Experimental investigation on rice husk ash as cement replacement on concrete production

Experimental investigation on rice husk ash as cement replacement on concrete production

Construction and Building Materials 127 (2016) 353–362 Contents lists available at ScienceDirect Construction and Building Materials journal homepag...

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Construction and Building Materials 127 (2016) 353–362

Contents lists available at ScienceDirect

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

Experimental investigation on rice husk ash as cement replacement on concrete production Josephin Alex, J. Dhanalakshmi, B. Ambedkar ⇑ Department of Chemical Engineering, SSN College of Engineering, Kalavakkam, Chennai 603 110, India

h i g h l i g h t s  Use of rice husk ash as cement additive for sustainable development.  Very fine RHA facilitates better pozzolanicity due to high specific surface area.  20 wt% of fine RHA addition can produce acceptable strength of cement concrete.

a r t i c l e

i n f o

Article history: Received 26 May 2016 Received in revised form 7 September 2016 Accepted 28 September 2016

Keywords: Rice husk ash Compressive strength Split tensile strength CO2 emission Concrete

a b s t r a c t The emission of CO2 has increased due to cement manufacturing and improper disposal of rice hush ash (RHA) leads to air pollution and land fill problem. To mitigate these issues, the RHA has been used as cement additive in concrete making. A Taguchi L27 fractional-factorial matrix was designed to assess the individual effects of key process variables like RHA loading, pozzolanicity, curing time, bulk density and RHA size. From the results, mechanical strength increased with decreasing RHA size and 20 wt% RHA replacement is optimum for 15 and 60 min grounded sample. The morphology of RHA are also discussed. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The major environmental issues arises due to changing climate patterns, resource depletion, etc. The climatic change becomes threat to air pollution, which is of main interest for research nowadays. The major causes for environmental pollution are industrialization, urbanization and population growth. In order to meet the infrastructure development for increasing population growth, the consumption of cement is also increased to about 1 m3 of concrete per year per person [1]. Cement is the primary material consumed in huge quantity annually and is been considered as very important material next to water. Cement is the principal raw material for the concrete production, manufacturing them leads to severe environmental pollution namely CO2 emissions [2,3]. The emission of CO2 is estimated as 1 tonne of CO2/1 tonne of cement produced and globally 5–7% of CO2 emissions [1,2,4] contributes to environ-

⇑ Corresponding author at: Associate Professor, Department of Chemical Engineering, SSN College of Engineering, Kalavakkam, Chennai 603 110, Tamilnadu, India. E-mail address: [email protected] (B. Ambedkar). http://dx.doi.org/10.1016/j.conbuildmat.2016.09.150 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

mental degradation namely global warming etc. In addition to this, production of cement requires high energy and cost and thus cement is unsuitable for the sustainable development [5]. The disposal of waste materials are also poses another serious environmental problem [2,6]. The utilization of waste materials such as industrial and agricultural wastes in the concrete production eventually reduces the environmental impact due to its improper disposal such as heaps of agricultural and industrial wastes are piled up or landfilled in rather productive lands, rendering the land useless. Thus, an alternative method is needed to reduce the emission of CO2 and making a path for solid waste management system as well. Therefore, a promising solution to mitigate the environmental impact of concrete is to minimize its cement content [7]. This can be achieved by using appropriate, cheap and easily available supplementary cementitious material (SCM), which are used as the partial replacement of cement without any adverse effect on the properties of the concrete. Many solid wastes such as industrial by products, natural pozzolanic materials, agricultural wastes etc., poses pozzolanic in nature is called as supplementary cementitious materials [2,6–9].

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Nomenclature CA CS CSH DOE FA HLA HP LOI

coarse aggregate compressive strength calcium silicate hydrates design of experiments fine aggregate high level analysis horse power loss on ignition

Rice husk is one of the agricultural waste, the outer covering of rice kernel is obtained by milling of paddy. About 200 kg of rice husk can be obtained from a tonne of paddy, constituting about one fifth of the total rice produced [10,11]. India is the major rice producing country and its annual paddy production for the year 2014 has been calculated as about 154.5 million tonnes [12], which produces approximately 31 million tonnes of rice husk which is quite very high. Because of its less nutritional value, it could not be used as animal feed, further the presence of siliceous composition shows unsuitable for natural degradation. Some other effective usage of rice husk as a fuel for boiler feed in order to produce steam and further the steam can be used for electricity power generation in rice mill. The generation of power is mainly used for rice mill operations and other processes, etc., [13]. During the combustion of rice husk yields 22 wt% of rice husk ash (RHA) [3]. RHA has high pozzolanicity and it can be used as SCM for concrete production [1,4,14,15]. Due to lack of disposal of RHA, it is often dumped in the land, exhibits adverse effect on the environment [10]. The RHA contains high silica composition [14] and pozzolanicity, which is similar to properties of silica fume while burning the rice husk at controlled condition [16]. Hence, RHA can be referred as a SCM by using partial replacement of cement with respect to RHA for concrete production. Thus, the addition of RHA as SCM is a promising solution to mitigate the environmental impacts due to the cement manufacturing process. The successive use of RHA as SCM has been reported by many researchers. The important factors of RHA to be considered as partial replacement of cement are fineness of the particle, temperature of combustion process and time required for chilling process etc., [14]. Madandoust et al., studied the use of RHA in the cement experimentally and observed that the pores of the mortar was filled with RHA increases the mechanical strength of the mortar [13]. It was also observed that RHA obtained during proper burning conditions, when used as cement additives in cement mortar, the strength increases for 10 wt% of RHA replacing the cement [15]. The high amorphous silica content of RHA exhibits the increasing strength of concrete containing RHA and showed better performance than the other natural pozzolanic material comparatively [8]. In another study, it was reported that the addition of 25 wt% of RHA increases the strength of the concrete significantly [3]. The burnt RHA from boiler feed has been considered and found that their pozzolanicity increases while re-combusting at 650 °C for one hour thereby increasing the mechanical strength of the concrete [17]. The total shrinkage of clear coloured RHA exhibits a similar effect to that of silica fume for 5 and 10 wt% RHA as SCM in the concrete [18]. The experimental investigation on the properties of the concrete has been performed by considering the unground and ground low carbon RHA and it was observed that ground RHA produces higher mechanical strength of the concrete when compared to unground RHA. It was also noticed that the ground RHA exhibits a similar mechanical behaviour of the concrete as that of silica fume [5]. In another study, it was reported that usage of RHA in a high strength concrete showed remarkable

NC OPC RH RHA SCM SSA TS

normal concrete ordinary portland cement rice husk rice husk ash supplementary cementitious material specific surface area tensile strength

increase in the concrete strength [19,10]. The application of RHA as SCM along with fibre shows that the mechanical strength of concrete increases and the voids and permeability of the concrete pavement decreases respectively [20]. Although the extensive research covering the aspects of strength, rheology, physical properties of RHA as a SCM, the influence of fineness on the pozzolanic reaction rate and the mechanical strength of the RHA at uncontrolled burning conditions is limited. Also, the experimental studies of the different types of RHA and its performance in terms of mechanical strength of concrete with respect to varying grinding time has not been reported in the literature. In the present study, the influence of three different types of RHA at uncontrolled burning conditions on the mechanical behaviour of concrete are investigated experimentally. The objective of the present work is to optimize the grinding conditions (15 and 60 min) and the amount of RHA replacement (10, 15 and 20 wt%) required for various types of RHA used as a supplementary cementitious material. Results of characterization of RHA, strength development and pozzolanicity behaviour of RHA are also reported.

2. Experimental programme 2.1. Materials used 2.1.1. Rice husk Three types of rice husk collected from Bhaba rice mills, Red Hills in Tamil Nadu, India was used in the present study.

2.1.2. Cement Ordinary Portland Cement (OPC) of 53 grade with a specific gravity of 3.12 conforming to Indian standard, IS 12269-1987 [21] was used.

2.1.3. Aggregates Natural sand of grading zone II with a fineness modulus 2.39, specific gravity 2.6 and absorption capacity 0.853 wt% was used as fine aggregate (FA). The coarse aggregate (CA) with a maximum size of 20 mm, specific gravity 2.79 and absorption capacity 0.518 wt% was used. The aggregates used were of conforming to IS 383-1970 [22].

2.2. Test instruments 2.2.1. Furnace A 4 kW power refractory furnace (Light weight) of heavy duty model was used. The heating element is made up of A1 Kanthal alloy in grooved bricks. This furnace is also equipped with variable temperature controller. The temperature range is from ambient temperature to 1150 °C.

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J. Alex et al. / Construction and Building Materials 127 (2016) 353–362 Table 1 Proximate analysis of different types of rice husks. Sl. No

Sample

Moisture (%)

Volatile matter (%)

Ash (%)

Fixed carbon (%)

1 2 3

RH-A RH-B RH-C

12.25 ± 0.29 12.38 ± 0.26 11.85 ± 0.27

52.29 ± 0.88 52.09 ± 0.47 51.12 ± 0.41

20.88 ± 0.85 20.65 ± 0.68 22.42 ± 0.46

14.58 ± 0.40 14.88 ± 0.48 14.61 ± 0.74

Table 2 Proximate analysis of different types of rice husk ashes. Sl. No

Sample

Moisture (%)

Volatile matter (%)

Ash (%)

Fixed carbon (%)

1 2 3

RHA-A RHA-B RHA-C

8.23 ± 0.48 7.49 ± 0.39 7 ± 0.03

18.32 ± 0.24 20.22 ± 0.46 21.13 ± 0.26

52.50 ± 0.41 50.75 ± 0.29 49.81 ± 0.37

20.95 ± 0.66 21.54 ± 0.52 22.06 ± 0.24

2.2.2. Sieve shaker The Almech Rotap sieve shaker was used to carry out the sieve analysis of RH, unground RHA samples and aggregates. It runs by 1/4 HP electric motor. It reproduces circular and tapping motion with a uniform mechanical action. It accommodates 6/7 full the 800 dia. The whole gear mechanism runs under oil operation at 220 V, single phase, 50 cycle AC only. 2.2.3. Particle size analyzer The Horiba LA-950 laser diffraction particle size distribution analyzer ranging from 10 nm to 3 mm was used to obtain the particle size of RHA. 2.2.4. SEM/EDX The Quanta 200 FEG scanning electron microscope (SEM), with a high resolution and energy dispersive X-ray spectrometry (EDX) was used to obtain scanning electron micrograph and elemental composition analysis of the RHA samples.

tion process, 40 kg of RHA was obtained from 200 kg of rice husk. Further, the RHA was ground in a ball mill by varying the grinding time at intervals of 0 (unground), 15 and 60 min. From this analysis, the three different size of RHA was obtained for each type of rice husk used (RHA-A, RHA-B and RHA-C). As a result, 9 RHA samples were used in the present study. It is important to characterize the different rice husk ashes (RHA) in order to assess as a cement additive in concrete making. 2.3.3. Characterization of RHA types and other materials Particle size distribution of unground RHA, FA and CA were determined by sieve analysis as per Indian standards, IS 2386 (Part 1) – 1963 [23]. The proximate analysis of different types of RHA were also performed using IS 1350 procedure to estimate the unburnt carbon left in the sample after the completion of uncontrolled burning of rice husk and is tabulated in Table 2. Specific gravity of OPC, RHA, FA and CA was calculated by pycnometry as per Indian standards, IS 2386 (Part III) – 1963 [24]. Bulk density of RHA, water absorption of CA and FA was also carried out as per Indian standards, IS 2386 (Part III) – 1963 [24] and IS 2720 (Part II) – 1973 [25] respectively. Loss on ignition (LOI) test for RHA was carried out as per Indian standard, IS 1727-1967 [26]. The main characteristics of different types of RHA, OPC and aggregates considered in this study are shown in Table 3. 2.3.4. Pozzolanicity testing The pozzolanic activity of RHA was analyzed using the modified Chapelle’s test [27]. This test allows the quantification of Ca(OH)2 consumed by 1 g of RHA when mixed with 2 g of CaO and 250 ml of distilled water. This suspension was boiled at 90 °C for 16 h in a continuous stirrer. The free available portlandite content of CaO in the suspension was determined by acid titration. The mechanism of reactions during titration are given in Eqs. (1) and (2).

CaO þ 2HCl ! CaCl2 þ H2 O

ð1Þ

CaðOHÞ2 þ 2HCl ! CaCl2 þ 2HO

ð2Þ

2.2.5. Strength testing machine Digital compressive strength machine with capacity of 3000 kN with ±2% accuracy was employed in compressive and splitting tensile strength test.

The pozzolanic activity of rice husk ash is calculated using Eq. (3).

2.3. Experimental procedure

where,

PA ¼ 2ððV1  V2 Þ=V1 Þð74=56Þ  1000

ð3Þ

2.3.1. Rice husk characterization The particle size distribution for different types of rice husk was obtained using the method of sieve analysis. From this data, the fineness modulus and can be calculated. The proximate analysis of different type of RH were determined and is shown in Table 1.

PA – Pozzolanic activity of RHA (mg Ca(OH)2 fixed/g RHA). V1 – Volume of 0.1 N HCl (ml), necessary for titrate 25 ml of the final solution obtained without RHA (blank test). V2 – Volume of 0.1 N HCl (ml), necessary for titrate 25 ml of the final solution obtained with RHA.

2.3.2. Preparation of RHA Thermal treatment of the rice husk samples were carried out in an open atmosphere at uncontrolled conditions. After the combus-

2.3.5. Design of experiments (DOE) The design of experiments were conducted to minimise the number of experiments and also could give full information on

Table 3 Characteristics of the materials used. Samples

Specific gravity

Fineness modulus

Max. size range (mm)

LOI (%)

Water absorption (%)

RH – A RH – B RH – C RHA-A RHA-B RHA-C OPC FA CA

– – – 2.11 ± 0.03 2.10 ± 0.11 2.14 ± 0.06 3.12 ± 0.01 2.6 ± 0.02 2.79 ± 0.02

9.43 9.28 8.94 3.3 2.25 3.58 – 2.39 7.13

1.09–1.55 1.09–1.55 1.09–1.55 0.55 and 0.075 0.075–0.221 0.075–0.282 – 0.23–0.89 7.38–30

– – – 2.26 ± 0.06 2.23 ± 0.13 2.46 ± 0.04 – – –

– – – – – – – 0.85 ± 0.05 0.52 ± 0.03

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Table 4 Parameters and different levels.

0.4

Parameters

Level 1

Level 2

Level 3

1 2 3

RHA loading, % Curing time, days RHA size, microns

10 7 174.36 168.65 214.92 A

15 14 103.31 106.89 115.22 B

20 28 39.94 45.56 55.66 C

RHA type

Table 5 L27 orthoganal array (DOE).

RH-B

RH-C

0.3

0.2

0.1

all the factors which to optimise the parameters. To accomplish this, Taguchi design of experiments (DOE) was used to determine the number of trial experiments to be required for process analysis. The factors and levels considered for this investigation are shown in Table 4. L27 orthoganal array was chosen for this DOE and is shown in Table 5. Total 27 trials for each trial, 2 specimen (1 cube and 1 cylinder) were cast and it was subjected to mechanical strength test. In addition for comparison purpose, mechanical strength of normal concrete (w/o addition of RHA) with three different curing time were determined. 2.3.6. Preparation and testing of cement concrete specimen with RHA Basically, manufacturing of cement consists of two important steps: 1. Production of clinker from raw materials. 2. Cement is produced from grinding the cement clinker. RHA production also consists of two important steps. 1. RHA is produced from combusting (controlled or un-controlled) the rice hush. 2. RHA is grounded to produce fine RHA of desired size. Cement concrete specimen with RHA samples obtained for three different size in each type of RHA were made as per Indian standards, IS: 10262-1982 [28]. The grinded RHA of about 10, 15 and 20 wt% (as specified in DOE Table 5) used as cement additive with a water to binder ratio of 0.45 in order to attain a compressive strength of 25 Mpa for 27 trial mixes considered in this study. The concrete were mixed in a laboratory concrete pan mixer and was compacted using vibrating table. The prepared cement concrete was cast in 150  150  150 mm iron mould and also in cylindrical moulds

2.4

1.55

1.85

1.29

1.09

0.93

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

0.73

1 1 1 2 2 2 3 3 3 3 3 3 1 1 1 2 2 2 2 2 2 3 3 3 1 1 1

0.55

1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3

0.45

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3

0.28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

0.35

RHA type

0.22

RHA Size

Average Particle Size (mm) Fig. 1. Size distribution analysis of different types of rice husk (RH).

0.3 RHA-A(Unground) RHA-A(15 minutes) RHA-A(60 minutes)

Mass Fraction

Curing time

0.2

0.1

0 0.01

0.1

1

10

100

1000

1000 0

Average size (microns) Fig. 2. Size distribution analysis of RHA-A with two different grinding time.

0.3 RHA-B(Unground) RHA-B(15 minutes) RHA-B(60 minutes)

Mass Fraction

RHA

0.08

0

TRIAL #

0.17

4

Mass Fraction Retained

RH-A

Sl. No

0.2

0.1

0 0.01

0.1

1

10

100

1000

10000

Average size (microns) Fig. 3. Size distribution analysis of RHA-B with two different grinding time.

of 100 mm in diameter and 200 mm in height for compressive and splitting tensile strength respectively. The cube and cylindrical specimen were demoulded after 24 h and placed in water tank for further curing to prescribed ages. The specimen were then tested for their respective mechanical properties as per Indian Standards, IS 516-1959 [29] and IS 5816-1999 [30] at various ages given in Table 5. Concrete specimen without RHA were also cast and their strength was analyzed to compare the strength development of the concrete blended with RHA.

J. Alex et al. / Construction and Building Materials 127 (2016) 353–362

time are shown in Figs. 2–4. Almost all the cases, the size distribution of RHA moves towards the smaller size were observed. And also the initial particle size reduction is faster than the next 60 min of grinding. The average sizes of the RHA types with different grinding time are discussed in later section.

0.2 RHA-C(Unground) RHA-C(15 minutes)

Mass Fraction

RHA-C(20 minutes)

3.2. SEM and EDX analysis

0.1

0 0.01

357

0.1

1

10

100

1000

10000

Average size (microns) Fig. 4. Size distribution analysis of RHA-C with two different grinding time.

3. Results and discussion High Level Analysis (HLA) is performed by averaging measured data for each level of a single parameter, then plotting the averaged data against each levels of that parameter. HLA has high statistical significance, since a large number of data points are available for further analysis. In our experiment, 27 compressive and tensile strength data points measured (9 for each level of each parameter) can be conveniently analyzed in this manner.

The SEM images of different types of RHA are shown in Fig. 5. The reason behind these analysis are the surface morphology may have a role in determining the concrete strength. For each sample two different magnification of SEM images were taken. The images are look like a honey comb structure with regular node, anti-node pattern and also their structure amorphous in nature. The observed surface morphology of all the samples are almost identical with minute changes like cracks and irregular surface. The observed center to center distance between two nodes are 100 lm. Semi-quantitative EDX analysis were also done to check the elements presence in the different types of RHA sample and RHA-A with two different grinding and are tabulated in Table 6. From the Table 6, it has been observed that silica (Si) is the major portion of its composition for all cases tested. This act as a prime pozzolan-material to strengthen the cement concrete in place of cement from RHA side. Grinding of the RHA sample does not make any significant change in elemental composition were observed. 3.3. Effect of grinding time on RHA particle size and specific surface area

3.1. Size distribution analysis of RH and RHA types Size distribution analysis of rice husk and rice husk ash are shown in Figs. 1–4. From Fig. 1, it has been observed that more than 70% of the particles are in 1–1.85 mm size range for all types of rice husk (RH). Size distribution analysis of the three different rice husk ash unground and corresponding two different grinding

The effect of grinding time on RHA particle size and specific surface area are shown in Fig. 6. From Fig. 6, it can be seen that the average size of the RHA particles decreases as increasing the grinding time. The specific surface area of the particles are calculated using Eq. (4) [31] and plotted against grinding time. Increase in grinding time renders decrease in RHA particle size leading to

(a) RHA - A un-ground

(c) RHA - B un-ground

(e) RHA - C un-ground

(b) RHA - A un-ground

(d) RHA - B un-ground

(f) RHA - C un-ground

Fig. 5. SEM images of different types of RHA.

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Table 6 EDX analysis of different types of RHA. Elements

C O Al Si K Fe

RHA-A, wt% Unground

15 min

60 min

3.28 35.01 0.6 60.17 0.39 0.55

3.86 35.24 0.06 59.7 0.58 0.56

3.64 35.12 0.02 60.68 0.44 0.1

RHA-B wt%

RHA-C wt%

4.01 36.45 0.44 57.96 0.65 0.49

4.84 39.38 0.54 54.08 0.68 0.48

Fig. 7. Effect of grinding time on bulk density and pozzolanicity of RHA.

Fig. 6. Effect of grinding time on RHA particle size and specific surface area.

the increase in specific surface area of the RHA particles. This condition is more suitable for making high strength cement concrete.

Aw ¼

n 6 X xi us qp i¼1 D pi

ð4Þ

where Aw = Specific surface area (m2/kg). us = Sphericity (RHA = 0.65, Sand = 0.75, Gravel = 0.6) [32,33]. qp = Density of the particle (kg/m3). xi = Mass fraction in a given increment.  pi = Average particle diameter, taken as arithmetic average of D smallest and largest particle diameter in increment (m). i = Individual increments. n = Number of increments. 3.4. Effect of grinding time on bulk density and pozzolanicity of RHA The bulk density and pozzolanicity for various types of RHA are plotted against grinding time and are shown in Fig. 7. It was observed that as the grinding time increases, the particle size decreases provides less voidage between RHA particles which leads to increase in the bulk density of RHA. The higher bulk density was obtained for type A RHA at a grinding time of 60 min comparatively. Variation of the pozzolanic activity of different types of RHA at different grinding time is also shown in Fig. 7. It was observed that the CaO consumption during the pozzolanic reactions increased with increasing the grinding time for all types of RHA. The activity of rice husk ash varying from 622.96 to 1085.46 mg Ca(OH)2/g RHA are indicative of their high reactivity. The consumption of calcium hydroxide by the unground RHA is relatively lower due to their coarse in nature. If the material is said to be pozzolanic, it must have a minimum reactivity of 330 mg Ca (OH)2/g of the sample [16]. The highest pozzolanicity was obtained for RHA ground at 60 min. Since the particle size reduces for increasing in the grinding time, as expected. Further, the increased

Fig. 8. Effect of grinding time on loss on ignition (LOI).

pozzolanic activity was observed for RHA-A with low carbon content [14]. Hence, it can be concluded that better pozzolanicity is higher the finest fraction of the RHA. In general the particle size, specific surface area, bulk density and pozzolanicity are interdependant properties which are more essential for making good cement concrete. 3.5. Effect of grinding time on loss on ignition (LOI) Loss on Ignition is used as the quality test to assess the use of RHA as SCM. LOI test is to estimate the un-burnt carbon present in the RHA. As per the Indian Standards of IS: 1727-1967 [26], the value of LOI should be less than 8 wt%. In this study, the estimated LOI are plotted against grinding time and are shown in Fig. 8 for various types of RHA. It was noted that the LOI of all the RHA samples are 2.23–2.6 wt% which is well below the standards specified above. Once the LOI is finished, the obtained RHA was in whitish grey colour. The change in colour indicates the completeness of the combustion process. It was also observed that grinding time does not have any influence on the LOI of RHA samples. Further, the RHA-C showed higher LOI than other samples comparatively. The reason being higher unburnt carbon content left in RHA-C sample during uncontrolled combustion and is shown in Table 2. 3.6. Effect of RHA size on compressive and tensile strength The compressive strength increases with decrease in the particle size. The increase in the strength is due to the high fineness. The

J. Alex et al. / Construction and Building Materials 127 (2016) 353–362

359

Fig. 9. Effect of RHA size on compressive and tensile strength. Fig. 10. Effect of RHA specific surface area on compressive and tensile strength.

smaller particles increases the strength due to the filler effect, which is responsible for the improved particle packing density as well as the greater compressive strength of the mix containing finer RHA particles. Further, the high strength gain can be attributed to the uniform microstructure of the finely ground fraction. A strength gain of 5.59%, 3.39%, 5.27% and 8.23%, 6.78%, 8.56% was observed for 15 and 60 min ground ash from that of the unground ash for type A, B and C respectively. For tensile strength, trend similar to that of compressive strength gain is followed, as shown in Fig. 9. A strength gain of 7.49%, 8.27%, 4.18% and 11.61%, 13.16%, 9.13% was observed for 15 and 60 min ground ash from that of the unground ash for the respective RHA types A, B and C.

3.7. Effect of RHA specific surface area on compressive and tensile strength

Fig. 11. Effect of bulk density on compressive and tensile strength.

Specific surface area is one of the important parameter affecting cement properties. The progress of compressive and tensile strength significantly enhanced with increase in specific surface area and are shown in Fig. 10. The increase in specific surface area of RHA contributes to the increased consumption of Ca(OH)2 in concrete i.e., the RHA with high specific surface area shows high reaction rate between calcium hydroxide and the silica, thereby exhibiting exceptional pozzolanicity and helps to increase the mechanical strength of the concrete.

3.8. Effect of bulk density on compressive and tensile strength The bulk density of RHA plays a major role in strengthening the cement concrete. Fineness of the RHA particle possess high bulk density due to less void fraction between the particles thereby providing intimate contact with other concrete materials leading to the strength development. From Fig. 11, it has been observed that addition of RHA-A makes cement concrete of highest compressive strength due to finess of particle sizes from the tested RHA types. Higher the compressive and tensile strength for larger bulk density RHA due to finess in size.

3.9. Effect of RHA pozzolanicity on compressive and tensile strength The importance of pozzolanicity is discussed in the previous Section 3.4. How the pozzolanicity affects the compressive and tensile strength is shown in Fig. 12. From the figure it has been observed that increasing RHA pozzolanicity linearly increases the compressive and tensile strength of cement concrete.

Fig. 12. Effect of RHA pozzolanicity on compressive and tensile strength.

3.10. Effect of % addition of RHA on compressive and tensile strength On studying the effect of the RHA dosage in concrete strength development (Fig. 13), it is found that 10 wt% cement replacement showed an remarkable percentage of strength gain (7.8%) as compared to normal concrete. The strength increase is due to the higher content of calcium silicate hydrate (CSH) in the RHA blended concrete specimens, due to the reaction of the calcium hydroxide produced from cement hydration with the active silica of the RHA. The CSH is the main carrier of strength in hardened cement. Further, on comparing the split tensile strength development of the RHA concrete of different dosages with that of normal concrete as shown in Fig. 13, it is evident that 10 wt% replacement

J. Alex et al. / Construction and Building Materials 127 (2016) 353–362

Compressive strength

Tensile Strength

Compressive strength

3 20 2.5 15

2 1.5

10

1 5 0.5 0

0 0

10

15

Tensile Strength

25

3

20

15

2.5

10

5

2

0

20

A

B

C

Type of RHA

Addition of RHA,% Fig. 13. Effect of % addition of RHA on compressive and tensile strength.

Fig. 15. Effect of type of RHA on compressive and tensile strength.

3

35 Compressive strength

Tensile Strength

30

2.5

25 2 20 1.5 15 1 10

Tensile Strength, N/mm2

Compressive Strength, N/mm2

Compressive Strength, N/mm2

3.5

25

3.5

30

4

Tensile Strength, N/mm2

Compressive Strength, N/mm2

30

Tensile Stremgth, N/mm2

360

0.5

5 0

0 7

14

28

Curing Time, Days Fig. 14. Effect of curing time on compressive and tensile strength.

gained more strength than other concretes. However 20 wt% replaced concrete also performed better than the normal concrete, thereby contenting to be the optimal replacement level. Taking normal concrete as the reference, the percentage gain in tensile strength for 10, 15 and 20 percentage of RHA replacement levels are 57.2, 56.02 and 55.04 respectively. 3.11. Effect of curing time on compressive and tensile strength It is evident from Fig. 14 that compressive strength increases with an increase in the curing period. A significant strength gain of 41.08% (14 days) and 62.98% (28 days) was observed from the 7-days strength. This is due to the chemical reaction between the water and the binding material. During curing, the water is not just absorbed but causes hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement forms chemical bonds with water molecules and become hydrates or hydration products, thus strengthening the concrete. As expected, the splitting tensile strength also increased with the days of curing as stated in compressive strength. A substantial strength gain of 46.91% (14 days) and 91.68% (28 days) was observed from the 7-days strength.

Fig. 16. Comparison on the compressive strength development of various RHA mixes and normal concrete (NC).

reported 0.7% and 2.43% increment in the strength as compared to that of type B and C respectively. It could be observed that similar to compressive strength gain, no significant strength variation is observed with respect to tensile strength development between the different types of RHA used, however the concrete with RHA of type B imparted 0.37% and 3.64% more strength as compared to that of type A and C respectively. 3.13. Statistical validation of experimental results T-test analysis was performed for assessing statistical significance of the obtained experimental data. This test may be used to identify a parameter which has significant effect on concrete strength. Critical value of t was found from T table using degrees of freedom. Confidence level of each parameter was arrived by comparing absolute T statistical value and T critical value. Table 7 shows that T statistical value and the corresponding confidence level of each parameter. From Table 7, RHA loading, curing time and RHA size plays a major role in ascertaining the concrete strength excluding RHA type due to insignificant variation in its physical properties.

3.12. Effect of type of RHA on compressive and tensile strength On scrutinizing the effect of the type of RHA on the strength development from the Fig. 15. It could be noted that no significant strength variation is observed between the different types of RHA used since the properties of RHA for various types remains same (Tables 1, 2 and 6), however the concrete with RHA of type A

3.14. Compressive strength of various RHA mixes and normal concrete (NC) Compressive strength is the maximum compressive stress that, under a gradually applied load, a given solid material can sustain without fracture. The rate of compressive strength development

J. Alex et al. / Construction and Building Materials 127 (2016) 353–362 Table 7 Statistical analysis of design parameters. Compressive strength

RHA loading, % Curing time, days RHA size, microns RHA type

Tensile strength

Tstat

Confidence

Tstat

Confidence

5.9 11.9 2.1 0.3

>99.9% >99.9% >95% <50%

2.4 8.8 1.13 0.19

>95% >99.9% >70% <50%

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In addition concerning to the strength gain, it seems that 20% of RHA ground for 15 and 60 min could be regarded as a suitable replacement and in case of the unground RHA, 15% wt replacement of cement might be considered satisfactory. 3.15. Splitting tensile strength of various RHA mixes and normal concrete (NC) Tensile strength is a measure of the ability of material to resist a force that tends to pull it apart. The comparison of the tensile strength development of the normal concrete to that of the concrete containing RHA is shown in Fig. 17. It could be noticed that, similar to the compressive strength, the splitting tensile strength also increased with increase in curing days for all the concretes. The splitting tensile strength of the RHA blended concrete outperformed the normal concrete in almost all levels of replacement, excluding the 7-day strengths of 60 and 15 min ground samples at 15 and 20 wt% replacement. 3.16. Compressive strength vs splitting tensile strength Fig. 18 shows the relationship between the compressive strength and the splitting tensile strength for the normal and blended concrete. It was observed that the split tensile strength increases with the increase in compressive strength for all samples.

Fig. 17. Comparison on the splitting tensile strength development of various RHA mixes and normal concrete (NC).

4. Conclusions

Fig. 18. Compressive strength vs splitting tensile strength.

in concretes containing various RHA sample used in different dosage is shown in Fig. 16. It is observed that the compressive strength increases with an increase in the curing period for all the concrete cubes and a decrease in the compressive strength takes place when the RHA dosage is increased. Considering the compressive strength of the normal concrete as reference, the percentage increase in the 7days strength development of A-U, B-U, C-U at 10 wt% of cement replacement were 12.89, 13.92 and 10.31 respectively and the respective strength development for A-60, B-60 and C-60 at 15 wt% replacement were 6.51, 7.22, 4.9. A comparison of the 14days compressive strength of the 10 wt% replacement by A-15, B15 and C-15 exhibited 8.27, 6.89 and 6.27% in strength. Whereas at 28-days, the percentage increase in the strength development of A-60, B-60 and C-60 at 10 wt% cement replacement was found to be 6.77, 5.33, 4.61 respectively. The increase in the strength is due to the high fineness of the RHA particles, which activates the pozzolanic property and improves the interfacial zone between the aggregates and the cement paste.

From the experimental results, the following conclusions could be made. The average particle size decreased with increasing grinding time whereas, the specific surface area increased with increasing grinding time for all types of RHA samples. RHA of type A, ground for 60 min was found to be the finest of all with an average size of 39.94 lm and specific surface area of 109.38 m2/kg. The bulk density also followed similar trend of the average particle size and RHA of type A subjected to 60 min of grinding, being the finest sample showed higher bulk density of the all the samples considered in this study. The grinding time does not have any significant impact on the loss on ignition of the RHA samples. The RHA of type C has comparatively more fixed carbon content. The pozzolanic activity of the material can be improved by grinding activity. The finer RHA fractions exhibits better Chapelle activity. The type of RHA used in this study does not have significant impact on the mechanical strength development of the concrete since the tested RHA composition is identical with each other. Although 15 min ground sample exhibits better results, grinding of RHA for 60 min was found to impart higher strength in concrete. In case of compressive strength development, the partial replacement of RHA ground samples at 20 wt% could be regarded suitable and for unground RHA 15 wt% might be considered satisfactory. For tensile strength development, 20 wt% replacement was considered to be optimal. Thus, it could be concluded that addition of RHA as SCM proves to be the better option for sustainable development, thereby solving the negative impacts during cement manufacturing process like CO2 emission, resource depletion, high cost and also the solid waste disposal problem associated with agricultural waste activity to a certain limit. Acknowledgement The authors earnestly acknowledge the financial support from the SSN Trust, Chennai, Tamilnadu, India.

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