Development of a high strength fly ash-based geopolymer in short time by using microwave curing

Development of a high strength fly ash-based geopolymer in short time by using microwave curing

Author’s Accepted Manuscript Development of a high strength fly ash based geopolymer in short time by using microwave curing Ahmed Graytee, Jay G. San...

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Author’s Accepted Manuscript Development of a high strength fly ash based geopolymer in short time by using microwave curing Ahmed Graytee, Jay G. Sanjayan, Ali Nazari www.elsevier.com/locate/ceri

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S0272-8842(18)30300-6 https://doi.org/10.1016/j.ceramint.2018.02.001 CERI17403

To appear in: Ceramics International Received date: 15 December 2017 Revised date: 1 February 2018 Accepted date: 1 February 2018 Cite this article as: Ahmed Graytee, Jay G. Sanjayan and Ali Nazari, Development of a high strength fly ash based geopolymer in short time by using microwave curing, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Development of a high strength fly ash based geopolymer in short time by using microwave curing Ahmed Graytee a,b,*, Jay G. Sanjayan a, Ali Nazari a a

Centre for Sustainable Infrastructure, Faculty of Science, Engineering and Technology, Swinburne

University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia. b

Al-Mustansiriyah University, Faculty of Engineering, Civil Engineering Department, Baghdad, Iraq.

*

Corresponding author.

E-mail addresses: [email protected], [email protected] (A. Graytee)

ABSTRACT: In this study, an experimental program was conducted to investigate the effect of microwave curing on the strength of geopolymer. Volumetric heating provided by microwave curing results in faster strength development as compared to conventional heat curing that depends on heat transmission from the outside to the core of a sample. Initially, the effect of different traditional curing methods (ambient air, water, and oven) on the strength development of Class F fly ash activated by sodium silicate and sodium hydroxide was investigated. Later, microwave curing and oven curing were introduced at higher temperatures in short periods of time (≤ 60min) as alternative curing methods. Results showed that the compressive strength of geopolymer pastes cured in a microwave oven was superior to those of the control cured in a conventional oven at 90 and 120oC for the same period. When the size of specimen size reduced form 50 mm to 25 mm, the strength development rate of the geopolymer decreased as the microwave curing time increased. A 56 MPa compressive strength was obtained for 25 mm size geopolymer paste cured in a microwave oven for five minutes.

Keywords: fly ash-based geopolymer, alkali-activated fly ash, microwave curing, elevated temperature curing, very early strength development, very short curing time

1. Introduction 1.1 Background Utilisation of Coal Combustion Product (CCP) from coal-fired power stations is of great interest. Due to the increase in energy demands, the amount of fly ash continues to increase each year. This increases the pressure on coal and utility industries and associated waste management to find solutions to the environmental problems that are associated with fly ash disposal, storage and management. Fly ash is one of the alumina-silicate by-products that can be activated by alkaline solutions to undergo dissolution, polymerization and solidification processes [1, 2]. This process can create a new material with a good physical and mechanical properties termed “geopolymer” [1, 3-5]. Geopolymers are well known for their good fire resistance [6-8], high acid resistance [9, 10] and good sulphate resistance [11] properties. Manufacturing of economical and good quality fly ash-based geopolymer structural elements, including aggregates, tiles and other building components helps with the fly ash disposal. Moreover, fly ash-based geopolymer can replace the application of ordinary Portland cement (OPC) and reduce associated CO2 emissions [6, 12]. Without the presence of an external energy source as heat, the polymerisation reaction takes place very slowly [13-15]. In conventional heat curing, thermal energy is transferred from the surface of the material to the core of the material through convection, conduction, and radiation of heat due to thermal gradients. An alternative curing method could be conducted by microwaves where these waves can penetrate building materials and generate heat throughout the volume of the material. Through microwave curing, energy is delivered directly to the material through molecular interaction with the electromagnetic field, and the electromagnetic energy is converted to thermal energy. In addition, temperatures of up to 100oC have been used in conventional curing methods to cure geopolymer specimens with long curing periods [4, 16, 17]. Long curing periods (more than 6 hours) are unaffordable and impractical for a manufacturing process. Effective microwave curing could accelerate the strength gain due to rapid and uniform heating [18, 19]. There is some literature that deals with microwave curing as an alternative curing method. However, these studies mostly deal with the strength of cement and concrete materials [20, 21] or the strength of fly ash mortars [22-24]. This study has investigated the strength

development of geopolymer with very short periods of curing (less than 60 minutes) using oven or microwave curing. Such curing, which is suitable for implementation in large-scale production facilities has not been reported in the literature so far.

1.2 Research outline In this study, microwave heating is used as a new curing method to cure the geopolymer paste specimens. The investigation included the following stages: - Firstly, the effect of different traditional curing methods (ambient air, water, and oven) on the strength development of Class F fly ash activated by sodium silicate and sodium hydroxide was investigated. - Secondly, microwave curing and oven curing were introduced at higher temperatures in short periods of time (≤ 60min) as alternative curing methods; then a comparison was made between oven heating and microwave heating. - Finally, the effect of specimen size was studied to investigate if reducing specimen size would make any difference on strength development of alkali-activated fly ash article during short time microwave and oven curing.

2. Experimental work 2.1 Materials Coal fly ash from Gladstone power station in Queensland was used as a raw material. It is a low calcium fly ash (class F) according to the ASTM C618 standard. Tables 1 and 2 show the chemical composition and the physical properties of the fly ash and its loss on ignition (LOI). Sodium hydroxide (NaOH) solution of 8M concentration was prepared using Caustic soda beads of 98% purity and tap water. Sodium silicate solution (Na2SiO3) supplied by PQ Australia was used. Table 3 shows the properties of sodium silicate solution. The fly ash alkali activation solution was made by mixing sodium hydroxide and sodium silicate solutions together with a Na2SiO3/NaOH weight ratio of 2.5. The alkali activation solution was used soon after preparation. Table 1: Chemical Composition and LOI of fly ash. Component

Al2O3

SiO2

CaO

Fe2O3

K2O

MgO

Na2O

P2O5

TiO2

MnO

SO3

LOI

Weight (%)

25.56

51.11

4.3

12.48

0.7

1.45

0.77

0.885

1.32

0.15

0.24

0.57

Table 2: Physical properties of the fly ash. Average diameter (µm)

Specific Gravity

Fineness (particles passing 45 µm)

7.66

2.35

88%

Table 3: Specification of sodium silicate. SiO2 %

Na2O %

SiO2/Na2O

Density (g/cm3 at 20oC)

Viscosity (cps at 20oC)

29.4

14.7

2

1.53

400

2.2 Specimen preparation and curing For all geopolymer mixes, an activator solution to fly ash weight ratio of 0.25 was used. The fly ash and the activator were mixed in a mechanical mixer for 10 minutes to form geopolymer paste. The paste was then moulded in cubic moulds with 50 mm and 25 mm edges. The paste in moulds was compacted for 2 minutes using a vibrating table. In the first stage of the study, specimens were de-moulded after 24 hours, and cured by two methods including 1) placing in curing cabinet at 23oC and 50% relative humidity, and 2) putting in a water tank at 23oC. Some other specimens were used for oven curing as well. They were placed unsealed in the oven at the required curing temperature for 1-2 hours to set, then de-moulded and left unsealed inside the oven for the rest of the curing period. Two different oven curing temperatures were used; namely 60oC and 90oC. In the second stage of the study, all specimens were de-moulded after 24 hours, and then placed unsealed inside the oven at the required curing temperature (90oC and 120oC) or inside the microwave oven at the required power for the designated curing period (between 5 to 105 minutes). After curing, the specimens were removed from oven or microwave oven and allowed to cool down to room temperature then tested for compressive strength. 2.3 Compressive strength test The compressive strength of the geopolymer specimens was tested using a Technotest 300 kN testing machine at a load rate of 0.5 and 0.25 N/mm2/second for 50 mm and 25 mm

cubic specimens respectively. Three specimens were tested for each case and the average value was recorded. 3. Results and discussion 3.1 Conventional curing methods 3.1.1 Effect of conventional curing methods on geopolymer strength Figure1 shows the compressive strength of geopolymer specimens in different curing conditions. Compressive strength values increase by increasing the curing period for all curing conditions. The early compressive strength of oven cured geopolymers is clearly superior to the compressive strength of air and water cured geopolymers. In addition, increasing the temperature improves the strength of geopolymers for the same curing period. The highest compressive strength of 86 MPa was recorded for geopolymer specimen cured at 90oC for 7 days. This could be considered as the potential maximum strength for the evaluated mix. Figure 1 also provides indications that the activation of fly ash can be done without heat treatment and that geopolymers could develop good strength in ambient temperature similar to OPC. In contrast, the results show that water curing might hinder geopolymer strength development. A possible explanation for this is that the leaching of activators in water makes it unavailable to the polymerisation of the geopolymer mixture [25]. These observations are advantages to geopolymer over OPC, which needs additional treatment after casting.

Figure 1: Strength development of geopolymer for different curing methods. Interestingly, the results show that moisture evaporation and surface drying has a minor effect on the strength development of geopolymers. These results contradict the work of some other researchers who stated that deterioration of strength occurs if moisture evaporation is allowed during or after heat curing [17, 26]. 3.1.2 Strength development rate at very early age Figure 2 shows the compressive strength of geopolymer specimens after oven curing at 90oC for different curing periods. For 30, 45 and 60 minutes curing time, the compressive strength of geopolymer paste was 32, 41 and 47 MPa respectively, which is 44%, 56% and 64% of the compressive strength of geopolymer cured for 24 hours (73 MPa) respectively. It is obvious that the compressive strength development rate increases during the first 60 minutes (8 to 12% rate increase), thereafter the development rate decreases during the following 45 minutes (3 to 4% rate increase). A geopolymer with reasonable strength could be made within one hour of oven curing at 90oC. Further work was conducted to investigate the possibility of producing stronger geopolymer in shorter curing time by using higher oven temperature as detailed below. Figure 3 depicts the strength development of geopolymers after oven curing at 90oC and 120oC for short times (≤ 60 min). For 30 and 45 minutes curing, an insignificant difference in

geopolymer strength was found, when temperature increased from 90oC to 120oC. Heat in oven curing system is distributed in the specimen from the outside to the inside. For this reason, a longer heating period is required to attain the suitable temperature inside the core of the specimen. Increasing the curing temperature improves the strength of the geopolymer but only after 60 minutes of curing. It is worthy to mention that this is for 50 mm cubic specimens and larger geopolymer specimens require more time for the heat to penetrate.

Figure 2: Geopolymer strength development as a function of oven curing time at 90oC.

Figure 3: Geopolymer strength development as a function of oven curing time at 90 oC and 120oC for 50 mm cubic specimens.

3.2 Microwave curing method 3.2.1 Microwave curing temperature profile Figure 4 shows the temperature development in geopolymer paste specimens subjected to microwave power of 200, 300, 400 and 600 W at different curing times (5, 10, and 15 min). The temperature of the specimens was recorded using an infrared thermometer. It is seen that by applying higher microwave power, the temperature of specimens increases. Further, the temperature of specimens increases at higher microwave curing time for all applied microwave powers. Application of 300, 400 and 600 W microwave power levels generates a high temperature in the specimen in short period of times.

Figure 4: Geopolymer temperature development as a function of microwave curing time and microwave power (Some specimens disintegrated in 13 minutes at 600 W).

3.2.2 Effects of microwave power on compressive strength The compressive strength of geopolymer specimens cured using different microwave power levels for 15 minutes are shown in Figure 5. The compressive strength of the geopolymer paste enhances as the microwave power increases up to 300 W then decreases at higher powers (i.e. 400 and 600 W). In addition, increasing the microwave power above 300 W results in large cracks on the surface of specimens (at 400 W) or deterioration of the specimens (at 600 W). Figure 6 shows images of some cured geopolymers. Several factors could explain this observation (i.e. strength degradations and specimen deterioration). Firstly, increasing microwave power brings the temperature of a specimen higher than 100oC (boiling temperature of water) in very short time (Fig. 4). This might reduce the amount of water available required for the polymerisation process. Secondly, rapid water evaporation, as reported in terms of moisture loss in Figure 7, increases pores and might lead to internal stresses, which causes microcracks and subsequent strength loss. Finally, at high microwave power levels, extreme local variations (hot spots) occur due to inhomogeneity of dielectric properties of the geopolymer constituents (i.e. fly ash, water,

sodium silicate and sodium hydroxide). It leads to temperature non-uniformity and thermal runway, which causes very high internal stresses and specimen deterioration [19, 23, 27]. The last explanation is the most likely reason for the strength degradations. In this study, reasonable strength properties were achieved due to small sizes of specimens and limited microwave power. No specimen cracking was observed when the specimens were subjected to a power level of 200 W. For this reason, 200 W microwave power was used for the remaining of experiments.

Figure 5: Compressive strength of geopolymer pastes as a function of microwave power level for 15 minutes curing time (with the exception of 13 minutes for 600 W).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 6: Images of cured geopolymer (a) oven-cured at 90°C for 45 min (b) oven-cured at 120°C for 45 min (c) microwave-cured at 200 W for 45 min (d) microwave-cured at 300 W for 15 min (e,f) cracks of microwave-cured specimens at 600 W.

Figure 7: Moisture loss of geopolymer pastes as a function of microwave power level for 15 minutes curing time.

Figure 8: Compressive strength of geopolymer pastes as a function of microwave curing time at 200 W microwave power level.

The compressive strength of geopolymer pastes cured at 200 W microwave power for different curing times is shown in Figure 8. In general, a raise in the microwave curing time increases the compressive strength of specimens. The highest compressive strength of 90 MPa was recorded for geopolymer paste cured for 60 minutes. It is interesting to see that about 50% of the compressive strength was reached (44 MPa) within the first 10 minutes. It can be inferred that producing high strength geopolymer in very short time is possible by using microwave heating by controlling the microwave power and specimen size.

3.3 Comparison between microwave curing and oven curing Figure 9 compares the compressive strength of oven-cured and microwave-cured geopolymer pastes. Compressive strengths of 70, 77 and 90 MPa for microwave-cured specimens and maximum compressive strengths of 44, 58 and 66 MPa for oven-cured specimens were recorded for 30, 45 and 60 minutes curing times respectively. It can be seen that increasing the curing time has a positive effect on the compressive strength of geopolymer pastes for all curing conditions.

Figure 9: Compressive strength of geopolymer pastes as a function of curing method for different curing times.

The strength of microwave-cured geopolymer pastes is superior to the strength of ovencured geopolymer pastes. These results are likely to be related to the difference in the way energy is delivered to the material. In oven curing, thermal energy is transferred to the material through convection, conduction, and radiation of heat from the surface of the material to the core. In contrast, in microwave curing, energy is delivered directly to the core of the material. Microwave can penetrate materials and heat is generated throughout the volume of the material. This phenomenon provides rapid and uniform heating which accelerates the strength gain and enhances the microstructure of specimens [18, 23]. 3.4 Effect of specimen size on geopolymer strength development There is a well-known phenomenon called the “size effect” where the compressive strength decreases as specimen size increases. This size effect is quite apparent in the compressive failure of quasi-brittle materials such as concrete, ceramic and composite materials, which fail by the formation of cracks [28-30]. It should be noted that most of the investigations about this size effect belong to traditional concrete and other construction materials [31, 32], while the effects of size of specimens on the mechanical properties of geopolymers have not been investigated yet. In addition, the size effect decreases as the homogeneity of the tested material increases and it disappears beyond a certain specimen size [33]. In this study, it will be clearly shown that the increasing in compressive strength when the specimen size decreased is due to curing effect rather than the size effect as discussed below. 3.4.1 Effect of specimen size in oven curing To study the effect of specimen size on geopolymer strength development, two different sizes namely, 50 mm and 25 mm cubic specimens were used and the results were compared. Figure 10 compares the strength values of geopolymers produced using different specimen sizes and cured at different oven curing temperatures. When specimen size is reduced from 50 mm to 25 mm, there is a clear trend of increasing strength of geopolymers cured at 90oC for 30 and 45 minutes curing periods (62% and 44% increase, respectively), but there was only 5% increase in geopolymer strength after 60 minutes of curing period. A possible explanation for this might be that the rate of heat exchange between geopolymer particles is faster for small size specimens than large ones. On the other hand, at 120oC, there was no significant increase in 25 mm specimen strength compared with 50 mm specimen strength (only 14% increase for 30 min, and 3-4%

for 45 and 60 min). This may be explained by the fact that at higher temperatures, heat penetration from the surface to the core will be faster. To investigate this hypothesis, thermocouples were inserted into the core of 25 mm and 50 mm cube geopolymer specimens to record the core temperatures during oven curing at 120oC. The results, as shown in Figure 11 and Table 4, indicate that the difference in core temperatures have decreased from around 30oC to only around 10oC after 30 minutes of curing. Table 4: Geopolymer specimens core temperature during oven curing at 120oC. Time (min)

0

5

10

15

30

45

60

75

90

105

120

135

23.5

59.2

80.4

92.6

100.7

105.2

108.9

111.8

113.7

114.9

115.9

116.2

23.5

32.2

46.8

59.6

90.4

98.2

99.2

101.3

103.7

105.7

107.4

108.7

0

27

33.6

33

10.3

7

9.7

10.5

10

9.2

8.5

7.5

25 mm specimen core temp. (oC) 50 mm specimen core temp. (oC) Core temp. difference o

( C)

These findings may be taken to indicate that the reducing of specimen size may improve strength, and consequently, reduce the curing time of geopolymers. However, this is true for low-temperature curing (i.e. 90oC) and curing time shorter than 45 minutes.

Figure 10: Effect of specimen size on geopolymer strength development using oven curing at different curing temperatures and times.

Figure 11: Effect of specimen size on the heat transfer from the outside to the specimen core during oven curing at 120oC.

3.4.2 Effect of specimen size in microwave curing Figure 12 compares the strength values of geopolymers produced using different specimen sizes and cured in a microwave oven at 200 W. In general, there was a significant increase in 25 mm specimen strength compared with 50 mm specimen strength for all curing periods. However, the geopolymer strength increasing rate decreased as the microwave curing time increased ( for 5, 10 and 15 min microwave curing time, the strength increase was 52%, 41% and 29% respectively). A possible explanation for these results might be that, in small specimens, the thermal differences and hotspots are less, and do not cause microcracks and deteriorations as much as in large specimens. Further, for large size specimens, microwave energy does not penetrate the whole volume of the specimen and most of the microwave energy is absorbed by the external layers of the geopolymer mix, then the heat moves to the core via conduction leading to a slower heating rate. On the other hand, for small size specimens, microwave energy penetrates the whole volume of the specimen, which leads to a faster heating rate. It should be noted that an equal batch weight is used for size effect investigation during microwave curing, namely three 50 mm cubes for 50 mm specimen tests, and two 50 mm and eight 25 mm cubes for 25 mm specimen tests. It can be seen that a high strength 25 mm size geopolymer specimens (62 MPa) could be made within 10 minutes of microwave curing. Further work was inducted to investigate the possibility of producing the same high strength geopolymer in shorter curing time by reducing the batch weight. Only three 25 mm specimens were cured for 5 minutes using both microwave oven at 200 W and conventional oven at 120oC. Interestingly, the geopolymer strength was 56 MPa for microwave cured specimen compared to only 11 MPa for oven cured specimen. These findings suggest that high strength geopolymer specimens could be produced in very short time by controlling microwave power, batch weight, specimen temperature, and specimen size.

Figure 12: Effect of specimen size on geopolymer strength development using microwave curing at 200 W. 4. Conclusions This study set out to compare the different ways of geopolymer curing, and to assess the feasibility of manufacturing economical and good quality fly ash-based geopolymer structural elements in very short time by introducing the microwave heating as an alternative curing method. The results indicate that microwave curing accelerates the early strength of geopolymer, and that higher microwave curing periods lead to higher geopolymer strengths. The strength development of geopolymers cured with microwave was much faster than oven curing due to the more effective heating process. For the same geopolymer strength, the specimens need to be cured in the oven for significantly longer times. Microcracking and damaging due to hotspots and thermal differentials can be reduced by using smaller specimens. A high strength geopolymer could be produced using microwave curing in less than 5 minutes, by controlling microwave power, batch weight, specimen temperature, and specimen size. Overall, microwave curing could be used as a new geopolymer curing method as an economic alternative way to the oven curing method.

Acknowledgments Ahmed Graytee is sponsored by the Iraqi government for his PhD study at Swinburne University of Technology and this financial support is gratefully acknowledged. He also thanks the technical staff of the Smart Structures Laboratory at Swinburne University of Technology for their assistance during experimental work.

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