Effect of the SiO2/Na2O ratio on the alkali activation of fly ash. Part II: 29Si MAS-NMR Survey
Introduction
The alkali activation of fly ash (AAFA) is a unique procedure in which the greyish-black powdery ash (FA) is mixed with alkaline activators (alkaline solutions) and then cured at a certain temperature. The formed paste by mixing the fly ash and the alkali dissolution sets and becomes hardened as Portland cement does [1], [2], [3], [4], [5]. Previous studies [6], [7], [8], [9] have shown that the primary reaction product formed in AAFA is an X-ray amorphous aluminosilicate gel (N–A–S–H) containing silicon and aluminium tetrahedra randomly distributed in tetrahedral networks. The cavities formed in these gels when the networks are cross-linked can accommodate alkaline cations that offset the electrical charge generated due to Si4+ replacement by Al3+ ions [1], [2], [3]. Moreover, the short-range structural order exhibited by the nano-crystalline materials that appear in a subsequent stage improves the mechanical properties of the hardened pastes. In some cases several types of zeolites (hydroxysodalite, Na-chabazite, zeolite Y, and so on) also form as secondary reaction products [7], [8], [9], [10]).
Prior papers stressed the importance of the characteristics of the prime materials (“reactive” aluminium plays a key role in the kinetics of aluminosilicate gel formation [8]), the curing conditions (time and temperature) [3], [4], [11], [12], [13] and the nature and concentration of the alkali activator used [10], [14]. In addition, the presence of soluble silica is known to improve the mechanical properties of the resulting material [4], [5], [6] at early ages. Palomo et al. found that after curing at 85 °C for 24 h, different types of fly ash activated with 8–12 M NaOH yielded a material with mechanical strength ranging from 35 to 40 MPa [2], and up to 90 MPa when waterglass was added to the NaOH solution (SiO2/Na2O = 1.23) [4].
In light of the difficulties encountered in the XRD characterization of alkaline aluminosilicate gel reaction products (zeolite precursors), conscientious efforts have been made in recent years to interpret the information furnished by other analytical techniques [3], [8], [9], [10], [11], [12], [13]. In one previous study [10] using infrared spectroscopy, for instance, the gel was found to comprise primarily bridge and terminal Si–O bonds. The presence of soluble silica showed to affect the rate of crystallization and type of zeolites formed as secondary reaction products ((D6R-type) Herschelite; zeolite Y; (S4R-type) zeolite P) [9], [10].
Earlier papers [6], [7], [8], [11] described the advantages of NMR techniques for characterizing three dimensional short-range ordered materials in which silicon is found in a variety of environments, predominantly Q4(nAl) (n = 0, 1, 2, 3 and 4).
The present study aimed to establish a correlation between the presence of soluble silica in the activating solution and the nature and composition of the reaction products formed (N–A–S–H gel and zeolites) as well as reaction kinetics. Three solutions with different SiO2/Na2O ratios and an 8 M NaOH control solution were used to activate the fly ash. 29Si MAS-NMR spectroscopy was used [10] in this study to reach a clearer understanding of the mechanisms controlling the activation processes involved and their respective kinetics.
Section snippets
Materials
A class F (ASTM C 618-03) fly ash from the steam power plant at Compostilla, Spain was used in the present study. The chemical composition of the initial ash is given in Table 1 and a much more exhaustive characterization in a previous paper [7].
The ash was activated with a series of alkaline solutions, all with a practically constant sodium oxide content (≈8%)
Results
Although the primary aim of this study was to acquire an understanding of the nanostructure of the aluminosilicate gel formed as a result of the alkali activation of fly ash, exploration of this question was preceded by the characterization of the prime materials used (ash and alkaline solutions).
Discussion
The alkali activation of fly ash is a process where the glassy constituents of the fly ash are transformed into compact cement [15]. This process can divided in different stages [7], [8], [29]: In the initial or dissolution stage, the fly ash comes into contact with the alkaline solution. The OH− ions present in the reaction medium sever the Si–O–Si, Si–O–Al and Al–O–Al covalent bonds of the vitreous fly ash. Previous studies showed that aluminium initially dissolves at a faster rate than
Conclusions
The primary reaction product of the alkali activation of fly ash is an amorphous gel consisting in a polymeric, cross-linked aluminosilicate network whose Si/Al ratio depends on curing time and the nature of the alkaline activator used.
The nature of the alkali activator plays an instrumental role in the kinetics, structure and composition of the gel initially formed. The addition of soluble silica affects the intermediate stages of the activation reaction but not the final result.
The present
Acknowledgements
Funding for this research was provided by the Directorate General of Scientific Research under project BIA2004-04835; a post-doctoral contract associated with the study was awarded by the CSIC and co-financed by the European social fund (REF. I3P-PC2004L).
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