correlation study on microstructure and mechanical

Advances in Concrete Construction, Vol. 11, No. 1 (2021) 73-80

DOI: https://doi.org/10.12989/acc.2021.11.1.073 73

Copyright © 2021 Techno-Press, Ltd. http://www.techno-press.org/?journal=acc&subpage=7 ISSN: 2287-5301 (Print), 2287-531X (Online)

1. Introduction

Global production of cement, with more than 4 billion

tons, accounts for the third largest source of anthropogenic

source of carbon dioxide with fossil fuels and land-use

changes being first and second respectively. Global CO2

emission from the production of cement was estimated at

1.45±0.20 Gt for the year 2016 (Andrew 2017). India with

cement production of 300,000 tons in 2016, is the second

largest producer of cement in the world (U.S. Geological

Survey 2019). Cement production also contributes to

greenhouse effects and acid rain with emission of SO2

(Sulphur dioxide), NOx (Nitrous oxide) (Valipour 2014),

consumption of subsequent amount of natural resources and

massive energy (Rashad 2011, 2013). Cement industry,

hence is faced tremendous challenges to address these

issues. Consequently, many researchers worked on partial

replacement of cement by utilization of by-products such as

fly ash, slag, silica fume, rice husk ash etc., which were

termed as supplementary cementitious materials (SCM).

Corresponding author, Associate Professor

E-mail: [email protected] aPh.D. Scholar

E-mail: [email protected] bAssociate Professor

E-mail: [email protected]

Another school of thought worked on developing alternate

binders which will contribute to lesser emission of CO2 and

consume lesser energy without compromising the quality

and efficiency. Geopolymers as alternate binders, shows

encouraging or even better properties than cement (Provis

2014).

Geopolymers are alkali aluminosilicate binders formed

by alkali silicate activation of aluminosilicate materials

(Davidovits 1994). Any material containing silica and

aluminum can be a source of geopolymer primer.

Researchers have studied different precursors like kaolinite

clays (Raheir et al. 1996, 1997, Barbosa et al. 2000),

metakaolin (Wang et al. 2005, Lee et al.2005, Praven et al.

2019), fly ash (Fernadez and Palomo 2005, Suresh et al.

2011), GGBS (Goriparthi 2007, Khater 2014), silica fume

(Khater 2013, Brew and MacKenzie, 2007), rice husk ash

(RHA) (Rattanasak et al. 2010, Kim et al. 2014, Singhal

and Jindal 2017).These precursors were normally activated

by hydroxides and silicates of sodium (Rattanasak and

Chindaprasirt 2009) and potassium (He et al. 2003, Shaikh

and Haque 2018). The production cost, viscous and

corrosive nature of these activators are the main hindrances

in adopting geopolymer widely and hence a search for a

new efficient activator is needed. Also, a limited research

are presently reported for RHA based geopolymer.

Rice is being cultivated on more than 165 million

hectares worldwide and with more than 756 million MT of

production in 2017. Asia accounts for more than 90% of

this production with India second at more than 168 MT of

Correlation study on microstructure and mechanical properties of rice husk ash-Sodium aluminate geopolymer pastes

N. Shyamananda Singh1a, Suresh Thokchom2 and Rama Debbarma1b

1Department of Civil Engineering, National Institute of Technology Agartala, Tripura, India 2Department of Civil Engineering, Manipur Institute of Technology, Imphal, Manipur, India

(Received September 12, 2019, Revised December 3, 2020, Accepted December 18, 2020)

Abstract. Rice Husk Ash (RHA) geopolymer paste activated by sodium aluminate were characterized by X-ray diffractogram

(XRD), scanning electron microscope (SEM), energy dispersion X-Ray analysis (EDAX)and fourier transform infrared

spectroscopy (FTIR). Five series of RHA geopolymer specimens were prepared by varying the Si/Al ratio as 1.5, 2.0, 2.5, 3.0

and 3.5. The paper focuses on the correlation of microstructure with hardened state parameters like bulk density, apparent

porosity, sorptivity, water absorption and compressive strength. XRD analysis peaks indicates quartz, cristobalite and gibbsite

for raw RHA and new peaks corresponding to Zeolite A in geopolymer specimens. In general, SEM micrographs show

interconnected pores and loosely packed geopolymer matrix except for specimens made with Si/Al of 2.0 which exhibited

comparatively better matrix. Incorporation of Al from sodium aluminate were confirmed with the stretching and bending

vibration of Si-O-Si and O-Si-O observations from the FTIR analysis of geopolymer specimen. The dense microstructure of

SA2.0 correlate into better performance in terms of 28 days maximum compressive strength of 16.96 MPa and minimum for

porosity, absorption and sorptivity among the specimens. However, due to the higher water demand to make the paste workable,

the value of porosity, absorption and sorptivity were reportedly higher as compared with other geopolymer systems. Correlation

regression equations were proposed to validate the interrelation between physical parameters and mechanical strength. RHA

geopolymer shows comparatively lower compressive strength as compared to Fly ash geopolymer.

Keywords: rice husk ash; sodium aluminate; XRD; SEM EDAX; FTIR

N. Shyamananda Singh, Suresh Thokchom and Rama Debbarma

Table 1 Physical properties of RHA

RHA

Natural Humidity (%) 3

Theoretical Density (g/cm3) 2.2

Colour Grey

Table 2 Chemical composition of RHA

Chemical Component (Weight %)

SiO2 92.19

Al2O3 0.09

Fe2O3 0.10

TiO2 0.71

MgO 0.41

K2O 0.05

Na2O 1.64

SO3 0.41

CaO 0.09

P2O5 0.01

LOI 4.14

production (FAO 2018). Rice husk being by-products of

milling process, accounts for around 20% of the rice

production (Jain et al. 1996). The silica content of ash

obtained from rice husk burned at 550°-700° is transformed

into amorphous stage (Boateng and Skeete1990). The

amorphous stage of RHA mainly consist of SiO2 (Mehta

and Pitt 1976). A strong network of Si-Al in three

dimension is one of the criteria for a strong geopolymer.

Sodium aluminate as alternative activator were proposed

and studied for fly ash based geopolymer by Phair and

Deventer (2002). Sturm et al. (2016) and Hajimohammadi

and van Deventer (2016) reported geopolymer based on the

“one -part” formulation of RHA and sodium aluminate.

This paper aims to validate the use of RHA and sodium

aluminate in the conventional “two part” geopolymer as

these were known for their versatility. Further, the paper

reports on the microstructure of the RHA geopolymer

specimens and correlates the observed microstructures with

the physical and mechanical parameters.

2. Experimental

2.1 Raw materials

The main raw material used in the study was RHA

collected from Kolkata, India. RHA samples were burned at

a temperature of (650-700)°C, collected and stored in air

tight containers. The physical characteristics for RHA is

tabulated in Table 1.

The X-Ray Fluorescence (XRF) results of RHA shown

in Table 2 indicate the bulk component as SiO2 at 92.19%

by weight. It also has traces of other oxides as well.

The SEM microstructure of the raw RHA shows rough,

flaky extended features with maximum percentage of Si and

trace amount of Al as evident from EDAX analysis in Fig.

1.

Sodium aluminate powder used as activator for RHA

Table 3 Series composition of RHA geopolymer samples

Specimen

Name Si/Al

RHA

(%wt)

Sodium Aluminate

(%wt)

Water

(%wt)

SA 1.5 1.5 40.65 18.70 40.65

SA 2.0 2.0 42.64 14.71 42.64

SA 2.5 2.5 43.94 12.13 43.94

SA 3.0 3.0 44.84 10.31 44.84

SA 3.5 3.5 45.51 8.972 45.51

was sourced from Sigma -Aldrich. The chemical

composition of sodium aluminate includes 54.62% of Al2O3

and 40.5% of Na2O and traces (≤ 0.05%) of Fe2O3.

2.2 Sample preparation

Based on the molar ratio of SiO2/Al2O3, five series of

geopolymer pastes with different sample designations were

prepared as shown in Table 3. Sodium aluminate was mixed

with required amount of distilled water and kept for 24

hours at room temperature. A constant Water/Binder ratio of

1:1 were maintained as RHA has low workability. 50 mm

cubes were used as per ASTM C109. After pouring into the

mould, the specimens were kept undisturbed for 1 hour. The

moulds were then placed into an electric oven maintained at

80°C for 24 hours. After cooling and demolding, samples

were organized systematically for relevant studies. The

nomenclature of the samples are such that the alphabets

denotes silicon and aluminum whereas the number denotes

the ratio of Si/Al. In all the series of the specimens, the ratio

of Na/Al was maintained constant at 1.

2.3 Testing procedure

2.3.1 Mechanical test The direct compressive strength of hardened

geopolymer specimens was determined at the ages of 7 and

28 days in a 3000kN capacity Servo-Hydraulic Computer

Controlled Compression Testing Machine. In each case,

three identical specimens were tested in accordance to

ASTM C-109-02 and average values were reported.

2.3.2 Bulk density and apparent porosity The bulk density and apparent porosity were determined

for 28 days old specimens. The specimens were dried in a

ventilated oven for 24 hours at a temperature of

80°C.Weight of the dried specimens were measured and

recorded as Wd. Specimens were then soaked in water for

24 hours. After removal from water, specimens were

suspended by a thin wire inside water and its weight

recorded as Wi.

The specimens were then wiped dry and its weight

measured in saturated surface dry condition as Ws. The bulk

density and apparent porosity of the specimens were then

determined using the relationships given below

Dry density (kg/m3) = Wd

WS−Wix 1000 (1)

Apparent porosity (%) = (ws −wd)

(ws − wi)x100 (2)

74


Fig. 1 SEM and EDAX of raw RHA sample

Where,

𝑊𝑑=Weight of the specimens after drying for 24 hours

in a ventilated oven.

𝑊𝑠=Weight of the specimens in saturated surface dry

condition.

𝑊𝑖=Weight of the specimens suspended by a thin wire

inside water.

2.3.3 Water absorption and water sorptivity The procedure followed for determination of water

absorption of geopolymer specimens was in accordance to

ASTM C-642. 28 days old specimens were dried at 80°C

for 24 hours, weighed and kept immersed in water for 24

hours. The specimens were then removed from water, wiped

clean and immediately weighed in saturated surface dry

(SSD) condition to find increase in weight.

The Sorptivity test determines the rate of capillary rise

absorption by the geopolymer paste cube. The specimens

were initially painted with water proof enamel paint on all

sides except the bottom and top surfaces, so as to allow

capillary uptake of water only from bottom. The slope of

the linear portion of the curve between cumulative mass

gained per exposed surface area and square root of time

taken was reported as the sorptivity of the geopolymer

paste.

2.3.4 X-ray diffraction (XRD) analysis X-ray diffraction analysis was made using D8 Advance

(Bruker) XRD machine with Cu-Kα radiation with the

following conditions: 40 kV, 30 mA. Fragments collected

from the compressive strength tests were powdered and

used for XRD in the scan angle (2θ) range of 2ᵒ to 45ᵒ.

Scanning was performed in continuous mode with step size

of (2θ) of 0.02 and scan step time of 1 sec. The slow

Fig. 2 Compressive strength after 7 and 28 days

scanning rate was used to improve resolution of peaks. The

reflection positions and d-spacing were calculated by

automated programs.

2.3.5 Scanning electron microscope RHA raw materials as well as the geopolymer paste

were microscopically examined by FEI Quanta 250.

Quantification of the elements present in the geopolymer

paste were performed by Energy Dispersion X-Ray

Analysis (EDAX) at an accelerating voltage of 20 kV.

2.3.6 Fourier transform infrared spectroscopy FTIR experiments were performed on powdered

samples with a Perkin Elmer, Simultaneous Thermal

Analyser STA 8000 device. Spectra were recorded in the

range 4000-400 cm-1 with a resolution of 4 cm-1 and 16

scans per spectrum.

3. Results and discussion

3.1 Compressive strength

A similar trend of increasing compressive strength of

geopolymer paste were observed for SA1.5 and SA 2.0 for

both the samples tested after 7 days and 28 days as shown

in Fig 2. With respect to SA2.5 to SA3.0, a sharp decline in

compressive strength values were observed for the

corresponding test days. The percentage increase in

compressive strength between SA1.5 and SA2.0 were

12.64% for 7 days and 15.84% for 28 days. Linear

increment of compressive strength was observed for Si/Al

1.5-2.0. Similar results were also reported in earlier studies

by Sturm et al. (2016) and Hajimohammaddi et al. (2016).

For SA 2.5, SA3.0 and SA3.5, with Si/Al ratio higher than

2.0, there were sharp decrease in the compressive strength.

This observation may be due to the formation of lesser

crystalline phase in the geopolymer matrix as evident from

the XRD in Fig.5. The maximum compressive strength was

observed for SA2.0 with average value of 16.96 MPa. The

increment in the compressive strength may be attributed to

the formation of a strong geopolymer network with more

active role of released aluminium from sodium aluminate

75


Fig. 3 Bulk density and apparent porosity

and hence gaining strength in the later stages of

geopolymerisation.

It was observed that RHA geopolymer paste achieve

relatively lower strength for the samples tested after 7 days

as compared with those tested after 28 days. In all the

series, an average strength increase of approximately 15%

occur from 7 days to 28 days. Slow release of silica from

RHA and extended time taken for silica and alumina to

form a stable nuclei causes the initial low strength of the

geopolymer paste. Similar research outcomes were also

highlighted by Hajimohammaddi et al. (2016) and Jiminez

et al. (2006).

3.2 Bulk density and apparent porosity

The values of bulk density and apparent porosity for

geopolymer pastes are presented in Fig. 3. Bulk density

increases with increase in Si/Al ratio up to 2. Further

increase of Si/Al beyond 2 resulted in decrease of bulk

density for RHA geopolymer pastes. However, apparent

porosity exhibits a reverse trend. With increasing Si/Al

ratio, apparent porosity values dropped in paste specimens.

For SA1.5 specimens with Si/Al ratio of 1.5, the bulk

density and apparent porosity values were 1.18 g/cc and

36.71% respectively. When the Si/Al is increased to 2, the

corresponding bulk density was 1.37 g/cc, which showed a

marked increase. In addition, apparent porosity in this

specimen reduced to 32.71%, which is far lesser than that of

SA1.5 specimen. The increase in bulk density of

geopolymer composites with Si/Al=2 can be attributed to

better dissolution of rice husk ash and subsequent formation

of more geopolymer gel. Moreover, the same reason should

be the cause of decrease in apparent porosity with

increasing Si/Al up to 2. When Si/Al was increased beyond

2, the unreacted RHA and partially formed geopolymer gel

tends to decrease the bulk density and thereby increasing

the apparent porosity.

3.3 Water sorptivity and water absorption

Water absorption and sorptivity tests were conducted on

the geopolymer specimens after 28 days from casting. The

effect of Si/Al ratio on water absorption and sorptivity for

RHA geopolymer paste specimens are shown in Fig. 4.

SA2.0 samples shows minimum water absorption at 32.29%

Fig. 4 Water absorption of geopolymer specimen

while maximum value of 48.63% was recorded for SA3.5

samples. The variation of the water absorption values with

compressive strength follows a trend such that specimen

with maximum compressive strength (16.96 MPa) yields

least water absorption (32.29%). This also validates the

observation that higher compressive strength indicate

denser microstructure with lesser voids in the matrix.

Sorptivity of RHA geopolymer tends to decrease with

increasing Si/Al ratio in general. A clear decrease in

sorptivity was visible when Si/Al ratio increased from 1.5 to

2.0. However, sorptivity values showed increasing trend

beyond Si/Al of 2.0 though not remarkably significant. The

sharp decrease of sorptivity for SA2.0 specimen may be

attributed to its better gel formation and improved matrix

among the RHA geopolymer specimens. Sorptivity values

for RHA specimens were found to be 20.14×10-3, 18.83×10-

3, 18.84×10-3, 19.50×10-3, and 19.61×10-3g/mm2/min0.5 for

SA1.5, SA2.0, SA2.5, SA3.0 and SA3.5 respectively. The

lower sorptivity for SA2.0 specimen could enhance its

durability properties in acids and sulphates.

3.4 X-ray diffraction analysis (XRD)

Fig. 5 presents a combined XRD for the geopolymer

specimens for comparison. The XRD pattern of raw RHA

shows peaks corresponding to d- spacing values around

4.08Å (21.71°), 3.35 Å (26.51°), 3.03 Å (29.37°) and 2.03

Å (44.56°). Previous studies by Kordatos et al. (2008),

Shinohara et al. (2004) and Hajimohammaddi et al. (2016),

the peak with d spacing value of 3.35 Å (26.51°)

corresponds to quartz (PDF 01-070-7344) while d spacing

values of 4.08Å, 3.03Å were assigned for cristobalite (PDF

00-039-1425) and 2.03 Å were associated with gibbsite

(PDF 00-033-0018). The characteristic peak of quartz at

around 3.35 Å (26.51°) remains same in the in raw RHA

and in all the geopolymer specimen. For SA1.5 specimen,

new peaks appear at d- spacing values of 4.33 Å (20.48°),

3.67 Å (24.22°), 2.93Å (30.43°), 2.59Å (34.50°) and 2.12Å

(42.60°) indicating the reaction between raw RHA and

sodium aluminate leading to the formation of Zeolite X

(PDF 00-012-0246). This is further evident from the FTIR

analysis with the formation of new bond. SA2.0 specimen

with the highest 28 days compressive strength of 16.96 MPa

shows maximum number of new peaks as compared to

other geopolymer specimens. New peaks were observed at

76


Fig. 5 XRD diffractogram of RHA and geopolymers

specimen

4.83Å (18.35°), 4.36Å (20.33°), 3.67Å (24.22°), 2.61Å

(34.21°), 2.45Å (36.58°) and 2.38Å (37.74°) for SA2.0

specimens which corresponds to Zeolite X.

The optimal amount of silica for the geopolymerisation

might be the reason for SA2.0 specimen (Si/Al=2) resulting

in maximum number of new peaks and highest compressive

strength among the RHA geopolymer specimens. The

maximum compressive strength may be due to the increased

interaction between the zeolite and the amorphous phase

(Jaarsveld et al. 1998). As the ratio of Si/Al is increased

beyond 2, the crystalline phase might have surpassed the

tolerance limit of the matrix causing reduction of

compressive strength. This lead to drastically lowering of

the compressive strength from 16.96 MPa for SA2.0

specimen to only 8.60 MPa for SA2.5 as observed in Fig.2.

The difference in the d- spacing of new peaks and its peak

intensities in all the geopolymer specimen studied might be

due to the difference in the kinetics of the

geopolymerisation as reported by Hajimohammaddi et al.

(2016).

3.5 Scanning electron microscopy with EDAX

The physico-mechanical properties of geopolymer

Fig. 6 SEM micrographs of RHA specimens (a) SA1.5 (b) 2.0 (c) SA2.5 (d) SA3.0 (e) SA3.5

77


Table 4 Elemental weight composition % by EDAX

Specimen Si Al Na O Ca Mg P K Si/Al Na/Al

SA1.5 20.3 16.15 16.97 41.05 0.88 1.11 2.05 1.49 1.257 1.051

SA2.0 25.5 12.14 13.49 40.82 2.78 1.12 2.18 1.97 2.1 1.111

SA2.5 30.63 9.94 8.95 44.05 1.75 0.95 2.14 1.59 3.081 0.9

SA3.0 33.23 8.11 9.5 42.4 1.16 1.62 2.15 1.83 4.097 1.171

SA3.5 27.29 7.76 11.36 44.13 2.6 1.34 3.42 2.1 3.517 1.464

specimen would depend on the microstructure of the

specimen. The observed properties can be correlated with

the changes in the microstructure. Fig. 6(a) shows SEM

micrographs for SA1.5 where interconnected pores and

loosely packed geopolymer matrix abound. Significant

improvement in the microstructure were observed for SA

2.0 samples shown in Fig 6(b). RHA geopolymer specimens

SA2.5, SA 3.0 and SA3.5 (Si/Al ˃2) presents an unstable

matrix with comparatively larger pores and even unreacted

RHA were observed in the SEM micrographs of Fig. 6 (c),

(d) and (e). The comparatively better microstructural

homogeneity matrix for SA2.0 correlates to improved

physico- mechanical properties. It resulted in maximum

compressive strength and minimum porosity, water

absorption and sorptivity for SA2.0 among the specimens.

The relatively higher value of porosity, absorption and

sorptivity of RHA geopolymers as compared with other

geopolymer system may be attributed to the higher water

demand to make the paste workable. The larger surface area

of raw RHA increases the water requirement for preparation

of geopolymer to maintain the required level of reactivity.

Physico- mechanical parameters may be further improved

by adopting optimal water/binders ratio and also by making

mortar specimens with appropriate amount of fillers.

The elemental weight percentage composition of the

geopolymer specimens as determined by EDAX is tabulated

in Table 4. It was observed that the samples except SA 3.5

indicate Na/Al ~1, comparable with the original batch

composition. Si/Al ratio of SA2.0 was found nearly equal to

that of original batch composition. However, other

geopolymer specimens shows Si/Al ratio different from the

initially adopted values. As Si/Al ratio is increased from 2

to 3.5, the quantity of unreacted RHA in the specimen were

found to increase as seen in SEM micrographs in Fig. 6(d)

and (e). Hence, the elemental Si/Al tabulated in Table4 is

slightly higher than the actual value taken during

preparation. The higher quantity of unreacted silica in RHA

is reportedly due to the rapid release of aluminum from

sodium aluminate (Hajimohammaddi et al. 2010). The

unreacted RHA caused defects in density which increased

the potential failure planes of the specimens under

compressive loading. Similar findings were also reported by

Duxson et al. (2005), where higher Si/Al ratios leads to

negative effect on the mechanical strength of the specimen.

3.6 Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectra for original RHA and prepared

geopolymer specimens in the wavenumber region of 4000-

400 are shown in Fig. 7. Raw RHA shows broad peak at

1062 cm-1 and minor peaks at 2894 cm-1, 1685 cm-1, 793

Fig. 7 IR spectra of original RHA and geopolymer specimen

Fig. 8 Correlation between compressive strength and

apparent porosity

cm-1, 621 cm-1 and 562 cm-1. The strong intense peak at

1062 cm-1 is assigned to the Si-O-Si asymmetric vibration

(Yousuf et al. 2009). Minor peak at 2894 cm-1corresponds

to C-H stretching bands as the raw rice husk were burned at

temperature of around 700°C to get RHA (Sharma et al.

2010). Peak at 1685 cm-1 corresponds to the deformation

vibration of chemically bonded water molecules due to the

presence of Gypsum (Suyanta et al. 2011). The band at 793

cm-1 was assigned for Si-O-Si symmetric stretching

vibration. A small band at 621 cm-1 was indicative of the

presence of cristobalite. The presence of cristobalite and

gypsum in the raw RHA was also inferred from XRD in

Fig. 5. Peak at 562 cm-1 are mainly due to stretching and

bending vibration of Si-O-Si and O-Si-O (Mozgawa et al.

2011).

All the RHA geopolymer specimens have peaks at

around 562 cm-1 confirming the formation of Zeolite X with

the stretching and bending vibration of Si-O-Si and O-Si-O

(Naskar et al. 2011). Si-O symmetric stretching vibrations

were observed in the range of 1005-1021 cm-1. As Si/Al

ratio is increased from 1.5 to 3.5, the Si-O peaks were

observed to have shifted to higher wavenumber. The band at

793 cm-1 and 621 cm-1 were characteristics peaks for Si-O-

Al bending vibration in tetrahedral and 6-coordinated Al

(Jakobsoon et al. 2002). The disappearance of these bands

would further authenticate the continuous replacement of Al

for Si in the framework.

78


Fig. 9 Correlation between compressive strength and water

absorption

3.7 Correlation of physical properties with compressive strength

A correlation between compressive strength and

physical parameters (apparent porosity and water

absorption) was studied and highlighted in Fig. 8 and Fig. 9

respectively. Both the parameters exhibit a decreasing trend

with increase in compressive strength. Polynomial

regression analysis were performed to establish the

correlation of the parameters with compressive strength.

The good correlation values of R2=97.84% and R2=97.42%

indicate that denser microstructure with lower voids in the

matrix leads to higher compressive strength and lower

porosity and water absorption among the specimens.

However, besides porosity, the permeability properties

of the specimens may depend on other factors like

tortuosity, specific surface, pore size distribution and

connectivity of pores (Lafhaj et al. 2006).

4. Conclusions

Through a series of tests, RHA geopolymer paste

activated with sodium aluminate were investigated. The

effects of microstructure of the geopolymers on the physico

mechanical properties were studied. Based on the present

study the following conclusions were highlighted:

1. Sodium Aluminate can be used as a viable source of

aluminum for the establishment of a strong geopolymer

network in the traditional two- system geopolymer. The

fast release rate of Al from sodium aluminate provides a

rapid formation of better homogenous geopolymer gel.

2. The porosity, water absorption and sorptivity of RHA

geopolymers were relatively higher than those of other

geopolymer system. This may be attributed to the higher

water demand to make the paste workable. The larger

surface area of raw RHA increases the water

requirement to maintain the required level of reactivity.

The physico- mechanical parameters can be further

improved by adopting optimal water/binders ratio.

3. XRD analysis indicates quartz, cristobalite and

gibbsite for raw RHA and new peaks corresponding to

Zeolite X in geopolymer specimens. The maximum

compressive strength for SA2 specimen may be due to

the increased interaction between the zeolite and the

amorphous phase as is evident from the formation of

many new peaks. As the ratio of Si/Al is increased

beyond 2, the crystalline phase of the geopolymer

matrix decreases and hence reduces the compressive

strength.

4. SEM micrographs shows interconnected pores and

loosely packed geopolymer matrix for specimen SA1.5,

SA2.5, SA3.0 and SA3.5. Comparatively large pores,

unstable matrix and even unreacted RHA were seen in

the SEM micrographs of SA3.0 and SA3.5. Better

microstructural homogeneity for SA2.0 correlates to

improved physico- mechanical parameters.

5. FTIR spectra for original RHA shows peaks

corresponding to wavenumber of Si-O-Si asymmetric

vibration, C-H stretching band, Si-O-Si symmetric

stretching and vibration validating the presence of

quartz, cristobalite and gibbsite. Formation of Zeolite X

with the stretching and bending vibration of Si-O-Si and

O-Si-O were also observed from the FTIR analysis of

geopolymer specimen.

6. Polynomial regression analysis between compressive

strength and physical parameters namely porosity and

water absorption established equations with high

correlation values of R2=97.84% and R2=97.42%

respectively.

7. The comparatively low compressive strength might

restrict RHA geopolymer from various practical

applications. However, the deficiency in strength could

be overcome by blending with other source materials.

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