Effect of chemical admixtures on properties of high-calcium fly ash geopolymer

International Journal of Minerals, Metallurgy and Materials Volume 18, Number 3, June 2011, Page 364 DOI: 10.1007/s12613-011-0448-3

Corresponding author: Ubolluk Rattanasak E-mail: ubolluk@buu.ac.th © University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2011

Effect of chemical admixtures on properties of high-calcium fly ash geopolymer

Ubolluk Rattanasak1), Kanokwan Pankhet1), and Prinya Chindaprasirt2) 1) Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Burapha University, Chonburi 20131, Thailand 2) Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand (Received: 12 February 2010; revised: 4 April 2010; accepted: 23 April 2010)

Abstract: Owing to the high viscosity of sodium silicate solution, fly ash geopolymer has the problems of low workability and rapid setting time. Therefore, the effect of chemical admixtures on the properties of fly ash geopolymer was studied to overcome the rapid set of the geo-polymer in this paper. High-calcium fly ash and alkaline solution were used as starting materials to synthesize the geopolymer. Calcium chloride, calcium sulfate, sodium sulfate, and sucrose at dosages of 1wt% and 2wt% of fly ash were selected as admixtures based on concrete knowledge to improve the properties of the geopolymer. The setting time, compressive strength, and degree of reaction were recorded, and the microstructure was examined. The results show that calcium chloride significantly shortens both the initial and final setting times of the geopolymer paste. In addition, sucrose also delays the final setting time significantly. The degrees of reaction of fly ash in the geopolymer paste with the admixtures are all higher than those of the control paste. This contributes to the obvious increases in compressive strength.

Keywords: geopolymers; chemical admixtures; setting time; fly ash

1. Introduction

Geopolymer binders are formed based on aluminosilicate chain reactions and possess good mechanical properties. Because of the high percentage of amorphous silica and alumina, fly ash is widely used as a source material. Geo-polymerization occurs in alkaline solution, particularly in the sodium hydroxide/sodium silicate system. Silica and alumina leach out from fly ash particles in the alkaline me-dium. Gel formation takes place, which overwhelms the fly ash particles [1]. This results in cementitious materials with good physical and mechanical properties.

Admixtures are added to concrete at the mixing stage to improve its properties, such as fluidity and/or setting be-havior. Similar to concrete, fly ash geopolymer has the problems of low workability and rapid setting time, owing to the high viscosity of sodium silicate solution. To improve workability, additional water is incorporated into the mix-ture [2], which changes the concentration of alkali hydrox-ide and lowers the compressive strength if a large amount of

water is used. However, geopolymer is suitable for repair work owing to its high strength development within a short curing duration. In many applications, a short setting time of geopolymer is needed. For example, the setting time is often more important than the strength in the sea-defense applica-tion, because the structure is required to resist wave action immediately after concrete placement.

Accelerators shorten the setting time of cementitious ma-terials and/or increase the rate of strength buildup [3]. Fur-thermore, they assist early removal of formwork and general reduction in the construction time schedules. In general, ac-celerators are divided into chloride-containing and chlo-ride-free materials. The widely used accelerator is calcium chloride. However, calcium chloride is criticized severely because it can induce corrosion in concrete reinforcement. However, it still remains an acceptable and highly effective admixture, especially for unreinforced concrete. Other chlo-ride-free chemicals are calcium and sodium salts, and the latter is generally preferred owing to its solubility.

U. Rattanasak et al., Effect of chemical admixtures on properties of high-calcium fly ash geopolymer 365

Retarders are used in concrete to delay its setting owing to, for example, high temperature or to avoid complications when undesirable delays between mixing and placing occur. It usually reduces the solubility of hydrating components in the matrix. Sucrose is widely used as a retarder at very low dosage owing to its osmotic rupture and formation of hydra-tion sheath [4]. However, using sucrose at high dosage may cause flash setting. It is of interest to note that some well-known accelerators can act as retarders depending on its concentration, particularly at low concentration.

There are many admixtures used in concrete, however, the lack of data on their effects on geopolymer limits their applications. Therefore, accelerators and retarders used in the geopolymer matrix for this research were selected based on the work in concrete. Calcium chloride, calcium sulfate, sodium sulfate, and sucrose were selected for incorporation into the fly ash geopolymer, and their effects were studied.

2. Materials and methods

2.1. Materials

Fly ash from Mae Moh Power Plant in the north of Thai-land with a mean particle size of 19 μm was used as the aluminosilicate source material. The major chemical com-position of fly ash is shown in Table 1. Fly ash has a high content of CaO and Fe2O3. In addition, quartz, mullite (3Al2O3⋅2SiO2), and hematite (Fe2O3) are identified on the X-ray diffraction (XRD) pattern of fly ash, as shown in Fig. 1. Fly ash contains an amorphous phase, which is detected from the board peak at 20°-25°. As reagents for geopoly-merization, 10 mol NaOH and sodium silicate solution (SiO2/Na2O=3.2 by weight ratio) were used. The viscosities of 10 mol NaOH and sodium silicate solution were 9.3 and 60.6 mPa·s, respectively. Analytical-grade calcium chloride (CaCl2), calcium sulfate (CaSO4), sodium sulfate (Na2SO4), and sucrose were selected as admixtures for the examination of setting time. They were incorporated to the geopolymer mixtures at dosages of 1wt% and 2wt% of fly ash [5]. Graded river sand with a fineness modulus of 2.8 and a spe-cific gravity of 2.65 was used for making mortar specimens.

Table 1. Composition of lignite fly ash wt%

SiO2 Al2O3 CaO Fe2O3 Na2O MgO K2O SO3 Loss on ignition

39.7 20.0 17.3 14.1 1.4 1.4 2.7 2.6 0.8

2.2. Preparation of geopolymer paste

Fly ash was mixed with NaOH solution for 5 min to al-low leaching of the ions. Sodium silicate solution (Na2SiO3)

Fig. 1. XRD patterns of original fly ash and geopolymers with admixtures.

was then added and mixed for a minute to form a uniform paste. The mass ratio of Na2SiO3/NaOH was 1.5, and that of solid/total mixture (S/T) was 0.6 [6]. In the final mixing step, the admixture was added to the mixture. The setting time of the paste was examined using Vicat needle in accordance with the ASTM C191 standard test method. The pastes were molded in 25 mm×25 mm diameter plastic cylinders. Cast samples were covered with a cling film to avoid moisture evaporation during heat curing. Curing was carried out in an oven at 65°C for 48 h to complete geopolymerization. Heat curing of 48 h was selected because it gave a relatively high-strength geopolymer for test at 7 d [6]. The scanning electron microscopy and energy dispersive X-ray (SEM-EDX) analyses were performed on the hardened sample.

2.3. Preparation of geopolymer mortar

Geopolymer mortar was prepared for the determination of compressive strength by adding sand to the fresh paste at a sand-to-fly ash ratio of 2:1 (by weight) and mixed for an-other minute. The mixture was then cast into 50-mm cubic mold in accordance with ASTM C109. The samples were wrapped and cured in the same way as the paste samples. The specimens were cured at room temperature for another 5 d and then tested for strength at the age of 7 d. The results were reported as the average of three samples with a stan-dard deviation (S.D.) of less than 5%.

2.4. Determination of the geopolymerization degree

The degree of reaction of fly ash in the geopolymer sys-tem was determined by identification of unreacted fly ash.

366 Int. J. Miner. Metall. Mater., Vol.18, No.3, Jun 2011

The unreacted fly ash was considered as microaggregate in the matrix. The method involved dissolution of powdered samples with 2 mol HCl and 3wt% Na2CO3 [7]. The hard-ened geopolymer pastes were ground to obtain particles that passed a 150-μm sieve. A 100-mL beaker filled with pow-dered samples (3 g) and 2 mol HCl (30 mL) was placed in a 60°C water bath and stirred for 20 min to accelerate the dis-solution. A vacuum filter was used to separate the solid phase. The remaining solid was then washed with warm water thrice to completely remove HCl. Acetone was ap-plied in the last filtration to remove water before drying at 70°C for 2 h.

The residual sample was then dissolved with 30 mL of 3wt% Na2CO3 in a beaker and placed in an 80°C water bath for another 20 min with occasional stirring. Again, the sam-ple was filtered and repeatedly washed with water and ace-tone before dried at 70°C for 2 h. Subsequently, the mass of the unreacted fly ash was determined. The degree of reac-tion on ignited basis is calculated as the following equation [8].

Degree of reaction=

( )sample residue

sample

1 LOI100

m mm

− × +⎡ ⎤⎣ ⎦ × (1)

where msample stands for the weight of powdery sample, g; mresidue stands for the weight of dried residue, g; and LOI stands for the loss of ignition of ground geopolymer paste tested in accordance with ASTM C114.

The degree of reaction of the original fly ash particles was also determined and assigned as “blank”. All the results were subtracted with blank to obtain the corrected degree of reaction. The results were reported as the average of the three samples with an S.D. of less than 5%.

3. Results and discussion

3.1. Characterization of geopolymers

The XRD results of geopolymers with admixtures are shown in Fig. 1. The XRD patterns of the geopolymers are clearly different from that of original fly ash. The peaks of quartz and mullite are dominant in the source material. The main difference of the XRD patterns between fly ash and geopolymer is the shift of the board amorphous peak from 22° to 30°. After the formation of the geopolymer, the peaks of quart and mullite are reduced, and those of calcium sili-cate are observed, indicating the occurrence of phase trans-

formation in a higher disorder material. The occurrence of calcium silicate results from the hydration of calcium and silicate materials which is similar to the hydration of Port-land cement [9]. In addition, the board peak at 30° of geo-polymer paste is clearly evident, confirming the increase of the degree of the amorphous phase.

3.2. Setting time

Vicat apparatus was used to measure the setting time of the geopolymer paste. The initial and final setting time are tabulated in Table 2. The initial and final setting time of the control geopolymer paste samples are 60 and 130 min, re-spectively. The incorporation of 1wt% and 2wt% CaCl2 sig-nificantly decrease the setting time. For 1wt% and 2wt% CaCl2 loading, the initial setting time is reduced to 26 and 35 min, and the final setting time is reduced to 60 and 45 min, respectively. It was apparent that CaCl2 accelerated the setting time of the geopolymer paste. The CaCl2 salt served as a source of calcium ions in the solution and induced car-bonation reaction with atmospheric CO2 resulting in calcium carbonate (CaCO3) at the paste surface, as shown in Eq. (2).

CaCl2 (s)+CO2 (g)+H2O (l) → CaCO3 (s)+2HCl (aq) (2)

The results of Fourier transform infrared spectroscopy (FTIR) indicated a high degree of carbonation in the geo-polymer pastes [6, 10]. In addition, the readily available free calcium ion from CaCl2 and high-calcium fly ash reacted with silicate to form calcium silicate hydrate (CSH) gel [3-4]. Owing to the high charge density and mobility of Cl−, early gel had more open and porous structure. This allowed greater alkaline/water diffusion through the gel, leading to more gel formation, and shortened the setting time of the matrix. In addition, it was also reported that CaCl2 increased the flocculation of CSH formed around the hydrating parti-cle [11].

Table 2. Initial and final setting time of the fly ash geopolymer

AdmixturesDosage /

wt% Initial setting time /

min Final setting time /

min Control 0 60 130

1 26 60 CaCl2 2 35 45

1 58 115 CaSO4 2 56 105

1 82 135 Na2SO4 2 90 130

1 60 210 Sucrose

2 60 230

U. Rattanasak et al., Effect of chemical admixtures on properties of high-calcium fly ash geopolymer 367

Incorporation of CaSO4 in the mixture had little effect on both the initial and final setting time. CaSO4 usually has a retarding effect on the Portland cement paste system. Owing to high alkaline solution in the geopolymer matrix, the hy-droxide ion (OH−) reacts with the free calcium ion from CaSO4 and fly ash, resulting in the formation of Ca(OH)2 as

Ca2++2OH− → Ca(OH)2 (3)

This reaction also occurs in the matrices containing CaCl2. The formation of reaction products can be explained with the solubility constant (Ksp). It was found that at 25oC, the values of Ksp of CaCO3 and Ca(OH)2 were 3.8×10−9 and 7.9×10−6, respectively. A low Ksp value led to the precipita-tion of CaCO3 from CaCl2 and CO2, which thus delayed the setting time, resulting in a small effect on the setting time.

Unlike CaSO4, Na2SO4 significantly delayed the initial setting time with little effect on the final setting time. The reaction between calcium aluminate and sulfate from Na2SO4 resulted in the formation of ettringite around the fly ash particles [4-5]. This formation hindered the leaching of silica and alumina from the fly ash particles, thus delaying the setting time of the paste. This was in accordance with the fact that the setting time of the system of calcium alu-minate and sulfate was longer than that of normal CSH [12].

The incorporation of sucrose had no effect on the initial setting time, but significantly retarded the final setting time. Sucrose is a disaccharide obtained from dehydrolysis of glucose and fructose units with an acetal oxygen bridge, as shown in Fig. 2. The CHOH group in the molecule makes it conducive to coating formation [4]. Sucrose was found to seal off the fly ash particles from the alkaline solution caus-ing retardation. In addition, sucrose also retarded the setting time by acting with calcium. This increased the viscosity of the solution and slowed down the reaction. Adsorption of the precipitated materials onto the surface of the fly ash par-ticles retarded geopolymerization, and the process became diffusion-controlled. Furthermore, it was also reported that sucrose inhibited precipitation of calcium aluminate hy-drates from mixtures, leading to prolonged setting time [13-14].

3.3. Compressive strength of geopolymer mortar

Fig. 3 shows the compressive strength of the geopolymer mortars with admixtures. The control geopolymer mortar gives a compressive strength of 25.8 MPa. In general, in-corporation of 1wt% admixtures increase the strength of the geopolymer, while increasing the dosage of admixtures to 2wt% results in almost no changes in compressive strength,

Fig. 2. Structure of sucrose.

Fig. 3. Compressive strength of fly ash geopolymer with ad-mixtures.

comparing with the control geopolymer mortar. The com-pressive strengths of the geopolymer with 1wt% CaCl2, CaSO4, Na2SO4, and sucrose are 28.0, 30.5, 32.2, and 30.1 MPa, respectively, while those with 2wt% CaCl2, CaSO4, Na2SO4, and sucrose are 26.9, 26.5, 25.0, and 27.4 MPa, re-spectively.

The strength of the geopolymer mortar was associated with the solubility of alumina and silica from the fly ash in the matrix [1]. As addition of admixture altered the dissolu-tion of fly ash and gel formation, the strengths were thus af-fected. Therefore, the use of admixtures in the fly ash geo-polymer helped to control the setting time and improve the compressive strength which depends on type and dosage.

3.4. Degree of reaction

To dissolve CaO and MgO from the hydrated product, 2 mol HCl was used, and to dissolve SiO2, Al2O3, and Fe2O3, 3wt% Na2CO3 was employed. The residue was the unre-acted fly ash residue [7]. The results of reaction degree of fly ash geopolymer paste are shown in Fig. 4. The control geopolymer shows 13.3% degree of reaction. Incorporation of 1wt% CaCl2, CaSO4, Na2SO4, and sucrose in the geo-polymer result in the higher degrees of reaction of 18.5%, 19.8%, 20.1%, and 15.5%, respectively. Increasing the ad-mixture dosage to 2wt% in the geopolymer matrix reduces

368 Int. J. Miner. Metall. Mater., Vol.18, No.3, Jun 2011

the degrees of reaction to 13.6%-16.5%. The degrees of re-action were consistent with the results of compressive strength.

Fig. 4. Reaction degree of fly ash geopolymer with admix-tures.

3.5. Microstructure of the geopolymer paste

A typical SEM image of hardened fly ash geopolymer with 1wt% admixture is shown in Fig. 5. The pastes contain a continuous mass of aluminosilicate matrix and partially reacted fly ash particles. Products with different shapes are formed on the surface of the fly ash particles. The regu-lar-shape products of CSH and Al-Si gel are observed on the fly ash particles of the control paste (Fig. 5(a)) and those of the CaCl2 system (Fig. 5(b)). For pastes containing CaSO4 and Na2SO4 (Fig. 5(c)-(d)), compared to the normal prod-ucts, a large amount of needle-shape products are also ob-served. These products are calcium aluminate sulfate hy-drate products. For the matrix with sucrose (Fig. 5(e)), the products appear as an agglomeration of small particles or unit of gel, and cover the fly ash surface.

As previously mentioned, the CHOH group of su-crose-made gel was conducive to seal off fly ash particles [3]. The difference in the gel formation affected the binding

Fig. 5. Morphology of fly ash geopolymer with 1wt% admixture: (a) no admixture; (b) CaCl2; (c) CaSO4; (d) Na2SO4; (e) sucrose.

U. Rattanasak et al., Effect of chemical admixtures on properties of high-calcium fly ash geopolymer 369

capacity and strength of the geopolymer. The SEM-EDX results also detected Al and Si in the gel on the fly ash sur-face and matrix, implying dissolution and polymerization. All the admixtures helped in gel formation, leading to the enhancement of compressive strength.

The incorporation of admixtures had a significant effect on the microstructure and degree of reaction of fly ash geo-polymer paste, and significantly contributed to the devel-opment of physical property, i.e., compressive strength of the geopolymer mortars.

4. Conclusion

A number of admixtures can be used in the fly ash geo-polymer to control the setting time. For CaCl2, CaSO4, Na2SO4, and sucrose, the incorporation of 1wt% fly ash in the mixture gives better results in terms of setting time and strength, in comparison with the use of 2wt% fly ash in the mixture. Compared with the control geopolymer paste, CaCl2 is effective as an accelerator and significantly accel-erates both the initial and final setting time. The accelerated action of CaCl2 involves the formation of calcium silicate hydrate and carbonation of the paste. On the other hand, Na2SO4 is effective as a retarder and significantly increases the initial setting time of the geopolymer paste. The forma-tion of ettringite appears to be responsible for the delay in the setting time, because it can hinder the leaching of silica and alumina from the fly ash particles. CaSO4 is not effec-tive and has a little effect on both the initial and final setting time of the paste, as it contributes to both acceleration and delay mechanisms. Sucrose has little effect on the initial set-ting time, but significantly delays the final setting time. This delay is believed to be owed to the formation of reaction products, covering the fly ash particles and decreasing the reaction.

Acknowledgments

The authors gratefully acknowledge the financial support from the Higher Education Research Promotion, Office of the Higher Education Commission and National Research University Project, Khon Kaen University. Appreciation is

also extended to the Center for Innovation in Chemistry (PERCH-CIC).

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