Accepted Manuscript

Effect of alkali activator concentration and curing condition on strength and

microstructure of waste clay brick powder-based geopolymer

Murat Tuyan, Özge Andiç-Çakir, Kambiz Ramyar

PII: S1359-8368(16)32812-8
DOI: 10.1016/j.compositesb.2017.10.013
Reference: JCOMB 5333

To appear in: Composites Part B

Received Date: 23 November 2016

Revised Date: 16 September 2017
Accepted Date: 10 October 2017

Please cite this article as: Tuyan M, Andiç-Çakir Ö, Ramyar K, Effect of alkali activator concentration
and curing condition on strength and microstructure of waste clay brick powder-based geopolymer,
Composites Part B (2017), doi: 10.1016/j.compositesb.2017.10.013.

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 ACCEPTED MANUSCRIPT
Effect of alkali activator concentration and curing condition on strength and

microstructure of waste clay brick powder-based geopolymer

Murat TUYAN, Özge ANDİÇ-ÇAKIR*, Kambiz RAMYAR

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Civil Engineering Department, Faculty of Engineering, Ege University, Izmir, Turkey

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Abstract

The effect of alkali activator concentration and curing conditions on consistency and strength

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of waste clay brick powder-based geopolymer composites was investigated. For this purpose,

geopolymer mortars with twenty different activator concentrations were produced and those

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mixtures having optimum alkali activator concentration were subjected to different curing
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conditions. Test results indicated that the optimum alkali activator concentration corresponded
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to Ms (SiO2/Na2O) ratio of 1.6 and Na2O content of 10% by weight of the binder. A maximum

compressive strength of 36.2 MPa was achieved by curing at 90°C, 40% RH for 5 days. In
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order to characterise the morphology and the structure of the resultant composites, x-ray
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powder diffraction analysis, thermogravimetric analysis, fourier transform infrared

spectroscopy analysis, scanning electron microscopy analysis and micro computed

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tomography analysis were performed. It was determined that the microstructure analysis

results were consistent with the compressive strength results. Denser structure was observed
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by microstructure analysis in the mixtures having high compressive strength.

Keywords: geopolymer, waste clay brick powder, compressive strength, microstructure,

curing condition, consistency

*
Corresponding author. Tel.: +90 232 38 60 26; Fax: +90 232 342 56 29
E-mail address: ozge.andic@ege.edu.tr (Ö. Andiç Çakır).

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1 Introduction

Cement industry undergoes a major change considering the greenhouse gas emissions, energy

consumption, sustainability and use of natural raw materials as a result of national and

international environmental restrictions and policies in recent years. Along with the changes

in cement industry, the studies also proceed on the development of alternative cementitious

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systems. Geopolymer-based mixtures being an important alternative to current cementitious

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systems has a potential for providing reduction in emissions and energy consumption by

meeting the properties of Portland cement mixtures.

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Geopolymer is classified as a synthetic aluminosilicate material that can be used particularly

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in place of Portland cement in the production of materials such as high-performance
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composites and ceramics [1]. There are many aluminosilicate materials found in nature,

additionally, some of them are generated from industrial wastes. Therefore, the use of these
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materials as binders is gaining more importance for the construction industry. Hence, many

studies have been conducted to investigate the properties of geopolymer-based composite

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materials [2-5] and to develop hybrid geopolymer-based composites [6-8] in the recent years.
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The most widely used aluminosilicate materials in the production of geopolymer concrete are
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fly ash, ground granulated blast furnace slag and metakaolin [9-12]. Among these materials,

fly ash and ground granulated blast furnace slag are the industrial by-products which are
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alternative sustainable materials for the construction industry. Owing to high CaO content
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(>30%) of ground granulated blast furnace slag, upon activation in geopolymer composites a

calcium silicate hydrate (C-S-H) gel-like structure is formed [13-16]. However, the

amorphous inorganic polymers form with the activation of fly ash by alkalis. Type F fly ash is

generally preferred for the geopolymer production. Since Type C fly ash has a lower

aluminium oxide and silica content than that of Type F fly ash, which results in a lower

degree of reaction. Moreover, the setting of Type C fly ash-based geopolymer is much faster

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than that of Type F fly ash-based geopolymer. The most important factors affecting the

properties of the fly ash-based geopolymer are the chemical and mineralogical composition of

fly ash, the type and concentration of the alkali solution and the curing condition. The degree

of reaction is lower in ambient temperature than that of high temperature especially in the

Type F fly ash-based geopolymer. It is shown that, in the high temperature curing condition,

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the increase in degree of reaction and kinetic energy provides a denser structure [17]. The

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type of the alkali solution is also of importance for the performance of geopolymer matrix.

Hydroxide and silicate-based solutions are generally used individually as alkali activator to

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produce geopolymers [18,19]. Moreover, the use of both sodium hydroxide and sodium

silicate considerably promotes the performance of geopolymer. Fernandez-Jimenez and

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Palomo [20] reported that 1-day compressive strength of the geopolymer mixture increased
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from 40 MPa to 90 MPa by using both sodium hydroxide and sodium silicate in place of the
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use of solely the sodium hydroxide solution. In addition to the type of alkali activator, its

concentration also affects the performance of the geopolymer significantly. In order to

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investigate the effect of NaOH concentration on mechanical properties of Type F fly ash-
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bearing geopolymer, Görkan and Kürklü [21] prepared geopolymer mixtures having 3 M, 6 M

and 9 M NaOH, respectively. It was observed that due to the rapid flocculation of the silica
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particles in high alkali concentration (9 M NaOH), the compressive strength of the

geopolymer mixture reduced. Somna et al. [22] investigated the influence of NaOH content
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on the compressive strength of the fly ash-based geopolymers having NaOH concentration

between 4.5 M and 16.5 M. The compressive strength of the mixtures increased significantly

with increasing NaOH concentration from 4.5 M to 9.5 M. However, increasing NaOH

beyond 9.5 M (up to 14.5 M) had not a significant effect on the compressive strength. A slight

reduction in the compressive strength of the geopolymer was reported upon increasing NaOH

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content to 16.5 M. The reduction in compressive strength was essentially thought to be due to

excessive hydroxyl ions caused by the precipitation of aluminosilicate gel at early ages.

In addition to industrial waste materials, studies on the production of geopolymer composite

with construction/demolition waste and ceramic waste were carried out. Komnitsas et al. [23]

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investigated the performance of the construction and demolition waste-based geopolymers. In

the study, three types of construction and demolition waste (concrete, brick and tile) were

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mixed with sodium hydroxide and sodium silicate solutions to produce geopolymer mixtures.

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The compressive strength values of brick and tile-based geopolymer were measured as 49.5

MPa and 57.8 MPa, respectively. However, concrete-based geopolymer showed a lower

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compressive strength (13 MPa). Sun et al. [24] investigated the mechanical and thermal
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properties of ceramic waste-based geopolymer. Ceramic waste material was activated by

alkali hydroxides and/or sodium/potassium silicate. The results showed that 28-day
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compressive strength of the geopolymer reached 71.1 MPa. The authors also suggested that

the ceramic waste-based geopolymer can be used as a fireproof material.

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Disposal of the waste clay brick generated from the breakdown of the bricks during

production is becoming a serious environmental issue for the brick industry. Therefore, a
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number of studies have investigated the evaluation of the waste clay brick in construction

industry [25-29]. In most of these studies, waste clay brick was used as a supplementary
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material to produce binary and ternary cementitious systems. In a study conducted by Reig et

al. [30], red clay brick waste-bearing geopolymer having 7 M Na+ concentration and

SiO2/Na2O of 2.0 was produced. The geopolymer mortar was subjected to steam curing at

65°C for 7 days to obtain a compressive strength of 50 MPa.

This study aims to evaluate the effect of alkali activator concentration and curing condition on

strength and microstructure of waste clay brick powder-based geopolymer system. In this

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experimental study, firstly, optimum alkali activator concentration of the waste clay brick

powder-based geopolymer mixtures was determined. Then, the effect of curing condition on

the strength properties of geopolymer mixtures having optimum alkali activator

concentrations was investigated. Finally, the effects of alkali activator concentration and

curing condition on microstructural characteristics of geopolymer were researched.

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2 Experimental Details

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2.1 Materials

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In this experimental study, a waste clay brick powder (WCBP) obtained from Turgutlu, Izmir

was used as a binder. The chemical composition and the physical properties of this material

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are given in Table 1 and Table 2, respectively. The particle size distribution and the
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morphology of the WCBP are shown in Figure 1 and Figure 2, respectively. The particle size

distribution shows that the WCBP was ground to a fineness that is similar to that of the
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common Portland cements having particles in the size range of 1-100 µm. [31]. It was

determined that the WCBP had an angular shape due to grinding of the waste brick. Sodium
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hydroxide (97% purity) and 3 module sodium silicate (SiO2/Na2O ≈ 3.0) (8% Na2O, 27%
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SiO2 and 65% H2O) were used as alkali activator. A siliceous standard sand with a maximum
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particle size of 2 mm in accordance with EN 196-1 standard [32] was used in mortar mixtures.

2.2 Mixture Proportions

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In this study, the upper 10% limitation for Na2O content of WCBP mixtures was due to

economic reasons. Besides, trial mixes and literature survey [33-35] revealed that the

optimum value of Ms was in the range of 1.6 to 2.2 depending on the characteristics of the

binder. The proportions of the mixtures are given in Table 3. The binder content (456 g),

aggregate content (1254 g) and water to binder (w/b) ratio (0.46) were kept constant in all of

the mixtures. These mixture design values were taken from EN 196-1 standard [32] and

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modified according to the specific gravity of binders while keeping the same volumetric

ratios.

2.3 Preparation of Geopolymer Mortars

In order to obtain a homogeneous dry mix, the WCBP and sand were mixed for one minute.

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Then, the alkali activator solution prepared one day before, and allowed to cool at room

temperature, was added to the mixer. After adding solution, extra water to provide the

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required w/b ratio was introduced in the mixer and mixing was continued for four minutes.

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The mixtures were cast in 50 mm cubic molds in two layers and each layer was compacted by

using a vibration table. The specimens were stored in laboratory condition (20°C±2) for

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24 hours before exposure to definite curing regimes. Finally, the specimens were demolded
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and related experiments were performed.
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2.4 Test Methods

In order to evaluate the consistency of different mixtures, the flow diameter of the mixtures
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was determined in accordance with ASTM C230 Standard [36]. The compressive strength of
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the geopolymer mortar mixtures was determined by a digital load-controlled hydraulic testing

machine in accordance with ASTM C109 Standard [37]. The rate of loading was set as 0.9
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kN/s. Three specimens were tested and the average strength was reported. In order to perform

the microstructural analysis by scanning electron microscope (SEM), samples with polished
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section were prepared from the paste mixtures. Investigations were performed on FEI

QUANTA 250 FEG-type environmental SEM by backscattered mode, 3-7 kV acceleration

voltage, spot size of 3.0 and applying appropriate pressure values in the device. X-ray

diffraction (XRD) analysis was performed on ground geopolymer paste samples with Philips

X'Pert Pro-type X-ray diffractometer. Kα1 wavelength of copper (λ= 1.5406 Å) was used in a

step scan mode range 5° ≤ 2θ ≤ 80°, with a step size of 0.033° 2θ and a scan step time of

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30.48 seconds. Device conditions were set at 40 mA and 45 kV. In order to determine the

mineralogical composition of the mixtures, X’Pert High Score Plus software was used for

quantitative phase analysis with XRD patterns. Thermogravimetric analysis (TGA) was

performed on a TA Instruments SDT Q600 device. The experiment was carried out on ground

geopolymer paste samples from 25°C to 1000°C, with a rate of temperature rise of 10°C/min.

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Fourier transform infrared spectroscopy (FTIR) analysis was performed with a

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ThermoScientific IS50 brand spectroscopy device. Ground geopolymer paste mixtures were

mixed with cobalt and measurements were made in the range of 400-4000 cm-1 wave number

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to determine the molecular structures and bonds of the geopolymer mixtures. In order to

investigate the void structure of the geopolymer paste mixtures, the analyses were performed

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on solid paste samples with Scanco Medical µCT 50-type Micro Computer Tomography
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(Micro CT) device equipped with Scanco Medical MicroCT software version 6.3. In the

analyses, an area of 0.5041 cm2 was scanned for each sample in a volume of 1 cm3 and the
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average pore diameter and percentage porosity of the geopolymer samples were determined.
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In addition to the Micro-CT analysis, the porosity of the geopolymer paste samples was
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determined using the Archimedes principle (Equation 1).

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Porosity % = 1 − #100 (1)
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where, γ is the density of water.

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3 Test Results and Discussion

3.1 Effect of alkali activator concentration on compressive strength of the mixtures

The compressive strength of geopolymer mortars having different alkali activator

concentrations is given in Figure 3. Increasing the Na2O content from 4% to 6% increased the

compressive strength. The effect was more pronounced in the mixtures having lower Ms ratios

(except for the mixture with Ms ratio of 2.2). This may be attributed to increase in the amount

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of dissolved Si and Al ions and forming more reaction products in the highly alkaline medium

[38]. Beyond 6%, further increase in Na2O content caused a slight variation in the

compressive strength. On the other hand, increasing the Ms ratio of the mixture from 1.6 to

1.8 had not a significant effect on the compressive strength. However, in the mixtures having

Ms ratios beyond 1.8, Ms ratio had a contradictory effect on the compressive strength, i.e. up

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to 6% Na2O content increasing the Ms ratio of 1.8 resulted in an increase in the compressive

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strength, however, in the mixtures of higher Na2O content the opposite was true as consistent

with the findings in the literature [21, 22]. Among the geopolymer composites, the highest

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and the lowest compressive strength values were obtained in the mixtures having 10% Na2O –

1.8 Ms ratio, and having 4% Na2O – 1.6 Ms ratio, respectively.

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The effect of Ms ratio on the compressive strength of mortar mixtures having 10% Na2O

content is presented in Table 4. It was observed that there was a significant increase (77%) in
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compressive strength with increasing Ms ratio from 0 to 1.2 due to the dissolved and partially

polymerized silicon [39]. The compressive strength of the mixture increased from 7.1 MPa to
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16.7 MPa with increasing Ms ratio from 1.2 to 1.6. However, the compressive strength
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decreased from 16.7 MPa to 13.1 MPa with increasing Ms ratio from 1.6 to 2.2. The decrease
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in compressive strength with increasing Ms ratio was explained in different ways by different

researchers: Law et al. [40] reported that the reaction products which formed rapidly in the
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highly alkaline medium may block the unreacted particles contacting with alkali solution. On
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the other hand, Barbosa et al. [41] indicated that excessive amount of sodium silicate may

hinder water evaporation and the formation of polymerization products during the

polycondensation process.

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3.2 Effect of alkali activator concentration on consistency of the mixtures

The flow diameter of the geopolymer mortars with different alkali activator concentrations is

given in Figure 4. The flow values of the mixtures indicate that the consistency of the

mixtures change in a narrow range (150±15 mm). In spite of having same water contents, the

flow diameter of the mixtures generally increased with increasing Na2O percent. Even though

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Phair and van Deventer [42] indicated that the high alkali concentration of the geopolymer

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mixtures increased the viscosity of the mixtures, an increase in flow diameter of the mixtures

was observed because the mixtures having high Na2O percent had higher sodium silicate

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content. The flow diameter of the mixtures with Ms ratio of 1.6 increased from 136 mm to 152

mm with increasing Na2O content from 4% to 10%. Similar increase in the flow diameter of

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the geopolymer mortar was observed in the mixtures having Ms ratio of 1.8, 2.0 and 2.2. The
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flow diameter of the mortars with 4% and 10% Na2O content slightly increased with
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increasing Ms ratio of the mixtures. However, a slight decrease in flow diameter of the

mixtures having 6% Na2O content was observed with increasing Ms ratio of the mixtures. In
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the mixtures with 8% Na2O content, the flow diameter of the mixtures slightly increased with
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increasing Ms ratio from 1.6 to 2.0. On the other hand, a slight decrease in flow diameter of

the mortars was observed with increasing Ms ratio from 2.0 to 2.2.
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The effect of Ms ratio on the flow diameter of geopolymer mixtures with 10% Na2O content is
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presented in Table 5. A slight increase in flow diameter of the mixtures was observed with
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increasing Ms ratio of the mixtures. The flow diameter of the mixtures increased from

138 mm to 149 mm with increasing Ms ratio from 0 to 1.2. Increase in Ms ratio of the

mixtures was obtained by the addition the sodium silicate solution in the alkali activator. In

spite of being a viscous liquid [43], a slight increase in the workability of the mixtures was

observed in this study with increasing sodium silicate solution content.

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3.3 Effect of water to binder ratio on compressive strength of the mixtures

In order to investigate the effect of water to binder (w/b) ratio on the compressive strength of

geopolymer mortars, the mixtures having different w/b ratios (0.46, 0.44, 0.42, 0.40) were

prepared. The mixtures with 10% Na2O content and Ms ratio of 1.6 were cured at 70°C in a

drying oven for 7 days. The effect of w/b ratio on the compressive strength and the flow

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diameter of the geopolymer mortars is shown in Figure 5. The compressive strength of the

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mixtures increased from 16.7 MPa to 20.7 MPa with decreasing the w/b ratio from 0.46 to

0.40. The flow diameter also decreased from 152 mm to 124 mm with decreasing w/b ratio

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from 0.46 to 0.40. The simultaneous reduction in consistency, thus, compactibility of the

mixtures upon reducing w/b ratio from 0.46 to 0.40 resulted in a relative lower increase in the

strength of the geopolymer mixtures.

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3.4 Effect of curing condition on compressive strength of the mixtures
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In order to investigate the effect of curing conditions on the compressive strength of

geopolymer mortars, the mixtures with 10% Na2O and Ms ratio of 1.6 were subjected to six
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different curing temperatures (50°C, 60°C, 70°C, 80°C, 90°C, 100°C) for four different
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durations (1, 3, 5 and 7 days). Additionally, in order to investigate the effect of curing
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condition at room temperature (25±2°C), the mixtures with 10% Na2O and Ms ratio of 1.6

were cured at room temperature for 3, 7, 28 and 90 days. The effect of curing conditions on
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the compressive strength of geopolymer mortars is given in Figure 6. As shown in Figure 6, a

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gradual increase in the compressive strength was observed in the mixtures cured at 50°C,

60°C and 70°C for all curing period. A slight increase in the compressive strength was

observed in the mixtures cured at temperatures beyond 70°C compared to those of the

mixtures cured at lower temperatures. The compressive strength of mixtures cured at 80°C for

5 days was almost similar to the mixtures cured at 80°C for 7 days. In the mixtures cured at

90°C, the maximum compressive strength was obtained in the specimens cured for 5 days. It

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was observed that the compressive strength of the mixtures cured at 100°C for 3 days was

slightly higher than those of the mixtures cured at 100°C for 5 and 7 days. The effect of

curing period at room temperature on the compressive strength of geopolymer mortars is

given in Table 6. The mixture cured at room temperature showed no hardening, therefore, no

strength at 3 and 7 days. The compressive strengths of the mixtures at 28 and 90 days were

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5.3 MPa and 18.7 MPa, respectively. It was observed that there was a significant difference

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between the compressive strengths of the mixtures cured at high temperature and those of the

mixtures cured at room temperature. While the maximum compressive strength of the

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mixtures cured at high temperature for 5 days was 36.2 MPa, the maximum compressive

strength of the mixtures cured at room temperature for 90 days was only 18.7 MPa.

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For the WCBP-based geopolymer mortars, it seems that the high temperature curing provided

a higher degree of reaction compared to that of curing at room temperature. However, a

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decrease in compressive strength of the mixtures was observed beyond a certain temperature.

This may be attributed to the deterioration of the reaction products with excessive high curing
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temperature and curing period [44, 45].

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3.5 Determination of optimum curing condition in terms of energy consumption

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In order to indicate the optimum curing condition in terms of energy consumption, energy

consumption of the drying oven was determined and the three mixtures which had maximum
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compressive strength among all mixtures were selected. The curing temperatures, curing

periods and energy consumption values of these mixtures are presented in Table 7. It was

observed that the geopolymer mixture cured at 80°C for 5 days have an optimum curing

condition in terms of energy consumption.

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3.6 Microstructural analysis

3.6.1 XRD analysis

XRD analysis results of the geopolymer mixtures are shown in Figure 7. Quartz (PDF No:

96-710-30-15), muscovite (PDF No: 96-901-2887), clinoptilolite-Ca (PDF No: 96-900-5428)

and Cyclohexanehexone (PDF No: 96-901-4869) minerals have been found in the raw WCBP

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and the WCBP-based geopolymer mixtures. Quartz peaks were found to be the major peaks in

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all of the geopolymer pastes. A very slight hump in the range of 20-35° (2θ), indicating a low

amorphousness, was observed in the diffractograms of the raw material and geopolymer

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mixtures. The reason for the low degree of amorphousness of the WCBP may be the low rate

of cooling of the brick after the calcination process in the brick production. It was found that

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the intensity of the crystal peaks in the geopolymer composites is slightly reduced compared
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to the raw WCBP, except for the peaks at 2θ angle of about 27°. In general, it was determined
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that in mixtures with a low Na2O and Ms ratio, the crystal peaks are more intense than the

other mixtures. In the mixtures with 10% Na2O ratio, no significant change in the hump in the
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range of 20 to 35° 2θ angle was determined with increasing the Ms ratio from 0 to 1.6. It was
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observed that the intensity of quartz peaks in the mixture having Ms ratio of 1.6 was lower

than that of the mixture having Ms ratio of 0. This can be arisen from the consumption of
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some of the SiO2 found in WCBP upon its activation. In the mixtures with Ms ratio of 1.6,

there was not a significant change in the hump in the range of 20-35° (2θ) with increasing
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Na2O content from 4% to 10%. Meanwhile, the intensity of the quartz crystal peaks reduced.

In the mixtures having 10% Na2O, there was no significant change in the peaks with

increasing the Ms ratio from 1.6 to 2.2. The intensity of the peaks in the mixture of 10% Na2O

and Ms ratio 1.6, which had the highest compressive strength among all mixtures, was lower

than that of the mixtures with low compressive strength. In all geopolymer mixtures, quartz

peaks may indicate the presence of unreacted silica. A slight reduction in the quartz and

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muscovite crystal peaks was observed with increasing the curing period from 1 day to 5 days

at the optimum curing temperature (80°C). In general, decrease in the intensity of the quartz

and muscovite peaks increased the compressive strength of the geopolymers. This can be

arisen from the transformation of quartz and muscovite minerals in the raw WCBP into the

strength-contributing amorphous geopolymerization products.

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3.6.2 TGA

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The results of thermogravimetric analysis of the WCBP-based geopolymer are given in Figure

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8. It was observed that irrespective of alkali activator concentration and curing condition,

there was no significant change in weight loss arisen from TGA. Generally, most of the

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weight loss of the samples occurred at the temperatures between 100°C and 200°C. The
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reason for this weight loss is the evaporation of free water absorbed in the specimen. Besides,

a small amount of weight loss occurred at temperatures between 650°C and 750°C. In the
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literature, the fact was attributed to the decomposition of the carbonates due to the

atmospheric carbonation occurring during the preparation of the samples prior to analysis
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[46]. TGA revealed that the WCBP-based geopolymers had a thermally stable structure upon
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heat treatment up to 1000°C. Accordingly, WCBP-based geopolymer may be more useful for
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producing elements that are resistant to high temperatures compared to Portland cement

systems due to its superior thermal properties. Furthermore, it may be stated that WCBP-
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based geopolymer was a single-component material [47].

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3.6.3 FTIR analysis

FTIR analysis results of the WCBP-based geopolymers are given in Figure 9. A significant

change in the molecular and bond structure was observed by changing the alkali activator

concentration and curing condition of the mixtures. In general, the peaks in the region of 526-

530 cm-1 were attributed to the asymmetric stretching vibrations of Si–O–Si and Al–O–Si

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bonds [48]. The peaks at around 537 cm-1 were attributed to the Si-O-Al bonds where Al is

present in octahedral co-ordination [49]. The peaks at around 558 cm-1 corresponded to the

double-ring linkage [50]. The peaks at about 668 cm-1 were attributed to the symmetric

stretching vibrations of Si-O bonds. This indicated that 6-coordinated Al(VI) changed into 4-

coordinated one [51]. A strong peak at around 1000 cm-1 was found in all of the mixtures.

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This peak was related to the asymmetric stretching vibrations of Si–O and Al–O bonds

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indicating an important evidence of geopolymerization process [52]. The peaks at around

1410 cm-1 and 1484 cm-1 were attributed to the stretching vibrations of O-C-O bonds

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indicating the presence of sodium bicarbonate due to the atmospheric carbonation of the

mixtures [53]. In the discussion of TGA results, the weight loss at about 700°C was attributed

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to the atmospheric carbonation of the mixtures. The fact was also supported by the FTIR
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analysis. The peaks in the region of 3500-4000 cm-1 and about 1700 cm-1 were related to the
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stretching and bending vibrations of O-H bonds. This showed the presence of the hydroxyl

groups in the geopolymer mixtures.

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In the geopolymer mixtures having 10% Na2O, Ms: 0 and 10% Na2O, Ms: 2.2, it was
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determined by FTIR analysis that these mixtures had much more hydroxyl groups in their
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structures than those of the other mixtures. As a result, the compressive strength of these

mixtures was lower than those of the other mixtures. The peaks below 1000 cm-1 increased
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with increasing curing period of the mixtures resulting in an increase in their compressive
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strength.

3.6.4 SEM analysis

SEM investigations of the WCBP-based geopolymers are presented in Figure 10. Unreacted

WCBP particles within the alkali solutions (A) and many voids (B) were observed in the

geopolymer matrix with 10% Na2O and Ms ratio of 0 (Figure 10a). In the discussion of FTIR

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analysis, it was mentioned that the mixture having 10% Na2O and Ms ratio of 0 had more

hydroxyl groups than the other mixtures relating with the unreacted WCBP particles. The

presence of unreacted particles in the mixture was also supported by the SEM image given in

Figure 10a. There was a denser structure in the mixture having Ms ratio of 1.6 compared to

that of the mixture having Ms ratio of 0 (Figure 10a and Figure 10c). Thus, increasing Ms ratio

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from 0 to 1.6 increased the compressive strength significantly. When the mixtures shown in

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Figure 10b and Figure 10c were compared, it was observed that a denser structure was

obtained by increasing Na2O content of the mixture. This fact proved that more

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geopolymerization products were formed and consequently, increased the compressive

strength of geopolymer. Moreover, no significant morphology difference was observed

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between the mixtures having Ms ratio of 1.6 and 2.2 (Figure 10c and Figure 10d). The
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mixtures shown in Figure 10e and Figure 10f had lower water to binder ratio and
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consequently lower void content than those of the other mixtures. Besides, comparison of

Figure 10e and Figure 10f indicates a significant improvement in the structure of the mixture
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upon prolonging the curing period. As it can be seen from Figure 10e, upon curing at 80°C for
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1 day, a considerable amount of needle-like products (C) were formed in the geopolymer

mixture. These products may be due to the presence of crystal phases of the geopolymer
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mixture determined by XRD analysis. It was seen that these products decreased with

increasing the curing period from 1 day to 5 days (Figure 10f). In XRD analysis, the intensity
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of the crystal peaks of quartz and muscovite slightly decreased with increasing the curing

period from 1 day to 5 days. It was determined that the excessive amount of needle-like

products in the mixture leading to decreased compressive strength. The increase in the curing

period from 1 day to 5 days caused an increase in the compressive strength of the mixture by

reducing the needle-like products. Thus, it can be stated that crystalline reaction product had a

little contribution to the strength of the mixture.

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3.6.5 MicroCT analysis

The average pore diameter and porosity results of the WCBP-based geopolymers are

presented in Table 8. In addition, the pore size distribution of the geopolymers is shown in

Figure 11. In the mixtures having 10% Na2O content, the average pore diameter increased

from 33.2 µm to 53.8 µm, meanwhile, the porosity decreased from 2.02% to 1.83% with

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increasing Ms ratio from 0 to 1.6. In the mixtures having Ms ratio of 1.6, the average pore

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diameter increased from 36.9 µm to 53.8 µm, consequently, the porosity decreased from

4.04% to 1.83% with increasing Na2O content from 4% to 10%. In the mixtures having 10%

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Na2O content, the average pore diameter remained constant (53.8 µm), but the porosity

increased from 1.83% to 1.94% with increasing Ms ratio from 1.6 to 2.2. Not surprisingly, the

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minimum porosity was obtained in the mixtures having higher compressive strength (10%
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Na2O and Ms ratio of 1.6). When the effect of the curing condition on the pore size and the
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porosity of the geopolymers was investigated, the average pore diameter decreased from 59.8

µm to 40.3 µm and the porosity decreased from 2.09% to 1.53% with increasing curing period
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from 1 day to 5 days. The pore size decreased and a denser structure formed by increasing the
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curing period at 80°C (Figure 10e and Figure 10f). Thus, a decrease in pore size also

increased the compressive strength of the geopolymer. The porosity values determined by
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Micro-CT analysis were found to show a good correlation with the porosity values obtained

by Archimedes principle (R2= 0.9865) while MicroCT values somewhat being higher than
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those obtained by Arcimedes principle.

4 Conclusions

In this study, the effect of alkali activator concentration and curing condition on the

consistency, mechanical properties and the microstructure of the waste clay brick powder-

based geopolymer was studied and the following conclusions were drawn:

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• The optimum alkali activator concentration from maximum strength and minimum

cost viewpoints was found to be 10% Na2O and 1.6 Ms ratio.

• The flow diameter of the mixtures increased with increasing Na2O content. For a

constant Na2O content, the flow diameter increased with increasing Ms ratio.

• Not surprisingly, the compressive strength of the mixtures increased by reducing water

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to binder ratio with a consequent reduction in the flow diameter of the geopolymer

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mixture. However, reducing the water to binder ratio to the values lower than 0.4

resulted in non-workable geopolymer mixtures.

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• The maximum compressive strength was obtained upon curing at 90°C for 5 days, and

in terms of the energy consumption, the optimum curing condition was at 80°C for 5

days.
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• More intense crystal peaks in the raw waste clay brick powder compared to those in
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the geopolymer mixtures were observed. Moreover, the amorphousness of the

mixtures increased with the activation of the raw material by alkalis.

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• TGA demonstrated the thermally stable structure of the waste clay brick powder-based
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geopolymers upon heat treatment.

• The asymmetric stretching vibrations of Si–O and Al–O bonds which was a strong
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evidence of the geopolymerization process was detected in the geopolymer mixtures.

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• An increase in Na2O content as well as an increase in Ms ratio up to a certain level

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(1.6) decreased the porosity of the mixture, causing a denser structure with consequent

increase in strength.

Acknowledgements

The authors would like to thank The Scientific and Technical Research Council of Turkey

(TUBITAK) for the financial support provided under Project: MAG 213M506 and Ege

University for funding by 13-MUH-071 project.

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Figure 1. Particle size distribution of waste clay brick powder

Figure 2. Morphology of waste clay brick powder

Figure 3. Effect of alkali activator concentration on compressive strength of the mixtures

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Figure 4. Effect of alkali activator concentration on flow diameter of the mixtures

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Figure 5. Effect of water to binder ratio on compressive strength and flow diameter of the

mixtures

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Figure 6. Effect of curing temperature and curing period on compressive strength of the
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mixtures
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Figure 7. XRD analysis of waste clay brick powder-based geopolymers

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(a) 10% Na2O, Ms: 0 (70°C, 3 days)

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(b) 4% Na2O, Ms: 1.6 (70°C, 3 days)

(c) 10% Na2O, Ms: 1.6 (70°C, 3 days)

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(d) 10% Na2O, Ms: 2.2 (70°C, 3 days)

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(e) 10% Na2O, Ms: 1.6 (80°C, 1 day)

(f) 10% Na2O, Ms: 1.6 (80°C, 5 days)

Figure 8. TGA results of waste clay brick powder-based geopolymers

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(a) 10% Na2O, Ms: 0 (70°C, 3 days)

(b) 4% Na2O, Ms: 1.6 (70°C, 3 days)

(c) 10% Na2O, Ms: 1.6 (70°C, 3 days)

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(d) 10% Na2O, Ms: 2.2 (70°C, 3 days)

(e) 10% Na2O, Ms: 1.6 (80°C, 1 day)

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(f) 10% Na2O, Ms: 1.6 (80°C, 5 days)

Figure 9. FTIR analysis of waste clay brick powder-based geopolymers

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(a) 10% Na2O, Ms: 0 (70°C, 3 days) (b) 4% Na2O, Ms: 1.6 (70°C, 3 days)

(c) 10% Na2O, Ms: 1.6 (70°C, 3 days)

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(e) 10% Na2O, Ms: 1.6 (80°C, 1 day) (f) 10% Na2O, Ms: 1.6 (80°C, 5 days)

Figure 10. SEM analysis of waste clay brick powder-based geopolymers

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(a) 10% Na2O, Ms: 0 (70°C, 3 days) (b) 4% Na2O, Ms: 1.6 (70°C, 3 days)

(c) 10% Na2O, Ms: 1.6 (70°C, 3 days) (d) 10% Na2O, Ms: 2.2 (70°C, 3 days)
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(e) 10% Na2O, Ms: 1.6 (80°C, 1 day) (f) 10% Na2O, Ms: 1.6 (80°C, 5 days

Figure 11. MicroCT analysis of waste clay brick powder-based geopolymers

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Table 1. Chemical composition of waste clay brick powder

Item (%) Material

SiO2 66.15
Fe2O3 5.98
Al2O3 15.36
CaO 2.95

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MgO 2.13
Na2O 0.89

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K 2O 2.77
SO3 0.11

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Loss on ignition 1.50

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Table 2. Physical properties of waste clay brick powder

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Property Material
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Specific gravity 2.70

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Blaine specific area (cm /g) 5570
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0.090 mm retained (%) 11.6

0.045 mm retained (%) 31.2
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*After grinding in the laboratory mill.

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Table 3. Mix proportions of geopolymer mortars having different alkali activator

concentrations

Na2O Sodium Sodium Mixing Total

Ms Aggregate WCBP*
(% by weight silicate hydroxide water water**
(SiO2/Na2O) (g) (g)
of binder) (g) (g) (g) (g)
4 1.6 1254 456 110 12.5 136 208
4 1.8 1254 456 123 11.1 128 208

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4 2.0 1254 456 135 9.9 120 208
4 2.2 1254 456 150 8.2 110 208
6 1.6 1254 456 165 18.9 101 208

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6 1.8 1254 456 180 17.3 92 208
6 2.0 1254 456 200 15.2 78 208
6 2.2 1254 456 220 13.0 65 208

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8 1.6 1254 456 220 25.2 65 208
8 1.8 1254 456 250 22.0 45 208
8 2.0 1254 456 265 20.4 36 208

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8 2.2 1254 456 290 17.5 19 208
10 0 1254 456 0 60.6 208 208
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10 0.4 1254 456 75 52.7 159 208
10 0.8 1254 456 140 45.7 117 208
10 1.2 1254 456 205 38.8 75 208
10 1.6 1254 456 265 32.5 36 208
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10 1.8 1254 456 280 27.6 26 208

10 2.0 1254 456 300 23.3 13 208
10 2.2 1254 456 320 18.0 0 208
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*waste clay brick powder, ** The summation of mixing water and water from sodium silicate
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Table 4. Effect of Ms ratio on compressive strength of geopolymer mortars having 10% Na2O
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content

10% Na2O
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Ms ratio 0 0.4 0.8 1.2 1.6 1.8 2.0 2.2

Average compressive strength (MPa) 4.0 5.4 7.1 7.8 16.7 17.0 12.4 13.1
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Standard deviation (MPa) 0.4 0.3 0.7 0.5 0.9 1.1 0.8 0.6

Table 5. Effect of Ms ratio on flow diameter of geopolymer mortars having 10% Na2O content

10% Na2O
Ms ratio 0 0.4 0.8 1.2 1.6 1.8 2.0 2.2
Average flow diameter (mm) 138 141 146 149 152 153 156 158
Standard deviation (mm) 2 3 2 2 3 3 2 3

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Table 6. Effect of curing period at room temperature on the compressive strength of the

mixtures

3 days 7 days 28 days 90 days

Average compressive strength (MPa) 0 0 5.3 18.7

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Table 7. Energy consumption of the mixtures

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Compressive strength (MPa) Curing temperature and period Energy consumption (kWh) kWh/MPa
36.2 90°C / 5 days 33.720 0.931
35.9 80°C / 5 days 29.040 0.808

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34.9 80°C / 7 days 40.656 1.116

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Table 8. Average pore diameter and porosity of waste clay brick-based geopolymers

10% Na2O, 4% Na2O, 10% Na2O, 10% Na2O, 10% Na2O, 10% Na2O,
M s: 0 Ms: 1.6 Ms: 1.6 Ms: 2.2 Ms: 1.6 Ms: 1.6
Mix
M

70°C, 70°C, 70°C, 70°C, 80°C, 80°C,

3 days 3 days 3 days 3 days 1 day 5 days
Average pore
33.2 36.9 53.8 53.8 59.8 40.3
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diameter (µm)
Porosity (%)
2.02 4.04 1.83 1.94 2.09 1.53
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(MicroCT)
Porosity (%)
1.55 2.88 1.42 1.60 1.48 1.22
(Archimedes)
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20
AC

0 0
0.40 0.42 0.44 0.46
Water to binder ratio
 ACCEPTED MANUSCRIPT

Compressive strength Flow diameter

PT
25 180

RI
160
Compressive strength (MPa)

SC
20 140

Flow diameter (mm)

U
120

AN
15
100

M
80

D
10

TE
EP 60

5 40
C

20
AC

0 0
0,40 0,42 0,44 0,46
Water to binder ratio
 ACCEPTED MANUSCRIPT

1 day 3 days 5 days 7 days

PT
45

RI
40

SC
Compressive strength (MPa)

35

U
30

AN
25

M
20

D
TE
15 EP
10
C

5
AC

0
50 60 70 80 90 100
Curing temperature (°C)
 ACCEPTED MANUSCRIPT

1 day 3 days 5 days 7 days

45

PT
40

RI
Compressive strength (MPa)

SC
35

U
30

AN
25

M
20

D
TE
15 EP
10
C

5
AC

0
50 60 70 80 90 100
Curing temperature (°C)
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC
 ACCEPTED MANUSCRIPT

PT
RI
U SC
AN
M
D
TE
EP
C
AC