An Experimental Investigation on Flyash-based Geopolymer Mortar under different curing regime for Thermal Analysis

Accepted Manuscript Title: An Experimental Investigation on Flyash-based Geopolymer Mortar under different curing regime for Thermal Analysis Author: Anuja Narayanan Prabavathy Shanmugasundaram PII: DOI: Reference:

S0378-7788(16)32056-4 http://dx.doi.org/doi:10.1016/j.enbuild.2016.12.079 ENB 7258

To appear in:

ENB

Received date: Revised date: Accepted date:

29-8-2016 27-12-2016 27-12-2016

Please cite this article as: Anuja Narayanan, Prabavathy Shanmugasundaram, An Experimental Investigation on Flyash-based Geopolymer Mortar under different curing regime for Thermal Analysis, (2016), http://dx.doi.org/10.1016/j.enbuild.2016.12.079 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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An Experimental Investigation on Flyash-based Geopolymer Mortar under different curing regime for Thermal Analysis Anuja Narayanana,c,∗, Prabavathy Shanmugasundaramb,c

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a Full-Time

Research Scholar Professor & Head c Department of Civil Engineering, Mepco Schlenk Engineering College, Sivakasi-626005, Tamil Nadu, India

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b Senior

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Abstract

In order to make our environment green, a lightweight inorganic aluminosil-

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icate polymer uses waste fly ash as source material instead of cement in concrete. Source material in combination with sodium-based alkaline liquid form a cement free material called geopolymer. This paper briefly presents the thermal

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performance of fly ash-based geopolymer mortar such as heat resistance behav-

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ior, thermal conductivity, compressive strength and dry density under different curing regime such as ambient temperature, heat chamber, hot air oven and autoclave. Here the silicate to hydroxide ratio was fixed as 2.5 and liquid to

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flyash ratio was taken as 0.4 throughout the process. From the results it was found that among all curing conditions, hot air oven curing shows best outcome. On further study it was found that specimen placed in hot air oven at 80◦ C for

6 hours gives higher compressive strength of 27.20MPa with the dry density of 1875kg/m3 and also thermal conductivity of 0.340W/mK than 24 hours curing for 1 day which make geopolymer as more energy efficient material. Keywords: compressive strength, curing time, dry density, flyash, geopolymer mortar, heat resistance

∗ Corresponding

Author Email address: [email protected] (Anuja Narayanan)

Preprint submitted to Journal of LATEX Templates

December 27, 2016

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1. Introduction

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Energy saving is an important concept that has an emerging need in the recent years. Next to water, Concrete is becoming a second important material nowadays. Development of infrastructure further increases the demand for con-

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crete which in turn increases the demand for cement. 1 ton of cement produces 1 ton of CO2 during its manufacture [1, 2, 3, 4, 5, 6]. CO2 is a harmful greenhouse

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gas that measures about 0.66 to 0.82 kg for every kilogram of its manufacture that may lead to global warming which in turn affects the environmental condition [7, 8]. Cement industry stands second in the production of greenhouse

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gas especially CO2 . By the year 2020, it is expected to reach 100% from the current level of production [9]. The annual worldwide production of cement will

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be approximately 4.38 billion tons in 2050 with 5% increase per year [5]. It is an urgent need to protect the environment from pollution by finding a suitable binder that can act as an excellent alternative in the place of cement

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in concrete to make a sustainable development in the construction industry. In 1979, Davidovits introduced a new 3D aluminosilicate material called Geopoly-

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mer which has a source material rich in silicon and aluminium that can be activated using alkaline solution to act as a binder [10, 11]. It was found that

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several industrial waste materials such as flyash, GGBS, red mud, etc. are commonly abundant in aluminosilicate which can be used for the manufacture of geopolymer concrete and this will reduce the embodied CO2 up to 80% when compared to OPC [12, 13, 14]. Alkali Activator solution plays a major role in the dissolution of Si and Al oxides [15]. On the other hand, it was found that flyash is a waste material obtained from the coal based thermal power plant, which causes a serious problem during its disposal [16, 17]. It requires a large area for disposal which may affect aquifers and freshwater. The annual generation of flyash is about 200 million tons in India with only 50% utilization [18]. While testing the constituents of flyash, it was found that flyash is a heterogeneous material of variable chemical composition that can affect the final geopolymer product [19].

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It is also rich in silicon and aluminium which has a great power to act as a binder and for this character, it can be replaced instead of cement in

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concrete [20]. Concrete can be made ecofriendly by utilizing waste materials from industry instead of cement. Traditional concrete is a poor insulator, so

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geopolymer prefers pozzolanic material such as flyash which is good in insu-

lating property. When compared to OPC, geopolymer has several advantages

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such as excellent mechanical properties, good fire resistance, high compressive strength, low creep, good bonding property, and good resistance to acid and sulfate attack. For these properties, it can be preferably used in wide variety

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of applications such as marine structures, pavers, precast products, fire resistance material, decorative stone artifacts, thermal insulation, low-tech building materials, low energy ceramic tiles, refractory items, thermal shock, refracto-

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ries, bio-technologies (materials for medicinal applications), foundry industry, cements and concretes, composites for infrastructures repair and strengthening, high-tech composites for aircraft interior and automobile, high-tech resin sys-

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tems, radioactive and toxic waste containment, arts and decoration, cultural

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heritage, archaeology and history of sciences [21, 22]. Important factors that affect the compressive strength of geopolymer are particle size distribution of

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fly ash, weight ratio of constituent oxides such as Al2 O3 , SiO2 and Na2 O, liquid to solid weight ratio, NaOH concentration, Curing temperature and time, Sodium Hydroxide to sodium silicate weight ratio, CaO content, pore size distri0

bution and age of geopolymer. Mostly some factors effect may vary during the process of geopolymerization [23]. Ambient temperature curing gives only low strength when compared to heat curing for geopolymer. Generally, the curing temperature of 60-90◦ C is given to the specimens for about 24-48 hours [24]. The required temperature can be applied by various curing methods. When compare to normal concrete, it is important to make geopolymer as energy proficient for practical utilization. Hence, in this study in order to make geopolymer as energy efficient material the specimen is subjected to various curing conditions such as Ambient Temperature, Heat Chamber, Hot Air Oven and Autoclave at a curing temper3

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ature of 80◦ C for knowing the best curing condition and also to view the effect of polymerization. Thermal tests such as heat resistance, thermal conductivity,

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compressive strength and dry density of geopolymer mortar are studied in detail by fixing the silicate to hydroxide ratio as 2.5 and liquid to flyash ratio as 0.4.

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Further, it is important to reduce the curing time from 24 hours, so it is varied

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from 1 to 24 hours at an interval of 3 hours to find the best level of curing.

2. Experimental Works

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2.1. Materials

Geopolymer is a lightweight inorganic aluminosilicate polymer in which the alkaline liquid reacts with the source material which is rich in silicon and alu-

an alkaline liquid [1, 22, 25].

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minium. Geopolymer has two main constituents such as a source material and

The source material of geopolymer may be of natural minerals such as clay,

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kaolinite or by-product minerals such as flyash, slag, silica fume, RHA, Red mud. The source material should be rich in silica and alumina which is helpful

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for binding action. It can be selected based on its availability, cost, and type of application and demand of end users [26, 27, 28]. Flyash is an industrial waste

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material that can be synthesis for the production of geopolymer which gives an effective result when compared to other materials. But other materials are also used by researchers for various works. In the market, flyash is available in two forms such as Low calcium flyash (Class F type) and High calcium flyash (Class C type). In this work, Low calcium flyash (Class F type) is preferred in which the calcium oxide is less than 10%, and it is generally glassy with some crystalline inclusions of mullite and quartz as significant and hematite as minor amount [29]. Due to its low calcium content, there are only low possibilities for alkali-aggregate reaction [30]. As at elevated levels, presence of higher amount of calcium hinders the polymerization reaction and alters the microstructure. Class F type gives better strength when compared to Class C type. Here Class F Flyash that is obtained from the Thoothukudi Thermal Power Station is used.

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Typical river sand which is locally available is preferred as fine aggregate. Characteristics such as fineness modulus, water absorption and dry density of

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fine aggregate are also considered to be major factors in determining the prop-

erties of mortar. The fineness modulus of the aggregate is determined by the

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process of sieve analysis.

The alkaline liquid preferably used for geopolymer concrete may be sodium

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hydroxide/potassium hydroxide and sodium silicate/potassium silicate [28].When NaOH is used along with water glass, the compressive strength of geopolymer material is higher than while using only NaOH. Sodium Hydroxide and sodium

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silicate is preferred here as it is less expensive when compared to Potassium hydroxide and Potassium silicate. Sodium hydroxide is available in flakes or pellets form in the market. Here Sodium hydroxide which is in pellet form with

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97% purity is dissolved in distilled water to obtain a required Molarity. The concentration of sodium hydroxide also affects the strength property. Increasing the concentration will increase the strength of the specimen [20]. NaOH

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Molarity of 8-16M is desirable and practicable. In this work, 10M concentration

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of NaOH is used throughout the process. Sodium Silicate which is available in gel form is used to improve the polymerization reaction rate and mechanical

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performance. Here the ratio of sodium silicate and sodium hydroxide is taken as 2.5. Both sodium hydroxide solution and sodium silicate gel are mixed together one day prior so that proper reaction will occur before using it. 2.2. Material Characterization All the materials used for the preparation of mortar specimen should be

tested before using it. As there is no standard mix design for geopolymer, several trials have been carried out, and adequate strength has been attained in 1:1.26 mix. The oxide composition of Class F Flyash is determined using X-ray Fluorescence Test. From the analysis, it was found that the material tested exhibits the chemical properties corresponding to Class F Flyash material as specified in ASTM C618. It has large quantities of reactive oxides such as 52.15% of Silicon dioxide (SiO2 ), 27.71% of Aluminium oxide (Al2 O3 ) and less 5

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than 6% of Calcium oxide (CaO) as shown in Table 1 . Here mainly the Si and Al components contribute to the strength development through geopolymeriza-

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tion process that occurs due to the use of alkaline liquid at elevated temperature

curing. The state-of-the-art of Thermal Field Emission Scanning Electron Mi-

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croscope (FE-SEM) (Zeiss EVO-40) accompanied by an EDS analyzer supplied by Carl-Zeiss, Germany was used for imaging process. Usually the samples are

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prepared by coating a thin gold film in vacuum conditions. The image is taken at a working distance of 9.2mm at a magnification of 125.00 KX (EHT-5.00kV). An optical micrograph clearly revealed that the fly ash particle exhibits a reg-

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ular spherical shape and the average size of the particle is of 10m. The surface of fly ash particle was found to be compact without any disfigurements. Scanning Electron Microscope (VEGA 3 TESCAN) with EDAX (Bruker) was used

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to define the chemical composition of fly ash under an acceleration voltage of 15kv and the observation was conducted at a (WD) working distance of 15mm. This analyses the elements that make up the sample and it is evident from the

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EDAX spectra that although there are many elements present in fly ash, Si

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and Al are found in significant amount. So it is confirmed that the flyash used here is copious with sufficient quantity of mullite and quartz to act as a suitable

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binding material. Figure.1(a) Figure.1(b) shows the SEM and EDAX test result of flyash.The properties of the other materials used for the sample preparation are shown in Table 2 .

(a)

(b)

Figure 1: (a) SEM image of Flyash (b) EDAX Test Result for Flyash

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Table 1: Oxide Compositions of Class F Flyash using XRF

SiO2

Al2 O3

Fe2 O3

CaO

MgO

TiO2

K2 O

Na2 O

CuO

LOI

(%)

52.15

27.71

5.09

0.51

1.01

3.94

1.46

0.27

0.24

6.80

Material

Property

Color

Spherical

Form

Powder 10µm

Specific Gravity

2.55 II

Fineness Modulus

2.675

Bulk Density

1591 kg/m3

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Zone (According to IS : 383 (1970))

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Sodium Hydroxide

Grey

Average Particle Size

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Fine Aggregate(River Sand)

2.3

Particle Shape

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Low Calcium Flyash (Class F Type)

Value

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Specific Gravity

99.18

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Table 2: Properties of Materials

Total

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Oxide

Water Absorption

0.75%

Size

<4.75mm

Form

Pellet

Color

White

Purity

>98%

Form

Gel

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Sodium Silicate

Superplasticiser

Color

Light Brown

Brand Name

Conplast SP430

Color

Brown

2.3. Specimen Preparation

In the process of mixing, two types of mixing such as conventional mixing

and separate mixing are carried out. In normal mixing, all the ingredients are mixed together at same time but in separate mixing, the dry materials such as flyash and fine aggregate are first mixed thoroughly for about 3 minutes and then sodium hydroxide, sodium silicate solution, and super plasticizer are added in proper proportion of not more than 1.5% and again wet mixing is carried out for about 4 minutes then the fresh mix is placed into the mold of size 70.6 mm x

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70.6 mm x 70.6 mm in three layers and it is well compacted using the tamping rod. Further, the specimen is placed on the vibrating table for about 5 minutes

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to get good compressive strength. When compared to the usual mixing process,

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the separate mixing process gives higher strength.

(b)

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(a)

(c)

(d)

Figure 2: (a) Heat Chamber Curing (b) Steam Chamber Curing (c) Hot Air Oven Curing (d) Ambient Curing

The fresh geopolymer specimens are well covered with a plastic sheet in order

to avoid the evaporation of moisture. Then the specimen is allowed to cure at room temperature for about 24 hours. After 24 hours, the specimens along with the mold covered with plastic sheets are placed in the Heating Chamber operating at 1200W(240V), Horizontal Type Steam Sterilizer (Autoclave) operating at a working pressure of 2.1kg/cm2 and Hot Air Oven working at 1000W(230V)

separately for about 24 hours at 80◦ C and separate specimens are also placed 8

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at the room temperature until the day of testing for finding the strength in an ambient condition which is shown in Figure.2(a) Figure.2(b) Figure.2(c) Fig-

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ure.2(d) . Through the curing process, geopolymer gains its strength through a rapid exothermic polymerization reaction. After 24 hours, the specimens are

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removed from the mold and placed at room temperature until the day of test-

ing. Three specimens were casted, and the average value of the three is taken.

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The specimens were tested on 1, 7, 14 and 28 days to check the variation in strength. Moreover, it is important to make geopolymer as energy efficient as 24 hours curing make it unsuitable for the particular purpose. Therefore after

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finding the best curing condition, the specimens are subjected to various curing time period from 1 to 24 hours to make geopolymer as a good energy efficient

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material. 2.4. Test Methods

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2.4.1. Heat Resistance

Figure 3: High Temperature Muffle Furnace

Electric Muffle Furnace (JCS-33A) is used to find the heat resistance prop-

erty of the mortar specimen and it is shown in Figure.3 . Here the specimens are subjected to various levels of temperature ranging from 100 to 900◦ C at an interval of 100◦ C for one hour. After the completion of one hour, the furnace is turned off to bring the temperature slowly down to the normal condition. After complete cooling, the specimens are placed for visual observation and then the residual compressive strength testing has been carried out. 9

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2.4.2. Thermal Conductivity Thermal conductivity is the material’s ability to conduct heat through it.

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Normal concrete has thermal conductivity in the range of 0.7-0.8W/mK which

is higher than geopolymer concrete[13]. To make the structure as thermal insu-

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lated, the thermal conductivity value should be decreased to a great extent. A

material should have low thermal conductivity to act as a good thermal insu-

2.4.3. Dry Density and Compressive Strength

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lating as well as energy efficient material.

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In the analysis of thermal property, density also plays an important role in insulation. When compare to normal concrete, geopolymer acts good in insulation as it uses fly ash which is a lightweight material of its own. Low density

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of the specimen gives good insulation. In order to find the dry density, the dry weight of the specimen is measured and it is substituted in Equation.1 .

(1)

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Density(ρ) = W/V

Where W is the dry weight of the specimen (kg), V is the volume of the speci-

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men (m3 ).

A UTE -100 Model Electronic Universal Testing Machine of 1000kN capacity

at a loading rate of 0.14MPa/s is used to determine the compressive strength of geopolymer mortar in accordance to the ASTM C39 [31] and the testing of the specimen is shown in the Figure.4. For each type of curing, the compressive strength varies and that can be used to determine the particular type of curing which gives higher compressive strength. Average of three specimens which are under the limit of IS: 456(2000) code specification are considered for strength determination [32]. It was found that for various curing conditions the compressive strength value differs even though the curing temperature and curing time are constant. The compressive strength is found by substituting the load value in Equation 2.

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(2)

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CompressiveStrength(f c) = P/A

Where P is the maximum load resisted by the specimen (kN), A is the Cross

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Sectional Area of the specimen (mm2 ).

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Figure 4: Compressive Strength Testing in UTM (1000kN Capacity)

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3. Results and Discussions

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3.1. Thermal Resistance

Figure 5: Variation of Compressive Strength of Geopolymer Mortar subjected to various Temperatures

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Usually when compared to traditional concrete, flyash-based geopolymer has excellent heat resistance property. When the specimen is subjected to the

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temperature of 100 to 900◦ C at an interval of 100◦ C for a period of 1 hour, the

compressive strength of the mortar specimen goes on decreasing from 27.01MPa

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to 9.22MPa and is clearly shown in Figure.5 . It is also seen that there is no change in shape or size of the specimen. Only the color of the specimen

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is changed from grey to brown due to elevated temperature. This is mainly because the specimen has been already subjected to heat curing condition.

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3.2. Thermal Conductivity

Here the thermal conductivity is tested for the specimens placed in Autoclave, Heat Chamber, Hot Air Oven and Ambient condition on 28th day and

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the result is clearly presented in Table 3 . The percentage variation of thermal conductivity of geopolymer mortar is clearly viewed from the Figure.6 and it reveals that hot oven curing gives low value of about 14.46%. While compar-

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ing the obtained values of thermal conductivity, the specimen placed in hot air

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curing condition gives lesser value than all other curing conditions and it shows

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a significant variation from the ambient condition.

Figure 6: Percentage Variation of Thermal conductivity of Geopolymer Mortar

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Table 3: Variation of Thermal Conductivity of Geopolymer Mortar

Thermal Conductivity (W/mK)

Autoclave

0.664

Heat Chamber

0.573

Hot Air Oven

0.363

Ambient

0.910

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3.3. Dry Density and Compressive Strength

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Curing

From the Figure.7 , it was found that the strength values for various liquid

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to flyash ratio such as 0.30, 0.35, 0.40, 0.45 and 0.5 are tested, and it was found that liquid to flyash ratio of 0.4 gives higher compressive strength of 29.27MPa for 24 hours curing. Beyond 0.4, the compressive strength goes on decreasing

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so the optimum value is taken as 0.4.

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It was found that the compressive strength for all the curing conditions in-

Figure 7: Compressive Strength for various Liquid to Flyash Ratio

creases till 28days. For the oven cured sample the compressive strength for 1, 7, 14 and 28 days are 20.01MPa, 21.19MPa, 23.56MPa and 29.27MPa and its dry density was found to be 1873kg/m2 , 1919kg/m2 , 1958kg/m2 and 1916kg/m2 . For the samples cured in the ambient condition, the compressive strength was

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(a)

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Figure 8: (a) Variation of Dry Density of Geopolymer Mortar (b) Variation of Compressive Strength of Geopolymer Mortar

found to be 11.01MPa, 13.89MPa, 15.55MPa and 22.32MPa and the dry density was about 1978kg/m2 , 1941kg/m2 , 2173kg/m2 and 2020kg/m2 for 1, 7, 14 and

28 days. For heat chamber curing, it is of 13.29MPa, 14.62MPa, 19.61MPa and 23.91MPa and the corresponding densities are 2148kg/m2 , 2105kg/m2 , 2071kg/m2 and 2083kg/m2 . For autoclave curing, the strength for 1,7,14 and 28 days are 13.34MPa, 14.75MPa, 16.38MPa and 21.64MPa and its densities are 2034kg/m2 , 2052kg/m2 , 2074kg/m2 and 2022kg/m2 . It has been found from 14

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(a)

(b)

(c)

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Figure 9: (a) Dry Density (b) Compressive Strength and (c) Thermal Conductivity for various curing time period for 1 day

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the result that for all the four curing conditions, the compressive strength value of hot air oven curing was found to be the highest and its corresponding den-

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sity was also found to be low when compared to the other curing types. The dry density and compressive strength value for 1, 7, 14 and 28 days are clearly

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figured out in the Figure. 8(a) Figure. 8(b).

On further study, it is important to reduce the curing time which in turn

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reduces the energy consumption of the specimen to make geopolymer as more energy efficient. To reduce the consumption of energy, the specimens are placed for about 1 to 24 hours in hot air oven curing at an interval of 3 hours. From

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the result, it was found that the specimen placed for 6 hours curing gives higher strength and it is shown in Figure.9(a) Figure.9(b) Figure,9(c). This is mainly due to the process of geopolymerization that takes place very effectively only

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during the first 8 hours in which the specimen gains strength and after that there is only a slight increase in its strength. Beyond 24 hours, the strength will

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goes on decreasing.

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4. Conclusion and Future Recommendations In this work, cement free geopolymer plays a vital role in reducing carbon

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dioxide emission which in turn reduces greenhouse gas emissions. Geopolymer mortar specimen subjected to different curing regime showed a reasonable variation in its density, strength, heat resistance and thermal conductivity. Flyash is a waste by-product material obtained from the coal-based thermal power plant is used as source material. Here, four types of curing conditions have been used and the thermal properties were mainly focused. From the experimental results, the following conclusions are drawn: • The use of fly ash as 100% replacement for cement was found to be capable of increasing the dry density by 2.83% and 8.59% for the specimens cured in autoclave and heat chamber respectively. On the other hand, it decreases the dry density by 5.31% for hot air oven cured sample when

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compared to the sample cured at ambient condition. The specimen with

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lesser density can be effectively used for insulation purpose. • A significant increase in compressive strength of about 20.71%, 21.16%

and 81.74% was achieved at 80◦ C for the liquid to flyash ratio of 0.4

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for heat chamber, autoclave and hot air oven cured samples compared to

strength than the other curing types.

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ambient condition. Hence hot air oven curing gives higher compressive

• Decrease in thermal conductivity of 27%, 37% and 60% has been achieved

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while curing the specimen in autoclave, heat chamber and hot air oven. More decrease in the conductivity value will make the specimen to behave more thermal insulative.

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• While subjecting the hot air oven cured sample to an elevated temperature of 100 to 900◦ C, there is a decrease in strength of about 66%. There is

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noticed.

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no damage and only a little discoloration from grey to brown has been

• To make geopolymer as more energy efficient construction material, its curing time is reduced from 24 hours to 6 hours. For 6 hours curing,

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there is an increase in compressive strength by 36% and a decrease in thermal conductivity by 6.34% has been achieved when compared to 24 hours curing. This implies that the strength gaining period of geopolymer was found to be very less when compared to normal concrete. The optimum enhancements in the above properties were achieved only by curing the specimen in hot air oven for 6 hours at 80◦ C so as to make

geopolymer a greener material. Future research work may be carried out in order to make the mix design of geopolymer as a standardized design. Ready Mixed Geopolymer Concrete may be manufactured which represents the successful implementation of a technically very challenging product. It can also be recommended for implementing geopolymer in

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large scale structures by confirming the structural adequacies to overcome

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crack propagation and corrosion deficiencies.

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Acknowledgement

This research work was carried out under the support of the INSPIRE

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Fellowship, Department of Science and Technology, New Delhi.

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