Or shredded phone books, newspaper, dandelion seeds, straw, scraps from cutting clothing, fiberglass, foamed sodium silicate, wadded-up aluminum foil, crushed lightweight firebrick, charcoal, sawdust, concrete foamed with detergent, LECA, pumice, vermiculite, perlite, rockwool, wood chips, palo borracho pod silk, cottonwood cotton, dry leaves, etc. Some of these would need to be treated with fire retardant, though being encased in concrete would cut down on the fire risk.