Medium-term phase stability of Na2O–Al2O3–SiO2–H2O geopolymer systems

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Abstract

The phase evolution and microstructural development of a series of geopolymer mixtures comprising SiO2, Al2O3, Na2O and H2O prepared by alkali reaction of metakaolin, have been studied. The study also included the effects of cure duration and its impact on physical properties such as compressive strength. The characteristic molar ratios of the geopolymer mixtures were of the range SiO2/Al2O3 [2.50–5.01] and Al2O3/Na2O [0.60–1.70], respectively. The formulations were subjected to continuous curing at 40 °C for 7 months, and were analyzed periodically by XRD and SEM techniques.

Amorphous Na–Al–Si phase(s), observed at early ages, gradually transformed to crystalline phase(s) with prolonged curing. The initial SiO2, Na2O, and Al2O3 contents of mixtures appeared to be critical factors governing the observed amorphous → crystalline transformation. Well-developed crystalline zeolitic phases, including chabazite, faujasite, zeolite A and zeolite P, were identified in some of the mixtures investigated. In most cases, with prolonged curing, some correlation emerged relating compressive strength development with corresponding phase changes. In essence, the mixture formulations that developed crystalline phases after prolonged curing tended to produce low strengths. The relevance of these findings on the phase development of mild- to warm-temperature prolonged curing of geopolymer systems is discussed.

Introduction

Inorganic polymerization of Al2O3- and SiO2-containing materials in strong alkali environments often results in products generally referred to as “geopolymers”[1]. The basic process of geopolymerization can be, in many ways, similar to the formation of zeolites, as both processes involve the dissolution of solid reactants, hydrolysis of the dissolved species and condensation of the gel phase. Furthermore, because both starting materials as well as the conditions of synthesis are similar, some classes of geopolymers can be broadly regarded as similar to zeolites in chemical composition [2], [3]. However, a key difference between zeolites and geopolymers is their respective relative levels of matrix phase crystallinity [2]. The geopolymer phase is typically “X-ray amorphous” in contrast to the well-developed crystalline structures of zeolites. The structural differences make these two classes of materials suitable for different applications [1], [2]. From a thermodynamic viewpoint, geopolymer phases can be considered as metastable with respect to zeolites [4].

The presence of nanocrystalline particles, which have been identified as zeolitic structures, embedded in the geopolymer gel has been reported [5], [6], [7]. Considering the similarities in chemistry and synthesis routes of geopolymers and zeolites, these findings are not totally surprising. However, for a geopolymer product, the establishment of synthesis conditions that can lead to crystal formation is important for assessing the potential impact on the final properties (both chemical and physical). In hydrated CaO–Al2O3–SiO2 systems (Portland and pozzolanic cements), for example, the formation of zeolitic phases from corresponding gel phases has been shown to have significantly altered the immobilization potential of these cements towards various waste metals [8], [9]. Similar changes may also be expected to occur in hydrated Na2O–Al2O3–SiO2 systems.

It has been postulated that the degree of crystallization in geopolymer systems is largely related to the system formulation and conditions of synthesis [5]. In geopolymer or zeolite formation, a number of factors seem to control the chemistry and nature of the final product. For instance, higher temperatures (≥ 100 °C) and pressures are generally considered prerequisites of zeolite formation. Nevertheless, some zeolite types can be synthesized at lower temperatures, or even under ambient conditions [9], [10], [11]. It is believed that the reaction rate is substantially faster for geopolymer formation [2], [11], [12], which results in amorphous or semi-crystalline matrices compared to highly crystalline and more ordered zeolite structures. Traditionally, the zeolite synthesis route involves an “aging time” (time between mixing and curing at high temperature) [10], and that this period assists aluminate and silicate species to rearrange in some orderly manner in the gel phase.

The concentration and type of alkaline activators, water content, and SiO2 and Al2O3 contents of the source materials, and exposure environments, are also reported to have an influence on the formation of crystalline zeolitic phases in hydrated Na2O–Al2O3–SiO2 systems [3], [4], [5], [6], [13], [14], [15]. In alkali-activated fly ash systems, the formation of zeolite P and chabazite has been shown to be favoured by the presence of Na+ rather than K+ ions [13], [15]. Silica-rich fly ashes, activated by NaOH, are reported to result in zeolite Na-P1 and/or hydroxy-sodalite phases [14]. In some studies, it was shown that alkali silicate activators tend to produce less or smaller crystals than alkali hydroxide activators [5]. When exposed to deionised water, sea water, Na2SO4 or acidic (H2SO4) solutions, amorphous or semi-crystalline geopolymer phases have been shown to crystallize into a zeolite material belonging to the faujasite family [6]. As indicated previously, it is also well established that the amount of water required in the synthesis of zeolites is typically much higher than that of geopolymers [10].

This work is aimed at investigating the effect of some key variables on the stability of hydrated Na2O–Al2O3–SiO2 systems using alkali-activated metakaolin systems. The variables chosen in this study are the Na2O, Al2O3 and SiO2 contents in the initial mixtures. A relatively warm-temperature cure, i.e. 40 °C, was selected for continuous curing of samples. Chemical and microstructural development of various formulations with slight variations in initial Na2O, Al2O3 and SiO2 contents were investigated with respect to their physical properties, in particular compressive strength. To consider practical implications, laboratory investigations into thermodynamic stability, phase transformations and the establishment of long-term stable phase assemblages should be carried out at ambient conditions. However, it is assumed that the lower curing temperature of 40 °C adopted in this study, does not significantly alter the basic chemical processes.

Section snippets

Materials and experimental

Metakaolin was obtained by calcining kaolin (Commercial Minerals, Australia, 47.3 wt.%; SiO2, 35.7 wt.% Al2O3, 3.1 wt.% other minerals). Laboratory-grade NaOH and Grade N sodium silicate (8.9 wt.% Na2O, 28.7 wt.% SiO2, 62.5 wt.% H2O) from PQ Australia were used as alkaline activators.

As shown in Table 1, three series of mixtures were prepared, with each set incorporating different Na2O, SiO2 or Al2O3 contents. Relatively low water contents representative of geopolymer synthesis were adopted.

Compressive strength development

Plots of compressive strength development with curing time of mixtures are shown in Fig. 2. There were clear differences in the strength development trends. At 3 days, mixtures Si-30 (or Al-10 or Na-10), Si-38 and Na-07 possessed higher strengths than the others, and their strength continued to increase with further curing up to 53 days, and remained almost constant thereafter. However, it should be noted that even though the patterns are similar, the magnitude of strength of these three

Discussion

The present study reasonably demonstrates how relatively small variations in initial molar concentrations of Na2O, SiO2 or Al2O3 in a geopolymer system can dramatically change its long-term properties, due to various phase transformations taking place within the system when subjected to prolonged warm-temperature curing conditions. Given the curing temperature used in the study, 40 °C, the findings can be relevant to the practical phase development of geopolymer systems under initial prolonged

Conclusions

Following conclusions can be made from this study.

  • 1.

    In Na2O–Al2O3–SiO2–H2O systems, amorphous → crystalline phase transformations are feasible at temperatures as low as 40 °C and high RH.

  • 2.

    The initial molar contents of Na2O, Al2O3 and SiO2 were shown to play a key part in controlling the above phase transitions. In particular, SiO2/Al2O3 molar ratios lower or higher than 3.8 and increasing the Na2O content tended to favour this transformation. Zeolite phases belonging to the faujasite group, as

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