ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT
AZOJETE, September 2019. Vol. 15(3) 598-610

Published by the Faculty of Engineering, University of Maiduguri, Maiduguri, Nigeria.
Print ISSN: 1596-2490, Electronic ISSN: 2545-5818

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598

ORIGINAL RESEARCH ARTICLE

MECHANICAL AND MICROSTRUCTURAL CHARACTERIZATION OF ALKALI-ACTIVATED
COCONUT SHELL ASH MORTAR
K. A. Olonade1* and T. Bello2

(1Department of Civil & Environmental Engineering, University of Lagos, Akoka, Nigeria.
2Nigerian Building and Road Research Institute, Building Research Department, Ota, Nigeria)

* Corresponding author’s email address: kolonade@unilag.edu.ng

ARTICLE INFORMATION

Submitted 7 April, 2018
Revised 12 June, 2019
Accepted 22 June, 2019

Keywords:
Alkali-Activation
coconut shell ash
Binder
Compressive
flexural strengths

ABSTRACT
In this paper, potential of alkaline activation of coconut shell ash (CSA)
as binder in mortar was investigated. Coconut shells were collected and
calcined at temperature ranges between 500 and 900°C at interval of
100 for 30, 60 and 90 minutes to determine optimum burning
condition that produces ash of higher silica content. Chemical
composition of the ashes was then determined using X-ray Florescence
(XRF) technique. Sodium hydroxide (NaOH) of different concentrations
of 10, 12, 14 and 16 Molar (M) was used as alkaline activator, while
mortar of mix ratio 1:3 was produced, while ratio of the alkali to CSA
was of 0.5. Mortar prisms of size 40 mm × 40 mm × 160 mm were cast
and cured in an oven at a temperature of 80 ℃ for 7, 28, 56, 90 and 120
days. Thereafter, flexural and compressive strengths were determined
at the end of each curing ages, following standard procedures.
Scanning Electron micrographs of the hydrated mortar at 120 days
were obtained using scanning electron microscopy (SEM) The results
showed the oxide composition did not follow any pattern with increase
in temperature but CSA produced at 800℃ for 1h gave the highest
combined silica, alumina and ferric oxides above 70%, which is
accepted as minimum value for pozzolanic materials. Both
compressive and flexural strengths of the activated mortar samples
were found to increase with increase in activator concentration up to
14M and decline thereafter. At 120 days, the compressive strength was
13.9 N/mm2 while the flexural strength was 6.88 N/mm2. These values
were higher than the strengths recommended by Nigerian Industrial
Standard (NIS) for load bearing blocks. It was concluded that activation
of CSA with NaOH could be used as binder for non-structural use. The
SEM results indicated that mortar made from 14 M had structure that
was densely packed compared to other mortar samples produced from
other concentrations

© 2019 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved.

1.0 Introduction
The production of modern cement (ordinary Portland cement) consumes huge energy and has
been implicated as the major contributor to greenhouse gas emission next to the automobile
industry. It is estimated that the cement industry contributes between 6 and 10% of the total
annual CO2 emitted (Metz, 2007, and Carstensen and Rapf, 2008); while about 5.5 million BTU of
energy is required to produce just one tonne of cement (Naik, 2008).

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Unfortunately, the demand for ordinary Portland cement (OPC) is ever increasing, especially in
developing countries, like Nigeria, where there is thirst for infrastructure development.
Oluwakiyesi (2011) reported that Nigeria has an estimated deficit of 16 to 18 million housing
units and that in 2009, the Presidential Committee on Implementation of Affordable Housing
estimated about N60 trillion to finance the deficit. This would require about 112 million metric
tonnes (mmt) of cement. Globally, the demand for cement in the year 2015 was 4.6 Gt/a
(CEMBUREAU, 2015) and could reach 13.5 Gt/a by 2050 (Edwards, 2016). If this trend continues
unabated, then the yearning for clean and sustainable environment would be a mirage in the
face of meeting infrastructure deficit.
In responding to this challenge, several efforts are being made to curtail the amount of OPC
consumed locally for global objective. One of such emerging and attractive approaches is alkali-
activation of waste materials. Large amount of industrial and agricultural waste materials is
produced, effective disposal of which remains a challenge with adverse effect on the ecosystem
at large. The volume of these wastes generated in the world is increasing over the years due to
increase in population, socio-economic activities and social development.
Alkali activation as defined by Martinez and Palomo (2001) is a chemical process where
amorphous structure is transformed into a skeletal structure that exhibits cementitious
properties. Once a material contains reactive silica or alumina, it could be considered for
activation using appropriate alkali solution (equations 1a, 1b and 1c). Several materials and alkali
have been studied with positive results.

Ca + 2 OH + SiO2 → C− S − H Gel as in OPC (1a)
Na + 2 OH + SiO2 → N− S − H Gel as in geopolymer (1b)
Na + 2 OH + Al3O2 → N− A− H Gel as in geopolymer (1c)

Where -: C − S − H and N− S − H (Gel) are Calcium-Silicate-Hydrate and Sodium-Silicate-
Hydrate Gels, respectively.
Bakharev (2006) produced concrete using alkali activated fly ash with NaOH and achieved a 2-
day compressive strength of 10 N/mm2 while the 28-day compressive strength was 60 N/mm2.
The Author also used sodium silicate (Na2SiO3) to chemically activate fly ash to achieve a 2-day
compressive strength of 2 N/mm2 and a 28-day compressive strength of 45 N/mm2. Similarly,
Morenoa et al. (2008) also activated class F fly ash with NaOH from different sources and
achieved a 28-day compressive strength of between 29 and 66 N/mm2. Another study by Sofia
et al. (2007) used class F fly ash with one or more of the following component; Sodium
Carbonate (Na2CO3), Sodium Silicate (Na2SiO3) and Sodium Hydroxide (NaOH), to achieve 7-day
compressive strengths in the range of 35 to 44 N/mm2 and a 28-day compressive strength in the
range of 47 to 57 N/mm2.
Rajesh et al. (2013) studied mechanical properties of alkali activated slag with different alkalis.
They reported that a significant compressive strength was achieved especially with sodium
silicate and sodium hydroxide solutions, which has one-day strength of 12.46 N/mm2 and 12.04
N/mm2 respectively compared with 9.5 N/mm2 for the control mix and 5.73 N/mm2 for the
Sodium Carbonate mix. It was concluded that NaOH was alkali for activating slag for higher
strength. Qureshi and Somnath (2013) used potassium oxide (K2O) and Sodium oxide (Na2O) to
activate blast furnace slag in order to investigate the relationship between alkali content and
compressive strength of concrete. Durability of alkali activated cement-based products was
equally studied (Bernal et al., 2011 and Atahan and Dikme 2011). Agricultural wastes are equally
being used, though sparsely. Palm Oil Boiler Ash (POFA) with sodium hydroxide and sodium-
silicate, using different concentrations of sodium hydroxide alkaline activator was studied (Zarina

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et. al., 2015). Based on authors’ knowledge, there is no reported work on the alkali activation of
coconut shell ash as binder; hence, this study was carried out to fill the research gap.
Coconut is a fruit from the coconut palm tree. It is grown in large quantity according to Food
and Agriculture Organization. About 0.3 mmt of coconut was grown in 2014 as against about 0.1
mmt and 0.12 mmt produced in the 60’s and 80’s respectively (Figure 1). It is equally widely
grown in Nigeria especially in the riverine areas of the country. During processing of coconut for
oil production, large quantity of coconut shells is produced, disposal of which remains a
challenge. Most often than not, they are piled up and burnt to give room for new set of coconut
shells produced. This study therefore investigated suitability of coconut shell ash (CSA) as
potential precursor for alkali activation.

Figure 1: Coconut production quantity in tons between 1961 and 2013 (FAOSTAT, 2016)

2.0 Materials and Methods
2.1 Materials
Coconut shells were obtained from coconut shell dumpsite in Badagry area of Lagos State,
Nigeria (Figure 2). River sand of maximum nominal particle size of 3.18 mm was used as the fine
aggregate. The sand was washed to remove dirt and deleterious substances. Specific gravity of
the sand was determined as well as sieve analysis was conducted on the sand to determine its
particle size distribution following the procedures specified in BS EN 1097-3 (1998). Sodium
hydroxide (NaOH) was used as alkali activator, while ordinary Portland cement (OPC) of grade
42.5 was used as binder for control sample.

Figure 2: A typical coconut shell dumpsite at Badagry, Nigeria

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2.2 Preparation of specimens
2.2.1 Coconut shell ash
The shells were thoroughly washed in water to remove all the organic matters that stuck to them
(Figures 3a and 3b). Thereafter, they were dried under the sun to minimize moisture and were
crushed into smaller sizes manually with the aid of harmer to facilitate burning.
The crushed shells were burnt in a furnace at different temperature ranging between 500 ℃ and
900 ℃ at an incremental interval of 100 ℃ for 30, 60 and 90 min, respectively. A total of 15
samples of the ash were prepared. For instance, Sample A1 represents CSA produced by burning
at 500 ℃ for 30 min (Table 1).

(a) (b)
Figure 3: Crushed coconut shell (a) before cleaning and (b) after cleaning

Table 1: Samples of ash produced at different temperatures and duration
Sample Temperature (℃) Duration (Min)
A1 500 30.00
A2 500 60.00
A3 500 90.00
B1 600 30.00
B2 600 60.00
B3 600 90.00
C1 700 30.00
C2 700 60.00
C3 700 90.00
D1 800 30.00
D2 800 60.00
D3 800 90.00
E1 900 30.00
E2 900 60.00
E3 900 90.00

2.2.2 Sodium hydroxide solution
Sodium Hydroxide (NaOH) in pellet form was purchased from Purechem Manufacturing
Company in Lagos and was used to prepare the various concentrations for the experiment. The
different concentrations prepared for this study were 10, 12, 14 and 16M. The choice of
concentrations was based on findings from previous studies (Ammar et al., 2013 and Qureshi
and Ghosh, 2013). Table 2 summarized the quantity used in preparing the NaOH solution.

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2.2.3 Alkali activated coconut shell ash mortar
The alkali activated coconut shell ash mortar (AACSAM) was prepared by mixing a measured
amount of coconut shell ash (CSA) with the alkali activator (NaOH-solution) in a ratio of 1:2
(alkali: CSA) by weight to produce an alkali activated coconut shell ash paste (binder). The
resulting binder was in turn mixed with fine aggregate to finally produce an AACSAM in a
binder/sand ratio of 0.5 by weight.

Table 2: Quantity of NaOH Per Liter in each Concentration
Molarity (M) of solution Amount of NaOH solid (g) Quantity of water (L)
10 400 1
12 480 1
14 560 1
16 640 1

The mortar produced was cast in moulds of sizes 40 × 40 × 160mm (ASTM C78, 2018). A total of
60 short mortar beams were cast and the moulds were removed after 24 hours (Figure 4a).
Thereafter, they were cured in oven at temperature of 80 ℃ for 7, 28, 56, 90 and 120 days (Figure
4b). The choice of temperature and mixing proportions were guided by principles found in the
literature that the sample be cured under a temperature of 80 ℃ till the period of testing
(Andrea and Herald, 2004).

(a) (b)
Figure 4: Alkali Activated Coconut Shell Ash Mortar (a) in moulds (b) stack in oven for curing

2.3 Testing procedures
2.3.1 Determination of chemical composition of CSA
Chemical composition of the 15 ash samples were determined individually, using the X-Ray
Fluorescence (XRF) technique. Weltje and Tjallingii (2008) give the detail procedure of XRF
technique. The test was conducted at Lafarge Cement PLC, Ewekoro plant. Statistical Package for
the Social Sciences (SPSS) software was used to conduct analysis of variance (ANOVA) on the
data to study the effect of the activation conditions (temperature and time) on the chemical
composition of CSA. The temperature and time that gave high amorphous silica content was
then used to produce the ash used for the activation.

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2.3.2 Determination of Compressive and Flexural Strengths of AACSAM
At the expiration of each curing period, the prism was weighed and the flexural strength of the
mortar prism was first determined. The experimental set-up for determining the flexural force is
shown in Figure 5. The amount of force that broke the sample into two halves was recorded as
flexural force. Thereafter, flexural strength was calculated using Equation 2. Each halves of the
prism from flexural testing was used as samples to determine the compressive strength. This is in
conformity with the provisions of BS EN 196 -1 (2015).

Figure 5: Experimental set-up for flexural testing

fs =
1.5Fl
b3

(2)

where:
fs = flexural strength, N/mm2
F = flexural force, N
b = side of prism square section, mm
l = span between supports, mm

3.0 Results and Discussion
3.1 Chemical properties of CSA at various burning conditions
Table 3 presents the chemical composition of the produced CSA at different temperature and
duration periods. It was observed that as the time of burning increased, the percentage oxides of
CSA varied, suggesting that duration of burning had effect on the chemical composition of CSA
produced. For instance, the silica (SiO2) content of the ash obtained at 500 ℃ for 30 minutes was
about 75% and 65% of what was obtained when the time was increased to 60 and 90 minutes,
respectively. The same trend (increased in SiO2 content as time increased), though in different
proportions, was observed for other temperatures from 600 ℃ to 900℃ except at 800 ℃ .
Looking at the effect of burning temperature, the results indicated that temperature had
significant impact in determining the amount of oxides obtained. For instance, Coconut shell
ash burnt for 30 min.. at 600 ℃ produced alumina content to be 5.4%, while it was 9.24% at 800
℃ for the same duration. The thrust of these variations implied that there was individual and
combined effect of temperature and duration on the chemical composition of CSA. Similar
observation was reported in the literature, but for cassava peel ash and rice husk ash (Salau and
Olonade, 2011 and Ramezanianpour et al., 2009).

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Table 3: Effect of Calcining Temperature and Duration on Chemical Composition of CSA
Temp. Time Oxides (%) LOI
(℃) (min) SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O

500
30 32.36 5.24 1.66 3.69 0.49 0.14 2.26 1.71 41.42
60 43.12 7.05 2.07 4.65 0.66 0.08 2.85 2.27 33.25
90 49.73 8.08 2.53 5.46 0.78 0.25 3.17 2.6 25.76

600
30 32.51 5.40 1.58 3.8 0.51 0.46 2.02 1.37 29.87
60 36.00 5.76 1.71 3.84 0.55 0.06 2.3 1.93 43.33
90 45.01 7.25 2.25 4.82 0.15 0.15 2.83 2.26 32.74

700
30 44.2 7.38 2.47 4.95 0.72 0.17 2.94 2.41 31.06
60 28.01 4.59 1.24 3.28 0.43 0.12 1.92 1.5 47.43
90 30.35 4.97 1.56 3.43 0.48 0.04 1.89 1.54 51.38

800
30 58.86 9.24 3.20 6.29 1.03 0.46 3.58 4.11 9.73
60 58.76 9.18 3.15 6.35 1.05 0.40 3.50 4.00 9.54
90 43.38 7.03 1.95 4.72 0.68 0.11 2.53 2.07 29.94

900
30 45.24 7.28 2.27 4.78 0.69 0.09 2.42 2.06 27.61
60 36.14 5.97 2.19 4.28 0.6 1.01 2.12 1.68 22.96
90 49.20 7.83 2.86 5.07 0.76 0.37 2.44 2.13 26.44

OPC 19.99 5.49 3.00 63.32 1.45 0.34 0.08 0.09 3.96

Nevertheless, the focus was to obtain CSA that contained reactive and higher content of
combined SiO2, Al2O3 and Fe2O3 that was above 70% as recommended by ASTM C618 (2006).
The results further suggested that burning coconut shells at 800℃ for 60 minutes could be the
appropriate treatment conditions to obtain CSA that would be an acceptable precursor for alkali
activation because the combined silica, alumina and ferric oxide was more than 70% minimum
recommended (Figure 4).

Figure 4: Combined silica, alumina and ferric at different temperatures and times
The summary of the multivariate tests conducted on the data presented in Table 3 is presented
in Table 4. It is shown that there is a statistically significant effect of temperature on the
combined silica, alumina and ferric oxide content of CSA (p < 0.05) , while duration of burning
did not have no significant effect statistically (p > 0.05)

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Table 4: ANOVA on the effect of Temperature and Time of oxide composition of CSA
Factor SS df MS F p-value F-crit
Temperature 108501.3 14 7750.094 1.072226 0.420296 2.063541
Time 4153178 2 2076589 287.2961 2.19E-19 3.340386
Error 202385.2 28 7228.044
Total 4464064 44 4464064

3.2 Physical and particle size distribution of Aggregate
Figure 2 shows the particle-size distribution curve of the sand. While less than 5% of the particle
size passed through sieve size 150 μm, about 98% of its particle size passed through sieve size
2.36 mm indicating aggregate fell within the class of fine aggregate (BS EN 1097-3, 1998). The
sand could be categorized as normal weight aggregate because its specific gravity was 2.67,
which is within the range of 2.43 – 2.75 for normal weight aggregate (BS EN 1097-3, 1998).

Figure 2: Particle size distribution curve of the fine aggregate

3.3 Strength characteristics of alkali activated coconut shell ash mortar
3.3.1 Compressive strength
Compressive strength of the AACSA mortar varied with concentrations of NaOH solution (Figure
5). At early age of 7 days, CSA activated with 16M of NaOH produced the highest strength (4.03
N/mm2), which was about 102%, 116% and 125% of that of 10, 12 and 14M concentrations,
respectively. However, at age 28 days, alkali activated mortar produced with 14M concentration
had gain more strength, which was about double its strength at 7 days (9.9 N/mm2). Other
mortar produced with 10, 12 and 16M gained marginal strength over their 7-day strength by just
4, 18 and 2%, respectively. After 28 days, the 14 M mortar sample maintained the lead up to 120
days followed by 12M mortar sample, while 16M mortar sample had the least strength. But,
comparing the 28-day strength of 14M mortar sample with that of normal cement mortar, 14M
mortar had strength of about 25% of that of the normal cement mortar. These results are in
agreement with Smith’s (1999) findings which showed
Nevertheless, the rate of strength development in AACSAM was low compared to normal mortar.
It was observed that normal mortar had reached about 80% of its 120 day-strength at 28 days

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compared to what 14 M mortar reached at the same period, which was about 37%. The
implication of this is that AACSAM will continue to gain more strength faster than normal mortar
as shown by their strength curves (Figure 5). It is known that pozzolanic reaction is a slow
process compared to hydration in OPC (Taylor, 2009). These results are comparable to what
Ammar et al. (2013) reported in their study with different materials.

Figure 5: Compressive strength of AACSAM and control
Despite the relative low strength obtained for alkali-activated coconut shell ash mortar
(AACSAM), the 28-strength of 14M mortar was still higher than the stipulated minimum strength
of 3N/mm2 for load bearing sandcrete blocks as prescribed by the Nigerian Industrial Standard,
(2004). Thus, the AACSAM could be considered for use in non-structural element. In future study,
use of other alkalis or combination of other alkali with NaOH would be investigated to improve
the strength of AACSAM.

3.2.2 Flexural strength
Figure 6 shows the flexural strength values of both the AACSAM and the normal cement mortar
(control). The strength pattern was similar to that of compressive strength. The strength
increased with age for all the samples but at different rate. While 16M mortar had highest
flexural strength (1.76 N/mm2) at age 7 days, 12M mortar had least strength (1.2 N/mm2).
Strength of 10 and 14M mortar were 91 and 86% of the strength of the 16M mortar, respectively.
As the age increased beyond 7 days, the strength pattern changed with 14M mortar sample
gaining strength steadily from 2.37 N/mm2 at 28 days up to 5.88 N/mm2 at 120 days. Other
mortar samples followed the same trend but with relative lower strength. Surprisingly, 16M
mortar sample that had initial high strength turned to give lowest after 28 days. The likely reason
for this observation is that the concentration of the alkali speed up the initial reaction for the
formation of geopolymer product that is responsible for increase in strength (Pacheco-Torgal, et.
al., 2008). Nevertheless, there was possibility of concentrated NaOH to be absorb moisture
(hygroscopic) from the atmosphere, which may lead to weaker zone in the matrix or increase the
moisture content of the matrix, leading to lower strength. These results agree with what Hashim
et al. (2015) reported about alkali activated slag.

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Figure 6: Flexural strength of AACSAM and control

4.3.3 Microstructure of alkali activated coconut shell ash mortar
The SEM images of CSA alkali activated mortar at ages 28 with different concentrations of NaOH
are illustrated in Figure 7. At 28 days of hydration, when NaOH concentration was 10M, the
microstructure was characterized by anhydrous CSA grains as most of the particles had not
reacted with the NaOH (Figure 7a). Subsequently, the matrix was more porous and thus with
little strength development. As the concentration of NaOH increased, some hydrated
compounds started forming, which were more pronounced at 14M concentration than found in
12M that had some traces of incomplete hydration (Figures 7b and 7c). Furthermore, at higher
concentration of 16M, apart from the fact that some CSA grains were hydrated, there were traces
of anhydrous CSA with some dark portions and fragile zones (Figure 7d), which is suspected to
be linked to hygroscopicity of high concentration of NaOH.

Figure 8 on the other hand shows microstructure of activated mortar samples cured for 120 days.
It is seen that more hydrated compounds were formed even at 10M concentration (Figure 8a)
due to longer period of hydration (120 days) as compared to 28 days (Figure 7a), only that
hydrated compound formed was not completed. The increase in hydrated products caused the
sample to be more compacted with relative low pores, leading to higher strength. The most
striking observation from these microstructures is that of 14M mortar samples that that no
traces of CSA grains. It seemed that all the CSA had been hydrated forming well-compacted
matrix, which suspected to be responsible for the highest strength observed (Figure 8c).
According to Türker et al., 2016, a complete alkali activation formed additional hydrated
compound characterized by a glassy shaped similar to calcium hydrate silicate found in hydrated
Portland cement; thus improve the strength properties.

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Figure 7: SEM images of AACSAM at 28 days of hydration with different NaOH concentrations.
ANA: Anhydrous NaOH, ACSA: Anhydrous CSA, HCSA: Hydrated CSA; IHCSA: Incomplete
Hydrated CSA

Figure 4.10: SEM images of AACSAM at 120 days of hydration with different NaOH
concentrations. HCSA: Hydrated CSA; IHCSA: Incomplete Hydrated CSA.
The hydrated products formed because of alkali activation of CSA could be amorphous alkaline
aluminosilicate with a 3D structure similar to a zeolitic gel type. The hydrated products caused
densification of the hydrated matrix due to increase in volume and filling effect of the pores
within the matrix. This action has been attributed to increase in the strength properties of
cementitious matrix (Yuya et. al, 2015). Thus, it could be safely concluded that more hydrated
products formed, as a result of reaction between CSA grains and 14M NaOH is responsible for
the increase in compressive and flexural strength obtained (Figures 5 and 6).

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4. Conclusion
In this study, characterization of coconut shell ash was performed and mortar samples were
prepared with different concentrations of NaOH. Strength of the mortar samples was equally
determined. From the study the following conclusions were drawn:
Coconut shell ash produced at 800 ℃ for 60 minutes possessed highest content of combined
silica, alumina and ferric for alkali activation.
Alkali activator concentration has influence on the strength development of AACSAM, as well as
its microstructure.
Sodium hydroxide of 14M concentration produced highest compressive strength, higher than
what was recommended for non-structural use.
There were indications that CSA could be further explored using different optimization
approaches to produce higher strength for structural application, like normal cement mortar.

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