Effects of Microwave Energy on Fast Compressive Strength Development of Coal Bottom Ash-Based Geopolymers

Sungil Hong,Hyo Kim

doi:10.1038/s41598-019-52160-2

Abstract

Microwave energy has been shown to be effective for geopolymer synthesis due to its fast and penetrative heating characteristics; however, the changes in the physicochemical properties of the geopolymer, resulting from the microwave irradiation, have not been fully elucidated. Therefore, this study is aimed at investigating the effect of the microwaving on the properties of coal bottom ash(CBA) geopolymers. We prepared geopolymer samples by casting a mixture of ground CBA and 14 M NaOH solution against cubic molds with a hand press machine, followed by pre-curing in a dry oven at 75 °C for 24 h and microwaving under various powers and durations. The compressive strength strongly depended on the moisture content, i.e., the strength increased from 21 to 65 MPa or higher as the moisture content decreased to critical values, after which the strength began to decrease. The results showed that microwave energy stimulated an additional geopolymerization by evaporating the redundant free water. This led to the strength gain, although the over-irradiation generated a high internal stress and poor structural integrity, which resulted in the strength loss. Therefore, the appropriate application of microwave energy is a promising option for synthesizing high-strength geopolymers in a cost- and time-effective manner.

Highlights

Since the first discovery of the alkali-activated binders by V
In the previous research[25], we demonstrated that the coal bottom ash (CBA)-based geopolymers could attain a high compressive strength by the additional use of microwave energy after a conventional dry-oven curing process
The compressive strength of the geopolymers were measured while manipulating the power and time of a household microwave oven; subsequently, the outcomes were corroborated by macroscopic observations, such as appearance inspection, temperature measurement, and moisture content evaluation as well as structural and morphological investigations by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD)

Summary

Introduction

Since the first discovery of the alkali-activated binders by V. Fernández-Jiménez and his group delineated the reaction mechanism of geopolymer synthesis, so called “geopolymerization”, into several successive steps that occur while overlapping each other to some extent They are as follows[12,13,14]: (1), dissolution of oligomers from the starting materials into the alkaline media; (2), polycondensation of these species to produce polymeric aluminosilicate structures; and (3), progressive incorporation of silicon into the matrix with a growth of crystal structures in a long term. In the previous research[25], we demonstrated that the coal bottom ash (CBA)-based geopolymers could attain a high compressive strength by the additional use of microwave energy after a conventional dry-oven curing process. This study proposed a practical synthesis method for high strength CBA-based geopolymers, which would extend the utilization of microwave energy on the geopolymer applications. The chemical composition of CBA is analogous to Class F fly ash because the sum of SiO2, Al2O3, and Fe2O3 is larger than 70%, as classified in ASTM C618

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Conclusion