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Perlite is a sterile, neutral product obtained from volcanic rock.

Such blends are typically heavy on the bark and may contain ingredients like sponge rock, coarse perlite and charcoal.

An initial investigation said a construction crew working on a new administrative building next to the gym had stored bags of perlite used in plastering and internal finishing work on the roof that had grown heavy by absorbing water from the recent heavy rains striking much of the country, particularly the northwest.

Perlite, a form of volcanic glass, that was piled on the roof soaked up rain water, causing it to collapse at 15:00 local time Sunday, investigators say.

A preliminary investigation found that construction workers illegally placed perlite, a mineral with high water content, on the roof of the gymnasium during construction of a teaching building adjacent to the gymnasium, Xinhua reported.