Effect of water addition, plasticizer and alkaline solution constitution on fly ash based geopolymer concrete performance

Construction and Building Materials 121 (2016) 694–703

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Effect of water addition, plasticizer and alkaline solution constitution on fly ash based geopolymer concrete performance Ali A. Aliabdo a,⇑, Abd Elmoaty M. Abd Elmoaty a, Hazem A. Salem b a b

Structural Engineering Department, Faculty of Engineering, Alexandria University, Egypt Faculty of Engineering, Alexandria University, Egypt

h i g h l i g h t s
Additional water improves workability up to 200% but reduces other properties by 27%.
Admixture improves workability up to 115% but reduces other properties by 25%.
The optimum molarity of NaOH solution and solution to fly ash ratio are 16 M and 0.40.
Increasing NaOH to Na2SiO3 ratio has negative effect on GPC properties.

a r t i c l e

i n f o

Article history: Received 5 April 2016 Received in revised form 29 May 2016 Accepted 14 June 2016

Keywords: Geopolymer concrete Fly ash Additional water Chemical admixture Molarity Alkaline solution

a b s t r a c t Using fly ash based geopolymer concrete (GPC) instead of portland cement concrete, partially or totally, can reduce CO2 emissions released during OPC production, which requires burning large quantities of fuel result in significant CO2 emissions. Development of GPC became a major concern to widen its usage. This paper is a study of the influence of additional water content, plasticizer content, sodium hydroxide solution molarity, alkaline solution to fly ash ratio and sodium hydroxide to sodium silicate ratio on fly ash based GPC. The studied properties of fly ash based GPC was workability, compressive strength, splitting tensile strength, modulus of elasticity, absorption, and porosity. The results showed that, generally, increasing additional water content increased workability, but decreased other fly ash based GPC properties and the optimum additional water content was found to be 30 kg/m3 which has slight effect on geopolymer properties. The increase of plasticizer content up to 10.5 kg/m3 had acceptable effects on GPC properties due to improved workability. The optimum molarity of sodium hydroxide solution was found to be 16 M. GPC properties was significantly affected by alkaline solution to fly ash ratio and 0.40 was expected to be the optimum ratio. Increasing sodium hydroxide solution to sodium silicate solution ratio reduces geopolymer concrete properties; nevertheless, low ratio is not economic due to sodium silicate solution cost. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The geopolymer cement can contribute effectively to reduce Ordinary Portland Cement (OPC) industry in the future. Provis et al. [1] emphasized that alkali-activated binders are not intrinsically or fundamentally ‘low-CO2’ unless designed effectively to achieve such performance, but when mix design and raw materials selection are carried out with a view towards optimization of environmental performance, the outcomes can result in very significant savings. The use of geopolymer technology not only has the potential to substantially reduces the CO2 emissions by the cement ⇑ Corresponding author at: Gamal Abd El-Naser st., Alexandria 21526, Egypt. E-mail address: [email protected] (A.A. Aliabdo). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.062 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

industries, but also utilizes the waste materials such as fly ash. It is to be noted that fly ash, one of the sources for making geopolymer binders, is available abundantly worldwide, and yet its usage to date is very limited [2]. Previous study published in the scientific literature indicated that fly ash based mixtures released 45% less CO2 than an average Portland cement concrete mixture [3]. The name ‘‘Geo-polymer” was coined by Prof. J. Davidovits in 1978 who found that the polymerization process involves a fast chemical reaction under alkaline conditions on Si-Al minerals, that results in 3D polymeric chain and ring structure consisting of Si-O-Al-O bonds. The main concept behind this geopolymer is the polymerization of the Si-O-Al-O bond which develops when Al-Si source materials like fly ash is mixed with alkaline activating solution (NaOH or KOH solution with Na2SiO3 or K2SiO3). The

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geopolymer can be in the form of -Si-O-Al-O- or –Si-O-Al-O-Si-Oor –Si-O-Al-O-Si-O-Si-O-. The schematic formation of geopolymer material can be shown as described by Eqs. (1) and (2) [4–6]:

concentrates on the effect of water addition, chemical admixture (plasticizer) and the alkaline solution constitution on the workability and mechanical properties of fly ash based GPC to achieve more development in GPC industry.

ð1Þ

ð2Þ

The last term in Eq. (2) reveals that water is released during the chemical reaction that occurs in the formation of geopolymers. This water, expelled from the geopolymer matrix during the curing and further drying periods, leaves behind nano-pores in the matrix, which enhances the performance of geopolymers. The water in a geopolymer mixture, therefore, plays no role in the chemical reaction that takes place; it merely provides workability to the mixture during handling. There are two main constituents of geopolymers, namely the source materials and the alkaline liquids. The source materials for geopolymers based on alumina-silicate should be rich in silicon (Si) and aluminum (Al). These could be natural minerals such as kaolinite, clays, etc., whose empirical formula contains Si, Al, and oxygen (O) [7]. Alternatively, by-product materials such as fly ash, silica fume, slag, rice-husk ash, red mud, etc could be used as source materials. The choice of the source materials for making geopolymers depends on factors such as availability, cost, type of application, and specific demand of the end users. Geopolymers are formed when aluminosilicates, such as fly ashes, dissolve in a strong alkaline solution, reorganize and precipitate in a hardened state [8]. Fly ash suitable for use in geopolymer consists mostly of glassy, hollow, spherical particles, which are cenospheres (thin walled hollow spheres) [9]. It is to be noted that the nanostructure of geoplymers is strongly dependent on the available calcium content of precursors; a high-calcium system such as alkali-activated blast furnace slag is dominated by a calcium aluminosilicate hydrate (C–A–S–H) gel with a tobermorite-like structure [10,11], while low calcium systems such as those based on metakaolin or fly ash tend to generate an alkali aluminosilicate (N–A–S–H) gel with a highly crosslinked, disordered pseudo-zeolitic structure [10,12,13]. These gels can coexist in binders based on blends of high-calcium and lowcalcium precursors [14–15]. Geopolymer concrete showed good properties such as high compressive strength, low creep, good acid resistance and low shrinkage [16]. Compressive strength is an essential property for geopolymer concrete where it also depends on curing time and curing temperature for geopolymer concrete. When the curing time and temperature increase, the compressive strength also increases with curing temperature in the range of 60° to 90 °C, between 24 and 72 h [17]. Limited efforts have been made to study the effect of adding water and chemical admixture during the production of GPC in addition to the NaOH to Na2SiO3 ratio [18,19]. So, this work

2. Materials and experimental program 2.1. Materials Any material that contains aluminum silicate of proper composition in amorphous form is a potential source material for the manufacture of geo-polymer binder. Geopolymer concrete consists of geopolymer cement paste and aggregates. Several mineral and industrial by-product materials have been investigated in the past. The calcined source materials such as fly ash, slag, calcined kaolin demonstrated a higher compressive strength when compared to non-calcined materials [20]. This section discusses in detail the used materials for producing fly ash based geopolymer concrete.

2.1.1. Fly ash In this study fly ash (ASTM Class F) was used as the main source material for aluminosilicates, the geopolymer binder. Specific gravity of the fly ash was 2.2 and 95% of fly ash was passing through the 45 lm sieve. The chemical composition as determined by chemical analysis is presented in Table 1.

2.1.2. Aggregates Natural siliceous sand with fineness modulus of 2.45 and pink limestone of 9.5 mm nominal maximum size, were used as natural aggregates. Table 2 presents the physical properties of the aggregates, and the sieve analysis is presented in Table 3.

2.1.3. Alkaline solution A mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solutions were used as the activator solution. Sodium hydroxide solution of desired concentration was prepared by mixing 97–98% pure NaOH pellets with tap water. Sodium silicate solution was obtained from a local commercial producer. The chemical properties of sodium silicate is shown in Table 4. The densities of sodium hydroxide solution and the total alkaline solution was about 1.15 and 1.5 g/cm3.

2.1.4. Admixture High range water reducer naphthalene-based admixture (ASTM Type F) was used to improve the workability.

Table 1 The chemical composition of fly ash. Chemical constituent

Content, mass%

Silicon dioxide (SiO2) Iron oxide (Fe2O3) Aluminum oxide (Al2O3) Calcium oxide (CaO) Magnesium oxide (MgO) Sulfur trioxide (SO3) Na2O K2O Loss on ignition (LOI)

62.30 2.10 28.10 0.50 1.00 0.40 0.50 1.00 2.50

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Table 2 Physical properties of the used aggregates in geopolymer concrete. Properties

Natural Aggregate Coarse Agg. Crushed pink limestone (N.M.S = 9.5 mm)

Fine Agg. Sand

Specific gravity (SSD) Unit weight (t/m3) Water absorption (%)

2.5 1.45 1.9

2.67 1.62 1.3

Materials finer than 200 sieve by washing (%) Fineness modulus

1.4 –

2.5 2.45

Table 3 Sieve analysis of the aggregates. ASTM designation, mm

1/200 , 12.5 mm 3/800 , 9.5 mm NO.4, 4.75 mm NO.8, 2.36 mm NO.16, 1.18 mm NO.30, 600 lm NO.50, 300 lm NO.100, 150 lm NO.200, 75 lm

Coarse Aggregate (mass% passing)

Fine Aggregate (mass% passing)

Test results

Limits according to ASTM C33

Percentage passing by weight (%)

Limits according to ASTM C33

100 100 30 6 3 – – – –

100 90–100 20–55 5–30 0–10 – – – –

100 100 96.1 91.9 83.2 59.7 19.7 4.2 0.20

100 100 95–100 80–100 50–85 25–60 5–30 0–10 –

Limits (According to Egyptian standard specifications)

62.5 for coarse agg. and 62.0 for fine agg. 63.0

were 10, 20, 30 and 35 kg per cubic meter of concrete. The (ASTM Type F) admixture contents were 2.5, 5, 7.5 and 10.5 kg per cubic meter of concrete. The alkaline solution molarities were 12, 16 and 18 M and the alkaline solution to fly ash ratios were 0.3, 0.35, 0.4 and 0.45. Finally, the sodium hydroxide to sodium silicate ratios were set at 0.3, 0.4, and 0.5. The fly ash content was chosen as 400 kg/m3. Curing temperature and curing period of all mixes was 50 °C for 48 h. Table 5 presents the mixes proportions of fly ash based geopolymer mixes, and the related parameters.

2.3. Mixing, sample preparation and curing

Table 4 Sodium silicate liquid specifications. Item

Specifications

Color Density g/cm3 Total solids content, by mass%

Colorless 1.45:1.55 42:47

2.2. Test parameters The effects of additional water content, chemical admixture content, sodium hydroxide solution molarity, alkaline solution to fly ash ratio and sodium hydroxide to sodium silicate ratio were studied in this work. The additional water contents

Geopolymer mixing process includes four main steps. These steps are activator solution preparation, dry mixing of solid materials (aggregate, and fly ash), mixing liquid components (activator, extra water and superplasticizer) and finally mixing all components in the mechanical mixer. To prepare the activator solution (first step), sodium hydroxide solution with certain molarity (12, 16, and 18) was prepared by mixing 97–98% pure NaOH pellets with tap water. A 16 M NaOH solution contains 16 * 40 g = 640 g of NaOH solids per litre of the solution, where 40 is the molecular weight of NaOH. The mass of NaOH solids was measured as 444 g per kg of NaOH solution with a concentration of 16 M. Adding NaOH and sodium silicate solutions results in exothermic reaction and, therefore, generates high temperature. Various methods have been proposed in the related literature to overcome the problem of temperature rise. Some investigators premix the alkali solutions and wait till it reaches the ambient temperature before adding into the dry mix [21–24], and others [25,26] recommend adding the alkali solutions during dry mixing itself. In the present study, the alkaline solution was first prepared by thoroughly mixing the NaOH and Na2SiO3 solutions 24 h prior to use and reached temperature of 25 ± 2 °C. The activator contents are given as percentage of fly ash content with sodium hydroxide to sodium silicate ratios of (0.3, 0.4 and 0.5). During dry mixing (second step), coarse and fine aggregates in saturated surface dry condition were well mixed with fly ash in a pan mixer. In the third step naphthalene based high range water reducing admixture (to improve the workability of the mixture) and additional water was added to the alkaline liquid and were thoroughly mixed. Finally, the liquid mixture was added to the dry mix and the whole mixture was mixed together for 5 min and the temperature of the mixtures was 27 ± 2 °C.

Table 5 mix proportions of fly ash based geopolymer concretes. Admix. (kg/m3)

Solution to fly ash ratio

NaOH to Na2Sio3 ratio

Molarity

Note

NaOH

Add. water (kg/m3)

40 40 40 40 40 40 40 40 40 40 40 51.5 46 40 34.5 32.5 40 47

35 30 20 10 35 35 35 35 35 35 35 35 35 35 35 35 35 35

10.5 10.5 10.5 10.5 10.5 7.5 5 2.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 47

0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.45 0.4 0.35 0.30 0.35 0.35 0.35

0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.30 0.40 0.50

16 16 16 16 16 16 16 16 18 16 12 16 16 16 16 16 16 16

400 kg fly ash content – 48 h heat curing at 50 °C

Mix No.

Fly ash content

Coarse Agg content (kg/m3)

Sand content (kg/m3)

Alkaline solution (kg/m3) Na2SiO3

1 2 3 4 1 5 6 7 8 1 9 10 11 1 12 13 1 14

400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400

875 880 895 910 875 880 885 887 875 875 875 840 860 875 895 875 830 875

875 880 895 910 875 880 885 887 875 875 875 840 860 875 895 875 830 875

100 100 100 100 100 100 100 100 100 100 100 128.5 114 100 85.5 107.5 100 93

Effect of additional water content

Effect of chemical admixture content

Effect of sodium hydroxide molarity Effect of solution to fly ash ratio

Effect of sodium hydroxide to sodium silicate ratio

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A.A. Aliabdo et al. / Construction and Building Materials 121 (2016) 694–703 Table 6 Summary of the tests performed and the relevant standards. Properties

Specifications

Specimen dimensions

Age of testing

Workability

ASTM C 143/C 143M

Compressive strength Tensile strength Modulus of elasticity Absorption Porosity

BS 1881: Part 3 ASTM C 496 – 96 ASTM C 469 – 94 ASTM C 642 – 06 ASTM D 7063/D 7063M – 11

Cone with the 200 mm base in diameter, the top 100 mm in diameter, and the height 300 mm Cubes: 100 * 100 * 100 mm Cylinder D = 75 mm and L = 150 mm Cylinder D = 100 mm and L = 200 mm Cubes 70 * 70 * 70 mm Cubes 70 * 70 * 70 mm

7, 28 days 28 days 28 days 28 days 28 days

The main problem during mixing process was that the alkaline solution need to be prepared 24 h prior the mixing. This problem makes the usage of fly ash based GPC limited and more suitable for precasting concretes. Also, the problem of temperature rise during alkaline solution preparation. After the mixing process, the fly ash based geopolymer concrete mixtures were placed in the molds to obtain specimens for different tests. The specimens were then vibrated using a vibrating table for 2 min to release any residual air bubbles. The molds were stored in the room temperature of 20–23 °C leaving the top surface exposed to air. The samples were de-molded 24 h after casting and then cured within a hot air oven for 48 h at 50 °C and were then kept at ambient temperature until testing.

can be seen that the concrete workability is influenced significantly by the additional water content where the increase of additional water content improves the concrete workability. From the test result, the increase in GPC slump is 66.7%, 165.6% and 200.0% for geopolymer concrete with 20, 30, and 35 kg/m3 additional water content compared with concrete mix with 10 kg/m3 additional water. This improvement in workability is apparently due to the increase of free water which has no role in chemical reaction.

2.4. Testing

3. Test results and discussions Different concrete mixes of fly ash based GPC were designed to study the effect of additional water, plasticizer, molarity of NaOH solution, alkaline solution to fly ash ratio and NaOH to Na2SiO3 ratio on the workability, compressive strength, splitting tensile strength, modulus of elasticity, absorption and porosity. The fly ash content was set at 400 kg/m3. The curing temperature and curing period of all mixes was 50 °C for 48 h. The following sections discuss in details the results and discussion of the parameters studied in this work. 3.1. Effect of additional water This section discusses the effect of additional water content on fly ash based GPC properties. The solution to fly ash ratio, NaOH solution molarity, NaOH to Na2SiO3 ratio and admixture content were kept constant at 0.35, 16 M, 0.4 and 10.5 kg per cubic meter of the mixture.

100 90 80 70

Slump (mm)

7 days compressive strength (Mpa)

60 50 40 30 20

28 days compressive strength (Mpa)

40 35 30 25 20 15 10 5 0 10

20 30 Additional Water (kg water/m3 concrete)

35

Fig. 2. Effect of additional water on fly ash based GPC compressive strength.

4.0

28 days Tensile strength(MPa)

3.1.1. Workability The effect of additional water content during mixing process on fly ash based GPC workability is shown in Fig. 1. From this figure, it

3.1.2. Compressive strength The effect of additional water content during mixing of fly ash based GPC on compressive strength is shown in Fig. 2. From this figure, it can be observed that the increase of additional water content decreases the concrete compressive strength, the reduction being not more than 10.0% up to 30 kg/m3 additional water. The reduction in 28-day concrete compressive strength due to use of

Compressive strength (MPa)

Series of tests were performed to determine the workability and mechanical properties of fly ash based GPC. Table 6 presents the relevant specifications, specimens’ dimensions and the age of testing for each test.

3.5 3.0 2.5 2.0 1.5 1.0 0.5

10 0.0

0 10

20

30

35

Additional Water (kg water/m3 concrete) Fig. 1. Effect of additional water content on fly ash based GPC slump.

10

20

30

35

Additional Water (kg water/m3 concrete) Fig. 3. Effect of additional water content on fly ash based GPC tensile strength.

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water content beyond which the increases in absorption and porosity were 11.8% and 13.8% for 35 kg/m3 additional water content compared with concrete mix with 10 kg/m3 additional water. Based on previous study, it may be concluded that the reduction in mechanical properties such as compressive strength, tensile strength, modulus of elasticity and the increase in absorption and porosity as a result of increasing additional water content could be due to the increase of voids resulting from the increase of water content which has no role in chemical reaction. Excess water also diluted the liquid and thus slowed down the dissolution and reaction of the geopolymer. Similar reduction in strength and improve in workability with the increase in extra water was reported [19].

28 days Modulus of elasticity (GPa)

25

20

15

10

5

0

10

20

30

35

Additional Water (kg water/m3 concrete)

35 kg/m3 is 24.0% compared to that of concrete mix with 10 kg/m3 additional water content. 3.1.3. Tensile strength The effect of additional water content on fly ash based GPC 28day tensile strength is shown in Fig. 3. From this figure, it can be observed that the fly ash based GPC tensile strength is more affected by water addition than is the compressive strength. The reduction in 28-day tensile strength of fly ash based geopolymer concrete is 7.9%, 21.0% and 27.6% for 20, 30, and 35 kg/m3 additional water contents compared with that of the mix with 10 kg/ m3 additional water content. 3.1.4. Static modulus of elasticity Fig. 4 reveals the effect of additional water on 28-day geopolymer concrete modulus of elasticity. From this figure it is clear that the increase of additional water content has no significant effect on the modulus of elasticity. From the test result, the reduction in 28day modulus of elasticity is 2.2%, 4.0% and 10.9% for fly ash based geopolymer concrete with 20, 30 and 35 kg/m3 additional water content compared with concrete mix with 10 kg/m3 additional water.

3.2. Effect of admixture content (plasticizer) This section discusses the effect of used naphthalene-based Type F chemical admixture according to ASTM on fly ash based geopolymer concrete properties. The solution to fly ash ratio, NaOH solution molarity, NaOH to Na2SiO3 ratio and additional water content were kept constant at 0.35,16 M, 0.4 and 35 kg per cubic meter of the mixture. 3.2.1. Workability The effect of using Type F (high range water reducer) chemical admixture is shown in Fig. 5. From this figure, it is obvious that admixture has a significant effect on slump. The usage of high content admixture achieves higher slump values compared with those of mixes using low admixture content. From the test result, the increase in fly ash based GPC slump is 50.0%, 90.5% and 114.5%

100 90 80 70

Slump (mm)

Fig. 4. Effect of additional water content on modulus of elasticity of fly ash based GPC.

3.1.5. Water absorption and porosity Table 6 summarize the water absorption and porosity test results according to ASTM C642-06 and ASTM D7063/D7063M-11. The effect of additional water content on 28-day absorption and porosity is presented in Table 7. From this table, it can be observed that the use of additional water has insignificant effect on fly ash based GPC absorption and porosity up to 30 kg/m3 additional

60 50 40 30 20 10 0

2.5

5.0

7.5

10.5

Chemical admixture content (kg/m3) Fig. 5. Effect of chemical admixture content on fly ash based GPC slump.

Table 7 water absorption and porosity test results for different parameters. Factor

Water absorption% Porosity% Factor

Water absorption% Porosity% Factor

Water absorption% Porosity%

Additional water (kg/m3)

Effect of (sol/fly ash) mass ratio

10

20

30

35

0.30

0.35

0.40

0.45

4.74 9.80

4.96 10.22

5.04 10.40

5.30 11.15

5.80 12.00

5.30 11.15

5.09 10.51

5.15 10.65

Admixture (kg/m3)

(Naoh/Na2SiO3) mass ratio

2.5

5

5.03 10.4

5.08 10.45

7.5 5.17 11.03 NaOH molarity

10.5

0.30

0.40

0.50

5.30 11.15

4.85 10.02

5.30 11.15

5.40 11.30

12 M

16 M

18 M

5.60 11.60

5.30 11.15

5.52 11.40

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for fly ash based GPC with 5, 7.5 and 10.5 kg/m3 admixture content compared with concrete mix with 2.5 kg/m3 admixture content. 3.2.2. Compressive strength The effect of chemical admixture content on GPC compressive strength after 7 and 28 days is shown in Fig. 6. From this figure it is obvious that, using low chemical admixture content yields higher compressive strength compared to high chemical admixture content. Generally, the use of chemical admixture content up to 7.5 kg/m3 has insignificant effect on 28 days GPC compressive strength. For example, the reduction in fly ash based GPC compressive strength is 4.2%, 8.6%, and 24.0% for geopolymer concrete with 5, 7.5, and 10.5 kg/m3 admixture content compared with concrete mix with 2.5 kg/m3 chemical admixture content. 3.2.3. Tensile strength The effect of chemical admixture content on fly ash based GPC tensile strength after 28 days is shown in Fig. 7. From this figure, it is obvious that low chemical admixture dose yields higher tensile strength compared with high chemical admixture doses and this negative effect of increasing chemical admixture content is more pronounced than test results of concrete compressive strength. From the test result, the reduction in fly ash based GPC tensile strength is 8.6%, 15.7% and 21.4% for geopolymer concrete with 5, 7.5 and 10.5 kg/m3 chemical admixture content compared with concrete mix with chemical admixture content 2.5 kg/m3.

7 days compressive strength (Mpa)

28 days compressive strength (Mpa)

Compressive strength (MPa)

35 30

3.2.4. Static modulus of elasticity Fig. 8 shows the influence of chemical admixture dose on 28day modulus of elasticity. From this figure, it is obvious that the dose of chemical admixture has insignificant influence on 28-day static modulus of elasticity. The reduction in 28-day modulus of elasticity is 0.5%, 3.7% and 7.2% for fly ash based GPC with 5, 7.5 and 10.5 kg/m3 chemical admixture content compared with that of the mix with 2.5 kg/m3 chemical admixture content. 3.2.5. Water absorption and porosity The effect of chemical admixture content on 28-day fly ash based GPC absorption and porosity is presented in Table 7. It is obvious that there are insignificant increases in both of the absorption and the porosity. The 28-day test results show that the increase in fly ash based geopolymer concrete absorption is 1.0%, 2.8% and 5.4%, and the increase in porosity is 0.4%, 6.1%, and 7.2% for 5, 7.5 and 10.5 kg/m3 chemical admixture contents compared with that of the mix with 2.5 kg/m3 chemical admixture content. The reduction in fly ash based GPC properties observed in compressive strength, tensile strength, modulus of elasticity and the increase in absorption and porosity as a result of increasing chemical admixture content may be due to increasing voids in the structure. The same reduction in strength and improve in workability with the increase in admixture (plasticizer) content was reported [19]. 3.3. Effect of sodium hydroxide molarity This section discusses the effect of sodium hydroxide solution molarity on fly ash based GPC properties. The solution to fly ash ratio, NaOH to Na2SiO3 ratio, chemical admixture content and additional water content were kept constant at 0.35, 0.4, 10.5 kg per cubic meter of the mixture and 35 kg water/m3 concrete.

25

3.3.1. Workability Fig. 9 shows the effect of sodium hydroxide solution molarity on fly ash based GPC workability. From this figure, it is clear that the increase of sodium hydroxide solution molarity reduces the slump. From the test results, the reduction in slump is 10.5%, and 20.0% for 16 M and 18 M sodium hydroxide solution, respectively, compared to that of the mix with 12 M sodium hydroxide solution.

20 15 10 5 0

2.5

5.0

7.5

10.5

Chemical admixture content (kg/m 3) Fig. 6. Effect of chemical admixture content on fly ash based GPC compressive strength.

3.3.2. Compressive strength The effect of sodium hydroxide solution molarity on cube fly ash based GPC compressive strength after 7, and 28 days is shown in Fig. 10. From this figure, it is obvious that sodium hydroxide

25

28 days Modulus of elascity (GPa)

28 days Tensile strength (MPa)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

20

15

10

5

0

2.5

5.0

7.5

10.5

Chemical admixture content (kg/m3)

0.0 2.5

5.0

7.5

10.5

Chemical admixture content (kg/m3) Fig. 7. Effect of chemical admixture content on fly ash based GPC tensile strength.

Fig. 8. Effect of chemical admixture content on modulus of elasticity of fly ash based GPC.

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100

2.5

28 days Tensile strength (MPa)

120

Slump (mm)

80 60 40 20

2.0 1.5 1.0 0.5

0

12

16

18

0.0 12

NaOH solution molarity

16

18

NaOH solution molarity Fig. 9. Effect of sodium hydroxide solution molarity on fly ash based GPC slump. Fig. 11. Effect of sodium hydroxide solution molarity on tensile strength of fly ash based GPC. 7 days

28 days

25

25 20 15 10 5 0 12

16

18

NaOH solution molarity Fig. 10. Effect of sodium hydroxide solution molarity on fly ash based GPC compressive strength.

28 days Modulus of elascity(GPa)

Compressive strength (MPa)

30

20

15

10

5

0 12

16

18

NaOH solution molarity

solution molarity has a noticeable effect on concrete compressive strength where the increase in molarity from 12 to 16 M enhances 7-day and 28-day geopolymer concrete compressive strengths. The observed optimum molarity for 48 h of curing at 50 °C is 16 M. Increasing sodium solution molarity from 16 M to 18 M reduces 7-day and 28-day compressive strengths. For example, the increase in 28-day cube GPC compressive strength is 45.7%, and 9.10% for GPC with 16 and 18 M sodium hydroxide solutions, respectively, compared with concrete mix with 12 M sodium hydroxide solution. 3.3.3. Tensile strength The effect of sodium hydroxide solution molarity on GPC 28day tensile strength is shown in Fig. 11. From this figure, it is clear that increase in sodium hydroxide solution molarity improves 28day tensile strength but this effect reverses above 16 M. The observed optimum molarity for 48 h of curing at 50 °C is 16 M as observed in the compressive strength results. This behavior agrees with compressive strength results. For example, the increase in 28day tensile strength is 44.7%, and 2% for GPC with 16 and 18 M sodium hydroxide solution, respectively, compared with that of the mix with 12 M sodium hydroxide solution. 3.3.4. Modulus of elasticity Fig. 12 reveals the effect of sodium hydroxide solution molarity on 28-day fly ash based GPC modulus of elasticity. From test result, it can be observed that 16 M is the optimum molarity for sodium hydroxide solution used in fly ash based GPC cured at 50 °C for 48 h which representing the highest value of modulus of elasticity and the significant rate of increasing achieved from 12 M to 16 M.

Fig. 12. Effect of sodium hydroxide solution molarity on modulus of elasticity of fly ash based GPC.

The increase in modulus of elasticity is 41.1%, and 33.7% for GPC with 16 and 18 M sodium hydroxide solution, respectively, compared with the mix with 12 M sodium hydroxide solution. 3.3.5. Water absorption and porosity The effect of sodium hydroxide solution molarity on 28-day absorption and porosity is shown in Table 7. From this results, it is clear that the sodium hydroxide solution molarity has insignificant role in concrete absorption and porosity, but generally, the increase in molarity from 12 M to 18 M decrease water absorption, and porosity. From test results, the reduction in fly ash based geopolymer concrete absorption is 5.4%, and 1.4% for mixes with 16 and 18 M sodium hydroxide solution, respectively, compared with the mix with 12 M sodium hydroxide solution. For porosity the reduction percentages are 3.9% and 1.7% for fly ash based GPC with 16 and 18 M sodium hydroxide solutions, respectively, compared with that of the mix with 12 M sodium hydroxide solution. The improvement in GPC properties as a result of increasing molarity up to 16 M agrees with those obtained by Hardjito [2] and Raijiwala [26]. The highest values being achieved at 16 M which agrees with those reported by Mandal [27]. The strength increased with the increase in NaOH concentration mainly through the leaching out of silica and alumina with the high concentration of NaOH [28]. Beyond 16 M GPC properties decreases due to a lower rate of polymerization taking place resulting in a decreased strength.

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3.4. Effect of alkaline solution to fly ash ratio This section discusses the effect of alkaline solution to fly ash ratio on fly ash based GPC properties. The molarity of NaOH solution, NaOH to Na2SiO3 ratio, chemical admixture content and additional water content were kept constant at 16 M, 0.4, 10.5 kg per cubic meter of the mixture and 35 kg water/m3 concrete. 3.4.1. Workability Fig. 13 shows the effect of alkaline solution to fly ash ratio on fly ash based GPC workability. From this figure, it is obvious that alkaline solution to fly ash ratio plays an important role on the slump values of fly ash based GPC. The increase in alkaline solution to fly ash ratio results in an increase in the slump values. The increase in fly ash based GPC slump is 28.5%, 71.4% and 114.3% for geopolymer concrete mixes with 0.35, 0.40 and 0.45 solution to fly ash ratio, respectively, compared with the mix with 0.30 solution to fly ash ratio. 3.4.2. Compressive strength Fig. 14 shows the effect of alkaline solution to fly ash ratio on fly ash based GPC 7- and 28-day compressive strengths. From this figure, it is clear that the increase of solution to fly ash ratio up to 0.40 increases the compressive strength, then the effect reverses. The increases in GPC compressive strengths were 52.2%, 77.8%, and 67.8% for mixes with 0.35, 0.40 and 0.45 solution to fly ash ratio, respectively, as compared with that of mix with 0.30 solution to fly ash ratio.

3.4.3. Tensile strength The effect of alkaline solution to fly ash ratio on 28-day tensile strength of fly ash based GPC is shown in Fig. 15. The increase in solution to fly ash ratio yields high tensile strength, matching with the trend of compressive strength test results. Also the increase in the tensile strength is almost proportional to the increase in alkaline solution to fly ash ratio up to 0.4 beyond which a decrease occurs. The increase of GPC tensile strength is 49.5%, 60.3%, and 52.2% for 0.35, 0.40 and 0.45 solution to fly ash ratio, respectively, compared with the mix with 0.30 solution to fly ash ratio. 3.4.4. Modulus of elasticity The effect of alkaline solution to fly ash ratio on fly ash based GPC modulus of elasticity after 28 days is shown in Fig. 16. It can be seen that the alkaline solution to fly ash ratio has an effect on the 28-day modulus of elasticity similar to those on compressive and tensile strengths. The increase in the modulus of elasticity is 17.5%, 25.8%, and 22.3% for the mixes with 0.35, 0.40 and 0.45 solution to fly ash ratio, respectively, compared with the mix with 0.30 solution to fly ash ratio. 3.4.5. Water absorption and porosity Table 6 presents the effect of alkaline solution to fly ash ratio on fly ash based GPC 28-ady absorption and porosity. It can be seen that the increase in alkaline solution to fly ash ratio reduces absorption and porosity. From test results, the reduction in absorption was 8.6%, 12.2% and 11.2% for GPC mixes with 0.35, 0.40 and 0.45 solution to fly ash ratio, respectively, compared with the mix with 0.30 solution to fly ash ratio. Also, the reduction in

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Fig. 15. Effect of solution to fly ash ratio on tensile strength of fly ash GPC.

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Fig. 16. Effect of solution to fly ash ratio on modulus of elasticity of fly ash based GPC.

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porosity was 7.1%, 12.4% and 11.3% for 0.35, 0.40 and 0.45 solution to fly ash ratio, respectively, compared with the mix with 0.30 solution to fly ash ratio. The improvement in GPC properties as a result of increasing alkaline solution to fly ash ratio agrees with Hardjito [2] and Sathia [29]. The increasing in the alkaline activator content (Alkaline activator/Fly ash ratio) increases the Si species content, because the alkaline activator contained sodium silicate (more Si species), increases the SiO2/Al2O3 ratio and also increases the GPC strength because increases the SiO2/Al2O3 ratio result in more Si-O-Si bonds which are stronger in comparison with Si-O-Al [30]. 3.5. Effect of sodium hydroxide to sodium silicate ratio This section discusses the effect of sodium hydroxide to sodium silicate ratio on fly ash based GPC properties. The solution to fly ash ratio, NaOH solution molarity, chemical admixture content and additional water content were kept constant of 0.35, 16 M, 10.5 kg per cubic meter of the mixture and 35 kg water/m3 concrete. 3.5.1. Workability Fig. 17 shows the effect of sodium hydroxide solution to sodium silicate solution ratio on fly ash based GPC workability. From this figure, it is obvious that NaOH to Na2SiO3 ratio has a pronounced effect on fly ash based GPC initial slump. The increase in NaOH to Na2SiO3 ratio results in an increase in slump. The increase in slump is 28.6% and 57.1% for mixes with 0.40 and 0.50 NaOH to

Na2SiO3 ratios, respectively, as compared with that mix with 0.30 sodium hydroxide solution to sodium silicate ratio. 3.5.2. Compressive strength The effect of sodium hydroxide solution to sodium silicate solution ratio on fly ash based GPC compressive strength at the age of 7 and 28 days is shown in Fig. 18. From this figure, it is obvious that sodium hydroxide solution to sodium silicate solution ratio has a significant effect on fly ash based GPC compressive strength where the increase in this ratio decreases the compressive strength. The decrease in cube GPC compressive strength after 28 days is 22.5% and 29.5% for GPC mixes with 0.40 and 0.50 NaOH to Na2SiO3 ratios, respectively, compared with 0.3 sodium hydroxide solution to sodium silicate ratio. This trend is the same after 7 and 28 days. 3.5.3. Tensile strength The effect of sodium hydroxide solution to sodium silicate solution ratio on 28-day tensile strength is shown in Fig. 19. From this figure, it is clear that increase in NaOH to Na2SiO3 ratio reduces 28day tensile strength. The reduction in 28-day tensile strength is 10.1% and 20.5% for mixes with 0.40 and 0.50 NaOH to Na2SiO3 ratios, respectively, compared with the mix with 0.30 sodium hydroxide to sodium silicate ratio. 3.5.4. Modulus of elasticity Fig. 20 reveals the effect of sodium hydroxide solution to sodium silicate solution ratio on the 28-day modulus of elastic-

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7 days compressive strength (Mpa)

Fig. 19. Effect of sodium hydroxide to sodium silicate ratio on tensile strength of fly ash based GPC.

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(NaOH/Na2SiO3) ratio Fig. 20. Effect of sodium hydroxide to sodium silicate ratio on modulus of elasticity of fly ash based GPC.

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ity. From test results, it can be observed that low sodium hydroxide solution to sodium silicate solution ratios yields high values for modulus of elasticity. The reduction in the modulus of elasticity is 3.3% and 6.8% for GPC mixes with 0.40 and 0.50 NaOH to Na2SiO3 ratios, respectively, compared with that of the mix with 0.30 sodium hydroxide solution to sodium silicate ratio. 3.5.5. Water absorption and porosity The effect of NaOH to Na2SiO3 ratio on fly ash based GPC absorption and porosity at the age of 28 days is presented in Table 7. The results clearly show that an increase in sodium hydroxide solution to sodium silicate solution ratio significantly increases water absorption and porosity. The increase in fly ash based GPC absorption is 9.3% and 11.3% for GPC mixes with 0.40 and 0.50 NaOH to Na2SiO3 ratios, respectively, compared to the mix with 0.30 sodium hydroxide solution to sodium silicate ratio. The relative increase in porosity is 11.2% and 12.8% for GPC mixes with 0.4 and 0.5 NaOH to Na2SiO3 ratios, respectively, compared with the mix with 0.30 sodium hydroxide solution to sodium silicate ratio. The same decrease in workability of the mixes with the decrease in NaOH to Na2SiO3 ratio was reported [19]. This was expected since sodium silicate was more viscous than sodium hydroxide. Also, the increase in strength with the decrease in NaOH to Na2SiO3 ratio was reported [19]. The improvement in GPC strength as a result of decreasing NaOH to Na2SiO3 ratio may be due to sodium silicate improving the polymerization process leading to reaction products with more Si and, hence, higher mechanical strength [31]. 4. Conclusion From the present study the following conclusions were obtained. – Increasing additional water content up to 35 kg/m3 increase workability of fly ash based GPC up to 200%. However, increasing additional water reduces fly ash based GPC properties up to 27%. The optimum additional water content could be 30 kg/m3 which has slight unfavorable effect on the properties of fly ash based GPC. – Increasing chemical admixture content improves GPC workability up to 115%. Although, it improves workability it results in slight increases in water absorption and porosity, and decrease in compressive and tensile strength and modulus of elasticity. – The increase in porosity due to the increase in high range water reducing admixture might be due to some air entraining effect of the high range water reducer. – Increasing sodium hydroxide molarity up to 16 M improves fly ash based GPC properties. Beyond 16 M the desirable properties decreases due to a lower rate of polymerization taking place due to the high NaOH concentration resulting in decreased properties. – Increasing alkaline solution to fly ash ratio improves fly ash based GPC properties. The optimum alkaline solution to fly ash ratio is 0.40. – Decreasing NaOH to Na2SiO3 ratio has favorable effect on fly ash based GPC properties. The optimum NaOH to Na2SiO3 ratio for the constituent materials used in this work could be 0.40. – More researches are needed to overcome the problems of needed period to prepare alkaline solution to use in the mixing process and the temperature rise during alkaline solution preparation.

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