Characterisation of High Calcium Wood Ash for Use as a Constituent in

Wood Ash-Silica Fume Ternary Blended Cement

Cheah Chee Ban1, a and Mahyuddin Ramli1, b 1School of Housing, Building and Planning, Universiti Sains Malaysia, 11800 Penang, Malaysia

avolrath15@hotmail.com, bmahyudin@usm.my

Keywords: High calcium wood waste ash; ternary blended cement; mechanical strength; biomass ash; chemical properties; physical properties.

Abstract. The potential stabilization of high calcium wood waste ash (HCWWA) derived from the

wood biomass energy production for use as cementitious material using another industrial by-

product, silica fume, was investigated. Throughout the study, both HCWWA and DSF were

characterized in term of their respective chemical composition and mineralogical phases. Besides,

physical characteristics of HCWWA and DSF in terms of particle grading and specific surface area

were established in order to evaluate their suitability as for use as constituent material in blended

cement. Additionally, compressive strength properties of high strength mortar produced using

HCWWA and DSF blended cement were investigated. Results indicated that the use of HCWWA

as a partial cement replacement did not have significant adverse effect on the workability of fresh

mortar. The enhancement of the compressive of mortar was observed for mortar mixes containing

DSF and HCWWA levels of cement replacement up to 16%.

Introduction

Compilation of chemical composition analysis data of ashes derived from combustion of 28 variety

of woody biomass indicated a high mean calcium oxide composition (43.03%) with certain types of

woody biomass ash having calcium oxide content up to 83.46% of total mass of ash [1]. The

reported finding justifies that wood biomass ash may possibly contains high calcium carbonate

composition which can be suitably used as an accelerating agent for cement hydration. The

exposure of wood ash up to temperatures of 800-950ºC during the incineration of wood biomass

may result in partial decomposition of calcium carbonate composition which occurs at temperature

range of 615-775ºC, forming quicklime minerals. A study performed to investigate the interaction

between quicklime and high silica powder (fly ash) in quicklime-fly ash blended cement concluded

that the addition of quicklime in fly ash concrete resulted in a notable degree of acceleration of fly

ash hydration at any given curing duration. The feasibility of the use of material with high calcium

carbonate content such as limestone powder as filler and supplementary binder material has been

established in several earlier pieces of research [2-7]. However there are controversial opinions on

the effect of the presence of calcium carbonate in blended cement. Some researchers [2, 4, 8-10]

considered calcium carbonate rich limestone powder as inert filler which does not undergo any form

of chemical reaction with Portland cement constituent phases and hydration products. Meanwhile,

recent findings justify that calcium carbonate rich filler participate actively in the reaction with

C3AF and C3A phases of the Portland cement to form hemicarbonate, monocarbonate or a mixture

of both [11-13]. In addition, calcium carbonate was also found to accelerate hydration of C3S phase

of Portland cement [11, 14]. Isothermal calorimetry analysis performed on cement paste containing

finely ground calcium carbonate mineral verified that the hydration of C3S phase occurs at a higher

rate in the presence of calcium carbonate. It was then concluded that the presence of calcium

carbonate as supplementary binder in blended cement paste contributes towards modification of

AFm and AFt phases and produces calcium carbosilicate hydrate and calcium carboaluminate as

final hydration product which alters the strength development behaviour the blended cement paste

[15].

Advanced Materials Research Vol. 346 (2012) pp 3-11Online available since 2011/Sep/27 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.346.3

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 132.174.255.116, University of Pittsburgh, Pittsburgh, USA-26/11/14,22:25:05)

These are indicative of the possibility of the reuse of high calcium wood waste ash as a

supplementary binder material in the production of binary or ternary blended cement for use as

binder in the production of concrete. The reuse of wood biomass ash as a supplementary binder

material contributes towards the reduction in cement consumption hence conservation of natural

resources. In addition, it also provides a means of ultimate disposal of wood biomass ash produced

from the biomass power station hence promoting sustainability of the energy source.

The main aim of this research was to study the feasibility of the incorporation of HCWWA in

combination with DSF as supplementary binder in ternary blended cementitious system with

superior or equivalent strength properties as compared to ordinary Portland Cement. Specifically,

the study was aimed in order to establish the chemical properties of HCWWA and DSF in terms of

their chemical compositions and mineralogical phases. Physical properties of HCWWA and DSF in

terms of particle grading and specific surface area were also determined. In addition, the

simultaneous influence of silica fume and finely ground high calcium wood waste ash (HCWWA)

on compressive strength properties of high strength mortars fabricated using HCWWA-DSF ternary

blended cement was investigated.

Materials

Portland Cement (PC). ASTM Type I Portland cement (PC) with median particle size of 3.9µm,

specific surface area of 1.0432m2/g and specific gravity of 3.02 were used in this study. Both

physical and chemical properties of cement used comply with specifications in ASTM Standard

C150 [16, 17]. The chemical composition of PC used is presented in Table 1.

High Calcium Wood Waste Ash (HCWWA). HCWWA is a by product acquired from a small

scale fully automatic boiler unit (commercially known as Bio-Turbomax boiler) used in rubber

wood timber product manufacturing industry. The wood biomasses used as fuel in the boiler were

derived from local rubber wood species dominantly Hevea Brasiliensis. The wood biomasses were

incinerated under self sustained burning condition within an atmosphere with turbulence air flow

supplied by an in-built air pump unit. Temperature of incineration was maintained within the range

of 800±10ºC. Raw ash obtained from the boiler unit were sieved through laboratory sieve with

opening size of 150µm to remove large agglomerated ash particles and unburned carbonaceous

materials. Ash passing the 150µm sieve was then grinded in a ring mill to fineness whereby mean

particle diameter (d50) of the material reaches 8.39 µm. The chemical compositions of HCWA are

presented in Table 1.

Densified Silica Fume (DSF). Silica fume used in this study was collected from local ferrosilicon

industry and had undergone densification process for increment of its bulk density. Densification of

raw silica fume was carried out by air flotation method within the storage silo for a total duration of

24 hours. At the end of the densification process, particle agglomerates with size ranging between

10 µm and 1000 µm were formed. Results obtained from laser particle diffraction analysis indicated

that DSF used in the study had median particle size of 28.21µm and specific surface area of 0.2170

m2/g. Specific gravity of DSF was determined to be 2.28. The chemical composition of DSF used is

summarised in Table 1.

4 Sustainable Construction Materials and Computer Engineering

Table 1 Chemical compositions and physical properties of Portland cement, HCCWA and DSF.

Chemical % by total mass

Compound Portland High Calcium Carbonate Densified

Cement (PC) Wood Ash (HCCWA) Silica Fume (DSF)

MgO 1.500 8.700 4.600

Al2O3 3.600 1.300 0.270

SiO2 16.000 2.700 84.000

P2O5 0.057 2.700 0.050

SO3 3.100 2.800 0.440

Cl n/d 0.100 2.400

K2O 0.340 12.000 2.700

CaO 72.000 61.000 0.660

TiO2 0.170 0.110 0.085

MnO 0.028 0.860 n/d*

Fe2O3 2.900 1.300 0.540

NiO n/d trace n/d

ZnO trace 0.100 0.100

SrO 0.035 0.220 n/d

ZrO2 0.018 n/d n/d

PbO 0.012 trace 0.011

CuO n/d 0.014 trace

Rb2O trace 0.052 0.015

C n/d 6.700 n/d

Na2O n/d n/d 4.200

Specific Surface

Area (m2/g) 1.0432 0.611 0.217

Loss on ignition

(%) 2.53 18.00 4.03

Median particle

diameter,d50 (µm) 6.14 8.39 28.21 *n/d indicates that the given compound was not detected in the sample

Aggregates. Fine aggregate used was locally sourced quartzitic natural river sand in uncrushed

form with a specific gravity of 2.83 and maximum aggregate size of 5mm. Fine aggregates were

graded in accordance to BS812: Part 102 [18] and the grading of fine aggregates used remained

compliant with the overall grading limits of BS 882 [19] as shown in Fig. 1.

Fig. 1. Grading of fine aggregates.

Superplasticizer and Mixing Water. An aqueous solution of polycarboxylic ether by the

commercial designation of Glenium Ace 388 was used as a superplasticizer in this study. Potable

water from a local water supply network was used to mix water for all mortar mixes produced.

Advanced Materials Research Vol. 346 5

Methods

Characterisation of Binder Materials. Chemical compositions of HCWWA, DSF and PC were

determined by X-Ray Fluorescence analytical methods using an X-ray spectrometer. Mineral phases

of oxide compounds detected by X-Ray Fluorescence analysis were identified by X-Ray diffraction

methods. Loss on ignition (LOI) of the materials was determined in accordance to procedures in

ASTM Standard C311 [20]. Particle size distribution, median particle size diameter (d50) and

specific surface area of the powders were determined using a laser particle size analyzer. Specific

gravity values of the samples were determined using Le Chatelier Flask and procedures prescribed

in ASTM Standard C188 [21].

Mixture Proportioning. The binder: sand and water/binder ratios were maintained constant at

1:2.25 and 0.32 respectively for all mortar mixtures cast. The Portland cement binder was partially

replaced using HCWWA at substitution levels of 0, 4, 6, 8, 10, 12, 16 and 20 % by total binder’s

weight. DSF was used as partial cement replacement material at a constant replacement level of

7.5% for all mixes containing HCWWA. Superplasticizer was dosed at appropriate dosages to

maintain the desired mortar slump of 70±20mm, as prescribed in BS EN 206: Part 1 [22] as S2

(medium workability) slump range. The flow of mortar mixes was maintained within the range of

26±5% to ensure adequate workability of the mix. The mix design of the control mortar mix (C)

was performed using absolute volume method prescribed in the design code ACI 211.1 [23] to

achieve a compressive strength of 45MPa at the age of 28days. The composition of mortar mixes

are summarised in Table 2.

Table 2 Proportion of constituent materials and rheological properties of mortar mixes.

Batch Cement DSF HCWWA Sand Water SP Mortar Slump Bulk

Designation (kg/m3) (kg/m

3) (kg/m

3) (kg/m

3) (kg/m

3) Dosage Flow (mm) Density

(%) (%)

(kg/m3)

C 708 53 0 1593 227 0.40 26.70 50 2246

W4 627 53 28 1593 227 1.10 26.40 50 2200

W6 613 53 42 1593 227 1.30 26.60 65 2204

W8 598 53 57 1593 227 1.40 26.90 85 2214

W10 584 53 71 1593 227 1.50 30.90 50 2217

W12 570 53 85 1593 227 1.70 22.50 50 2193

W16 542 53 113 1593 227 1.70 38.20 90 2246

W20 513 53 142 1593 227 1.80 26.70 60 2220

Mortar Mixing and Curing. Each batch of mortar was produced using an epicyclic type

mechanical mixer. Dry hydraulic binder of mortar namely HCWWA, DSF and Portland cement

were initially dry mixed at a low mixing speed for a duration of 3 minutes prior to the addition of

other constituent materials. Further mixing sequence and durations were performed in accordance to

standard procedures prescribed in ASTM Standard C305 [24].

Measurement of Rheological Properties. An ASTM Flow test was performed on fresh mortar

mixtures using flow table in accordance to standard procedures described in ASTM Standard C109

[25]. A slump value of fresh mix was determined in accordance to procedures prescribed in BS

1881: Part 102 [26]. Fresh mortar flow and slump values obtained are presented in Table 2.

Determination of Compressive Strength and Bulk Density Tests. Mortar cube specimens with

edge dimensions of 50 mm were moulded, cured and tested in accordance to procedures described

in ASTM Standard C109 [25] for the determination of compressive strength. All mortar specimens

fabricated were water cured for durations of 3, 7 and 28 days prior to test. Bulk densities of

hardened mortar mixtures were determined in accordance to methods in BS 1881: Part 114 [27].

6 Sustainable Construction Materials and Computer Engineering

Results and Discussion

Physical Properties of HCWWA and DSF. Particle grading curves of HCWWA, DSF and PC are

presented in Fig.2. Median particle sizes of ground HCWWA were found to be 8.39µm with a

corresponding specific surface area of 611.0m2/kg as compared to the PC sample which has a

median particle size of 6.14µm and a specific surface area of 1123.3m2/kg. In addition, the particle

size distribution curve indicates close resemblance of particle size distribution between HCWWA

and PC within the range of 10 to 150µm. However, for particles with sizes ranging between 0 to

10µm, HCWWA particles were generally coarser as compared to PC particles. Specific gravity of

ground HCWWA particles was determined as 2.52. Meanwhile, median particle diameter and

specific surface area of DSF were determined to be 28.21µm and 217m2/kg, respectively. The

particle grading of DSF particles is significantly coarser as compared to both HCWWA and PC

particles. The observation is consistent with findings of other researchers [28]. The specific gravity

of DSF particles was determined to be 2.28.

Fig. 2. Particle size distribution curve

Chemical Composition. The results of X-ray fluorescence analysis on HCWWA, DSF and PC are

presented in Table 1. From the test results, it can be observed that the major oxide compounds

present in HCWWA are CaO, MgO and K2O. A large composition of CaO of the ash indicates that

the ash could be hydraulically reactive [29]. Loss on ignition of HCWWA was found to be 18%.

The significant loss on the ignition of HCWWA is probably due to the partial thermal

decomposition of calcium carbonate phases into calcium oxide followed by the release of carbon

dioxide at the test temperature of 750ºC [30]. The results of X-ray fluorescence analysis on DSF

indicated that the dominant oxide compound present in DSF is silica which constitutes 84% by total

mass of the material. The other oxide compounds are as indicated in Table 1. The sum of

composition of essential pozzolanic oxide namely SiO2, Al2O3 and Fe2O3 of DSF was found to be

84.81%. Loss on ignition of DSF was found to be 4.96 % which is in compliance with the upper

limit prescribed in ASTM Standards C1240 [31].

X-Ray Diffraction Analysis and Mineralogical Phases. The X-Ray diffraction pattern of

HCWWA is shown in Fig. 3. The X-Ray diffraction pattern obtained indicated that the major

chemical phases of the HCWWA were Calcium Carbonate (CaCO3) and Portlandite (Ca(OH)2). The

presence of these chemical phases is in close agreement with the result of X-Ray Fluorescence

analysis, Table 1, which indicates presence of significant quantities of CaO (61%) in HCWWA.

Calcium carbonate minerals, when used as supplementary binder in parallel with Portland cement,

may act to accelerate the hydration rate of the C3S mineralogical phase of Portland cement [11, 32].

The mechanism is beneficial for rapid strength gain of the cementitious mix containing HCWWA at

the early stages of hydration. In addition, HCWWA which is rich in Portlandite minerals can be

suitably used in combination with silica rich powder material such as silica fume as an early dosage

of Portlandite to accelerate formation of C-S-H gel by pozzolanic reaction. The X-Ray diffraction

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000

Cu

mu

lati

ve

pa

ssin

g (

%)

Particle size (µm)

Cement

HCCWWA

DSF

Advanced Materials Research Vol. 346 7

pattern of DSF is presented in Fig.4. The broad diffused band between 12º and 40º on 2θ scale of

the XRD pattern of DSF is an indication that the silica minerals were glassy and amorphous in

nature [33, 34]. Silicate minerals in a glassy or amorphous state are highly reactive with Portlandite

produced from the hydration of cement to form additional calcium silicate hydrate (CSH) gel which

is the main strength contributor of cementitious mix.

Fig 3. XRD patterns of HCWWA

Fig. 4. XRD patterns of DSF

Superplasticizer Requirement and Workability of Fresh Mortar. The flow and slump values of

fresh mortar produced along with their respective required dosage of superplasticizer to achieve

slump range of 70±20mm are presented in Table 2. A significant rise of the required dosage of

superplasticizer from 0.4% to 1.1% was recorded upon the incorporation of DSF and HCWWA as a

supplementary binder at 7.5% and 4% respectively. The results also indicated that as the content of

HCWWA was increased from 4% to 12%, the required dosage of superplasticizer to maintain a

constant level of workability was increased gradually from 1.1% up to 1.7%. When HCWWA

content was increased beyond 12% up to 20% by total binder mass, no significant increase in

dosage of superplasticizer was required to maintain a constant level of workability was observed.

Generally, with the combined inclusion of DSF and HCWWA as a supplementary binder material,

the slump and flow of mortar mixes could be maintained at a given level with minor adjustments in

the dosage of superplasticizer incorporated into the mix.

Bulk Density and Compressive Strength of Hardened Mortar. Bulk densities of hardened

mortar are presented in Table 2. Generally, it was observed that the inclusion of HCWWA as a

supplementary binder in mortar had resulted in the marginal reduction of bulk density of mortar

mixes produced with reference to the control mortar mix. The compressive strength development of

the hardened mortars containing various level of cement replacement by HCWWA is presented in

8 Sustainable Construction Materials and Computer Engineering

Fig. 5. At the age of 3 days, compressive strength of mortar W4, W6 and W8 was found to be

marginally lower in comparison to the control mortar (C). The trend observed is probably attributed

to the dominating effect of dilution in the cement content of mixes containing HCWWA and DSF.

At the same age, mortar mixes W10, W16 and W20 were found to have higher compressive

strength as compared to the control mortar. The presence of higher content of HCWWA resulted in

a corresponding higher dosage of calcium carbonate within the mixes. Hence, the effect of the

accelerated hydration of the C3S phase by way of the calcium carbonate mineral became the

dominant factor against the dilution of cement content. The compressive strengths of all HCWWA

mortar mixes were found to be higher as compared to the control mortar by the age of 7 days as

indicated in Fig. 5. The higher rate of strength gain of the HCWWA mortar as compared to the

control mortar between the age of 3 days and 7 days was mainly attributed to the combined effect of

early pozzolanic reaction and acceleration of hydration of C3S in the presence of calcium carbonate

mineral [11]. The Portlandite mineral which is present in HCWWA could initiate the pozzolanic

reaction with the amorphous silica content of DSF at an early age of hydration hence contributing to

the higher yield of calcium silicate hydrate (C-S-H) crystals and corresponding higher rate of early

strength development. At 28days, the compressive strength of the control mortar (C), W4, W6, W8,

W10, W12, W16 and W20 was found to be 49.5, 49.6, 52.9, 54.1, 50.3, 50.8, 51.8 and 48.1MPa,

respectively. The compressive strengths of mortars containing various levels of cement replacement

using HCWWA and constant DSF content of 7.5% was found to exceed the target design

compressive strength of 45MPa. The mortar mixes could be classified as high strength mortar by

definition of ASTM Standard C387 [35] The optimum content of HCWWA ensures the best

compressive strength performance of the mortar mix produced with 8% by total weight of binder as

shown in Fig.5.

Fig. 5. Compressive strength development of HCCWA mortars

Conclusions

As referred to the results acquired throughout the laboratory investigation, the following

conclusions can be derived:

1. HCWWA has fine particle distribution, high calcium carbonate and high free lime content,

which can be suitably used for accelerating the hydration rate of the C3S mineral phase of

Portland cement and is potentially reactive with amorphous silica mineral to form secondary

calcium silicate hydrate.

2. Densified silica fume which is rich in amorphous silica content can be used to stabilize

HCWWA which is rich in quicklime content as a supplementary binder material in concrete or

mortar.

3. The presence of HCWWA in the mortar mix at replacement levels beyond 10% contributed

towards the enhancement rate of compressive strength gain at early stages of curing.

0

10

20

30

40

50

60

0 4 6 8 10 12 16 20

Co

mp

ress

ive

Str

eng

th

(MP

a)

% Wood ash of Total Binder Weight

3 Days

7 Days

28 days

Advanced Materials Research Vol. 346 9

4. The optimum level of cement replacement with HCWWA to optimize the 28-day compressive

strength performance is 8% by total weight of binder.

5. HCWWA can be used in conjunction with DSF and ordinary Portland cement to produce ternary

blended cement which has superior or equivalent compressive strength properties as pure

ordinary Portland cement.

Acknowledgement

The research study was jointly funded by Universiti Sains Malaysia Fellowship Programme and

Universiti Sains Malaysia Research University Postgraduate Research Grant Scheme (USM-RU-

PRGS).

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Ternary Blended Cement 10.4028/www.scientific.net/AMR.346.3

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