Setting and Hardening of Geopolymeric Cement Pastes Incorporated with Fly Ash

ACI Materials Journal, Sep/Oct 2009 by Zhang, Yunsheng, Li, Zongjin, Sun, Wei, Li, Wenlai

In this study, a fully automated electrodeless resistivity apparatus was developed and used for continuously monitoring the changes in electrical resistivity with time of geopolymeric cement pastes with fly ash. The results showed that the incorporation of fly ash increased the geopolymerization reaction rate to some extent and ultimate electrical resistivity of geopolymeric cement pastes. Comparatively, the fly ash content had a little impact on electrical resistivity. The physicochemical process that occurred during the geopolymerization process was also analyzed on the basis of the electrical resistivity evolution curves. To aid in understanding the resistivity measurement, an environment scanning electron microscope with energy dispersive X-ray (EDX) analysis (ESEM-EDXA) was employed to observe in place the geopolymerization process at early ages. In addition, geopolymeric products were characterized by using X-ray diffraction (XRD) and infrared analysis (IR). Based on the aforementioned analysis, a describable model involving dissolution, reorientation, and polycondensation reactions was established for a better understanding of the geopolymerization reaction during the setting and hardening process of geopolymeric cement incorporated with fly ash.

Keywords: electrical resistivity; fly ash; geopolymeric cement; hydration.

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INTRODUCTION

Geopolymer is a novel family of amorphous alumino-silicate cementitious materials that was first introduced to the inorganic cementitious world by Davidovits1 in 1978. Geopolymeric cement materials can be synthesized simply by mixing reactive alumino-silicate materials with less or no CaO component (such as metakaolin, Class F fly ash) and strongly alkaline solution (such as NaOH or KOH or water glass), and then curing at room temperature. Under a strongly alkaline solution, reactive alumino-silicate materials are dissolved into the solution to form free SiO4 and AlO4 tetrahedral units. With the development of reaction, water is gradually split out and these SiO4 and AlO4 tetrahedral units are linked alternatively to yield polymeric precursors (-SiO4-AlO4-, or -SiO4-AlO4-SiO4-, or -SiO4-AlO4-SiO4-SiO4-) by sharing all oxygen atoms between two tetrahedral units, and thereby forming amorphous geopolymeric products with a three-dimensional network structure.

Geopolymeric cement possesses the following characteristics as compared to portland cement2-5: the production of geopolymeric cement only requires 1112 to 1472 °F (600 to 800 °C), which is much lower than portland cement (2552 °F [1400 °C]), and thus providing a great reduction in energy consumption. In addition, CO2 emission is 80 to 90% less than from portland cement. Reasonable strength can be gained in a short period at room temperature. In most cases, 70% of the final compressive strength is developed in the first 12 hours. Low permeability, comparable to natural granite, is another property of geopolymeric cement. It is also reported that resistance to fire and acid attacks for hardened geopolymeric cement pastes is substantially superior to those for portland cement.6,7 Apart from its high early strength, low permeability, and good fire and acid resistance, geopolymeric cement also attains higher unconfined compressive strength and lower shrinkage than portland cement.8 Other documented properties include good resistance to freezing-and-thawing cycles as well as excellent solidification of heavy metal ions.8-11 These properties make geopolymeric cement become a strong candidate for substituting portland cement.

The electrical resistivity change with time is regarded as a fingerprint of hydration process of cement-based materials.12 Through monitoring the electrical resistivity of fresh cement materials, different periods in the hydration process can be identified. Thus, the measurement of electrical resistivity can be used to understand the physicochemical reactions that occur during the setting and hardening process of cementbased materials. Moreover, it can be used as a quality index for cement-based materials at an early age. Previous research on the electrical property of cement-based materials can be divided into two stages. In the first stage, a direct current (DC) is applied to two electrodes that are connected to a cement-based specimen for electrical resistivity measurement.13 Because any application of DC to a cement-based specimen produces a polarization effect, this method cannot provide accurate results for the electrical resistivity of cement-based materials. In the second stage, to overcome the disadvantage of direct current, high-frequency (1000 Hz) alternating current (AC) is applied for measuring the electrical resistivity.14-20 In this case, the polarization effect is eliminated, but the electrode-paste contact problem caused by the gradual shrinkage of cement-based materials during the setting and hardening process cannot be solved. In addition, gas (H2 or O2) is also released out of the interfacial zone between the electrode and cement paste after applying AC voltage for a very long time due to electrolyzing effect, which will further weaken the electrodes-paste contact.