Effects of seeding nucleation agent on geopolymerization process of fly-ash geopolymer

Abstract

Geopolymer, an inorganic aluminosilicate material activated by alkaline medium solution, can perform as an inorganic adhesive. The geopolymer technology has a viability to substitute traditional concrete made of portland cement (PC) because replacing PC with fly ash leads to reduced carbon dioxide emissions from cement productions and reduced materials cost. Although fly ash geopolymer stimulates sustainability, it is slow geopolymerization reaction poses a challenge for construction technology in term of practicality. The development of increasing geopolymerization reaction rate of the geopolymer is needed. ƒThe purpose of this study is to evaluate seeding nucleation agents (NA) of fly ash geopolymer that can accelerate polymerization reactions such that the geopolymer can be widely used in the construction industry. Results from the present study indicate that the use of NA (i.e., Ca(OH)₂) can be potentially used to increase geopolymerization reaction rate and improve performance characteristics of the fly ash geopolymer product.

Keywords

fly ash / geopolymer / nucleation agent / portland cement replacement

Introduction

Increasing uncertainties of global energy production and supply, increasing price of energy, and significantly high CO₂ emissions from traditional industries are the major stimulants for many industries to seek alternative and more sustainable systems. The construction industry is not only one of the world’s largest industries but also one of the largest polluters [¹,²]. Modern construction industry is evaluating a wide range of sustainable practices and technologies with emphasis on using alternative resources including the use of industrial by-product such as fly ash.

Fly ash is defined as the main residues generated during the pulverized coal combustion and collected by the electrostatic precipitators or a bag filter in a thermo-electric power station [³]. The production of coal combustion process was estimated approximately 500 MT/year and this process reportedly generates approximately 75 to 80% of fly ash [⁴,⁵]. Although fly ash has been used in many applications, only 30 to 40% of its total production is used [⁶,⁷]. Hence, the amount of fly ash has been increasing throughout the world due to increased demands of electric power, and the disposal of fly ash has become a major concern. The proper disposal of fly ash costs approximately US$1.2 billion annually [⁸]. As a result, attempts have been made, both in research and industries, particularly in the construction industry, to expand the usage of fly ash.

Geopolymer is an alumino silicate-reactive material synthesized in highly alkali medium solutions and under elevated temperature. Common alkali medium solutions used in the synthesis are NaOH, KOH, sodium silicate, and potassium silicate solutions. Alkalinity of medium solutions is reported to be an important factor for the dissolution of aluminate and silicate compounds to form geopolymer products [⁹,¹⁰]. In the synthesis of geopolymers, NaOH was reported to have a significant effect on both the compressive strength and structure of geopolymers. The NaOH concentration of the geopolymeric system acts on the dissolution process, as well as on the bonding of solid particles in the final structure [¹¹]. When fly ash is introduced with NaOH solution, silicate, aluminate, and others minor compounds are dissoluted. It is known that the alkalinity or pH level of NaOH solution highly affects the amount of ions dissoluted into solution. After being cured, geopolymer products can perform as inorganic adhesives similarly to PC. Due to similar performance characteristics of geopolymer product to PC, this leads to a potential of using geopolymer to be used in the construction industry. Implementation of this material in construction grants benefits by reduced materials cost and reduced CO₂ emissions associated with cement production [¹²,¹³]. Li et al. [¹⁴] reported that the production of geopolymer led to 80% reduction of CO₂ footprint and required 60% less energy as compared to PC systems. McLellan et al. [¹⁵] studied the costs and carbon emission of geopolymer pastes compared to PC and reported a 97% reduction in greenhouse gas emission and approximately 72% reduction in costs on a production-only basis in some geopolymer mixes.

Fly ash-based geopolymer concrete has been used in construction industry. Schmucker and MacKenzie [¹⁶] reported that high compressive strength of geopolymer can be synthesized from class-F fly ash. Several studies [¹⁷-²¹] have reported on the performance of fly ash geopolymer concrete using NaOH and KOH. Fly ash geopolymer concretes are reported to have excellent compressive strength [²²], suffer less drying shrinkage, exhibit low creep [²³], have high resistance on acids and salts attack [²⁴,²⁵], and are not subjected to the alkali-aggregate reactions [²⁶]. However, one practical challenge that hinders the use of geopolymer in today’s construction industry is its time consuming reaction process. Fly ash geopolymer concretes generally show delayed setting behaviors and slow compressive strength development at ambient temperatures. This is caused by very slow reactions of geopolymerization [²⁷]. Although, slow geopolymerization reactions can be minimized by curing the product at elevated temperatures (between 40 to 95°C), increasing alkalinity of medium solutions (up to 10 M), and using very fine particles of fly ash, these mentioned methods are not practical in the construction field with regards to additional curing works, safety concerns, and increased materials cost. This results in reduced sustainability.

Thermal curing has widely been applied to improve the performance of geopolymerization reactions [²⁸-³³]. Haddad and Alshbuol [²⁸] studied the effects of different curing temperature ranging from 40 to 120°C for 24 to 48 h and reported the compressive strengths in the range of 8.9 to 30.8 MPa after 28-day setting time. The same study reported 25 to 32% increase in compressive strength when curing at 40°C and 80°C, respectively. However, curing at 120°C negatively impacted internal structure and subsequently compressive strength of the geopolymer concrete. Albitar et al. [²⁹] demonstrate the significant improvement of compressive strength from 6 to 65 MPa when curing at 70°C for 24 to 48 h. Similar experimental conditions conducted by other researchers (80°C for 4 to 20 h) showed compressive strength ranging from 11.75 to 19.40 MPa [³⁰] and 8 to 33.07 MPa [³¹] after 28 days, depending on the NaOH concentration.

This study presents the results of evaluating the effects of seeding nucleation agent (NA) on geopolymerization reaction rate of class-F fly ash geopolymer. Several types of NA are investigated and performance characteristics of geopolymer containing NA are determined. The study also aims to identify the efficient NA for class-F fly ash geopolymer that promotes accelerated behaviors at ambient temperatures. The addition of NA to geopolymer system is a viable technique to improve the rate of geopolymerization reactions. This study demonstrates that the technique has potential in decreasing materials cost from high alkalinity of medium solutions while eliminating curing process at elevated temperatures. Along with process design and optimization, this technique can widely be used in the construction industry.

Materials and methods

Materials

Class F-fly ash was used for all mixtures in this study procured from Centralia, WA. Type I/II PC was also used in this study for comparing the results with the fly ash geopolymer. The chemical composition of fly ash and PC are shown in Table 1. Scanning Electron Microscopy (SEM) image of fly ash particles with Energy Dispersive Spectrometer (EDS) spectra for semiquantitatively chemical analysis is shown in Fig. 2. Pellet NaOH reagent was ACS grade and used for the synthetic of geopolymerization. NaOH solution was prepared and used as activating solutions. Research works reported the use of typical NaOH concentrations between 4 M to 12 M [¹⁰,³¹,³⁴]. However, 4M NaOH concentration was selected in the present study as the manufacturing process could potentially benefit from milder alkalinity operating condition [³⁵]. Sodium silicate solution was reagent grade and contained 10.6% Na₂O and 26.5% SiO₂and 62.9% water by mass. The bulk geopolymer composition had a Na₂O/SiO₂ = 1.5. ASTM Type II de-ionized (DI) water (1 MW·cm at 25°C) was used for all mixtures and experiments.

Fine aggregate, used for preparing mortar specimens, was procured from a local source in Corvallis, OR and met ASTM requirement C33, Standard Specification for Concrete Aggregates. The fineness modulus of the fine aggregate was 2.5 determined following ASTM C136, Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates procedures.The specific gravity of the fine aggregate was 2.47 and the absorption was 3.08%. The specific gravity and absorption were determined following ASTM C128, Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Fine Aggregate. The ratio of fine aggregate to solid binder (that is, fly ash and PC) was 1.35 for mortar preparation.

LiCl, LiNO₃, Ba(OH)₂, BaCl₂, and Ca(OH)₂ reagents were ACS grade and used as NA in this study. All chemical reagents were used as supplied, thus without any pre-treatment. Lithium based compounds were selected in this study as they are existing compounds commonly used as flash accelerators in the cementitious systems [³⁶-⁴²]. Researchers also reported thatwhen lithium compounds were dissoluted into solution in the presence of aluminosilicate compounds derived from fly ash, an intercalated doubled layer of lithium-aluminosilicate compounds were nucleated and precipitated [⁴³,⁴⁴]. These compounds are served as NA in the hydration process of cementitious systems [⁴⁵].

However, Luong et al. [³⁹] reported that the most important factor of the degree of acceleration in cementitious systems was not subjected to the pH level of the solution, but more likely a specific cation. Silicate compound is the main composition of Class-F fly ash- 51% (in Table 1); thus it is assumed in this study that the silicate phase can be governed the performance of fly ash geopolymer systems. Researchers [⁴⁶-⁵⁰] reported that adding alkaline and alkaline earth cations into silicate systems, its solubility changes. Several studies exhibited that solubility increased in the presence of cations with the trend: Mg<Ca≈ Li ≈ Na ≈ K<Ba ions. Based on the literatures, silicate dissolution in the presence of Ba²⁺ ion is one of the fastest rate in the system. Therefore, this Ba addition may promote faster dissolution mechanism for geopolymer systems; consequently, accelerated compressive strength development can be obtained. However, research is needed. Research studies [⁵¹,⁵²] reported that calcium compounds can perform as accelerating agents for fly ash geopolymer systems by decreasing its setting time. Studies also reported that the addition of calcium compounds into geopolymer led to increased calcium composition and therefore resulted in tricalcium aluminate formation (3CaO· Al₂O₃). Calcium compounds are also reported to promote extra nucleation sites for precipitation of dissolute phases and result in rapid hardening [²²,⁵³].

Methods

Preparation of geopolymer

The mixture design of fly ash geopolymer paste and mortars was adopted from Davidovits [⁵⁴]. The NA and silicate solution were mixed with 4M NaOH solution prior to introducing fly ash into the systems. The alkali medium solution and fly ash were mixed followed ASTM C305, Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency.

Evaluation of the reactivity of NA

The initial setting time of all systems was determined following ASTM C191, Standard Test Methods for Time of Setting of Hydraulic Cement by Vicat Needle. Initial setting time refers to the time when the paste begins to stiffening or losing its plasticity [⁵⁵,⁵⁶]. The pastes were tested at different curing temperatures (23°C, 30°C, 40°C, and 60°C). The mixtures were prepared with 1% addition of the NA. A control mixture (without NA) was mixed and tested for comparison. Test data was based on triplicate samples. After finish testing, data was thermodynamically applied using Arrhenius equation for determining the relative rate of geopolymerization in the presence of the NA, as follows:

(1)k=A\times e^{(-Ea/R\cdot T)},

where k is the rate coefficient (s^-1), A is a pre-exponential constant (s^-1), E_a is the activation energy (J·mol^-1), R is the gas constant (8.314 JK^-1mol^-1), and T = temperature (K).

Characterization of fly ash geopolymer including NA

Based on test results of the reactivity of NA described in Results and Discussion section, Ca(OH)₂ and LiCl are the most effective NA for fly ash geopolymer. The author then focused on the study of fly ash geopolymer with Ca(OH)₂ from the industrial economic standpoint. The geopolymer containing Ca(OH)₂ and the PC systems were evaluated in this section. The initial setting time, compressive strengths cured at 1, 7, and 28 days, and time-variant concentrations of calcium and aluminate ions in solution were determined in this study.

Initial setting times of geopolymer pastes with different percent addition of NA were evaluated following ASTM C191. The initial setting time of geopolymer pastes are compared to the initial setting time of PC pastes at the water-cement (w/c) ratios of 0.3 and 0.4.

The 1-, 7-, and 28-day compressive strength of the PC and fly ash geopolymer containing 0, 1%, and 3% Ca(OH)₂were evaluated following ASTM C109/C109M, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimens).After casted, test specimens were kept in molds under saturated relative humidity for 24 h and then demolded. Demolded specimens were then kept at the ambient temperature (23°C) until testing. Test data was based on triplicate specimens.

The rates at which calcium, aluminate, and silicate ions dissolute and precipitate from geopolymerization reactions directly influence fresh and hardened characteristics of the system. To understand performance characteristics of geopolymer systems, concentrations of aluminate and calcium ions in solution were determined in this section. Note that the analysis of silicate ion concentration was not performed here due to equipment limitations. Concentrations of calcium and aluminate ions were determined using flame atomic absorption spectroscopy (AAS). The fly ash geopolymer systems containing 0 and 3% Ca(OH)₂ were determined. Mixing for all systems was performed using a magnetic stirrer rotating at 400 rpm throughout the test. The water to solid binder ratio for this study was 4.0. The time elapsed after introducing the cementing materials to the solution is referred to here as the “hydration time.” Solutions used for evaluating hydroxyl ions were analyzed at 5, 10, 15, 30, 45, 60, 90, 120, 150, 180, 210, 240, 300, 360, and 420 min.

At each hydration time, 10 and 1 mL of filtered solution was used for analyzing aluminateand calcium ion concentration, respectively. Because high concentrations of calcium ions occur at early ages, filtered solutions for analyzing calcium concentrations were diluted for AAS analysis. Extracted and filtered solutions for analyzing calcium ion concentrations were diluted with 9 ml of deionized water to obtain the solution within the detection range of the AAS. After decanting and diluting, 1 mL of lanthanum acid solution (50 g/L lanthanum oxide (La₂O₃) in 3M hydrochloric acid (HCl)) was added to the solutions. Aluminate ion concentrations were determined using the AAS with nitrous oxide-acetylene gas at a wavelength of 309.3 nm ignited at the temperature of 2600 to 2800°C. Calcium ion concentrations were determined using AAnalyst 100 AAS (Perkin Elmer Instrument, Waltham, MA) using air-acetylene gas at a wavelength of 422.7 nm ignited at the temperature of 2100 to 2400°C. A blank sample (DI water) was also analyzed and used as a background correction. Triplicate samples were determined for each condition.

Microstructure assessment

To investigate the effect of Ca(OH)₂ addition in fly ash geopolymer systems, the microstructure of the geopolymer mortars containing 0% and 3% Ca(OH)₂ was observed using SEM. The geopolymer and PC mortar specimens were casted and cured under water. After 28 days of curing, the specimens were crushed and soaked with ethanol. The ethanol-soaked specimens were oven-dried at 110°C for approximately 4 h and examined using SEM.

Statistical analysis

Two sample t-test and analysis of variances (ANOVA) analyses were performed to evaluate the samples with two groups and more than two groups, respectively. The Shapiro-Wilk test was used to determine normal-distribution and the Levene’s test was used to determine equal variance of the data prior to the analyses. The statistical hypotheses are defined as:

(2)\mathrm{Null\; hypothesis\; (}{H}_{\mathrm{0}}\mathrm{)\; :}{\mu }_{\mathrm{1}}\hspace{0.17em}={\mu }_{\mathrm{2}}\hspace{0.17em}=\hspace{0.17em}\mathrm{...}\hspace{0.17em}=\hspace{0.17em}{\mu }_a,

(3)\mathrm{Alternative\; hypothesis\; (}{H}_a)\hspace{0.17em}:\hspace{0.17em}{\mu }_{\mathrm{1}}\hspace{0.17em}\ne \hspace{0.17em}{\mu }_j\hspace{0.17em}\mathrm{for}\hspace{0.17em}\mathrm{some}\hspace{0.17em}i\hspace{0.17em}\ne \hspace{0.17em}j.

The 95% confidence interval was used in the analyses. If the H₀ is rejected (p-value≤0.05), results can be concluded that there is statistically significant effect at the 5% level between the means of group population. On the other hand, if the H₀ is not rejected (p-value>0.05), results can be concluded that there is no statistically significant effect at the 5% level between the means of group population.

Results and discussion

Evaluation of the reactivity of NA

Reactivity of NA can be determined by measuring the initial setting time of the pastes which is consequently applied in the Arrhenius equation (as shown in Eq. (1)). The slopes of straight lines indicate the activation energy. Because the activation energy here is determined from the initial setting time of geopolymer paste, it represents the energy that the geopolymer paste must acquire before the paste begins stiffening. Higher slope corresponds to larger activation energy requiring for transforming from liquid to solid phase.

Figure 2 shows the plot of the initial setting time as an inverse function of temperature. Results indicate that the slope of the control system is less than the slopes of the LiCl and Ca(OH)₂ systems, but higher than the slopes of the BaCl₂ and Ba(OH)₂ systems. This indicates that the LiCl and Ca(OH)₂ systems require less energies for paste stiffening than the control system. TheLiCl and Ca(OH)₂ can be used to NA for accelerating the geopolymerization reactions of class-F fly ash geopolymer.

Characterization of fly ash geopolymer containing NA

In this study Ca(OH)₂ was selected to perform further characterization due to its economic pracability at process scale. In this section, the geopolymer containing different percent addition of Ca(OH)₂ was evaluated.

Initial setting time of paste

The initial setting time of the pastes as a function of percent addition of Ca(OH)₂was determine and shown in Fig. 3. Results indicate that increased percent addition of Ca(OH)₂ in the geopolymer system results in faster initial setting time. Ca(OH)₂ addition was limited to 3% as the threshold setting time of commonly used w/c ratio of 0.3 and 0.4 in PC systems was surpassed [⁵⁷]. However, results from Figs. 3 and 4 exhibit that although the initial setting time of the 3% Ca(OH)₂ system is similar to the PC systems, its compressive strengths are still lower than the compressive strengths of the PC systems.

Figure 4 shows the results of the 1-, 7-, 28-day compressive strengths of the geopolymer mortars containing different amount of Ca(OH)₂ and PC mortars. Results indicate that compressive strength increases with age increases. A comparison between different percent additions of Ca(OH)₂ of the fly ash geopolymer mortars indicates that increased percent addition of leads to higher 1-, 7-, and 28-day compressive strengths. In addition, when comparing the fly ash geopolymer systems to the PC system, results indicate that the compressive strengths of the geopolymer systems at all ages are still lower than the compressive strengths of the PC system. From Fig. 4, at 28-day setting time, compressive strengths of the geopolymer mortars are in the range of 9 to 23 MPa. The results obtained in this study are consistent with those reported by other researchers using thermal curing process [²⁸-³¹]. The products at this range are suitable for most non-structural and some reinforced concrete applications [⁵⁸-⁶⁰].

Calcium ion concentration in solution of the geopolymer and PC systems with different hydration times was shown in Fig. 5(a). Results indicate that calcium ion concentrations of the 0 and 3% Ca(OH)₂geopolymer systems are significantly low when compared to the calcium ion concentration of the PC system at all hydration times. This indicates that the dissolution of calcium ions of the fly ash geopolymer system is very low and 3% addition of Ca(OH)₂ in the geopolymer system does not influence the dissolution of calcium ions in solution.

Figure 5(b) shows the effect of hydration time on the aluminate ion concentration in solution of the fly ash geopolymer and PC systems. Results indicate that aluminate ion concentrations of 0 and 3% Ca(OH)₂ geopolymer systems are significantly higher than aluminate ion concentrations of PC system at all hydration times. Increased percent addition of Ca(OH)₂ leads to lower aluminate ion concentration in solution. The highest aluminate ion concentrations of the 0% Ca(OH)₂ geopolymer system are presumably because the 0% Ca(OH)₂ geopolymer systems contain highest composition of Al₂O₃ among the3% Ca(OH)₂ and PC system, leading to the fact that aluminate ions of the 0% Ca(OH)₂ geopolymer systems can dissolute into solution higher than the 3%Ca(OH)₂ geopolymer and PC systems.

Microstructure assessment

Figures 6(a) and (b) show the SEM images of fly ash geopolymer mortar containing 0% Ca(OH)₂. Results indicate that the fly ash particles in the geopolymer system have not much bonding or calcium-based gel to bridge between each particle and plenty of inter-particle space were observed. Note that calcium-based gel was examined using EDS, but the results are not shown here. These less bonding and higher inter-particle space result in lower 28-day compressive strength as shown in Fig. 4. The SEM images of the 3% Ca(OH)₂ geopolymer system are shown in Figs. 6(c) and (d). SEM images here show that fly ash particles are partially covered by the calcium-based gel and less inter-particle space was virtually observed. These result in higher 28-day compressive strength.

Compared the geopolymer systems to the PC system, Figs. 6(e) and (f) show the microstructure of PC systems. The figures show that individual particle is less seen and covered with the calcium-based gel. As a result, the highest 28-day compressive strength of the PC system is obtained.

Based on tested results, the NA addition of the fly ash geopolymer system can clearly accelerate geopolymerization reactions, and consequently results in increased hardened characteristics and formation of calcium-based gel. Figure 7 shows the ternary CaO-Al₂O₃-SiO₂ diagram of class-F fly ash geopolymer and PC systems. The diagram exhibits that the fly ash composition is toward SiO₂ side, but the PC composite is likely in CaO side. Thus, as shown, more calcium phases can be occurred in the PC system. Addition of NA (i.e., Ca(OH)₂) moves the composition of fly ash geopolymer toward CaO side.

Conclusions

Modern construction technology requires economy, convenience, and performance. The use of fly ash geopolymer in construction industry provides cost-saving and high performance product. However, its convenience remains a practical challenge because its geopolymerization reactions are slow. This consequently results in a necessity of elevated curing temperature or strong alkalinity of medium solution to promote faster geopolymerization reactions. In this paper, a NA for fly ash geopolymer was developed to promote the faster geopolymerization reactions in ambient temperature and lower alkalinity of medium solution. Tested results indicated that:

1) The geopolymerization reaction rate of Ca(OH)₂>LiCl>control>Li(NO)₃, BaCl₂>Ba(OH)₂ systems.

2) Increased percent addition of Ca(OH)₂ of fly ash geopolymer system resulted in reduced setting time and increased compressive strength.

3) Based on microstructure assessment, the system that had more calcium composition led to more gel formation and less inter-particle space.

4) As the composition of the fly ash systems shifts toward the PC system, performance characteristics tend to be similar to performance characteristics of the PC system

The findings in this article provide a promising alternative to more convenient solution for fly ash geopolymer. Although many performance characteristics of fly ash geopolymer are not similar to the conventional cement for modern construction technology, the finding of this paper is valuable and is a preliminary research work for better construction technology development. Future studies on concrete testing and in-field evaluation are required.

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