The effects of high volume nano palm oil fuel ash on microstructure properties and hydration temperature of mortar

Construction and Building Materials 93 (2015) 29–34

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

The effects of high volume nano palm oil fuel ash on microstructure properties and hydration temperature of mortar Nor Hasanah Abdul Shukor Lim a, Mohamed A. Ismail b,⇑, Han Seung Lee b, Mohd Warid Hussin c, Abdul Rahman Mohd. Sam c, Mostafa Samadi a a b c

Construction Material Research Group (CMRG), Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia Dept. of Architectural Engineering, Hanyang University, Ansan, Republic of Korea Construction Research Centre (UTM CRC), Institute for Smart Infrastructure and Innovative Construction, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia

h i g h l i g h t s  High volume of nano POFA as cement replacement improves strength.  Nano POFA reduces hydration temperature of massive concreting in early age.  Treatment of POFA increases pozzolanic properties and activity index.

a r t i c l e

i n f o

Article history: Received 3 February 2015 Received in revised form 12 May 2015 Accepted 15 May 2015

Keywords: Microstructure Palm oil fuel ash Mortar strength High volume Hydration temperature

a b s t r a c t The effect of high volume nano palm oil fuel ash in the mortar was investigated. This study covers basic properties like the morphology of the composite, the hydration temperature, strength activity index, thermal conductivity and microstructure properties with regards to the variations in the mix design process of mortar. The effects of fineness of the ash on the strength properties of mortar were also investigated. To get a better performance in terms of strength development, the ash used has gone through heat treatment and was ground up to less than 1 lm. The incorporation of more than 80% nano size palm oil fuel ash as cement replacement has produced a mortar having a compressive strength more than OPC mortar at a later age. The overall results have revealed that the inclusion of high volume nano palm oil fuel ash can produce a mortar mix with high strength, good quality and most importantly that is more sustainable. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Affordable products with advanced properties are necessary towards the higher human development and sustainable economic growth. Therefore, reusing the abundant waste materials has become necessary especially waste coming from palm oil manufacturing. It is estimated that ‘‘the total potential palm biomass from 4.69 million hectares of palm oil planted area in Malaysia in 2009 is 77.24 million tonnes per year comprising of 13.0 million tonnes of Oil Palm Trunks (OPT), 47.7 million tonnes of Oil Palm Fronds (OPF), 6.7 million tonnes of Empty Fruit Bunches (EFB), 4.0 million tonnes of Palm Kernal Shell (PKS) and 7.1 million tonnes of Mesocarp Fibre (MF) (all dry weight)’’ [1,2]. These wastes are usually used as fuel in palm oil mill to generate electricity [3] and after ⇑ Corresponding author. Tel.: +82 31 4005181; fax: +82 31 4368169. E-mail address: [email protected] (M.A. Ismail). http://dx.doi.org/10.1016/j.conbuildmat.2015.05.107 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

the combustion in the boiler there are approximately 5% of ash generated [4] and another solid waste being produced. The need towards sustainability and sustainable environment has made the use of pozzolanic material in mortar popular. One of the latest additions of pozzolanic material is palm oil fuel ash (POFA) [5–7]. This POFA is the source of silicate that produced after the combustion of palm oil fibre, shell and mesocarp as fuel to generate electricity [8]. Few studies were done by other researchers on the replacement of partial weight of cement by POFA [9,10], but there is still high amount of ash abundant in the landfill which lead to environmental problems. It is reported that the maximum strength gain occurred at the replacement level of 30% with the size of 45 lm but further increment in the ash content would reduce the strength of mortar gradually [5]. Besides, POFA was also used as nano filler [11]. The hydration temperature of mortar describes the hardening behaviour of it. The mix properties and component of materials

30

Nor Hasanah Abdul Shukor Lim et al. / Construction and Building Materials 93 (2015) 29–34

used in mortar have too much effect on hydration temperature. The heat released in hydration process of mortar containing blended ash is originating from two reactions firstly heat released from hydration of cement and secondly heat released from admixture reactions [12]. Therefore, exploring POFA as nano material would create an advanced waste material. In this paper, high volume of POFA with size less than 1 lm was used as cement replacement up to 80%. This helps to reduce the hydration temperature and carbon dioxide gasses emission from cement production process.

Table 1 Mix proportions of mortar. Materials (kg/m3)

Binder

OPC GPOFA UPOFA Fine aggregate w/b ratio

Mortar mix OPC mortar

GPOFA mortar

UPOFA mortar

525 – – 1578 0.4

105 420 – 1578 0.4

105 – 420 1578 0.4

Activity index ðAIÞ ¼ ðA=BÞ  100

2. Materials and methods 2.1. Materials The cement used in this study complies with Portland cement Type I as stated in the ASTM C 150-12 [13]. Palm oil fuel ash (POFA) was obtained from the burning of palm oil shell and husk (in equal volume) from a southern part of Malaysia. The collected ashes were dark in colour and the losses on ignitions (LOI) were 20.9% for ground POFA. The POFA was then dried in the oven for 24 h at 105 ± 5 °C and sieved through a 150 lm size sieve to remove coarser particles. Then the POFA was ground up until 90% passing 45 lm sieve according to ASTM C618-12 [14] and mention in this paper as GPOFA for ground POFA. Meanwhile, the POFA was then heated to 500 °C for 1 h in a furnace to remove the excessive unburned carbon [15,16]. The carbon content is an important factor to consider. These particles result in the increasing of water demand because it is absorbed by carbon particles [15,17]. Then it was subjected to further grinding using ball mill until it reaches the meridian size of less than 1 lm and in this paper refer to UPOFA for ultrafine POFA. To ensure the uniformity and fineness of POFA, all the treatment processes were controlled [16]. In the preparation process for all specimens, the fine aggregates were used in the saturated dried surface condition. The fine aggregate was sieved through 2.35 mm sieve and retained at 300 lm before storing in the airtight container. Fig. 1 shows the sieve analysis test on the fine aggregates. The grading curve for fine aggregates was within the limit line prescribed according to ASTM C33-03[18]. 2.2. Testing procedures All mortar specimens were prepared with sand to binder with the ratio of 3:1, whereby the sand was prepared into saturated surface dried condition. The mixing was carried out in a room temperature of approximately 28 ± 2 °C [19]. The mix proportions are given in Table 1 based on weight of materials according to BS EN 998-1:2010 [20]. The test specimens of 70  70  70 mm cubes were prepared. The specimens were compacted in two-layer with rod tamping as described in ASTM C109-13 [21]. Additional vibration of about 10 s was applied using the vibrating table. The test specimens were cured in water for 7, 14 and 28 days. The morphology of UPOFA was investigated by using Field Emission Scanning Electron Microscopy (FESEM). The surface of the specimens obtained from the compressive strength test was coated with gold prior to their morphological observation. A thermo gravimetric analysis (TGA) was carried out to investigate the thermal stability of the composites. The powder (about 25 ± 5 mg) was heated at a heating rate of 20 °C/min from 30 to 800 °C under nitrogen. Fourier transforms infrared spectroscopy (FTIR) which is a tool for qualitative and quantitative materials and identification of the composite group and chemical bonding in mortar specimens. The activity index for UPOFA and GPOFA were checked for pozzolanic materials. As prescribed by ASTM C 311-13 [22], activity index is defined as:

3. Results and discussions 3.1. Chemical and physical properties The chemical and physical properties of OPC, GPOFA and UPOFA are shown in Tables 2 and 3, respectively. It reveals that the OPC and POFA have similar characteristics base on the chemical composition. UPOFA sample contains higher percentage of silica content than GPOFA and OPC. Obviously, the presence of higher silica content influences the pozzolanic reaction to produce extra calcium silicate hydroxide gels thus makes the mortar more durable and denser. The findings show that the GPOFA used is classified as Class C pozzolan meanwhile UPOFA as Class F pozzolan [14] which conform to the observations made by a previous research [23]. Results show that the heat treatment reduced the LOI of POFA from 20.9% to 1.3% hence removed the unburned elements in the POFA. The fineness of UPOFA is 146% more than OPC used in this study. The percentage of particle retain on sieve size 45 micron are 4.5, 4.9 and 0.13 for OPC, GPOFA and UPOFA, respectively. Thus, the UPOFA have large surface area and significantly smaller particle size in comparing with OPC and GPOFA.

Percentage retained (%)

100 90 80 70

fine aggregate

60

lower limit

upper limit

50 40 30 20 10 0 1

10 Sieve size (µm) Fig. 1. Sieve analysis of fine aggregate.

ð1Þ

where A = average compressive strength of POFA mortar cube and B = average compressive strength of control mortar cube. The compressive strength of the mortar cubes was determined using a 3000 kN compression machine according to ASTM C109-13 standards [21]. The test was performed on mortar cubes at the ages of 7, 14 and 28 days. For hydration temperature measurement, plywood with the size of 300  300  450 mm cube was used as the exterior mould. It was packed with 76 mm thick polystyrene acting as the insulator. Each mortar mix was cast into PVC pipe with the size of 150 mm diameter and 300 mm heights. The same method was used by other researchers [10,23] to determine the hydration temperature. A thermocouple (Type K) was inserted into the centre of each box and was connected to a data logger system. Recording the temperature was continued up to 7 days for the whole mortar types. Fig. 2 shows the equipment used in the test.

Fig. 2. Test equipment for measuring hydration temperature.

31

Nor Hasanah Abdul Shukor Lim et al. / Construction and Building Materials 93 (2015) 29–34 Table 2 Chemical composition of OPC, GPOFA and UPOFA. Chemical composition (%)

OPC

GPOFA

UPOFA

SiO2 Al2O3 Fe2O3 CaO K2O MgO CO2 SO3 LOI

16.40 4.24 3.53 68.30 0.22 2.39 0.10 4.39 2.4

53.5 1.9 1.1 8.3 6.5 4.1 0.30 2.36 20.9

69.3 5.30 5.10 9.15 11.10 4.10 0.10 1.59 1.3

(a) before treatment (GPOFA)

(b) after treatment (UPOFA)

Fig. 4. POFA colour before and after treatment. Table 3 Physical properties of OPC, GPOFA and UPOFA. Physical properties

OPC

GPOFA

UPOFA

5.4

Specific gravity Particle retained on 45 lm sieve Median particle d10 Median particle d50 Blaine fineness (cm3/g)

3.15 4.58 – – 3999

2.42 4.98 1.69 14.58 4935

2.56 0.13 0.02 1.10 7205

5.3

Weight (mg)

3.2. Morphological structure of POFA

5.1 5 4.9 4.8

The morphological structure of UPOFA sample was examined using Field Emission Scanning Electron Microscope (FESEM) technique. UPOFA consists of thinner, irregular and crushed particles as a typical microstructure of the ash being shown in Fig. 3. It can be observed that POFA has irregular, thinner and crushed particle having clustered arrangement of the spherical particles with little air space between them [9,10]. 3.3. Thermogravimetric analysis The colour of POFA changed from dark black to greyish brown after the carbon particles were removed as shown in Fig. 4 and this is similar to the findings observed by Altwair et al. [24]. Fig. 5 shows the TGA result for the POFA used. The weight loss in the temperature of 100 ± 5 °C was caused by the evaporation of moisture content. The decarbonation of POFA occurs between temperatures of 400 and 600 °C. This may be due to the calcium carbonate decomposition. From the figure, 0.3578 mg of CO2 evaporated during the process as shown below:

CaCO3 ðsÞ ! CaO ðsÞ þ CO2 ðgÞ

Content 6.8391% 0.3578 mg

5.2

ð2Þ

4.7 4.6 0

100

200

300

400

500

600

700

800

900

1000

Temperature (ºC) Fig. 5. TGA results for POFA.

Since the suitable temperature for heat treatment is between 400 and 600 °C, in this study, POFA was heated up to 500 °C for the duration of only 1 h [14,15].

3.4. Fourier transforms infrared spectroscopy (FTIR) results Fig. 6 shows the FTIR result of UPOFA and GPOFA. As shown in the figure, there is large difference between the treated and untreated POFA. The water component of O–H stretching band at 3465 cm1 and the bending of the chemically bonded water, H– O–H at 1650 cm1 were altered by the exposure to the treatment process. The Si–O asymmetric stretching vibration mode at 1040 cm1 also showed high shift after treatment. This suggested that the treatment process alters the chemical structures and

Si-O GPOFA UPOFA

H-O-H

O-H

3900

3400

2900

2400

1900

1400

Wavenumbers (cm-1) Fig. 3. Field Emission Scanning Electron Micrograph of POFA. Fig. 6. FTIR spectra of UPOFA and GPOFA.

900

400

32

Nor Hasanah Abdul Shukor Lim et al. / Construction and Building Materials 93 (2015) 29–34

Table 4 Strength activity index results.

50

GPOFA

UPOFA

7 days 28 days

75 84

97 105

increases the chemical composition of the materials. The value of silica content increased from 0.7 to 3.4 absorbance.

OPC

45

POFA(N)

Temperature (°C)

Activity index (%)

POFA(B) 40

35

3.5. Strength activity index 30

Table 4 and Fig. 7 show the compressive strength and the strength activity indices of OPC and POFA mortars at the given ages. The strength activity index of UPOFA and GPOFA was calculated based on the control sample (OPC mortar). The strength activity index for 7 days of UPOFA and GPOFA were 97% and 75% of OPC, respectively, which satisfied ASTM C 311-13 [22]. At the age of 28 days, UPOFA exhibits 105% of strength activity index of the OPC which is caused by the pozzolanic reaction. The data clearly showed that UPOFA increased the compressive strength even at early ages as compared to GPOFA. This may be due to the fineness of the particle and less amount of carbon content of UPOFA. However, the activity index at 7 days is slightly lower than the control sample. This may be due to dilution effect and delayed onset of the pozzolanic reaction of Ca(OH)2 with POFA [25]. The UPOFA mortar gives more percentage than the control sample at 28 days. Again, this showed that the packing effect of small particle size and pozzolanic reaction was fully involved [26]. 3.6. Heat of hydration test The results of variation of the temperature versus time were recorded for the different mortar types and presented in the Table 5. A peak temperature of 50 °C was recorded for OPC mortar at 9 h while 34.1 °C and 34.7 °C were recorded at 11 h for GPOFA and UPOFA, respectively. The development of temperature was also obtained in the mid depth of mortar during the hydration process for all mortar samples and the results are shown in Fig. 8. It has been observed that during the initial reading, the increase in the temperature for the GPOFA and UPOFA mortar was almost similar. However, the comparison of the results of hydration temperature in the control sample and the other two types of mortar was remarkably different. The use of POFA could successfully reduce the total temperature rise compared to the normal OPC mortar regardless the size of POFA. It has been researched that the fineness

25 0

20

40

60

80

100

120

140

Fig. 8. The development of hydration temperature.

Table 5 The hydration temperature of OPC, GPOFA and UPOFA mortar. Properties

OPC

GPOFA

UPOFA

Initial temperature (°C) Peak temperature (°C) Time since mixing to peak temperature (h)

28.6 50.0 9

28.7 34.1 11

29.4 34.7 11

of binder has an effect on the rate of heat development of samples to some extent [10,26]. In the case of the reduction in hydration temperature, GPOFA and UPOFA mortar performed more effectively than the OPC mortar, especially in the early steps where the peak in the temperature was measured. This was also reported by Sata et al. [27] for the normal concrete mixed with POFA. They have observed a decreased of 30% temperature in mortar incorporating with 80% POFA as compared to OPC mortar. The hydration temperature of POFA mortar depends on the amount of calcium hydroxide Ca(OH)2 in the hydration process of cement and reactivity of materials used. Hydration of cement was affected physically and chemically by adding other pozzolanic materials [12,28–30]. Fig. 9 shows the compressive strength of OPC, GPOFA and UPOFA mortars. The results show that the compressive strength of mortar increased with the increasing age of curing. However, using high volume UPOFA shows higher result compared to others at all curing periods. This is due to the higher silica content after 50 45

110 UPOFA

GPOFA

Compressive strength (MPa)

105

OPC

Activity index (%)

100 95 90 85 80

OPC

80% UPOFA

80% GPOFA

40 35 30 25 20 15 10 5

75

0

70 7

14

Age (days) Fig. 7. Strength activity index of mortar.

28

160

Time after casting (hr)

7

14 Age (days)

Fig. 9. Compressive strength of mortars.

28

33

Nor Hasanah Abdul Shukor Lim et al. / Construction and Building Materials 93 (2015) 29–34

(a)

(b)

Fig. 10. Field Emission Scanning Electron Microscope (FESEM) of (a) GPOFA and (b) UPOFA.

UPOFA mortar Q G E

Q – Quartz E – Ettringite G – Gypsum Ca(OH)2 – Calcium Hydroxide

G

Ca(OH)2

Q E

Q Ca(OH)2

GPOFA mortar

G

Q G E

Q E

Ca(OH)2

Q

Ca(OH)2

Fig. 11. XRD analysis of UPOFA and GPOFA mortars at 28 days.

the treatment process that helps the pozzolanic activities. The hydration process was also examined by FESEM to study the paste morphology. Fig. 10 shows the FESEM for mortar containing GPOFA and UPOFA. The figure shows that the fine UPOFA materials act both as a binder and filler in the mortar [31,32]. Therefore, there are fewer voids that can be observed in UPOFA specimens and more calcium silicate hydrate crystal (C–S–H) which makes the mortar more durable and denser. Fig. 11 exhibits the XRD patterns for the UPOFA and GPOFA mortar after 28 days curing. The amount of Ca(OH)2 in the pastes point out its consumption in the pozzolanic reaction. More Ca(OH)2 is consumed during pozzolanic reaction as the UPOFA contents increase because of the higher content of amorphous SiO2 available to react with Ca(OH)2. This reaction is more effective with high amount of Ca(OH)2, which comes from the hydration process of OPC. The addition of UPOFA resulted in an increase in C–S–H, detected along the 2 Theta axis between 13.7 and 50.5 °C of 2 Theta. The XRD graphs and SEM images confirmed this reaction. In addition, the rate of pozzolanic reaction may be influenced by the fineness of the UPOFA [33] (i.e., the fineness of POFA affected the production rate of C–S–H) [24]. Moreover, the fine POFA provides more silica for pozzolanic reaction because it has a larger surface area.

4. Conclusions The following conclusions have been drawn from the experimental results of this research: (1) High volume POFA with some modification can be used up to 80% as cement replacement and achieve higher strength than OPC mortar at a later age. (2) Up to 105% of strength activity index can be achieved when POFA is ground to a smaller size. (3) The fineness size of POFA which is less than 1 lm gives a better effect to the mortar both as binder and filler. (4) More than 80% nano palm oil fuel ash can be used to produce mortar with an increased strength than normal mortar. (5) High volume UPOFA reduces the heat of hydration of mortar and can be used particularly for mass mortar application where thermal cracking due to extreme heat rise is of great concern. Acknowledgements The authors are grateful to the Ministry of Education, Malaysia (MOE) and Research Management Centre (RMC), Universiti Teknologi Malaysia (UTM) for financial support under grant:

34

Nor Hasanah Abdul Shukor Lim et al. / Construction and Building Materials 93 (2015) 29–34

Q.J.130000.2522.03H36 and Q.J.130000.2509.06H56. The authors are also thankful to the staff of Materials & Structures Laboratory, Faculty of Civil Engineering for the facilities and support for the experimental work. References [1] Foo YN, Foong KY, Basiron Y, Sundram K. A renewable future driven with Malaysian palm oil based green technology. J Oil Palm Environ (JOPE) 2011;2:1–7. [2] Ronald Z. Opportunities and challenges in the development of a viable Malaysian palm oil biomass industry. J Oil Palm Environ (JOPE) 2013;4:41–6. [3] Hashim MA, Chu KH. Adsorption of metal ions on oil palm ash: application of a surface reaction rate model. In: Proceedings of 9th Asian Pacific confederation of chemical engineering congress. Christchurch, New Zealand. 2002. [4] Tay JH, Show KY. Use of ash derived from oil-palm waste incineration as a cement replacement material. Resour Conserv Recycl J 1995;13:27–36. [5] Hussin MW, Abdul Awal. Influence of palm oil fuel ash on strength and durability of concrete. In: Proceedings of the 7th international conference on durability of building materials and components, Stockholm, Sweden 19–22 May; 1996:1: p. 291–98. [6] Abdul Awal, ASM. A study of strength and durability performances of concrete containing palm oil fuel ash [Ph.D. thesis]. University Teknologi Malaysia; 1998. [7] Sumadi SR. Relationships between engineering properties and microstructural characteristics of mortar containing agricultural ash [Ph.D. thesis]. Universiti Teknologi Malaysia; 1993. [8] Abdul Khalil HPS, Fizree HM, Bhat H, Jawaid M, Abdullah CK. Development and characterization of epoxy nanocomposites based on nano-structured oil palm ash. Compos Part B: Eng 2013;53:324–33. [9] Awal ASMA, Abubakar SI. Properties of concrete containing high volume palm oil. Malaysian J Civil Eng 2011;23(2):54–66. [10] Taha M, Mohammad I, Salihuddin RS, Bhutta MAR, Mostafa S, Seyed MS. Binary effect of fly ash and palm oil fuel ash on heat of hydration aerated concrete. The Sci World J 2014:461241. [11] Abdul Khalil HPS, Fizree HM, Jawaid M, Allatas OS. Preparation and characterization of nanostructured materials from oil palm ash: a bio agricultural waste from oil palm mill. Bioresour Technol 2011;6:4537–46. [12] Wang XY, Lee HS. Modeling the hydration of concrete incorporating fly ash or slag. Cem Concr Res 2010;40(7):984–96. [13] ASTM C 150. Standard specification for Portland cement. West Conshohocken: ASTM, International; 2012. [14] ASTM C618. Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete. West Conshohocken: ASTM International; 2012. [15] Chandara C, Sakai E, Mohd Azizli KA, Ahmad ZA, Saiyid Hashim SF. The effect of unburned carbon in palm oil fuel ash on fluidity of cement pastes containing superplasticizer. Constr Build Mater 2010;24:1590–3.

[16] Megat Johari M, Zeyad M, Muhamad Bunnori N, Ariffin KS. Engineering and transport properties of high-strength green concrete containing high volume of ultrafine palm oil fuel ash. Constr Build Mater 2012;30:281–8. [17] Sata V, Jaturapitakkul C, Kiattikomol K. Influence of pozzolan from various byproduct materials on mechanical properties of high-strength concrete. Constr Build Mater 2007;21:1589–98. [18] ASTM C33. Standard specification for concrete aggregates. Conshohocken: ASTM International; 2003. [19] Ariffin MAM, Bhutta MAR, Hussin MW, Mohd Tahir M, Aziah N. Sulfuric acid resistance of blended ash geopolymer concrete. Constr Build Mater 2013;43:80–6. [20] British Standards Institution. Specification for mortar for masonry. Rendering and plastering mortar. BS EN 998-1:2010. [21] ASTM C109. Standard test method for compressive strength of hydraulic cement mortars (using 2-in. or [50-mm] cube specimens). West Conshohocken: ASTM International; 2013. [22] ASTM C311. Standard test methods for sampling and testing fly ash or natural pozzolans for use in Portland-cement concrete. West Conshohocken: ASTM International; 2013. [23] Awal ASMA, Hussin MW. Effect of palm oil fuel ash in controlling heat of hydration of concrete. The twelfth east Asia-Pacific conference on structural engineering and construction. Proc Eng 2011;14:2650–7. [24] Altwair NM, Azmi M, Johari M, Fuad S, Hashim S. Strength activity index and microstructural characteristics of treated palm oil fuel ash. Int J civil Environ Eng 2011;10:100–7. [25] Jaturapitakkul C, Kiattikomol K, Songpiriyakij S, Ash F. A study of strength activity index of ground coarse fly ash with Portland cement. Sci Asia 1999;25:223–9. [26] Awal ASMA, Shehu IA. Evaluation of heat of hydration of concrete containing high volume palm oil fuel ash. Fuel J 2013;105:728–31. [27] Sata V, Jaturapitakkul C, Kiattikomol K. Utilization of palm oil fuel ash in highstrength concrete. J Mater Civil Eng 2004;16(6):623–8. [28] Walter AG, John AD. Filler cement: the effect of the secondary component on the hydration of Portland cement: Part I. A fine non-hydraulic filler. Cem Concr Res 1990;20:778–82. [29] Walter AG, John AD. Filler cement: the effect of the secondary component on the hydration of Portland cement: Part 2: fine hydraulic binders. Cem Concr Res 1990;20:853–61. [30] Martin C, Philippe L, Erick R. Mineral admixtures in mortars: quantification of the physical effects of inert materials on short-term hydration. Cem Concr Res 2005;35:719–30. [31] Bamaga SO, Hussin MW, Ismail MA. Palm oil fuel ash: promising supplementary cementing materials. KSCE J Civil Eng 2013;17:1708–13. [32] Tangchirapat W, Jaturapitakkul C, Chindaprasirt P. Use of palm oil fuel ash as a supplementary cementitious material for producing high-strength concrete. Constr Build Mater 2009;23:2641–6. [33] Kroehong W, Sinsiri T, Jaturapitakkul C, Chindaprasirt P. Effect of palm oil fuel ash fineness on the microstructure of blended cement paste. Constr Build Mater 2011;25:4095–104.