Microstructure development of alkali-activated fly ash cement: a descriptive model

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Abstract

The microscopic study of a set of alkali-activated and thermally cured fly ash samples enabled the authors to establish a descriptive model for the microstructural development of fly ash-based cementitious geopolymers.

The morphology of most fly ash particles (perfect spheres) not only makes microscopic research highly productive but facilitates the formulation of hypothesis explaining the fly ash activation over time through a series of consecutive steps that can be successfully fitted to real situations.

Introduction

The copolymerization of the individual alumina and silica components that takes place when aluminosilicate source materials are dissolved at a very high pH yields amorphous [1], [2], [3], [4], [5] “zeolitic precursor” (sometimes called “geopolymer”).

Whilst the product of the fly ash activation reaction is similar in cementitious properties to ordinary Portland cement (OPC), the process itself entails certain financial and environmental advantages over traditional OPC manufacture.

In such activation, the fly ash is mixed with alkaline activators (alkaline solutions) and the resulting paste is solidified by curing. In this process, the glassy constituent of the fly ash is transformed into well-compacted cement. The advent of advanced instrumental techniques, such as MASNMR, SEM, TEM, etc., has helped to clarify important aspects of the structure and morphology of these new materials. Hence, previous studies [3], [6], [7] have found that the main reaction product of such systems is a short-range ordered amorphous aluminosilicate gel: a three-dimensional structure where the Si occurs in a variety of environments, with a predominance of Q4(3Al) and Q4(2Al). This material could consequently be considered to be a “zeolite precursor”. Indeed, small amounts of certain zeolites such as hydroxysodalite, herschelite, etc. are often detected in these systems [3], [6].

It is within this intertexture where the main objective of this work was defined: To establish a singular and conceptual model capable to describe the general process of alkaline activation of fly ashes, independent on the experimental conditions at which the activation is produced; that is to say, independent on the particular characteristics of fly ashes, on the type and concentration of the activating dissolution, etc.

The authors of the present paper, according to previous results [3], [6], [7], [8], [9], consider that experimental conditions at which alkali activation is conducted will specially affect the chemical composition of the reaction products and also to the kinetics of reactions but not to the mechanisms controlling the setting and hardening process of the material.

The proposed model is mainly based on some data gathered through the different electron microscopy techniques since pictures strongly facilitate the comprehension of the steps through which the alkaline activation of fly ashes flows. Nevertheless, the bases supporting the model are in good agreement with other results provided by XRD, FTIR, MAS-NMR, etc. These results have not been included in the paper as they can be extracted from the bibliography [3], [6], [7].

Section snippets

Experimental

This survey was conducted on a class F fly ash with the following chemical composition: 53.09% SiO2, 24.80% Al2O3, 8.01% Fe2O3 and 2.24% CaO. The ash was mixed with an 8 M solution of NaOH, (“solution/ash” ratio=0.35). The resulting paste was poured into small plastic moulds and oven-cured at 85 °C for 5 h, 24 h and 60 days. The hardened pastes were removed from the oven at the specified ages and immediately frozen in acetone for subsequent testing:

  • (1)

    The degree of reaction at each particular

Results

In Fig. 1, the values of the degree of reaction of a fly ash activated with the 8 M NaOH dissolution have been plotted versus the time of thermal curing at 85 °C. Results show, as expected, the degree of reaction continuously increasing with time. However, it should be remarked from this figure the especially high degree of reaction reached during the first hours of the thermal curing.

The micrographs presented in this paper depict the typical microstructure developed by the material at specific

Discussion

It is important to remark at the beginning of the discussion section that authors of this paper have enough background from their investigation (some already published) as to strongly believe that most of type F fly ashes are suitable to be alkali-activated [3], [7], [11], [12]. In good agreement with other authors findings, we have demonstrated that there are some key factors influencing the potential reactivity of the fly ashes (reactive silica content, the vitreous phase content and the

Conclusions

Electron microscopy is a very useful tool for monitoring the microstructural development, over time, of the cementitious matrix generated as a result of the alkali activation of fly ash. Additionally, the spherical shape of fly ashes facilitates the formulation of a simple conceptual model capable of describing the general process of alkali activation of the ashes in the form of series of consecutive steps. This process is non-uniform and is governed by dissolution in the early stages of the

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