stability correlated with rheology and chord length measurement of the floccules [30,69]. However, proper proportioning of fly ash and small addition of nG in cement can reduce lateral hydraulic formwork pressure significantly because nG does allow no flocculation as revealed in this study (Table 6). Fig. 6 shows relationship between additive of nG and flows of FFA mortar [54,55]. Fig. 6 presents polynomial equation to show the strong relationship between amount of nG and flow. The R-squared value is on the figure and shows well compatibility between amount of the nG and flow of FFA mortar. As figure exhibits, one may predict mortar flow from amount of the nG additive given at least one of them (Fig. 6). Fig. 6 also proves how nano-size-nG with flaky particle decreases the flow while the FFA particle increases it (Table 6, Fig. 6). FFA particle has lower packing fraction as well as smaller change in packing fraction while the nG shows opposite trends. The study reveals another significant result that nG is an effective material for improving the formwork pressure-stability as seen in result of M7 mortar (Table 6). This study also presents novel information that is a direct connection between macro behavior and flocculation behavior. Nano materials can provide further progress for cement-based material industry to combine cement grain and additive homogeneously to improve flocculation behaviors, and create superior materials modified fresh properties properly for needing of construction industry. 3.2.1. Advance treatment for coagulations in FFA-C systems Since nG brings together FFA and cement effectively, M7 mortar displays the aforementioned rapid coagulation. The coagulation contributes to the irreversible stiffening in M7 and its rate is too high as not to permit the mixing and placement these mortar in formwork. This study fills, therefore, no M7 mortar to mold easily. To reduce this rapid coagulation, author carries out additional study to add SP for mixtures of M4 and M7 mortar. Table 7 states types of cement, codes of mortar, mixture proportions of mortar and SP content, and flow results of mortar. Author decides to reach optimum SP ratio when flow of M7-SP increases from 135 (mm) to 160 (mm) (Table 7). On the other hand, this M7-SP flow value is equal flow of M4 mortar without nG. Fig. 7 shows effect of optimum SP content on the coagulations of FFA-SC/SP and FFA-SC/nG/SP mortar combination compared by

Fig. 6. Relationship between amount of nG and flow of FFA mortar [54,55].

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Fig. 7. Effect of optimum SP content on the coagulations of FFA-SC/SP and FFA-SC/nG/SP mortar combination, and comparison of flow between FFA-SC mortar combinations and FFA-SC/nG mortar combinations [54,55].

mortar containing FFA-SC and FFA-SC/nG. Water-to-cement ratio is 0.76 as well as optimization of SP is 1% of binder mass in order to measure standard flow for FFA-SC and FFA-SC/nG combination. The highest ratio of flow change is in the M4-SP containing FFA-SC, over 33% greater. Ratio of 0.022 nG-to-w provides equal flow for M7-SP mortar when compared to M4. The lowest flow is in M7 mortar since it has no SP. The rapid coagulation reduced by SP is over 15% lesser in M7-SP than that of M7 mortar without SP. These results reveal that FFA-SC system blended 1.1% nG needs 1% SP by mass of binder as an optimum ratio in order to protect the FFA-SC system from rapid coagulation (Table 7, Fig. 7) [54,55]. 3.3. Flexural strength gains Axial force of flexure specifies flexural strengths of mortar containing pure cement, FFA and nG particle at 1st-d, 2nd-d, and 3rd-d after water curing at 22 ± 3  C and 98% relative humidity. Table 6 summarizes flexural strength gains of mortar in view of types of cement, codes of mortar, and standard deviations. Presented Fig. 8 is a representative for increasing ratios of flexural strength at 1st-d, 2nd-d, and 3rd-d. Water-to-cement ratio is 0.76 for preventing its positive effect on strength gain. However, curings of warm waters are not done so that strengths of mortar are not

positively influenced. This test, therefore, observes gains of standard flexural strength for FFA-SC system reinforced by nG. Since cement-based material is non-resilience, this article expresses the results such changes of flexural strength as ups-and-downs at early age [70e72]. Author does, therefore, signify no effects of cement hydration microstructure on flexural strength gain. His studies explain that increasing of loss on ignition (LOI) and alkali causes the reduced strength gain in cement paste. Effect of admixture containing high calcium oxide on strength gain deals with fluctuation of silicon oxide (SiO2), sodium oxide (Na2O), and alkali as well as increasing of LOI. This result unveils that more than 6% of admixture containing high calcium oxide has no positive effect on early strength gains of mortar [70e72]. In comparison with Table 6 and Fig. 8, one can indicate that the highest flexural strength is in the M5 mortar containing 35% class F fly ash, 65% ASTM type I cement and 1.1% nG, 3.81 (MPa) at 3d (Table 6). This is in coherence with author's previous study [44] when considering strength gain for mortar containing 35% marble powder and 65% pure Portland cement and 1.1% nG at 7d and 28d. After 1st-d casting of mortar, M1 and M2 display over 6% and 21% lower flexural strength than that of CM, except M3 mortar. M3 mortar presents 2.5% greater flexural strength with respect to CM at 1d. M4, M5, M6, and M7 mortar also displays, respectively, over 21%, 51%, 60% and 12% greater flexural

Fig. 8. Effects of nG on flexural strength gain, ratio of flexural strength gain ¼ 100 (Strength of mix with nG/Strength of control mix, w/c ¼ 0.76 [54,55].

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strength when compared to CM (Table 6, Fig. 8). In contrast with these results, one points that while effect of FFA provides over 21% greater flexural strength gain for pure Portland cement, nG leads to a growth of 2.5%, and that combination effect of FFA accompanying with nG is approximately 41% growth for flexural strength in FFASC system at 1d [54,55]. On the other hand, chemical composition changings of cement paste explain mechanism of admixture containing high calcium oxide. Blending of high calcium oxide based admixture provides to continue the strength gain in mortar after 28d since it activates hydrations of calcium based compound in cement paste. High calcium oxide based admixture, therefore, tends to be an activator for early strength gains of mortar. With high blending ratios of this admixture in this mortar, increasing ratio of strength gain is greater than that of pure Portland cement mortar [70e72]. After 2nd-d, M1, M2, and M3 mortar displays, respectively, over 31%, 12%, and 42.5% greater flexural strength than that of CM. M4, M5, M6, and M7 mortar also presents, respectively, over 58%, 92%, 47% and 18% higher flexural strength when compared to CM, respectively (Table 6, Fig. 8). In comparison with these results, this article unveils that while FFA impact has over 58% greater flexural strength for Portland cement, nG impact has over 28% greater, and that combination effect of FFA and nG on flexural strength gain is 52% growth for FFA-SC system at 2d [54,55]. After 3rd-d, flexural strength for M1, M2, and M3 mortar is over 5%, 8%, and 20% greater than that of CM. M4, M5, and M6 mortar also presents, respectively, over 31%, 48%, and 16% greater flexural strength when compared to CM. M7 mortar shows over 2.5% lower flexural strength than that of CM (Table 6, Fig. 8). With peering to examine these results, this study states that while FFA impact provides over 11% greater flexural strength for pure Portland cement, nG impact has over 32% higher and that combination effect of FFA and nG is 32% growth for flexural strength in FFA-SC system at 3d. These results also reveal that the increasing of FFA and nG leads to an increasing approximately 50% in the average flexural strength of FFA-SC system. As it meets the target strength class for FFA-SC system according to Strength Activity Index, the study chooses the mixture proportion of M5 as an appropriate receipt (Tables 5 and 6, and Fig. 8) [54,55]. According to results, there are at least two mechanisms on early flexural strength gains of FFA-SC system containing nG particle. First, nG with flaky structure can open space between cement particles to combine cement and FFA properly, providing the nucleation and growth of hydration product called calcium carbon hydroxide (CeCeH). In this case, nG particle

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increases the strength gain for FFA-SC mortar at early age. Second but perhaps less likely, the nG particle could provide potentially an additional source of carbon for activation of calcium oxide in cement hydration. Since cement consists of 65% calcium oxide (CaO) content, the rest of 35% cement oxide is not enough to saturate this CaO content, during cement hydration for gaining of hardness and strength. nG particle may, therefore, produce the calcium carbon oxide (CaCO) since the nG particle can trigger theefree 30%eCaO content in cement as seen in Fig. 4 [46,47]. 3.4. Compressive strength gains Axial force of compression measures compressive strengths of mortar containing FFA and nG and pure Portland cement at 1st-d, 2nd-d, and 3rd-d after water curing at 22 ± 3  C and 98% relative humidity. Table 6 compares compressive strength gain of mortar in view of types of cement, codes of mortar, and standard deviations. Fig. 9 presents the rising ratios of compressive strength at 1st-d, 2nd-d, and 3rd-d. Water-to-cement ratio is 0.76 for preventing its positive effect on strength gain. However, curings of warm waters are not done so that strengths of mortar are not positively influenced. This test, therefore, observes gains of standard compressive strength for FFA-SC systems strengthened by nG. This article also discusses results of compressive strength for cement mortar at 1-d, 2-d, and 3-d as seen in Fig. 9 [54,55]. Studies also explain mechanisms of admixture containing artificial pozzolan for strength gain effected by chemical composition changings of cement paste. Blending of this admixture provides to enhance positively the compressive strength for mortar up to 90th-d since it activates hydration of silica based compound in cement paste. It is, therefore, an activator for compressive strength gains of mortar at later age. With high blending ratio of artificial pozzolan, the mortar has greater compressive strength than that of pure Portland cement mortar [70e72]. Mortar containing high calcium oxide based admixture has lower compressive strength than that of pure Portland cement based mortar. This deals with the increasing of calcium-to-silicon (Ca/Si) ratio from 1.76 up to 2.8. On the contrary to this knowledge, compressive strength is the greatest in mortar containing artificial pozzolan. This attributes the increase of silicon-to-calcium (Si/Ca) ratio from 0.53 up to 0.96 [70e72]. The results show for the FFA-SC combinations that regardless of the cement combinations introduced, the compressive strength gain tends to increase proportionally by increasing the nG particle. In comparison with Table 6

Fig. 9. Effect of nG on compressive strength gain, ratio of compressive strength gain ¼ 100 (Strength of mix with nG/Strength of control mix, w/c ¼ 0.76 [54,55].

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and Fig. 9, one notices that the highest compressive strength is in the M5 mortar containing 35% class F fly ash, 65% ASTM type I cement and 0.22% nG, 10.1 (MPa) (Table 6, Fig. 9). This is in coherence with author's previous study [44] when considering 7-day and 28-day strength gain. After 1st-d casting of mortar, compressive strengths related to M1, M2, and M3 are, respectively, over 7%, 4.5%, and 0.5% greater than that of CM. M4, M5, M6, and M7 mortar has, respectively, over 63%, 48%, 8%, and 8.5% higher compressive strength when compared to CM (Table 6, Fig. 9). With peering to compare these results, the article can make a logical knowledge that while FFA provides over 63% higher compressive strength for pure Portland cement, nG confers over 4.1% greater, and that combination effect of FFA and nG has over 52% greater compressive strength gain for FFA-SC system at 1d. After 2nd-d, compressive strengths related to M1, M2, and M3 are, respectively, over 9%, 35%, and 74% greater than that of CM. M4, M5, M6, and M7 mortar also presents, respectively, over 67%, 93%, 39%, and 45% greater compressive strength when compared to CM (Table 6, Fig. 9). In contrast with these results, this study shows that while FFA impact provides over 67% greater compressive strength for pure Portland cement, nG impact has over 39% higher, and that combination effect of FFA and nG is approximately 60% growth for compressive strength gain in FFA-SC system at 2d [54,55]. After 3rd-d, compressive strength for M1, M2, and M3 mortar is over 0.5%, 3.5%, and 8.5% greater than that of CM. M4 and M5 mortar also confers over 4% and 31% higher compressive strength when compared to CM. M6 and M7 mortar shows over 6% and 3% lower compressive strength than that of CM (Table 6, Fig. 9). Author carries out section 3.4.1 as additional study to overcome the reduced compressive strength in M4 and M7 mortar. With comparison these results, this article can make a logical inference that while FFA impact provides over 4% greater compressive strength for pure Portland cement, nG impact is over 4.4% higher and that combination effect of FFA and nG is over 30% growth for compressive strength in FFA-SC system at 3d. Results also reveal that the increased FFA and nG leads to an increasing of approximately 46% in average compressive strength for FFA-SC system. As it meets the target strength class for FFA-SC system according to Strength Activity Index, the study chooses the mixture proportion of M5 mortar as a proper receipt for enhancing the reduced early compressive strength (Table 5, Table 6, and Fig. 9) [54,55]. On the other hand, blending of artificial pozzolan accelerates the tendency of strength gain at later age because it reduces the LOI in the cement paste. Using of 35%-artificial pozzolan displays the greatest compressive strength and flexural strength for mortar at later age. Effect of artificial pozzolan on strength gain deals with increasing of SiO2 in cement paste. Average of SiO2 tends to increase in cement paste continuously. Strength gain acts similar as SiO2 in mortar containing artificial pozzolan. Since mortar containing artificial pozzolan achieves nearly 52.5 MPa compressive strength at 90d, blending up to 35% artificial pozzolan has positive effect on strengths of cement mortar [70e72]. During the 1-d of hydration, the formation of the Ca(OH)2 is mainly responsible for the strength gain in the FFA-SC system as proved the reduced Ca(OH)2 content in Fig. 4. Portland cement paste has the greatest Ca(OH)2 content when compared to other cement combination. This strength evolution deals with the formation of other hydration products at early age, most probably ettringite and calciumesiliconehydrates (CeSeH) and calciumealuminaehydrates (CeAeH) and calcium hydrates (CH) because of absence the nG particle [70e72]. The effect of this possible set of hydration product on the strength development of pure Portland cement could be potentially greater than that of the Ca(OH)2 in the FFA-SC system containing nG. According to the Ca(OH)2 content graph in Fig. 4, this strength gain is because the aforementioned hydration

products do not saturate the Ca(OH)2 fully in pure Portland cement system at 1d in order to provide full amounts of ettringite, CeSeH and CeAeH for high strength-possibly because of low reactivity of the cement raw material. Compressive strength results show in FFA-SC system that the increased FFA and nG particle gives over 32% higher rising movement for average compressive strength when compared to CM. Chosen M5 mortar preparation is, therefore, a proper recipe containing 35% FFA, 65% ASTM type I cement and 0.22% nG as it meets the target strength class according to Strength Activity Index (Table 6, Fig. 9) [54,55]. In addition to mechanisms of nG particle on strength gain mentioned in section 3.3 Flexural Strength Gain, FFA/nG combination plays an effective role on early strength gain since FFA particle has pozzolanic effect and nG particle has increasing effect on binder properties of cement. This combination effect provides more than over 27% strength gain while mono effect of nG particle is lower over 8.4 times than that of this combination effect (Table 6, Fig. 9) [54,55]. 3.4.1. Advance treatment for reductions of compressive strength gains Since nG is responsible for the rapid coagulation in FFA-SC system, M7 mortar displays the reduced compressive strength gain at early age. In order to overcome the reduced strength gain, author carries out this additional study to add the SP for M4 and M7 mortar. Author also examines optimum SP ratio for FFA-SC system after many mortars are cast by the SP. Table 7 summarizes types of cement, codes of mortar, mixture proportions of mortar and SP content, and flow results of mortar. Table 8 also specify the compressive strength gain after author adds superplasticizer for FFA-SC mortar. Fig. 10 shows effect of optimum SP content on the reduced compressive strength gain of FFA-SC and FFA-SC/nG mortar combination compared with FFA-SC and FFA-SC/nG containing no SP [54,55]. Water-to-cement ratio is also 0.76 as well as optimized SP is 1% of binder mass in order to measure growth for compressive strength gain of FFA-SC and FFA-SC/nG combination. The highest compressive strength growth is in the M4-SP mortar, more than over 75%. Average mono increasing effect of SP on strength gain of FFA-SC is more than over 5%, while average combination effect of FFA/nG/SP is more than 41%. In other words, ratio of 1.1-nG/SP increases the compressive strength of M7-SP mortar more than over 41% when compared to M4 (Table 8, Fig. 10). However, M7 compressive strength is over 69% lesser than that of M7-SP. This result reveals that FFA-SC system blended 1.1%-nG (5g-nG) needs 1% SP by mass of binder as optimum ratio in order to increase compressive strength. The aforementioned result proves proficiency of author's decision regarding on optimization of SP content for FFA/nG mortar. Fig. 11 shows rising movements for compressive strength increased by nG particle and SP content [54,55]. Results show that the increased FFA and nG gives 31% average rising movement for compressive strength gain in FFA-SC mortar combination when compared to CM. Chosen M7-SP preparation is, Table 8 Compressive strengths of FFA-SC and FFA-SC/nG mortar with and without SP. Types of cement

Codes of Compressive strength (MPa) mortar 1-d Standard 2-d Standard 3-d deviation deviation

35FFAeASTM I M4 35FFAeASTM I M4-SP 35FFAe5nGeASTM I M7 35FFAe5nGeASTM I M7-SP

5.7 6.1 3.7 4.3

0.4 0.2 0.06 0.08

7.8 7.6 6.7 7

0.5 0.1 0.5 0.02

8 8.9 6.7 11.3

Standard deviation 0.9 0.4 0.5 0.05

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Fig. 10. Effect of optimum SP content on the reduced compressive strengths of FFA-SC/SP and FFA-SC/nG/SP mortar combination and comparisons between FFA-SC mortar and FFASC/nG mortar [54,55].

Fig. 11. Rising movements for compressive strength increased by nG particle and SP content [54,55].

therefore, a proper mortar receipt containing 35% FFA, 65% ASTM I type cement, 1.1% nG and 1% SP as it achieves to overcome the target strength class (Fig. 11) [54,55]. 4. Conclusions This experimental study scope presents novel information regarding on advance treatment of Portland pulverised fly ash cement by nG particle and SP to overcome the rapid coagulation and the reduced strength gain. Author also writes it to develop better understanding related how to effect of nG particle on the properties of FFA-SC system. Results obtained these experiments support following novel conclusions:  This study defines mechanisms of nG particle as consumer of the Ca(OH)2 content, reducer of thixotropy and increaser of strength gain for FFA-SC system as well as pure Portland cement. Since the nG consumes partially the Ca(OH)2 content in FFA-SC system, this increases flexural and compressive strengths of FFA-SC system considerably. As nG is blended from 0.22% to 1.1% in

mortar, FFA-SC system decreases the flow, more than over 30%. The nG particle, therefore, combines FFA particle and pure Portland cement in mortar, instead of separating them. Since nG and FFA reduces the flow in mortar, the reduced flow causes rapid coagulation and the reduced early strength gain in M7 mortar. To overcome this rapid coagulation and the reduced strength gain, mortar needs 1%-SP for plasticizing. Author decides 1%-SP by mass of cement as an optimum ratio for these mortars of FFA/nG combination since M7 mortar reaches the flow value of M4 mortar and show growth for compressive strength notably. However, thee0.22%-nG particle increases both the flexural strength and the compressive strength in M5 mortar without SP.  FFA/nG combination reacts with cement after dormant period and can also assist to increase final strength properties of mortar at early age. The study indicates that the addition of 3 (kg) FFA and 6.6 (gr) nG allow for reduction about 1050 (g) cement (efficient factor of 0.35 in cement). Thus, a 50 kg-cement bag can contain 17.5 (kg) FFA and 111.1 (g) nG and 32.5 (kg) pure Portland cement.

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