MICROSCOPIC AND CHEMICAL CHARACTERIZATION OF ELEPHANT GRASS AND CORN LEAVES AND THEIR ASHES

Josefa Roselló 1,a, Lourdes Soriano2,b, Holmer Savastano Jr 3,c, María V. Borrachero 2,d,

Pilar Santamarina 1,e, Jorge L. Akasaki 4,f, Jordi Payá 2,g*

1 Departamento de Ecosistemas Agroforestales, Universitat Politècnica de Valéncia,

Valencia, Spain

2 Instituto de Ciencia y Tecnología del Hormigón (ICITECH), Universitat Politècnica de

València, Valencia, Spain

3 Departamento de Engenharia de Biossistemas, Faculdade de Zootecnia e Engenharia de

Alimentos, Universidade de São Paulo, Pirassununga, SP, Brazil

4Departamento de Engenharia Civil, Universidade Estadual Paulista UNESP Campus de

Ilha Solteira, SP, Brasil

ajrosello@upvnet.upv.es, blousomar@upvnet.upv.es, cholmersj@usp.br,

dvborrachero@cst.upv.es, empsantam@eaf.upv.es, fakasaki@dec.feis.unesp.br,

gjjpaya@cst.upv.es

Keywords: corn, elephant grass, leaves, silica, ash, phytolith Abstract. Many agrowastes are being used for energy production by combustion in thermal power

plants. This process generates huge amounts of ashes, which have a potential pozzolanic activity for

blending with Portland cement or hydrated lime. Elephant grass (Pennisetum purpureum) and corn

straw (Zea mays) are two types of biomass whose ashes could be valorized in construction

materials. The aim of the present research was to analyze the chemical composition of leaves and

the corresponding ashes in terms of inorganic elements characterization (silicon, potassium,

calcium, sodium, chloride,..). Microscopic studies (FESEM) on the dried leaves and the

characterization of the ashes obtained at different calcination temperatures have been carried out.

Energy dispersive X-ray spectroscopy (EDX) has been applied for quantifying the percentage of the

main chemical elements. Elephant grass ashes (EGA) maintained the spodogram after calcinations

at 450 and 650ºC. Silica (SiO2) was the main oxide in EGA (55.7%); also CaO, MgO and K2O were

in high percentage and chloride content was very low. In corn leaf ashes (CLA) the spodogram was

maintained at 350 and 450ºC. Silica was also the main oxide (62.6%). The amount of K2O was also

high (26.8%) and chloride content was significant (2.4%) which must taken into account for the use

of CLA in concrete. Both ashes could be interesting for OPC-based blended cements because their

potential pozzolanic properties.

Introduction

Supplementary cementing materials (SCMs) in the production of cements and concretes are

widely studied by many researchers [1]. The key of their use is due, in many cases, to the presence

of amorphous silica and alumina in the compositions of SCMs. These SCMs are usually obtained as

byproducts or wastes from different human activities: industrial, agricultural, energetic and

decontamination processes. Silica (SiO2) and alumina (Al2O3) amorphous phases are usually

present and they reacts towards hydrated lime Ca(OH)2 by means of the pozzolanic reaction: this is

the reason because this type of SCMs are called pozzolans. This pozzolanic reaction yields

hydraulic cementing products, similar to those found in the hydration of ordinary Portland cement

(OPC). The use of these SCMs in concrete lets to reduce the OPC consumption and additionally

avoid the disposition of wastes in landfills. From the technical point of view, the use of these

pozzolans in OPC-based binders has advantages such as lower heat of hydration, higher

compressive strength and lower permeability/porosity of the capillary network. These advantages

makes that these pozzolanic materials could play an important role in the performance of concrete,

specifically in its durability.

Global warming, attributed to the CO2 increase in the atmosphere, is becoming the more relevant

worldwide issue [2]. Portland cement fabrication is responsible of 5% of CO2 worldwide emissions

and consequently, another advantage in the use of SCMs from wastes is considered in

environmental concerns: the reduction of the greenhouse gaseous (GHG) emissions. Agricultural

wastes can be converted into interesting power sources by combustion. This process converts

biomass in water vapor, carbon dioxide and ashes, among others. CO2 emission has not net

environmental impact because is not from fossil origin, this carbon released during combustion was

previously captured by plants in a recent period.

One of the products produced during combustion is the inorganic residue, the ash. This ash must

be collected from the boiler (bottom ash) or from the mechanical/electrostatic precipitators (fly ash).

In both cases, the chemical nature of the ash could be interesting for blending with OPC. In many

cases, ashes are rich in amorphous silica, which is a main reagent for producing the pozzolanic

reaction.

Different types of biomass ashes [3, 4] are being studied for their reusing in OPC binders. The

most well-known ash is that produced by combustion of rice husks. Rice Husk Ash (RHA) has been

extensively analyzed when mixed with OPC. Also Sugar Cane Bagasse Ash (SCBA) and Palm Oil

Fuel Ash (POFA) have been tested in the last decade. However, there is an increasing interest on

assessing the behavior of new ashes from biomass [5]: wood, sugarcane straw, bamboo leaves,

coffee husks and cashew nut rinds [6], among others. Few studies have been also carried out by

using ashes from maize (corn) and elephant grass plants.

Maize (Zea mais) is grown throughout the world, and it is one of the most important cereals

cultivated both for human and animal consumption (for grain and forage) and in the last years for

bioethanol production. More than 875 million tons of grains were worldwide produced in 2012 [7].

Corn cob is a waste product obtained from maize and it can be used as biomass for combustion.

Corn Cob Ashes (CCA) have been previously studied [8-10]. Adesanya and Raheem [8] used CCA

by blending with OPC (in 0-25% by mass of CCA). SiO2 percentage ranged in 65-68%, and Al2O3

ranged in 6-9%. This means that the sum of both oxides reached 70%, minimum required according

to ASTM C-618 for pozzolans. Replacement of OPC by CCA produces an increasing in the initial

and final setting times of pastes. Compressive strength values at early curing ages for mortars

containing CCA were lower than those found for control mortar [9]; however, an important strength

gain was observed for longer curing period (120 days), and 8% replacement of OPC by CCA was

the optimum for yielding the best strength development. Corn Stover Ashes (CSA; from leaves and

canes) were also used for OPC blending [10]: the produced ashes contained high proportion of K2O

(25.41%) and SiO2 (28.42%) and showed low pozzolanic reactivity. When these ashes were washed

with hydrochloric acid, the potassium content strongly diminished (4% of K2O) and their reactivity

was significantly enhanced, yielding higher compressive strength than control mortar.

Elephant grass (Pennisetum purpureum) is cultivated for feeding ruminants. It grows rapidly (40

ton/ha/year) and is considered as an efficient carbon dioxide sink. Also, it can be used in

thermoelectric power plants as biomass source. In this process, huge amount of elephant Grass

Ashes (EGA) is produced. The chemical composition [11] of these ashes showed the high

percentage of silica (56-68%) and alumina (22-23%). K2O content was also important, although the

percentage depends on the biomass treatment prior to burning (7.4% without treatment, 3.5% with

hot water, and 2.0% with hydrochloric acid). EGA presented similar pozzolanic activity to other

biomass ashes, such as SCBA and RHA. This reactivity let to prepare 20% OPC replaced mortars

by using these EGA, maintaining compressive strength after 28 days of curing. In other study [12],

it was found that different cultivars yielded different ashes: Napier variety produced higher rich

silica ashes than Cameroon variety (80 vs 50% in SiO2). Both ashes obtained at 700ºC showed high

pozzolanic activity in EGA/Ca(OH)2 systems: more than 80% of calcium hydroxide was fixed in

1:1 mixtures after 7 days of curing at 22ºC.

Plants absorb silicon from the soil as silicic acid and also deposited as amorphous hydrated silica

or opal (SiO2.nH2O) in the cell wall, inside the cells (intracellular spaces or cell lumen) and in the

intercellular spaces of different plant organs such as leaves, pods, stems, inflorescences (i.e.

herringbones) and epidermal appendages (trichomes) [13]. Different siliceous structures have been

found in plants. There are special siliceous cells that are called phytoliths or silico-phytoliths [14].

The aim of this research is to study in depth, from the microscopic point of view, the ashes

obtained from Elephant grass leaves and corn (maize) leaves, in order to analyse the silica content

and the presence of other compounds, and also the presence of silica bodies (phytoliths). Both

leaves could be used as biomass for power plants and the corresponding ashes could be potential

pozzolanic materials for blending with OPC.

Experimental section

Leaf samples of elephant grass (Pennisetum purpureum, EGL) were collected in Pirassununga

(São Paulo, Brazil); and corn leaves (Zea mais, CL) were collected in Ilha Solteira (São Paulo,

Brazil). To correctly identify structures rich in silica (phytoliths) and their location within the plant,

studies on both collected material (dried) and ash resulting from combustion in a muffle furnace at

selected temperatures were performed. Leaves were previously washed with deionized water.

Preparation of samples: a) Dried material studies: small samples of leaves were dried at 105°C

for 24 h in a laboratory oven (Memmert UN model); b) Calcined material (ash) studies: The dried

samples were calcined for 1 hour at the selected temperature (depending on the leaf, 350, 450 and

650ºC) in a muffle furnace (Carbolite RHF model 1500). The obtained ashes were: Elephant Grass

Ash (EGA) and Corn Leaf Ash (CLA).

Microscopy equipment: Field emission scanning electron microscopy (FESEM) was used for

microscopy and microanalysis studies by using a ZEISS ULTRA 55 microscope. Samples for

taking images were studied at 20kV and covered with gold (working distance 11-11.5 mm); and at

1-3 kV and not covered (working distance 3.4-4.9 mm). Samples for chemical analysis (EDX) were

not covered and studied at 15kV. The EDX analysis was carried out by analysing a 115 m x 85 m

area: from the 5 tests, mean values and their standard deviations were calculated.

Results and Discussion

Microscopic characterization of dried Elephant Grass Leaves (EGL). FESEM studies were

carried out on EGL samples dried at 105ºC. Selected micrographs are shown in Figure 1. They

show the lower leaf surface (abaxial surface) and the upper leaf surface (adaxial surface). In the

abaxial surface, many structures (Figure 1a) have been found: hairs, stomas and phytoliths. In

Figure 1b, these structures are showed in detail. Phytoliths showed different shapes: bilobated,

trialobated and tetralobated. Adaxial surface was similar, as can be seen in Figure 1c.

(a) (b) (c)

Figure 1. FESEM micrographs of abaxial (a and b) and adaxial (c) surfaces of Elephant Grass

Leaves (EGL) dried at 105ºC.

Microscopic characterization of Elephant Grass leaf Ashes (EGA). FESEM studies were carried

out on EGA samples obtained at 450ºC and 650ºC. Selected micrographs of EGA ashes obtained at

450ºC are shown in Figure 2. Figure 2a shows a general view of the spodogram obtained when a

large part of organic matter has been removed by calcination. It can be observed that the structure

remained, due to the presence of important percentage of inorganic matter. In figure 2b, a detailed

zone of the spodogram is observed: hairs and phytolits showed similar shape than that observed in

Figure 1, meaning that these structures are rich in inorganic elements. These phytoliths are arranged

in a similar way than found in sugarcane leaves [15,16], that is, they are aligned and arranged so

that the major axis thereof is parallel to the major axis of the epidermal cells (Figures 2c and 2d).

(a) (b)

(c) (d)

Figure 2. FESEM micrographs of EGA obtained at 450ºC.

When the leaves were ashed at 650ºC, the spodogram was maintained, suggesting the strong

connected inorganic skeleton. Figure 3 shows some FESEM micrographs for EGA obtained at

650ºC. At this calcination temperature, organic matter has been completely removed. Despite this,

the spodogram was maintained (adaxial surface, Figures 3a and 3b). In the abaxial surface (Figures

3c and 3d), some indications of partial fusion processes (sinterization) were observed: inorganic

matter surrounding the phytoliths apparently melted during calcination. Probably, localized higher

temperatures than 650ºC were reached because the heat emitted during combustion reaction of

organic matter. However, phytoliths remains unaltered, probably due to the high content of silica

[16].

(a) (b)

(c) (d)

Figure 3. FESEM micrographs of EGA obtained at 650ºC: a and b): adaxial surface; c and d)

abaxial surface.

Chemical composition of EGA. EDX analyses were carried out in order to quantify the chemical

composition of EGA. Five EDX were taken on different 104 m

2 zones. Table 1 summarizes the

calculated compositions (carbon was not assessed in the analysis).

The highest percentage was found for SiO2, which denotes the siliceous nature of the ash. This

percentage was found in the range 47-79%, with a mean value of 55.70%. This percentage is

notably lower than usual found for rice husk ash [5] and it is also lower than that found for bamboo

leaf ash [16], however is significantly higher than percentage found for sugarcane straw ash [16].

Magnesium, calcium and potassium oxides were also high, and it is noticed the low value for

chloride (0-0.3% range). Similar values for SiO2, Cl and K2O were found in EGA Cameroon variety

[12] and in EGA samples analysed for Cordeiro et al [11]. However, significant differences were

found for CaO and MgO (see Table 2). Also, significant differences were found in the content of

Al2O3: Cordeiro et al [11] found percentages higher than 20% and Nakanishi et al [12] obtained

values lower than 1%. Our results were more similar to the last case (2-4%). Probably, samples

analyzed in [11] were contaminated by soil.

EDX analysis on phytolith showed that they are highly concentrated in silica: 96.20% SiO2,

2.86% K2O, 0.66% MgO and 0.28 Cl.

Table 1. EDX analyses (% by weight) in Elephant Grass Ash (EGA) calcined at 450ºC.

Oxide Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Mean

value

Maximum

value

Minimum

value

Standard

deviation

SiO2 47.89 49.92 49.97 51.46 79.24 55.70 79.24 47.89 13.2

K2O 10.94 10.70 10.71 12.07 5.27 9.94 12.07 5.27 2.7

Cl 0.23 0.27 0.30 0.28 0.00 0.22 0.30 0.00 0.1

CaO 10.83 7.73 9.00 12.17 2.45 8.44 12.17 2.45 3.8

MgO 22.21 24.59 25.22 18.51 8.36 19.78 25.22 8.36 6.9

P2O5 0.98 1.00 0.00 0.00 0.00 0.40 1.00 0.00 0.5

SO3 1.48 1.25 1.29 1.63 0.61 1.25 1.63 0.61 0.4

Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0

Fe2O3 1.85 1.62 1.19 1.74 1.65 1.61 1.85 1.19 0.2

Al2O3 3.59 2.93 2.32 2.15 2.43 2.68 3.59 2.15 0.6

Table 2. Comparison of chemical compositions for EGA (% by weight).

Ref [11] Cordeiro et al. Ref [12] Nakanishi et al

Oxide

This work

EGA

EGA1

(not

treatment)

EGA2

(hot water

treatment)

EGA3

(HCl

treatment)

EGA

Cameroon

variety

EGA

Napier

variety

SiO2 55.70 56.2 59.6 67.8 49.40 80.00

K2O 9.94 7.4 3.5 2.0 8.60 7.05

Cl 0.22 nd nd nd 0.46 0.95

CaO 8.44 nd 2.6 nd 10.40 1.85

MgO 19.78 nd nd nd 4.22 1.11

P2O5 0.40 nd nd nd 9.91 0.56

SO3 1.25 2.3 1.8 1.6 0.47 0.65

Na2O 0.00 nd nd nd nd nd

Fe2O3 1.61 6.1 5.9 4.0 0.83 0.77

Al2O3 2.68 22.1 22.1 23.1 0.47 0.44

LOI nd 4.4 3.0 2.6 14.60 6.31

Obtained results in this research showed that EGA is a good candidate to be a pozzolanic mineral

addition in OPC mixtures. It may be taken into account the significant differences among ashes,

depending on the source of leaves and the treatment process.

Microscopic characterization of dried Corn Leaves (CL). FESEM studies were carried out on

CL samples dried at 105ºC. Selected micrographs are shown in Figure 4. They show the lower leaf

surface (abaxial surface) and the upper leaf surface (adaxial surface). In the abaxial surface stomas

(Figure 4a) have been observed, but hairs were not in a significant presence. In a detailed zone

(Figure 4b) phytoliths were observed. In Figure 4c, adaxial surface was depicted, and also

phytoliths are identified, most of them showing tetralobulated shape.

Microscopic characterization of Corn Leaf Ashes (CLA). FESEM studies were carried out on

CLA samples obtained at 350ºC and 450ºC. Selected micrographs of CLA ashes obtained at 350ºC

are shown in Figure 5. Figure 5a shows a general view of the spodogram obtained when part of

organic matter was removed by calcination. In Figure 5b, a detailed zone of the spodogram is

observed: phytoliths showed similar shape than that observed in Figure 4, meaning that these

structures are rich in inorganic compounds. The arrangement of these phytoliths is aleatory

differently to the arrangement found for EGA. At 450ºC, the spodogram was maintained (Figure 6).

The inorganic skeleton showed the original arrangement of stomas and phytoliths.

(a) (b) (c)

Figure 4. FESEM micrographs of abaxial (a and b) and adaxial (c) surfaces of Corn leaves (CL)

dried at 105ºC (samples were covered with gold).

(a) (b)

Figure 5. FESEM micrographs of CLA obtained at 350ºC (samples were covered with gold).

(a) (b)

Figure 6. FESEM micrographs of CLA obtained at 450ºC.

Chemical composition of CLA. EDX analyses were carried out in order to quantify the chemical

composition of CLA. Five EDX measurements were taken on different 104 m

2 zones. Table 3

summarizes the calculated compositions (carbon was not assessed in the analysis). SiO2 was also

the main oxide in the composition of ashes, ranging from 44-80%, with a mean value of 62.61%.

This value was similar (slightly higher) to that found for EGA. However, there were main

differences in composition: K2O was the second oxide in percentage, with a mean value of 26.85%.

This fact may be taken into account when this ash is used in OPC concrete, because the possibility

of activating the alkali-silica reaction towards reactive aggregates. Also, another important chemical

parameter found was the chloride content: CLA ranged in 0.65-4.65%, with a mean value of 2.42%.

Values higher than 1% could activate the corrosion of steel reinforcements in concrete. This

chloride content was similar to that found for sugarcane straw ash [16]. For CLA, magnesium and

calcium contents were very low, in contrast to EGA sample.

Table 3. EDX analyses (% by weight) in Corn Leaf Ash (CLA) calcined at 450ºC.

Oxide Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Mean

value

Maximum

value

Minimum

value

Standard

deviation

SiO2 72.64 52.88 79.56 44.02 63.93 62.61 79.56 44.02 14.4

K2O 16.08 37.67 18.21 38.25 24.03 26.85 38.25 16.08 10.6

Cl 1.07 3.93 0.65 4.65 1.80 2.42 4.65 0.65 1.8

CaO 2.41 0.00 0.00 3.46 2.81 1.74 3.46 0.00 1.6

MgO 2.37 1.40 0.42 2.55 2.01 1.75 2.55 0.42 0.9

P2O5 3.91 2.75 0.79 4.70 3.65 3.16 4.70 0.79 1.5

SO3 1.21 0.73 0.00 1.61 1.09 0.93 1.61 0.00 0.6

Na2O 0.31 0.64 0.38 0.75 0.68 0.55 0.75 0.31 0.2

Fe2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0

Al2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0

There were many important differences when comparing chemical composition of CLA analyzed

in this work and ashes from corn cob (CCA, [8]) and ashes from corn stover (CSA, [10]) as can be

seen in Table 4. Thus, SiO2 content in CLA was similar to that found for CCA, however K2O and

CaO contents were completely different: CLA contained 26.85% of K2O versus 4.20 for CCA,

which suggests that chemical composition of ashes depends largely on the selected part of the maize

plant.

On the other hand, in CSA produced from combustion of canes and leaves, SiO2 was

comparatively lower than that found in CLA. K2O was similar, however, taking into account the

high loss on ignition value (LOI = 22.78%), the real content of K2O in CSA in terms of inorganic

matter would be significantly higher. Apparently, the potassium in the ash would be easily removed

by hydrochloric treatment, as can be seen in CSA treated sample. This treatment also removed other

oxides, in this manner the relative percentage of SiO2 is notably increased.

EDX analysis on phytolith showed that they are highly concentrated in silica: 92.78% SiO2,

4.39% K2O, 0.64% MgO, 1.73% P2O5 and 0.46 Cl.

Conclusions

Combustion of elephant grass leaves yielded ashes (EGA) that maintained the spodogram

(inorganic skeleton after organic matter removal by calcinations) at 450 and 650ºC. Phytoliths

(silica bodies) were present in different shapes and maintain their shape after calcinations without

melting after heating at 650ºC. Silica was the main oxide in EGA, reaching 55.7% by weight. This

result showed that EGA would be a good candidate to be a pozzolanic mineral addition in OPC

mixtures as previously reported. However, it may be taken into account the noticeable differences

among ashes when comparing to results from other papers: depending on the source of leaves and

the treatment process, chemical composition varies significantly.

Corn leaf ashes (CLA) also maintain the spodogram at temperatures of 350 and 450ºC. Chemical

composition revealed that silica was the main component (62.6%). However, the percentages of

K2O and chloride indicated that attention may be paid for further uses in concrete, because of alkali-

silica and metallic corrosion problems concerning to reactive aggregates and reinforcement

respectively.

Table 4. Comparison of chemical compositions for Corn Leaf Ash (CLA), Corn Cob Ash (CCA)

and Corn Stover Ash (CSA).

Ref [8] Adesanya et al. Ref [10] Ataie & Riding

Oxide

This work

(CLA)

Sample

CCA1

Sample

CCA2

Sample

CCA3

CSA (not

treatment)

CSA (HCl

treatment)

SiO2 62.61 67.33 65.39 66.41 28.42 76.54

K2O 26.85 4.20 4.92 5.64 25.41 3.93

Cl 2.42 nd nd nd nd nd

CaO 1.74 10.29 12.89 11.53 7.83 4.05

MgO 1.75 1.82 2.33 2.02 3.30 1.00

P2O5 3.16 nd nd nd 6.78 3.31

SO3 0.93 1.11 1.10 1.01 nd nd

Na2O 0.55 0.39 0.48 0.36 0.4 0.6

Fe2O3 0.00 3.74 5.61 3.97 0.56 0.79

Al2O3 0.00 7.34 9.14 5.97 0.89 1.59

LOI nd nd nd nd 22.78 6.09

Acknowledgements

The Authors would like to thanks Electron Microscopy Service of the Universitat Politècnica de

València.

Literature References

[1] R. Siddique, M. I. Khan, Supplementary Cementing Materials, Springer Science & Business

Media, 2011.

[2] L. Barcelo, J. Kline, G. Walenta, E. Gartner, Cement and carbon emissions, Mater. Struct. 47

(2014) 1055-1065.

[3] E. Aprianti, P. Shafigh, S. Bahri, J.N. Farahani, Supplementary cementitious materials origin

from agricultural wastes – A review. Constr. Build. Mater. 74 (2015) 176-187.

[4] V. Sata, J. Tangpagasit, C. Jaturapitakkul, P. Chindaprasirt, Effect of W/B ratios on pozzolanic

reaction of biomass ashes in Portland cement matrix, Cem. Concr. Comp. 34 (2012) 94–100.

[5] J. Payá, J. Monzó, M.V. Borrachero, Outstanding aspects on the use of rice husk ash and similar

agrowastes in the preparation of binders, In: Proceedings of the First Pro-Africa conference: Non

conventional building materials based on agroindustrial wastes. Pirassununga, São Paulo (Brazil),

Savastano Jr H (Ed) pp. 179-181, ISSN 2177-6334, 2010.

[6] S.A. Lima, J.A. Rossignolo, Análise da pozolanicidade da cinza da casca da castanha do caju

pelo método de difratometria de raios X. Revista Matéria, 14 (2009) 680 – 688.

[7] P. Ranum, J. P. Peña-Rosas, M. N. Garcia-Casal, Global maize production, utilization, and

consumption, Ann. N.Y. Acad. Sci. 1312 (2014) 105–112.

[8] D.A. Adesanya, A.A. Raheem, Development of corn cob ash blended cement, Constr. Build.

Mater. 23 (2009) 347–352.

[9] D.A. Adesanya, A.A. Raheem, A study of the workability and compressive strength

characteristics of corn cob ash blended cement concrete, Constr. Build. Mater. 23 (2009) 311–317.

[10] F. F. Ataie, K. A. Riding, Use of bioethanol byproduct for supplementary cementitious

material production, Constr. Build. Mater. 51 (2014) 89–96.

[11] G. C. Cordeiro, C. P. Sales. Pozzolanic activity of elephant grass ash and its influence on the

mechanical properties of concrete. Cem. Concr. Comp. 55 (2015) 331–336.

[12] E. Y. Nakanishi, M. Frías, S. Martínez-Ramírez, S. F. Santos, M. S. Rodrigues, O. Rodríguez,

H. Savastano Jr, Characterization and properties of elephant grass ashes as supplementary

cementing material in pozzolan/Ca(OH)2 pastes, Constr. Build. Mater. 73 (2014) 391–398

[13] C.J. Prychid, P.J. Rudall, M. Gregory, Systematics and biology of silica bodies in

monocotyledons, The Botanical Review 69 (2003) 377–440.

[14] D.R. Piperno, Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists,

AltaMira Press, Oxford, 2006.

[15] J. Roselló, L. Soriano, P. Santamarina, J.L. Akasaki, J. Payá, J.L.P. Melges. Residuos agrícolas

ricos en sílice: una caracterización para fines de utilización como puzolanas, 56º Congreso

Brasileiro do Concreto. Natal, Rio Grande do Norte (Brasil) 7 -10 oct 2014. ISSN 2175-8185.

[16] J. Roselló, L. Soriano, M.P. Santamarina, J.L. Akasaki, .L.P. Melges, J. Payá, Microscopic

characterization of silica-rich agrowastes to be used in cement binders: bamboo and sugarcane

leaves. Submitted to Microscopy and Microanalysis (2015).