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• Modified vermiculite achieved a better expansibility under microwave irradiation. • A novel quantitative analysis method of microwave energy absorption was proposed. • Influence mechanism of Mg2+ modification on microwave absorption was analyzed. This work investigated the effect of microwave power on vermiculite expansibility for different exposure time and compared the changes after Mg2+ modification. The expansion ratio of the modified vermiculite sample reached up to 5.29 at 800 W with a significant increase of 23.89 % in contrast to the raw vermiculite sample. Aimed to quantify the microwave energy absorption of vermiculite, a novel method was proposed by employing water as a medium material due to its sensitivity to microwave energy. Using this method, the effect of microwave power on microwave energy absorption of vermiculite was studied. Meanwhile, the influence mechanism of Mg2+ modification on microwave energy absorption of vermiculite was analyzed by means of energy dispersive X-ray spectroscopy and thermogravimetric-differential scanning calorimeter. Mg2+ modification could significantly improve the microwave energy absorption ability of vermiculite, which was mainly ascribed to the increase of interlayer water content through Mg2+ hydration reaction. [ABSTRACT FROM AUTHOR]