Optimization and characterization of cast in-situ alkali-activated pastes by response surface methodology

Construction and Building Materials 225 (2019) 776–787

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Optimization and characterization of cast in-situ alkali-activated pastes by response surface methodology Bashar S. Mohammed a,⇑, Sani Haruna a,b, Mohamed Mubarak bn Abdul Wahab a, M.S. Liew a a b

Department of Civil and Environmental Engineering, Universiti Teknologi Petronas, Seri Iskandar, Perak, Malaysia Civil Engineering Department, Bayero University, Kano, Nigeria

h i g h l i g h t s
High calcium fly ash and GGBS based one-part alkali-activated pastes were produced.
Compressive strength of 50 MPa was obtained for HC based one-part alkali-activated pastes and 92 MPa for GGBS paste.
Strength growth of the developed cast in-situ one-part alkali-activated binder is similar to ordinary Portland cement.
Mathematical models have been established for cast in-situ alkali-activated pastes.

a r t i c l e

i n f o

Article history: Received 7 February 2019 Received in revised form 11 July 2019 Accepted 21 July 2019

Keywords: Cast in-situ Alkali-activated paste Anhydrous sodium silicate Response surface Strengths Water absorption

a b s t r a c t Due to its low carbon footprint and the capacity to be used for in situ applications, cast in-situ geopolymers are recognized as a feasible substitute of Portland cement. One of the limiting factors of using geopolymer in concrete sectors is dealing with viscous and dangerous alkaline alternatives, making it hard to adopt for mass concrete manufacturing. The advanced binder comprises of aluminosilicate components and granular sodium metasilicate to which water has been added like OPC. This document reports on the novel experimental method of producing cast in-situ alkali-activated binders using fly ash, ground granulated blast furnace slag (GGBS) and anhydrous sodium metasilicate. The defined method is described with a logical experimental study conducted to examine a feasible manufacturing method for casting in-situ geopolymer production. Replacement concentrations for slag were 0–100 percent by fly ash weight, while activator is used at 8%–16% of the complete binder content. The strengths, absorption rate and microstructural behaviour of cast in-situ alkali-activated pastes were regarded for up to 28 days. The resistance development of one-part/cast in-situ alkali-activated binders was discovered to be comparable to that of OPC. Microstructural assessment disclosed that the incorporation of GGBS in the paste resulted in structural changes of the in-situ geopolymer paste that could be attributed to the creation of C-A-S-H gel owing to the existence of extremely reactive alumina and silica in the source materials. Using the variance analysis, the impact of slag and sodium metasilicate activator on the behaviour of cast in-situ geopolymer pastes was acquired. The defined models were discovered to be important for all P-value reactions of <5%. Results of numerical optimizations showed that the best mixture can be obtained by replacing 100 percent fly ash with slag and 11.19 percent sodium metasilicate with total binders weight. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Concrete use is increasing quickly every day owing to the enhanced need for shelter and industrialization. Most infrastructural installations are currently being created using concrete. Ordinary Portland cement (OPC) is one of the key substances used in the production of standard concrete. Besides water, concrete is

⇑ Corresponding author. E-mail address: [email protected] (B.S. Mohammed). https://doi.org/10.1016/j.conbuildmat.2019.07.267 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

the most widely used substance around the globe [1]. Development of replacement low carbon binders has been acknowledged as one of the feasible alternatives for decreasing CO2 emissions [2]. Geopolymer is a cement less binder and an appropriate environmentally-friendly green material used as a replacement for OPC binder. Geopolymerisation mechanism of geopolymer represents an extremely fast chemical reaction in an alkaline setting on silica-alumina minerals resulting in a 3-dimensional polymer sequence and ring structure consisting of SiAOAAlAO bonds [3–5]. However, the manufacturing of alkali-activated concrete does not involve any OPC to be used. The binder was actually gen-

B.S. Mohammed et al. / Construction and Building Materials 225 (2019) 776–787

erated by the response of an aluminosilicate material with powerful alkaline liquids. It is obvious that during the manufacturing of OPC, in addition to depleting the natural resources, enormous quantities of carbon dioxide were emitted into the surrounding atmosphere. A replacement green material is needed to mitigate these impacts. It is therefore crucial to use substitute materials for the manufacturing of environmentally-friendly concrete [6]. Collectively, cement gel geopolymer connects aggregates and unreacted material to produce geopolymer concrete [7,8]. Alkali activated binders release up to 80 percent less CO2 (greenhouse impact gas) compared to Portland cement production [9–11]. Despite the fact that the geopolymer system has been established since the nineteenth century [12] and embodies countless useful characteristics, it has not yet been used as extensively as OPC in concrete manufacturing. The impediments in terms of the two-part geopolymer binders have hindered its prevalent use in the concrete sector, although the concrete produced from this binder has superior engineering characteristics with extra environmental advantages [13]. High calcium fly ash was revealed to have been used to generate 65 MPa compressive strength geopolymer mortar without any high temperature curing [14,15]. The activator used, however, was still in fluid shape. Efforts have been created to generate one-part geopolymer blend [16–18]. Their results focused on generating one-part binders that are commercially and environmentally appropriate. One-part or cast in-situ alkali-activated binders are a latest approach to alkali-activated materials manufacturing that was lately established with the aim of decreasing problems in dealing with silicate solution activated geopolymers. It was generated by mixing aluminosilicate source materials with strong activators [17,19]. The development of cast in-situ alkali-activated binder was acknowledged as a feasible solution for the use of alkaliactivated binders for large-scale production owing to the constraints of standard geopolymer [20]. Unlike ordinary geopolymer binders where alternatives were used to activate the activation process, the activator stays in dry powdered shape in one-part alkali activation. The response starts whenever water is added to the OPC-like binder. This technique helps to avoid corrosive and viscous alternatives for large-scale geopolymer concrete manufacturing and encourages the business suitability of alkali-activated binders. The idea is called just adding water. Numerous attempts have been made to synthesise aluminosilicate materials together with alkaline alternatives at greater temperatures to produce one-part binders [17,18,21–24]. Koloušek et al. [21] created onepart geopolymer systems by burning kaolinite with powdered hydroxides. After 7 days of curing, which was inferior to standard geopolymer technologies, they recorded strength gain of only 1 MPa. In addition, Ye et al. [19] used red mud and silica fumes to create one-part geopolymer. They acquired a compressive strength of about 32 MPa at 28 days of ambient healing which proves to be a crucial improvement to the results reported by [21]. Suwan and Fan [25] noted that heat generated as a result of the disintegration of strong activators into one-part geopolymer could be useful for healing and development of strength. Peng et al. [26] recorded a compressive strength of <5 MPa at 28 days of ambient cured one-part alkali-activated paste samples produced with calcined bentonite, dolomite and Na2CO3. They achieved a higher compressive strength of 38.3 MPa by curing the samples at 80 °C. However, a compressive resistance of nearly 19 MPa was noticed in the lack of Na2CO3 alkali activator. Ambient healed one-part alkali-activated binder was generated by [27,28]. They used fly ash and slag as the main components in their job and used silicate powder and sodium hydroxides as activators. They revealed that the binder generated with 100 percent slag achieved nearly 50 MPa compressive strength, while that of 100 percent fly ash provides 9.45 MPa power after 28 days healing at room

777

temperature. Nematollahi et al. [29] developed one-part geopolymer with the following characteristics: density varies from 1800 kg/m3 to 1900 kg/m3 and compressive resistance up to 50 MPa in ambient healing conditions. The main components used, however, were a coalescence of fly ash and slag to achieve the specified characteristics. Luukkonen et al. [30], at 28 days, acquired more than 100 MPa compressive strength for one-part alkali-activated slag mortar activated with sodium metasilicate. The strength, however, deteriorates slightly over time. One-part alkali-activated binders displayed superior opportunities to standard two-part geopolymers because it will be ideal for large-scale applications so that most quality control can be achieved easily [31]. The most common materials used in one-part alkali-activated binder were fly ash either alone or in conjunction with ground granulated blast furnace slag [32]. However, the use of elevated calcium fly ash mixed with slag in one-part geopolymer has not been recorded. Because of its elevated reactivity and specific surface area, the use of slag as part of the source materials will improve the mechanical strength characteristics of the one-part alkali activated binders. Response surface methodology (RSM) is a mathematical analysis method in which the output outcome is connected to autonomous variables to explore the impacts, connection and connection between dependent variables (responses) and autonomous factors [33]. Response surface method has been used in multiple fields of research to establish models and optimise mixtures. The implementation of statistical design method in sectors started in the 1930 s owing to Box & Wilson’s development of reaction surface methodology in 1951 [33]. Response surface methodology is commonly used to model and optimise experimental reactions in both geopolymer and normal Portland cement (OPC) manufacturing. Mohammed et al. [34], predicted the compressive strength of a mixed paper concrete by using RSM and maximising the model using numerical optimisation, the study was endorsed by [35– 37]. RSM method has been used in latest research by Mohammed and Adamu [38] to establish mix design models for roller compacted concrete. They have further optimised mixtures of roller compacted rubbercrete concrete (RCR) by minimising water absorption and improving power. Zahid et al. [39] used the RSM method to design and optimise engineered geopolymer composite. They achieved elevated compressive strength, ductility and further forecast the mechanical and post-cracking behaviour of the engineered cementitious geopolymer composites (EGC). In latest research, one-part alkali activated binders were synthesised with anhydrous sodium metasilicate powder from distinct aluminosilicate source materials [17,29,30]. However, there are few studies that used RSM method to design and model the behaviour of elevated calcium fly ash one-part geopolymer binders synthesized with anhydrous metasilicate powder. In this current research, one-part or cast in-situ alkali-activated pastes were prepared and healed at ambient temperature. Instead of the standard combination of sodium silicate and sodium hydroxide, solid anhydrous sodium metasilicate was used as an activator. The strength and microstructures of the cast in-situ alkali-activated paste were examined at separate activator and slag content. Additionally, an optimisation research on sodium metasilicate and GGBS factors was conducted using RSM to provide the greatest strength for the one-part alkali-activated paste.

2. Materials and methods 2.1. Materials 2.1.1. Fly ash High calcium fly ash (HCFA) was used as the main binder in this inquiry. Chemical constituents of the elevated calcium fly ash used

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were identified by X-Ray fluorescence (XRF) and displayed in Table 1. The HCFA was obtained from the Manjung Power Plant in Perak, Malaysia. It was used as the source of aluminosilicate materials to produce one-part alkali-activated binder. Field emission scanning electron microscope (FESEM) examined the morphology of fly ash and shown by a micrograph in Fig. 1a. Based on the FESEM micrograph, all fly ash particles appeared to be spherical in distinct dimensions, allowing it to mix freely in mixtures. It is well observed that for HCFA, CaO was more than 10 percent. Furthermore, the sum of SiO2, Al2O3 and Fe2O3 was <70%. Therefore, fly ash meets the C-class fly ash requirement as per ASTM 618-10 [40].

2.1.3. Alkali activator powder Granular anhydrous sodium metasilicate (51 percent Na2O, 45 percent SiO2 and 4 percent H2O) was used as a solid activator to activate fly ash. Portray Sdn Berhad Selangor, Malaysia provided it. The use of granular activator in alkali activated binders was easier and less time consuming than the frequently used alkaline solutions as there is no need to prepare NaOH solution before mixing. The granular activator was used by the total binders at 8 percent, 12 percent, and 16 percent by weight.

2.1.2. Ground granulated blast furnace slag In this investigation, ground granulated blast furnace slag (GGBS) was used as a secondary binder. It is a by-product of an iron industry and consists mainly of aluminosilicate calcium materials. The use of GGBS as a fly ash replacement in one-part alkaliactivated materials was expected to enhance its mechanical properties due to its high reactivity and specific surface area. The GGBS composition of oxides used in this work was presented in Table 1. In Fig. 1b, the FESEM micrograph indicates that the slag consists of a bundle of angular grains. In this study, GGBS slag replaced fly ash at 0 percent, 25 percent, 50 percent, 75 percent, and 100 percent of source material weight.

Response surface methodology was used in this study to investigate the combined effect of GGBS and anhydrous sodium metasilicate on the behaviours of cast-in-situ alkali-activated pastes and to establish an association between parameters and output responses. Numerical optimization was performed to investigate the most suitable mixtures by boosting the strengths and curtailing water absorption. In the response surface analysis, a different set of models were adopted in the development of statistical associations between results and independent parameters. The most commonly used and consistent model adopted was the central composite design as it can be used when the design involved two independent variables [33,38,41–45]. Design expert software has been used for experimental design. Based on the face-cantered central composite design (FCCD) for two independent variables, mix design formulations of cast in-situ alkali-activated pastes were randomly selected. GGBS ranging from 0 to 100 percent and anhydrous sodium metasilicate activator ranging from 8 to 16 percent by weight of the aluminosilicate precursors were the parameters considered in the design. The results of this work were: compressive strength, split tensile strength, and water observation. The software developed thirteen mixtures for each response with five randomised duplications. The five duplications are the central points used by the software to improve the experiment’s accuracy against any likely errors.

Table 1 Chemical Composition Fly Ash and slag (percentage by weight). Chemical oxide

HC fly ash

Slag (GGBFS)

Anhydrous sodium metasilicate

SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 MnO S2 Cl LOI Specific gravity Blaine fineness (m2/Kg)

37.3 14.90 16.5 17.9 2.08 0.7 2.8 0.26 1.07 0.13 – – 0.17 2.35 386

33.86 14.73 0.48 36.62 6.33 2.10 0.39 0.16 0.73 0.33 0.80 0.01 1.72

46 – – – – – – 51 – – – – –

415

2.2. Development of statistical models using response surface methodology

2.3. Mixing procedure, curing, and testing of specimens High calcium fly ash, ground granulated blast furnace slag, and sodium metasilicate granules were made of alkali-activated cement used in this work. Cast-in-situ alkali-activated paste was produced by mixing solid precursors and granular sodium metasilicate with drinking water at a constant water-to-solid ratio (w / s)

Fig. 1. (a) FESEM image of high calcium fly ash (b) FESEM image of ground granulated blast furnace slag.

B.S. Mohammed et al. / Construction and Building Materials 225 (2019) 776–787

of 0.25 as shown in Table 2. HCFA and slag were thoroughly mixed in a small Hobart mixer for about 2 min. Granular anhydrous sodium metasilicate was then added to the dry mixture and continued mixing at 145 rpm for another 3 min. Gently added potable tap water to the mix and continue mixing for 3 more minutes until it becomes homogeneous and consistent. The wet mixture of the one-part alkali-activated paste was cast into the steel moulds of 50 mm cube. After 24 h, hardened pastes were removed from the moulds and cured in ambient condition. Specimens are left alone in ambient mode until the test period. The parameters considered in this investigation are I percentage of anhydrous granular sodium metasilicate and (ii) variation of GGBS content in the total binder. For easy identification, each mix was assigned as depicted in Table 2 with a specific means of identification. For example, F100S0-A8 is a mixture made with 100 percent fly ash, 0 percent slag, and 8 percent Na2SiO3 by the binder’s total weight. 2.3.1. Sample preparation and test procedure In this study, a digital compressive strength testing machine with a load capacity of 3000 kN was used to measure the compressive strength of the in-situ alkali-activated paste. Each test cube was exposed to a force at a load rate of 0.9 kN/s until it failed. The cubes specimens were weighed to obtain their densities at the date of testing. The compressive strength of the specimens was evaluated at 3, 7 and 28 days in accordance with ASTM

F100S0-A8 F100S0-A12 F100S0-A16 F75S25-A8 F75S25-A12 F75S25-A16 F50S50-A8 F50S50-A12 F50S50-A16 F25S75-A8 F25S75-A12 F25S75-A16 F0S100-A8 F0S100-A12 F0S100-A16

Source materials Fly ash

GGBFS

1 1 1 0.75 0.75 0.75 0.5 0.5 0.5 0.25 0.25 0.25 – 0.8 0.9

– – – 0.25 0.25 0.25 0.5 0.5 0.5 0.75 0.75 0.75 1 0.2 0.1

C109/109 M [46]. At each testing age, three sets of the specimens were used to conduct the compressive strength test. Similarly, the splitting tensile strength was obtained on the basis of BS EN 12390–6:2009, whereby 9 cylinders of 200 mm height by 100 mm diameter were assigned for all the mixes considered and tested at the age of 3, 7 and 28 days respectively. 2.4. Microstructural analysis Fourier transform infrared (FT-IR) spectra were used to observe particle absorption and transmission, to develop a sample molecular impression. The observations were made after the compressive strength test of 28 days. Energy dispersive spectroscopy (EDS) was also used to examine the elemental structure of in-situ alkaliactivated pastes. Field emission scanning electron microscope (FESEM) was also used to explore the crack surface characteristics of the developed pastes. The FT-IR spectra were recorded in the range of 4000 cm1 to 500 cm1 using the Perkin Elmer FT-IR spectrometer. Measurements were taken using 2 cm1 and 40 scans resolution. Before viewing, the specimens were coated with 200 A° Gold-Palladium for FESEM analysis. Carl Zeiss’ ultra-high resolution SUPRA 66VP was used to capture the micrograph image of the cast-in-situ alkali-activated pastes.

3. Result and discussions

Table 2 Mix formulations of in-situ alkali-activated paste. Mix ID

779

Na2SiO3/b

w/s

0.08 0.12 0.16 0.08 0.12 0.16 0.08 0.12 0.16 0.08 0.12 0.16 0.08 0.12 0.12

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

3.1. Influence of sodium metasilicate anhydrous and GGBS on compressive strength The compressive strength profile of the cast in-situ geopolymer paste at different percentages of GGBS and anhydrous silicate activator from 3 to 28 days was shown in Fig. 2. The mix formulations with 100 percent GGBS and 12 percent solid activator content exhibit the highest compressive strength of nearly 93 MPa. The compressive strength increases with an increase in the binder’s GGBS content. The lowest compressive strength of <20 MPa was obtained when 100 percent fly ash was activated with 8 percent sodium metasilicate activator. However, with the same amount of slag, the strength was found to be slightly lower at 16 percent activator content than the strength gains at 12 percent. The strength development of the one-part alkali-activated paste was found to be similar to that of the OPC, since the strength gain at

Fig. 2. Compressive strength profile of in-situ geopolymer.

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3 and 7 days was calculated to be 48.9 percent and 65.6 percent respectively. The inclusion of GGBS in the mixes increased the compressive strength of the cast in-situ alkali-activated mixtures and subsequently reduced the paste setting time. This could be associated with the rapid reaction process due to the presence of the slag’s high specific surface area in the system. The development of compressive strength of cast in-situ alkali-activated paste at a later age could also be attributed to the continuity of the geopolymerisation process and multi-condensation of the aluminosilicate system thus forming a three-dimensional structure. Similarly, the incorporation of GGBS in the mix results in the formation of Calcium alumino silicate hydrate gel (C-A-S-H) due to the existence of high calcium content in source ingredients. This led to an enhancement of the mechanical properties of the activated one-part alkali paste. The compressive strength of the pastes was increased by 46.2 percent, 52.2 percent, and 86.2 percent respectively for F75S25-A12, F50S50-A12, and F0S100-A12. The strength development in the paste could be attributed to the presence of C-A-S-H. The presence of C-A-S-H gel results in the development of a more compact and dense structure, this finding is in agreement with that reported by [13,47]. On the surface of the slag-based pastes, however, micro cracking was visible.

a

In addition, the contour plot of the established model was presented graphically using a 2-dimensional plot as shown in Fig. 3(a). It is observed that the entire contour lines were elliptical in shape indicating an ideal interaction between the activator and GGBS. The model has been found to exhibit an ideal synergy between the independent variables. Reddish and yellowish portion on Fig’s contour plot. (a) indicates an excellent combination that yields optimum strength values. In Fig, the bluish portion. 3(a) represents the desired water absorption value of the one-part alkali activated pastes respectively. As shown in Fig’s 3-dimensional surface diagram. (b), the inclusion of the slag in the mixes increased the compressive strength of the cast in-situ alkali-activated mixes and subsequently reduced the setting time of the paste. The addition of slag in the binder significantly influences the setting time of the pastes thus making it hardened faster, which is associated with the rapid reaction process due to the presence of high calcium oxide in the system. 3.2. Influence of sodium metasilicate anhydrous and GGBS on splitting tensile strength The split tensile result of the one-part alkali-activated paste was shown in Fig. 4 for all the design mixes considered. The test was

b

Design-Expert® Software Factor Coding: Actual Compressive strength (MPa) Design Points 92.4

B: Sodium metasilicate Anhydrous (%)

17.22

Design-Expert® Software Factor Coding: Actual Compressive strength (MPa) Design points above predicted value Design points below predicted value 92.4

Compressive strength (MPa)

16

17.22 14

100

X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous 80

Compressive strength (MPa)

X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous

80 5

12

60 10

40

60

40

20

0

16

100 80

14 8

60

12 0

20

40

60

80

40

100

B: Sodium metasilicate Anhydrous10(%)

20 8

0

A: GGBS (%)

Fig. 3. (a) 2-D contour plot and (b) 3-D response surface models for compressive strength.

Fig. 4. Split tensile strength profile.

A: GGBS (%)

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carried out in accordance with BS EN 12390-6 [48]. As shown in the Fig. 4, the split tensile strength of the pastes increased proportionally with the increase of GGBS and sodium metasilicate. A slight reduction in strength of 6 percent, 7.7 percent, 3.9 percent and 4.6 percent was observed as the activator dosage increased from 12 percent 16 percent for F100S0, F75S25, F50S50, and F0S100 respectively. It is observed that as the dosage level of sodium metasilicate increased, the split tensile increased with a slight reduction in strength when the dosage of sodium metasilicate reached 14 percent. During the reaction process, the decrease in splitting tensile was associated with the formation of micro cracks in the specimens, which could be attributed to the dissolution of granular sodium meta silicate [16]. However, as the GGBS replacement level increased, an increase in strength was observed. The split tensile strength of slag-modified pastes was 6.8 percent higher than that of fly ash-based pastes, 30.3 percent, and 38.6 percent for 25 percent, 50 percent, and 100 percent slag respectively. It is interesting to note that the strength of F50S50-A16 and F25S75-A16 was almost the same. This could be associated with the GGBS ’ pore filling ability and self-cementing nature resulting in microstructural refinement, bond improvement, and therefore increased tensile strength.

Design-Expert® Software Factor Coding: Actual Splitting tensile strength (MPa) Design Points 3.66

B: Sodium metasilicate Anhydrous (%)

1.1

3.3. Water absorption Water absorption test was used to assess the durability performance of in-situ alkali-activated paste. The water absorption values of cast in-situ alkali activated pastes reported in this work were obtained in the range of 3.1%–7.2%. Fig. 6 indicates variation in water absorption with respect to activator dosage and GGBS proportion. It can be deduced from Fig. 6 that water absorption decreases with increased activator dosage and vice versa. The

Design-Expert® Software Factor Coding: Actual Splitting tensile strength (MPa) Design points above predicted value Design points below predicted value 3.66

Splitting tensile strength (MPa)

16

The mix with 100 percent GGBS and 12 percent solid activator content shows the maximum split tensile strength of more than 3.5 MPa. This observation was consistent across all of the mixes. In the 3-dimensional surface diagram, the variation of the splitting tensile across the mixes could be well observed in the 2dimensional contour plot shown in Fig. 5(a). The desired variables combination was indicated in Fig. 5b by a reddish yellowish portion. This indicates an excellent interaction between anhydrous sodium metasilicate and GGBS. The 3D dimensional diagram in Fig. 5(b) shows the influence of the two parameters (GGBS and sodium metasilicate activator) after 28 days of ambient healing with respect to the split tensile strength.

1.1

4

14

X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous

Splitting tensile strength (MPa)

X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous

3.5 5

12

3

10

2.5 2

3.5 3 2.5 2 1.5 1

16

1.5

100 80

14 8

60

12 0

20

40

60

80

40

100

10

B: Sodium metasilicate Anhydrous (%)

20 8

0

A: GGBS (%)

(b) 3-D response surface diagram

(a) 2-D contour plot

Fig. 5. Response surface diagrams for splitting tensile strength.

Fig. 6. Water absorption profile of cast in-situ alkali-activated pastes.

A: GGBS (%)

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absorption of the F100S0-A8 mix was significantly reduced by 32.8% as the activator content increased from 8% to 12%. The subsequent increase in activator content to 16 percent reduced water absorption by 33.3 percent of the same mix. It is worth mentioning that the system’s replacement of 50 percent of fly ash with GGBS reduced water absorption by 36.7 percent at the same activator content. The mixture of 8 percent anhydrous sodium silicate and 100 percent fly ash clearly shows a very high-water absorption by capillarity actions. It is interesting to note that water absorption was proportional to the geopolymeric ingredients and activator dosage, the higher the activator dosage, the more resistance to water penetration, and subsequently, the less environmental damage caused to the material. As shown in the Fig. 6, the absorption of F75S25-A12, F50S50-A12, and F50S50-A16 was almost constant. It was observed that samples with higher water absorption have a minimum compressive and tensile strength compared to samples with lower water absorption. Similar findings have also been reported [49]. It was noted that the water absorption of all specimens was below the 10 percent permissible limits [50]. The variation of paste water absorption depended on the activator and slag content in the binder as shown in Fig. 7(b). A decreasing trend in

Design-Expert® Software Factor Coding: Actual Water absorption (%) Design Points 7.2

B: Sodium metasilicate Anhydrous (%)

3.1

4. Microstructural analysis 4.1. Fourier transform infrared spectroscopy (FTIR) of cast in-situ geopolymer paste The result of the one-part alkali-activated paste FT-IR spectra was shown in Fig. 8. The in-situ geopolymer control spectra differs from that of the modified pastes. This asserts that the microstructure of the one-part alkali-activated pastes changes relative to that of the fly ash-based paste. New peaks were observed in the slagmodified pastes ranging from 1773 cm1 to 650 cm1. This can be attributed to the presence of slag particles in the one-part alkali-activated system. The absorption peak at 3447 cm1 corresponding to Si-OH OH groups and adsorbed water molecules on the binder material surface. The peak at 1658 cm1 corresponds

Design-Expert® Software Factor Coding: Actual Water absorption (%) Design points above predicted value Design points below predicted value 7.2

Water absorption (%)

16

water absorption is noticed when the activator content increases as shown in Fig. 7(a). In Fig. 7(a), the bluish portion provides the desired water absorption value for the one-part alkali-activated pastes respectively.

3.1 14

X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous

X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous

8

Water absorption (%)

7

5

12

5

4

10

6

5

4

3 100

6 80

16 60

14

8 0

20

40

60

80

40

12

100

B: Sodium metasilicate Anhydrous (%)

A: GGBS (%)

(a) 2-D contour plot

A: GGBS (%)

20

10 8

0

(b) 3-D response surface diagram

3950

3450

2950

2450

1950

668

1000 945 987

3442

F100S0-A12 F75S25-A12 F50S50-A12 F0S100-A12

1487 1418

1658

1503

Relative transmitance

Fig. 7. Response surface diagrams for water absorption.

1450

950

Wave number (cm-1) Fig. 8. Combined FT-IR spectra of fly ash-slag based cast in-situ alkali-activated pastes.

450

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to the aluminosilicate stretching vibration of OH1 and H-O-H [51,52]. Triple peaks were noticed in the one-part alkali-activated specimens at 1503 cm1, 1487 cm1, and 1418 cm1. These peaks were designated to carbonate asymmetric stretching, which suspects the existence of sodium pirssonite as a result of unreacted Na2O atmospheric carbonation [53]. The appearance of the peak at 1401 cm1 corresponds to asymmetric stretching of undistorted + CO2 3 as a result of carbonation of free Na into Na2CO3 during the healing period [54]. However, the peak was slightly shifted to about 1000 cm1, 987 cm1 and 945 cm1 respectively. Increasing the content of GGBS causes spectral fluctuation in the observed pastes, which could be associated with polymerisation of reactive alumina and silica in the source materials. This led to a rapid geopolymerisation reaction and consequently increased compressive strength. These findings were well correlated with the previous work [55]. The two peaks at 695 cm1 668 cm1 are associated with symmetric Si-O-Al stretching that were almost equal in all spectra. The slight changes in the position of the absorption peaks were attributed to changes in the aluminosilicate structures in the source materials and the dissolution of anhydrous sodium silicate that led to the formation of rich Al in-situ alkaliactivated paste. In addition, the absorption peak at 464 cm1 corresponds to O-Si-O symmetric bending vibrations that occurred due to the presence of anhydrous silicate activator and water molecules. 4.2. Field emission scanning microscopy (FESEM) and energy dispersive spectroscopy (EDX) The morphological behaviour of F100S0-A8, F75S25-A12 and F50S50-A16 one-part alkali-activated paste was examined using field emission scanning electron microscope (FESEM) and EDX. The results of this investigation revealed that a significant improvement in compressive strength was achieved by adding GGBS. The presence of GGBS in the geopolymer structure led to the formation of calcium aluminosilicate hydrate gel (C-A-S-H), which contributed to higher strength growth. The F100S0-A12 micrograph shown in Fig. 9A shows a perfect bond between the partially reacted fly ash and the synthesized binder, with micro cracks observed along the interface. The appearance of large micro cracks was believed to have contributed to the development of low strength. Similarly, in Fig. 9b, F75S25-A12 is showing a similar trend. The appearance of denser slag molecules has contributed with few micro cracks to the formation of more C-A-S-H gels, resulting in high strength growth. Most of the fly ash and slag molecules that reacted with the alkali activator were noticed to be present, while some of the fragments were partially engulfed with the reaction products. Increasing GGBS dosage changes the microstructure of the F50S50-A12 micrograph in Fig. 9(c) by forming a denser and compact structure. In F25S75A12 micrographs, however, minor and micro cracks were observed as depictions in Fig. 9(d). This could be attributed to the drying shrinkage of the gel. During the polycondensation process, which develops capillary tension within the gel matrices [49], the shrinkage could be due to water evaporation and nonuniform internal pressure. The system also emanates more reaction heat at higher activator content, resulting in a rapid hardening process. It is worth mentioning that as shown in Fig. 9c, F50S50-A16 exhibited a more uniform phase and possess a smaller number of unreacted fly ash and slag. The FESEM images are well correlated with the mechanical strength result of in-situ alkali-activated pastes. The emergence of silicate-activated gel was observed using EDS analysis for some selected mix. The analysis revealed the

783

presence of Na-Si-Al phases through interparticle bonds between the elements. Additional quantities of Ca, Fe, and Mg were also noticed as residues in the paste as shown in Fig. (9A1, 9B1, 9C1 and 9D1). During the reaction process, the element residues did not completely dissolve and have a composition and morphology similar to that of conventional geopolymer. This correlates well with the findings of the previous works [56,57]. 5. Statistical interpretation of the test results All the established models have been statistically analysed and validated. The analysis was conducted at a level of 5 percent significance to examine the importance of experimental factors. Compressive strength, splitting tensile strength and water absorption were considered as the dependent variables in this analysis, while sodium metasilicate and GGBS (slag) were selected as the independent factors. The resulting p-values shown in Table 3 show that all factors were important at a confidence level of 95 percent and accepted as crucial parameters on the test result. The model’s quality was examined using R2 –value. As shown in Table 3, the high R2 values of 0.8843, 0.9913 and 0.9646 for compressive strength, split tensile strength and water absorption models indicate a good measure of correspondence between the predicted and experimental results. It is also worth mentioning that the predicted R2 values are in good agreement with the adjusted R2 as the difference between them is <0.2. As shown in Table 4, all models have sufficient precision values of more than 4, indicating that the models could be used to navigate the design space. The model’s quality could also be assessed on the basis of lack of fit; the smaller lack of fit value indicates models of worthiness. It was noteworthy that, as shown in Table 3, the lack of fit P-value of all models was more than 0.05, which indicates insignificance, and thus implies excellent fitness for all model’s response. Normal probability plot is a graphical representation used to evaluate the distribution of data and validate its sufficiency [35,37,41]. As shown in Fig. 10, the points were distributed almost in a straight line for all dependent variables and thus shows that the data was normally distributed for all residual responses. A plot of predicted vs. actual results graphically examined the competency and fitness of the response models. In Fig. 11, the predicted Vs actual results plot shows that the predicted response model was precise. The points are fitted smoothly on a straight line indicating a good relationship in the established models between experimental and predicted outcomes. Consequently, the established response models were relevant and appropriate in estimating the strengths and water absorption of cast in-situ alkaliactivated pastes. However, the compressive strength of cast-insitu paste can be predicted using ANOVA as shown in Eq. (1). It is worth mentioning that all models of response were quadratic. Relationships and influence between variables (activator content and GGBS with respect to their actual quantities) and responses were achieved through variance analysis and presented in Eqs. (1)–(3).

Comp:str: ¼ 182:753 þ 0:493 A þ 36:208 B þ 0:004 AB  0:002 A2  1:368 B2

ð1Þ

Split tensile str: ¼ 5:003 þ 0:031 A þ 1:055 B þ 0:0004 AB  0:0001 A2  0:0037 B2

ð2Þ

water absorption ¼ þ13:491  0:050 A  1:183 B þ 0:0003 A2 þ 0:043 B2

ð3Þ

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Fig. 9. FESEM and EDS images of (a) F100S0-A12 (b) F75S25-A12, and (c) F50S50-A12 (d) F75S25-A12 mixtures.

5.1. Optimisation and validation study Numerical multi-objective optimisation was used to determine the optimum amount of anhydrous sodium metasilicate and GGBS, with the aim of maximising the strengths and minimising the

water absorption of the developed pastes. The optimisation study focuses on identifying the desired values of independent variables to achieve the optimisation goals. The responses affected by the multiple variables were enhanced using the RSM technique, which described the meaning function for the target output to enhance

785

B.S. Mohammed et al. / Construction and Building Materials 225 (2019) 776–787 Table 3 Anova response models. Responses

factors

S.S

Df

M.S

F-Value

P-Value

Remark

Compressive strength (MPa)

Model A-GGBS (slag) B-sodium metasilicate anhydrous AB A2 B2 Residual Lack of fit

3749.9 616.11 1824.9 2.66 31.55 1110.05 74.85 50.2

5 1 1 1 1 1 7 3

749.98 616.11 1824.9 2.66 31.55 1110.05 1.07E + 01 16.73

228.89 188.04 170.67 0.25 2.95 103.82

<0.0001 <0.0001 <0.0001 0.6334 0.1295 <0.0001

Significant

2.72

0.1794

Insignificant

Splitting tensile strength (MPa)

Model A-GGBS B-sodium metasilicate anhydrous AB A2 B2 Residual Lack of fit

6.37 2 2.27 0.027 0.23 0.8 0.066 0.021

5 1 1 1 1 1 7 3

1.27 2 2.27 0.027 0.23 0.8 0.00945 7.16E-03

159.85 250.62 284.9 3.42 29.06 100.26

<0.0001 <0.0001 <0.0001 0.107 0.001 <0.0001

Significant

0.84

0.5405

Insignificant

Water absorption

Model A-GGBS B-sodium metasilicate anhydrous AB A2 B2 Residual Lack of fit

13.29 6.93 2.31 0.12 0.87 1.09 1.49 0.96

5 1 1 1 1 1 7 3

2.66 6.93 2.31 0.12 0.87 1.09 0.21 0.32

12.51 32.61 10.85 0.58 4.11 5.12

0.0022 0.0007 0.0132 0.4725 0.0822 0.0581

Significant

2.45

0.2031

Not significant

Df: degree of freedom, P: Probability; F: Fisher statistical value; SS; sum of squares; MS: mean square.

Table 4 Validation properties of response model.

Design-Expert® Software Compressive strength

Compressive strength (MPa)

Splitting tensile strength (MPa)

Water absorption (%)

Standard deviation Mean C.V % R2 Predicted R2 Adjusted R2 Adequate precision

3.27 66.95 4.88 0.9856 0.8843 0.9753 33.25

0.089 2.96 3.01 0.9913 0.9666 0.9851 42.18

0.46 4.16 11.08 0.8994 0.9646 0.7147 27.848

Predicted vs. Actual

Color points by value of Compressive strength: 92.4

100

17.22

80

Predicted

Response

60

40

20

0

C. V: coefficient of variation. 0

20

40

60

80

100

Actual

Fig. 11. Predicted vs actual plot of the develop models.

Table 5 Optimisation benchmark.

Fig. 10. Normal probability plot of the develop models.

the responses [58,59]. Based on the purpose of optimisation, the results of numerical optimisation solutions were presented in Table 6. The design expert software acquired the optimum desired mixture fractions by synthesising 100 percent of GGBS with 11.19 percent of sodium metasilicate. Enhanced responses with a unified desirability of 85.3 percent have been achieved. The results of

Factors and responses

Goals

Lower limit

Upper limit

GGBS Sodium metasilicate Compressive strength (MPa) Split tensile strength (MPa) Water absorption

In range In range Maximise Maximise Minimise

0 8 17.22 1.1 3.1

100 16 92.4 3.59 7.2

numerical optimisation for the established models were presented in the 3D diagram shown in Fig. 12. In addition, the independent factors and responses were presented graphically through the optimisation ramps shown in Fig. 13. To validate the appropriateness of the optimisation results and the entire response models, an additional set of investigations were carried out using the optimised mixture proportions and two more different mixes to validate the optimised mixture proportion within the design mixes. The criteria for optimisation are summarised in Table 5. The error between experimental and predicted values was evaluated using Eq. (4) and expressed in percentage as shown in Table 6.

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Table 6 Model verification. Responses

Solutions

GGBS

Sodium metasilicate

Predicted outcomes

Experimental outcomes

Error (%)

Compressive strength (MPa)

1 2 3

100 50 25

11.19 12 16

89.27 77.9 59.31

91.15 81.33 63.5

2.06 4.22 6.6

Split tensile strength (MPa)

1 2 3

100 50 0

11.19 12 16

3.52 3.35 3.01

3.7 3.48 3.15

4.86 3.74 4.44

Water absorption

1 2 3

100 50 25

11.19 12 16

3.21 3.63 4.45

3.58 4.21 4.6

3.63 3.8 3.26

cast-in-situ applications. Based on the results of this study’s experimental findings, the following conclusions can be outlined here:

Design-Expert® Software Factor Coding: Actual Desirability 1.000 0.853

0.000 X1 = A: GGBS X2 = B: Sodium metasilicate Anhydrous 1.000 0.800

Desirability

0.600 0.400 0.200 0.000

100 16

80 14

60 12

40 10

20

A: GGBS (%)

B: Sodium metasilicate Anhydrous (%) 8

0

Fig. 12. Desirable mixture of slag and sodium metasilicate activator.

Errorð%Þ ¼

Experimental model  predicted model  100% Experimental model

ð4Þ

6. Conclusions This study focuses on optimisation and characterisation by response surface technique of cast in-situ alkali-activated pastes. Granular anhydrous sodium metasilicate has been successfully used as a single activator to produce geopolymer suitable for

0

1. The strength growth of one-part/cast in-situ alkali-activated binders was found to be similar to that of ordinary Portland cement as the strength gain of 48.9 percent and 65.6 percent were realized at 3 and 7 days respectively. 2. Higher percentage replacement of fly ash with slag results in a significant increase in strength growth. However, in the developed in-situ alkali-activated pastes, as observed by the FTIR analysis incorporating slag in the binder causes structural aluminosilicate modifications. The modifications were associated with the polymerisation of reactive alumina and silica in the source materials. 3. Increasing the activator amount by weight of the precursor materials beyond 12 percent slightly affects the reaction process thereby increasing the amount of energy. This results in the formation of minor cracks during the hardening process. 4. The water absorption rate is proportional to the activator dosage. Water absorption by capillary pores is high at low activator content and vice versa. 5. All response models were quadratic, indicating a good relationship between the variables and their responses. 6. The outcome of the RSM optimisation study shows that the best strengths were achieved by combining 100 percent GGBS and 11.19 percent solid sodium metasilicate with the total weight of the primary ingredients. 7. The predicted results were well correlated with the experimental results. The validation results were very close to the experimental values obtained.

100

8

A:GGBS = 100

17.22

16 B:Sodium metasilicate Anhydrous = 11.1923

92.4

1.1

Compressive strength = 89.2693

3.66 Splitting tensile strength = 3.51513

Desirability = 0.853

3.1

7.2 Water absorption = 3.21111

Fig. 13. Optimisation ramps.

B.S. Mohammed et al. / Construction and Building Materials 225 (2019) 776–787

Declaration of Competing Interest None.

Acknowledgement The authors would like to admit the assistance offered under the YUTP 0153AA-H30 grant by Universiti Teknologi PETRONAS, Malaysia for research financial support.

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