Strength and microstructure analysis of concrete containing rice husk ash under seawater attack by wetting and drying cycles

Advances in Cement Research

http://dx.doi.org/10.1680/adcr.13.00010

Paper 1300010

Received 24/01/2013; revised 18/05/2013; accepted 19/06/2013

ICE Publishing: All rights reserved

Advances in Cement Research

Strength and microstructure analysis ofconcrete containing rice husk ash underseawater attack by wetting and dryingcyclesJaya, Bakar, Johari et al.

Strength and microstructureanalysis of concrete containingrice husk ash under seawaterattack by wetting and dryingcyclesRamadhansyah Putra JayaSenior Lecturer, Faculty of Civil Engineering and Construction ResearchAlliance, Universiti Teknologi Malaysia, Johor Bahru, Malaysia

Badorul Hisham Abu BakarProfessor, School of Civil Engineering, Universiti Sains Malaysia, NibongTebal, Malaysia

Megat Azmi Megat JohariAssociate Professor, School of Civil Engineering, Universiti Sains Malaysia,Nibong Tebal, Malaysia

Mohd Haziman Wan IbrahimSenior Lecturer, Faculty of Civil & Environmental Engineering, UniversitiTun Hussein Onn Malaysia, Batu Pahat, Malaysia

Mohd Rosli HaininProfessor, Faculty of Civil Engineering and Construction Research Alliance,Universiti Teknologi Malaysia, Johor Bahru, Malaysia

Dewi Sri JayantiLecturer, Faculty of Agriculture, Department of Agricultural Engineering,Universitas Syiah Kuala, Banda Aceh, Indonesia

Concrete containing rice husk ash (RHA), subjected to seawater under wetting and drying cycles, was studied

through an investigation of the compressive strength and microstructure of various types of blended cement paste.

Five levels of cement replacement (0%, 10%, 20%, 30% and 40% by weight) were studied. The total cementitious

content used was 420 kg/m3: A water/binder ratio of 0.49 was used to produce concrete with a target strength of

40 MPa at age 28 days. The performance of blended cement concrete was evaluated based on compressive strength

and chloride ion permeability. Microstructural changes in the specimens were determined by differential thermal

analysis, X-ray diffraction and scanning electron microscopy. The addition of RHA was found to decrease calcium

hydroxide formation by hydration and, consequently, gypsum and ettringite were reduced during seawater attack.

RHA at 40% cement replacement improved resistance to seawater attack and effectively decreased ettringite and

gypsum formations.

IntroductionThe durability of concrete has significant effects on the construc-

tion industry worldwide, and aggressive environments are known

to influence concrete durability (Zuquan et al., 2007). Early

concrete failure (cracking, strength, porosity and permeability

problems) may be caused by external or internal factors. Internal

causes may be related to the choice of materials or inappropriate

combination of materials (Ganjian and Pouya, 2009). External

factors can be physical or chemical in nature, for example

weathering, extreme variation in temperatures, wetting and drying

cycles, abrasion and exposure to aggressive environments (Hekal

et al., 2002; Memon et al., 2002). The most aggressive chemicals

that affect the durability of concrete are chlorides and sulfates

and their associated cations.

Most studies on concrete exposed to aggressive environments

(e.g. Al-Amoudi, 2002; Nehdi and Hayek, 2005; Wee et al.,

2000) have focused on concrete incorporating supplementary

cementitious materials such as fly ash, blast-furnace slag, palm

oil fuel ash and silica fume. Memon et al. (2002) indicated that

rice husk ash (RHA) as cement replacement can enhance the

strength and durability of concrete, but studies on concrete

containing RHA subjected to aggressive seawater and wetting and

drying cycles are rarely reported. In addition, to the authors’

knowledge, no research has evaluated the effect of pozzolanic

materials in concrete exposed to a marine environment in the

high-humidity and tropical climate of South East Asia. The main

aim of this study was therefore to examine the effects of strength,

permeability and microstructure of RHA waste as a supplemen-

tary cementitious material on the durability performance of

concrete exposed to an aggressive seawater environment. Com-

pressive strength, chloride ion permeability, thermal analysis,

X-ray diffraction (XRD) and scanning electron microscopy

(SEM) images of concrete were examined.

Materials and experimental procedure

Cement

Type I ordinary Portland cement (OPC, Blue Lion Cement) was

used as the major binder material in the production of moderate-

1

strength concrete (40 MPa). The mean particle size of the OPC

was 10.11 �m, the density was 3.12 g/cm3 and the surface area

was 359 m2/kg, as tested and determined by the Blaine test. The

chemical composition and loss on ignition (LOI) of the OPC are

given in Table 1. Chemical analysis of the cement was conducted

using an X-ray fluorescence apparatus in accordance with BS EN

197-1 (BSI, 2011).

Aggregate

Local natural sand derived from granite was used as the fine

aggregate in the concrete mixtures. A single size (20 mm) of

crushed granite was used as the coarse aggregate. The coarse and

fine aggregates each had a specific gravity of 2.66, and water

absorption rates of 0.48% and 0.86%, respectively.

Rice husk ash

Rice husk was incinerated in a gas furnace at a heating rate of

108C/min until the temperature reached 7008C; the rice husk was

maintained at this temperature for 6 h. After the burning process

was completed, the ash was left inside the furnace to cool and

was removed the following day. The ash was then ground in a

laboratory ball mill using porcelain balls (Ramadhansyah et al.,

2012a). The RHA after grinding had a median particle size of

9.52 �m, Blaine fineness of 5290 cm2/g and specific gravity of

1.95 (Abu Bakar et al., 2011). The chemical composition and

LOI of the RHA are reported in Table 2.

Superplasticiser

The superplasticiser Glenium C380 was used in this study. This

superplasticiser is a new-generation C380 containing polycarbox-

ylate ether polymers. It is free from chloride and complies with

the ASTM C494-12 (ASTM, 2012a) requirements for type A and

F admixtures. Table 3 presents the technical data for this type of

superplasticiser.

Sample preparation and curing conditions

A laboratory study investigated the effects of seawater attack on

the OPC and RHA blended cement concretes and pastes. A

control mix was prepared using OPC. RHA replacement levels of

10%, 20%, 30% and 40% by weight of cement were applied,

these are referred to as RHA10, RHA20, RHA30 and RHA40,

respectively. The RHA was thoroughly mixed with OPC in

blended cement, and water was added to the mixer. Super-

plasticiser was added to the mix to maintain slump flow values.

When the mixtures were prepared, the concrete was cured in

water maintained at room temperature for a minimum of 28 days

to achieve a strength of 40 MPa. After 28 days of curing under

water, the specimens were subjected to seawater wetting–drying

cycles.

The location used for seawater collection was the Malacca Straits,

Malaysia (Figure 1). Selection of this area was based on the

availability of an open area and coverage with good wind circula-

tion, as well as locations without interference from walls, trees or

other elements (MMD, 2013). The average monthly temperature at

the site is 26–328C. The chemical composition of the seawater

used during the immersion studies is shown in Table 4.

In the cyclic test, the specimens were subjected to an average of

15 h of wetting and 9 h of drying per day to simulate Malaysian

tidal zone conditions (MMD, 2013). During the wetting cycles,

100 3 100 3 100 mm cube specimens were placed in a square 10 l

plastic container filled with 9 l of seawater. The same type of 10 l

square plastic containers was used for all cyclic tests. For the dry

cycles, the specimens were placed in an enclosed chamber of

relative humidity 60–75% at temperatures ranging from 258C to

318C. This procedure completed one wetting–drying cycle, and

was repeated until the cycle periods were complete. Repetitive

wetting–drying cycles were intended to simulate marine structures

where durability is essential (Ramadhansyah et al., 2012b; Sah-

maran et al., 2007). The seawater in the laboratory was refreshed

every 2 weeks or as required based on a change in pH value. Tests

were conducted at ages of 3, 7, 28, 56, 90 and 180 days.

SiO2: % Al2O3: % Fe2O3: % CaO: % MgO: % Na2O: % K2O: % SO3: % LOI: %

17.0 3.90 3.20 70.0 1.50 0.02 0.53 3.60 0.25

Table 1. Chemical composition of OPC

SiO2: % Al2O3: % Fe2O3: % CaO: % MgO: % Na2O: % K2O: % SO3: % LOI: %

92.99 0.23 0.26 0.41 0.73 0.02 1.35 0.08 3.93

Table 2. Chemical composition of RHA

Form Colour Specific

gravity

pH Suitability

Liquid Light

brown

1.07–1.16 6.0–7.5 All Portland

cements

Table 3. Technical data of superplasticiser C380

2

Advances in Cement Research Strength and microstructure analysis ofconcrete containing rice husk ash underseawater attack by wetting and dryingcyclesJaya, Bakar, Johari et al.

Compressive strength test

Compressive strength is commonly considered to be the main

property of concrete. In this study, compressive strength tests of

all the concrete mixes were conducted on 100 3 100 3 100 mm

cubes. The specimens were compressed using a compression

machine with a maximum capacity of 3000 kN and a loading rate

of 150 kN/min. The tests were conducted according to the British

Standard test method BS EN 12390-3 (BSI, 2002) and the

compressive strength reported is the average of three samples.

Chloride ion permeability test

The permeability of concrete to chloride ions is evaluated based

on the total charge passed as determined by the rapid chloride ion

permeability test. Testing was conducted according to the ASTM

test method C 1202-12 (ASTM, 2012b). Concrete specimens of

100 mm diameter and 50 mm thickness were extracted from the

central portion of cylindrical specimens of diameter 100 mm and

height 200 mm. The dry concrete specimens were conditioned

under a vacuum pressure of 1 mmHg (133 MPa) maintained for

3 h. The specimens were treated under vacuum saturation for 1 h

after adding water and soaked in water for a further 18 h. The

specimens were then placed into the test device. Tests were

conducted at ages of 3, 7, 28, 56, 90 and 180 days and the results

obtained at each age are reported as the average of three tested

specimens.

Thermal analysis

Differential thermal analysis (DTA) locates the ranges correspond-

ing to the thermal decomposition of different phases in paste,

whereas thermogravimetric analysis (TGA) simultaneously meas-

ures weight loss caused by decomposition (Alarcon-Ruiz et al.,

2005). Simultaneous TGA and DTA were performed using a

Shimadzu DTG-60/60H instrument. On the specified day of testing,

hardened cement paste samples were crushed into powder form

(passing a 75 �m sieve). About 15–20 mg of sample was placed in

a platinum pan and heated in a nitrogen atmosphere from 258C to

10008C at 108C/min. Weight loss, which occurs in hardened cement

paste due to dehydration and decomposition of its components

when heated gradually, was recorded at varying temperatures.

X-ray diffraction

OPC and RHA samples were characterised using a Bruker DX 8

XRD device. In accordance with BS EN 13925-1 (BSI, 2003),

Figure 1. Location of the seawater collection site

Sodium (Naþ): ppm 2002–2402

Potassium (Kþ): ppm 375

Calcium (Ca2þ): ppm 285

Magnesium (Mg2þ): ppm 1055

Chloride (Cl�): ppm 16618–18020

Sulfate (SO�24 ): ppm 1098

pH 7.90–8.20

Temperature range: 8C 26.5–32.0

Table 4. Analysis of the seawater used in the study

3


the samples were scanned in steps of 0.0348 (2Ł) with a fixed

counting time of 1 s. The X-ray scan ranged from 2Ł ¼ 108–908

using copper (KÆCu) with a wavelength º of 1.5406 nm as the

X-ray source. EVA software was used to analyse the phase of the

samples.

Scanning electron microscopy

SEM is typically conducted under high vacuum because gas

molecules interfere with the electron beam as well as the emitted

secondary and backscattered electrons used for imaging. A Zeiss

Supra 35VP field-emission scanning electron microscope charac-

terised the samples in this study. To determine the morphological

characterisation, the samples were cut into small sizes (except for

powder samples). The samples were horizontally placed on a

substrate holder (1808) for surface analysis and vertically at 908

for a cross-sectional view (thickness). Magnification was set at

50003, 10 0003, 20 0003 and 50 0003 to examine the micro-

structure of the samples, with operating power of 3 kV and 5 kV.

Results and discussion

Compressive strength of OPC and RHA blended cement

concrete

After 28 days of curing under water, the specimens were

transferred into seawater for wetting–drying cycles. The general

trend presented in Figure 2 reveals that the strength of concrete

with RHA cement replacement initially increased during the

period 3–56 days. Thereafter, the strength decreased until the

specimens eventually disintegrated after 180 days. Meanwhile,

the compressive strength of the OPC concrete increased from 3 to

7 days and then slowly increased to 28 days. After 28 days of

wetting–drying cycles, the strength linearly increased to 56 days.

After 56 days of cycles, the strength of OPC concrete decreased

significantly. The strength reached the deteriorated stage after 180

days. Two mechanisms explain the increase in strength. First,

when pozzolanic and cementitious materials are added, calcium

hydroxide is transformed into additional calcium silicate hydrate

(C-S-H) gel. This transformation results in an increased strength

of blended cement. Second, the reaction of sulfate and chloride

ions with hydrated cement components induce ettringite forma-

tion, which contributes to the increase in strength of the OPC and

RHA blended cement concrete. However, the decrease in com-

pressive strength observed in this study was attributed to two

main factors

j deterioration resulting from the reaction of magnesium

sulfate with cement hydrates

j repetitive crystallisation cycles of MgSO4.nH2O by the

wetting and drying of the hardened pastes, which can produce

internal stresses in pores and lead to crack formation.

Thus, subjecting specimens to seawater wetting–drying cycles

can affect salt crystallisation, resulting in protective layer degra-

dation and facilitating the ingress of ions (Ganjian and Pouya,

2009; Kjellsen and Atlassi, 1999).

As shown in Figure 2, the RHA20 blended cement concrete

exhibited a significant increase in compressive strength and

increased resistance to seawater attack compared with the OPC

concrete and RHA10, RHA30 and RHA40 blended cement

concretes up to 56 days. However, at the age of 90 and 180 days,

RHA10 blended cement concrete significantly improved in

strength compared with the other concretes. For instance, at

28 days, the compressive strength of RHA20 blended cement

concrete was 53.30 MPa, whereas the compressive strengths of

OPC concrete and RHA10, RHA30 and RHA40 blended cement

concrete were 49.15, 45.10, 50.00 and 47.90 MPa, respectively.

These figures corresponded to compressive strength reductions of

7.78%, 15.38%, 6.19% and 10.13%, respectively. At age

180 days, the compressive strength of RHA10 blended cement

concrete subjected to seawater was 45.50 MPa, whereas the

values for OPC, RHA20, RHA30 and RHA40 were 43.7, 44.0,

42.0 and 41.5 MPa, respectively. These figures indicate that

compressive strengths varied by approximately 3.95, 3.29, 7.69

and 8.79%. Thus, incorporation of 10% and 20% of RHA in

blended cement helped produce the highest level of strength

compared with OPC, RHA30 and RHA40, even when subjected

to seawater wetting–drying cycles. The use of RHA blended

cement at 10–20% replacement showed the highest concrete

resistance against seawater attack. This finding suggests that the

RHA can be used to replace cement replacement in the construc-

tion of marine structures.

Chloride ion permeability

According to ASTM C1202-12 classification ranges (ASTM,

2012b) and the results summarised in Table 5, the addition of

RHA in cement concrete generally causes a decrease in chloride

permeability. The results also indicate that the Coulomb (C)

values sharply decrease as the replacement level increases. Total

21018015012090603030

35

40

45

50

55

0

Com

pres

sive

str

engt

h: M

Pa

Time: days (after subjected to seawater)

OPC

RHA10

RHA20

RHA30

RHA40

Figure 2. Compressive strength of OPC and RHA blended cement

concretes subjected to seawater wetting–drying cycles

4


chloride ion permeability is significantly reduced by the partial

replacement of cement with RHA up to 40%, even when

subjected to seawater wetting–drying cycles. Total chloride per-

meability for RHA40 blended cement concrete was considerably

reduced from 805 C to 518 C as exposure increased from 3 to

180 days, which shows that total chloride permeability decreased

by 35.65%. The decrease in chloride permeability with age can

be attributed to the continual hydration of cement products

(Kurian et al., 2010). According to Table 5, OPC concrete under

seawater attack during cyclic wetting–drying presented a ‘high’

risk of chloride ion permeability at 3, 7, 28, 56 and 90 days.

However, after 90 days of exposure, chloride ion permeability

changed from ‘high’ to ‘moderate’. Meanwhile, RHA reduced the

chloride ion permeability of concrete from a ‘moderate’ to a

‘very low’ rating from higher to lower replacement levels.

Generally, the chloride ion permeability of the concrete contain-

ing RHA was ‘low’ for all RHA levels at an early age and ‘very

low’ at later ages. The reduction in the chloride ion permeability

of specimens can be attributed to the crystallised formation of

calcium hydroxide and increased calcium or silica ratio (Nehdi et

al., 2003).

Differential thermal analysis and thermogravimetric

analysis

In a marine environment, chloride and sulfate ions from seawater

can penetrate into concrete (Abdelkader et al., 2010). Generally,

chloride ions are attributed to the formation of Friedel’s salt

(Islam et al., 2010), whereas sulfate ions react with calcium

hydroxide to form gypsum, which then reacts with hydrated

calcium aluminate to produce secondary ettringite (Marchand et

al., 2002).

In this study, the formation of Friedel’s salt, calcium hydroxide,

gypsum and ettringite were evaluated by thermal analysis. The

results of TGA and DTA of specimens subjected to seawater

showed a similar pattern for OPC and RHA blended cement

pastes and therefore a 10% replacement level of RHA was

selected for discussion. As shown in Figure 3, five endothermic

peaks were generally observed.

j The first endothermic peak, which was detected at 67.63–

110.788C with a peak at 82.938C and a 5.06% weight loss,

can be associated with the dehydration of ettringite.

Santhanam et al. (2002) and Wee et al. (2000), respectively,

found endothermic peaks at 80 and 1008C, caused by the

dehydration of ettringite.

j The second endothermic peak, at 100.81–147.048C with a

peak at 115.848C and a 2.38% weight loss, results from the

dehydration of gypsum. The findings of Li et al. (2000)

showed that the dehydration of gypsum formation is indicated

by an endothermic peak at 110–1308C.

j The third endothermic peak was observed between 254.708C

and 362.168C, with a peak at 295.358C and 2.30% weight

loss; this can be attributed to the decomposition of calcium

silicate hydrate (Saikia et al., 2006), calcium aluminate

hydrate (Zornoza et al., 2009) and Friedel’s salt (Csizmadia et

al., 2001; Vedalakshmi et al., 2008). The endothermic peak

corresponding to brucite (small peak) was also identified and

verified by the DTA curve. The endothermic peak

corresponding to brucite occurred at temperatures ranging

from 350.108C to 400.098C. The weight loss corresponding to

RHA: % 3 Days 7 Days 28 Days 56 Days 90 Days 180 Days

CIP: C Risk CIP: C Risk CIP: C Risk CIP: C Risk CIP: C Risk CIP: C Risk

0 4732 H 4551 H 4209 H 4151 H 4001 H 3654 M

10 2210 M 2150 M 2117 M 2059 M 1703 L 1674 L

20 1169 L 1157 L 1133 L 1129 L 1106 L 1091 L

30 1081 L 1064 L 854 VL 835 VL 818 VL 815 VL

40 805 VL 800 VL 789 VL 762 VL 531 VL 518 VL

Table 5. Chloride ion permeability (CIP) and risk of penetration

(H, high; M, moderate; L, low; VL, very low) of OPC and RHA

blended cement concretes

1

2

3

4

1000800600400200�60

�50

�40

�30

�20

�10

0

0

20

40

60

80

100

120

0

Der

iv. w

eigh

t: %

.min

Wei

ght:

%

Temperature: °C

5DTA

TGA

Figure 3. TGA and DTA curves of 10% RHA cement paste fired to

various temperatures

5


brucite was 0.73%, with a peak appearing at approximately

374.858C.

j The fourth endothermic peak was identified at 410.86–

459.698C, with a peak in the region of 435.268C and a 3.35%

weight loss; this can be attributed to the dehydroxylation of

calcium hydroxide.

j Finally, the endothermic peak between 656.31 and 743.608C

with a peak at 714.838C and a weight loss of 8.75% is as a

result of calcium carbonate from the seawater.

Tables 6 and 7 indicate thermogravimetric mass losses corre-

sponding to the ettringite and gypsum contents of the specimens.

A weight loss trend is observed with increasing ettringite and

gypsum formation as the exposure period is also increased. The

results also show that the weight losses of the control specimens

were higher than those found in RHA blended cement paste,

increasing with time up to 180 days. For instance, the weight loss

of the control OPC specimens was 3.76% at the end of 3 days,

increasing to 5.32% at 180 days. For RHA10, the weight losses

are 3.20% and 5.06% at the end of 3 and 180 days, respectively.

The inclusion of RHA as cement replacement apparently reduces

the overall amount of C3A in the system and consequently the

amount of ettringite and gypsum that can form on seawater attack

is also decreased. Calcium hydroxide may be reduced when

ground RHA is used as partial replacement of cement and further

decreased when the replacement levels of RHA are increased.

This finding suggests that RHA blended cement paste contains

more silicon dioxide than OPC. Thus, OPC becomes less effec-

Mix Age: days Temperature: 8C Peak

temperature: 8C

Weight loss: % Onset

temperature: 8C

Final

temperature: 8C

OPC 3 64.18–116.87 76.86 3.76 60.61 81.31

7 63.80–113.40 80.64 3.96 69.43 87.50

28 68.88–110.26 86.06 4.27 74.94 91.61

56 64.97–106.75 79.18 4.51 75.31 84.98

90 59.77–114.19 75.99 5.07 68.53 84.15

180 58.51–118.87 79.02 5.32 70.86 84.35

RHA10 3 64.67–100.60 75.00 3.20 69.04 82.64

7 60.59–101.02 74.40 3.73 68.72 81.55

28 68.04–108.42 79.73 4.00 72.75 83.42

56 61.70–106.81 73.77 4.18 66.06 73.14

90 67.14–110.59 79.47 4.86 72.95 87.78

180 67.63–110.78 82.93 5.06 73.40 89.11

Table 6. Thermogravimetric mass losses of OPC and RHA cement

paste, endothermic peak corresponding to ettringite

Mix Age: days Temperature: 8C Peak

temperature: 8C

Weight loss: % Onset

temperature: 8C

Final

temperature: 8C

OPC 3 114.55–154.38 128.20 1.92 122.46 138.79

7 109.82–156.90 129.32 2.09 114.11 134.75

28 117.36–162.22 133.73 2.33 127.14 143.90

56 119.58–165.29 137.18 2.35 127.67 143.54

90 113.13–158.53 128.93 2.63 115.27 135.30

180 110.28–162.73 141.47 2.86 135.51 142.90

RHA10 3 113.46–145.40 127.31 1.30 126.36 132.26

7 109.96–141.21 130.70 1.31 112.74 132.00

28 109.37–149.26 128.61 1.50 116.09 130.74

56 109.34–151.10 131.21 1.68 117.38 135.85

90 110.60–163.84 127.79 2.20 110.30 138.61

180 100.81–147.04 115.84 2.38 121.96 132.25

Table 7. Thermogravimetric mass losses of OPC and RHA cement

paste, endothermic peak corresponding to gypsum

6


tive in reducing calcium hydroxide formation because of the

lower silicon dioxide content. According to Zhang and Malhotra

(1996), calcium hydroxide and calcium silicate hydrate are the

major hydration and reaction products in RHA blended cement.

Thus, concrete containing RHA had a lower calcium hydroxide

content than the control specimen because of the pozzolanic

reaction. Finally, RHA mixes exhibit the best performance against

seawater attack.

X-ray diffraction

Figure 4 shows the XRD pattern of OPC and RHA blended

cement pastes subjected to seawater by wetting–drying cycles,

and reveals relatively large intensity peaks for calcium hydroxide

in the control specimen paste cured in seawater. Furthermore, as

also shown in the figure, a large quantity of calcium hydroxide

was converted to gypsum or brucite. The presence of a calcite

phase in the examined pastes implies a carbonation effect on the

surface of specimens, which is less pronounced in the mixes with

RHA. Meanwhile, the ettringite phase was detected in pastes both

with and without RHA. As shown in Figure 4, the peak

corresponding to ettringite was observed at 2Ł ¼ 118. The

ettringite was formed by the reaction of gypsum and calcium

aluminate. In this study, the specimen was found to contain small

amounts of brucite (with the peak at approximately 188 2Ł). The

combination of gypsum and brucite was also detected at 278 2Ł.

Skaropoulou et al. (2006) reported on the reaction of calcium

hydroxide with magnesium sulfate to form gypsum and brucite.

The XRD analysis also revealed that gypsum and ettringite peaks

in each specimen were strong, indicating that these formations

are the main corrosion products. However, the addition of RHA

in the blended cement reduced the deterioration of concrete due

to seawater attack. The replacement of cement with RHA in

pastes exposed to seawater also exhibited lower ettringite,

gypsum and brucite intensity peaks compared with those of the

OPC paste, as shown in Figure 4. As expected, the RHA pastes

contain more silicon dioxide than the control specimen. RHA is

assumed to be the reactant to produce secondary calcium silicate

hydrate by consuming calcium hydroxide. The formation of

pozzolanic gel begins during the hardening process. The de-

creased calcium hydroxide content of the cement matrix and the

increased amount of calcium silicate hydrate gel, together with

the filler effect of RHA, contribute to the protection of concrete

against seawater attack.

Scanning electron microscopy

The microstructures of OPC and RHA blended cement pastes

subjected to seawater wetting–drying cycles were observed using

SEM. In addition, energy dispersive X-ray (EDX) analysis was

conducted on the samples to determine the existing compounds

after reactions with seawater. Figure 5 shows the EDX analysis of

the OPC paste exposed to seawater. The main elements present were

magnesium, silicon, sodium, aluminium, chlorine, calcium and

iron. A silicon peak was clearly observed, indicating the presence of

calcium silicate hydrate in the paste. The SEM images of the OPC

paste further show various microstructure formations such as

calcium hydroxide, calcium silicate hydrate gel, gypsum and

ettringite, as shown in Figure 6. As noted earlier, seawater attack on

concrete has been attributed to the reaction of magnesium sulfate

with calcium hydroxide, resulting in the formation of gypsum and

magnesium hydroxide. The resulting gypsum reacts with calcium

OPC RHA10 RHA20 RHA30 RHA40

908070605040302010

Cc

02 : degreesθ

Et

CH

CH

Et/Gp

Cc

CcGp/Gp

Et/Gp

Gp

Gp/Br

CH/Cc

Gp/Et Cc

Gp

Gp/EtCc

Gp

Figure 4. XRD patterns of OPC paste subjected to seawater

wetting–drying cycles: Br, brucite; Cc, calcite; CH, calcium

hydroxide; Et, ettringite; Gp, gypsum

7


hydroxide and forms ettringite. As shown in Figure 6, ettringite

exists as long slender needles, calcium hydroxide is precipitated as

hexagonal plates, the calcium silicate hydrate gel exhibits a fine

layered network and the gypsum comprised elongated rod-shaped

formations. Calcium silicate hydrate gel is known to exhibit a

layered structure (Moon et al., 2007). The presence of all forma-

tions was verified by TGA/DTA and XRD analysis.

Figure 7 illustrates the microstructure of 10% RHA blended

cement paste under seawater attack. The morphology of the flaky

calcium silicate hydrate crystals can be attributed to the com-

bined effects of chloride and sulfate ions in a seawater environ-

ment. In cement chemistry, C, S and H represent CaO, SiO2 and

H2O, respectively. As shown in Figure 8, EDX analysis indicates

that the Si and Ca peaks were the strongest. The results also

indicate that the lower peaks of magnesium hydroxide and

chloride in RHA mixes are attributed to the lower calcium

hydroxide resulting from pozzolanic reaction. Therefore, the use

of RHA in blended cement may protect concrete from deteriora-

tion as a result of seawater attack during wetting–drying cycles.

The replacement of cement with RHA reduces the amount of

calcium hydroxide in the paste and decreases gypsum and

ettringite formation (a reduction in calcium hydroxide often leads

to reduced gypsum and ettringite formation (Yigiter et al.,

2007)).

ConclusionsThe following conclusions are drawn from the data obtained from

this study.

j The compressive strength of RHA blended cement concrete

subjected to wetting–drying cycles in seawater for a period of

up to 180 days can be classified into two stages – an initial

increase during the period 3–56 days followed by a decrease

until specimen disintegration after 180 days. However, the

compressive strength of OPC concrete can be split into four

stages – an increase between 3 and 7 days, a slow increase

up to 28 days, a linear increase up to 56 days and

deterioration after 180 days. These findings may be attributed

to the reaction of MgSO4 with cement hydrates and the

repetitive crystallisation cycles of MgSO4.nH2O by wetting

and drying.

Ettringite

Gypsum

Ca(OH)2

Ettringite

C-S-H gel

Figure 6. Microstructure of OPC paste subjected to seawater

wetting–drying cycles

Ettringite

C-S-H gel

Gypsum

C-S-H crystals

C-S-H gel

Figure 7. Microstructure of 10% RHA blended cement paste

subjected to seawater wetting–drying cycles

0

69

138

207

1 2 3 4 5 6 7

Mg

Ca

Si

Na Al Cl Fe

Inte

nsity

(a.u

.)

Energy: KeV

Figure 8. EDX analysis of 10% RHA blended cement paste

subjected to seawater wetting–drying cycles

0

66

132

1 2 3 4 5 6 7

Mg

Si

NaAl

Cl

Ca

FeInte

nsity

(a.u

.)

Energy: KeV

Figure 5. EDX analysis of OPC paste subjected to seawater

wetting–drying cycles

8


j The pozzolanic reaction that results from the addition of

RHA reduces the content of calcium hydroxide formed

during hydration, which consequently reduces the leaching of

lime and the content of gypsum and ettringite during seawater

attack. The durability of the concrete is thus enhanced. This

finding suggests that RHA could be used as an effective

mineral addition in the design of durable concrete structures.

j Concrete containing 10% and 20% RHA replacement levels

exhibits excellent durability to seawater attack. The test

results also indicate that the amount of calcium hydroxide in

RHA blended cement was lower than that of Portland cement

because of the pozzolanic reaction of RHA.

AcknowledgementSupport from Universiti Sains Malaysia in the form of a research

grant for this study is greatly appreciated.

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Strength and microstructure analysis of concrete containing rice husk ash under seawater attack by wetting and drying cycles