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Optimization of local modified metakaolin-based geopolymer concrete by Taguchi method

  • Nazar F. Al Obeidy EMAIL logo , Wasan I. Khalil and Hisham K. Ahmed
Published/Copyright: March 6, 2024
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Open Engineering
From the journal Open Engineering Volume 14 Issue 1

Abstract

Geopolymer in recent concrete (GP) has gained significant attention due to its sustainability and environmental friendliness. Local metakaolin-based geopolymer is weaker than geopolymers made from other materials due to its low Si/Al ratio. A consistent mixture design is also lacking because geopolymers are affected by many variables. This study presents a method to find the optimal geopolymer mixture based on locally modified metakaolin as a source of aluminates and silicates using the Taguchi method. Metakaolin was modified using different contents of materials rich in silica, such as silica fume (SF), or materials rich in calcium, such as calcium oxide (CaO) of 5, 10, and 15%. The inclusion of 5% SF, 10% CaO, and a combination of 5% SF and 5% CaO as substitutes for metakaolin increases the compressive strength by 16.8, 6.9, and 22.8%, respectively, compared to the reference mixture without any modifications. In other words, adding 5% SF and CaO increased the molar ratio of SiO2/Al2O3 (R) from 1.782 to 1.914, resulting in the highest compressive strength of 53.3 MPa after 7 days of sun curing. To obtain the optimal mixture that achieves the highest compressive strength, the impact of three main variables, including the concentration of sodium hydroxide (SH), alkali solution-to-binder (Al/B) ratio, and sodium silicate-to-sodium hydroxide (SS/SH) ratio, must be considered using Taguchi method. A total of nine mixtures were investigated. It was found that 13 M of SH, 0.65 Al/B, and 2.5 SS/SH give a high compressive strength of 58.6 MPa at 7 days. Also, it was found that the concentration of SH plays a more important role in increasing the compressive strength than the alkali-to-binder ratio and SS/SH ratio. The scanning electron microscopy images show that the 5% weight replacement of metakaolin by silica and CaO could source fewer and smaller pores and reduce the microcracks’ width.

Keywords: geopolymer concrete; Taguchi method; sustainability; metakaolin

1 Introduction

The process of manufacturing Portland cement and the increase in its demand in the construction sector cause many problems related to sustainability in terms of the depletion of non-renewable natural resources and carbon dioxide emissions [1,2,3,4,5].

Geopolymer concrete (GPC) has mechanical properties comparable to or many times greater than ordinary Portland cement concrete [6]. In addition to not requiring cement, geopolymer concrete uses some industrial wastes such as fly ash, silica fume, and slag that harm the environment and convert them to material with associative properties. As a result, geopolymer concrete is becoming a sustainable material. Geopolymer concrete is made of two major components: an alkaline solution containing sodium hydroxide or potassium and materials with a high content of silica (SiO2) and alumina (Al2O3), such as metakaolin, fly ash, and silica fume (SF). These elements react together to develop the polymerization process and form a gel that binds with aggregates and, at last, produce geopolymer concrete. Geopolymer concrete does not need water curing but sun curing or curing at a temperature in an oven to significantly accelerate the polymerization method. It is widely used in the production of pre-cast members [2,7,8].

Geopolymer concrete’s compressive strength is mainly influenced by the Si/Al ratio (R-value). Some studies revealed that concrete based on metakaolin as a source of silica and alumina has a high compressive strength when this proportion exceeds a particular threshold [9]. The geopolymeric structure mainly comprises SiO4 and AlO4, which are linked to each other by covalent bonds formed by oxygen atoms. Davidovits [10] classifies geopolymeric (polysialate) into three main types depending on the ratio of Si/Al. Duxson et al. [11] studied the effect of Si/Al ratios (1.15, 1.4, 1.65, 1.95, and 2.15) on the compressive strength and found that the compressive strength tends to increase with the increase in the R ratio. Some previous studies illustrated that an increase in R-value leads to an increase in the number of bonds of Si–O–Si, which are stronger than the bonds of Si–O–Al or Al–O–Al, and at the same time, it was found that geopolymer concrete has the highest value of compressive strength when the ratio of Si/Al is 1.95 rather than 2.15. This is because the amount of non-reacted metakaolin is greater when the ratio is 2.15 than 1.95 and thus forms a defect factor in the geopolymer structure which negatively affects its mechanical properties [11,12].

The geopolymer mixture is sensitive to main variables such as the concentration and type of alkali solutions, the source of alumina and silica, the percentage of the alkali solution/binder, as well as the ratio of sodium or potassium silicate/sodium or potassium hydroxide [6,13]. Due to the lack of specific specifications for the design of geopolymer concrete, most researchers have resorted to using experiments and trying different methods to select mix proportions that achieve the optimum strength of geopolymer concrete. There is no consensus on how these factors impact geopolymer concrete, and it may be impossible to investigate all these aspects in one study. However, effective experimental methods like the Taguchi technique can study how some of these variables affect the quality of geopolymer concrete [14,15]. The Taguchi method is a statistical approach for exploring many variables with a small number of tests that use the orthogonal array. It helps to analyze data by optimizing the signal-to-noise ratio (S/N ratio) and predicting the optimum results. It is a simple, practical, low-cost technique [15,16]. Although the Taguchi approach is often employed in engineering applications, it is thought to have very few applications in developing geopolymer mixtures. The effect of the main variables, such as binder content, alkaline activator-to-binder content ratio (Al/B), sodium silicate-to-sodium hydroxide (SS/SH) ratio, and sodium hydroxide concentration (SH), on the geopolymer concrete based on slag as an aluminosilicate source material and the design of the mixture to achieve optimal strength were investigated by Hadi et al. [15]. When the number of experiments was 9, it was found that the binder content, Al/B, SS/SH, and SH concentration were 450 kg/m3, 0.35, 2.5, and 14 M, respectively, which achieved 60.4 MPa compressive strength, while the setting time was found to be short. Therefore, the setting time was improved by using fly ash, SF, and metakaolin to replace slag by weight partially.

The metakaolin-based geopolymer has a low compressive strength compared to other geopolymers based on slag and fly ash due to the low value of (R). This is shown by Ali et al. [17], who found that the compressive strength of the metakaolin-based geopolymer concrete was only 2.7 MPa at 7 days, which improved by 82, 86, 93, and 95% when the metakaolin content was partially replaced by fly ash and GGBS at 37, 70, 90, 72, and 95%, respectively.

Benalia et al. [18] developed a pozzolanic metakaolin binder and various proportions of slag to create a geopolymer mortar. The mechanical properties of mixtures with a high slag ratio at room temperature (20°C) were superior to those of mixtures based on metakaolin.

It is clear from the preceding that metakaolin must be developed as a binder to obtain the best mechanical properties for geopolymer concrete. All previous studies tended to combine metakaolin with other binders that do not contain silica in large quantities. This necessitates additions of these materials over 50% to achieve the best strength. In contrast, the present study focused on small and concentrated replacement ratios to modify the value of (R) and increase strength. This study is also concerned with determining the optimal mix proportion of the geopolymer mixture with modified metakaolin using the statistical Taguchi method for optimization, as well as the participation percentage of each major component and its effect on the strength of GPC.

The novelty of this study arises from the absence of existing research on the modification of local metakaolin as a source of silica and alumina for enhancing the compressive strength of geopolymer concrete. This is achieved by adjusting the value of (R) by including silica-rich materials such as SF or calcium-rich materials such as calcium oxide (CaO) to increase the bond’s strength or generate additional gel. Furthermore, these proportions are combined and explored to determine the optimal utilization value. Then, selecting the optimum mix proportion of geopolymer concrete under sun curing conditions, the average curing temperature was 46°C at daytime and 29°C at night [19] until the test time. The Taguchi method is used to achieve the highest compressive strength and desirable workability and recognize the suitable age of geopolymer concrete.

The research gives the appropriate design life for the geopolymer mixture based on metakaolin under certain environmental conditions, establishes a method for the possibility of modifying metakaolin in the manufacture of the geopolymer mixture, gives the optimal design for the geopolymer mixture by reducing the number of attempts, and displays microscopic images of various magnifications of geopolymer concrete based on metakaolin and modified metakaolin to compare between them starting from 1 mm up to 1 μm.

2 Research methodology

The following flow chart shows the methodology of the research (Figure 1).

Figure 1 Experimental program used during this study.
Figure 1

Experimental program used during this study.

3 Experimental work

3.1 Materials

As shown in Figure 2a, Iraqi kaolin from the Al-Anbar region was used as a source of silica and alumina to produce geopolymer concrete. It was subjected to several processes, including grinding and burning in an electric furnace at 700°C for 2 h to produce metakaolin. Metakaolin, as shown in Figure 2b, is preserved in plastic containers to avoid air humidity. The chemical and physical properties of metakaolin are shown in Tables 1 and 2, respectively. The results show that the metakaolin used in this study agrees with the American specifications ASTM C618 [20] as a natural pozzolan, class N.

Figure 2 Some of the materials used in this research. (a) Kaolin raw material. (b) Metakaolin. (c) Sodium silicate. (d) Natural coarse aggregate. (e) Natural fine aggregate. (f) Sodium hydroxide.
Figure 2

Some of the materials used in this research. (a) Kaolin raw material. (b) Metakaolin. (c) Sodium silicate. (d) Natural coarse aggregate. (e) Natural fine aggregate. (f) Sodium hydroxide.

Table 1

Chemical properties of metakaolin

Oxide Composition Weight (%) Requirements of ASTM C 618 [20]
SiO2 62.410 SiO2 + Al2O3 + Fe2O3 = 98.327 ≥ 70
Al2O3 35.026
Fe2O3 0.891
K2O 0.908
TiO2 0.531
CaO 0.143
SO3 0.027 ≤4%
MnO 0.002
LOI 0.71 ≤10%

LOI: loss of ignition.

Table 2

Physical properties of metakaolin

Physical properties MK Requirements of ASTM C 618 [20]
Strength activity index at 7 days (%) 113 ≥75%
Retained on 45 µm (%) 18.5 ≤34%
Specific surface area (m2/kg) 14,300 —
Specific gravity 2.64 —
Color White–pinky powder —

Sodium hydroxide (NaOH) is the main component of the alkaline solution employed in this investigation from Mahaco industrial company [21], which was tested according to the ASTM E 291 [22], and sodium silicate (Na2SiO3) from DUBAI CHEM Company [23]. The properties of these two compounds are shown in Tables 3 and 4, while Figure 2c and f gives the images of these materials.

Table 3

Properties of sodium silicate

Description Value [23]
Na2O percent by weight 13.10–13.70
SiO2 percent by weight 32.00–33.00
SiO2/Na2O ratio 2.4 ± 0.05
Specific gravity 1.534–1.551
Viscosity (CPS) 20°C 600–1,200
Appearance Hazy
Table 4

Properties of sodium hydroxide prills

Chemical composition Results [21] Specification [24]
Sodium hydroxide content 99.5% 99.0% wt. min*
Na2CO3 0.54% 0.94% wt. max
NaCl 0.02% 0.022% wt. max
Iron 1.3 ppm 5 ppm wt. max
Na2So4 0.003% 0.038% wt. max
Appearance White, flaky to pellet shape

*On a dry basis.

The natural fine aggregate in Figure 2e used in this study was brought from the Al-Ukhaidir area/Iraq, with a maximum size of 4.75 mm. The sieve analysis and the properties of the fine aggregate are shown in Table 5. The results illustrate that the used fine aggregate complies with the Iraqi standard (IQS) No. 45/2016, zone II [25].

Table 5

Properties of fine aggregate

Sieve size (mm) Cumulative passing (%) Limits of IQS No. 45 for zone II [25]
10 100 100
4.75 94 90–100
2.36 82 75–100
1.18 68 55–90
0.6 51 35–59
0.3 27 8–30
0.15 8 0–10
Material passing from sieve 75 µm (%) 3 ≤5%
Sulfate content (%) 0.085 ≤0.5%
Fineness modulus 2.71 —
Absorption (%) 1.8 —
Specific gravity 2.6 —
Bulk density (kg/m3) 1,744 —

Natural crushed coarse aggregate from the Al-Badrah region/Iraq, with a maximum size of 10 mm that agrees with Iraqi specifications No. 45/2016 [25], was used in this study and shown in Figure 2d. The properties of the natural coarse aggregate are presented in Table 6.

Table 6

Properties of coarse aggregate

Sieve size (mm) Cumulative passing (%) Limits of IQS No. 45 [25]
10 97 85–100
5 12 0–25
2.36 — 0–5
Material passing from sieve 75 µm (%) 0.3 ≥3
Dry density (kg/m3) 1,627 —
Specific gravity 2.62 —
Absorption (%) 0.6 —
Sulfate content (%)

The water used for dissolving the sodium hydroxide and as extra water in the mixture was potable (tap water).

A high-range water reducer (HRWR) with the commercial mark of Flocrete SP33 [26] was used. It is free from chlorides and agrees with ASTM C494 [27] types A and F. Table 7 shows the main properties of HRWR.

Table 7

Properties of high-range water reducer

Property Description [26]
Appearance Dark brown liquids
Specific gravity 1.17–1.21
Chloride content Nil
pH 6.5
Recommended dosage 0.8–2.8 L/100 kg binder

Sika company [31] produces the SF used in this study. Table 8 shows the physical characteristics, strength activity index, and chemical composition of SF. The results show that the SF used meets the ASTM C1240 [32] chemical and physical requirements.

Table 8

Properties of SF

Property Results Requirements of ASTM C1240 [28]
 Specific surface area (m2/kg) 19,200 ≥15,000
 Strength activity index with Portland cement at 7 days (%) 122 ≥105
 Retained on sieve 45 µm, max (%) 9 ≤10
 Specific gravity 2.2 —
 Color Gray —
Oxides composition Results (%)
 SiO2 88.593 ≥85
 Al2O3 — —
 Fe2O3 5.564 —
 K2O 4.777 —
 TiO2 — —
 CaO 0.666 —
 SO3 0.027 —
 MnO 0.276 —

The CaO used in this study is from the Karbala factory/in Iraq [33]. The physical and chemical properties of CaO are given in Table 9.

Table 9

Properties of CaO

Property Results
 Specific surface area (m2/kg) 16,350
 Specific gravity 3.3
 Color White
Oxides composition Results (%)
 SiO2 4.314
 Al2O3 —
 Fe2O3 0.461
 K2O 1.667
 TiO2 —
 CaO 93.40
 SO3 0.10
 MnO 0.025

3.2 Selection of geopolymer concrete mix proportions

The preliminary selection of the geopolymer mixture was based on previous studies [2,29]. The binder content (metakaolin) was 414 kg/m3, the concentration of sodium hydroxide was 12 M, the sodium silicate/sodium hydroxide was 2.5, and the alkali solution/binder ratio was 0.55. Many trial mixes were conducted to determine the geopolymer mixture’s practical age and curing condition. Table 10 shows that the suitable age was seven days with the curing condition in the sun during August. The average temperature was measured at 12:00–2:00 pm with 46°C at daytime and 29°C at night until the test age of 7 days was reached with a compressive strength of 43.4 MPa. Finally, SF, CaO, and binary replacement of silica and CaO were used to modify metakaolin-based geopolymer concrete.

Table 10

The compressive strength of geopolymer concrete at different ages

Curing age (days) Compressive strength (MPa)
7 43.4
14 50.3
28 50.8

3.3 Optimum mix design for geopolymer concrete

Compressive strength is the most important property of hardened concrete, and it expresses the degree of its quality and appropriateness [30,31]. The compressive strength is obtained from the trial mixes of geopolymer concrete using the Taguchi method to find the optimum geopolymer mix proportions [32], which gives the highest compressive strength under sun-curing conditions at seven days. Program Qualitek-4 was used to achieve this purpose [32]. Figure 3 explains the stages of using the Taguchi method. Three main variables were used, including (SH), the (SS/SH), and the (Al/B), with three levels for each, as shown in Table 11. The binder content and workability were kept constant at 372 kg/m3 and a slump of 180 ± 10 mm. This program reduced the number of experiments from 27 to 9.

Figure 3 Stages of Taguchi method for design of experiments of geopolymer concrete.
Figure 3

Stages of Taguchi method for design of experiments of geopolymer concrete.

Table 11

Variables and proportions used in the Taguchi experiment design

Variables Level 1 Level 2 Level 3
SH (M) 12 13 14
SS/SH 2.5 3 3.5
Al/B 0.55 0.65 0.75

3.4 Mixing procedure, preparation, and curing of specimens

Mixing is one of the most essential phases in creating geopolymer concrete. The geopolymer mixture should have a homogenous and uniform consistency. The mixing process was carried out in the following steps:

  • Before adding the ingredients, the electric rotary mixer of 0.1 m3 capacity was cleaned and wetted with water.

  • As a partial substitute for metakaolin, 5% SF and 5% CaO were manually blended with metakaolin for 2 min.

  • The dry materials (modified metakaolin, fine aggregate, and coarse aggregate) were blended in a mixing bowl for 2 min.

  • The alkaline solution was added to the dry components of the mixture while the mixer was rotating, and then the ingredients were mixed for 2–3 min.

  • At this point, additional water and superplasticizer were manually combined before being progressively added to the mixture.

  • After that, the electric mixer was paused for 2 min to rest, and the blades were cleaned.

  • The mixer was then worked for one minute to get a homogenous metakaolin geopolymer concrete mixture with a satisfactory consistency. The whole mixing time varies between 9 and 10 min. Figure 4 shows the details of the mixing process.

Figure 4 The flow chart of the mixing process.
Figure 4

The flow chart of the mixing process.

Analyzing the workability, casting, and compaction of geopolymer concrete required extra attention since geopolymer concrete is mixed with alkaline, which makes the fresh geopolymer concrete sticker than fresh Portland cement concrete. Moulds were cleaned and polished with grease. The fresh concrete was consolidated by vibrating for 60 s to remove air. After troweling the surfaces of the specimens, they were covered with polyethene sheets and kept in the laboratory for 24 h. After that, the moulds were opened. In this study, the practical age of geopolymer concrete to give the best compressive strength was considered to be seven days with curing of specimens at a temperature range between 46°C at daytime and 29°C at night in summer.

3.5 Testing method and specimen details

Eighty-four 100 mm cubes were prepared in this investigation to determine the average of three compressive strength values for each case (determining the appropriate age, modifying metakaolin, and obtaining the optimal mix proportions) for geopolymer concrete. The specimens were tested under compression using a testing machine with a capacity of 2,000 kN and a loading rate of 0.25 N/mm2 s, as depicted in Figure 5.

Figure 5 Compressive strength test.
Figure 5

Compressive strength test.

4 Results and discussion

4.1 Effect of metakaolin modification on the compressive strength

The compressive strength of geopolymer concrete is significantly influenced by the Si/Al (R) ratio, with a higher R-value resulting in increased compressive strength [33]. The Si/Al ratio for the metakaolin used in this study, according to Table 1, is equal to 1.78. The increase in R-value caused by utilizing a silica-rich substance, such as SF, is illustrated in Table 12. The compressive strength was enhanced to 46.4 MPa by replacing 10% of the metakaolin weight with SF, and subsequently, it was lowered by replacing the metakaolin with 15% SF. This increase in compressive strength is because the rise of R-value strengthens the Si–O–Si bond in the microstructure of geopolymer concrete, which is stronger than the Si–O–Al bond [11,12]. Due to the convergence in compressive strength of mixtures with 5 and 10% SF content, 5% was the sensible replacement decision.

Table 12

Modified metakaolin by SF

SF content (%) MK (kg/m3) SF (kg/m3) Change in R * value Compressive strength at 7 days (MPa)
0 414 — 1.781 43.4
5 393 21 1.908 46.02
10 372.5 41.5 2.03 46.4
15 352 62 2.16 33.8

*SiO2/Al2O3 by weight in the source material.

In the case of using CaO in the process of modifying metakaolin, it can be observed in Table 13 that increasing the CaO by 5% increases the compressive strength up to 53 MPa while increasing the CaO by more than 5% decreases the compressive strength. The inclusion of calcium ions in the geopolymeric structure aids in forming an additional gel of Ca–Al–Si, resulting in an increase in setting time and strength [34]. However, when this proportion increases, the strength decreases. This is due to the increase in gel in the form of C–S–H, which may interfere with the polymerization process and alter the microstructure [35].

Table 13

Modified metakaolin by CaO

CaO content (%) MK (kg/m3) CaO (kg/m3) Compressive strength at 7 days (MPa)
0 414 — 43.4
5 393 21 50.7
10 372.5 41.5 48.5
15 352 62 46.9

Accordingly, using the binary of the optimum proportions of each SF and CaO (5% SF and 5% CaO) adjust the R-value from 1.78 to 1.914 for the metakaolin used in geopolymer concrete, thus leading to an increase in the compressive strength from 44.4 to 53.3 MPa as shown in Table 14. This optimum R-value (1.914) is comparable to the R-value calculated by Duxson et al., which gave the highest compressive strength [11]. The compressive strength was reduced when the R-value exceeded 1.914, as illustrated in Table 14. This is because the amount of non-reacted metakaolin increases, forming a defect factor in the geopolymeric structure and reducing its mechanical characteristics [11,12].

Table 14

Modified metakaolin by SF and CaO

SF content (%) CaO content (%) MK (kg/m3) CaO (kg/m3) SF (kg/m3) Change in R value Compressive strength at 7 days (MPa)
0 0 414 — — 1.781 43.4
5 5 372 21 21 1.914 53.3
5 10 351.5 41.5 21 1.921 48.3
5 15 331 62 21 1.926 42.9

4.2 Mix proportions optimization for modified metakaolin-based GPC by Taguchi method

The compressive strength was relied on as a primary criterion in evaluating the resulting experimental mixtures according to the Taguchi method in geopolymer concrete. Generally, compressive strength is an essential indicator in assessing the properties of concrete [31]. The number of trial mixes was reduced from 27 to 9 using the Taguchi method, as shown in Table 15. Figure 6 shows the compressive strength values for the nine geopolymer mixtures at seven days of age. Mixture (4) with SH = 13, SS/SH = 2.5, and Al/B = 0.65 has the highest compressive strength, while mixture (1) with variables of SH = 12, SS/SH = 2.5, and Al/B = 0.55 has the lowest compressive strength. The difference between mixtures (1) and (4) was increased sodium hydroxide (SH) concentration and the alkaline solution/binder ratio (Al/B). The increase in the compressive strength of geopolymer concrete with increasing the concentration of sodium hydroxide is due to the increase in the decomposition of the alumina and silica in metakaolin, thus enhancing the polymerization process, which leads to an increase in the compressive strength of geopolymer concrete [36]. However, some researchers gave an optimum value for this increase because increased sodium hydroxide concentration from the specified limit leads to decreased workability. Then, an increase in the porosity leads to a reduction in the compressive strength that is inversely proportional to the porosity [31]. Figure 7a shows that the optimum value of calcium hydroxide concentration is 13 M, which gives the highest compressive strength of 58.6 MPa.

Table 15

Mix proportion of trail mixes for geopolymer concrete

Mix no. Variables MK* CA* FA* SS* SH* SP* W* Compressive strength (MPa)
(kg/m3)
M1 SH = 12, SS/SH = 2.5, Al/B = 0.55 372 955 632 162 71 8.3 85 33.4
M2 SH = 12, SS/SH = 3, Al/B = 0.65 372 912 603 201 74 4 50 43.4
M3 SH = 12, SS/SH = 3.5, Al/B = 0.75 372 870 575 241 75 4 28 44.0
M4 SH = 13, SS/SH = 2.5, Al/B = 0.65 372 911 603 192 83 4 54 58.6
M5 SH = 13, SS/SH = 3, Al/B = 0.75 372 869 575 232 84 4 30 58.0
M6 SH = 13, SS/SH = 3.5, Al/B = 0.55 372 956 633 176 57 8.3 66 53.1
M 7 SH = 14, SS/SH = 2.5, Al/B = 0.75 372 868 574 221 95 4 32 57.0
M8 SH = 14, SS/SH = 3, Al/B = 0.55 372 956 632 170 63 8.3 85 53.5
M9 SH = 14, SS/SH = 3.5, Al/B = 0.65 372 913 604 209 66 4 54 57.4
R SH = 12, SS/SH = 2.5, Al/B = 0.55 414 911 574 193 83 4.8 50 44.4

*MK: metakaolin, CA: coarse aggregate, FA: fine aggregate, SS: sodium silicate, SH: sodium hydroxide, SP: superplasticizer, W: water.

Figure 6 The compressive strength of different geopolymer concrete mixtures.
Figure 6

The compressive strength of different geopolymer concrete mixtures.

Figure 7 The main parameters affecting geopolymer concrete’s compressive strength under ambient curing conditions. (a) Effect of SH concentration on the compressive strength. (b) Effect of Al/B ratio on compressive strength. (c) Effect of SS/SH ratio on the compressive strength.
Figure 7

The main parameters affecting geopolymer concrete’s compressive strength under ambient curing conditions. (a) Effect of SH concentration on the compressive strength. (b) Effect of Al/B ratio on compressive strength. (c) Effect of SS/SH ratio on the compressive strength.

The alkali solution-to-binder ratio significantly impacts the compressive strength of geopolymer concrete [37]. When this ratio increases to a specific value, the compressive strength increases due to improved workability and, thus, reduced porosity [31]. However, the increase of alkaline content beyond the optimal limit leads to a decrease in compressive strength. This is attributed to the increased amount of water in the alkaline solution, which hindered the polymerization process by decreasing the alkali concentration and increased porosity due to water evaporation from pores, which reduces the compressive strength [38].

Table 16 shows the percentage of participation of each variable on the compressive strength of geopolymer concrete at seven days of age. The participation of SH, Al/B, and SS/SH are 83.632, 0.914, and 13.143, respectively. It can be observed that the SH concentration is the most influencing factor on the geopolymer concrete, with the participation of 83.632%, and the optimum value of SH concentration was 13 M. As shown in Figure 7a, when the sodium hydroxide concentration increases, the compressive strength increases up to a specific limit and then decreases.

Table 16

The participation and optimum levels of the main variables on the compressive strength of geopolymer concrete at 7 days

Parameters SH SS/SH Al/B
Percentage of participation (%) 83.632 0.914 13.143
Optimum level 13 3 0.65

It can also be demonstrated that the Al/B ratio, with a participation rate of 13.143%, is the second important factor. Figure 7b and Table 16 explain that an Al/B ratio of 0.65 is optimal. This means that an Al/B ratio higher than 0.65 reduces the compressive strength of geopolymer concrete.

The SS/SH ratio has the lowest participation rate, 0.914%. The optimum SS/SH ratio is 3, as shown in Table 16 and Figure 7c. This means a high percentage of SS to SH may produce geopolymer concrete with a high compressive strength.

4.3 Scanning electron microscopy (SEM) of metakaolin and modified metakaolin geopolymer concrete

The fractured surfaces of the GPC after seven days are represented in Figures 8–11, which were created using images from an SEM. The primary reaction product of metakaolin activation is Na2OAl2O3SiO2H2O (NASH) geopolymer gel, which may be considered a zeolite predecessor [39]. Figure 5 shows the images for GPC specimens with 100% metakaolin. It can be seen that N–A–S–H gel has formed, and the presence of more microcracks and large voids. The matrices of geopolymer specimens with 5% SF as a replacement to metakaolin by weight seemed dense and compact matrix with fewer microcracks than the reference specimens, as shown in Figures 8 and 9. This implies a strong Si–O–Si bonding, resulting in a compressive strength of 46 MPa. It may be noted that the geopolymer matrix without SF has more numbers of pores present. Due to the inclusion of SF’s inclusion, geopolymer mixtures have an enhanced polymerization process due to their amorphous nature and high specific surface area.

Figure 8 SEM for the fracture surface of specimens without wastes (R 0) with different magnification levels.
Figure 8

SEM for the fracture surface of specimens without wastes (R 0) with different magnification levels.

Figure 9 SEM for the fracture surface of specimens containing 5% SF with different magnification levels.
Figure 9

SEM for the fracture surface of specimens containing 5% SF with different magnification levels.

Figure 10 SEM for the fracture surface of specimens containing 5% CaO with different magnification levels.
Figure 10

SEM for the fracture surface of specimens containing 5% CaO with different magnification levels.

Figure 11 SEM for the fracture surface of specimens containing 5% SF and 5% CaO with different magnification levels.
Figure 11

SEM for the fracture surface of specimens containing 5% SF and 5% CaO with different magnification levels.

Alkaline activation of metakaolin and CaO-containing compounds results in the formation of NASH and CSH [39]. Changes in the composition and microstructure of geopolymer concrete result from the interaction between these gels [40]. The increase in the calcium demonstrates the co-existence of N–A–S–H and C–S–H gels. Figure 10 shows the images of SEM for specimens produced from a mixture of metakaolin with 5% CaO as a weight replacement, and it can be observed that the matrix seems more condensed. In addition, there are fewer microcracks and pores, as compared with the specimens without CaO. Thus, this was reflected positively on the mechanical properties of the geopolymer mixture by increasing the compressive strength to 50.7 MPa. The optimum binary inclusion of SF 5%, and CaO 5% as a replacement by weight of metakaolin are expressed in the images shown in Figure 11. It can be seen that the dense of two geopolymer gels with very small pores and homogeneous microstructure improves the mechanical properties of the modified geopolymer concrete which have the highest compressive strength up to 53.3 MPa.

5 Conclusions

The following conclusions may be drawn from the findings and experimental work reported in this study:

  1. The optimal service life for geopolymer concrete is seven days in summer weather conditions, with an average daytime temperature of 46°C and a nighttime temperature of 29°C.

  2. The optimum percentages for including silica fume, CaO, or both to obtain the highest compressive strength compared to the reference mixture with the strength of 43.4 MPa were 5, 10, or 5% as a replacement by weight of metakaolin. These proportions resulted in 46, 50.7, or 53.3 MPa compressive strengths.

  3. Metakaolin can be modified by increasing the SiO2/Al2O3 (R-value) to a specific limit to obtain higher strength than using only local metakaolin in geopolymer concrete as a base material. The percentage increase of strength was 22.8% compared with the reference specimens without replacement when the metakaolin is modified with silica fume and CaO by 5% replacement by weight.

  4. The optimum geopolymer concrete mixture possesses parameters that include sodium hydroxide concentration of 13 M, sodium silicate/sodium hydroxide ratio of 2.5, alkaline solution/binder ratio of 0.65, and the amount of modified Metakaolin binder of 372 kg/m3 by binary percentages of 5% SF and 5% CaO as a replacement by weight gave the highest compressive strength of 58.6 MPa at 7 days age and under weathering conditions in summer.

  5. SEM shows that the geopolymer concrete matrix, including 5% SF and 5% CaO, exhibited uniform distribution. In contrast, reference geopolymer concrete without additives exhibited less uniformity and slightly more pores with wider cracks. This explains the good mechanical behavior of geopolymer concrete based on locally modified metakaolin mixes.

Acknowledgments

The authors would like to gratefully acknowledge the National Center for Construction Laboratories staff for their assistance in conducting the tests.

  1. Funding information: Authors declare that the manuscript was done depending on the personal effort of the author, and there is no funding effort from any side or organization.

  2. Conflict of interest: The authors state no conflict of interest.

  3. Data availability statement: Most datasets generated and analyzed in this study are comprised in this submitted manuscript. The other datasets are available on reasonable request from the corresponding author with the attached information.

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Received: 2023-09-26
Revised: 2023-11-09
Accepted: 2023-11-10
Published Online: 2024-03-06

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/eng-2022-0561
Keywords for this article
geopolymer concrete; Taguchi method; sustainability; metakaolin
Creative Commons
BY 4.0

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  17. Application of the Taguchi method and RSM for process parameter optimization in AWSJ machining of CFRP composite-based orthopedic implants
  18. Improved correlation of soil modulus with SPT N values
  19. Technologies for high-temperature batch annealing of grain-oriented electrical steel: An overview
  20. Assessing the need for the adoption of digitalization in Indian small and medium enterprises
  21. A non-ideal hybridization issue for vertical TFET-based dielectric-modulated biosensor
  22. Optimizing data retrieval for enhanced data integrity verification in cloud environments
  23. Performance analysis of nonlinear crosstalk of WDM systems using modulation schemes criteria
  24. Nonlinear finite-element analysis of RC beams with various opening near supports
  25. Thermal analysis of Fe3O4–Cu/water over a cone: a fractional Maxwell model
  26. Radial–axial runner blade design using the coordinate slice technique
  27. Theoretical and experimental comparison between straight and curved continuous box girders
  28. Effect of the reinforcement ratio on the mechanical behaviour of textile-reinforced concrete composite: Experiment and numerical modeling
  29. Experimental and numerical investigation on composite beam–column joint connection behavior using different types of connection schemes
  30. Enhanced performance and robustness in anti-lock brake systems using barrier function-based integral sliding mode control
  31. Evaluation of the creep strength of samples produced by fused deposition modeling
  32. A combined feedforward-feedback controller design for nonlinear systems
  33. Effect of adjacent structures on footing settlement for different multi-building arrangements
  34. Analyzing the impact of curved tracks on wheel flange thickness reduction in railway systems
  35. Review Articles
  36. Mechanical and smart properties of cement nanocomposites containing nanomaterials: A brief review
  37. Applications of nanotechnology and nanoproduction techniques
  38. Relationship between indoor environmental quality and guests’ comfort and satisfaction at green hotels: A comprehensive review
  39. Communication
  40. Techniques to mitigate the admission of radon inside buildings
  41. Erratum
  42. Erratum to “Effect of short heat treatment on mechanical properties and shape memory properties of Cu–Al–Ni shape memory alloy”
  43. Special Issue: AESMT-3 - Part II
  44. Integrated fuzzy logic and multicriteria decision model methods for selecting suitable sites for wastewater treatment plant: A case study in the center of Basrah, Iraq
  45. Physical and mechanical response of porous metals composites with nano-natural additives
  46. Special Issue: AESMT-4 - Part II
  47. New recycling method of lubricant oil and the effect on the viscosity and viscous shear as an environmentally friendly
  48. Identify the effect of Fe2O3 nanoparticles on mechanical and microstructural characteristics of aluminum matrix composite produced by powder metallurgy technique
  49. Static behavior of piled raft foundation in clay
  50. Ultra-low-power CMOS ring oscillator with minimum power consumption of 2.9 pW using low-voltage biasing technique
  51. Using ANN for well type identifying and increasing production from Sa’di formation of Halfaya oil field – Iraq
  52. Optimizing the performance of concrete tiles using nano-papyrus and carbon fibers
  53. Special Issue: AESMT-5 - Part II
  54. Comparative the effect of distribution transformer coil shape on electromagnetic forces and their distribution using the FEM
  55. The complex of Weyl module in free characteristic in the event of a partition (7,5,3)
  56. Restrained captive domination number
  57. Experimental study of improving hot mix asphalt reinforced with carbon fibers
  58. Asphalt binder modified with recycled tyre rubber
  59. Thermal performance of radiant floor cooling with phase change material for energy-efficient buildings
  60. Surveying the prediction of risks in cryptocurrency investments using recurrent neural networks
  61. A deep reinforcement learning framework to modify LQR for an active vibration control applied to 2D building models
  62. Evaluation of mechanically stabilized earth retaining walls for different soil–structure interaction methods: A review
  63. Assessment of heat transfer in a triangular duct with different configurations of ribs using computational fluid dynamics
  64. Sulfate removal from wastewater by using waste material as an adsorbent
  65. Experimental investigation on strengthening lap joints subjected to bending in glulam timber beams using CFRP sheets
  66. A study of the vibrations of a rotor bearing suspended by a hybrid spring system of shape memory alloys
  67. Stability analysis of Hub dam under rapid drawdown
  68. Developing ANFIS-FMEA model for assessment and prioritization of potential trouble factors in Iraqi building projects
  69. Numerical and experimental comparison study of piled raft foundation
  70. Effect of asphalt modified with waste engine oil on the durability properties of hot asphalt mixtures with reclaimed asphalt pavement
  71. Hydraulic model for flood inundation in Diyala River Basin using HEC-RAS, PMP, and neural network
  72. Numerical study on discharge capacity of piano key side weir with various ratios of the crest length to the width
  73. The optimal allocation of thyristor-controlled series compensators for enhancement HVAC transmission lines Iraqi super grid by using seeker optimization algorithm
  74. Numerical and experimental study of the impact on aerodynamic characteristics of the NACA0012 airfoil
  75. Effect of nano-TiO2 on physical and rheological properties of asphalt cement
  76. Performance evolution of novel palm leaf powder used for enhancing hot mix asphalt
  77. Performance analysis, evaluation, and improvement of selected unsignalized intersection using SIDRA software – Case study
  78. Flexural behavior of RC beams externally reinforced with CFRP composites using various strategies
  79. Influence of fiber types on the properties of the artificial cold-bonded lightweight aggregates
  80. Experimental investigation of RC beams strengthened with externally bonded BFRP composites
  81. Generalized RKM methods for solving fifth-order quasi-linear fractional partial differential equation
  82. An experimental and numerical study investigating sediment transport position in the bed of sewer pipes in Karbala
  83. Role of individual component failure in the performance of a 1-out-of-3 cold standby system: A Markov model approach
  84. Implementation for the cases (5, 4) and (5, 4)/(2, 0)
  85. Center group actions and related concepts
  86. Experimental investigation of the effect of horizontal construction joints on the behavior of deep beams
  87. Deletion of a vertex in even sum domination
  88. Deep learning techniques in concrete powder mix designing
  89. Effect of loading type in concrete deep beam with strut reinforcement
  90. Studying the effect of using CFRP warping on strength of husk rice concrete columns
  91. Parametric analysis of the influence of climatic factors on the formation of traditional buildings in the city of Al Najaf
  92. Suitability location for landfill using a fuzzy-GIS model: A case study in Hillah, Iraq
  93. Hybrid approach for cost estimation of sustainable building projects using artificial neural networks
  94. Assessment of indirect tensile stress and tensile–strength ratio and creep compliance in HMA mixes with micro-silica and PMB
  95. Density functional theory to study stopping power of proton in water, lung, bladder, and intestine
  96. A review of single flow, flow boiling, and coating microchannel studies
  97. Effect of GFRP bar length on the flexural behavior of hybrid concrete beams strengthened with NSM bars
  98. Exploring the impact of parameters on flow boiling heat transfer in microchannels and coated microtubes: A comprehensive review
  99. Crumb rubber modification for enhanced rutting resistance in asphalt mixtures
  100. Special Issue: AESMT-6
  101. Design of a new sorting colors system based on PLC, TIA portal, and factory I/O programs
  102. Forecasting empirical formula for suspended sediment load prediction at upstream of Al-Kufa barrage, Kufa City, Iraq
  103. Optimization and characterization of sustainable geopolymer mortars based on palygorskite clay, water glass, and sodium hydroxide
  104. Sediment transport modelling upstream of Al Kufa Barrage
  105. Study of energy loss, range, and stopping time for proton in germanium and copper materials
  106. Effect of internal and external recycle ratios on the nutrient removal efficiency of anaerobic/anoxic/oxic (VIP) wastewater treatment plant
  107. Enhancing structural behaviour of polypropylene fibre concrete columns longitudinally reinforced with fibreglass bars
  108. Sustainable road paving: Enhancing concrete paver blocks with zeolite-enhanced cement
  109. Evaluation of the operational performance of Karbala waste water treatment plant under variable flow using GPS-X model
  110. Design and simulation of photonic crystal fiber for highly sensitive chemical sensing applications
  111. Optimization and design of a new column sequencing for crude oil distillation at Basrah refinery
  112. Inductive 3D numerical modelling of the tibia bone using MRI to examine von Mises stress and overall deformation
  113. An image encryption method based on modified elliptic curve Diffie-Hellman key exchange protocol and Hill Cipher
  114. Experimental investigation of generating superheated steam using a parabolic dish with a cylindrical cavity receiver: A case study
  115. Effect of surface roughness on the interface behavior of clayey soils
  116. Investigated of the optical properties for SiO2 by using Lorentz model
  117. Measurements of induced vibrations due to steel pipe pile driving in Al-Fao soil: Effect of partial end closure
  118. Experimental and numerical studies of ballistic resistance of hybrid sandwich composite body armor
  119. Evaluation of clay layer presence on shallow foundation settlement in dry sand under an earthquake
  120. Optimal design of mechanical performances of asphalt mixtures comprising nano-clay additives
  121. Advancing seismic performance: Isolators, TMDs, and multi-level strategies in reinforced concrete buildings
  122. Predicted evaporation in Basrah using artificial neural networks
  123. Energy management system for a small town to enhance quality of life
  124. Numerical study on entropy minimization in pipes with helical airfoil and CuO nanoparticle integration
  125. Equations and methodologies of inlet drainage system discharge coefficients: A review
  126. Thermal buckling analysis for hybrid and composite laminated plate by using new displacement function
  127. Investigation into the mechanical and thermal properties of lightweight mortar using commercial beads or recycled expanded polystyrene
  128. Experimental and theoretical analysis of single-jet column and concrete column using double-jet grouting technique applied at Al-Rashdia site
  129. The impact of incorporating waste materials on the mechanical and physical characteristics of tile adhesive materials
  130. Seismic resilience: Innovations in structural engineering for earthquake-prone areas
  131. Automatic human identification using fingerprint images based on Gabor filter and SIFT features fusion
  132. Performance of GRKM-method for solving classes of ordinary and partial differential equations of sixth-orders
  133. Visible light-boosted photodegradation activity of Ag–AgVO3/Zn0.5Mn0.5Fe2O4 supported heterojunctions for effective degradation of organic contaminates
  134. Production of sustainable concrete with treated cement kiln dust and iron slag waste aggregate
  135. Key effects on the structural behavior of fiber-reinforced lightweight concrete-ribbed slabs: A review
  136. A comparative analysis of the energy dissipation efficiency of various piano key weir types
  137. Special Issue: Transport 2022 - Part II
  138. Variability in road surface temperature in urban road network – A case study making use of mobile measurements
  139. Special Issue: BCEE5-2023
  140. Evaluation of reclaimed asphalt mixtures rejuvenated with waste engine oil to resist rutting deformation
  141. Assessment of potential resistance to moisture damage and fatigue cracks of asphalt mixture modified with ground granulated blast furnace slag
  142. Investigating seismic response in adjacent structures: A study on the impact of buildings’ orientation and distance considering soil–structure interaction
  143. Improvement of porosity of mortar using polyethylene glycol pre-polymer-impregnated mortar
  144. Three-dimensional analysis of steel beam-column bolted connections
  145. Assessment of agricultural drought in Iraq employing Landsat and MODIS imagery
  146. Performance evaluation of grouted porous asphalt concrete
  147. Optimization of local modified metakaolin-based geopolymer concrete by Taguchi method
  148. Effect of waste tire products on some characteristics of roller-compacted concrete
  149. Studying the lateral displacement of retaining wall supporting sandy soil under dynamic loads
  150. Seismic performance evaluation of concrete buttress dram (Dynamic linear analysis)
  151. Behavior of soil reinforced with micropiles
  152. Possibility of production high strength lightweight concrete containing organic waste aggregate and recycled steel fibers
  153. An investigation of self-sensing and mechanical properties of smart engineered cementitious composites reinforced with functional materials
  154. Forecasting changes in precipitation and temperatures of a regional watershed in Northern Iraq using LARS-WG model
  155. Experimental investigation of dynamic soil properties for modeling energy-absorbing layers
  156. Numerical investigation of the effect of longitudinal steel reinforcement ratio on the ductility of concrete beams
  157. An experimental study on the tensile properties of reinforced asphalt pavement
  158. Self-sensing behavior of hot asphalt mixture with steel fiber-based additive
  159. Behavior of ultra-high-performance concrete deep beams reinforced by basalt fibers
  160. Optimizing asphalt binder performance with various PET types
  161. Investigation of the hydraulic characteristics and homogeneity of the microstructure of the air voids in the sustainable rigid pavement
  162. Enhanced biogas production from municipal solid waste via digestion with cow manure: A case study
  163. Special Issue: AESMT-7 - Part I
  164. Preparation and investigation of cobalt nanoparticles by laser ablation: Structure, linear, and nonlinear optical properties
  165. Seismic analysis of RC building with plan irregularity in Baghdad/Iraq to obtain the optimal behavior
  166. The effect of urban environment on large-scale path loss model’s main parameters for mmWave 5G mobile network in Iraq
  167. Formatting a questionnaire for the quality control of river bank roads
  168. Vibration suppression of smart composite beam using model predictive controller
  169. Machine learning-based compressive strength estimation in nanomaterial-modified lightweight concrete
  170. In-depth analysis of critical factors affecting Iraqi construction projects performance
  171. Behavior of container berth structure under the influence of environmental and operational loads
  172. Energy absorption and impact response of ballistic resistance laminate
  173. Effect of water-absorbent polymer balls in internal curing on punching shear behavior of bubble slabs
  174. Effect of surface roughness on interface shear strength parameters of sandy soils
  175. Evaluating the interaction for embedded H-steel section in normal concrete under monotonic and repeated loads
  176. Estimation of the settlement of pile head using ANN and multivariate linear regression based on the results of load transfer method
  177. Enhancing communication: Deep learning for Arabic sign language translation
  178. A review of recent studies of both heat pipe and evaporative cooling in passive heat recovery
  179. Effect of nano-silica on the mechanical properties of LWC
  180. An experimental study of some mechanical properties and absorption for polymer-modified cement mortar modified with superplasticizer
  181. Digital beamforming enhancement with LSTM-based deep learning for millimeter wave transmission
  182. Developing an efficient planning process for heritage buildings maintenance in Iraq
  183. Design and optimization of two-stage controller for three-phase multi-converter/multi-machine electric vehicle
  184. Evaluation of microstructure and mechanical properties of Al1050/Al2O3/Gr composite processed by forming operation ECAP
  185. Calculations of mass stopping power and range of protons in organic compounds (CH3OH, CH2O, and CO2) at energy range of 0.01–1,000 MeV
  186. Investigation of in vitro behavior of composite coating hydroxyapatite-nano silver on 316L stainless steel substrate by electrophoretic technic for biomedical tools
  187. A review: Enhancing tribological properties of journal bearings composite materials
  188. Improvements in the randomness and security of digital currency using the photon sponge hash function through Maiorana–McFarland S-box replacement
  189. Design a new scheme for image security using a deep learning technique of hierarchical parameters
  190. Special Issue: ICES 2023
  191. Comparative geotechnical analysis for ultimate bearing capacity of precast concrete piles using cone resistance measurements
  192. Visualizing sustainable rainwater harvesting: A case study of Karbala Province
  193. Geogrid reinforcement for improving bearing capacity and stability of square foundations
  194. Evaluation of the effluent concentrations of Karbala wastewater treatment plant using reliability analysis
  195. Adsorbent made with inexpensive, local resources
  196. Effect of drain pipes on seepage and slope stability through a zoned earth dam
  197. Sediment accumulation in an 8 inch sewer pipe for a sample of various particles obtained from the streets of Karbala city, Iraq
  198. Special Issue: IETAS 2024 - Part I
  199. Analyzing the impact of transfer learning on explanation accuracy in deep learning-based ECG recognition systems
  200. Effect of scale factor on the dynamic response of frame foundations
  201. Improving multi-object detection and tracking with deep learning, DeepSORT, and frame cancellation techniques
  202. The impact of using prestressed CFRP bars on the development of flexural strength
  203. Assessment of surface hardness and impact strength of denture base resins reinforced with silver–titanium dioxide and silver–zirconium dioxide nanoparticles: In vitro study
  204. A data augmentation approach to enhance breast cancer detection using generative adversarial and artificial neural networks
  205. Modification of the 5D Lorenz chaotic map with fuzzy numbers for video encryption in cloud computing
  206. Special Issue: 51st KKBN - Part I
  207. Evaluation of static bending caused damage of glass-fiber composite structure using terahertz inspection
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